The Physical Geography of Southeast Asia (Oxford Regional Environments)

The Physical Geography of Southeast Asia

THE OXFORD REGIONAL ENVIRONMENTS SERIES PUBLISHED

The Physical Geography of Africa edited by William M. Adams, Andrew S. Goudie, and Antony R. Orme The Physical Geography of North America edited by Antony R. Orme The Physical Geography of Northern Eurasia edited by Maria Shahgedanova FORTHCOMING

The Physical Geography of South America edited by Tom Veblen, Kenneth Young, and Antony R. Orme The Physical Geography of Fennoscandia edited by Matti Seppälä The Physical Geography of Western Europe edited by Eduard Koster The Physical Geography of the Mediterranean Basin edited by Jamie Woodwand

The Physical Geography of Southeast Asia Edited by

Avijit Gupta

3

3

Great Clarendon Street, Oxford ox2 6dp Oxford University Press is a department of the University of Oxford. It furthers the University’s objective of excellence in research, scholarship, and education by publishing worldwide in Oxford New York Auckland Bangkok Buenos Aires Cape Town Chennai Dar es Salaam Delhi Hong Kong Istanbul Karachi Kolkata Kuala Lumpur Madrid Melbourne Mexico City Mumbai Nairobi São Paulo Shanghai Taipei Tokyo Toronto Oxford is a registered trade mark of Oxford University Press in the UK and in certain other countries Published in the United States by Oxford University Press Inc., New York © Oxford University Press 2005 The moral rights of the author have been asserted Database right Oxford University Press (maker) First published 2005 All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, without the prior permission in writing of Oxford University Press, or as expressly permitted by law, or under terms agreed with the appropriate reprographics rights organization. Enquiries concerning reproduction outside the scope of the above should be sent to the Rights Department, Oxford University Press, at the address above You must not circulate this book in any other binding or cover and you must impose this same condition on any acquirer British Library Cataloguing in Publication Data Data available Library of Congress Cataloging in Publication Data Data available ISBN 0-19-924802-8 10 9 8 7 6 5 4 3 2 1 Typeset by Graphicraft Limited, Hong Kong Printed in Great Britain on acid-free paper by Antony Rowe Ltd., Chippenham, Wilts.

Foreword The Physical Geography of Southeast Asia is the fourth in a series of advanced books that is being published by Oxford University Press under the rubric of Oxford Regional Environments. The aim of the series is to provide a durable statement of physical conditions on each of the continents, or major regions within those continents. Each volume includes a discussion of the systematic framework of the region (for instance, tectonism, climate, biogeography), followed by an evaluation of dominant environments (such as mountains, forests, and deserts) and their linkages, and concludes with a consideration of the main environmental issues related to the human use and misuse of the land (such as resource exploitation, agricultural and urban impacts, pollution, and nature conservation). While books in the series are framed within an agreed context, individual books seek to emphasize the distinctive qualities of each region. We hope that this approach will provide a coherent and informative basis for physical geography and related sciences, and that each volume will be an important and useful reference source for those concerned with understanding the varied environments of the continents. Andrew Goudie, University of Oxford Antony Orme, University of California, Los Angeles

To Anthea For long companionship in search for the ideal beach, the well-behaved volcano, and the perfect chicken rice

Preface Southeast Asia is a fascinating part of the world not only because of its physical environment but also because of its long cultural and social history and its recent economic development. The latter is important to an earth or a natural scientist, and certainly to a physical geographer, because of its widespread environmental impact. Plate tectonics and people are both important contributors to the physical geography of Southeast Asia. Southeast Asia carries a well-established mental image but it is difficult to define on the ground. It is separated from India and China by a vast area of near-continuous high mountains, but the exact boundary of the region is not clearly defined physically and is often taken as the political line of separation between states. This may not be satisfactory, as less than two centuries ago the northern boundary of the various kingdoms was ill-defined although the physical region definitely existed. All the major rivers of mainland Southeast Asia (except the Chao Phraya) start outside the region. Similarly, the line on the map that separates Southeast Asia from Australia is not Wallace’s line or the furthest extension of the volcanoes but a north–south line drawn politically across the large island of New Guinea. The region, however, is perceived as an assemblage of large river basins sloping south and east, a number of peninsulas and archipelagos, and seas that exist between islands and are connected by straits of various widths. It is an area that displays great physical variations. For working purposes we have defined the region as the combined sovereign area of the countries Myanmar, Thailand, Malaysia, Singapore, Indonesia, Lao PDR, Cambodia, Viet Nam, Brunei Darussalam, East Timor, and the Philippines. Southeast Asia has been perceived as a laboratory for studying plate tectonics. Its outer boundary, an arc formed by the islands of Indonesia and the Philippines, is an area of active subduction and a wonderful volcanic landscape. Several of the volcanoes are infamous across the world, such as Krakatau, Tambora (whose eruption of 1815 is probably the biggest in historic time), and more recently Pinatubo. The mainland part was shaped by escape tectonics associated with the collision of India with Asia, which began in the Tertiary. The locations of several major river valleys were thus determined. The changing sea levels of the Pleistocene times moved the seas for hundreds of kilometres across the shallow Sunda and Sahul Shelves. This impacted significantly on the biophysical environment of Southeast Asia. Closer to our time, the physical environment was significantly degraded anthropogenically, first by the establishment of plantation and extraction economies of the colonial times, and then, since the second half of the twentieth century, by widespread deforestation, agricultural expansion, resettlement, and urbanization. The forms and processes on land and offshore for very large parts of the region should no longer be seen as natural. The effect of such economic activities on local vegetation and fauna, particularly on rainforests, peat swamps, mangroves, and coral reefs, has been horrific. The book discusses these characteristics of the region in twenty-four chapters which can be broadly grouped into three units. Part I (Chapters 1–7) introduces various biophysical components of Southeast Asia, establishing the regional framework. Part II (Chapters 8–13) discusses in detail several aspects of the physical geography of Southeast Asia that are widespread, fascinating, and representative of the regional environment. Part III (Chapters 14–24) focuses in detail on specific topics both as natural phenomena and as examples of the environmental impact of anthropogenic alterations. There is also in this last part a hint towards the future environmental trends in Southeast Asia.

viii Preface

The contributors are extremely knowledgeable about the region, having worked for decades on their areas of interest. The coverage of topics is wide, but environment is a complex and multifaceted subject, and it is extremely difficult to cover all topics satisfactorily and still keep the book within manageable dimensions. Southeast Asian place names, and especially their spelling in English, have changed over the last few decades. For example, the city in Java which the Dutch called Batavia was subsequently known as Djakarta and then as Jakarta. The acceptance of new names or spelling is not uniform. These days everyone, for example, calls the island ‘Sulawesi’ but the sea adjacent to it is referred to as both the Sulawesi Sea and the Celebes Sea. Then there is the question of whether it is correct to write ‘Vietnam’ or ‘Viet Nam’. ‘Laos’ is widely used, but the country is ‘Lao PDR’ (People’s Democratic Republic). In this book we have followed the line of least resistance, i.e. used the name and spelling the reader is most likely to find on the ground or on a map or in a local newspaper, except in rare cases where individual idiosyncrasies have intervened. For example, we have written ‘Jakarta’ (instead of ‘Djakarta’) and ‘Johor Baru’ (instead of ‘Johore Bahru’), but ‘Sumatra’ (not ‘Sumatera’); we have also used ‘Krakatau’, and not the externally imposed irrational spelling of ‘Krakatoa’. The reader, however, should keep these variations in mind when consulting old maps or research publications. Certain terms have been used interchangeably depending on location, common use, etc. These are gunung or mount/mountain, gua or cave, pulau or island and sungai or river. Lastly, some familiarity with local languages is extremely rewarding in Southeast Asia. The new volcanic cone that is rising in the middle of the old blown-apart crater of Krakatau is ‘Anak Krakatau’, baby Krakatau or the son of Krakatau, a hint of a possible sequel to a disastrous episode. A. G. Leeds January 2003

Acknowledgements The authors, editor, and publishers would like to thank the following, who helped with the preparation of this volume and kindly gave permission for the copyright material. We are grateful to Marie-Françoise André, Nick Chappell, Chris Cocklin, Ian Douglas, Goh Kim Chuan, Jerry Mueller, Jan Nossin, John Pitts, Tom Spencer, Jean-Claude Thouret, Ian Turner, and Wong Poh Poh for carefully reviewing earlier drafts of the chapters and for many helpful suggestions. A number of figures were either drawn or redrafted by Lee Li Kheng and the graphics office of the Department of Geography, University of Leeds. Jean-Claude Thouret and Franck Lavigne would like to thank Spot Image for allowing them to obtain and interpret one SPOT XS image of 1990 on the Merapi (Figure 16.2). They would also like to acknowledge the help received from colleagues at the Volcanological Survey of Indonesia at Bandung and Yogyakarta. Avijit Gupta would like to acknowledge two visits to the Centre for Remote Imaging, Sensing and Processing, National University of Singapore while working on this book. The following have kindly given permission for the use of copyright material: Figs. 1.1, 1.3, 1.4, 1.5(a), 1.6(B), and 1.6(C): from Geological Evolution of South-East Asia, Hutchison, pp. 9– 42 (1996) with permission from The Geological Society of Malaysia. Fig. 1.6A: from Journal of Asian Earth Sciences, 6, Harris, p. 377 fig. 4. (1991) with permission from Elsevier Science. Figs. 1.7 and 1.9: from South-East Asian Oil, Gas, Coal and Mineral Deposits, Hutchison, p. 17, fig. 2.1 and p. 89 fig. 4.7 (1996) by permission of Oxford University Press. Fig. 1.5(b) and 1.6(D): from Tectonic Evolution of Southeast Asia, Hall and Blundell (eds.) (1996), Society Special Publication No. 106, with permission from The Geological Society of London. Fig. 1.8: from ‘Pre-Cretaceous evolution of SE Asian terranes’, Metcalfe, p. 100, in Tectonic Evolution of Southeast Asia, Hall and Blundell (eds.) (1996), Society Special Publication No. 106, with permission from The Geological Society of London. Fig. 4.6: from Geomorphology, 44, Nos. 3–4 Gupta et al., ‘Evaluation of part of the Mekong River Using Satellite Imagery’ p. 227 (2002) with permission from Elsevier Science. Fig. 5.4: from Tropical Climatology, McGregor and Nieuwolt, p. 154, fig. 8.6 (1998) 2e. Reproduced with permission of John Wiley & Sons Limited. Fig. 6.1: from FAO-Unesco Soil Map of the World, fig. 7 (1979), with permission from Food and Agriculture Organization of the United Nations and United Nations Educational, Scientific, and Cultural Organization. Plate 1: from Biological Conservation, 95, No. 2, Blasco et al., ‘A framework for the worldwide comparison of tropical woody vegetation types’, pp. 175–89 (2000) with permission from Elsevier Science. Fig. 9.5: from Speelman, ‘Geology, hydrogeology and engineering geological feature of the Serayu riverbasin’. Ph.D. thesis, p. 86 (1979), Free University of Amsterdam with permission. Fig. 10.1: from Cave and Karst Science, 27/2:61–70, ‘An assessment of protected karst landscapes in Southeast Asia’, Day and Urich (2000) with permission. Fig. 10.10: from Précis de Karstologie, Salomon, p. 148, fig. 60 (2000) with permission from Presses Universitaires de Bordeaux. Table 14.2: from The Structure, Function and Management Implications of Fluvial Sedimentary Systems Dyer, Thoms, and Olley (eds.), ‘Sediment movement on steep slopes to

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Acknowledgements

the Mekong River: an application of remote sensing’, Gupta and Chen, Table 2, p. 404, IAHS Publ. No. 276 (2002) with permission from The International Association of Hydrological Sciences Press. Fig. 14.4: from Erosion and Sediment Yield: Global and Regional Perspectives, Walling and Webb (eds.), ‘Erosion and sediment yield in Southeast Asia: a regional perspective’, Gupta, Fig. 1, p. 217, IAHS Publ. No. 236 (1996) with permission from The International Association of Hydrological Sciences Press. Fig. 14.5: from Variability in Stream Erosion and Sediment Transport, Olive, Loughran, and Kesby (eds.), ‘Spatial distribution of sediment discharge to the coastal waters of South and Southeast Asia’, Gupta and Krishnan, Fig. 1, p. 458, IAHS Publ. 224 (1994) with permission from The International Association of Hydrological Sciences Press. Fig. 15.1: from Outline of the Geomorphology of Indonesia. H. Th. Verstappen, ITC Publications 79 (2000) with permission from H. Th. Verstappen. Fig. 15.2: from Volcanoes of the Philippines, Phivolcs Press, by permission of Phivolcs. Fig. 15.3: from Earth’s Changing Surface, M. J. Selby, p. 68 fig. 4.5 (1985) by permission of Oxford University Press. Fig. 15.5: from ITC Journal, (1996), 2: 110–24, ‘Cartographic modelling of erosion in pyroclastic flow deposits of Mount Pinatubo, Philippines’, Daag and van Westen, by permission of C. J. van Westen. Fig. 15.12: from unpublished maps by Sijmons, van Westen and Dayao, by permission of K. Sijmons. Figs. 16.1, 16.2, 16.5, 16.6, 16.7, 16.8, 16.9, 16.10: from the Journal of Volcanology and Geothermal Research, 100, Thouret et al., ‘Toward a revised hazard assessment’, pp. 479–502 (2000) with permission from Elsevier Science. Fig. 16.14.: from GeoJournal, 49/2, (1999) p. 180, ‘Lahar hazard micro-zonation and risk assessment in Yogyakarta city, Indonesia’, Lavigne, fig. 7, with kind permission of Kluwer Academic Publishers. Fig. 19.2: from Physical Adjustments in a Changing Landscape: the Singapore Story, Gupta and Pitts (eds.), ‘The control of water quality in Singapore’ Appan, p. 376 (1992) with permission from Singapore University Press and A. Appan. Figs. 24.4 and 24.6: from World Atlas of Coral Reefs, Spalding, Ravilious, and Green (2001) by permission of University of California Press. Full references for these figures have been included in their respective chapters. Although every effort has been made to trace and contact copyright holders, this has not always been successful. We apologize for any apparent negligence.

Contents

List of Figures List of Plates List of Tables List of Contributors I. The Physical Framework 1. The Geological Framework charles s. hutchison 2. The Quaternary in Southeast Asia   3. Landforms of Southeast Asia   4. Rivers of Southeast Asia   5. The Climate of Southeast Asia    6. Soils of Southeast Asia .  7. Vegetation  . 

xiii xix xx xxi 1 3 24 38 65 80 94 105

II. Specific Environments

121

8. Granitic Terrains . .  9. Volcanic Islands  .  10. Karst in Southeast Asia   11. The Coastal Environment of Southeast Asia . .  12. The Mekong River Basin   13. Southeast Asian Deltas  . 

123 142 157 177 193 219

III. Environment and People

237

14. Accelerated Erosion and Sedimentation in Southeast Asia   15. Volcanic Hazards in Southeast Asia  .  16. Hazards and Risks at Gunung Merapi, Central Java: A Case Study -    

239 250 275

xii Contents

17. Hydrology and Rural Water Supply in Southeast Asia    18. The Urban Environment in Southeast Asia   19. Water in Cities       20. The Urban Geomorphology of Kuala Lumpur   21. Subsidence and Flooding in Bangkok      22. Urban Pollution in Southeast Asia   23. Coastal Zone Development in Southeast Asia . .  24. Coral Reefs of Southeast Asia: Controls, Patterns, and Human Impacts .   . .  Index

300 314 336 344 358 379 389 402

429

List of Figures 1.1 1.2 1.3 1.4

1.5 1.6 1.7 1.8 1.9 2.1 2.2 2.3 2.4 2.5 2.6 3.1 3.2 3.3 3.4 3.5

3.6 3.7 3.8 3.9 3.10 3.11 3.12 3.13 3.14 4.1 4.2

4.3 4.4 4.5

Major physiographic features of Southeast Asia Chronological summary of the major geological changes in Southeast Asia The oceanic lithospheric areas of Southeast Asia The active plate margins of Southeast Asia, showing the positions of the trenches, volcanic arcs, main transform faults, and Benioff zones (a) Main structural features of Myanmar; (b) The major right-lateral wrench faults of Sumatra Structural interpretations of the Timor collision zone Distribution of oil- and natural-gas-bearing basins of Southeast Asia Distribution of continental blocks, terranes, and principal sutures of Southeast Asia Cathaysian and Gondwanaland Carboniferous–Permian entities The extent of dry land at maximum and average sea levels Sea-level fluctuations of the last 300 000 years The Sepik floodplain of Papua New Guinea infills a vast Holocene estuary Glaciation and glacial features on Mount Jaya Vegetation change at Bandung, a 120 000 year record of climate Human farming has been a major agent for geomorphic change. Paddy fields in Sulawesi Physiographic provinces of Southeast Asia Northern Mountainous Region The Mekong River in north Lao PDR Western Myanmar Hills and Central Myanmar Lowlands Tenasserim coast, Tenasserim Hills, Central Plain of Thailand, Khorat Upland, Coastal Plain of southeastern Thailand, and Elephant and Cardamom Hills Shan Highland and hills of northern Thailand and Lao PDR Central Highland of Malay Peninsula and the coastal plains of Kra Isthmus and the Malay Peninsula Steep granitic hills, Penang, Malaysia Annamite Chain, Mekong Lowland, and North Viet Nam Plain The island of Borneo The islands of Indonesia and details of Sumatra Sumatra: ignimbrite topography near Bukittinggi Volcanic landscape, Java The Philippines Location map (a) Granite under erosion at a cascade in a small forest stream, Johor, Malaysia; (b) Downstream of the cascade granite corestones form boulders in the channel Bar formation in the Tembeling River, Pahang, Malaysia Tidal river through a flat coastal plain, Pontian Kechil, Johor, Malaysia River channel in an active volcanic arc, Luzon, Philippines

4 5 6

8 10 12 16 18 19 27 28 28 29 30 35 41 43 44 45

47 48 49 53 54 57 58 59 60 61 66

68 69 69 70

xiv List of Figures

4.6 4.7

Annual hydrographs of the Mekong and two of its tributaries Annual hydrograph of the Mun River at Rasi Salai, Thailand, for 1996 4.8 Annual hydrograph of the Rajang River at Benin Nanga, Sarawak, Malaysia, for 1994 4.9 Diagrammatic sketch of the Irrawaddy leaving an alluvial reach to enter a rock-cut course along the Sagaing Fault 4.10 Flood signs in the Mekong, falling stage of the flood hydrograph, upstream of Luang Prabang, Lao PDR 4.11 Diagrammatic sketch of a rock-cut reach of the Mekong downstream of Savannakhet, Lao PDR 4.12 The Phapheng Falls on the Mekong 5.1 Mean annual rainfall distribution in Southeast Asia 5.2 The southwest and northeast monsoon wind systems in Southeast Asia 5.3 Monthly distribution of rainfall for selected stations in Southeast Asia 5.4 Main areas of tropical cyclone and common tracks 6.1 Soils of Southeast Asia 7.1 The location of Wallace’s line, which marks the boundary between the Oriental and Australian faunal regions 7.2 The forest cover of Southeast Asia 8.1 Distribution of granitic rocks in Southeast Asia 8.2 Regolith exposed in quarry face in upland east of Gopeng, Perak 8.3 Core-boulders exposed (a) near Gamencheh, Negeri Sembilan, and (b) near Sungai Batang, Padang 8.4 (a) Sketch of bare rock surface exposed in southern Myanmar. (b) Bare rock slopes exposed in steep face of valley in southwestern outskirts of Georgetown, Pulau Pinang 8.5 Slope development in the context of bornhardt–nubbin model: (a) stages in valley deepening and regolith development and (b) end result 8.6 (a) Boulder-strewn surface cleared for plantation agriculture near Tampin, in Negeri Sembilan. (b) Boulder-strewn hill in ranges east of San Diego, southern California 8.7 Flutings (a) on boulder near Tampin, (b) on boulder exposed on southern littoral of Pulau Ubin, in the Strait of Johor 8.8 Flared and fluted sidewall of boulder exposed by accelerated soil erosion near Tampin, in Negeri Sembilan 8.9 (a) The peak of Kinabalu. (b) The Donkey’s Ears and spall plates near the crest of Kinabalu 9.1 Vertical airphoto of the lava flows on the northeast slopes of the Ciremai volcano, west Java, Indonesia 9.2 Vertical airphoto of fluvio-volcanic slopes, west of Sasangani Bay, Mindanao, Philippines 9.3 Vertical airphoto showing less dissected lahar deposits and details in an area near Figure 9.2 9.4 Ash deposits and burnt vegetation on the slopes of the Galunggung volcano, west Java, after the 1982–3 eruption 9.5 Groundwater resource of the Slamet volcano, central Java 9.6 A geothermal powerplant in the volcanic Dieng Plateau in central Java, Indonesia

71 72 73 74 76 77 78 80 81 84 86 103 105 115 124 126 128

130

132

133 134 136 138 145 146 147 148 149 150

List of Figures xv

9.7

Sulphuric mud deposits in the crater bottom of the Sorikmerapi volcano, Sumatra, Indonesia 9.8 Geomorphic sketch map of the islands of Bali and Lombok, Indonesia 9.9 The nested Batur caldera, Bali, seen from the top of Mount Agung 9.10 Vertical aerial view of the top area of Mount Rinjani, Lombok, and the east–west-stretching tectonic depression with the lake and the cinder cones of Mount Baru (I and II) 9.11 Ground view of Lake Rinjani looking west towards the top of the volcano 10.1 Karst areas of Southeast Asia 10.2 Radar image of the Kinta Valley, Malaysia 10.3 The Gunung Tempurung karst massif in the Kinta Valley, Malaysia 10.4 A well-preserved phreatic tube with solutional scallops and an incised canyon at the highest levels of Gua Tempurung 10.5 Tin-bearing solution pipes in the alluviated karst of the Kinta Valley, Malaysia 10.6 Distribution of limestone massifs in the Gunung Mulu National Park, Sarawak 10.7 Hidden Valley, a major karstic valley bisecting the Gunung Api massif 10.8 The entrance of Deer Cave (Gunung Mulu National Park), one of the largest natural tunnels at over 1500 m long and 150 m wide 10.9 Pinnacle karst at 1200 m altitude on the flanks of Gunung Api, Gunung Mulu National Park 10.10 Sequence of karst landforms in the Guilin area of China 11.1 Major geological and climatic elements influencing the coastal environments of Southeast Asia 11.2 Beaches, mangroves, and coral reefs in Southeast Asia 11.3 Granite coast with boulders precariously balanced and subject to rockfall on the exposed side of Pulau Perhentian, Malaysia 11.4 A double tombolo formed as a result of the monsoons from opposite directions affecting three rocky islets on a reef flat, Ko Nang Yuan, Gulf of Thailand 11.5 Coastal dunes at Parangtritis, south coast of Java 11.6 Owing to drier conditions, cactus has replaced the more typical Ipomoea formation on the north coast of Gili Trawangan, Lombok, Indonesia 11.7 Along various sectors of Bunaken Island, north Sulawesi, Indonesia, mangroves have established on the reef flat and protect the sandy beach 11.8 Major zones in a mangrove forest 11.9 Typical profile of a fringing reef in Southeast Asia 12.1 Major rivers and locations in the Mekong Basin 12.2 Mean annual rainfall over the Mekong below China 12.3 Mean monthly discharges at key gauging stations on the main river and tributaries in the lower Mekong Basin 12.4 The original plans for hydropower development in the Lower Mekong Basin 12.5 The Tonlé Sap Basin

150 151 152

153 154 158 159 160 162 163 165 166

166 168 171 178 181 183

184 185

185

186 187 189 194 195 196 198 205

xvi List of Figures

12.6 12.7

Water balance of the Tonlé Sap The Mekong Delta showing the approximate limit of brackish water penetration 13.1 Schematic model of typical Southeast Asian delta, showing the main morphological components on the delta, and typical landforms associated with river-, tide-, and wave-dominated sectors 13.2 Southeast Asia showing the Sunda Shelf and the probable extension of river systems across it at times of lower sea level, and deltas discussed in the text 13.3 Irrawaddy Delta, showing major distributaries and extent of salt water 13.4 Mekong Delta, incorporating data on pollen cores and radiocarbon dates on coastal progradation 13.5 Mahakam Delta in east Kalimantan: ecological zonation 13.6 Rajang and Baram Deltas in Sarawak, showing distribution of peat swamp and idealized long section 13.7 Chao Phraya, Thailand, with stratigraphical long section 13.8 Solo River, reconstruction of the progradation of the delta, northeastern Java 14.1 Land clearance on steep slopes, in the valley of the Nam Ou, tributary to the Mekong, northern Lao PDR 14.2 Construction in Singapore, exposed sediment 14.3 Volcanic-material-choked river channel draining the slopes of the Merapi volcano, Java 14.4 Distinct sediment plumes in the coastal waters of Southeast Asia 14.5 Deviation of sediment yield from the regional expectation, South and Southeast Asia 14.6 Mangroves affected by excessive sandy sediment 15.1 Volcanoes of Indonesia 15.2 Volcanoes of the Philippines 15.3 Scheme to illustrate subduction 15.4 Collapsed roof of San Marcelino market, 25 km southwest of Mount Pinatubo, February 1992 15.5 Volcanic deposits around Mount Pinatubo 15.6 Pasig-Potrero, effect of lahar after upstream breach of dyke 15.7 Santo Tomas plain, western Zambales, before and after the Pinatubo eruption 15.8 Risk zonation around Mount Pinatubo 15.9 Mapanuepe Lake formed by blocking of the valley by Pinatubo lahars 15.10 Bacolor village in Pasig-Potrero Valley, buried in lahar deposits 15.11 Tangkuban Perahu volcano in the Sunda caldera, with the nearby city of Bandung, and Lembang Ashri on the south flank below the crater (west Java, Indonesia) 15.12 Volcanic deposits of Mayon volcano (Luzon, Philippines) 16.1 Sketch map of the basic geographic context and geologic features of Merapi volcano and its region 16.2 Sketch map of structures and deposits of the Merapi stratocone, interpreted on one satellite SPOT2 XS (multiscanner) image of 9 August 1990 16.3 Dome-coulée of Merapi volcano in August 1994

206 208

221

221 223 224 226 227 229 231 239 242 244 246 247 247 251 252 253 256 258 259 260 261 262 263

265 270 277

278 279

List of Figures xvii

16.4

16.5

16.6 16.7

16.8 16.9 16.10

16.11 16.12 16.13 16.14 17.1 18.1 18.2 18.3

18.4 18.5 18.6 19.1 19.2 20.1 20.2 20.3 20.4 20.5 21.1 21.2 21.3 21.4

Map of the destruction area and block-and-ash flow deposits of the December 1930 eruption of Merapi, with locations of entirely and partially destroyed villages Sketch map of areas affected by Recent and Modern Merapi or New Merapi volcaniclastic debris and limits of the three VSI danger zones Azimuths and travel distances for pyroclastic flows released at Merapi over the period 1904–2000 Hazard-zone map for the Merapi-type eruption scenario based on the 1961–98 scenario, and approximate hazard zones in the event of the mixed effusive–Peléean eruption scenario Hazard-zone map for the Plinian and the worst-case eruption scenarios Part of an ortho-image which covers a digital elevation model illustrating the simulation of gravity-driven pyroclastic currents Plot of height (vertical drop) and length (run-out distance) for the simulated dome collapse, the longest 1930 pyroclastic flow, and the 1994 channelled pyroclastic flows at Merapi Population vulnerability zonation on the Merapi flanks Risk zonation for people living on the Merapi slopes Flowchart of vulnerability assessment study within lahar-related hazard zones Example of hazard and risk assessment in Prawirodirjan, a suburb of Yogyakarta city Location map Location map Trends in the water supply of DKI Jakarta, Indonesia, showing increase in groundwater abstraction since 1980 Penetration of saline water beneath Jakarta, Indonesia, showing the inland movement of the boundary between the saline and fresh groundwater in (a) the first deep aquifer system at 40 to 140 m depth, and (b) the second deep aquifer system at more than 140 m below the surface The Manila area, Philippines Sediments of the Bandung area, Indonesia, showing the inter-fingering of volcanic fan debris with lacustrine deposits Urban suitability, Kuching, Sarawak Location map Singapore: impounding reservoirs and waterworks Kuala Lumpur: geology and place names Growth of Kuala Lumpur, initially along the Klang Valley and now along a north–south axis Distribution of bare areas in the Kuala Lumpur area based on aerial photograph and satellite image interpretation Sequence of changes to a small river in Kuala Lumpur as a result of urban construction Types of foundation problem associated with subsurface conditions in Kuala Lumpur Physical units of Thailand Geomorphological map of the Lower Central Plain Geomorphology and stratigraphical section of the Lower Central Plain Ground surface elevation map of Bangkok

279

281 282

285 286 288

289 291 293 294 295 303 315 321

322 325 329 331 337 341 345 346 350 353 354 359 360 361 363

xviii List of Figures

21.5 21.6 21.7 21.8 21.9 21.10 21.11 21.12 21.13 21.14 21.15 21.16 21.17 23.1 23.2 23.3 23.4

24.1 24.2

24.3 24.4 24.5

24.6 24.7 24.8

Flood protection infrastructures of Bangkok metropolitan area Hydrological profile of the Bangkok aquifer system in a north–south direction Chronological record of groundwater extraction rate in the Bangkok Plain Piezometric levels in the Nakhon Luang aquifer, 1981 Land subsidence of Bangkok, 1981 Piezometric levels in the Nakhon Luang aquifer, 1998 Map of accumulative land subsidence in Bangkok, 1978–97 Land subsidence rate in Bangkok, 1997 Bangkok land subsidence versus time at selected locations Chronological change in accumulative land subsidence along a north–south section of Bangkok Piezometric drawdown in the shallow soft and stiff clays of Bangkok Differential settlements between structures caused by land subsidence Schema of differential settlements between underground structures caused by land subsidence Coastal development in Southeast Asia: selected major uses and impacts Corals removed from reefs for construction or lime-making, Lombok, Indonesia A sea wall within reach of waves in front of a resort encourages beach erosion, Phuket, Thailand Pedestrian walk equipped with lighting being developed on a bund on the landward side of a protective belt of mangroves, south Johor, Malaysia Tectonic units in Southeast Asia (a) Late Glacial–Early Holocene sea-level curve for the Sunda Shelf derived from shoreline (delta plain, mangrove, tidal flat) and marine facies. (b) Holocene sea-level curves for Singapore, Strait of Malacca, and Peninsular Malaysia. (c) 14° corrected age (correction = 410 years) for fossil reef flat corals against height above highest living open-water corals at Ko Taphao Yai, Phuket, south Thailand (a) Seasonal reversals in surface currents, Java Sea. (b) Schematic pathways of the Indonesian Throughflow Coral bleaching events recorded from Southeast Asia in 1998 (a) Monthly mean sea-surface temperatures (1990–2000) for the sea area around Phuket, south Thailand. (b) Percentage of total coral cover by visual bleaching categories on line transects at Phuket, May 1991, 1995, 1998 Patterns of diversity in hermatypic Scleractinian corals in Southeast Asia and adjacent waters Model of Late Holocene reef growth in a muddy environment with assumed slight sea-level fall over last 5000 years (a) Temporal and spatial comparison of water transparency (Secchi disc depths) between four patch reefs in Jakarta Bay. (b) Temporal and spatial comparison of maximum depth of living coral at same stations

365 367 368 369 370 371 372 373 374 374 375 376 377 391 396 397

399 404

406 409 412

413 414 415

421

List of Plates between pp. 264–5 1 Bioclimatic types and potential natural vegetation in Southeast Asia 2 False colour SPOT image with interpretation showing the volcanic and tectonic features in part of Banten, western Java, adjacent to Sunda Strait 3 The distribution of coral reefs and mangrove forests in Southeast Asia, shown in relation to terrestrial and marine topography

List of Tables 3.1 4.1 6.1 7.1 7.2

11.1 12.1 14.1 14.2 15.1 16.1 16.2 16.3 16.4 16.5

17.1 17.2 17.3 18.1 18.2 18.3 18.4 18.5 19.1 20.1 20.2 20.3 22.1 22.2

22.3 22.4 22.5 23.1

Physiographic provinces of Southeast Asia Characteristics of the major rivers in Southeast Asia Reference soil groups Major vegetation types of Southeast Asia Human population, total forest area, percentage forest cover, and annual percentage loss of forest area for the countries of Southeast Asia Synopsis of coasts of Southeast Asian countries Existing and proposed hydropower dams in the Mekong River Basin A summary set of sediment measurement Middle Mekong Basin: seasonal erosion and sediment transfer Volcanic eruptions in the Philippines Population at risk: population density and growth around Merapi, 1976–1995 Eruptive events and reported damage at Merapi, 1672, 1822–1997 Vulnerability parameters for the people living on the Merapi slopes Estimate of potential damage for four eruptive scenarios of the Merapi volcano Nomenclature and classification of vulnerability types, based on thirty-eight interviews in the lahar-prone areas of the Boyong, Code, and Krasak Rivers Water resources and their use in Southeast Asia Irrigated acreage in Southeast Asia Small-scale surface irrigation: Indonesia, Philippines, and Myanmar Population size and growth in some of Southeast Asia’s largest cities, 1960–2015 Indicators of the urban infrastructure provision in the largest cities of Viet Nam Air quality in major Southeast Asian cities Results of urban catchment studies in Pulau Pinang, Malaysia Opportunities for disease vectors created by urban construction and possible solutions Estimated level of urbanization in Southeast Asia Rock weathering grades Results of urban catchment studies in Kuala Lumpur Erosion on a bare construction site at Mengkuang Heights in Ulu Klang near Kuala Lumpur, Malaysia Degree of traffic congestion in selected Southeast Asian cities Frequency of occurrence of days with specific air pollution index categories for four urban centres in the Klang Valley during the haze period September–October 1997 Progress of the ten-year clean-up programme for urban rivers in Singapore Percentage of impervious surface as a function of land use Activities to help determine priorities for action for cities in Southeast Asia Population of coastal cities in Southeast Asia with more than 1 million inhabitants

42 65 95 106

114 182 200 241 243 268 275 283 290 292

296 307 308 308 316 317 319 327 333 336 347 351 351 380

382 384 385 387 390

Contributors Richard T. Corlett is Associate Professor in the Department of Ecology and Biodiversity at the University of Hong Kong. Previously he taught at Chiang Mai University in northern Thailand (1980–2) and at the National University of Singapore (1982–7). His major research interest is the ecology of human impact on species-rich tropical ecosystems in Southeast Asia. Ian Douglas is Emeritus Professor at the University of Manchester. He has worked at the University of Malaya, the Universiti Kebangsaan Malaysia, the Danum Valley Field Centre in Sabah, the Sub-Institute of Geography in Ho Chi Minh City, and the Mekong River Commission, and with partners at Universitas Gadjah Mada and Kaesetsart University. His main interests are tropical rainforests and urban environmental change. R. Dudal is Professor Emeritus, Soil Science, Institute for Land and Water Management, University of Leuven. He worked in Land and Water Development with the FAO (1955– 84). During this period he was assigned to the Soil Science Research Institute, Bogor, Indonesia (1955–9), and also carried out short-term missions in Thailand, Lao PDR, Cambodia, Viet Nam, and the Philippines. He was the correlator of the FAO/ UNESCO Soil Map of the World (1961– 81). His main interests are in soils and land use in the tropics, land evaluation, soil classification, soil fertility, and soil conservation. David Gillieson is Professor of Geography in the School of Tropical Environment Studies and Geography, James Cook University, Cairns. His research interests focus on karst processes, cave management, and tropical environmental monitoring using remote sensing and geographic information systems. He has worked in Southeast Asia, New Guinea, and Australia. Goh Kim Chuan is Professor and Associate Dean at the National Institute of Education, Nanyang Technological University, Singapore. Previously he was Head of Geography at the Universiti Sains Malaysia, Penang, at the Universiti Brunei Darussalam, and until recently, at the National Institute of Education, NTU, Singapore. His research is in hydrology, climatology, and geography and environmental education. Avijit Gupta is with the School of Geography, University of Leeds. He also has a visiting research association with the Centre for Remote Imaging, Sensing and Processing, National University of Singapore, an institution where he taught in the Department of Geography (1975–97). His research interests are in the geomorphology of tropical rivers, large rivers, floods, urban environment, and application of geomorphology and remote sensing in environmental evaluation. He has worked in South and Southeast Asia, the Caribbean, and the United States. Geoffrey Hope is a Research Professor in the Department of Archaeology and Natural History, Research School of Pacific and Asian Studies, Australian National University. His principal field of interest is in vegetation history and the historical biogeography of Australian and Pacific biota. He works with archaeologists on the effects of prehistoric people on their environment as they colonized the Southeast Asian region and spread into the Pacific. In addition to his work in pollen analysis, he is helping to develop new techniques for palaeoenvironmental analysis using carbonized particles, diatoms, and opaline phytoliths. Charles S. Hutchison spent most of his academic career at the University of Malaya (1957–92), where he was Professor of Geology. He is the recognized authority on the geology and tectonics of Southeast Asia. He currently teaches courses part-time at

xxii Contributors

the Universiti Brunei Darussalam. He has been a Visiting Professor at the University of Kansas and Cornell University, worked for ESRI at the University of South Carolina, for CCOP in Bangkok, and for an oil company in Trinidad. Franck Lavigne is a Senior Lecturer at the Institute of Geography, Université Paris 1 Panthéon-Sorbonne. Since 1992 he has worked on the physical geography of Indonesia. His research is on volcanic geomorphology, landslides, riverbed degradation, soil erosion, and sediment load, mostly in volcanic settings. Noppadol Phienwej has taught geotechnical engineering for fifteen years at the School of Civil Engineering, Asian Institute of Technology, Bangkok. He was the chair of the Geotechnical Committee of the Engineering Institute of Thailand (1998–2001). His research interests are in underground excavations, tunnelling, land subsidence in Bangkok, and landslides. Jan J. Nossin was Professor of Applied Geomorphology at ITC, the International Institute for Geo-Information Sciences and Earth Observation, Enschede, the Netherlands. His specialization is in the application of remote sensing in geomorphology. He has research experience in Malaysia, Singapore, Indonesia, the Philippines, and other countries. Together with a number of students from the Philippines, he monitored the geomorphic effects of the 1991 eruption of Pinatubo for a number of years. Similar cooperative research work had been carried out also on the Sunda volcanic complex in Indonesia. Prinya Nutalaya was Professor of Engineering Geology in the School of Civil Engineering, Asian Institute of Technology, Bangkok. His research interests are in engineering geology, geo-environment, and nature and anthropogenic hazard assessment and mitigation. He is known for his pioneer work on land subsidence in Bangkok and earthquakes in Thailand. He is a former President of the Association of Geosciences for International Development and also of the Geological Society of Thailand. Sham Sani is currently an honorary Senior Research Fellow with the Institute for Environment and Development (LESTARI) in the Universiti Kebangsaan Malaysia, where he was Professor of Geography (1983–2000) and Vice-chancellor in the 1990s. He was the Tun Abdul Razak Professor at the Centre for International Studies, Ohio University, Athens (1991–3). Sham Sani is one of the fifty foundation members of the Academy of Sciences, Malaysia. His research interests include urban climate, air pollution, environmental management, sustainability indicators, and urban environment. M. D. Spalding is a freelance marine ecologist and Research Associate of the Cambridge Coastal Research Unit. He was formerly a senior marine scientist at the UNEP–World Conservation Monitoring Centre. His main research interest is in coral reefs and mangrove forests. He has worked with the World Resources Institute on modelling threats to reefs in Southeast Asia. His field research has largely focused on reef fish, including biodiversity patterns, community dynamics, and management approaches. T. Spencer is Director of the Cambridge Coastal Research Unit and University Senior Lecturer in the Department of Geography, Cambridge University. He has undertaken research in Borneo, the Indian Ocean, East Africa, the South Pacific Ocean, the Caribbean, and the southern North Sea. Current research interests include hydrodynamics, sedimentation, and ecosystem function in tidal wetland and coral reef environments, with a particular focus on system responses to sea-level change. Jean-Claude Thouret is a Professor with the Université Blaise Pascal and a Research Fellow with the Laboratoire Magmas et Volcans (CNRS) at Clermont-Ferrand, France. His work in Indonesia includes research on the active volcanoes of Merapi, Galunggung, Kelud, and Semeru. His research interests are in geomorphology of volcanoes (rates of growth and denudation), eruption history, and pyroclastic and lahar deposits. His current

Contributors xxiii

research in the volcanic areas of Peru and Java involves application of geomorphology, mapping, and GIS in hazard and risk assessment, especially in densely populated areas and for cities prone to volcanic and flood hazards. C. R. Twidale, after working with CSIRO, served on the faculty of the University of Adelaide for more than forty years. His main research interests are in granitic landforms, ancient palaeoforms, dating and models of landscape evolution, desert landscapes, neotectonism, and the history of geomorphological ideas. He now holds an Honorary Visiting Research Fellowship in the University of Adelaide and an Honorary Professorship in the Institute of Geology, University of Coruña. Herman Th. Verstappen is Emeritus Professor of Geomorphology at ITC, the International Institute of Geo-Information Sciences and Earth Observation, Enschede, the Netherlands. He was with the Survey Department of the Geographical Institute, Jakarta (1949–57), and also taught as a part-time Lecturer in the Geography Department, Universitas Gadjah Mada. Indonesia has remained his main regional interest, but he has also carried out research in South Asia, Africa, and Latin America. His main research areas are remote sensing, applied geomorphology, natural disaster reduction, and tropical geomorphology, especially concerning volcanoes and coral reefs. He served as the President of the International Geographical Union (1992–6). P. P. Wong is Associate Professor in the Department of Geography, National University of Singapore. He is a coastal geomorphologist, and his publications are on coastal geomorphology, coastal tourism, ecotourism, and coastal management. He has carried out field research in Barbados, Taiwan, Southeast Asia, and the Indian Ocean islands. He is a professional member of the Geological Society of Malaysia and the International Ecotourism Society. Colin D. Woodroffe is Associate Professor in the School of Geosciences, University of Wollongong, and an active member of both the Oceans and Coastal Research Centre and the Research Centre for Landscape Change. His research interests include the geomorphology of tropical coastal environments. He has worked on islands in the Caribbean, Indian, and Pacific Oceans; and has examined estuarine and deltaic environments around the coasts of Australasia and parts of Southeast Asia.

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I

The Physical Framework

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1

The Geological Framework Charles S. Hutchison

Introduction This chapter outlines the principal geological features of the region, extending from Myanmar and Taiwan in the north, southwards to include all the ASEAN countries, and extending as far as northern Australia (Figure 1.1). The present-day lithospheric plates and plate margins are described, and the Cenozoic evolution of the region discussed. Within a general framework of convergent plate tectonics, Southeast Asia is also characterized by important extensional tectonics, resulting in the world’s greatest concentration of deep-water marginal basins and Cenozoic sedimentary basins, which have become the focus of the petroleum industry. The pre-Cenozoic geology is too complex for an adequate analysis in this chapter and the reader is referred to Hutchison (1989) for further details. A chronological account summarizing the major geological changes in Southeast Asia is given in Figure 1.2. The main geographical features of the region were established in the Triassic, when the large lithospheric plate of Sinoburmalaya (also known as Sibumasu), which had earlier rifted from the Australian part of Gondwanaland, and collided with and became sutured onto South China and Indochina, together named Cathaysia. The result was a great mountain-building event known as the Indosinian orogeny. Major granites were emplaced during this orogeny, with which the tin and tungsten mineral deposits were genetically related. The orogeny resulted in general uplift and the formation of major new landmasses, which have predominantly persisted as the present-day regional physical geography of Southeast Asia.

General Plate Tectonics The Indo-Australian Plate is converging at an average rate of 70 mm a−1 in a 003° direction, pushed from the

active South Indian Ocean spreading axis. For the most part it is composed of the Indian Ocean, formed of oceanic sea-floor basalt overlain by deep water. It forms a convergent plate margin with the continental Eurasian Plate, beneath which it subducts at the Sunda or Java Trench. The Eurasian continental plate protrudes as a peninsular extension (Sundaland) southwards as far as Singapore, continuing beneath the shallow Straits of Malacca and the Sunda Shelf as the island of Sumatra and the northwestern part of Borneo (Figure 1.1). A Cenozoic arc of volcanic and non-volcanic islands, related to the Sunda subduction system, links Sumatra with Papua, which geologically is an extension of the continental lithosphere of Australia, continuous with it beneath the shallow Arafura Sea. A complicated volcanic archipelago, subduction, and fault system links the Bird’s Head of northwestern Papua, through Sulawesi and the Philippines, with Taiwan. The Philippine Sea Plate, pushed westwards by the Pacific Plate, converges on the Eurasian Plate at Taiwan in a 307° direction at 86 mm a−1 (McCaffrey 1996). The present-day physical geography of Southeast Asia is governed by these basic characteristics.

Indian Ocean Based on magnetic anomaly identification, calibrated by drill site data, three distinct episodes of sea-floor spreading can be discerned (Curray et al. 1982). 1. Anomalies M10 to M25 (Neocomian–Oxfordian) have been identified (Heirtzler et al. 1978) in the Argo Abyssal Plain, between the Sunda ( Java) Trench and western Australia (Figure 1.3). They trend 60° and increase in age towards Australia, dating the original Late Jurassic–Early Cretaceous rifting of India from

Fig. 1.1. Major physiographic features of Southeast Asia (Source: simplified by Hutchison 1989 after Mammericks et al. 1977 and Curray et al. 1982)

The Geological Framework 5 Quaternary Ma Pliocene 5

54

Palaeocene

65

Jurassic

Permian

286

Carboniferous

360

Triassic

Major geological events in Southeast Asia A B

C D

Lateritization of granite and karstification of E limestone outcrops

G

F Collision of India with Eurasia, extinction of Wharton Ridge spreading, & Sarawak Orogeny

Southeast Asian oil- and gas-bearing basins: rifting and filling by lacustrine, followed by shallow marine strata

Phase II Indian Ocean spreading

38

Eocene

146

Oligocene

Cretaceous

Mesozoic

23

208

Tertiary

Cenozoic

Miocene

245

Era

main northern sector Phase I Argo Abyssal Plain

Collision of Burma Plate with the Shan Highlands causing tin–tungsten granites in Phuket & Myanmar Continental conditions persisted over South China, Indochina, northern Thailand, & eastern peninsular Malaysia, resulting in redbed deposition (e.g. terrain rouge and Khorat Group)

H

Indosinian Orogeny: closure of Palaeotethys Ocean by collision of Sinoburmalaya with Cathaysia; uplift, folding, & mountain-building. Major tin-bearing granite emplacements from Belitung, through peninsular Malaysia, to north Thailand and Indochina. Cathaysian terrains of South China, Indochina, eastern Thailand & peninsular Malaysia, & south Sumatra, contain abundant warm water fusulinid limestones and tropical Gigantopteris plant sites. Marine glacial tilloids occur in many localities of Sinoburmalaya, e.g. Sumatra, Langkawi, Phuket, western Myanmar, and Yunnan, indicating proximity to glaciated Gondwanaland. This terrain also contains cold-water non-fusulinid limestones.

505

Cambrian

544

Proterozoic

2500

Ordovician

Archaean

3800

Precambrian

Silurian

410

Devonian

440

Palaeozoic

Key to letters above

Deposition of shallow water marine strata widely over Southeast Asia, from the Shan Highlands of Myanmar and the Yangzi Platform of China, to Thailand and peninsular Malaysia in the south. There are prominent limestones from Ordovician to Devonian, which have been karstified in the Cenozoic. The underlying Precambrian basement is rarely exposed.

Formation of the Precambrian terrain of the Assam area of northeastern India

A = Andaman Sea Basin spreading B = South China Sea Basin spreading C = oil-bearing Baram Delta Basin of Sarawak, Brunei, & Sabah: sedimentation D = Sulu Sea Basin spreading E = Sabah Crocker Range uplift, & inversion of Malay & West Natuna basins F = Celebes Sea Basin spreading G = West Philippine Sea Basin spreading H = Banda Sea Basin spreading

Formation of the Precambrian terrain of the central Viet Nam Kontum Massif & the southern China Yangzi Platform basement

NOTE: Precambrian time not to scale

Fig. 1.2. Chronological summary of the major geological changes in Southeast Asia (Source: compiled from Hutchison 1989, 1996b)

Fig. 1.3. The oceanic lithospheric areas of Southeast Asia. The Indian Ocean, an integral part of the Indo-Australian Plate, is converging on the Sunda Trench in a 003° direction at an average rate of 7 cm a−1. The back-arc region contains a number of marginal basins, also floored by sea-floor basalt (Source: modified after Hutchison 1989)

The Geological Framework 7

Australia. South of the Abyssal Plain, the anomalies trend 30° in the Wallaby Plateau. Thus, while India separated towards the northwest, some other continental fragment(s) may have moved northerly. Micro-continents containing tin-bearing granites, indicating continental crustal origin, have been identified in the Borneo–Sula region (Hutchison 1989), and the Jurassic–Lower Cretaceous ophiolitic basement of northeast Borneo represents the uplifted sea floor, once continuous with the Argo Abyssal Plain, now separated from it by the younger Sunda Trench. 2. The spreading pattern was completely reorganized between magnetic anomalies M0 and 34, which is the Cretaceous magnetic quiet period (110– 80 Ma ago). From anomaly 34 to 19 (84– 44 Ma ago) India made its spectacular rapid northwards flight with rates of 15 to 17 cm a−1; the anomalies are aligned east–west, offset by major north–south transform faults. One of the faults is the Investigator Ridge; others lie close to and parallel to the Ninety-East Ridge. The prominent Ninety-East Ridge is the trace of a single mantle hotspot, which now lies under the Kerguelen Plateau in the south Indian Ocean. The furthest end of it is the Rajmahal Traps, 200 km north-northwest of Calcutta, where the basalt has an age of 105 Ma. The average rate of relative motion of the hotspot trace was about 11 cm a−1 (Curray et al. 1982). 3. Around magnetic anomaly 19 time (44 Ma ago), spreading completely ceased at the Wharton Ridge in the northern Indian Ocean (Curray and Munasinghe 1989). This striking event in Southeast Asia coincides with the prominent widespread unconformity within the Bengal Fan and with the unconformable continental beginning of many Southeast Asian Cenozoic basins (e.g. Central Sumatra). The cause for this event may be sought in the Eocene collision of India with Eurasia. As India came into full collision, its northwards motion was spectacularly slowed, and spreading became impossible at the Wharton Ridge. A spreading axis between Australia and Antarctica had been in existence since 95 Ma ago. It then propagated westwards to begin the Southeast Indian Ocean Ridge at 44 Ma ago as spreading closed down southeast of India (Veevers 1984). From then to the present, the Indian Ocean of Figure 1.1, India, and Australia, all three belong to a single plate, pushed northwards from the South Indian Ocean spreading axis.

Bengal and Nicobar Fans The Bay of Bengal contains the largest subaerial delta in the world, the Ganga–Brahmaputra, filling the Bengal Basin, grading outwards into the Bay of Bengal and

Nicobar Deep Sea Fans, the largest turbidity complex in the world. It extends over 3000 km from the continental shelf and the Swatch-of-No-Ground submarine canyon in the north, to its distal end, where the sediments feather out at about 5–7° south of the Equator (Figure 1.1). Most of the fan sediments have been derived by erosion of the Himalaya Mountains and transported by the two great rivers. The sediments of the fan have filled in almost all submarine irregularities, except for the Ninety-East Ridge, which has remained uncovered and has split the seafloor turbidity currents into two lobes: the Bengal Fan, extending south and west to Sri Lanka, and the Nicobar Fan, extending down to the Sumatra Trench. The presence of Nicobar Fan sediments west of the Investigator Ridge has resulted in strongly scraped-up fan sediments to form the outer non-volcanic arc islands of the Sumatra accretionary prism: Nias, Mentawai, etc. Further south, the feathering out of the fan correlates with a lack of offshore islands. The Bengal Fan surface slopes smoothly down southwards (Figure 1.1) at an angle of between 6 minutes at a water depth of 2 km to 3 minutes at a water depth of 5 km at the toe of the fan. The smooth surface demonstrates the effectiveness of turbidity currents in reaching equilibrium as the sediment load is transported great distances. The still active Nicobar Fan teaches us that sediments eroded from the Himalaya Mountains collision zone may end up far from their source, uplifted on the Nias islands. To reach their destination, the turbidity currents have had to course along the narrow gap between the Ninety-East Ridge and the Sumatran Trench to the west of the Andaman Sea. This narrow gap effectively is the remarkable secondary source of the Nicobar Fan.

Active Plate Margins A nearly continuous arc–trench convergent plate margin extends throughout the region. The correct terminology of the direction is that an observer, standing on the volcanic arc and facing the related trench, has the fore-arc in front of him and the back-arc behind. Fore-arc sedimentary basins lie between the volcanic arc and the accretionary prism. All the deep marginal basins and the oil- and gas-producing sedimentary basins, characterized by shallow-water sediments, lie in the back-arc. Hence the term ‘back-arc basin’ is of such diversity as to be useless for the petroleum industry (Hutchison 1996b). In Myanmar, the plate margin has been uplifted to form the Indo-Burman Ranges because of the collision of India to the west, and the volcanic arc

8 Charles S. Hutchison

Fig. 1.4. The active plate margins of Southeast Asia, showing the positions of the trenches, volcanic arcs, main transform faults, and Benioff zones (Source: modified after Hutchison 1989)

through Mount Popa is extinct. However, an active plate margin continues southwards west of the Andaman Sea, west of Sumatra, and south of Java, curving northwards in the Banda Sea (Figure 1.4). The sector around Timor has been converted to a collision zone by the arrival of Australia. The continuity is completely stopped by the major leftlateral Sorong Fault, north of which rather complicated

arc–trench systems, commonly of opposed polarity, link the Molucca Sea through the Philippines to Taiwan, where the volcanic arc has collided with the continental margin of China and the arc–trench system converted to an actively uplifting mountain thrust belt. The convergent plate margins are characterized by an inclined earthquake or Benioff zone, which outcrops on the sea floor in the trench and dips downwards, at

The Geological Framework 9

first at a shallow angle, attaining an average 45° dip in the back-arc. The Benioff zone depth beneath the volcanic arc is characteristically in the range 90 to about 200 km. Volcanoes that occur closer to the trench above the shallower depth range produce lavas known as tholeiitic, low in potassium and high in magnesium. Volcanoes occurring farther from the trench produce calc-alkaline lavas, and those that occur farthest from the trench produce high-potassium or alkaline lavas (Kuno 1966). There is thus a regular relationship between Benioff zone depth (km) and lava chemistry in the volcanic arcs (Hutchison 1976, 1982), expressed as a regression equation (km v. silicon v. potassium). It is normal for individual volcanoes to commonly erupt not only the basic lavas, basalt and andesite, but also periodically the more explosive acid dacite, resulting from magma differentiation at depth.

Java–Sumbawa The convergence rate of the Indo-Australian Plate at the Sunda (Java) Trench is around 70 mm a−1 in the direction 003° (Malod and Mustafa Kemal 1996). This direction is approximately perpendicular to the trench so that almost all the energy of convergence is converted to normal subduction and a very active earthquake (Benioff) zone dips northwards beneath Java and the Java Sea to a depth of 600 km (Figure 1.4)—at greater depths friction no longer exists beneath the subducting and overlying plate. This sector is particularly active, and the volcanoes are not confined to any particular depth of the underlying Benioff zone. They overlie depths commonly in the range 118–92 km. The world’s greatest recorded eruption was in 1815 at Tambora on Sumbawa, and the fourth on the list was Krakatau, west of Java, which erupted in 1883 (Hutchison 1982, 1989).

Sumatra Convergence of the Indo-Australian Plate along a direction 003° reduces from 7.8 cm a−1 near Sumbawa to 6 cm a−1 near the Andaman Islands, and to only about 1.5 cm a−1 in the Himalayan collision zone (McCaffrey 1996; Samuel and Harbury 1996). The Sumatran Trench has long been recognized as the type example of an obliquely convergent margin (Fitch 1972). The obliquity increases northwestwards from the Sunda Strait. Oblique convergence is partitioned into two components, perpendicular to the trench (subduction) and parallel to the trench, which causes the plate margin to develop right-lateral strike-slip motion along major wrench faults. The rate of convergence may be resolved into these two components by the parallelogram of forces (Figure 1.5).

Myanmar Cratonic India began its collision with Asia around 50 Ma ago and rotated anticlockwise to cause a major indentation at the Assam–Yunnan syntaxis. Subduction was converted to collision, and the accretionary prism active along the length of Sumatra was strongly uplifted to form the Indo-Burman Ranges, immediately west of which Precambrian India outcrops in the Shillong Plateau and Mikir Hills of Assam (Figure 1.5). Accretionary prism and trenches (subduction systems) are known to be regions of low geothermal gradient. Accordingly an uplifted and eroding accretionary prism should contain outcrops of glaucophane schist (lowtemperature –high-pressure metamorphism) and they have been described from several locations in the Naga Hills (Hutchison 1989). Discontinuous bodies of ophiolite, representing dismembered oceanic basement, also occur along the eastern margin of the ranges. The IndoBurman Ranges accordingly have all the characteristics of a suture, or uplifted plate margin, formed between India and the Burma Plate (Hutchison 1975). The volcanic rocks lie on three distinct lines (Chhibber 1934; Stephenson and Marshall 1984). The Western Line, through Narcondam and Barren Islands, is tholeiitic and still active. The Central Line, through Mount Popa, is calc-alkaline and high-K calc-alkaline and became extinct in the Pleistocene. The volcanoes of this line are subduction-related and the Benioff zone was easterly dipping. An Eastern Line, through Thaton and Madaw Island, is alkaline and probably related to continental rifting in the back-arc area related to the Sagaing–Namyin Fault system. A broad zone of earthquakes, wedge-shaped in east–west cross-section, extends eastwards from the western margin of the Indo-Burman Ranges, but there is no longer any welldefined Benioff zone. A formerly eastwardly dipping zone is inferred because the earthquakes become deeper eastwards. The Burma or West Burma Plate is bounded on the west by the Indo-Burman Ranges suture, and on the east by the Sagaing right-lateral wrench fault, beyond which the terrain of the Shan Highlands belongs to the Sinoburmalaya or Sibumasu Plate, a northward continuation of western Thailand, western Peninsular Malaysia, and part of Sumatra. Most of the Burma Plate is covered by Cenozoic strata, deposited in basins formed in the fore-arc (the Interdeep or Western Trough) and in the back-arc (the Back Deep or Eastern Trough), but these terminologies are not useful because the collision of India on the west has caused a change from a convergent plate margin to a collision zone from the

10 Charles S. Hutchison

Andaman Sea Extension 460 km

Zone of Constant Obliquity

(a)

(b)

Fig. 1.5. (a) Main structural features of Myanmar (Source: after Mitchell and McKerrow 1975)

(b) The major right-lateral wrench faults of Sumatra (Source: after Malod and Mustafa Kemal 1996) The extinct Wharton spreading ridge is converging on the Sumatra Trench at a velocity of 60 to 70 mm a−1.

The Geological Framework 11

Eocene onwards. The sediments were from the protoIrrawaddy River, confined to a southerly course towards the sea in the Gulf of Martaban, where the strata contain significant productive gas deposits.

Timor Collision Zone The continental margin of northwestern Australia began its collision with the Banda Arc in early Pliocene time. The previous ongoing subduction had dragged down the thin lithosphere of the Australian continental shelf at the Timor Trench to the point at which its low density made further subduction impossible. This resulted in extinction of the volcanic arc from Alor, through Wetar, to Romang, and isostatic rebound of the down-dragged Australian crust resulted in regional uplift, currently continuing as evidenced by widespread occurrence of hundreds of metres of uplifted Pliocene to Holocene coral reef terraces on these islands (Vita-Finzi and Hidayat 1991). The volcanic geomorphology of Wetar has been eroded and the plutonic roots exposed. Studies of the outcropping geology of Timor have led to three main structural interpretations: The imbricate model. This is championed by Hamilton (1979). Timor is interpreted essentially as a large accretionary prism composed of mélange or chaotically imbricated material. The Kolbano unit (Figure 1.6c), of deformed Cretaceous to Pliocene bathyal sediments, is interpreted by most workers as the pre-collision subduction-related accretion prism. The overthrust model. Alpine-style thrust sheets were proposed (Figure 1.6), based on the pioneering work of Audley-Charles (1968). Geological units were interpreted as allochthonous material of non-Australian origin, and para-autochthonous units derived from the underlying Australian continent, such as the Permian Maubisse–Aileu unit (Barber 1981). The rebound model. The surface geology is interpreted to have resulted from isostatic rebound along steep faults, following cessation of subduction (Figure 1.6). The model was proposed by Chamalaun and Grady (1978). A revised model by Harris (1991) indicates that these different hypotheses were all correct and each focused on different outcrops and therefore on different phases of the deformation. His model, largely based on an analysis of Taiwan, which occupies a very similar tectonic position to Timor, shows that these various models can be incorporated into one (Figure 1.6a). The most recent model by Richardson and Blundell (1996), based on the interpretation of a deep regional seismic section across the Timor Trench to the volcanic

arc, is deceptively simple (Figure 1.6d ). As subduction was converted to collision, the Australian continental margin acted as a bulldozer, shortening and uplifting the continental and oceanic parts of the collision zone, making use of deep-seated faults, dipping southwards antithetic to the subduction. The thrusts propagated progressively northwards and are active today in the back-arc region (Richardson and Blundell 1996).

Molucca Sea Collision Zone Earthquake foci define two Benioff zones dipping away from the Molucca Sea beneath two active volcanic arcs, Halmahera and Sangihe. The appropriate trenches would be expected to outcrop beneath the Molucca Sea, but instead there is a great thickness of low-density sediments interpreted as the collided accretionary prisms of the two arc–trench systems. Uplifted scraps of ophiolite occur on Talaud Ridge in the central axis of the sea. There may have formerly been a spreading axis within the Molucca Sea, but it became inactive. There is therefore no push mechanism for the two opposed subduction systems. Subduction systems may continue to be driven by a downwards pull as the sea-floor basalt of the subducted slab changes to high-density eclogite in response to increased burial pressure. The subducted slabs will therefore continue to sink into the Mantle, leaving only the low-density collided accretionary prisms at the surface.

The Philippines The Philippine Sea Plate converges along a direction 307° at a rate of 86 mm a−1 oblique to the Philippine Trench (McCaffrey 1996). Fitch (1972) pointed out that this oblique convergence was the cause of the major left-lateral Philippine Fault, along which the slip-rate has been measured at around 26 mm a−1 (McCaffrey 1996). The straight and deep Philippine Trench is a young feature along which subduction has not proceeded deep enough to have produced a volcanic arc. The Manila Trench is part of a well-established eastward-dipping Benioff zone and active volcanic arc, but it may be in process of closing down in favour of the Philippine Trench, because it has collided with the continental shelf of China in Taiwan along strike to the north.

Seas of the Back-Arc Region The region is covered extensively by seas (Figure 1.1), which are of three distinct types: continental shelf sea, marginal sea or basin, and ocean. Different water depths, resulting from the density of the underlying crust, characterize the sea. Lowdensity quartz-rich and acid rocks, such as sandstone

12 Charles S. Hutchison

Fig. 1.6. Structural interpretations of the Timor collision zone (A) Restored section (Source: based on field and seismic data, from Harris 1991)

(B and C) Pliocene collision followed by extinction of the volcanic arc and isostatic rebound (Source: Barber 1991)

(D) Back-thrusting model based on interpretation of a regional deep seismic profile (Source: Richardson and Blundell 1996)

The Geological Framework 13

and granite, which characterize continental crust, are isostatically buoyant, whereas high-density basic rocks, such as sea-floor basalt and gabbro, characteristic of oceanic crust, are isostatically drawn down by the gravity pull of the inner Earth to form basins filled by deeper water. Because high-density oceanic crustal rocks more strongly attract the overlying column of seawater than low-density continental rocks, the average sea surface is of variable elevation, which today may be directly measured routinely by altimeter measurements made from orbiting satellites. The different kinds of seas and other geological features may be directly seen on gravity variation maps derived from satellite altimeter data (Foss and Savage 1992).

Continental Shelf Seas This type of sea is distinguished by its shallow water depth of < 200 m, resulting directly from the lowdensity (quartz-rich) nature of the underlying crust, which is a seaward extension of the continental landmass. Because of Tertiary rifting, the crust is thinner beneath the shelf seas than beneath the landmass— exact elevation controlled by isostasy. The geographically extensive Sunda Shelf (Figure 1.1) is underlain by continental crust which has been thinned by rifting in the Late Eocene to Early Oligocene (45–30 Ma ago). The rifting has been attributed to the Indian collision. During maximum glaciation before the end of the Pleistocene (> 10 000 years ago) water locked up by the ice sheets caused the sea level to be some 120 m lower than the present day. Borneo, Sumatra, and Java would then have formed an extensive peninsular landmass continuous with Peninsular Malaysia and Indochina, named Sundaland. The geographical extent of this landmass had earlier experienced several fluctuations throughout the Pleistocene. This was not the first time that Sundaland existed— there was also a large peninsular landmass, extending as far south as Java and as far east as western Sulawesi, in early Eocene time (~50 Ma ago), but within it there was a deep sea gulf extending westwards from the Pacific through northern Borneo (Hutchison 1992a). The deep-water sedimentary fill of that ‘Rajang Gulf ’ was uplifted to form land in Sarawak in the late Eocene and in Sabah not until the Late Miocene (Hutchison 1996a). The Eocene Sundaland was not caused by glaciation, for indeed eustatic sea levels were then at a worldwide high. Sundaland must therefore have resulted from increased elevation following crustal thickening caused by a Mesozoic mountain-building collisional event.

The very broad present-day continental shelf is anomalous—it would have been considerably narrower (from 120 m to 200 m isobath) during maximum Pleistocene glaciation, and in future, should the ice sheets melt completely, the shelf will be even wider than today as the seas transgress the land.

Marginal Seas In comparison with the oceans, these are small basins floored by sea-floor basalt and characterized by water deeper than 3 km. They may be likened to small oceans, resulting similarly from sea-floor spreading. As sea-floor basalt cools and becomes denser, it sinks isostatically and the overlying water deepens. Marginal seas always occur marginally to the continent and between it and the main ocean. Another requirement is that there must be a convergent plate margin between the main ocean and the continental plate. The Atlantic Ocean, which is wholly divergent, has no marginal seas. Southeast Asia and the western Pacific represent the type locality for such seas. Marginal seas may have been formed by separation into two parts of a formerly larger single basin by a superimposed younger arc–trench subduction system. Another popular hypothesis is that marginal seas result from rifting of an active volcanic arc (Karig 1971). The rifting resulted from forward migration of a trench because of increased sediment influx, creating a new volcanic arc on the front (ocean) side of the basin and leaving behind an extinct remnant arc.

South China Sea Basin Most of the South China Sea is of shallow water (< 200 m) depth and underlain by attenuated continental crust. The deeper part lying between Viet Nam and the Philippines is a marginal basin, characterized by sea-floor spreading, now extinct (Figures 1.1 and 1.3). The magnetic anomalies trend east–west in the eastern part to southwest–northeast in the southwest. Anomalies 11 to 5c have been identified (Briais, Patriat, and Tapponnier 1993), representing north–south to northwest–southeast sea-floor spreading from 33 to 17 Ma ago (Oligocene to Middle Miocene). Unfortunately the recent ODP drill sites were not selected to authenticate the anomaly identification. Basalts have been dredged from the extinct fossil spreading axis (anomaly 5c) of the Scarborough Seamounts and radiometrically dated 10 to 15 Ma. These Middle Miocene volcanics were intruded through the extinct but still weak spreading axis (Hutchison 1996b). The South China Sea Basin lithosphere is now actively subducting beneath Luzon, the convergent plate

14 Charles S. Hutchison

margin being the Manila Trench. The eastward subduction of totally extinct lithosphere is only possible if the Manila Trench migrates forwards (westwards) until the whole marginal basin is subducted. However, the northwards continuation of the Manila Trench is located beneath thrust fault structures in Taiwan, where it has collided with the continental shelf of China and accordingly cannot migrate further westwards. The Manila Trench opposite Luzon may, however, migrate westwards only if an east–west striking right-lateral wrench fault system develops to uncouple it from Taiwan. Earthquake first motion studies suggest that this may well be the case (Hutchison 1989).

Sulu Sea Only the southeast Sulu Sea is of oceanic crust, bounded along its northwest margin by the Cagayan Ridge, formed of an Oligocene –Lower Miocene rifted volcanic arc. Along its southeast and east margins, the marginal basin is actively subducting beneath the Sulu Archipelago, Zamboanga, and Negros. Two Ocean Drilling Program (ODP) sites were drilled on the flanks of the Cagayan Ridge and a K–Ar age of 14.7 Ma was obtained from a dredged andesite sample. The claystone immediately overlying the volcanic rocks contain late Lower to early Middle Miocene fossils. This age therefore represents the cessation of the volcanic arc and onset of sea-floor spreading along the rifted southeastern margin (Hutchison 1996b). The first sediments to be deposited upon the pillow basalt of the marginal basin were drilled and contain late Lower to early Middle Miocene fossils. They are of volcanic brown clay and rhyolitic tuff composed of glass shards and pumice. Rifting of the active Cagayan Ridge volcanic arc therefore caused the marginal basin. Hinz and Block (1990) could discern no magnetic anomalies, where Lee and McCabe (1986) had previously identified 17 to 20. Representatives of these volcanic rocks outcrop onland in Sabah at Sandakan and east of Lahad Datu. The marine mélange deposits, which occur extensively in the Dent Peninsula, have been interpreted as formed in the westwards, now uplifted onland extension of the rifted late Lower to early Middle Miocene marginal basin (Hutchison 1992b).

Celebes (Sulawesi) Sea Magnetic anomalies 18 to 20 (42–5 Ma: Middle Eocene) had been identified by Weissel (1980), and subsequent drilling revealed a basement of mid-ocean-ridge basalt immediately overlain by clays containing Middle Eocene fossils (Hutchison 1996b). Nichols and Hall (1997)

have deduced that the Celebes Sea Basin was formerly an integral part of the West Philippine Sea Basin, now isolated from it by the younger subduction system of the Molucca Sea.

West Philippine Sea Basin Magnetic anomalies trend 110° and their identification, ranging from 25 (56 Ma ago) to 7a (26 Ma ago), Palaeocene to Oligocene, was aided by the data from several drill holes resulting in direct dating (Shih 1980). The Oligocene fossil spreading axis is interpreted to be the Central Basin Fault. The basin contains a large seamount called the Benham Plateau, whose basement probably represents a micro-continent, which has strongly indented Luzon Island because it resists being subducted. The West Philippine Sea marginal basin is actively subducting westwards at the Philippine Trench, northwards at the Okinawa Trench, and along its eastern margin is delineated by the Palau–Kyushu Ridge.

Banda Sea Water depths of 4 to 5 km indicate that the underlying sea-floor basalt is dense because it is old and therefore cold. The eastern arcuate sea contains the enigmatic Weber Deep, in which water depth exceeds 7 km. It separates the inner non-volcanic from the outer volcanic island chains of the Banda Arc, and has the greatest free-air gravity anomaly of the whole region (−275 mgal). It was formerly wrongly interpreted as the eastwards extension of the Java Trench. It is not a plate margin, but its geological significance remains unknown (Bowin et al. 1980). Northeast-trending magnetic anomalies ranging from M14 (131 Ma: Neocomian) in the south to M0 (112 Ma: Barremian) in the north, have been identified (Hartono 1990). However, there has never been any drilling, but metamorphic rocks, dredged from the Banda Ridges and K–Ar dated 10.8 and 22.5 Ma (Silver et al. 1985), display the characteristics of arc volcaniclastics, rather than of continental origin (Vroon, van Bergen, and Forde 1996). Although the Banda Sea needs further research, it appears to represent, in part at least, a segment of oceanic lithosphere that was formerly an integral part of the early Cretaceous Argo Abyssal Plain of the eastern Indian Ocean, now separated from it by the younger Sunda arc–trench system. The basement ophiolite outcrops of Sabah have the same age and probably represent uplifted Banda Sea or Indian Ocean lithosphere. Part of the sea has experienced younger volcanic activity, and dredged volcanic rocks have yielded K–Ar dates in the range 0.40 to 8.75 Ma (Silver et al. 1985).

The Geological Framework 15

Andaman Sea Unlike other marginal basins, much of this sea is of relatively shallow water, and underlain by continental crust which represents a geological continuation of Myanmar, Thailand, and Sumatra. The western part of the sea is built of the Andaman–Nicobar Ridge, which is a continuation of the Arakan Yoma (Indo-Burman Ranges) of western Myanmar and the Nias–Mentawai accretionary prism of the Sunda subduction system. The geology of the sea has been summarized by Curray et al. (1982). The eastern side of the sea is formed by the Mergui Terrace, of continental crust, representing an offshore continuation of the geology of the Malay Peninsula. The Mergui Terrace, which includes Phuket and Phang-Nga, has subsided because of thinning of the continental crust as a result of the Andaman Sea rifting. The large number of Ratburi limestone islands offshore Phang-Nga are a result of this subsidence, and the continental alluvial tin deposits of Phuket, overlain by a thin cover of marine mud, are mined by sea-going dredges. The Andaman Sea is a rift basin, and magnetic anomalies in the rift valley, yet unconfirmed by drilling, show that sea-floor spreading began 13 Ma ago. The upturned margins of the rift valley show that spreading is still active. However, abundant sediments, brought down by the Irrawaddy River, have filled the spreading axis rift as quickly as it opened. Spreading is in a NNW–SSE direction. The West Andaman Fault is of major significance—it links up northwards with the great left-lateral Sagaing Fault of Myanmar and southwards with the great Sumatra Fault. Step-like segments of the spreading axis link up the fault in a rectilinear en echelon fashion. The Andaman Sea therefore appears to have begun life as a leaky transform fault along which basaltic magma welled up along trans-extensional segments of the major fault. With time, the leaking sectors have amalgamated into continuous spreading axes. However, the Andaman Sea is still dominantly of left-lateral strike slip tectonics.

Tertiary Sedimentary Basins The Tertiary basins, unlike the marginal basins, experienced slow subsidence, and rifting did not progress to the stage of sea-floor spreading. The distribution of these basins is shown in Figure 1.7. The outstanding mountain-building event of Southeast Asia was the Late Triassic Indosinian Orogeny, when Sinoburmalaya (Figure 1.8) collided with and became sutured onto Cathaysia (Indochina and South China).

Southeast Asia, including the present landmasses and most of the areas now covered by shallow (< 200 m) shelf seas, then formed an extensive landmass, which persisted, despite high worldwide eustatic sea levels, into early Tertiary Eocene time. The landmass extended as far south as the south coast of Java (Hutchison 1992a), westwards to the west coast of Sumatra, and eastwards to include western Sulawesi. It was on this greater Sundaland that the Tertiary sedimentary basins were formed. Within this greater Sundaland landmass, there existed a major deep-water gulf of the western Pacific–Indian Ocean known as the Danau or Proto South China Sea (Hutchison 1996a), floored by Mesozoic sea-floor basalt (ophiolite) and infilled by thick-turbidity sandstone–shale flysch sequences, subsequently uplifted by the Sarawak Orogeny in the Eocene to form the Rajang Group and by the Sabah Orogeny in the Miocene to form the Crocker Ranges. The Baram Delta, a unique oil-bearing basin, was not formed on the Sundaland Eocene landmass, but owed its origin to the Miocene uplift of the West Crocker Formation (Hutchison 1996a). Granitic Mount Kinabalu was also emplaced at this time. Greater Sundaland experienced rifting and graben formation throughout the late Eocene and early Oligocene, following close-down of the Indian Ocean Wharton Basin spreading axis and collision of India with Eurasia. The rifting is generally accepted to be genetically related to propagation of a set of major wrench faults from the Assam–Yunnan syntaxis, a process known as extrusion or escape tectonics, elegantly modelled in laboratory plasticene experiments by Tapponnier et al. (1982). An extensive river system developed over Sundaland, reaching the sea in southwest Sumatra and southern Java. A major river flowed eastwards, resulting in Eocene –Oligocene lacustrine deposits in the Ketungau and Mandai Basins of Borneo and building out the major Kutei Basin delta into the Makassar Strait, still active today as the Mahakam Delta. The rift grabens could be likened to the great rifts of present-day East Africa. Large elongated lakes developed, and the equatorial climate resulted in the waters becoming very rich in algae. After burial beneath Miocene and Pliocene sediments, the algal muds matured to become important source rocks for the oil of the region (Katz 1991). The most spectacular Eocene–Oligocene graben, the Bengkalis, extends southwards from the Strait of Malacca and ends abruptly at the faulted margin of the Tiga Puloh Mountains of Sumatra. It is filled with Pematang Formation fluvial and lacustrine deposits, which were buried beneath marine Miocene sandstones and shales. The very high geothermal gradient of the Central Sumatra Basin, 61°C km−1, facilitated

16 Charles S. Hutchison

Fig. 1.7. Distribution of oil- and natural-gas-bearing basins of Southeast Asia (Source: Hutchison 1996b) The numbers refer to the order of importance for cumulative oil production up to the end of 1991. Number 12 is off the map towards the southeast.

The Geological Framework 17

the maturation of the source rocks, and the oil migrated out into Miocene sandstone reservoir beds, where it was trapped beneath Telisa Formation shales. The Central Sumatra Basin was uplifted by a Late Miocene–Pliocene inversion (Sabah Orogeny) and it is onland unlike many Southeast Asian basins. Lacustrine formations, filling grabens, and elongate rift basins form an important part of the Malay Basin, Gulf of Thailand, South China Sea, Java Sea, and the Barito Basin of the Meratus Mountains of south Borneo (Hutchison l996b). Following the early rift and graben stage, these basins later sagged and were more widely inundated by the sea, which deposited shallow marine formations, which also contributed to the petroleum source. In many of these basins, in particular West Natuna and Malay, the Late Miocene compressional inversion (Sabah Orogeny) was followed by erosion of the uplifted highs, forming an unconformity, after which subsidence beneath the sea was renewed (Hutchison 1996b). Along the western margin of Peninsular Malaysia, and continuing along the length of Thailand as far as the Myanmar border, is a line of small lacustrine intermontane rift basins of Eocene – Oligocene age, which always remained isolated from the sea. Both the Fang and Phitsanulok Basins of Thailand produce oil, derived entirely from lacustrine algal source rocks. But most of the basins, such as Batu Arang in Peninsular Malaysia, are too small to be productive. It used to produce coal for the railways. The petroleum industry of the region is restricted to basins of Tertiary age. Central Sumatra is the most outstanding producer of oil, with 7577 × 106 barrels of oil (1 barrel = approximately 159 litres or 0.14 tonne). Its giant oilfield of Minas, discovered in 1944, has since produced a spectacular 3603 million barrels of oil (Hutchison 1996b). The Baram Delta produced 3062 × 106, Kutei 2031 × 106, south Sumatra 1705 × 106, the Malay Basin 1073 × 106, and north Sumatra 937 × 106 barrels up to the end of 1991.

Baram Delta This prolific Neogene delta, first discovered at Miri, Sarawak, in 1910, is unique in the region because it was not formed on the Sundaland landmass. Its origin is related to the uplift of the Oligocene–Lower Miocene West Crocker Formation, composed of deep marine sandstone–shale turbidites, to form the Crocker Ranges and Mount Kinabalu (4101 m) of Sabah. The uplift, named the Sabah Orogeny (Hutchison 1996a), began in the Middle Miocene and continues to the present day. The cause of the spectacular Late Miocene –Pliocene uplift

was probably underthrusting of the Spratly Islands– Dangerous Grounds continental lithosphere southeastwards beneath Sabah. The plate margin was the now extinct Northeast Borneo (Palawan) Trough. Subduction was followed by isostatic rebound of the low-density underthrust material (Hutchison et al. 2000, 2001). Rapid erosion of the thick sandstone beds of the Crocker Ranges provided the medium-grained sands for the Baram Delta, transported only a short distance (< 100 km) into the muddy deep-water environment (Setap Shale), which lay close to the shore. The first delta sands are of Middle Miocene age, and sedimentation continued in cycles, related to sea-level changes and uplift of the Crocker Ranges, into the Pliocene and Quaternary (Hutchison 1996b). The outer toe of the Baram Delta extends as far as the Northwest Borneo Trough, where it is still active depositionally and tectonically, being continuously modified in deep water by diaspirism and folding (Hutchison 1996b). The sandy Baram Delta, centred on Brunei, produces oil from Miocene and Pliocene reservoirs. Anticlinal structures, related to major listric faults, are typical of deltas (Petronas 1999). The oil source was predominantly from coastal plant material rapidly transported and buried in the delta, and much of the sedimentary sequence is rich in amber nodules. However, it may also be concluded that the underlying prodelta muds of the Setap Shale may have contributed significantly.

Pre-Tertiary Geology Southeast Asia is composed of a mosaic of different blocks or micro-continents that have rifted and drifted from the southern mega-continent of Gondwanaland (Gatinsky and Hutchison 1987; Metcalfe 1990, 1996). The former attachment is deduced to have been near the Canning Basin of northwestern Australia (Hutchison 1989). The boundaries between the blocks are suture zones containing dismembered ophiolites (Hutchison 1975), or major wrench faults (Figure 1.8). The suture zones contain obducted remnants of deep-water sediments, interpreted to be the relicts of the predominantly subducted Palaeotethys Ocean. Ribbon-bedded cherts within the Palaeotethyan sutures have yielded radiolaria ranging in age from Upper Devonian to Middle Triassic (Metcalfe 1998). The Bentong–Raub line is the main Palaeotethys suture of Peninsular Malaysia. It continues northwards beneath the Gulf of Thailand to reappear at Sra Kaeo, whence it has been offset left-laterally by the Three Pagodas Pass Fault, and continues northwards as the Nan–Uttaradit suture of northern Thailand. West

18 Charles S. Hutchison

Fig. 1.8. Distribution of continental blocks, terranes, and principal sutures of Southeast Asia 1. 2. 3. 4.

South China Indochina Sinoburmalaya, or Sibumasu East Malaya, which is continuous with Indochina beneath the subsided Tertiary basins of the South China Sea 5. West Burma 6. Southwest Borneo 7. Semitau 8. Sikuleh 9. Natal 10. West Irian Jaya 11. Buru-Seram 12. Buton 13. Bangai-Sula 14. Obi–Bacan (Source: after Metcalfe 1996)

of the suture is the Sinoburmalaya (Sibumasu) Block of western Peninsular Malaysia, western Thailand (Shan-Thai), and the Shan Highlands of Myanmar. It continues into Sumatra as far south as the Tiga Puloh Mountains. East of the suture is the East Malaya– Indochina Block.

15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29.

North Palawan Spratly Islands–Dangerous Grounds Reed Bank Luconia Macclesfield Bank Paracel Islands Kelabit–Long Bawan Mangkalihat Paternoster West Sulawesi East Sulawesi Sumba Banda allochthon Qiongzhong and Yaxian terranes of Hainan Simao terrane

The Sinoburmalaya Block is partly constructed of Carboniferous–Permian formations which contain pebbly mudstone spreads, interpreted to be marine glacial deposits (Stauffer and Mantajit 1981), named the Phuket Group of Thailand, Singa Formation of Langkawi, and various names in Sumatra (Figure 1.9).

Fig. 1.9. Cathaysian and Gondwanaland Carboniferous–Permian entities (Source: after Hutchison 1993) Pebbly mudstones of glacial origin are found only in Sinoburmalaya in a belt extending northwards from the Tiga Puloh Mountains of Sumatra, through Langkawi and Phuket, and can be traced through Mandalay to Yunnan in China.

20 Charles S. Hutchison

These formations were deposited when Sinoburmalaya was an integral part of or lay close to glaciated Gondwanaland. There are strong stratigraphic similarities to the Canning Basin of Australia. These formations are conformably overlain by poorly fossiliferous platform limestones of Upper Permian to Triassic age. They are known as the Ratburi Limestone in Thailand and the Chuping Formation in Malaysia. Generally, the oldest outcropping rocks in Sinoburmalaya are of Cambrian age. By contrast the Indochina Block lay in equatorial latitudes during the Permian, attached to southern China (Cathaysia). This large East Asian continent was characterized by Late Permian Gigantopteris plants. It also contains important Permian–Triassic limestones. In contrast to Sinoburmalaya, these limestones are commonly associated with andesitic volcanic rocks and are highly fossiliferous, including abundant fusulinids and corals. A good example is the Saraburi Limestone of eastern Thailand. The oldest rocks of the whole of Southeast Asia are the high-grade Precambrian metamorphic rocks of the Kontum Massif of eastern Viet Nam, which have given a radiometric age as great as 2300 Ma (Hutchison 1989). The Mesozoic continental Khorat Basin of eastern Thailand and Cambodia owes its tectonic stability to an underlying basement of these ancient rocks. West of the Gondwanaland terrain of Sumatra, and in fault contact with it, lies a west Sumatra terrain, which has similarities to that of the eastern part of Peninsular Malaysia including Cathaysian plants and volcanic rocks (Figure 1.9). Sumatra is therefore composite, like Peninsular Malaysia. The sutures between these various terrains became reactivated as wrench fault zones during the Tertiary as a result to the Eocene collision of India with Eurasia. The Triassic collision between Sinoburmalaya and the Indochina Block (the Indosinian Orogeny) eliminated the intervening Palaeotethys Ocean and caused crustal thickening, resulting in the emplacement of large composite S-type granite batholiths, such as the Main Range of Peninsular Malaysia (Cobbing et al. 1992). The main Triassic granite belt can be traced from northern Thailand and eastern Myanmar, through the Malay Peninsula as far as Belitung Island. The collision of the West Burma Plate (Lhasa Block of Tibet) with Southeast Asia caused an important belt of Late Cretaceous granites, extending northwards from Phuket into Myanmar. At the time of the collision, there was no Andaman Sea, and the West Burma Plate (also known as the Burma Plate) has subsequently moved right-laterally northwards as the sea opened.

Mineral Deposits The region may be broadly subdivided into an eastern province, formed of andesitic island arcs and obducted ophiolite bodies—the Philippines and eastern Indonesia —and continental Sundaland to the west. The mineral deposits reflect this subdivision, and have been reviewed by Hutchison and Taylor (1978) and Hutchison (1996b). Major chromite concentrations are mined in the Zambales ophiolite of western Luzon. Tropical weathering of the eastern Sulawesi ophiolite has concentrated nickel into mined surficial laterite formations. The andesitic volcanic arcs of the Philippines are abundantly mineralized in porphyry copper deposits, the largest of which is the Atlas on Cebu Island, with reserves of around 900 000 Mt containing an average 0.45 per cent copper. Gold is an important by-product, averaging 0.3 ppm. On the outer fringe of the Philippine province is the minor but unique Mamut porphyry copper deposit of Sabah, only one-seventh of the size of the Atlas. The porphyry has intruded on the margins of the Miocene Mount Kinabalu batholith, but there are no coeval volcanic rocks, and both sedimentary and ophiolite rocks are mineralized in addition to the porphyry. Epithermal gold deposits (Mitchell and Leach 1991) are mostly vein systems related to minor andesitic to dacitic intrusions which have driven near-surface hydrothermal circulations. The Baguio district of Luzon is the most important, having produced more than 800 t of gold, more than any other goldfield of the western Pacific. Placer deposits are unimportant. The Lebong and West Coast mining districts of western Sumatra have been historically important since the early years of last century. They contain more silver than gold, and the mineralization is of heavily fractured andesitic and dacitic volcanic rocks (van Bemmelen 1949). The epithermally mineralized region of east central Kalimantan forms a prominent linear zone, 450 km long, within which the mineralization is related to Lower Miocene minor intrusives. All have supported a welldeveloped alluvial gold-mining industry. The largest is at Kelian. The continental core of Sundaland has been the source of more than 70 per cent of the world’s supply of tin in this century, but mining is now in sharp decline because of fallen demand. The great majority of the mines produced alluvial cassiterite from Pleistocene and Quaternary continental alluvium, predominantly onland. The nature of the tin placers has been described by Taylor (1986).

The Geological Framework 21

The ultimate source of the cassiterite was hydrothermal quartz veins and greisen systems developed in the immediate contact zones of ilmenite-series granite batholiths. Many of the deposits are related to Late Triassic granites formed during the Indonisian Orogeny, for example, the Main Range of Malaysia and the tin islands of Indonesia (Cobbing et al. 1992). Concentration into the rich placer deposits had to wait until the granites were exhumed in the Miocene. The Eastern Belt Mid- to Late Triassic granites of Peninsular Malaysia, of mixed ilmenite and magnetite series, brought together an interesting tin and iron mineralization, with the best example at Pelepah Kanan. Mining of the large iron ore deposits of the east coast, such as Bukit Besi, commonly had to cease because of high tin values. The large underground mine at Sungai Lembing near Kuantan had a long successful history of mining laterally extensive Cornish-type lodes, which emanated far from the granite contacts into the overlying sedimentary envelope. The Western Belt of granites, extending northwards from Phuket along the coastal zone of Myanmar, is of Late Cretaceous age. The associated Quaternary continental tin placers have foundered as a result of the opening of the Andaman Sea and lie offshore Phuket beneath a thin layer of marine mud. A curiosity of Phuket is that seagoing dredges, which recover the cassiterite, occasionally encounter pockets of placer diamonds, whose origin remains a mystery (Hutchison 1996b). Another curious, though more spectacular, diamond occurrence is in southeast Kalimantan in the Barito Basin. At Martapura, more than 1 M carats have been washed from Quaternary river terraces. The immediate source has been traced to Cretaceous basal conglomerate, but a search for the ultimate kimberlite source in Kalimantan has been unfruitful. The west coast zone of Late Cretaceous granites has an important association of tungsten with tin mineralization in Myanmar, at Mawchi Mine, and especially at Hermyingi Mine, Tavoy, which in 1917 was the world’s foremost tungsten producer. The granite has been precisely dated 59 ± 2 Ma (Cobbing et al. 1992). The Triassic granites of the Main Range of Malaysia also had an outstanding scheelite–fluorite mine at Kramat Pulai, near Ipoh. Smaller tungsten deposits occurred associated with the east coast granite belt as far south as Belitung. There are many small lead deposits in northern Thailand and the Shan States of Myanmar, but the Bawdwin lead–zinc mine in the Shan States is the most famous. It has an association with rhyolite and metaquartzite of the Palaeozoic Bawdwin Volcanic Series.

The region of northern Thailand is cut by numerous Tertiary faults which have been hydrothermally mineralized in gold, fluorite, and barite. The metals may have been driven out from the contiguous Tertiary sedimentary basins, such as the Gulf of Thailand, which has a remarkably high geothermal gradient of 50°C km−1. The gold lodes are unrelated to volcanic activity, having been localized in fault and fracture zones. There are numerous occurrences in Peninsular Malaysia, for example at Raub and Lubok Mandi, where the outcropping lodes have been eroded and the gold concentrated as young overlying placers. The underlying lodes are also mined. The Bau gold-mining district of Sarawak continues to produce gold from limestone which has been intruded by numerous Miocene adakitic dykes. The famous mining district also formerly produced antimony and mercury. But the greatest concentration of mercury mineralization was on Palawan Island (Philippines), located along the faulted contact between the Calamian micro-continent and ophiolite.

References Audley-Charles, M. G. (1968), The Geology of Portuguese Timor, Geological Society Memoir 4 (London). Barber, A. J. (1981), ‘Structural Interpretations of the Island of Timor, Eastern Indonesia’, in A. J. Barber and S. Wiryosujono (eds.), The Geology and Tectonics of Eastern Indonesia, Geological Research and Development Centre Special Publication 2 (Bandung), 183–97. Bowin, C., Purdy, G. M., Johnston, C. R., Shor, G., Lawver, L., Hartono, H. M. S., and Jezek, P. (1980), ‘Arc – Continent Collision in the Banda Sea Region’, American Association of Petroleum Geologists Bulletin, 64: 868–915. Briais, A., Patriat, P., and Tapponnier, P. (1993), ‘Updated Interpretation of Magnetic Anomalies and Seafloor Spreading Stages in the South China Sea: Implications for the Tertiary Tectonics of Southeast Asia’, Journal of Geophysical Research, 98: 6299–328. Chamalaun, F. H., and Grady, A. (1978), ‘The Tectonic Development of Timor: A New Model and its Implications for Petroleum Exploration’, Australian Petroleum Exploration Association Journal, 18: 102–8. Chhibber, H. L. (1934), The Geology of Burma (London: Macmillan). Cobbing, E. J., Pitfield, P. E. J., Darbyshire, D. P. F., and Mallick, D. I. J. (1992), The Granites of the Southeast Asian Tin-Belt, British Geological Survey, Overseas Memoir 10 (Keyworth, Notts.). Curray, J. R., and Munasinghe, T. (1989), ‘Timing of Intraplate Deformation, Northeastern Indian Ocean’, Earth and Planetary Science Letters, 94: 71–7. —— Emmel, F. J., Moore, D. G., and Raitt, R. W. (1982), ‘Structure, Tectonics and Geological History of the Northeastern Indian Ocean’, in A. E. M. Nairn and F. G. Stehli (eds.), The Ocean Basins and Margins: The Indian Ocean (New York: Plenum Press), vi. 399–450. Fitch, T. (1972), ‘Plate Convergence, Transcurrent Faults and Internal Deformation Adjacent to Southeast Asia and the Western Pacific’, Journal of Geophysical Research, 77: 4432–60.

22 Charles S. Hutchison Foss, C. A., and Savage, J. (1992), ‘The Bouguer Gravity Variation over South East Asia as Derived from Satellite Altimeter Data’, Tectonic Framework and Energy Resources of the Western Margin of the Pacific Basin: Programme and Abstract of Papers (Kuala Lumpur: Geological Society of Malaysia and Houston, Circum Pacific Council for Energy and Resources), 42. Gatinsky, Y. G., and Hutchison, C. S. (1987), ‘Cathaysia, Gondwanaland and the Palaeotethys in the Evolution of Continental Southeast Asia’, Geological Society of Malaysia Bulletin, 20: 179–99. Hamilton, W. (1979), ‘Tectonics of the Indonesian Region’, U.S. Geological Survey Professional Paper 1078 (Denver, Colo.). Harris, R. A. (1991), ‘Temporal Distribution of Strain in the Active Banda Orogen: A Reconciliation of Rival Hypotheses’, Journal of Southeast Asian Earth Sciences, 6: 373–86. Hartono, H. M. S. (1990), ‘Late Cenozoic Tectonic Development of the Southeast Asian Continental Margin in the Banda Sea Region’, Tectonophysics, 181: 267–76. Heirtzler, J. R., Cameron, P., Cook, P. J., Powell, T., Roeser, H. A., Suhardi, S., and Veevers, J. J. (1978), ‘The Argo Abyssal Plain’, Earth and Planetary Science Letters, 41: 21–31. Hinz, K., and Block, M. (1990), ‘Summary of Geophysical Data from the Sulu and Celebes Seas’, in C. Rangin, E. Silver, M. T. von Breyman, et al. (eds.), Proceedings of the Ocean Drilling Program: Initial Reports (College Station, Tex.), ch. 5, p. 124. Hutchison, C. S. (1975), ‘Ophiolite in Southeast Asia’, Geological Society of America Bulletin, 86: 61–86. —— (1976), ‘Indonesian Active Volcanic Arc: K, Sr, and Rb Variation with Depth to the Benioff Zone’, Geology, 4: 407–8. —— (1982), ‘Indonesia’, in R. S. Thorpe (ed.), Andesites: Orogenic Andesites and Related Rocks (Chichester: Wiley), 207–24. —— (1989), Geological Evolution of South-East Asia, Oxford Monographs on Geology and Geophysics, 13 (Oxford: Clarendon Press). —— (1992a), ‘The Eocene Unconformity in Southeast and East Sundaland’, Geological Society of Malaysia Bulletin, 32: 69–88. —— (1992b), ‘The Southeast Sulu Sea: A Neogene Marginal Basin with Outcropping Extensions in Sabah’, Geological Society of Malaysia Bulletin, 32: 89–108. —— (1993), ‘Gondwanaland and Cathaysian Blocks, Palaeotethys Sutures and Cenozoic Tectonics in South-East Asia’, Geologisches Rundschau, 82: 388–405. —— (1996a), ‘The “Rajang Accretionary Prism” and “Lupar Line” Problem of Borneo’, in R. Hall and D. Blundell (eds.), Tectonic Evolution of Southeast Asia, Geological Society Special Publication 106 (London), 247–61. —— (1996b), South-East Asian Oil, Gas, Coal and Mineral Deposits, Oxford Monographs on Geology and Geophysics 36 (Oxford: Clarendon Press). —— and Taylor, D. (1978), ‘Metallogenesis in S. E. Asia’, Geological Society of London Journal, 135: 407–28. —— Bergman, S. C., Swauger, D. A., and Graves, J. E. (2000), ‘A Miocene Collision Belt in North Borneo: Uplift Mechanism and Isostatic Adjustment Quantified by Thermochronology’, Geological Society of London Journal, 157: 783–93. —— —— —— —— (2001), ‘Discussion of a Miocene Collision Belt in North Borneo: Uplift Mechanism and Isostatic Adjustment Quantified by Thermochronology’, Geological Society of London Journal, 158: 396–400. Karig, D. E. (1971), ‘Origin and Development of Marginal Basins in the Western Pacific’, Journal of Geophysical Research, 76: 2542–61. Katz, B. J. (1991), ‘Controls on Lacustrine Source Rock Development: A Model from Indonesia’, Proceedings of the 20th Annual Convocation of the Indonesian Petroleum Association (Jakarta), i. 587–619.

Kuno, H. (1966), ‘Lateral Variation of the Basalt Magma Type across Continental Margins and Island Arc’, Bulletin of Volcanology, 29: 195–222. Lee, C. S., and McCabe, R. (1986), ‘The Banda– Celebes– Sulu Basin: A Trapped Piece of Cretaceous–Eocene Oceanic Crust?’, Nature, 322: 51– 4. McCaffrey, R. (1996), ‘Slip Partitioning at Convergent Plate Boundaries of SE Asia’, in R. Hall and D. Blundell (eds.), Tectonic Evolution of Southeast Asia, Geological Society Special Publication 106 (London), 3–18. Malod, J. A., and Mustafa Kemal, B. (1996), ‘The Sumatra Margin: Oblique Subduction and Lateral Displacement of the Accretionary Prism’, in R. Hall and D. Blundell (eds.), Tectonic Evolution of Southeast Asia, Geological Society Special Publication 106 (London), 19–28. Mammericks, J., Fisher, R. L., Emmel, F. J., and Smith, S. M. (1977), ‘Bathymetry of the East and Southeast Asian Seas’, Geological Society of America Map, MC–17. Metcalfe, I. (1990), ‘Allochthonous Terrane Processes in Southeast Asia’, London: Royal Society Philosophical Transaction, A331: 625–40. —— (1996), ‘Pre-Cretaceous Evolution of SE Asian Terranes’, in R. Hall and D. Blundell (eds.), Tectonic Evolution of Southeast Asia, Geological Society Special Publication 106 (London), 97–122. —— (1998), ‘The Palaeo-Tethys in East Asia’, in Ninth Regional Conference on Geology, Mineral and Energy Resources of Southeast Asia— GEOSEA ’98, Programme and Abstracts (Kuala Lumpur: Geological Society of Malaysia), 27–8. Mitchell, A. H. G., and Leach, T. M. (1991), Epithermal Gold in the Philippines: Island Arc Metallogenesis; Geothermal Systems and Geology (London: Academic Press). —— and McKerrow, W. S. (1975), ‘Analogous Evolution of the Burma Orogen and the Scottish Caledonides’, Geological Society of America Bulletin, 86: 305–15. Nichols, G., and Hall, R. (1997), ‘Stratigraphic and Sedimentological Constraints on the Tectonic History of the Celebes Sea Basin’, in Tectonics, Stratigraphy and Petroleum Systems of Borneo, University of Brunei Darussalam, 22–25 June; Programme and Abstracts (Brunei: Universiti Brunei Darussalam), 34. Petronas (1999), The Petroleum Geology and Resources of Malaysia (Kuala Lumpur: Petroliam Nasional Berhad (Petronas)). Richardson, A. N., and Blundell, D. J. (1996), ‘Continental Collision in the Banda Arc’, in R. Hall and D. Blundell (eds.), Tectonic Evolution of Southeast Asia, Geological Society Special Publication 106 (London), 47– 60. Samuel, M. A., and Harbury, N. A. (1996), ‘The Mentawi Fault Zone and Deformation of the Sumatran Forearc in the Nias Area’, in R. Hall and D. Blundell (eds.), Tectonic Evolution of Southeast Asia, Geological Society Special Publication 106 (London), 337–51. Shih, T. C. (1980), ‘Marine Magnetic Anomalies from the Western Philippine Sea: Implications for the Evolution of Marginal Basins’, in D. E. Hayes (ed.), The Tectonic and Geologic Evolution of Southeast Asian Seas and Islands, Geophysical Monograph 23 (Washington: American Geophysical Union), 49–75. Silver, E. A., Gill, J. B., Schwartz, D., Prasetyo, H., and Duncan, R. A. (1985), ‘Evidence for a Submerged and Displaced Continental Borderland, North Banda Sea, Indonesia’, Geology, 13: 687–91. Stauffer, P. H., and Mantajit, J. (1981), ‘Late Palaeozoic Tilloids of Malaya, Thailand and Burma’, in M. J. Hambrey and W. B. Harland (eds.), Earth’s Pre-Pleistocene Glacial Record (Cambridge; Cambridge University Press), 331–7.

The Geological Framework 23 Stephenson, D., and Marshall, T. R. (1984), ‘The Petrology and Mineralogy of Mt. Popa Volcano and the Nature of the Late Cenozoic Burma Volcanic Arc’, Geological Society of London Journal, 141: 747–62. Tapponnier, P., Peltzer, G., Le Dain, A. Y., Armijo, R., and Cobbold, P. (1982), ‘Propagating Extrusion Tectonics in Asia, New Insights from Simple Experiments with Plasticene’, Geology, 10: 611–16. Taylor, D. (1986), ‘Some Thoughts on the Developments of the Alluvial Tinfields of the Malay–Thai Peninsula’, Geological Society of Malaysia Bulletin, 19: 375–92. van Bemmelen, R. W. (1949, 1970), The Geology of Indonesia, Ia: General Geology of Indonesia and Adjacent Archipelagoes, 2: Economic Geology, Ib: Portfolio and Index, 1st and 2nd edns. (The Hague: Martinus Nijhoff).

Veevers, J. J. (ed.) (1984), Phanerozoic Earth History of Australia (Oxford: Clarendon Press). Vita-Finzi, C., and Hidayat, S. (1991), ‘Holocene Uplift in West Timor’, Journal of Southeast Asian Earth Sciences, 6: 387–93. Vroon, P. Z., van Bergen, M. J., and Forde, E. J. (1996), ‘Pb and Nd Isotope Constraints on the Provenance of Tectonically Dispersed Continental Fragments in East Indonesia’, in R. Hall and D. Blundell (eds.), Tectonic Evolution of Southeast Asia, Geological Society Special Publication 106 (London), 445–53. Weissel, J. K. (1980), ‘Evidence for Eocene Oceanic Crust in the Celebes Basin’, in D. E. Hayes (ed.), The Tectonic and Geologic Evolution of Southeast Asian Seas and Islands, Geophysical Monograph 23 (Washington: American Geophysical Union), 37– 47.

2

The Quaternary in Southeast Asia Geoffrey Hope

Introduction We live in the Quaternary period and are a product of its wide fluctuations in climate and rapid environmental change. From at least the Mid-Miocene, about 25 million years ago, the expansion of the Southern Ocean has supported a powerful westerly wind system. These winds prevent tropical heat from reaching the Antarctic region, which in turn has allowed the gradual refrigeration of the world’s oceans as ice built up on Antarctica (and eventually formed an ice shelf over the sea; Nunn 1999). Earlier in the Tertiary, when the ocean column was warm from top to bottom, seasonal cooling was offset by rising warm water, and the ocean currents effectively transported heat to the poles. For the last 2 million years the main mass of the oceans has remained at maximum density, around 4°C, with warmer surface waters of the tropical and temperate regions floating only in the upper few hundred metres above the thermocline. The Quaternary is the period of refrigerated ocean which marks an ice age, with the Earth in such a delicate thermal equilibrium that relatively minor changes in the amount of solar radiation received by a given hemisphere in a given season cause major fluctuations of ice volume in terrestrial ice caps. The marked asymmetry of land and sea in the two hemispheres means that the effects of changes in the season of closest approach to the sun, of the degree of tilt of the planet and the eccentricity of the orbit, cause instability in the long-term climate. I thank Peter Bellwood, Dave Bulbeck, Richard Corlett, Rien Dam, John Flenley, Avijit Gupta, Simon Haberle, Peter Kershaw, Liew Ping-Mei, Bob Morley, Paddy Nunn, Sue O’Connor, Dan Penny, Colin Prentice, Michael Prentice, Anne-Marie and François Semah, Sander van der Kaars, and Sun Xiangjun for information and useful comments on aspects of the manuscript.

The Quaternary is defined by successive expansions and retreats of ice caps, with the maximum episodes of ice and of warmth (the interstadials) each lasting around 10 000 years. Intermediate times are cooler than present, and these persist for around 100 000 years. The lock-up of ice is reflected by global changes in sea level, ocean levels falling about 125 m during glacial maxima and rising up to 6 m above present during some interglacials. The Antarctic ice cap retains about 75 m of the ocean’s water even during the interglacial phases. The beginning of the Quaternary is defined in various ways, such as the appearance of cold-water foraminiferal faunas in the north Atlantic (about 2.3 million years ago) or the base of the Olduvai normal magnetic sub-chron, commencing at about 1.77 million years ago (Nunn 1999). It is the period in which modern species evolve, and is characterized biologically by widespread extinction and the appearance of generalists which could handle environmental variability. It is divided into two uneven parts, the Pleistocene epoch from 1.65 Ma to 10 000 years ago, and the Holocene epoch (10 400 bp–present), which represents the period for which climates have been in an interglacial and similar to the present. The Pleistocene is often subdivided into Early (1.65– 0.79 Ma bp, part of the Matuyama reversed polarity chron), the Middle (780–130 ka bp), and the Late Pleistocene (130–10 ka bp), the period since the previous interglacial. The Middle and Upper have normal magnetization of the current Brunhes chron. The Quaternary timescale is also subdivided into 65 oxygen isotope stages, based on changes in marine carbonates that reflect ice cap fluctuations (Martinson et al. 1987). Thus Stage 5e is the last interglacial, 128–111 ka ago, while the Holocene, our present interglacial, is Stage 1, marked by low negative delta O18 values which commenced only 10 000 radiocarbon years ago.

The Quaternary in Southeast Asia 25

Until relatively recently the evidence for environmental change in the tropics during the Quaternary was scattered, and the obvious absence of former ice sheets led to the suggestion that the tropical regions had evaded the fluctuations of temperate areas, acting as a kind of planetary refuge. In a landmark paper Verstappen (1975) reviewed the abundant clues that environmental change has been widespread throughout tropical Southeast Asia. He pointed out the wide range of influences which are also summarized in this chapter. However, the relative mildness of Quaternary change in the Asian tropics is demonstrated by the fact that Southeast Asia preserves very high biodiversity, including taxa of great antiquity, in contrast, for example, to Canada, where mass extinction took place as the cold developed and ice caps formed. Rainfall and seasonality have also varied in Southeast Asia, but not as severely as in parts of Australia, where rainfall at times may have been only 20 per cent of present. As well as significant climate fluctuations, sea-level change has had greater effects in this region than probably anywhere else in the world because of the very extensive shallow shelf areas such as the Sunda and Sahul Shelves. These have been exposed as vast plains at times of glaciation, when the build-up of ice on land causes the ocean to fall. The continuing geological instability of the island arcs and plate boundaries in the region have added to the process of change. The mixing of Asian and Australian biota proceeded at a faster pace as the areas became closer and climates more similar through the later Tertiary. Finally the period is marked by the appearance of hominids possibly more than 1 million years ago, and the spread of modern humans within the past 100 000 years or so. At least initially the major impact of humans may have been through fire and the predation of island fauna. However, during the last few millennia landscapes made by human activity have become dominant throughout the region, and people now drive the major geomorphic, and at times atmospheric, processes in many parts of the region. Kalimantan and mainland Southeast Asia have been stable landmasses since the Tertiary, but they have experienced pronounced weathering and erosion. Island Southeast Asia has been tectonically and volcanically active, and uplift has led to downcutting and the deposition of hundreds of metres of sediments. However, the techniques used to date the earlier Quaternary have only become available recently, and some, like cosmogenic dating, are still being developed. Many age estimates are based on correlation to dated units or their position relative to marker horizons such as volcanic ash or marine sediments. These are relative chronologies, and they may provide only maximum or minimum estimates

of age. A few techniques, such as tree ring dating (for which teak has been used), ice cores, and coral growth bands (Gagan and Chappell 2000), allow annual or even monthly records to be built up, although these rarely cover long time periods at these resolutions. This chapter reviews the Quaternary historical biogeography of the biota of the region, with an emphasis on the data from the last 50 000 years, as this is much better understood than events in the Early or Middle Pleistocene. It then considers the causes of geomorphologic and biotic change by discussing sea levels, climate, neotectonics, extreme events, and anthropogenic influences up to the colonial period.

Biota The outstanding feature of the Southeast Asian region is the diversity of its environments and biota. This results from the combination of the collision of the IndoAustralian Plate with Asia over the Cenozoic and a long period of evolution in a changing world. Walker (1982) pointed out that the plant biodiversity per unit area is highest in the Malay Peninsula, which has had a stable geological history under continuously tropical climates. Here are very complex trophic levels, with the rainforest partitioned into canopy, trunk, and lower worlds with their own species and relationships of epiphytes, climbers, and parasites. The flora, of about l2 000 species, includes many endemic genera but widespread families which also occur in the other tropical regions of the world. New Guinea also has high biodiversity (an estimated 18 000 higher plant species), but this is developed in a much larger area by spatial partitioning due to complex topography, great altitudinal and climatic ranges, and variable geology. This is shown by the level of endemic species of more than 90 per cent, even though it shares many genera with other areas. Because the biodiversity per unit area of New Guinea is lower than that of Malaya, Walker hypothesizes that the niches in New Guinea are undersaturated, in relative terms. Whether Malaya is actually saturated will now never be known, as forest destruction has abruptly ended this 60 million years experiment (Morley 2000). The flora of Southeast Asia is termed the Malesian flora, and at the generic and family levels it stretches out into the Pacific in an increasingly depauperate form. It is bounded by the radically different Australian flora to the south and by Eurasian temperate floras to the north. Both these floras intrude to some extent. For example, Australian-derived Leptospermum occupies leached soils and the tops of mountains in Kalimantan and Thailand, and Casuarina occurs everywhere on strands and swamps. From the north, Pinus occurs

26 Geoffrey Hope

naturally as far as Sulawesi in drier montane habitats, and the Himalayan flora contributes a large part of high mountain shrubberies in New Guinea with Rhododendron and Vaccinium species. Dipterocarpaceae and oaks reach New Guinea but not Australia. Van Steenis (1979) has analysed the origins at much greater detail, defining twenty-eight groups of plants. Some of these groups have scattered occurrences which are mainly related to climate or local conditions rather than origin of the floras. The famous Wallace’s line, by which bird species are largely partitioned into island groups with strong Australian affinities and mainland Asian birds, is at least partly ecological rather than a major limitation on dispersal. The study of detailed species patterns of taxa such as palms shows that centres of diversity exist in both mainland Asia and New Guinea, with the islands in between relatively poor in species. Thus it seems clear that although immigration of biota has taken place, the similarities in the flora may have much older origins, with local speciation and limited migration. The likely cause of the similar family and generic make-up of Australian, Afro-Asian, and American tropical floras is that all are derived from tropical precursors in western Gondwana in the Cretaceous period. Asia has acquired its tropical Gondwanic taxa with the collision of India, which brought families such as Rutaceae (e.g. citrus) and Aquifoliaceae (e.g. Ilex) into southern Asia. Southern Gondwanic taxa, such as Agathis, Nothofagus, Phyllocladus, and Dacrycarpus, have reached montane New Guinea and spread west to only a limited extent, although Podocarpus has been more successful, reaching mainland Asia (Morley 2000). The faunas are much more distinct, reflecting the post-Gondwanic radiation of mammals. Rodents entered Australia about 4. 5 million years ago and also radiated throughout Asia. They reach high diversity in Australia only in the newer habitats of the deserts, which have greatly expanded in the last 2 million years (Heinsohn and Hope 2003). Curiously the ratio of rodents to marsupials in the Australian deserts is matched in New Guinea, suggesting that this island also presented new environments to mammals within the past few million years. This picture of evolving biota is thrown into new turmoil with the cyclic changes of the Quaternary and the transfer of species with temporary land bridges. The rainforest species had evolved for ever increasing specialization but now environmental plasticity was rewarded. The biogeography of mammals in island Southeast Asia has been interpreted by some authors as showing that past conditions must have provided barriers to the spread of rainforest. For example, Brandon-

Jones (2001) suggests that the absence of leopard, tiger, and wild dog from Borneo is evidence for very different vegetation and climates in the past, with drought events being so extensive as to exclude rainforest except on coastal islands and ranges. He suggests that there may have been a central Sundaland grassland with a possible semi-arid core. Were conditions so extreme? The use of modern distributions is complicated by the arrival of the greatest Pleistocene generalist, humans, who have changed distributions through introductions and extinctions that have accelerated to the present.

Influences Sea-Level Change Times of sea-level depression radically change the geography of Southeast Asia. Figure 2.1 shows the extent of land at maximum lowering. Thailand, Malaysia, and all the greater Sunda islands are joined as far east as Bali, the coastline running north to Kalimantan and some nearby Philippine islands, then west to the eastern coast of Thailand. Hainan Island and Taiwan are joined to the Chinese mainland so that the South China Sea exits through the relatively narrow Bashi Channel in Luzon Strait, or southward west of Mindanao towards Sulawesi. At maximum sea-level lowering the exposed shelves add 2 million km2 to the land area and even at −40 m major connections persist (Voris 2000). The Indonesian archipelago east of the Wallace line shows few connections, although some islands are enlarged. However, New Guinea extends further west and the Arafura Sea disappears, adding a third to the area of the Australian continent, an increase of around 1. 9 million km2. The vast plains are termed Sunda and Sahul and total the area of the Indian subcontinent. Across them large rivers made their way to the sea, including Asian rivers such as the Chao and Mekong, and the North Sunda or Molengraff River, which drained Sumatra and western Borneo north-eastward. In some cases these rivers cut down to the lower sea level, forming valleys into which the sea later flooded. Coral reefs also became exposed and developed solutional features which subsequently flooded to form deep holes. This process has been repeated many times in the Quaternary (Figure 2.2). The most widespread Holocene change that has had a major influence on human settlement is the adjustment of the coastline to the stable high sea level of the last 6000 years. The length of coastline in Southeast Asia more than doubled as the plains flooded and new islands formed (Bellwood 1997). Where major rivers reach the shallow shelf areas, a rapid construction of

The Quaternary in Southeast Asia 27

Fig. 2.1. The extent of dry land at maximum (−120 m) and average (−60 m) sea levels

deltas and beach ridges has advanced the coast seawards, for example the Irrawaddy and Mekong Deltas from mainland Asia (Woodroffe 2000), which have prograded up to 5 km in the last century. The Gulf of Thailand has a smaller river system but was flooded by the sea 6000 years ago to Ayutthaya, 70 km north of Bangkok, since when mangrove muds and clays have infilled over 100 km. In east Kalimantan, coring of the Mahakam River Delta shows at least 50– 80 m of sediment buildup in the Holocene. In some cases the weight of sediment and water has caused a general subsidence, which has been compensated for by new sediment or the formation of freshwater swamps behind beach or mangrove barriers. Thus marine sediments and old coastal forms can be found surprising distances inland, yet dating shows that they are only a few thousand years old. (Figure 2.3.) For example, in south Sulawesi Gremmen (1990) found mangrove 40 km inland and demonstrated that the Tempe depression, now occupied by a shallow lake, was an infilled estuary that had nearly divided the southwestern arm of the island. Spectacular limestone

cliffs rising from coastal plains in Thailand, China, Viet Nam, and Indonesia may also be old shorelines, with caves developed at the point where water used to issue across wave-cut platforms. Even areas lacking strong sediment supply have infilled through the growth of coastal barriers and biological successions. In northern Australia estuaries up to 100 km from the present coast were rapidly colonized by mangroves. In many cases these forests infilled to high-tide level by about 4000 years ago, and there has been only minor accumulation since then on the grassy plains, and mangroves have retreated to line tidal channels (Woodroffe 2000). The sea rose rapidly from 14 500 to 9000 years ago, and may even have risen by as much as 16 m over 300 years around 14 000 bp (Hanebuth, Stattegger, and Grootes 2000). It once more covered old coral areas and reef-building commenced, with consequent calcareous sand spits and beach formation. This is shown by widespread sequences indicating successive phases of open coast, estuarine muds, mangroves, and freshwater swamps such as the widespread sago and Nipa palm.

28 Geoffrey Hope

Fig. 2.2. Sea-level fluctuations of the last 300 000 years (Source: based on the Oxygen isotope record of Martinson et al. 1987) Connections between Kalimantan and the Malay Peninsula are established at −25 m, and between Australia and New Guinea at −12 m (Tjia 1980)

Fig. 2.3. The Sepik floodplain of Papua New Guinea infills a vast Holocene estuary

The Quaternary in Southeast Asia 29

It is important to realize that modern reefs are of very recent origin, though often built on a formation of reefs from former interglacial periods.

Climate Change and Ecological Response Montane Clear evidence that the tropics experienced climate change first came from the discovery of changes in the extent of tropical glaciers in the Late Pleistocene. In tropical Southeast Asia the lower atmosphere cools with altitude at a rate of about 6°C per 1000 m, and the snowline coincides with the 0°C mean annual temperature at 4650 m. Glaciers are found today above 4620 m on Mount Jaya (Carstensz), which at 4884 m is the highest tropical mountain in the region (Figure 2.4). But glaciation was much more widespread at the peak of the last ice age about 18 000 years ago, with ice on Mount Kinabalu in Sabah (Flenley and Morley 1978) and an estimated 1400 km2 in New Guinea (Hope 1986; Peterson et al. 2001). Snow was probably common on peaks above 3000 m in Sumatra, Java, Lombok, and Sulawesi. Glaciation extended to mountains in southern Yunnan. The snowline occurred at 3500–3650 m in New Guinea and perhaps 3800 m on Mount Kinabalu, a lowering of about 1000 m. This suggests that summit temperatures were 6–7°C cooler than present, an estimate in agreement with general global cooling at the time of the glacial maximum. Pollen analyses of swamps and lakes at altitudes above 2000 m also confirm that cooling took place. Walker and

Flenley (1979) established that open grasslands were widespread above 2200 m in New Guinea, and this has been supported at various sites across the island (Hope 1996). Hope (1989) warns against assuming that the modern treeline (at about 3900 m) is responding to the same climatic controls as that of the past. The evidence suggests that modern subalpine shrublands and low forest are occupying a zone that was more open in the past, for reasons which are unclear. However, it is possible that woody plants near treeline may be sensitive to the low carbon dioxide levels that occur during glaciations (Street-Perrot et al. 1997), or that lethal frosts may have occurred frequently enough to discourage tree growth. The modern upper limit for a range of upper montane trees is 3200 m, and in New Guinea these are sometimes killed during drought and frost events associated with El Niño activity (Brookfield and Allen 1989). This boundary was 800–1000 m lower at glacial maximum and thus in agreement with the lowered snowlines. The rainfall at higher altitudes must have been similar to present for moisture-dependent rainforest was widespread in the mountains of the equatorial tropics. In Sumatra Newsome and Flenley (1988) found little evidence for forest change at 1600 m over the last 30 000 years, but this site lies in the centre of the montane forest zone. Maloney and McCormac (1996) found evidence for some climate changes at Pea Bullok in north Sumatra. Van der Kaars and Dam (1995, 1997) analysed a long core from 665 m altitude on the Bandung Basin, west Java, and found evidence for cooling and for periods of aridity over the past 120 000 years, although

Fig. 2.4. Glaciation and glacial features on Mount Jaya

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Fig. 2.5. Vegetation change at Bandung, a 120 000 year record of climate (Source: simplified from van der Kaars and Dam 1995)

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The Quaternary in Southeast Asia 31

forest cover was maintained (Figure 2.5). In Sulawesi a 60 ka core from an ultramafic basin at 380 m records a phase of expanded grassland around 20 000 years bp, a time of maximum sea-level lowering (Hope 2001). Fire is present throughout the record, and the grassland may reflect drier climates. Hope and Tulip (1994) also recorded cooler conditions, but with moisture, at 750 m in the Cyclops Mountains of New Guinea. The vegetation has thus apparently reacted to temperature change, with drier climates in the west and probably to the south.

Lowland During times of low sea level the Sunda Shelf cut off warm water from the Indian Ocean in the northern summer, while the Arafura Shelf prevented cooler water from the Pacific from crossing to the Banda Sea during the southern winter. Even in the absence of external influences these changes from shallow sea to land would have a major regional and probably global effect on climate. The shelf areas today are a major source of moisture and latent heat, as they trap the solar radiation and transport it as water vapour and clouds. Increasing land area and cooler seas, however, mean that more continental climates must have occurred across the exposed land and newly connected neighbouring land masses during times of low sea level. This led to greater aridity to the lee of mountain ranges and less vegetation cover, and higher reflectivity with less deep atmospheric warming and cloudiness. The summer monsoons of East Asia and northern Australia seem to have been weakened by these changes, while the southeast and northeast trades (the winter monsoon) were possibly strengthened, extending the dry season in many localities (Sun et al. 2003). Verstappen (1975) pointed out that the widespread coastal plains of the western Malayan Peninsula are coarser than sediments forming now, and extend below present sea level, often with lenses of freshwater peat such as an organic section over 20 m in thickness, near Ipoh. These sands and gravels, well known as sources of alluvial tin, built up during glacial times when sea level was lower, and mark the courses of rivers as pointed out by Molengraff and Weber (1919). Periods of reduced and more seasonal rainfall resulting in a woodland formation with exposed slopes may be responsible for widespread sandy alluvium found around Singapore and along the eastern and western coasts of the Malayan Peninsula, and are possibly matched by red sands and gravels in southern Vietnam (Nghi et al. 1991). Erosion seems to have been active in transporting slope mantles onto the Sunda Shelf at these times, whereas today slopes are stable under dense forest (Gupta et al. 1987). Environ-

ments with gravel-choked braided streams are found in the seasonal tropics, such as Timor or northern Thailand and Lao PDR, where a dry season of several months occurs. Hence it is possible that this affected land right to the Equator at times in the past. Long pollen records close to sea level are still uncommon in equatorial regions. The widespread peatlands of Kalimantan and Sumatra have provided mainly Holocene records (e.g. Morley 1981), although some peat development took place in the Late Pleistocene (Anshari, Kershaw, and van der Kaars 2001; Kershaw et al. 2001). S. van der Kaars (2001) has completed a 30 ka record from the Sunda Strait, which has complex shifts in forest dominance through that time. Marine palynological and sedimentary records include several long cores from marine basins in eastern Indonesia (S. van der Kaars 1991; S. van der Kaars et al. 2000). These clearly show the fluctuations in vegetation with more open conditions during glacial times, and an expansion of rainforest and mangroves during interglacials. Fire is more widespread at times, but not at the peak of glacials (Kershaw et al. 2001). The cores from eastern Indonesia include a signature from Australia, reflecting the continuing southeast wind vector that brings eucalypt pollen and charcoal into the area. A new record from the South China Sea at 7°N in 1800 m water depth north of Kalimantan (Sun et al. 2000) confirms that lowland rainforest cover was maintained throughout the peak of the last glaciation. The transition from glacial to Holocene is not marked at all. However, there are continuous changes in the input of montane gymnosperms such as Dacrydium and Podocarpus, which may represent increases in cool environments. The fluctuations occur on about a thousand-year timescale, and the increases in lowland rainforest coincide with peaks in mangrove types. This variability suggests that constant change in forest composition has been taking place, although the position of the core on the continental shelf of a large delta may contribute by adding sediment packets from a range of sources. At Lake Sentarum, east Borneo, a compressed peat sequence shows rainforest dominance throughout the 30 000-year record. The results vindicate the predictions made by Verstappen (1975, 1980) that pollen would eventually show that, in the equatorial region, forest cover has remained extensive, but that more open woodlands were widespread in some areas. This conclusion matches recent interpretations from the Amazon, where hypotheses of very widespread forest retreat at the last glacial maximum have been shown to be wrong, although the savanna certainly expanded at that time (Markgraf 2000).

32 Geoffrey Hope

More dramatic vegetation shifts are evident in northern Thailand, where full glacial conditions are associated with a shift from subtropical forest to pine woodlands and even shrublands and grasslands. Pine forests and even open steppe occurred on the continental shelf of the South China Sea during glaciations. In a million-year record the fluctuations between steppe and tropical forest seem to become more extreme from 300– 400 ka onwards, suggesting a gradual tendency to more extreme climatic fluctuations. Sun et al. (2003) explain this as a consequence of the increasing height of the Tibetan Plateau, which has acted as a ‘third pole’. However, the survival of a rich subtropical biota in mainland Southeast Asia and southern China shows that these dramatic shifts were not experienced all the way to the coast. The long record from Nong Pa Kho in northeastern Thailand has also confirmed continuous forest cover at full glacial times (Penny 2001). However, pines were more dominant and there was probably an extension of cool forest with Quercus and Castanopsis. Dry-season deciduous elements are absent but deciduous forest may have extended southward into the Malay Peninsula during cold periods. The analysis of oxygen isotopes from foraminiferal tests in ocean cores has provided estimates of thermal change near sea level in the region. For the area north of New Guinea and westwards through the Indonesian archipelago the warmest open seas on Earth are found today. The isotope studies show relatively small change through time over the last 150 ka (when shifts due to ice cap formation as sea levels fall are taken into account) (e.g. Thunell et al. 1994; Barmawijaja et al. 1993). This has been claimed as evidence that temperatures at sea level dropped only 1–2°C, and that the ocean circulation was little changed. This result contrasts with cores from the South China Sea, where cooling of up to 8°C is inferred at 15 000 years bp (Thunell and Miao 1996). This area was possibly chilled in winter by a southward movement of the cold fronts from the Mongolian high-pressure cell. The land record from Indonesia also suggests cooling near sea level of 4–5°C at 18 000 years bp (Hope and Tulip 1994; S. van der Kaars 2001). The Holocene transition occurred about 14 000– 9000 years ago, while carbon dioxide levels doubled around 13 000–11 000 years ago. Glaciated areas and subalpine grasslands were invaded by high-altitude forests which became richer as slower-moving tree species arrived around 10 000 years ago. The transition at lower altitudes is marked in several places by a temporary increase in Phyllocladus but in general by increases in complex warm forests at the expense of

gymnosperms. Seasonally dry areas contracted as evergreen lowland closed forests displaced woodlands. This is demonstrated in cave faunas from the Aru Islands by the abrupt replacement of savanna kangaroos by forest wallabies and possums about 14 000 years ago (O’Connor et al. 2004). Parts of Thailand and China have increases in pine dominance that reflect warmer conditions. Walker and Sun (1988) point out that some areas in southern Yunnan may have become drier while others became wetter, owing to a reduced land– sea temperature gradient. In some places, such as New Guinea, forests reached their highest altitudes before 6000 years ago and then were replaced by grassland in the last 4000 years. This is also a time of minor ice advances in Papua (Hope and Peterson 1975). The changes in the Late Holocene thus certainly include some climate shifts. However, it is also possible that the optimal development of forest in the Early Holocene represents relative climatic stability compared to the high variability experienced at present, with major droughts correlated with El Niño events. The interpretation of vegetation change is complicated by the influences due to human occupation which become more noticeable throughout the Holocene.

Neotectonics Island Southeast Asia is forming as the result of the current plate collision between the Indo-Australian, Eurasian, Philippine, and Caroline and Pacific Plates, although the major features have not changed much in the last 2 million years (Hall 2001). However, even over the short period of the Quaternary there have been dramatic changes to landscape caused by uplift, downthrow, and wrench faulting. The most obvious expression of this, beside the active volcanoes and shattering earthquakes in the Philippines and Indonesia, is the young age of some mountains and islands formed by rapid uplift of deep marine sediments. This can be seen on many islands in Nusa Tenggara and Timor, where series of terraces of raised coral show the former position of the sea as the land rose over the past 250 000 years. Uranium series dating shows uplift rates of 50–300 m in 100 000 years (Nunn 1999). This is borne out by the higher mountains; Mount Jaya (Carstensz) is formed from 25-million-year-old deep-sea limestones, intruded by Pleistocene granites. Mount Kinabalu (4101 m) in Borneo and Mount Wilhelm (4509 m) in Papua New Guinea are isolated Pleistocene granodiorite peaks which have lost the substantial sedimentary cover they must once have had. Similarly, the large volcanic cones of Lombok, Java, Sumatra, and Luzon are also Quaternary in age.

The Quaternary in Southeast Asia 33

There is a clear demarcation between these active landscapes in the island arcs and the stable continental shields of Asia and Australia. Mainland Southeast Asia, and parts of Malaysia, Thailand, and Kalimantan, are also stable. But even in these areas small changes have occurred, an example being the remarkable Aru Islands in the Arafura Sea on the western edge of the Australian Plate. These low islands are the result of gentle doming of seabed muds and marls. With subsequent slow subsidence in this region of 8 m tides, old joint cracks are now marine channels cutting across the islands in many places, with reversing tidal flows (Verstappen 1959). The best-studied example of uplift in the Pleistocene is the Sangiran Dome in central Java, where a Pliocene estuary has gradually emerged and is now cut through by the Cemoro River to expose a sequence 160 m in thickness (Bellwood 1997). Above the estuarine sediments are lake clays (Pucangan Formation) and river alluvium (Kabuh Formation), both famous for faunal and hominid remains. The division between these formations is dated at about 0.9–0.7 million years ago. Between the mountain blocks are intermontane valleys and sunklands, which in some cases are the landward end of marine gulfs that continue to sink, like the Lingayan Gulf, Luzon, or the Gulf of Martaban east of Yangon. Sediment brought by rivers load the shelf areas and enhance the subsidence (Woodroffe 2000). These subsiding valleys are the downthrown sides of faults. In Sulawesi the 2000 km long Irian Fault cuts across the island, and large lakes such as Danau Matano and Danau Poso, both at about 400 m altitude but 600 m deep, occupy deep points in the structure. They have probably been infilling with sediment for the whole of the Pleistocene.

Volcanism and Impacts Throughout the Quaternary singular events have occurred that have large local effects that are not universal across the region, as climate or sea-level changes are. Quaternary volcanic landscapes including active and extinct cones, lava fields, and ash blankets are widespread in western Indonesia, particularly Sumatra, Java, and the Lesser Sunda Islands, and also in eastern New Guinea and the Philippines. The largest eruption of the Quaternary anywhere on earth was the explosion of Lake Toba, in central Sumatra, only 73 000 years ago. This explosion left ash and gas traces in the Greenland ice cap (Paul Mayewski, personal communication), as an estimated 2800 km3 was thrown out and the ash cloud reached the stratosphere. Ash from this event forms soil horizons throughout northern Sumatra and as far as India (Rose and Chesner 1987; Chesner et al. 1990).

Even relatively minor eruptions in Java are known to deposit very fine ash in central Australia so that the volcanoes have influenced large parts of the Southeast Asian region. Volcanic outflows and ash have also blocked drainage lines and created lakes and valley infills. The Mid-Pleistocene vulcanism of the central highlands of Papua New Guinea reversed the flow of the Wahgi River and created several lakes around Tari, mostly now infilled but including Lake Kutubu. Around 730 000 years ago a substantial meteor strike impacted the Gulf of Thailand (Fudali and Ford 1979). The explosion is dated from a crater in western Tasmania which was formed by a fragment from the original object which passed over Australia and left a shower of fused silica tectites in its path. Its impact in western Tasmania caused Darwin Crater, which is infilled by over 60 m of sediment, which records several glacial cycles (Colhoun and van der Geer 1988), but no magnetically reversed sediments.

Human Influences The oldest remains of tool-using humanoids in Southeast Asia are known from Java; a date of 1.8 million years has been claimed, but more acceptable dates for the Sangiran occurrences of Homo erectus date from 1.2 to 0.75 Ma bp (Bellwood 1997; Storm 2001). Bellwood reviews the available evidence for Homo erectus; he considers that the early dates of 1.8 Ma bp in China are problematic, but it seems clear that the species was widespread in Southeast Asia by the Mid-Pleistocene. Changes from the earliest skulls to more recent suggest that larger brain size and local variations were evolving. Homo erectus has been claimed from Flores about 0.84 Ma bp (Morwood 2001), which means that even water gaps were being overcome. The Pacitanian stone tool tradition that is arguably associated with Homo erectus includes heavy tools made by flaking riverine pebbles as well as flakes struck from cores. Finds have been reported from the Philippines, Malay Peninsula, Sulawesi, and Flores, but except for Java and Flores the actual association with old deposits is dubious. These early humans presumably lived by hunting and gathering, and may have had some effect on vegetation through the use of fire, judging from some cave deposits from China. However, the environmental effects are unknown. Fauna found in the Early Pleistocene in Java but absent from the region today include an elephant relative, the stegodon, hippopotamus, and enormous horned cattle, but their extinction is not well dated and may relate to environmental change from open seasonal woodlands to modern rainforest (van den Bergh, Vos, and Sondaar 2001). It is true that stegodons and some

34 Geoffrey Hope

other animals such as giant tortoises occupied Sulawesi and other islands in the Mid-Pleistocene. The Komodo dragon was also widespread in Sumba and Flores (Morwood 2001), but large mammalian carnivores are absent. These animals became extinct or regionally restricted, probably in the Mid-Pleistocene, because no Late Pleistocene remains have been located. In Papua species of kangaroo and diprotodontid were lost about 25 000 years ago (Hope 1998). It seems possible that early human settlement contributed to the loss of these large island faunas, but this remains to be demonstrated. Modern humans do not yet have a long record in mainland or island Southeast Asia. In fact the oldest dated skeletal remains of Homo sapiens come from Australia, with an age (based on Uranium series, OSL, and ESR on bone and sediment) of 58 000– 64 000 years (Thorne et al. 1999). This may provide a minimum age for the spread of Homo sapiens into Southeast Asia, but such old dates are not yet established, the oldest dated skull from Niah Cave in Sarawak being less than 40 000 years old. In Java, Semah and Semah (1999) have dug a deep section with a sequence of stone tools and calcareous layers dated by Uranium series at more than 75 000 years bp. Modern tools appear well after that date. The present assumption is that Homo sapiens appeared in Southeast Asia around 100 000 years ago. Bellwood (1997) reviews the range of views for the origins of modern populations; limited evidence supports the idea that the ancestral population was Indo-Malay–Melanesian and that these peoples crossed sea barriers to eastern Indonesia and New Guinea by 50 000 years ago. Storm (2001) hypothesizes that in Java Homo erectus was replaced or absorbed as part of a wider replacement of woodland species by those adapted to rainforest. A flake industry is known from a few localities on the Malay Peninsula, and older sites occur in southern China and Thailand. Large tools occur in Java and New Guinea dated to more than 30 000 years ago. In Malaya and Sumatra inland sites are more or less absent through this early period. However, in island Indonesia and New Guinea, as well as many sites in Southeast Asia, flaked tool industries continue up to the Holocene. Mainland populations presumably changed over time, with arguable additions of Mongoloid elements from the north in the Late Pleistocene and Early Holocene. The Hoabinhian is a pebble tool tradition that becomes widespread in Malaya and Sumatra about 13 000 years ago, but sites back to 18 000 years ago are known from Viet Nam and Lao PDR. A concentration on pebble tools with the manufacture of waisted blades, possibly used as adzes, is typical, and in some areas polished blades

become common. Bellwood (1997) considers that while the people of this time did not practise sedentary field agriculture, they may have cleared and encouraged desirable species. This Hoabinhian lifestyle continued up to 1000 years ago in isolated valleys, even after ricebased agriculture started to spread into valley floors about 4000–3000 years ago. The pre-agricultural impact of people was considerable, mainly through burning (Kershaw et al. 2002). Penny (2001) found consistent increases in charcoal after 6400 years ago, which was followed by the disappearance of many tree taxa at Kumphawapi in Thailand. The record of human disturbance from pollen sequences is a very mixed one, ranging from evidence for mammal extinction and erosion following fire in the highlands of Papua at 33 000 years (Hope 1998) to virtually no apparent effect in remote forested sites. Some grasslands and glacial-time woodlands may have been maintained into the Holocene by fire as at Sentani, Irian Jaya (Hope and Tulip 1994), while others were abandoned by people after a few thousand years. Evidently some human landscapes have great antiquity. Humans also exploited fauna, causing range restrictions and local extinctions. One curious case is the appearance of kangaroos in the northern Moluccas about 10 000 years ago, probably as a result of human introduction from New Guinea (Bellwood 1997). These species went extinct about 2000 years ago, as a result of predation and perhaps the use of dogs. While the general role of indigenous versus imported populations and techniques is speculative, there is no doubt that Austronesian-speaking people from southern China moved into the area after 5000 years ago, spreading across Indonesia and coastal New Guinea and finally reaching the furthest limits of the Pacific at Hawai’i, Easter Island, and New Zealand, and the Indian Ocean in Madagascar around 1000–700 years ago. With this diaspora came maritime exploitation, agriculture, and pottery to areas previously lacking these skills, although some of these technologies preceded the Austronesian arrival in areas of New Guinea and mainland Southeast Asia. Agriculture involving cereal production was occurring in China by 8000 years ago, and it had spread to Taiwan, Thailand, and Sumatra (Maloney 1996) by at least 5000 years ago. Apparently independently, agriculture appeared in the highlands of New Guinea about the same time, based on root crops. Around the major montane valleys deforestation was in train by 7000 years ago, and sediment yields from exposed slopes had increased several times (Golson and Hughes 1980). Fixed agriculture is associated with population increases; from a patchy record in the Early Holocene, by the Late Holocene most sites show

The Quaternary in Southeast Asia 35

Fig. 2.6. Human farming has been a major agent for geomorphic change. Paddy fields in Sulawesi

signs of the impact of people on forests and mires, with charcoal inputs and the appearance of domesticates and secondary plants as well as increased clay and silt deposition (Haberle 1994). There is some controversy between the view that deforestation has led to major changes in river silt loads and the view that long-term erosion rates have not changed, as they are a function of weathering rates under regional climates. However, at the local catchment level, human intervention is dramatic. This is shown over many parts of Indonesia by the spread of low-fertility grasslands (alang alang) onto formerly farmed areas. In most cases the humic topsoil has been completely stripped from slopes, leaving indurated red clays exposed. The sediments often infill valleys with deep alluvial deposits. Once the sediment supply is finished, the deposits may start to gully. The process of infilling of estuaries has also accelerated owing to the supply of sediment reaching the sea. In the modern era some of these effects have multiplied through urbanization, logging on steep slopes, and mining. The effect of forest clearance is to expose highly absorbent humic layers to drying and oxidation. This generally leaves a less absorbent mineral soil which sheds water more rapidly. In turn this generates fast run-off that floods streams rapidly and increases their erosive power. Humans have, however, made remarkably effective controls against erosion by mastering water flows in crop agriculture, particularly aquatic crops such as rice (Figure 2.6). The advent of larger-scale operations such as oil palm, sugar, and forest ‘mining’

have been more damaging for geomorphic stability. The huge peat fires of 1984 and 1997 have removed part of the Holocene legacy of metres of peat from millions of hectares in Kalimantan and Sumatra, largely at the instigation of large companies. The resulting smoke plumes lasted for months and represent a global-scale climate incident.

Quaternary Environments It is clear that Southeast Asia has changed continuously throughout the Quaternary. The sea levels have cyclically exposed and drowned the shallow shelves providing linkages between continents and their nearby island, followed by phases of isolation and coral development. Climate change has also been cyclic but has varied in degree and persistence through the Quaternary. Ecological responses to the changes of the Quaternary have included sequential invasion of islands, the development of endemicity, increased speciation, and extinction. Extinction increased markedly as humans caused environmental changes and directly killed fauna. The rate of change in the past is much slower than that currently occurring. This suggests that the least stressed habitats of Southeast Asia, such as core areas of tropical forest, are not well suited to surviving the more rapid change of the present. However, the rapid recovery of both terrestrial and marine ecosystems at the end of the last ice age gives hope that intelligent management of environments is possible if human population and aspiration can be kept within limits.

36 Geoffrey Hope

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—— (1989), ‘Climatic Implications of Timberline Changes in Australasia from 30 000 b.p. to Present’, in T. H. Donnelly and R. J. Wasson (eds.), CLIMANZ 3: Proceedings of the Third Symposium on the Late Quaternary Climatic History of Australasia (Canberra: CSIRO Division of Water Resources), 91–9. —— (1996), ‘Quaternary Change and Historical Biogeography of Pacific Islands’, in A. Keast and S. E. Miller (eds.), The Origin and Evolution of Pacific Island Biotas: New Guinea to Eastern Polynesia. Patterns and Process (Amsterdam: SPB Publishing), 165–90. —— (1998), ‘Early Fire and Forest Change in the Baliem Valley, Irian Jaya, Indonesia’, Journal of Biogeography, 25: 453– 61. —— (2001), ‘Environmental Change in the Late Pleistocene and Later Holocene at Wanda Site, Soroako, South Sulawesi, Indonesia’, Palaeogeography, Palaeoclimatology, Palaeoecology, 171: 129– 45. —— and Peterson, J. A. (1975), ‘Glaciation and Vegetation in the High New Guinea Mountains’, Bulletin of the Royal Society of New Zealand, 13: 153– 62. —— and Tulip, J. (1994), ‘A Long Vegetation History from Lowland Irian Jaya, Indonesia’, Journal of Palaeogeography, Palaeoclimatology, Palaeoecology, 109: 385–98. Kershaw, A. P., Penny, D., van der Kaars, S., Anshari, G., and Thamotherampili, A. (2001), ‘Vegetation and Climate in Lowland Southeast Asia at the Last Glacial Maximum’, in I. Metcalfe, J. M. B. Smith, M. Morwood, and I. Davidson (eds.), Faunal and Floral Migrations and Evolution in SE Asia–Australasia (Lisse: Balkema), 227–36. —— van der Kaars, S., Moss, P., and Wang, X. (2002), ‘Quaternary Records of Vegetation, Biomass Burning, Climate and Possible Human Impact in the Indonesian–Northern Australian Region’, in A. P. Kershaw, N. J. Tapper, B. David, P. M. Bishop, and D. Penny (eds.), Bridging Wallace’s Line, Advances in GeoEcology 34 (Reiskirchen: Catena Verlag), 97–118. Maloney, B. K. (1996), ‘Palaeoecological Evidence for Possible Early Dry Land and Wet Land Rice Cultivation in Highland North Sumatra’, Asian Perspectives, 35/2: 165–92. —— and McCormac, F. G. (1996), ‘Palaeoenvironments of North Sumatra: A 30 000 Year Old Pollen Record from Peabullok, Sumatra’, Bulletin of the Indo-Pacific Prehistory Association, 14: 73–82. Markgraf, V. (ed.) (2000), Interhemispheric Climate Linkages (San Diego: Academic Press). Martinson, D. G., Pisias, N. G., Hays, J. D., Imbrie, J., Moore, T. C., and Shackleton, N. J. (1987), ‘Age Dating and Orbital Theory of the Ice Ages: Development of a High Resolution 0–300 000-Year Chronostratigraphy’, Quaternary Research, 27: 1–29. Molengraff, G. A. F., and Weber, M. (1919), ‘Het verband tussen den plistocenen ijstijd en het ontstaan der Sunda Zee en de invloed daarvan op de verspreiding der koraalriffen en op de land-en zoetwaterfauna’, Verslag Koninklijke Nederlandsche Akademie van Wetenschappen, 28: 497–544. Morley, R. J. (1981), ‘Development and Vegetation Dynamics of a Lowland Ombrogenous Peat Swamp in Kalimantan Tengah, Indonesia’, Journal of Biogeography, 8: 383– 404. —— (2000), Origin and Evolution of Tropical Rain Forests (London: Wiley). Morwood, M. (2001), ‘Early Hominid Occupation of Flores, East Indonesia, and its Wider Significance’, in I. Metcalfe, J. M. B. Smith, M. Morwood, and I. Davidson (eds.), Faunal and Floral Migrations and Evolution in SE Asia–Australasia (Lisse: Balkema), 387–98. Newsome, J. C., and Flenley, J. R. (1988), ‘Late Quaternary Vegetational History of the Central Highlands of Sumatra’, II: ‘Palaeopalynology and Vegetational History’, Journal of Biogeography, 15: 555–78.

The Quaternary in Southeast Asia 37 Nghi, T., Toan, N. Q., Thanh, D. T. V., Minh, N. D., and Vuong, N. V. (1991), ‘Quaternary Sedimentation of the Principal Deltas of Vietnam’, Journal of Southeast Asian Earth Sciences, 18: 427–39. Nunn, P. D. (1999), Environmental Change in the Pacific Basin (New York: Wiley). O’Connor, S. L., Spriggs, M., and Veth, P. (eds.) (2004), The Archaeology of the Aru Islands, Eastern Indonesia, Modern Quaternary Research in Southeast Asia, (Canberra, Pandanus Press). Penny, D. (2001), ‘A 40 000 Year Palynological Record from North-East Thailand: Implications for Biogeography and PalaeoEnvironmental Reconstruction’, Palaeogeography, Palaeoclimatology, Palaeoecology, 171: 97–128. Peterson, J. A., Hope, G. S., Prentice, M., and Hantoro, W. (2001), ‘Mountain Environments in New Guinea and the Late Glacial Maximum “Warm Seas/Cold Mountains” Enigma in the West Pacific Warm Pool Region’, in P. Kershaw, B. David, N. Tapper, D. Penny, and J. Brown (eds.), Bridging Wallace’s Line, Advances in GeoEcology 34 (Reiskirchen: Catena Verlag), 173–87. Rose, W. I., and Chesner, C. A. (1987), ‘Dispersal of Ash in the Great Toba Eruption, 75ka’, Geology, 15: 913–17. Semah, A.-M., and Semah, F. (1999), ‘Plestosen atas dan batas Holosen di daerah Jawa Tengah/Timur’, Abstract Pertemuan Ilmiah Arkeologi VIII dan Kongress Ikatan Ahli Arkeologi Indonesia Ke8, Yogyakarta 1999, 114. Storm, P. (2001), ‘The Evolution of Humans in Australasia from an Environmental Perspective’, Palaeogeography, Palaeoclimatology, Palaeoecology, 171: 129–45. Street-Perrot, F. A., Huang, Y., Perrott, R. A., Eglinton, G., Barker, P., Ben Khelifa, L., Harkness, D. A., and Olago, D. O. (1997), ‘Impact of Lower Atmospheric Carbon Dioxide on Tropical Mountain Ecosystems’, Science, 278: 1422–6. Sun, X., Li, X., Luo, Y., and Chen, X. (2000), ‘The Vegetation and Climate on the Emerged Continental Shelf of the South China Sea’, Palaeogeography, Palaeoclimatology, Palaeoecology, 160: 301–16. —— Luo, Y., Huang, F., Tian, J., and Wang, P. (2003), ‘Deep-Sea Pollen Record over the Last Million Years from the South China Sea and East Asian Monsoon’, Marine Geology, 201: 97–118. Thorne, A., Grun, R., Mortimer, G., Spooner, N. A., Simpson, J. J., McCulloch, M., Taylor, L., and Curnoe, D. (1999), ‘Australia’s Oldest Human Remains: Age of the Lake Mungo 3 Skeleton’, Journal of Human Evolution, 36: 591–612. Thunell, R. C., and Miao, Q. (1996), ‘Sea Surface Temperatures of the Western Equatorial Pacific during the Younger Dryas’, Quaternary Research, 46: 72–7. —— Anderson, D., Gellar, D., and Miao, Q. (1994), ‘Sea Surface Temperature Estimates for the Tropical West Pacific during the Last Glaciation, and their Implications for the West Pacific Warm Pool’, Quaternary Research, 41: 255–61. Tjia, H. D. (1980), ‘The Sunda Shelf, Southeast Asia’, Zeitschrift für Geomorphologie, ns 24/4: 405–427. van den Bergh, G., Vos, J. de, and Sondaar, P. Y. (2001), ‘Late Quaternary Palaeogeography of Mammal Evolution in

the Indonesian Archipelago’, Palaeogeography, Palaeoclimatology, Palaeoecology, 171: 385– 408. van der Kaars, S. (1991), ‘Palynology of Eastern Indonesian Marine Piston-Cores: A Late Quaternary Vegetational and Climatic record for Australasia’, Palaeogeography, Paleoclimatology, Palaeoecology, 85: 239–302. —— and Dam, M. A. C. (1997), ‘Vegetation and Climate Change in West Java, Indonesia during the last 135 000 years’, Quaternary International, 37: 67–71. —— Wang, X., Kershaw, P., Guichard, F., and Setiabudi, D. A. (2000), ‘A Late Quaternary Palaeoecological record form the Banda Sea, Indonesia: Patterns of Vegetation, Climate and Biomass Burning in Indonesia and Northern Australia’, Palaeogeography, Paleoclimatology, Palaeoecology, 155: 135–53. van der Kaars, W. A. (1997), ‘Marine and Terrestrial Pollen Records of the Last Glacial Cycle from the Indonesian Region: Bandung Basin and Banda Sea’, Palaeoclimates: Data and Modelling, 4: 1–11. —— —— (2001), ‘Late Quaternary Palaeocology, Palynology and Palaeolimnology of a tropical Lowland Swamp: Rawa Danau, WestJava, Indonesia’, Palaeogeography, Paleoclimatology, Palaeoecology, 171: 185–212. —— —— and Dam, M. A. C. (1995), ‘A 135 000-Year Record of Vegetational and Climatic Change from the Bandung Area, WestJava, Indonesia’, Palaeogeography, Paleoclimatology, Palaeoecology, 117: 55–72. van Steenis, C. G. G. J. (1979), ‘Plant Geography of East Malesia’, Botanical Journal of the Linnean Society, 79: 97–178. Verstappen, H. Th. (1959), ‘Geomorphology and Crustal Movements of the Aru Islands in Relation to the Pleistocene Drainage of the Sahul Shelf’, American Journal of Science, 257: 491–502. —— (1975), ‘On Palaeo Climates and Landform Development in Malesia’, Modern Quaternary Research in Southeast Asia, 1: 3–35. —— (1980), ‘Quaternary Climate Changes and Natural Environment in SE Asia’, Geojournal, 4: 45–54. Voris, H. K. (2000), ‘Maps of Pleistocene Sea Levels in Southeast Asia: Shorelines, River Systems and Time Durations’, Journal of Biogeography, 27: 1153– 67. Walker, D. (1982), ‘Speculations in the Origin and Evolution of Sunda–Sahul Rainforest’, in G. T. Prance (ed.), Biological Diversification in the Tropics: Proceedings of the Fifth International Symposium of the Association for Tropical Biology (New York: Columbia University Press), 554–75. —— and Flenley, J. (1979), ‘Late Quaternary Vegetational History of the Enga Province of Upland Papua New Guinea’, Philosophical Transactions of the Royal Society of London, B286: 265–344. —— and Sun, X. (1988), ‘Vegetational and Climatic Changes at the Pleistocene –Holocene Transition across the Eastern Tropics’, in P. Whyte and J. S. Aigner (eds.), The Palaeoenvironment of East Asia from the Mid-Tertiary (Hong Kong: Centre of Asian Studies, University of Hong Kong), 579–91. Woodroffe, C. D. (2000), ‘Deltaic and Estuarine Environments and their Late Quaternary Dynamics on the Sunda and Sahul Shelves’, Journal of Asian Earth Sciences, 18: 393– 413.

3

Landforms of Southeast Asia Avijit Gupta

Introduction Southeast Asia is a corner of the continent of Asia which ends in an assemblage of peninsulas, archipelagos, and partially enclosed seas. Towards the northwest, the physical contact of this region with the rest of Asia is via a mountainous region that includes the eastern Tibetan Plateau, the eastern Himalaya Mountains, the hills and plateaux of Assam (India) and of Yunnan (China). From this high region a number of large, elongated river basins run north–south or northwest– southeast. These are the basins of rivers such as the Irrawaddy, Salween, Chao Phraya, Mekong, and Sông Hóng (Red). An east–west traverse across the mainland part of Southeast Asia, therefore, is a repetition of alluvium-filled valleys of large rivers separated by mountain chains or plateaux. To the south and to the east are coastal plains, rocky peninsulas, and a number of deltas. Beyond lies the outer margin of Southeast Asia, the arcuate islands of Indonesia, and the Philippines with steep volcanic slopes, intermontane basins, and flat coastal plains of varying size. This assemblage of landforms has resulted from a combination of plate tectonics, Pleistocene history, Holocene geomorphic processes, and anthropogenic modifications of the landscape. Most of the world has been shaped by such a combination, but unlike the rest of the world, in Southeast Asia all four are important. The conventional wisdom of a primarily climate-driven tropical geomorphology is untenable here. The first two factors, plate tectonics and the Pleistocene history, have been discussed in Chapters 1 and 2 respectively. In the Holocene, Southeast Asia has been affected by the following phenomena: • The sea rose to its present level several thousand years ago.

• The present natural vegetation, a major part of which includes a set of rainforest formations, achieved its distribution. • A hot and humid climate became the norm, except in the high altitudes and the extreme northern parts. • The dual monsoon systems blowing from the northeast in the northern hemispheric winter and from the southwest in the summer (and in general producing a large volume of precipitation) became strongly developed. • Countries away from the equator (Myanmar, Viet Nam, and the Philippines) became prone to tropical storms, which could reach hurricane force. • Anthropogenic alteration of vegetation, slopes, and river systems intensified over the last 200 years. An appreciation of all these factors is necessary in order to comprehend the current landforms and geomorphic processes in Southeast Asia. The physiography not only has given rise to a very distinctive set of landforms, but also has influenced migrations and settlements, economic practices, and social and political patterns of the region. This chapter provides a description of the landforms of Southeast Asia to form the background for analytical discussions in the subsequent chapters. This is achieved by classifying the region into largescale physiographic provinces.

Previous Work In spite of Southeast Asia being an assemblage of fascinating landforms, geomorphological studies of this region are limited in number. This has resulted in an uneven coverage that varies in quality and distribution. Certain parts of Southeast Asia have been well studied:

Landforms of Southeast Asia 39

Sumatra (Verstappen 1973, 2000); Singapore (Gupta and Pitts 1992); the east coast of the Malay Peninsula (Nossin 1964a; Swan 1968; Zakaria 1970; Wong 1981), some of the karst areas ( Jennings 1976); the Mekong Valley (various published and unpublished documents of the Mekong River Commission); the Bangkok region (Rau and Nutalaya 1982; Nutalaya et al. 1996). In contrast, we know very little about many areas beyond a straightforward description. Geomorphology of the urban areas is better known, and a data bank on erosion and sedimentation rates from different kinds of physical environment has been building up over the years. But we do not know much about the major river systems, the mountainous areas of the north, or the islands of eastern Indonesia. Such knowledge that we have in many instances has arrived indirectly, as a by-product of geological or archaeological investigations. The geology of Southeast Asia is much better known (Hutchison 1989). Such knowledge is the accumulated result of nearly 150 years of investigation by various organizations: national geological surveys; university geology departments; petroleum companies searching for oil and gas both onshore and offshore; multinational bodies such as the Committee for the Co-ordination of Joint Prospecting for Mineral Resources in Asian Offshore Areas (CCOP), United Nations Economic and Social Commission for Asia and the Pacific (ESCAP), and the International Geological Correlation Programme (IGCP). The regional structure and lithology is well understood. Topographical maps and satellite scenes including radar images are available. The current research objectives should involve building an account of the physiography of the region, and putting more emphasis on process-based investigation. Process geomorphology has arrived late in Southeast Asia. The Pleistocene in Southeast Asia is known for the environmental changes that occurred during this epoch. The changes included those of sea level and climate, which were most significant in the vicinity of the two large shelf sea regions, Sunda and Sahul. There, large areas emerged from below the sea to extend the landmass of Southeast Asia. This led to elongation of drainage systems, modification of rainfall patterns, and changes in the distribution of flora and fauna. The Quaternary studies include those by individual researchers, national geological surveys, and international organizations (Suntharalingam 1980; Wezel and Rau 1987; Thiramongkol 1989; Javelosa 1994). The details of a regressive sea have been studied over years (Molengraff 1921; Haile 1971; Aleva et al. 1973; Biswas 1973; Batchelor 1979; Verstappen 1975; Emmel and Curray 1982; Gupta et al. 1987). The studies were partly land-based, but a number were derived from

oceanographic cruises or from offshore searches for economic deposits such as tin or petroleum. Several accounts of active plate margin features such as volcanic landforms and related processes are available (Verstappen 1973, 2000; Simkin and Fiske 1983; Concepcion 1993; Dam 1994; Lavigne 1998; Thouret 1998). In contrast, very few case studies (Douglas 1968; Peh 1981; Bishop 1987) deal with the rivers of Southeast Asia. A set of landform and stream network studies was carried out in the 1960s and early 1970s (Nossin 1964b; Douglas 1967; Eyles 1968, 1969; Swan 1970, 1972; Morgan 1972). Dykes (2002) has described the role of mass movements and long-term uplift towards landform development in Brunei. Given the number of archipelagos and islands, coastal geomorphology is better covered (Carter 1959; Nossin 1961, 1962, 1964a,b, 1965a; Swan 1968; Tjia 1970; Wong 1978, 1981; Teh 1980; Bird and Rosengren 1984; Bird and Schwartz 1985; Ongkosongo 1988; Sharifa 1988). Several deltas have been studied (Nossin 1965b; Zakaria 1970; Koopmans 1972; Bassoulet et al. 1986; Hoekstra 1989a,b; Staub and Esterle 1993). The limestone landscape of Southeast Asia, especially the tower karst, has attracted geomorphological investigations (Lehmann 1936; Verstappen 1960; Wilford and Wall 1965; Jennings 1976; Brook and Ford 1978; Crabtree and Friederich 1982; Pham 1985). Two aspects of anthropological modification of landforms and processes have been extensively studied: disturbances to rainforests (Douglas et al. 1992a,b) and the urban environment (Douglas 1978; Leigh 1982; Rau and Nutalaya 1982; Balamurgan 1991; Gupta and Pitts 1992; Nutalaya et al. 1996). Certain aspects of the landforms and geomorphic processes of Southeast Asia seem to attract researchers more than the other topics.

Structural Control The foundation of the landforms of Southeast Asia has been laid by a sequence of plate movements over time. Hutchison (1989) described Southeast Asia as the most outstanding geological laboratory in the world for studying active plate tectonics. The plate margins that extend from Myanmar to Indonesia and from the Philippines to the Ryukyu group of islands beyond the region are undergoing subduction with active seismicity, vulcanism, well-developed Benioff zones, ocean trenches, transform faults, and accretionary wedges (see Chapter 1). Such activities have not only created typical landforms but have also subjected them to a set of processes such as periodic slope failures and the transfer of large volumes of sediment to local river and coastal systems.

40 Avijit Gupta

General tectonic stability and the major granitic areas of Southeast Asia occur in the southeastern corner of the Eurasian Plate, an area described as Sundaland. The margin of Sundaland is defined clearly in the west, running along the Sagaing Fault (Myanmar) and the Mergui Ridge southwards to Sumatra and then turning eastwards along Java up to the south Makassar Strait. Beyond this, the eastern margin of Sundaland is fragmented, tectonically altered, and complex. South and southeast of Sundaland lies the active subduction zone of Southeast Asia, and beyond that another stable continental plate, the Australian Craton. A number of small continental fragments, formerly part of either of the two stable continental plates, lie in the subductionrelated island arc zone in the middle, fragmented and conveyed to their present location by oceanic spreading such as that of the South China Sea. The continental fragments have provided areas of tectonic stability in the middle of the large island arc system, allowing shallowwater shelf sedimentation, where limestone has formed. Elsewhere, the fragments are identified as anomalous basement rock outcrops in a tectonic and volcanic subsidence zone. A number of the eastern Indonesian islands have such a geological history. The spreading of the South China Sea has also been associated with fracture systems on the mainland, such as the Red River Fault. The northward movement of India and its Eocene collision with the Eurasian Plate, which started in the Eocene and subsequently resulted in the formation of the Himalaya Mountains and the high plateau of Tibet, also modified Southeast Asia (see Chapter 1). For example, the Arakan Yoma and the Chin Hills of western Myanmar were uplifted along with the Shan Plateau, pre-existing major faults (e.g. the Sagaing Fault of Myanmar) were modified, and the current north–southtrending alternate mountains range and valley pattern described earlier came into existence (Tapponier et al. 1982). There were also associated vulcanism and granitic emplacements. Such a geological history not only determines the location of mountains, stable granitic landforms, sharply demarcated river valleys, tectonic areas, and volcanoes, but also explains the occurrence of clastic sedimentary rocks in Southeast Asia as flysch or molasse deposits associated with past structural basins and the subsequent characteristic topography. The location of carbonate deposits and the associated development of karst topography is similarly demarcated. It also explains the large valleys filled with alluvium of major rivers, such as the Irrawaddy or the Mekong, and the structural control of river courses such as that of the Sông Hóng or the section of the Irrawaddy along the Sagaing Fault.

The final set of landforms that emerged in Southeast Asia therefore is strongly controlled by geological history as reflected in structure and lithology. The geomorphic processes that operate on such a geological background are introduced in the following section.

Geomorphic Processes Studies of geomorphic processes in Southeast Asia are restricted to a number of small-area case studies or special environments such as a tropical delta (Hoekstra 1989a,b) or coastal landforms (Wong 1981). In general, weathering, fluvial action, and slope processes operate over most of the region. The long and varied coastlines of this assemblage of peninsulas and archipelagos are affected by a range of coastal processes. Wind action is limited to the coasts, and three very small ice fields have survived since the Pleistocene on top of the high mountain peaks of the Central Range of Irian Jaya (Williams and Ferrigno 1989). Glaciated landforms, a relict of the Pleistocene, occur on top of Mount (Gunung) Kinabalu in Sabah. Other evidences of extinct Pleistocene processes exist only as relict sediments such as the aeolian deposits of the Khorat Upland (Thailand) or the braided river deposits such as the Old Alluvium of Singapore or Malaysia, but not as distinctive landforms. The natural rates of operating geomorphic processes have been strikingly altered by anthropogenic modifications of the physical environment. Such modifications have been particularly intensive for the last few decades. In many parts of Southeast Asia, the current rates of erosion and deposition and the nature of geomorphic processes are not representative of the conditions in the natural state. For example, measured average sediment yield from undisturbed forest ranges between 0 and about 300 t km−2 yr −1. This figure increases at least tenfold when the vegetation is cleared (Gupta 1996). Such an alteration obviously affects the river systems and the coastal processes (see Chapter 14).

Physiographic Provinces The physiography of Southeast Asia can be divided into several provinces (Table 3.1 and Figure 3.1). The boundaries between the provinces are determined by both elevation and breaks in slope. Types of geomorphic processes may change in relative intensity between the provinces, and regional geology is reflected significantly in the landforms of a particular province. The provinces vary considerably in size, and some of them, especially the coastal plains, may not be continuous. Their boundaries do not coincide with political

Fig. 3.1. Physiographic provinces of Southeast Asia

42 Avijit Gupta Table 3.1 Physiographic provinces of Southeast Asia Northern Mountainous Region Western Myanmar Hills Central Myanmar Lowland Tenasserim Coast Central Range of Hills Shan Highland Hills of Northern Thailand and Lao PDR Tenasserim Hills Central Highland of Malay Peninsula Central Plain of Thailand Khorat Upland Coastal Plain of Southeast Thailand Elephant and Cardamom Hills Coastal Plains of Kra Isthmus and Malay Peninsula Kra Isthmus Western Coastal Plain of Malay Peninsula Eastern Coastal Plain of Malay Peninsula Annamite Chain Mekong Lowland North Viet Nam Plain Island of Borneo Arcuate islands of Indonesia The Philippine Islands

ones, and a physiographic province may stretch across a national frontier. The terms used in this chapter to describe such provinces may therefore differ from those conventionally used in studies of landforms within a state. For example, the Central Range of Hills in this chapter represents a near-continuous assemblage of uplands, hills, and mountains from northern Southeast Asia to the tip of the Malay Peninsula in Singapore. Different parts of it are known in Myanmar and Thailand as the Tenasserim (Tanosi) Hills or Shan Plateau and as the Main Range in Malaysia. The objective of this chapter is to provide a summary description of the physiography of each of these units as an introduction to the landforms and geomorphic processes in Southeast Asia, and to provide a base for the subsequent material in the book.

Northern Mountainous Region This region, mapped arbitrarily as the area higher than 1000 m, stretches across the north from Myanmar through Lao PDR to Viet Nam (Figure 3.2). It is a continuation of the mountainous area of the Assam Himalaya in India and that of Yunnan in China, the highest region in terms of both average elevation and mountain peaks. The highest point in mainland Southeast Asia, Hkakbo Razi (5881 m) is located within this physiographic province, on the Myanmar–China border. This is a region of alternate mountain ranges and steep valleys, the relief being progressively less southward and eastward. Very few mountain peaks rise above 3000 m

beyond northwest Myanmar. Mountain peaks in Lao PDR barely reach 2000 m, although higher elevation is found in northern Viet Nam, where the highest point, Fan Si Peak, reaches 3143 m. Relief is impressive throughout the province with deep upper gorges of large rivers such as the Irrawaddy, Salween, Mekong, and Sông Hóng being located within a stretch of several hundred kilometres. In Yunnan, immediately to the north of Southeast Asia, these four gorges are even closer, all occurring within a distance of about 250 km. A number of steep-sided, sharp-crested, near-parallel northeast– southwest ridges separated by deep gorges characterize this province in northern Lao PDR. The V-shaped valleys in places transform into narrower gorges. Destruction of vegetation, high rainfall events, and tectonic movements together have given rise to extensive mass movements on the steep slopes. The relief between the valley bottoms and the ridge tops ranges between 600 and 1200 m. Valley bottoms often show considerable sediment storage, sediment that travels episodically during the rainy season. The Mekong and its tributary the Nam Ou flow through wider valleys, but flanked by steep ridges (Figure 3.3). Locally steep hills, apparently remnants of former continuous mountain ranges, rise from the valley floors of the two rivers. Both rivers show straight reaches and sharp direction changes indicating structural control. Some of the ridges are relatively less dissected and show rounded tops. The only area which does not show advanced dissection is the 1100–1400 m high limestone plateau of Trannih (part of which is better known as the Plain of Jars), located north of Xieng Khuang. A heavily dissected limestone terrain, however, occurs across the Lao PDR–Viet Nam border. Alternating steep ridges and structure-guided major river valleys continue to characterize this physiographic region towards the east in Viet Nam. A southwest to northeast traverse crosses Sip Song Chau Thai Ridge, the Black River, Fan Si Pan Ridge, the Sông Hóng and Nui Con Voi Ridge in succession (Figure 3.2).

Western Myanmar Hills The Western Myanmar Hills (Figure 3.4), a continuation of the Naga and Patkoi Hills of India, occur as a series of north–south-running fold mountains with parallel intermontane valleys. In general, the region can be mapped as bounded by the 500 m contour. This physiographic province can be divided into the Chin Hills towards the north and the Arakan (Rakine) Yoma towards the south. The summits of the ridges of the Chin Hills usually vary between 2000 and 2600 m in elevation, with the highest peaks rising over 3000 m.

Landforms of Southeast Asia 43

Fig. 3.2. Northern Mountainous Region

44 Avijit Gupta

Fig. 3.3. The Mekong River in north Lao PDR

In the north, where the hills are at the India– Myanmar boundary, the eastward drainage flows to the Chindwin. In the south, where the ranges are entirely within Myanmar, it flows west to the Kaladan and east to the Irrawaddy. Within the Western Myanmar Hills, streams tend to follow the north–south-running ridges and valleys with short sections cutting across the ridges. This has given rise to an impressive trellis pattern, mostly in the Kaladan Basin, which raises the question of possible stream captures and modifications from a former drainage system. The Arakan Yoma is lower and narrower. The summits of the ridges are usually below 1500 m, and the ridges are dissected by short, steep streams that tend to flow west or northwest across them. The longitudinal valleys provide the opportunity for the tributaries to expand at right angles to the master stream. Towards the south, the Arakan Yoma forms the western boundary of the plains of the Irrawaddy and peters out at an elevation of about 400 m. In the south, small patches of coastal plain occur between the northwest–southeast-running coast and the north–south-running near-parallel hills. Locally, the hills of Arakan approach the coast to form steep cliffs. The neighbouring longitudinal valley may have a swampy floor with mangroves. Where larger rivers flow out to the coast along such a longitudinal valley, those carrying more sediment have filled in valleys and formed deltas at the end with mangroves and tidal creeks. The Kaladan is a good example. The Holocene sea-level rise has led to partial submergence of the end

of the ridges along this coast, transforming the higher parts into elongated islands and the drowning of longitudinal valleys which were then filled with sediment.

Central Myanmar Lowland The overall dimensions of this lowland (Figure 3.4) are about 1100 km north–south and 160 km east–west. Foothills that rise to 1000 m surround it on three sides. Beyond the foothills, it is flanked to the west by the Naga Hills of India and the Chin Hills and Arakan Yoma of Myanmar, and to the east by the Kachin Hills and the Shan Plateau. Locally, the contact with the Kachin Hills and the Shan Plateau is sharp, probably faultguided, and the transitional foothill zone is either limited in width or absent altogether. Two major rivers of Myanmar, the Irrawaddy and the Chindwin, flow through this semi-elliptical lowland in the centre of the country. The two headstreams of the Irrawaddy, the Mali Hka and the Nmai Hka, have eroded narrow steep valleys in the mountains of northern Myanmar. To the west of these valleys, the upper Chindwin flows through the Hukawang Valley, which is a transition zone from the Western Myanmar Hills to the Central Myanmar Lowland. South of the Hukawang Valley, rising to over 1500 m, the Mangin and Gangow Ranges extend south-southwest, separating the valleys of the Chindwin and the Irrawaddy before they join near Mandalay. As a result, the northern part of the Lowland is never more than 80 km wide continuously. A similar pattern is found further south, where the low hills of Pegu Yoma separate the plains of the

Landforms of Southeast Asia 45

Fig. 3.4. Western Myanmar Hills and Central Myanmar Lowlands

46 Avijit Gupta

Irrawaddy and the Sittang. These hills also include several extinct volcanoes, which exist in a much-dissected state. The 1518 m high Mount Popa, located west of Meiktila, is probably the best known of these. Towards the south, the lowland merges into the large delta of the Irrawaddy, the eastern margin of which extends up to the plains of the Sittang. Geologically, the Central Lowland of Myanmar is a structural low filled mostly with sedimentary material of the Tertiary period that is overlain by Quaternary alluvium of the Irrawaddy, Chindwin, and the Sittang. The rivers, particularly the Irrawaddy and the Chindwin, have their channels and floodplains incised below a set of terraces, which in turn are inset below the flat surface of the lowland at a higher level. The central part of the province is the dry zone of Myanmar, where the rainfall is not only seasonal but also much reduced. Both the Chindwin and the Irrawaddy, which originate in the northern mountains and carry a large sediment load, have wide, braided reaches for much of their course. The Irrawaddy, however, passes through three bedrock gorges in the sector that starts 75 km south of Myitkyina and ends 80 km north of Mandalay. This is where the river cuts through several ridges, follows the Sagaing Fault for part of its course, and at places is only 50 m wide. The gorge sections are usually straight, with a few bars and steep rock-cut cliffs. In the alluvial sections, the channel could be up to several kilometres wide, and the extensive floodplain is marked by abandoned river channels, oxbow lakes, and a profusion of channel bars and islands. South of Mandalay, the floodplain of the Irrawaddy is between 5 and 15 km wide and bounded by sets of probable Pleistocene terraces, the top one of which occurs about 100 m above the present river. The delta is recognized to begin nearly 300 km from the sea. The tide-dominated delta is characterized by innumerable creeks, mangrove swamps, and a sea-face of about 240 km. The Sittang, coming to the sea almost at the eastern margin of the province, has not built a large delta.

Tenasserim Coast This narrow coastal province (Figure 3.5) continues for about 750 km from the mouth of the Sittang up to where, south of Mergui near the Myanmar–Thailand border, a low range of hills reaches the Bay of Bengal. Towards the north, it is sharply separated from the Tennaserim Hills to the east by the Three Pagodas Fault. The northern section is the widest (up to 90 km), and remains a flat coastal plain throughout. Almost at the northern end of this physiographic province, the Bilin River, after draining a longitudinal valley, flows to the

Gulf of Martaban, depositing huge quantities of sediment and building bare mudbanks interlaced with tidal channels. The longest river in Myanmar, the 2800 km long Salween (which flows through the Shan Highland and the Tenasserim Hills), does the same about 90 km further south, immediately after it has been joined by the Gyaing from the east at the landward margin of the coastal plain. In spite of the large amount of sediment that the Salween brings to the coast, it has not built a delta. It reaches the sea via two channels separated by the large island of Bilu. The enormous sediment load of the Salween is filling up both channels. From Moulmein (on the Salween) southwards, isolated steep-sided ridges rise from the coastal plain alluvium and also form a number of islands offshore. The varying lithology of the region caused by granitic intrusion into sedimentary rocks together with structural depressions is responsible for such topography. The coastal plain here changes from a flat appearance to a pattern of alternate near-continuous ridges and longitudinal valleys that come at an angle to the sea. The highest point on these ridges reaches 1174 m at Paungchon Taung, north of Tavoy. The longitudinal valleys are drained by streams, which have built mudflats and tidal deltas at their mouths. The largest of these is the Tavoy, which, being confined to a structural valley, has built a narrow but nearly 50 km long delta at its mouth, distinguished by small elongated islands along the channel. South of the Tavoy mouth the coastal plain first narrows to less than 10 km, and then opens up at the large tidal delta of the Great Tenasserim, another large river flowing out of a longitudinal valley in the Tenasserim Hills to the east. Off the edge of this delta and running south along the coast, the islands of Mergui Archipelago illustrate the drowning of a former ridge and valley landscape. The southernmost 450 km of the coastal plain alternates between narrow alluvial strips backed by steep ridges (which are spurs of the hilly region to the east) and wide flat wetlands.

Central Range of Hills The Central Range of Hills (Figures 3.5–3.7) is a combination of hill ranges, high plateaux, and steep river valleys that extend from the mountains of Yunnan through Myanmar, Thailand, Malaysia, and Singapore to a series of islands south of Singapore in the Indonesian waters. It displays considerable variations in geology, elevation, and local relief, and locally includes patches of lowland and structural depressions. This unit forms the divide between the drainage westward to the Bay of Bengal and the Malacca Strait (the Rivers Irrawaddy, Salween, and Perak) and that flowing eastward to the Gulf

Landforms of Southeast Asia 47

Fig. 3.5. Tenasserim coast, Tenasserim Hills, Central Plain of Thailand, Khorat Upland, Coastal Plain of southeastern Thailand, and Elephant and Cardamom Hills

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Fig. 3.6. Shan Highland and hills of northern Thailand and Lao PDR

Landforms of Southeast Asia 49

Fig. 3.7. Central Highland of Malay Peninsula and the coastal plains of Kra Isthmus and the Malay Peninsula

50 Avijit Gupta

of Thailand and the South China Sea (Chao Phraya, Kelantan, Pahang, and Mekong). In the Kra Isthmus the hills are so low and discontinuous that it is better to recognize the isthmus as a coastal plain with scattered hill ranges. The Central Range of Hills returns south of this break to form the backbone of the Malay Peninsula. The Central Range of Hills can be divided into four subprovinces. Starting from its contact with the Northern Mountainous Region and moving roughly south, these are: 1. 2. 3. 4.

the Shan Highland, the hills of northern Thailand and Lao PDR, the Tenasserim Hills, the Central Highlands of the Malay Peninsula.

Shan Highland The Shan Highland (also known as Shan Hills or Plateau) runs north–south from the Northern Mountainous Region. Rising steeply from the Central Lowland of Myanmar, which lies to the west, it stretches for hundreds of kilometres across eastern Myanmar and northwestern Thailand. The general surface elevation is nearly 1000 m, but the surface is heavily dissected by a large number of steep river gorges. In contrast, a series of near-parallel ranges rise above the plateau surface with peaks reaching about 2500 m. A series of parallel faults has given rise to scarps, cliffs, and hot springs. A large part of the plateau is in limestone, where the river gorges are particularly deep and karst features are common. The dissection of the northern Shan Highland has been carried out by the headwaters of three major drainage systems: the Irrawaddy, the Salween, and the Chao Phraya. The valleys are deep and narrow. The 2800 km long Salween flows in a succession of nearly 1000 m deep gorges for almost its entire course through the highlands (Chhibber 1934). Only for the last 250 km its valley widens.

Hills of Northern Thailand and Lao PDR Northern Thailand is a mosaic of generally steep hill ranges, intermontane basins, and steep gorges. Floors of the intermontane basins occur at a wide range of elevations, from 500 m to as low as nearly 200 m, and with a layer of alluvium of varying thickness masking hard rock. Rock outcrops and rocky thresholds at basin outlets also indicate the structural control of these basins. Most of this sub-province (Figure 3.6) is drained by four main headwater streams of the Chao Phraya: Ping, Wang, Yom, and Nan. Only a small northern section

slopes to the Mekong. The rivers meander in and out of the flat-floored, elongated intermontane basins or flows through rock gorges connecting the basins. The ranges separating the larger rivers of the region are in places steep, high, and continuous. Towards the east, they are lower. The elevation is low also in the drainage basins of the Wang and the Yom, where the divides are in Permo-Carboniferus limestone and impressively steep in spite of relatively lesser relief. Towards the Lao border the divide to the Mekong drainage is much higher, with peaks rising to 1500–2000 m and streams flowing in narrow, steep-sided valleys at 400–500 m elevation. The Mekong alternates between narrow valleys with incipient floodplains and precipitous river gorges.

Tenasserim Hills Near the 16th parallel, the Central Range of Hills break up into narrow, steep-sided ranges and intermontane valleys collectively known as the Tenasserim Hills (Tenasserim Yoma), which extend southward to the Kra Isthmus (Figure 3.5). The westernmost of the ranges is separated steeply from the Tenasserim Coast by the Three Pagodas Fault. East of this range lies the relatively wide valley of the lower Salween and the Gyaing. Further to the east, in Thailand, hill ranges and limestone uplands of limited areal extent alternate with narrow (barely 2 km wide) valleys. The ridges become progressively lower eastward, and finally isolated small hills rise in line through the alluvium of the Central Plain of Thailand. On the Thailand side of the Tenasserim Hills, the highland is dissected by the Kwai Yih and the Kwai Noi. Both rivers flow east to meet at Kanchanaburi to form the Mae Klong, which flows across the Central Lowland of Thailand to the Gulf. The headwater hills of these basins lie at an elevation of about 600 m. Limestone uplands locally occur at 900 m elevation. At the Myanmar border further to the west, with peaks over 1000 m, is the highest range of the area, the Blauktaung. The southernmost extension of the Blauktaung Range reaches the northern end of the Kra Isthmus.

Central Highland of Malay Peninsula This physiographic sub-province, flanked by coastal plains on either side, forms the backbone of the Malay Peninsula (Figure 3.7). The highland consists of a number of hill ranges on granite and plateaux, escarpments, and hills on sedimentary rocks. The highest and the most persistent range of hills is located in the western part of this sub-province, granitic in origin, and in Peninsular Malaya known as the Main Range. The sedimentary rocks include varieties of conglomerate, sandstone, shale,

Landforms of Southeast Asia 51

and limestone, gently folded or uniclinal in structure. The central part of the highland consists of a number of cuestas and plateaux, usually steeply bounded by antidip escarpments or faults, reflecting the local lithology and structure. The presence of limestone has also given rise to areas of conical hills and pinnacles, although not on a large scale. A number of granitoid batholiths of various sizes also occur in this area, further complicating the topography. A number of small first- to third-order tributaries of the major streams (the Perak, Pahang, Kelantan, etc.) run down steep slopes, rapidly eroding the regolith and increasing the local relief. A lower and less continuous granitic range (the Eastern Range) lies near the eastern boundary of the subprovince. Towards the south in Johor, where the Malay Peninsula narrows, granitic rocks form several low but steep hills and continue in the same fashion further south forming the islands of Singapore, Bangka, Belitung, etc. Here the hills are very low, in hundreds of metres, but the local relief separates the hills from the surrounding coastal plains. The highest peak in the Central Highland is Gunung Tahan (2187 m). Several other peaks rise to nearly 2000 m, but in general, the Highland is not very high. The ruggedness comes from the steep slope of the granitic plutons, sedimentary escarpments, karst topography, and steeply incised stream valleys, all until recently covered in dense tropical rainforest. The physiography is also controlled by past tectonics. For example, the Pahang River, draining east to the South China Sea, flows through a graben-type structure (Hutchison 1989). A part of its basin is marked by a number of lakes and other evidences of deranged drainage. A series of northwest– southeast-trending faults (e.g. the Bok Bak Fault and the Bukit Tinggi Fault) cut across the peninsula (see Chapter 1). Hutchison (1989) has described the Main Range granite to be progressively downfaulted southwestward to sea level at the Malacca Strait. The granite is also offset laterally by faulting to run southward through the Indonesian island of Bangka.

Central Plain of Thailand A large north–south lowland, drained by the Chao Phraya River and its tributaries, extends through most of central Thailand (Figure 3.5). This is an elongated structural low, like the Central Myanmar Lowland, filled by riverine alluvium towards the north and marine deposits towards the south. The lowland is bounded by the hills of northern Thailand to the north, the Shan Highland and the Tenasserim Hills to the west, the Gulf of Thailand to the south, and the Khorat Upland to the east. The Central Plain is more than 500 km long in the

north–south direction and widens to almost 300 km near its southern limit. The Central Plain of Thailand has been tectonically active for most of the Tertiary and Quaternary periods, during which it has been filled by several thousand metres of alluvial and deltaic sediments originating from rising boundary highlands. The alignment of rivers and hill ranges and configuration of the bounding highlands seem to have been determined by faulting (Rau and Nutalaya 1982). The Quaternary seas have repeatedly regressed and transgressed across the southern part of the plains. Further south is the Gulf of Thailand, structurally a graben (Hutchison 1989). Low, rocky spurs, isolated hills, and piedmont fans mark the transition from the surrounding highlands. This pattern is found all along the western, northern, and eastern edges of the plains. These hills and rocky outcrops in limestone, granite, and quartzite rise steeply from underneath a riverine alluvial cover. Here, the major headwater streams (the Ping, Yom, and Nan) which combine downstream to form the Chao Phraya, and its long tributary from the east, the Pa Sak, flow in narrow valleys filled with alluvium and flanked in many places by alluvial terraces. North of Nakhon Sawan, however, where the Ping, the Yom, and the Nan join, meandering streams with levees and backswamps and abandoned river courses traverse the plain. A number of isolated hills rise for several hundred metres above its surface, mainly south and east of Nakhon Sawan. Further south, the Central Plain is lower, flatter, and wider, and the lower subsurface alluvium marine in origin. Here the Chao Phraya deposited most of the riverine alluvium to form its delta, but the Mae Klong and the Bang Pakong have also contributed parts of it at the southwestern and southeastern corners respectively. The very low and flat nature of the plain leads to widespread flooding each year during the southwest monsoon.

Khorat Upland This 150 000 km2 steep sided upland with a gently sloping, saucer-shaped upper surface (Figure 3.5) is separated steeply from the Central Plain of Thailand to its west by the Phetchabun Mountains and the Phang Hoei Range, and sharply from the Mekong lowlands of Cambodia to the south by the Sankamphaeng and the Dong Rek Ranges. It is underlain by gently folded and uniclinally dipping Mesozoic and Tertiary rocks, and owes its uplifted southern margin to tectonic movements. The rocks are overlain by gravel, ferricrete, sands of various origin including wind action, and alluvium (part of which has been identified as flood deposits). This sequence has been taken to indicate a series of

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climate changes in the Quaternary, which ranged from warm humid to near arid conditions (Nutalaya et al. 1989; Udomchoke 1989). Summits along the upland rim may rise to over 1000 m, but the surface elevation is much lower, about 200 m near its northwestern corner and as low as 50 m at the southeastern end. The gradient at the top is gentle, although several low ridges as well as a number of lakes, whose areas fluctuate between the wet and the dry seasons, occur on top. The northwest–southeast-trending Phu Phan Hills separate the upland into the Khorat and Sakon Nakhon Basins (Thiramongkol 1983). The sedimentary rocks on top have been eroded to form cuestas and hogbacks, and these in turn have been dissected to form both water and wind gaps (Apisit and Prasit 1982; Löffler, Thompson, and Liengsakul 1984; Parry 1996). The Mun River drains almost the entire upland eastward to the Mekong. A small portion towards the north and east drains directly via a number of short streams to the Mekong. A few streams drain part of the western margin of Khorat to the Chao Phraya system. The rivers of Khorat flow on a gentle gradient on top of the upland, but through steeply cut wide shallow valleys. Towards the north, the upland stretches across the Mekong River into Lao PDR to form the low, flat surface near Vientiane.

Coastal Plain of Southeastern Thailand This small province is bordered on the north and northwest by the Central Plain of Thailand, on the west and south by the Gulf of Thailand, on the southeast and east by hill ranges extending from the Cardamom Hills of Cambodia and the Banthat Range between Thailand and Cambodia (Figure 3.5). From the low range of hills and uplands that bound this coastal plain, small streams flow out to the sea. Short rivers flow in valleys between low ranges of hills, constricted by terraces and fans. Towards the central part of the province, alluvial fans cover the contact between the granite hills to the north and the plains behind the coast. Several of the fans and hills reach the coast to form headlands in alluvium and rock respectively, and rocky hills occur offshore as islands. The peaks of these islands (Si Chang, Samet, Chang, and Kut) rise to several hundred metres. The coastline is much indented. Rocky headlands are common, and mangrove swamps occur at the head of small bays where rivers reach the coast. Elsewhere, sandy beaches stretch along the coastline. The coast has a straight north–south orientation towards the west, which changes to an east–west one beyond Sattahip. A structural control seems to be responsible for this change in orientation. Towards the southeast, the coastal

plains pinch out as the Cardamom Hills of Cambodia stretch to the sea.

Elephant and Cardamom Hills This small but high relief physiographic province (Figure 3.5) separates the Coastal Plain of southeastern Thailand from the southern plains of the Mekong River. The northern part of this hilly region is known as the Cardamom Hills, and the southern part, the Elephant Hills. The hills also encircle a small patch of coastal plain in Cambodia. Within half a kilometre of the sea, the hills begin to rise very steeply and an elevation of 1000 m is reached in less than 10 km. The highest peak is slightly over 1700 m. Hills are flat-topped and steep-sided, with small structure-guided streams flowing westward in narrow gorge-like valleys. In their lower courses these streams follow a meandering course through the coastal alluvium. Small alluvial fans mask the contact between steep hillsides and coastal plains. In contrast, the hills slope much more gently to the northeast, towards the Tonlé Sap area. The local streams are also longer and flow over a gentler gradient.

Coastal Plain of Kra Isthmus and Malay Peninsula South of the Tenasserim Hills is the narrow Kra Isthmus, a coastal plain with a couple of low hill ranges in its middle. Further south, in the wider Malay Peninsula, the Central Range of Hills again form a core with coastal plains on either side.

Coastal Plain of Kra Isthmus Kra Isthmus (Figure 3.7) lies at the extreme southern end of Thailand. It is essentially a narrow coastal plain in the middle of which two low ranges of hills (Phuket and Nakhon Si Thammarat Ranges) are situated. The hills are about 1000 m high with the highest point rising to 1835 m at Khao Luang, west of Nakhon Si Thammarat. The western coast of Kra Isthmus, opening out to the Andaman Sea, is indented with a number of estuaries. In contrast, the eastern coast (next to the Gulf of Thailand) is a wide, unbroken coast with well-developed straight beaches, inland lagoons, and coastal terraces. Parallel beach ridges and depressions are common. Both coastal plains continue south into Malaysia. Further inland, small fans occur at the contact of the two hill ranges with the coastal plain. An older (Pleistocene) and a younger set of fans occur, differentiated by the presence of ferricrete. The hill ranges run at an angle to the trend of the isthmus and have formed a number of islands offshore. Phuket and the Langkawi

Landforms of Southeast Asia 53

Fig. 3.8. Steep granitic hills, Penang, Malaysia

Group are the best-known examples. The major islands are granitic, but a number of others are in limestone with well-developed karst features.

Western Coastal Plain of the Malay Peninsula The two coastal plains, one on each side of the Central Highland of the Malay Peninsula (Figure 3.7), differ in their morphology and extent. The western coastal plain is continuous, wide, and characterized by fine-grained sediment and mangrove communities. Several relatively long rivers originating in the central highland flow out to the Strait of Malacca through this coastal plain. From north to south these are the Muda, the Perak, and the Muar. The coastal plain is wider near such rivers but narrows considerably where uplands such as the granitic hills of Kulim, Bubu, and Melaka approach the coastline at an angle. The large island of Penang is also on granite (Figure 3.8). Further south, smaller granitic hills of Pengkalan and Batu Pahat rise steeply from the alluvial cover of the coastal plain. This coastal plain therefore includes both wide, marshy, flat areas and small, steep, hilly tracts. Towards the southern end of the peninsula, the hills are lower and the eastern and western coastal plains converge, but even in south Johor and on the island of Singapore, a distinctly rolling hilly tract lies between the two coastal plains. The natural morphology of the coastal plain has been distinctly altered in many places by the draining of the swamps, urbanization, and coastal reclamation.

Eastern Coastal Plain of the Malay Peninsula The eastern coastal plain, in contrast, is narrow, sandy, backed by beach ridges or low dunes in many places, and mangroves are present only in a few favourable locations. This is an exposed coast, seasonally modified by the wind and the waves of the northeast monsoon. Towards the north is the delta of the Kelantan River. South of the delta, it is a straight coast backed by lines of old beach ridges (permatang) separated by narrow lagoons which interrupt and modify the lower course of smaller streams reaching the South China Sea. Further to the south, the coast is narrow, backed by highlands, and of the headland and bay type with spits and bay mouth bars indicating the presence of longshore drifts. The finegrained flat triangular delta of the Pahang, the largest river on the peninsula, interrupts this morphology, but beyond it the headland and bay-type pattern continues. Southwards in Johor, although rivers such as the Endau and the Johor flow out to the sea, the coastal plain is extremely narrow and over a considerable stretch is backed by rocky cliffs. It is still a headland and bay coast, except that the bays are distinctly J-shaped. Inland a number of streams tend to flow through freshwater or mangrove swamps.

Annamite Chain The Annamite Chain (Figure 3.9) runs from the Northern Mountainous Region to the southern parts

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Fig. 3.9. Annamite Chain, Mekong Lowland, and North Viet Nam Plain

Landforms of Southeast Asia 55

of Viet Nam through Lao PDR in a continuation of mountains and plateaux that form the eastern divide of the Mekong Basin. The lower boundary of the Annamite Chain is arbitrarily drawn on Figure 3.9 following the 500 m contour. Almost throughout its entire length, peaks rise to more than 2000 m, and the range is crossed only by a limited number of passes. The eastern slopes are steeper. A series of plateaux occur to the west of the crestline. The most extensive of these is the Khammuan Plateau, from the edge of which a steep drop takes one almost to the Mekong River. This is a limestone plateau with welldeveloped karst features and deep river gorges. Boloven and Kontum are two other important plateaux that occur towards the south, where rugged hills appear again. This range of mountains is being eroded in the west by large tributaries of the Mekong and in the east by short rivers that run down to the South China Sea. The contact between the mountain and the plains, especially towards the Mekong River, is softened by alluvial fans, isolated rocky hills, and rock outcrops. The Annamite Chain stretches to the South China Sea at the extreme southeastern end. There, lines of hills forming wide headlands enclose several patches of coastal plains. A continuous and wide coastal plain is found only north of Da Nang.

Southward of the passage between the Dong Rek Range to the west and the Boloven Plateau to the east, the Mekong emerges on to a wide plain, nearly 500 km across with isolated small rocky hills emerging out of the alluvial cover. The river at this point is flowing below an elevation of 100 m, and some of these hills may rise for another several hundreds of metres. Usually they are lower. Towards the west, an extension of this wide plain lies between the Dong Rek Range and the Cardamom Hills. The drainage in this wide western embayment of the Mekong Lowland collects in the lake of Tonlé Sap (Figure 3.9), whose area extends and shrinks remarkably between the wet and dry seasons. Tonlé Sap is connected to the Mekong via the Tonlé Sap River, which reverses its flow at different seasons according to the stage of the Mekong (see Chapter 4). East of the Mekong channel, the Srepak, the San, and the Kong Rivers extend the lowland in three east–west-trending embayments that push into the Annamite Chain. The first major distributary of the Mekong, the Bassac, separates from the main channel near Phnom Penh. The active delta, however, is in Viet Nam. The elevation of the plain is below 10 m, about 150 km from the coast. It has been suggested that silting by the Lower Mekong filled a shallow sea which once extended up to Tonlé Sap. The delta of the Mekong is described in Chapter 13.

Mekong Lowland

North Viet Nam Plain

The lowlands of the Mekong (Figure 3.9), like those of the Irrawaddy or the Chao Phraya, extend north–south from the Northern Mountainous Region to the South China Sea. They are bounded in the west first by the gently rising Khorat Upland and then to the south by the steeper slopes of the Cardamom and Elephant Hills. The eastern margin is marked by the Annamite Chain. This contact is locally softened by small alluvial fans and basins of the tributaries of the Mekong, which have extended the lowlands by eroding back the hills and plateaux of the Annamite Chain. Towards its north, the lowland is narrow, in many places below 200 m in elevation, and the Mekong flows very close to the Annamite Chain. Up to the border of Cambodia with Lao PDR, the Mekong Lowland alternates between wide, flat stretches where larger tributaries join the Mekong and deep, narrow valleys between two ranges of hills. In these narrow reaches, the river often flows over rapids and rock exposures. Immediately to the north of the Cambodian border, the Mekong flows in a wide anastomosing reach between rocky islands, where it widens to about 15 km during the rainy season. Downstream lies the large Khone Falls.

The Annamite Chain, coming at an angle to the coast, divides the lowlands of Viet Nam fronting the South China Sea in two large units (Figure 3.9). In the north is a coastal plain with a large extension inland along the structure-guided valley of the Sông Hóng, and to the south lies the huge delta of the Mekong. The middle part of the Viet Nam coast alternates between rocky headlands and patches of coastal plain, and has been included in the Annamite Chain physiographic province. The North Viet Nam Plain reaches its widest extent (nearly 200 km) at the delta of the Sông Hóng and the wide lowland immediately upstream of the delta. Offshore and northeast of the delta, a series of narrow, low islands extend parallel to the coast trending northeast. About 250 km upstream from the mouth of the Sông Hóng, the valley narrows to less than 5 km in a steep-sided structure-guided course through the mountains. The valley of a major tributary from the north, the Sông Ch¯ay, displays the same morphology. In general, this lowland is below 200 m in elevation, although the 100 m contour runs close to the coast. The plains are also interrupted by isolated highlands of various sizes, some of which may rise to over 1500 m,

56 Avijit Gupta

thereby bringing in significant local relief and steep slopes in the midst of a coastal plain. South of the plains of the Sông Hóng, a number of short streams run down from the Annamite Chain to reach the coast. Several larger ones carry out a surprising right-angled turn after reaching the plains and flow for 100 km or so parallel to the coast before turning sharply to join the sea. The coast is a straight one with subdued headlands, and spits and bars extending northwards. The best examples of these coastal features occur near Hué, where tens of kilometre-long lagoons are separated from the sea by well-developed spits. Further south, near Da Nang, the Annamite Chain comes close to the coast to interrupt the continuation of the coastal plains towards the south, which do not appear again as an extensive body north of the Mekong Delta.

Island of Borneo Borneo demonstrates an equidimensional shape and a NNE–SSW-trending highland in the centre surrounded by a coastal plain (Figure 1.1). The plain can be as wide as 250 km (Figure 3.10). Except for eastern Sumatra, such wide, flat coastal plains are uncommon for the Southeast Asian islands. The highest point is the granitic pluton of Mount Kinabalu (4101 m) in the Malaysian state of Sabah, rising steeply from a low, rolling plain. About 5 km2 of area on top of this mountain were glaciated during the Pleistocene. Mount Kinabalu is almost at the north end of the series of ranges and plateaux that constitute the central highlands of Borneo. Running south-southwest from this large mountain are the ranges Crocker, Witty, Tama Abu, Apo Duat, Iran, and Schwaner. These ranges may peak at elevations near 2000 m and beyond, but the ranges are not distinguished by their continuity, being separated from each other by low saddles that hardly reach 1000 m in elevation. This is well illustrated towards the southern end of the Iran Range, where the headwaters of the three major rivers of Borneo— the Rajang, the Mahakam, and the Barito—approach each other. Other ranges run out towards the coast at an angle from the central highland. Towards the northeast, the Brassey Range extending from the highland overlooks the Bay of Lahad Datu. From the centre of the island, the ranges Kapuas Hulu, Hose, and Dulit extend towards the northwest. Beyond the southwestern limit of the Schwaner Range, the highland dies out in a series of small isolated hills that rise above 1000 m only locally. A low, narrow, but continuous range of hills occurs in the middle of coastal plains in the extreme southeast, the Meratus Mountains, an area of uplifted and folded rocks formerly occupying a graben structure (Hutchison 1989).

Other isolated hills lie scattered in the middle of the coastal plains, and in some cases as an elevated spine along a small peninsula. Apart from Mount Kinabalu, several other areas of the central highland of Borneo are granitic, such as the Schwaner Mountains. The highland in the centre has not been properly investigated near the border of Sarawak (Malaysia) with Kalimantan (Indonesia). Discontinuous Cenozoic lavas and pyroclastic cones have built this part of the highland (Hutchison 1989). Elsewhere, a variety of sedimentary rocks, flysch, limestone, etc., along with neotectonic movements, bring variations to the landforms. The geomorphology of the island of Borneo is fascinating but not well studied. The coastal plain of Borneo is a plain of deposition, extended in places by the deltas of major rivers such as Mahakam, Kayan, and Kapuas. The present coastline is formed by the Holocene sea-level rise, when many former valleys were submerged. Evidence for this is best displayed towards the northeast, especially around Lahad Datu Bay and the coast immediately to its south. The wide range of fluctuation of stream levels between seasons and the flatness of the terrain have resulted in large swamps (comparable to those of Sumatra) in lower courses of some of these rivers such as the Barito. The large rivers of Borneo also appear to have three different types of channel pattern. In the central highland their upper courses are steep, narrow, and with sharp bends, all illustrating geological control. On the low, flat plains beyond the mountains they flow in small tight meanders. For the last 30 or 40 km the rivers run straight through a very flat and swampy mangrovefringed coastal plain. The Barito peat swamp is a good example. The Mahakam River occupies a rift structure, probably not the only major river system to do so.

Arcuate Islands of Indonesia The last two physiographic provinces that include most of the islands of Indonesia and that of the Philippines are the expressions of the seismic and volcanic active outer arc systems that wrap round the stable cratonic core of Southeast Asia. The island arcs and the intervening small seas of Indonesia are currently being compressed by the Australian Plate, which moves northwards at the annual rate of 8 cm (Hutchison 1989). The effect of the location of a subduction belt is clearly seen in the westernmost large island of Sumatra (Figure 3.11). The floor of the Indian Ocean off the southwest and west coast of Sumatra drops to more than 6000 m in the Java Trench (subduction trench). A line of islands occur parallel to the coast (fore-arc ridge) separated from Sumatra by a narrow sea (fore-arc basin). The backbone

Landforms of Southeast Asia 57

Fig. 3.10. The island of Borneo

of Sumatra is the long range of Barisan Mountains (magmatic arc), which rises within tens of kilometres of the Indian Ocean coast. East and northeast of this slightly curving mountain range is the low eastern plain of Sumatra, filling a back-arc basin by sediments

derived from the Barisan Mountains. The back-arc basin reaches the edge of the Sundaland Craton. Granitic rocks do occur on Sumatra, particularly towards the south. A very large part of the broad eastern plain of Sumatra is almost at sea level, with extensive mangrove swamps

58 Avijit Gupta

Fig. 3.11. The islands of Indonesia and East Timor and details of Sumatra

fronting the Strait of Malacca. The northeastern coast of Sumatra is wide, flat, and composed of fine sediment with wide river estuaries running through it. In contrast, the tectonic southwestern coast is narrow, steep, and

locally discontinuous. This is a common pattern for the larger islands of western Indonesia. A linear complex fault-bound depression runs approximately through the middle of the mountains

Landforms of Southeast Asia 59

Fig. 3.12. Sumatra: ignimbrite topography near Bukittinggi

described by Verstappen (1973) as the Median Graben. A series of lakes and basins occur in this depression. From north to south the bigger of such lakes are Tawar, Toba, Maninjau, Singkarak, and Ranau. Local drainage often ends in these lakes forming very flat deltas. The mountainous region is marked by numerous stratovolcanoes, faulted block mountains, and fluvio-volcanic fans of varying dimensions. The mountains are essentially a coalescence of andesitic material, although limestone topography is seen in central Sumatra, and granitic intrusions are present in the south. A number of the volcanoes are more than 2000 m in elevation, and several very large ones rise to over 3000 m. Past violently explosive eruptions have deposited huge volumes of tuffs and ignimbrite over parts of the Barisan Mountains. Ignimbrite has given rise to characteristic landforms with steep slopes and river canyons (Figure 3.12). One such vertically dissected ignimbrite plateau occurs near Bukittinggi in central Sumatra, but the best-known example is that of Lake Toba, which is the remnant of several supervolcanic explosions, ash from which is found deposited even as far as western India. The drainage shows evidence of very recent changes and river captures due to volcanic eruptions, tilting, and faulting (Verstappen 1973, 2000). Krakatau, which erupted in 1883 leading to huge destruction of life and property and large-scale coastal modification from a series of gigantic tsunamis, is located in the Sunda Strait between Sumatra and Java. The pattern of a central volcanic range with coastal plains of different widths on either side persists east-

ward through the islands. The central range in Java is not continuous but is separated in several sectors by low, wide saddles. The same pattern of stratovolcanoes rising to more than 2000 m with a few peaks over 3000 m, fault-bound basins, and large fluvio-volcanic fans continues (Figure 3.13). The city of Bandung is located in one of the faulted basins flanked by volcanic ranges. The complex evolution of this large intermontane basin involved tectonic subsidence, large volcanic eruptions, volcano-tectonic faulting and collapse of large volcanoes, drainage system disruptions and adaptations, fan formation, and lacustrine sedimentation in the intermontane basin (Dam 1994; Dam et al. 1996). Such activities have probably happened repeatedly across the Indonesian islands. The closeness of some of the active large volcanoes and densely populated centres is a serious hazard in Java, which is discussed in detail in Chapters 9, 15, and 16. Part of the central range in Java, however, is in limestone with typical karst features. The southern coastal plain of Java is extremely narrow, and locally the volcanic ranges reach the coast to form headlands. The southern slope offshore is steep, and this is a high-energy coast, a characteristic which is common for Indonesian islands. In contrast, the northern coastal plain is wide, and rivers flowing out of the central range have extended the island over the shallow Sunda Shelf. The smaller island of Bali to the east shows a similar pattern of landforms, a volcanic central highland surrounded by narrow coastal plains and limestone platforms.

60 Avijit Gupta

Fig. 3.13. Volcanic landscape, Java

Further to the east, the islands are smaller, and in certain cases, such as in Lombok, one single large volcano may emerge from the plains. Lombok is the first of a group of small islands collectively described as Nusa Tenggara (Southeast Islands). These are mountainous islands with a narrow coastal plain from which volcanic slopes rise steeply. Volcanic craters may rise to about 3000 m within tens of kilometres of the coastline. Of these islands, Flores has the most volcanoes, although one of the world’s biggest volcanic eruptions of historic times happened in 1815 on Sumbawa, when the volcano Tambora erupted. There are thousands of such islands in Indonesia, volcanic or otherwise. The continuation of the east–west-running subduction zone from south of Java to eastern Indonesia meets the convergence caused by the movement of the Australian Plate. This has given rise to the complicated geology and topography of Timor and the changed orientation of the islands. The tectonics in the Molucca Sea area is very complicated owing to the convergence of the Eurasian, Australian, Pacific, and Philippine Sea Plates at this location (Hutchison 1989). Such complication probably explains the appearance of Sulawesi and Halmahera islands with narrow mountainous arms stretching in several directions. The geomorphology of these islands must be fascinating, but proper investigations have probably not yet been carried out. Sulawesi has mountain peaks over 3000 m in elevation within 25 km of the coast, lakes occupying floors of mountain-girt basins, alluvial fans, and steep river gorges. Excellent coral reef

development occurs offshore. Coral reefs are common off many of these islands, especially on the Sunda Shelf.

Philippine Islands The several thousand islands of the Philippines (Figure 3.14) are associated with tectonic and volcanic activities at plate margins ( Javelosa 1994) as in Indonesia. The activities, however, are complicated because of the location of the Philippines between two subduction arcs (see Chapter 1), and the complex collision of multiple plates towards the southwest. The Manila Trench occurs offshore, west of the northern islands of the Philippines; and the longer Philippine Trench is located to the east of the islands. Hutchison (1989) has described the situation as a rare event, an orogenic zone being bounded on both sides by two currently active but opposing arc–trench systems. The landforms of the islands reflect this complex plate tectonics pattern. These islands are marked by conspicuous volcanic mountains, linear ridges, and prominent valleys. Sedimentary rocks deposited in fore-arc basins, fault-guided ridge boundaries, recent uplift, and ongoing large-scale volcanic activities (e.g. the Pinatubo eruptions of 1991) have all left their mark on the physiography of the islands. A 1200 km long fault system, the left-lateral strike-slip Philippine Fault Zone, runs from Luzon to Mindanao and beyond. Ridges peaking over 2000 m in elevation occur in north Luzon, the largest island located to the north, and Mindanao, the second largest island, which is situated in the south. On only a few of the other islands

Fig. 3.14. The Philippines

62 Avijit Gupta

do individual peaks reach this level. A number of volcanoes are currently active. Besides Pinatubo, the well-known and dangerous ones are Mayon, Taal (located in a lake in southern Luzon), and Hibok-Hibok (Camiguin Island to the north of Mindanao). Structurerelated lineaments cut across volcanic mountains, some of which are associated with active mass movements (Javelosa 1994). The repeated triggering of pyroclastic flows, debris flows, and hyperconcentrated flows along the major river systems down the flanks of Mount Pinatubo after the eruptions indicated the role of lahars in landform evolution and sediment transfer in this area of monsoonal rainfall and convectional rainstorms (Thouret 1999). Coastal plains are rather narrow, being wider than 15 km only in a few places. Extensive lowlands are limited to the central plain of Luzon, the Cagayan Valley (northeast Luzon), the Bicol Plain (southeast Luzon), the Agusan and Cotobato Lowlands of Mindanao, western Negros, and eastern Panay. Alluvial fans occur between the hills and the plains. A number of the inland valleys are bounded by block-faulted ridges and tilted rocks such as the Cagayan Basin in north Luzon with the Central Cordillera to the west and the Sierra Madre Range to the east (Hutchison 1989). The surface of the depression has been modified by the activities of the Cagayan River, which flows along its central axis. The Central Valley of Luzon is a filled-in fore-arc basin bounded by the uplifted Zambales ophiolitic highland to the west and the volcanic Central Cordillera to the east. Fluvio-volcanic terraces, formed by streams downcutting into unconsolidated volcanic material, are common in these structure-guided river valleys. Landforms are controlled primarily by tectonics, and tectonic history has influenced past sedimentation in the basins. Landforms are controlled also by rock types, the best example being the conical karst in limestone such as the Chocolate Hills in Bohol Island (Javelosa 1989). The coastal plains show evidence of recent uplift in raised coral reefs and inland barrier beach complexes.

Conclusions This chapter introduces the distribution of major association of landforms (the physiographic provinces) in Southeast Asia. It provides the basic framework necessary for following the subsequent chapters that discuss various aspects of the physical geography of Southeast Asia. The major river basins, where most of the population and infrastructure of Southeast Asia occur, are discussed. The chapter identifies the areas that are prone to natural hazards or vulnerable to the

impact of development-related activities. Lastly, it draws attention to the importance of shallow coastal seas on the landforms, climate, vegetation, and people of Southeast Asia.

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Löffler, E., Thompson, W. P., and Liengsakul, M. (1984), ‘Quaternary Geomorphological Development of the Lower Mun River Basin, North East Thailand’, Catena, 11: 321–30. Meade, R. H. (1996), ‘River-Sediment Inputs to Major Deltas’, in J. D. Milliman and B. U. Haq (eds.), Sea-Level Rise and Coastal Subsidence: Causes, Consequences, and Strategies (Dordrecht: Kluwer Academic Press), 63– 85. Molengraff, G. A. F. (1921), ‘Modern Deep-Sea Research in the East Indian Archipelago’, Geographical Journal, 58: 95–121. Morgan, R. P. C. (1972), ‘Observations on Factors Affecting the Behaviour of a First-Order Stream’, Institute of British Geographers Transactions, 56: 171– 85. Nossin, J. J. (1961), ‘Relief and Coastal Development in Northeastern Johore (Malaysia)’, Journal of Tropical Geography, 15: 27–38. —— (1962), ‘Coastal Sedimentation in Northeastern Johore (Malaya)’, Zeitschrift für Geomorphologie, 6: 296–316. —— (1964a), ‘Beach Ridges on the East Coast of Malaya’, Journal of Tropical Geography, 18: 111–17. —— (1964b), ‘Geomorphology of the Surroundings of Kuantan, Eastern Malaya’, Geologie en Mijnbouw, 43: 157– 82. —— (1965a), ‘Analysis of Younger Beach Ridge Deposits in Eastern Malaya’, Zeitschrift für Geomorphologie, 9: 186–208. —— (1965b), ‘The Geomorphic History of the Northern Pahang Delta’, Journal of Tropical Geography, 20: 54– 64. Nutalaya, P., Sophonsakulrat, W., Sonsuk, M., and Wattanachai, N. (1989), ‘Catastrophic Flooding—an Agent for Landform Development of the Khorat Plateau; a Working Hypothesis’, in N. Thiramongkol (ed.), Workshop on Correlation of Quaternary Successions in South, East and Southeast Asia, Proceedings (Bangkok: Chulalongkorn University), 95–115. —— Yong, R. N., Chumnankit, T., and Buapeng, S. (1996), ‘Land Subsidence in Bangkok during 1978–1988’, in J. D. Milliman and B. U. Haq (eds.), Sea-Level Rise and Coastal Subsidence: Causes, Consequences, and Strategies (Dordrecht: Kluwer Academic Press), 105–30. Ongkosongo, O. S. R. (1988), ‘Indonesia’, in H. J. Walker (ed.), Artificial Structures and Shorelines (Dordrecht: Kluwer Academic Press), 393– 408. Parry, J. T. (1996), ‘The High Terrace Gravels, Northeast Thailand— a Re-evaluation and an Integrated Theory of their Origin’, Zeitschrift für Geomorphologie, 40: 145–75. Peh, C. H. (1981), ‘The Suspended and Dissolved Sediment Load of Three Small Forested Drainage Basins in Peninsular Malaysia’, Malaysian Forester, 44: 438–52. Pham, K. (1985), ‘The Development of Karst Landscape in Vietnam’, Acta Geologica Polonica, 35: 305–19. Rau, J. L., and Nutalaya, P. (1982), ‘Geomorphology and Land Subsidence in Bangkok, Thailand’, in R. G. Craig and J. L. Craft (eds.), Applied Geomorphology (London: Allen & Unwin), 181– 201. Sharifa Mastura, S. A. (1988), ‘Critical Erosion: A Case Study of Kundor Beach, Melaka’, Ilmu Alam, 17: 45– 60. Simkin, T., and Fiske, R. S. (1983), Krakatau 1883: The Volcanic Eruption and its Effects (Washington: Smithsonian Institute). Staub, J. R., and Esterle, J. S. (1993), ‘Provenance and Sediment Dispersal in the Rajang River Delta/Coastal Plain System, Sarawak, East Malaysia’, Sedimentary Geology, 85: 191–201. Suntharalingam, T. (1980), ‘A Systematic Investigation of the Quaternary Deposits in the Coastal Plains of Beruas, Perak’, Geological Survey of Malaysia, Annual Report 1981, 186–92. Swan, S. B. St (1968), ‘Coastal Classification with Reference to the East Coast of Malaya’, Zeitschrift für Geomorphologie, 7: 114–32.

64 Avijit Gupta Swan, S. B. St (1970), ‘Relationship between Regolith, Lithology and Slope in a Humid Tropical Region, Johore, Malaya’, Institute of British Geographers Transactions, 51: 189–200. —— (1972), ‘Land Surface Evaluation and Related Problems with Reference to a Humid Tropical Region: Johore, Malaya’, Zeitschrift für Geomorphologie, 16: 160–81. Tapponier, P., Peltzer, G., Le Dain, A. Y., Armijo, R., and Cobbold, P. (1982), ‘Propagating Extrusion Tectonics in Asia: New Insights from Simple Experiments with Plasticene’, Geology, 10: 611–16. Teh, T. S. (1980), ‘Morphostratigraphy of a Double Sand Barrier System in Peninsular Malaysia’, Malayan Journal of Tropical Geography, 2: 45–56. Thiramongkol, N. (1983), ‘Reviews of Geomorphology of Thailand’, in N. Thiramongkol and V. Pisutha Arnond (eds.), Geomorphology and Quaternary Geology of Thailand (Bangkok: Chulalongkorn University), 6–23. —— (ed.) (1989), Workshop on Correlation of Quaternary Successions in South, East and Southeast Asia, Proceedings (Bangkok: Chulalongkorn University). Thouret, J. C. (1999), ‘Volcanic Geomorphology—an Overview’, Earth Science Reviews, 47: 95–131. —— Abdurachman, K. E., Bourdier, J.-L., and Bronto, S. (1998), ‘Origin, Characteristics, and Behaviour of Lahars Following the 1990 Eruption of Kelud Volcano, Eastern Java (Indonesia)’, Bulletin Volcanology, 59: 460–80. Tjia, H. D. (1970), ‘Monsoon Control of the Eastern Shoreline of Malaya’, Geological Society of Malaysia Bulletin, 3: 9–15. Udomchoke, V. (1989), ‘Quaternary Stratigraphy of the Khorat Plateau Area, Northeastern Thailand’, in N. Thiramongkol (ed.),

Workshop on Correlation of Quaternary Successions in South, East and Southeast Asia, Proceedings (Bangkok: Chulalongkorn University), 69–94. Verstappen, H. Th. (1960), ‘Some Observations on Karst Development in the Malay Archipelago’, Journal of Tropical Geography, 14: 1–10. —— (1973), A Geomorphological Reconnaissance of Sumatra and Adjacent Islands (Indonesia) (Groningen: Wolters-Noordhoff Publishing). —— (1975), ‘On Paleo-Climates and Landform Development in Malesia’, in G. J. Bartstra and W. A. Casparie (eds.), Modern Quaternary Research in Southeast Asia (Rotterdam: Balkema), v. 3–35. —— (2000), Outline of the Geomorphology of Indonesia: A Case Study on Tropical Geomorphology of a Tectogene Region (Enschede: International Institute for Aerospace Survey and Earth Sciences). Wezel, F. W., and Rau, J. L. (eds.) (1987), Progress in Quaternary Geology of East and Southeast Asia, CCOP Technical Publication 18 (Bangkok). Wilford, G. E., and Wall, J. R. D. (1965), ‘Karst Topography in Sarawak’, Journal of Tropical Geography, 21: 44–70. Williams, R. S., Jr., and Ferrigno, J. G. (eds.) (1989), ‘Glaciers of Irian Jaya, Indonesia and New Zealand’, U.S. Geological Survey Professional Paper 1386-H. Wong, P. P. (1978), ‘The Herbaceous Formation and its Geomorphic Role, East Coast, Peninsular Malaya’, Malayan Nature Journal, 32: 129–41. —— (1981), ‘Beach Changes on a Monsoon Coast, Peninsular Malaysia’, Geological Society of Malaysia Bulletin, 14: 59–74. Zakaria, A. S. (1970), ‘Geomorphology of Kelantan Delta’, MA thesis, University of Malaya.

4

Rivers of Southeast Asia Avijit Gupta

Introduction Southeast Asia, in general, is a subcontinent with surplus water, as evidenced by the formerly widespread tropical rainforests. Most of the region receives at least 2000 mm of rainfall annually, and a positive water balance prevails for the majority of months. Four very large rivers (the Irrawaddy, Salween, Mekong, and Sông Hóng or Red) originate close to each other on the eastern Tibetan Plateau north of the region, and flow through large structure-guided valleys towards the southeast like outstretched fingers (Figure 4.1). Other major rivers of the region (Chao Phraya, Pahang, Brantas, Mahakam, etc.) start and end within Southeast Asia. The upland slopes are drained by a large number of tributaries, and short, wide estuaries wind through coastal plains. Table 4.1 lists selective physical dimensions of the large rivers of Southeast Asia. Except the Mekong, a part of whose discharge consists of seasonal snowmelt from the Tibetan Plateau, the rivers are rain-fed; and the majority tend to show a seasonal pattern of discharge corresponding to either the southwestern or the northeastern monsoon, depending on the location.

The wide riverine lowlands of the previous chapter are structural depressions, filled in mostly by the alluvium of the major rivers that occupy them. The Irrawaddy and its main tributary, the Chindwin, flow through the Central Myanmar Lowland. The channel of the Chao Phraya is located within the Central Plain of Thailand. Further to the east, the Mekong has filled the eponymous lowland. The Salween, in contrast, flows almost entirely in 1000 m gorges cut into plateaux and mountains. The Sông Hóng flows in a narrow valley except for the last 250 km from the coast, where it traverses the coastal plain of north Viet Nam. The present coastline of Southeast Asia, however, is a temporary pause in the geological evolution of the drainage system, and as described in Chapter 2, only appeared in the Holocene. The rivers of the ice age Pleistocene used to continue further. What now are individual major streams in many instances used to be parts of the channel network of a much larger system. The lower part of these systems is now submerged under the South China Sea, the Java Sea, and the Malacca Strait (Molengraff 1921; Emmel and Curray 1982; Gupta et al. 1987). Much of the present drainage of Southeast Asia therefore is the dismembered upper parts of larger networks.

Table 4.1 Characteristics of the major rivers in Southeast Asia Measures

Irrawaddy

Salween

Chao Phraya

Mekong

Sông Hóng

Basin area (km2) Channel length (km) Mean annual discharge at mouth (m3 × 109) Average annual suspended sediment discharge (t × 106)

413 700 2010 430 (12) 260 (5)

271 900 2820 300 (17) 100 est. (15)

117 500 1110 16.7

795 000 (21) 4880 (12) 470 (9) 160 (10)

155 000

Note: World rank, where appropriate, given within parentheses. Sources: Various, including Meade (1996); Revenga et al. (1998); Tran et al. (2002).

120 160 (9)

66 Avijit Gupta

Fig. 4.1. Location map

The slowing down of the sea-level rise late in the Holocene and the sediment deposited consequently by the rivers of Southeast Asia resulted in the building of deltas of various sizes. A large part of Southeast Asia’s mangrove community grew around the deltaic distributaries. For a long time such deltas and coastal lowlands served as undisturbed important wetland habitats. The wetlands have been drained mostly over the last 200 years and also irrigated locally to form the rice granaries of the region supporting a high population. The population is primarily rural, in spite of a number of urban settlements including important cities such as Bangkok and Ho Chi Minh City (Saigon) being located in a deltaic environment. Before the coming of the modern road and rail networks, the rivers of Southeast Asia were the main arteries of communication,

especially for transport inland from the coast. This function is still carried out by a number of rivers, as on the island of Borneo. The biggest river in Southeast Asia, the Mekong, with a series of rapids and waterfalls along its course, is the exception. The river traffic on the Mekong is mostly local and short-distance. For a long time people have lived in these river valleys, and water management has been a common practice. In certain instances, as in the case of the hydraulic civilizations of Southeast Asia, remarkable engineering feats were performed very early. The bestknown example is the Khmer Empire, which lasted from the seventh to sixteenth centuries, and which has left behind the temple of Angkor Wat and other magnificent structures. The importance of river basins, even of those that were small in size, was shown in the

Rivers of Southeast Asia 67

pre-colonial history of the Malay Peninsula, where individual kingdoms were located in separate basins, and movement inland from relatively large coastal settlements was along river valleys. The rising population and the economic development of the twentieth century, however, led to considerable modification of the physical environment that impacted on the river systems. Modification of the environment has been multifaceted, including, among other activities, destruction of natural vegetation, land clearance for agriculture, increased water pollution, and withdrawal of water to meet both rural and urban demands. Such impacts are discussed later in the book, but it should be mentioned that many rivers of Southeast Asia no longer exist in a natural state.

Geomorphic History of Southeast Asian Drainage The location, characteristics, and behaviour of the major rivers of Southeast Asia are derived from the regional tectonic history, the Quaternary sea-level changes, the Holocene delta-building activities, and the nature of the monsoon rainfall. The major rivers flow through structure-guided basins, filled at least partially with riverine sediment. Quite a few of these basins are grabens, and formed as a result of extrusion tectonics towards the southeast associated with the collision of the Indian Plate with the Eurasian one, which produced major shear faults towards the east and the southeast. This also led to the opening of the South China Sea (Molner and Tapponier 1975; Hutchison 1989). The Hanoi Rift System, which stretches from China to the Gulf of Tonkin, is probably the best example of these openings. The Hanoi Rift System is filled with Neogene fluvio-lacustrine and littoral sediment, and at present occupied by the Sông Hóng and its tributaries such as Sông Ch¯ay and Sông Ca. In China and in northwest Viet Nam, the Sông Hóng has an almost linear course for more than 400 km along the narrow valley formed by the fault system. Further downstream, where the lowland is wider, the river tends to follow the southwestern boundary of the rift. The faults are still active, having offset river courses up to 6 km (Hutchison 1989). The delta sediment is Quaternary and so probably are some of the terraces found along the Sông Hóng. Even smaller rivers are structurecontrolled. The Pahang River in the Malay Peninsula and the Mahakam in Borneo are localized in grabens. Common features of the major rivers include association with a linear basin that probably came into existence in the Tertiary, filling of the basin by hundreds of metres

of sediment, some but not all of which is fluvial, and building of a large delta. On this common pattern, regional complications have been superimposed. The sediment has been deposited by more than one river (the Irrawaddy Basin). The low relief of the basin has been interrupted by linear ridges and volcanic activities (the Irrawaddy Basin again). Part of the sediment in the subsurface could be lacustrine, probably reflecting past fault-guided disruptions in drainage as in the Mekong and the Sông Hóng Basins, or marine as in the case of the Chao Phraya. Rock barrier may outcrop in the middle of the alluvium (the Mekong Valley). Hutchison (1989) is of the opinion that the Mekong has in the past contributed sediment to the central lowland in Thailand before changing to its present course. According to him, abandoned channels and deltas of an older Mekong may occur in the region from Chiang Rai to the southern Gulf of Thailand and also between Tonlé Sap and Vung Tau Graben which occurs beyond the present Mekong Delta. This observation is not surprising to anyone trying to follow the present course of the Mekong on a map. The deposits of the older Mekong remain unsearched for, and very little work has been done on the development of drainage nets of these major rivers as they filled linear depressions or extended their courses across the emerged parts of Sundaland during the Pleistocene glacial stages. Determination of the history of drainage development in Southeast Asia remains a fascinating but incomplete exercise. Molengraff (1921) mapped the extension of the rivers across the Sunda Shelf in the Quaternary. The trunk streams were longer than the current major rivers of Southeast Asia, and the deltas or fans of such rivers now lie submerged. Current boreholes sunk in deltas (Nguyen et al. 2002) support this pattern. Molengraff recognized a long river draining northeastward across the exposed Sunda Shelf, collecting drainage from a large landmass the higher parts of which now form the islands of Borneo, Java, Sumatra, and the eastern Malay Peninsula. A comparable river drained eastward along the floor of the present Java Sea, collecting drainage from the southern slopes of Borneo and the northern slopes of Java. Emmel and Curray (1982) traced a large submerged system along the floor of the Malacca Strait. This river system changed downstream from a braided to a meandering pattern and ultimately went out to the sea through a delta. Many of the short rivers that currently run down the slopes of the hill ranges of this region directly to the sea are probably the truncated upper reaches of former longer tributaries to trunk streams that disappeared with the rise in sea level in the Early Holocene. The sediment of these rivers also

68 Avijit Gupta

reflects a different palaeohydrology in the Pleistocene (Gupta et al. 1987), although more investigation is definitely required on this topic. Three major rivers of Southeast Asia are discussed below. Even for major rivers information is not always adequately available. For example, the Salween is not discussed, as the data on this 2800 km long river are extremely limited. Geomorphological studies on smaller rivers are also few in number. Information is usually available as engineering reports which have been commissioned to meet a hazard (Australian Consortium of Consultants 1974), discussions on proposed structural controls (Takanashi 1981), or case studies carried out by individuals on rivers or a particular hazard (Bishop 1987). Excluding the major rivers of Southeast Asia as listed in Table 4.1, the rest can be broadly conceptualized as belonging to one of the three following classes. This is an attempt to generalize on the basis of a limited number of case studies and brief field visits. Streams that originate in uplands, with or without floodplains. Such streams usually carry a mixed load with a coarse bedload fraction as described from west Malaysia by Douglas (1968, 1970) or from north Thailand by Bishop (1987, 1989). The channel sediment reflects the basin geology and is predominantly sand. Granitic or sandstone boulders are often seen in the upper parts of these streams (Figure 4.2), embedded in sand or resting on rock, the finer material having been transported downstream by the preceding high flow. The rivers often (a)

Fig. 4.2. (a) Granite under erosion at a cascade in a small forest stream, Johor, Malaysia; (b) Downstream of the cascade granite corestones form boulders in the channel (b)

Rivers of Southeast Asia 69

Fig. 4.3. Bar formation in the Tembeling River, Pahang, Malaysia

have steep banks and a rapid and pool sequence in the upper reaches, replaced by point bars and flood bars in sand and pebbles downstream. The Tembeling, a major headwater of the Pahang, is a good example (Figure 4.3). Even where the gradient is gentle, as in the case of the Mun River, which drains most of the Khorat Upland, the banks are steep and the mixed load with a coarse bedload fraction characterizes the channel. Lowland rivers on very gentle gradient flowing through finegrained coastal plains and forming estuaries in their lower

courses. This is a set of rivers that flow through the fine-grained coastal plains and deltas of Southeast Asia. Most of these rivers open out in their lower courses to form wide estuaries lined with mangrove and swamp vegetation. The deltas and their channel systems are discussed in Chapter 13. Rivers of this type occur in the wide coastal plain of eastern Sumatra, filling a backarc basin fronting the Strait of Malacca, the coastal plain of Borneo, the western coastal plain of the Malay Peninsula (Figure 4.4), part of the Tenasserim coast,

Fig. 4.4. Tidal river through a flat coastal plain, Pontian Kechil, Johor, Malaysia

70 Avijit Gupta

Fig. 4.5. River channel in an active volcanic arc, Luzon, Philippines Grey area indicates deposition of volcanic material in the channel. Scale approximate. (Source: interpreted from SPOT image on the web and topographical maps)

and other locations. The estuaries are tidal, and the local relief is between the levees bordering the channels and the backswamps. Mangroves and peat cover very large areas of these swamps. Volcanic area rivers, a high proportion of which have low width–depth ratios with channels cut deep into volcanic deposits. Rivers that drain the volcanic areas of the islands of Indonesia and the Philippines are influenced by the local geology, tend to follow the general structural trends, and also are controlled by the type of volcanic sediment that reaches them episodically in large volumes. Such streams may be significantly modified by pyroclastic flows and lahars as described in Chapter 16. The smaller channels are cut into fine-grained weathered material, which also forms the bulk of their load. Such channels are usually steep-sided and in many places with a low width–depth ratio. The ignimbrite areas of Sumatra, for example, are deeply dissected by river systems which are locally described as canyons (Figure 3.12). Huge

volumes of material from pyroclastic flows and lahars episodically fill the channels that drain out of active volcanoes such as the Merapi in Java. The trunk streams of such river systems display the storage and transfer of the material as mixed load to the coast, which is a short distance away on these islands. The Progo River, flowing south to the Indian Ocean from the Merapi volcanic area, is a good example. A very large eruption may completely alter the landscape and bury the former river system, as occurred on the island of Luzon, the Philippines, following the eruption of Pinatubo in 1991. Other rivers on the Luzon are also examples of adjustment to underlying geological structure and a large volume of volcanic sediment (Figure 4.5).

Water and Sediment Good hydrological data in Southeast Asia has a patchy distribution. Reasonable records are available for several

Rivers of Southeast Asia 71

large basins such as the Mekong and the Chao Phraya, for rivers which have been impounded, or for basins with development projects. Several years of data have been recorded for a number of small streams related to either development or research projects. Overall, however, the data network has wide gaps, and the figures are usually not easy to acquire. The data that are available, however, do allow certain generalizations across the region. Except very large rivers such as the Mekong which rise on the Tibetan

Plateau, the entire drainage network of Southeast Asia is rain-fed. The rainfall of the region is related to two opposite patterns of monsoon winds (see Chapter 5), the northeastern in the northern hemispheric winter (approximately November–January) and the southwestern in the northern hemispheric summer (approximately June–September). The annual discharge pattern for rivers therefore is also seasonal (Figure 4.6), the wet or highflow period depending on which monsoon the drainage basin is exposed to. The Irrawaddy, the Chao Phraya,

Mekong at Pa Mong Dam Site, Thailand 1993 Discharge in cu. m/sec

20 000 16 000

Daily Discharge

12 000 8 000 4 000 0 Jan

Feb

Mar

Apr

May

Jun

Jul

Aug

Sep

Oct

Nov

Dec

Oct

Nov

Dec

Nam Lik at Muong Kasi, Lao PDR 1993 Discharge in cu. m/sec

160 120

Daily Discharge

80 40 0 Jan

Feb

Mar

Apr

May

Jun

Jul

Aug

Sep

Nam Ngum at Ban Na Luang, Lao PDR 1993 Discharge in cu. m/sec

2000 1600

Daily Discharge

Fig. 4.6. Annual hydrographs of the Mekong and two of its tributaries

1200

The drainage areas measure Mekong at Pa Mong (299 000 km2 ), Nam Lik at Muong Kasi (374 km2 ), and Nam Ngum at Ban Na Luang (5220 km2 ). (Source: Mekong River Commission data, from Gupta et al. 2002)

800 400 0 Jan

Feb

Mar

Apr

May

Jun

Jul

Aug

Sep

Oct

Nov

Dec

72 Avijit Gupta

Fig. 4.7. Annual hydrograph of the Mun River at Rasi Salai, Thailand, for 1996 Drainage area 44 600 km2. (Source: Mekong River Commission data)

and the Mekong are all open to the southwestern monsoon and therefore run high during the late northern hemispheric summer, and their stages drop from December to April or May. In contrast the rivers of the eastern coast of the Malay Peninsula or Viet Nam experience high flow during the northern hemispheric winter. Seasonality is less pronounced, though still identifiable, for rivers whose basins are located close to the Equator, owing to the rainfall pattern derived from the movement of the Intertropical Convergence Zone, but towards the southern margin of the region, as in Timor, the seasonality returns. The seasonality is very strong for rivers such as the Mun (Figure 4.7), which flows across the relatively dry Khorat Upland. In the lowlands, as in the case of the Mekong in Cambodia, this results in a remarkable expansion and shrinkage of the channel width. As Figure 4.6 indicates, the seasonality is more pronounced for smaller tributaries rather than major rivers. In both cases, spikes of short-duration high discharge show up on the annual hydrograph; the spikes in the case of small tributaries may rise from a very low stage. Even in the case of big rivers such as the Rajang, which drains the west coast of Borneo,

such fluctuations in flow are prominent (Figure 4.8). Pandjaitan (1981) has described strong seasonality and high-flow spikes from Timor on a much smaller stream, the Benain River at Nunbei. The rivers therefore have fluctuating stage, velocity, shear stress, and stream power. This suggests a storage and transfer pattern in the channel sediment at least for the bedload, and certainly in smaller streams. In mixed-load channels this indicates seasonal deposition of sand and pebble to form bars. The wet monsoon and the high-flow period is also the time-span when the rivers not only run high but their basins may receive rainstorms leading to largescale flooding. Stage changes of 10–20 m are not unusual. A number of these floods have been studied, as, for example, the 1972 flood on the Pahang River or the 2000 flood on the Mekong. The morphology and behaviour of the rivers therefore are controlled also by the seasonality in discharge. The seasonal storage and transfer of the sediment-forming bars and inset accumulation in the channel have become more pronounced in the last several decades owing to accelerated erosion on the slopes and arrival of extra sediment in the channels as described in Chapter 14.

Rivers of Southeast Asia 73

Fig. 4.8. Annual hydrograph of the Rajang River at Benin Nanga, Sarawak, Malaysia, for 1994 Drainage area 21 273 km2. (Source: data from Sarawak Hydrological Year Book 1993, 1994)

Description of Three Major Rivers The Irrawaddy The two headwaters of the Irrawaddy River of Myanmar, the eastern Nmai Hka and the western Mali Hka, rise in the Northern Mountainous Region and come together about 50 km north of Myitkyina to form the Irrawaddy (Figure 3.4). The Nmai Kha is the longer branch. Rising in Tibet at over 5000 m, it follows a steep, narrow gorge through several nearly straight structure-guided sectors south, dropping to an elevation of 500 m in 300 km. The shorter Mali Kha rises at an elevation of about 3000 m and flows through a wider valley. Both rivers flow over rocks and rapids, and waterfalls are common. South of Myitkyina, the Irrawaddy is already below the 200 m contour, about 1600 km from the sea. Its downstream passage as a low-gradient alluvial river is

interrupted by three steep gorges in Palaeozoic rocks. South of Myitkyina, the river meanders freely, with a number of abandoned channels and oxbow lakes next to the active channel, exhibiting a number of sandy bars in the dry season. The River Magaung joins it from the west, draining the Gangow Ranges. The Irrawaddy enters the first of its gorges several kilometres below Sinbo, where the river is nearly a kilometre wide. The 56 km long rocky, tortuous gorge with sharp bends, rapids, and pools locally narrows to 50 m. The river widens again below the gorge in a display of meanders, abandoned channels, and sandbars. Near Bhamo, the tributary Dayang Jiang, draining part of the Shan Highlands, joins the Irrawaddy from the east. Below Bhamo, the river turns west and enters the second gorge to cross an upland area before the next alluvial reach, where the Irrawaddy resumes its wide,

74 Avijit Gupta

wide, meandering course marked by active deposition. Here the Irrawaddy begins to flow through the dry zone of Myanmar with low annual rainfall and marked seasonality. The river is flanked by wide plains through which low hills emerge. Abandoned channels and channel bars again become common, and tributary streams tend to dry up for part of the year. Mid-channel sandbars, reflecting seasonality in discharge, and a large load of sandy sediment characterize the channel. Beyond Mandalay, the Irrawaddy, skirting the southern end of the low hills of Sagaing, turns first west and then southwest, and meets its largest tributary, the Chindwin, from the west over a distance of 30 km via several channels. Below the confluence with the Chindwin, the seasonal character of the Irrawaddy becomes more pronounced. The tributaries tend to hang. The river flows through high sand river cliffs topped with red gravel. Gravel also occurs on the flat surface on top of the banks. The river appears not to be able to move freely and could be structure-guided. The mid-channel bars are large and elongated, and occur at different levels, the lower ones in bare sand and the higher ones vegetation-covered. The river receives numerous short tributaries draining the Arakan Yoma to the west and the Pegu Yoma to the east. This appearance continues to the apex of the delta, which starts near Henzada. Old, high terraces occur along the lower course of the Irrawaddy. The delta is discussed in Chapter 13.

The Chao Phraya

Fig. 4.9. Diagrammatic sketch of the Irrawaddy leaving an alluvial reach to enter a rock-cut course along the Sagaing Fault Scale approximate. (Source: interpreted from SPOT image on the web and topographical maps)

meandering course and turns south, skirting the Gangaw Ranges. This reach is marked by a very wide floodplain, many abandoned channels, and numerous bars and islands. A large volume of sediment is in transport and storage. The third gorge of the Irrawaddy is a narrow, straight southward course along the Sagaing Fault (Figure 4.9). The river is confined by steep forested hillsides, but sandbars still occur in the channel. Figure 4.9 clearly illustrates the effect of increased gradient and stream power on the transporting capacity of the river. As the river emerges out of the gorge, it reverts back to a

The Chao Phraya originates as four rivers that drain the hills of northern Thailand. The four headwaters (Ping, Wang, Yom, and Nan) rise at an elevation of about 1000 m and flow north–south. The 400 km Wang is the shortest stream, the rest are 700 km long or more. The rivers flow through steep gorges and sharp, nearright-angled bends, over rapids, and along stretches of flatter gradient. Such characteristics plus the localized straight reaches indicate geological control. A number of dams and reservoirs interrupt the flow of these rivers, including the Bhumipol Dam on the Ping and the Sirikit Dam on the Nan. The two western streams, the Ping and the Wang, join below the Bhumipol Dam; the eastern two, the Yom and Nan, about 30 km north of Nakhon Sawan. The augmented Ping and Nan come together at Nakhon Sawan to form the Chao Praya, although the waters of the sediment-rich Nan take about a kilometre to diffuse with that of the clearer Ping. At Nakhon Sawan, 370 km from its mouth, the Chao Phraya is only 23.5 m above sea level. The Ping, the Nan, and their joined flow, the Chao Phraya, travel

Rivers of Southeast Asia 75

through alluvial plains through which small rocky ridges emerge. The rivers meander freely at this stage, displaying point bars, oxbows, and a string of abandoned channels indicating frequent migration. At Ayutthaya, the Chao Phraya is joined by a major tributary, the 570 km long Pa Sak from the east draining the western edge of the Khorat Upland. Here the river is 3.5 m above the mean sea level and at Bangkok only a metre or so. The Chao Phraya follows a meandering course with abandoned channels on a very gentle gradient through the central plain of Thailand. The lower river flows in a semi-graben (Hutchison 1989) filled by marine sediment from the Gulf of Thailand succeeded by fluvial and deltaic alluvium contributed by the river. The upper sedimentary succession in the structural depression reflects alternate high and low stands of the sea during the Pleistocene. On the surface, the Chao Phraya is an alluvial river running on its own silt. Unlike the central lowland of Myanmar, the flat plain is a continuous one, uninterrupted by hill ranges. Several rivers leave the Chao Phraya at certain locations on the flat plain. The first, Tha Chin, leaves the mainstream near Chai Nat and flows south nearly parallel to it. Running 35– 40 km west of the Chao Phraya, it reaches the sea at Samut Sakhon. An older channel of the Chao Phraya leaves the main channel also near Chai Nat to rejoin it downstream. The Lop Buri River, another old channel separated from the present Chao Phraya at Sing Buri, flows east to the Pa Sak. The delta is a maze of watercourses: the main channel, distributaries, and numerous canals. The Chao Phraya is a seasonal river carrying the rain of the southwestern monsoon. The seasonality is pronounced in the middle of the hot, dry summer in April, when the upper headwaters run very low. The Chao Phraya itself starts to rise in May and commonly peaks in September. By November, the rains have stopped for weeks and the river has started to drop. The floods in the delta are augmented by the arrival of high tides in the middle of the wet season, when the river is high.

The Mekong The Mekong, the twelfth-longest river in the world, is the biggest river in Southeast Asia. Only the lower 2400 km of this river, about half its total length, are inside Southeast Asia. The rest of it is in China, draining the panhandle part of its basin. The area drained by the lower half of the river is much bigger, 609 000 km2 or 77 per cent of the total area of the basin. Inside Southeast Asia, the morphology and channel material of the Mekong change several times. From

the Chinese border to a little upstream of Vientiane (Figure 3.9), the river flows on granitic rocks, folded Palaeozoic sedimentary and metamorphic rocks with local volcanic exposures, and Mesozoic sedimentary deposits. The structural trend (in north Lao PDR) is north-northeast, a direction also followed by a number of faults. The Mekong Basin in northern Lao PDR consists of a number of dissected, steep, narrow-crested, near-parallel ridges separated by deep valleys. Between 600 and 1200 m of relief separate the valley bottoms from the ridge tops. The annual hydrograph of the Mekong is seasonal (Figure 4.6), matching the rainfall over the basin. The annual rainfall is high over the northern and eastern basin (2000– 4000 mm) but much lower over the western sector and the southern lowlands (dropping to about 1000 mm locally). Although the Mekong receives summer snowmelt from the Tibetan Plateau in May, it is primarily rain-fed, and its tributaries inside Southeast Asia are entirely so. About 80 per cent of the river’s discharge arrives between June and November, 20–30 per cent in September alone. Large floods tend to occur late in the wet season (Figure 4.10) and usually tail off slowly. Such floods have occurred in 1955, August 1988, September 2000, and September 2001. Although the upper Mekong flows through a rockcut channel and the lower Mekong on thick alluvium, certain reaches in both sections depart from the norm. The channel from the Chinese border almost up to Vientiane is in rock. The rock-cut channel of the Mekong tends to fall into three basic types: 1. A smooth trapezoidal channel which becomes asymmetrical at bends. 2. A 100–200 m wide deep inner channel flanked by rock benches veneered by sediment. The channel floor is uneven and alternates between rock protrusions and elongated pools. 3. A wide scabland-type channel with rock protrusions acting as rapids separated by scour pools. Bars are formed by sediment accumulation behind rock protrusions. The inner channel may or may not be present. Bars in the rock-cut Mekong are generally controlled by the channel relief, only locally by channel geometry. The river also does not move freely, but follows a set of structure-guided near-straight reaches bounded by sharp bends. The large hairpin bend upstream of Luang Prabang is a good example. About 250 km downstream of Luang Prabang, the morphology of the river changes to a set of six incised, near-symmetrical meanders, and the river travelling

76 Avijit Gupta

Fig. 4.10. Flood signs in the Mekong, falling stage of the flood hydrograph, upstream of Luang Prabang, Lao PDR

south sharply turns east to cut surprisingly through a range of hills. This sudden change of direction has not yet been explained. Very little sediment is visible along this reach, but for the next 130 km which takes it to Vientiane the channel at low flow is full of sediment. The river runs in 10–20 km long straight reaches separated from each other by sharp bends. Further downstream, the Mekong flows through an alluvial section for about 400 km. The first 20 km or so near Vientiane, the alluvium appears to be thick, and the river flows through a shallow, low-gradient channel about 1 km wide. The channel and the thalweg both meander, but during the dry season multiple small channels appear between sandy mid-channel bars. The alluvium apparently thins downstream; the river continues to meander, and after flowing northwards turns east. The Mekong then follows a winding course along the foothills of the Northern Mountainous Region to reach the Annamite Chain. The river subsequently turns southeast to flow parallel to this mountain chain at about 20 km distance from it in 50–60 km long straight reaches separated by sharp bends. It is in alluvium, but the sharp bends and the flow direction suggest that the Mekong is not free from structural control. Apart from seasonally exposed side bars inset against steep banks, bars in the river are lozenge-shaped with pointed ends, either in midstream or skewed to a side but rarely attached to a bank. These measure 1–3 km along the main axis and 200–400 m across the widest part. Local rock protrusions appear on the bed at low flow. The tributaries, however, meander freely in alluvium.

Downstream of Savannakhet, the Mekong reverts to a steep rock-cut channel. Flowing northwest–southeast, the river is 1.5–3 km wide, and several lozenge-shaped islands occur at intervals, locally widening the channel. Downstream the islands disappear, the channel gradient steepens, the river narrows to less than a kilometre, and numerous rock exposures crowd the Mekong channel. The channel of the Mekong here is cut into the Mesozoic sedimentary rocks of the southern Lao PDR. The Mekong also passes through a hairpin bend and several sharp turns with rapids, rock exposures, whirlpools, and deep inner gorges (Figure 4.11). The river narrows to about 500 m, and very deep pools occur at the bends, the deepest reaching 90 m below the low-water stage. The surface of the water displays rapids, eddies, and fast and turbulent flow. At the end of this turbulent course in rock, the main right-bank tributary of the Mekong, the Mun, joins it from the west, draining most of the Khorat Upland and building a small fan at the confluence. For the next 150 km the steep river banks are in alluvium, although rocks are exposed in the channel and presumably form the bed. In spite of flowing through an alluvial valley flat, the river does not meander. The visible depositional forms in the channel are very large mid-channel islands, rock-cored, up to 15 km long, with scroll marks and flood channels on top. The larger of these islands are invariably located skewed in the river with a narrow channel separating them from a bank. The next 200 km of river length is remarkable, with the Mekong alternating between nearly straight

Rivers of Southeast Asia 77

Fig. 4.11. Diagrammatic sketch of a rock-cut reach of the Mekong downstream of Savannakhet, Lao PDR Rocks in channel shaded. Scale approximate. (Source: interpreted from SPOT image on the web and topographical and other maps)

and anastomosed patterns. The best example of the anastomosed pattern occurs immediately to the north of the Lao PDR–Cambodia border, the so-called 4000 Islands, where the Mekong cuts through a zone of Mesozoic basalt. Numerous islands, their number varying with river stage, occur in this 50 km long reach, where the Mekong is nearly 15 km wide. The islands are rock-cored and probably formed by accumulation of river-borne sediment around high spots on the bed of the channel. At the downstream end of this anastomosed reach in rock, the Mekong flows over a series of waterfalls and cataracts, which include the Phapheng Falls (Figure 4.12). This line of cataracts (the Khone Falls) has functioned as a barrier to navigation upstream from the Mekong mouth. South of the border, the river continues to alternate between straight single-channel reaches and anastomosed ones with islands of various size, rock exposures, rapids, and waterfalls. The Mekong here is flowing over thick alluvium in the centre of the low-

lands of Cambodia, but it still displays possible structureguided characteristics. To illustrate, the river goes through a series of nearly 50 km long straight reaches separated by a couple of right-angle bends. Where the river is flowing south in these straight reaches, midchannel islands are common, but the two westwardflowing stretches are nearly island-free. The gradient of the river, however, is extremely gentle, the overall gradient for the last 650 km to the sea being 0.00005. Beyond this section, the river is more than 3 km wide, alluvial with a sandy bed, and with a maximum depth of about 5 m. The Mekong finally has a free alluvial channel and meanders laterally to build a wide floodplain. Three large tributaries of the Mekong (the Srepak, the San, and the Kong) that drain the Annamite Chain to the east join the main river in Cambodia. The first distributary (the Bassac) leaves the Mekong at Phnom Penh, 330 km from the sea. For about 200 km it runs parallel to the Mekong, and no other distributary or interconnecting

78 Avijit Gupta

Fig. 4.12. The Phapheng Falls on the Mekong

channel appears. The head of the active delta probably starts about 125 km from the sea inside Viet Nam, where the first inter-connecting channel appears between the Bassac and the Mekong. The Tonlé Sap River joins the Mekong also at Phnom Penh, connecting the river with the large lake of Tonlé Sap. The flow in the Tonlé Sap River changes direction seasonally, flowing upstream to the lake when the Mekong runs high during the wet monsoon. This final reach of the Mekong has the standard appearance of a low-gradient large river flowing through a basin filled with alluvium, comprising the main channel, levées, short tributaries, and backswamps. Overbank flooding starts in August and September, but the river stage falls slowly, and after a large flood the backswamps may stay inundated until November. This is a very large wetland, partly converted to paddy fields. The Mekong Basin is primarily in forests or rural. In the highlands of China, Lao PDR, Myanmar, and Thailand the slopes are generally either in forest (a substantial part of which is in a degraded state) or under shifting cultivation. Wet rice is grown in the alluvial plains of the lower Mekong in Cambodia and Viet Nam, the delta being the most fertile region. The present body for planning the development of this multinational basin is the Mekong River Commission. For decades, the development of the basin has been planned in a structured fashion with the cooperation of the four member states (Cambodia, Lao PDR, Thailand, and Viet Nam) in dialogue with the two other riparian states, China and Myanmar. Dams have been com-

pleted on two of the left-bank tributaries of the Mekong in Southeast Asia, and China has designed a series of seven dams across the upper Mekong, at least two of which have been completed, but information regarding these is limited. The Mekong River needs to be monitored on a regular basis in view of the many proposed structural projects in its basin and across its channel. Development of the Mekong Basin is necessary, but many of the projects have the potential to cause considerable environmental damage. Chapter 12 discusses the Mekong Basin and these issues in greater detail.

Conclusions This introduction to the river systems of Southeast Asia is designed to highlight their major characteristics. In spite of the number of rivers, their regional characteristics, and the intensity of their utilization as a resource, not much information is easily available about these systems. Information that is available tends to be spatially uneven and concentrated on certain rivers or specific locations. It is therefore difficult to produce a balanced account, but the following points can be stressed. • Southeast Asia is a water-surplus region, which is reflected in its drainage network. • The location, characteristics, and behaviour of the rivers of Southeast Asia are derived from the regional tectonic pattern, the Quaternary sea-level changes, the nature of the monsoon rainfall, and anthropogenic changes brought to their basins and channels.

Rivers of Southeast Asia 79

• The major rivers follow structural depressions, and a number of them tend to alternate (at least for a part of their course) between rock-cut and alluvial channels. • A large number of the rivers (or the lower parts of them) flow through flat coastal plains which originally developed into mangrove or peat swamps. • Streams that drain the plate margin volcanic islands of Indonesia and the Philippines are to a large extent controlled by structural lineations and the nature of volcanic sediment. • The discharge pattern of the streams indicates a wet and dry seasonality, the timing of which depends on whether the basins are exposed to the northeastern or the southwestern monsoon. Floods are common in the wet season. • The rivers have been used for a very long time as an economic resource, a practice that continues. The original land cover in most of the basins has been greatly altered. A number of engineering controls in the form of dams, reservoirs, and canals have been put in place and others are being planned. Both types of change have significantly affected the river systems.

References Australian Consortium of Consultants (1974), ‘Pahang River Basin Study’, 6 vols., unpub. Bishop, P. (1987), ‘Geomorphic History of the Yom River Floodplain, North Central Thailand, and its Implications for Floodplain Evolution’, Zeitschrift für Geomorphologie, 31: 195–211. —— (1989), ‘Late Holocene Alluvial Stratigraphy and History in the Sisatchanalai Area, North Central Thailand’, in Narong Thiramongkol (ed.), Proceedings of the Workshop on Correlation of Quaternary Successions in South, East and Southeast Asia (Bangkok: Chulalongkorn University), 117–34.

Douglas, I. (1968), ‘Erosion in the Sungei Gombak Catchment, Selangor, Malaysia’, Journal of Tropical Geography, 26: 1–16. —— (1970), ‘Measurements of River Erosion in West Malaysia’, Malaya Nature Journal, 23: 78–83. Emmel, F. J., and Curray, J. R. (1982), ‘A Submerged Late Pleistocene Delta and Other Features Related to Sea Level Changes in the Malacca Strait’, Marine Geology, 47: 197–216. Gupta, A., Rahman, A., Wong, P. P., and Pitts, J. (1987), ‘The Old Alluvium of Singapore and the Extinct Drainage System to the South China Sea’, Earth Surface Processes and Landforms, 12: 259–75. —— Lim, H., Huang, X., and Chen, P. (2002), ‘Evaluation of Part of the Mekong River Using Satellite Imagery’, Geomorphology, 44: 221–39. Hutchison, C. S. (1989), Geological Evolution of Southeast Asia (Oxford: Clarendon Press). Meade, R. H. (1996), ‘River Sediment Inputs to Major Deltas’, in J. D. Milliman and B. U. Haq (eds.), Sea-Level Rise and Coastal Subsidence: Causes, Consequences, and Strategies (Dordrecht: Kluwer Academic Press), 63– 85. Molengraff, G. A. F. (1921), ‘Modern Deep-Sea Research in the East Indian Archipelago’, Geographical Journal, 58: 95–121. Molner, P., and Tapponier, P. (1975), ‘Cenozoic Tectonics of Asia: Effects of a Continental Collision’, Science, 189: 419–26. Nguyen, V. L., Ta, T. K. O., Tateishi, M., Kobayashi, I., Tanabe, S., and Saito, Y. (2002), ‘Holocene Evolution of the Mekong River Delta, Viet Nam’, Abstract, International Workshop on Asian Deltas: Their Evolution and Recent Changes, 14 Mar. 2002, Tsukuba, Japan. Pandjaitan, B. T. D. (1981), ‘Erosion, Potential and its Damage in Timor’, in T. Tingsanchali and H. Eggers (eds.), Southeast Asian Regional Symposium on Problems of Soil Erosion and Sedimentation Proceedings (Bangkok: Asian Institute of Technology), 129– 41. Revenga, C., Murray, S., Abramovitz, J., and Hammond, A. (1998), Watersheds of the World: Ecological Value and Vulnerability (Washington: World Resources Institute and Worldwatch Institute). Takanashi, K. (1981), ‘Basic Concepts for Debris Control Work in Mt. Kelud, East Java, Indonesia’, in T. Tingsanchali and H. Eggers (eds.), Southeast Asian Regional Symposium on Problems of Soil Erosion and Sedimentation Proceedings (Bangkok: Asian Institute of Technology), 477–89. Tran, D. T., Saito, Y., Dinh, V. H., and Tran, V. D. (2002), ‘Recent Changes in the Coastal Evolution of the Red River Delta, and Impacts from Human Activities’, Abstract, International Workshop on Asian Deltas: Their Evolution and Recent Changes, 14 Mar. 2002, Tsukuba, Japan.

5

The Climate of Southeast Asia Goh Kim Chuan

Introduction Southeast Asia lies between the continental influence of the rest of Asia to the north and the more oceanic influence of the Indian and Pacific Oceans to the south and the east respectively. While its overall net energy balance is very much determined by its latitudinal position, which is approximately between 20°N and 10°S, the locational factors referred to above largely give the regional climate its distinctive character. Within the broad latitudinal extent defined above, the Southeast Asian region has often been conveniently separated into two sub-areas: continental and insular Southeast Asia. In some ways these sub-regions represent a valid delineation into the more seasonal climatic region influenced by the monsoon system of winds and the uniformly humid equatorial climate. The former comprises Myanmar, Thailand, Lao PDR, Cambodia, and Viet Nam, while the latter includes Malaysia, Singapore, Indonesia, and the Philippines. The continental Southeast Asia experiences greater seasonality, more extremes in both temperature and rainfall, and more pronounced dry spells; whereas the insular parts, termed the ‘maritime continent’ (Ramage 1968), with a much greater expanse of sea than land (the sea area of Indonesia, for example, is four times its land area), have more equable climate. The northern and southern continental interactions in winter and summer and the differential heating due to the asymmetric character of the two sub-regions give rise to the monsoon development (Hastenrath 1991), which, to a large extent, influences the rainfall characteristics of the region as a whole (Figure 5.1). In a sense, more than temperature variations, this monsoonal influence gives the Southeast Asian climate its distinctive

character. Figure 5.2 shows the two monsoon wind systems that affect Southeast Asia. In addition to these annual reversals of the monsoon winds, the seasonal migration of the Intertropical Convergence Zone (ITCZ)— closest to the Equator during the northern hemispheric winter and farthest north during summer—is a significant factor in influencing the monthly weather regime of the region. Being a belt of low-pressure trough coinciding with the band of highest surface temperature, the ITCZ attracts the moist easterlies from both hemispheres towards its trough resulting in uplift of air, intense convection, and precipitation. This whole process provides a mechanism for the transfer of latent heat from the low to the higher latitudes (Houze et al. 1981; Hastenrath 1991).

Fig. 5.1. Mean annual rainfall distribution in Southeast Asia

The Climate of Southeast Asia 81

Fig. 5.2. The southwest and northeast monsoon wind systems in Southeast Asia

Moisture-laden winds that blow towards the region are influenced by relief. The height and alignments of mountain barriers modify rainfall distribution, and exacerbate the effects of seasonality in the continental areas of Southeast Asia. For example, the Chin Hills and the Arakan Yoma of Myanmar tend to cause greater uplift of the southwest monsoon winds and greater deposition of moisture on the windward side and, by the time these winds reach the Dry Zone of Myanmar in Central Myanmar Lowland, they are generally dry. Even within the insular parts of Southeast Asia, islands with significant relief tend to act as barriers to neighbouring lands during a particular monsoon season. For example, the Barisan Range in particular, and the Sumatran landmass in general, are effective barriers to the southwest monsoon wind, causing much less rain to fall on the Malay Peninsula than it would otherwise have received. The reverse is true during the northeast monsoon, when the Malay Peninsula acts as a barrier to the winds that would otherwise have brought much more moisture to eastern Sumatra. All this interplay of factors has brought about considerable climatic differentiation within Southeast Asia (Martyn 1992).

The Monsoon Wind Systems Winter Monsoon Cheang (1987) has discussed extensively the features of the monsoon wind systems that affect Southeast Asia.

Based on sixteen years of data on wind steadiness and twenty-four-hour rainfall and concentrating on the 850 mb level where winds are friction-free, he was able to predict satisfactorily the time of arrival of the winter monsoon in Peninsular Malaysia and its retreat from it. The onset of the winter monsoon normally is in midNovember along the east coast of Peninsular Malaysia, in early December towards the south of the peninsula, and in late December in northern Borneo. Three features are recognized as being associated with the onset of this monsoon. First, a large-scale monsoon trough moves south from its location in late summer around September in the northern South China Sea. This trough then becomes quasi-stationary near the equatorial South China Sea with steady northeasterly trades to its north. Secondly, a cold surge precedes the onset of the winter monsoon in Peninsular Malaysia by a day or two. Thirdly, a reversal of the easterly wind to a westerly one happens at the 200 mb level over southern China owing to the reversal of the north-south temperature gradient across the Asian continent. The retreat of the monsoon from Peninsular Malaysia varies from year to year, between early February and the end of March. For the rest of Southeast Asia, the beginning of the winter monsoon similarly varies from one location to another. For example, in the eastern Philippines the monsoon arrives in November in the northern part, in December in the central part, and in January in the southern, a distinctive feature of all being a marked

82 Goh Kim Chuan

increase in rainfall. In Indonesia, a distinct increase in rainfall in November and December marks the onset of the monsoon. One important feature of the winter monsoon in Southeast Asia is the arrival of cold surges from Siberia, whose effects are felt over the South China Sea. A combination of high-pressure build-up of air mass in winter and the blocking effect of the Himalayan mountain range brings about the phenomenon of cold surges, although Das (1986) doubts the significant role of the Himalaya. This is helped by strong baroclinic conditions between the cold continental air mass to the north and the warm tropical air mass to the south and the downstream acceleration of the upper-level westerly jet stream over East Asia. The resultant strong pressure gradient that develops across the East China coast creates bursts of low-level cold air from the continent towards the South China Sea. The extent of the effect of these cold surges is evident from the diffused clouds that cover the northern parts of the South China Sea, the northern and central Philippines, and even the West Pacific for much of this period. Observations obtained during Winter Monsoon Experiment in the late 1970s and early 1980s showed that over the sea just north of Borneo, the general convective activity intensified when the region was affected by synoptic scale surges in the low-level northeasterlies over the South China Sea and westward-propagating near-equatorial disturbances moved into the region. Heavy rainfall on the north coast of Borneo is associated with these cold surges, during which a general strengthening of the low-level winds over the South China Sea occurs simultaneously with a cold-front passage at Hong Kong (Houze et al. 1981). These surges are most frequent from November to February, and may occur at intervals of a week to about twenty days (Cheang 1987). In some cases wind velocity may exceed 40 knots, and cold winds may reach the southern part of the South China Sea in less than a day. The effects of the cold surges on land in Southeast Asia also depend on the extent of sea surface traversed as well as the wind velocity. By the time a cold surge reaches the equatorial South China Sea, it has lost its continental air mass characteristics. Extreme cold surges can reduce air temperature in northern Southeast Asia considerably, and places located at 17°N and beyond can experience near-freezing conditions in the highlands. Nearer the equatorial South China Sea, cold surges enhance convection associated with pre-existing disturbances over the area. Heavy rainfall and severe flooding may occur in such places as southern Thailand, Malaysia, and Singapore. The heavy

rainfall and localized flooding experienced in Singapore on 10 December 1969 was associated with such a cold surge (Chia and Chang 1971). Similar effects are felt over the West Pacific. Here, the northeasterly trades are strengthened by cold surges resulting in increased convectional activity and higher rainfall in the central and southern Philippines, as mentioned earlier. While the early cold surges tend to enhance convective activity, bringing in its wake increased rainfall to Southeast Asia, the middle and late winter cold surges bring about dry conditions. This is because of the blocking effect of the surface subtropical high-pressure centre whose east–west axis is now located between latitudes 15° and 20°N, over the northern region of the South China Sea and northern Viet Nam. Instead of travelling directly south, cold surges now make a longer detour across South Korea and south Japan and around the subtropical high in the West Pacific before finally veering back to the South China Sea as easterlies. These easterlies finally turn to become northeasterly over the Malaysia–equatorial South China Sea region. Another contributory factor to this dry condition is the passage of the upper-level westerly trough close to Malaysia. In general, dry spells in the Malaysia–South China Sea region are marked by the presence of low pressure over China, an active southern near-equatorial trough, and by two large-amplitude, upper-level westerly troughs, one over the Bay of Bengal and another over the Central Pacific. Two synoptic patterns tend to favour the tropical disturbances over Malaysia. The northern near-equatorial trough extends across Peninsular Malaysia from the South China Sea to the Bay of Bengal at the 850 and 700 mb levels, and a belt of easterlies of about 10 degrees latitude in width appears through the entire troposphere. These features are usually present in early winter. Heavy rains seldom occur in Peninsular Malaysia, Borneo, and Indonesia after mid-January, mid-February, and early March respectively. Along the eastern parts of the Philippines, heavy rains seldom occur in the northern, central, and southern parts after January, February, and March respectively.

Summer Monsoon The onset of the summer monsoon in Southeast Asia coincides with the disappearance of the northern hemisphere near-equatorial trough and the formation of the summer monsoon trough over the Viet Nam–Cambodia– Lao PDR region allowing the equatorial westerlies to penetrate north. In Viet Nam the onset is 17 May, with the date in certain years being as early as 1 May and in others as late as 3 June. The summer monsoon retreats

The Climate of Southeast Asia 83

from the northern part of Southeast Asia in September, and this process may take a month because of a low-frequency north–south oscillation of the monsoon trough throughout the summer monsoon period. An important feature of the summer monsoon in Southeast Asia is the monsoon trough. This trough is a region of low pressure which varies from a long trough extending from West Pacific across Southeast Asia to the Bay of Bengal, or at other times as two shorter troughs, one over China and India and another over the West Pacific and Southeast Asia. The trough is also a heat source characterized by wet and windy weather and closely linked to the monsoon disturbances that produce significant precipitation in summer in Southeast Asia. The summer monsoon shows oscillations in rainfall periods associated with two types of disturbance: a westward-propagating wave with a wavelength of 3000 km and phase speeds of about 6–7 degrees per day, and a much larger planetary-scale wave with wavelength of about 10 000 km. Under favourable conditions some of these waves can develop into tropical storms and typhoons. In fact, a significant number of tropical depressions, storms, and cyclones in the Bay of Bengal appear to originate over the West Pacific or the South China Sea (Cheang 1987). Three types of disturbance normally account for the most heavy rainfall events during the summer monsoon in Southeast Asia. These are tropical waves, midtropospheric cyclones over Indochina and the South China Sea, and the convergence zone in the southwesterlies over Indochina. For both monsoons, the rainfall distribution in Southeast Asia, to a large extent, is influenced by topographic configurations of mountains and islands. The greater Indonesia region offers a variety of sub-climates and annual rainfall regimes, presumably owing in large part to distortions of the monsoons by the mountain islands of the maritime continent.

Climatic Characteristics Temperature The persistence of clouds prevents much more solar radiation from being received in the Southeast Asian region and much more diffused radiation than the direct components. Also, relative to the seasonal regimes experienced at higher latitudes, intra-annual variation is small in this region. Exell (1976) indicated that in Thailand there is significant persistence in the daily global total of solar radiation. The pattern of temperature in the course of the year thus reflects the pattern of radiation received and insola-

tion curve in the tropics, the highest temperatures coinciding with the period when the sun is directly overhead (McGregor and Nieuwolt 1998). Also, in general, seasonal differences increase with latitude. Temperature ranges in Southeast Asia are very small. Within the insular part of Southeast Asia no place shows a mean annual range of temperature greater than 5°C. This relatively low mean annual range is also consistent with the radiation characteristics of the region. This is due to the small differences in the amount of solar radiation received and the important influence of large ocean surfaces acting as heat reservoirs. At stations nearest to the Equator, temperature shows very little variation throughout the year, as, for example, in Singapore. Obviously temperature decreases with elevation, so significant changes interrupt the generally uniform temperatures of the region. Thus, in places such as the Cameron Highlands in Malaysia (about 1800 m) or Baguio in the Philippines (about 2OO0 m), a reduction of some 10–14°C is experienced in the diurnal temperature as compared to stations at sea level. Frost occurs on the highlands of Pangalengan (1500 m) on west Java in July, August, and September. Frost has also been observed in the Dieng Plateau (2100 m) in east Java, Pangrango (3223 m) in west Java, Lalidjiwo (2500 m), and the Yang Plateau (2180 m) in central Java, Kalisat (1100 m) in east Java, and in the central range of Papua (Sukanto 1969). Within the continental Southeast Asia, especially in areas furthest from the Equator, greater seasonal variation in the net energy balance prevails. This is reflected in the temperature values between January and July where the mean annual range exceeds 10°C.

Evaporation Evaporation loss is a function of the energy available and the availability of moisture to evaporate. In Southeast Asia, the seas in the equatorial locations experience the lowest evaporation owing to a combination of factors. Factors such as high cloud cover, regular rainfall, greater sea surface, and generally low-velocity wind cause the air masses to have in general high relative humidity if not near-saturation. Nevertheless, the changing seasons bringing stronger winds during the southwest and northeast monsoons do influence the seasonal evaporation loss. Evaporation rates on land are highest in the equatorial region with as much as 1200 mm yr −1 (McGregor and Nieuwolt 1998) owing to the higher air temperature as a result of stronger sensible heat.

Rainfall Figure 5.3 shows the monthly rainfall distribution for a number of stations in Southeast Asia. Rainfall

Fig. 5.3. Monthly distribution of rainfall for selected stations in Southeast Asia

The Climate of Southeast Asia 85

distribution in the region is a function of several factors: prevalent monsoon, orographic influence, distribution of land and sea, and depressions developing close to the low-level near-equatorial trough. Superimposed on the seasonal rainfall distribution that provides the region its spatial variation is the influence of elevation. The seasonal effects are very much related to the position of the ITCZ, especially its rapidity of movement north and south of the Equator, and the subtropical high pressure cells (McGregor and Nieuwolt 1998). Jackson shows three types of rainfall sub-region within Southeast Asia. The first is close to the Equator, where the nearness of the ITCZ ensures a zone of continuous rainfall throughout the year. This zone experiences two maxima and two periods of less rain. The second is the zone of low rainfall between 15 and 25°C, where the subtropical high-pressure cells are located. This zone is characterized by a longer and more intense dry season that coincides with the cooler (winter) half of the year. The third is the transition zone in between these two characterized by the alternate influence of the above two (Jackson 1989). The spatial distribution of rainfall is interrupted by the influence of elevation that enhances rainfall amounts. For most of insular Southeast Asia, in a situation where the rainfall from both the monsoons and convectional storms are significant in a year, any rainfall–elevation relationship for the region is not a straightforward one. In general, annual rainfall increases from the sea level up to about 1000–1500 m but decreases beyond this point. In some places a second but a lower peak may be experienced at higher elevation. In Indonesia, the amount of precipitation varies with altitude. Maximum annual rainfall is 7069 mm in Baturaden, central Java, at 700 m, while the minimum is 574 mm in the Palu Valley of Sulawesi (sea level). However, such a relationship does not hold true of all mountain ranges in the region. The rainfall of the Philippines averages 2533.4 mm annually, but the eastern coast of Mindanao receives the highest rainfall, with an annual mean of 4305.2 mm at Hinatuan, while General Santos, in a valley in southern Mindanao, has the lowest annual mean of 933.8 mm. The western coastal areas receive more rainfall than the eastern owing to the stronger southwest monsoon winds. Here, August has the highest average monthly rainfall, April the lowest. However, if the rainfall during a single monsoon season is analysed, then the relationship with elevation is more discernible. For example, the rainfall–elevation relationship is more apparent in the Barisan Range of Sumatra when only the southwest monsoon rains are considered or in the highlands of Kelantan, Malaysia,

with the northeast monsoon rain. The significance of the relationship, however, has not been statistically analysed. The diurnal rainfall pattern, however, varies from place to place. In general two patterns exist: one is the late afternoon and another the early morning peak. The late afternoon peak results from intense build-up of heating of the land during the day, which induces convective activity in both the coastal and highland areas and sea breeze circulation. The early morning pattern is the result of night-time radiative cooling processes in the cloud masses creating subsidence of air at the margins but moisture convergence in the cloud mass at the lower levels. This ascent in the cloud mass is enhanced with strong convective cloud development and the resultant precipitation. Large and small islands in the western Pacific display this type of diurnal rainfall pattern, with the maximum occurring at between 0300 and 0600 hours local time. Diurnal rainfall also varies with seasons and with location near the coast. In Singapore, for example, rainfall tends to be concentrated between 0300 and 1200 hours local time in the southwest monsoon months of May to September. For the rest of the year afternoon peaks dominate (MeGregor and Nieuwolt 1998). The number of rain-days (2 mm per day) is very much related to the total monthly rainfall. Obviously, a greater number of rain-days is associated with the wet season, specially in areas of Southeast Asia further away from the Equator. By the same token, the length of dry spells is very much dictated by the dry-season half of the year. It is in the Philippines that heavy rainfalls are experienced. These are often associated with tropical storms and cyclones. The Philippines located in the extreme east of the region and exposed to the influence of the vast Pacific Ocean experience semi-permanent cyclones and anticyclones, air streams (air masses), ocean currents, linear systems, tropical cyclones, and thunderstorms (Flores and Balagot 1969). The influence of the ITCZ is greatly felt in the months of May to October, when conditions of widespread cloudiness, convective precipitation, and moderate to strong surface winds prevail. The maritime polar air exerts its influence from November to April coinciding with the northeast monsoon. Much rainfall is deposited in the Philippines because of the amount of moisture that this air is able to gather from the Pacific before hitting the archipelago. In fact, the cold continental polar air mass with a temperature of about 20°C and low humidity with mixing ratio of about 0.5 g kg−1 when it left the Asian landmass ends up with a surface temperature of about 25°C and a mixing ratio of 12 g kg−1 by the time it reaches the Philippines.

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Fig. 5.4. Main areas of tropical cyclone and common tracks Annual numbers in percentages. (Source: adapted from McGregor and Nieuwolt 1998)

Dry Spells Seasonal droughts are common in the continental Southeast Asia, very often associated with the latter part of the northeast monsoon season. While the insular parts of Southeast Asia are often associated with wetter clime, prolonged dry spells are not uncommon. Over the Malaysian region, occasions of dry spells may be associated with upper wind systems. The displacement of the upper subtropical ridge southwards of its normal position with the upper trough penetrating southwards to equatorial latitudes results in the dry air prevailing in the equatorial part of the upper trough, accounting for the clear weather. However, synchronous rain over Indochina and the northern part of the South China Sea region implies that, during this period, the local Hadley cell could have shifted northwards with the updraft portion over the Indochina region. On other occasions, dry spells may be associated with a very rapid deepening of the upper trough well into the Southern Hemisphere, with the upper equatorial winds, which should normally be easterlies, being replaced by extremely strong southerly flow (> 20 ms−1). Convective activity would now be centred south of the Java–New Guinea area, reflecting the southward shift of the updraft portion of the local Hadley cell with a strong upper-level return current into the Northern Hemisphere (WMO 1976).

Tropical Storms and Typhoons Heavy rainfalls in Southeast Asia are associated with highly unstable convective systems such as tropical storms and typhoons. Of all the countries in the region, the Philippines is the most affected by typhoons, which

give its climate a distinctive character. Tropical cyclones also affect the Viet Nam coast and, on rare occasions, Thailand. The Philippines is located within a belt of highest frequency of typhoons in the world (Figure 5.4) with an annual mean of 22 (Chin 1958). Neumann (1993) shows that for the period 1968– 89 typhoon occurrence (> 33 ms−1 sustained wind) in the Pacific Northwest averaged 16 per year, while tropical storms (> 17 ms−1 sustained wind) averaged 26 annually. June to December accounts for 89 per cent of the mean annual number of typhoons. The typhoon tracks vary according to seasons. Typhoons cross the Visayas between April to June, northern Luzon or the Batanes Islands in July to September, and the Visayas between October to March. The typhoons’ velocities also vary from 2 to > 10 ms−1, with an average speed of 6 ms−1. The cyclones account for much of the rainfall in the Philippines from May to December. Given the influence of typhoons it is not surprising that the maximum rainfall in the Philippines can be very high. Baguio, at 1482 m above mean sea level and on the western slope of the Cordillera Central ranges, holds the annual, monthly, and twenty-four-hour absolute maximum rainfall records for Luzon and the entire Philippines. Its absolute maximum annual rainfall was 9038.3 mm in 1911, the maximum monthly rainfall 3462 mm in August 1919, and the absolute maximum twenty-four-hour rainfall 1168.1 mm on 14–15 July 1911. Typhoons that originate in the western Pacific and that frequently hit the Philippines also travel westwards towards Viet Nam, although by the time they veer to the north and encounter land their effects may no longer be as strong as when they cross the Philippines.

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Nevertheless, in cases of strong typhoon events close to the Vietnamese coast, severe flooding can be experienced, particularly in parts of countries that form the lower Mekong region. While the areas of heavy rain may be concentrated in a narrow zone around the core of the typhoons, the actual amounts received may vary with size, intensity, and movement of the phenomenon, and so may the spatial extent of rainfall. Depending on the location and intensity of the typhoons, the effects on weather of such events can be felt as far south as Borneo and Peninsular Malaysia. Gan and Tan (1970) observed that the weather in the Malaysian region is affected by the intensification and dissipation of tropical storms in the western North Pacific–South China Sea region during the summer monsoon and winter autumn transition. The effect is reversed during the winter monsoon in so far as the east coast of Peninsular Malaysia is concerned, when the storm formation frequently gives rise to periods of improved weather. Lim (1981) has examined the relationship between weather conditions in Malaysia and tropical storm position in the western Pacific. He observed that during the summer monsoon, rainfall on the west coast of Peninsular Malaysia is enhanced by the presence of typhoons close to Viet Nam, whereas a reduction is largely noted when they are further away. For the east coast of Peninsular Malaysia there is some evidence which suggests that the presence of typhoons, particularly those occurring between 10 and 15°N, leads to drier conditions because of the suppression of local effects by the induced strong synoptic winds. In Sarawak, rainfall is inclined to be below normal in the presence of the tropical storms except when the storms occur in the vicinity of the central Philippines. Typhoons are likely to reduce rainfall when they are north of 15°N. Between 10 and 15°N this effect is only observed when typhoons are located to the east of the Philippines. Sabah, which is more susceptible to the influence of typhoons than any region in Malaysia, experiences mostly well above normal rainfall when tropical storms are found close to it, and below normal rainfall when tropical storms are far from it. During autumn transition (September to October) the above normal rainfall is largely noticeable in Sabah when a tropical storm is sufficiently close to its shores. In the early winter monsoon (November to December), the east coast states of Peninsular Malaysia normally experience spells of torrential rain associated with cold surges. However, the presence of tropical storms in certain areas in the neighbourhood of the Philippines can produce wetter than normal conditions in the

east coast. Storms in these locations often strengthen the pressure gradient directed across the South China coast, and this in turn initiates monsoon surges which would translate into heavier than usual rainfall there (Saddler and Harris 1970). One example of the effect of a typhoon on the rainfall in Malaysia can be seen in the case of Typhoon Ryan, which initially developed as a tropical depression on 15 September 1995. It became a severe tropical storm with the central surface pressure dropping 1000 mb on the 15th to 940 mb on the 21st–22nd. It became a typhoon on 20 September at 0200 local time (Ooi and Lim 1997). The sudden large drop in central pressure of Ryan between 16 and 18 September caused the northwesterly wind belt in southern Thailand and northern Peninsular Malaysia to widen and strengthen from 20 to 35 knots. The westerly winds produced by this sudden intensification brought large supplies of moisture and rainfall to the northwest part of Peninsular Malaysia. Intense rainfall covered an area stretching for about 200 km in length with its core of torrential rain centred around Butterworth, Penang. Three hundred and fifty mm of rainfall with a more than fifty-year return period was recorded on 17 September at the Butterworth Meteorological Station, the highest one-day record since 1969. A large part of the northwestern states of Peninsular Malaysia was flooded. Typhoon Bart and its associated severe tropical storm Cam in May 1996 (Ooi and Lim 1997) were another example of the large-scale influence of tropical storms outside their normal tracks influencing wind patterns and rainfall over the Malaysian region. When Typhoon Bart moved close to Luzon Island, westerly winds over the northern Peninsular Malaysia strengthened and a low was simultaneously induced close to Viet Nam. This low later developed into the severe tropical storm Cam, which sustained strong westerly winds over the peninsula and the southern South China Sea for a couple of days. Rain initially fell over central Peninsular Malaysia and western Sarawak from 8 to 10 May 1996, then spread to cover almost the whole of Malaysia from 11 to 15 May, and finally narrowed down to the northwestern part of the peninsula.

El Niño and La Niña Events Southeast Asia is affected by El Niño events. Its location in the western Pacific makes it vulnerable when the pressure gradient favours the eastward flow of air over the Pacific Ocean resulting in prolonged periods of dry spells occurring in the region. However, the impact of El Niño is not uniform throughout the region.

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Based on their analysis of annual rainfall for 135 stations in Southeast Asia, Kripalani and Kulkarni (1997) found that besides the inter-annual rainfall variation there are certain epochs (decadal nearer the Equator and on a three-decade scale away from the Equator) of above normal and below normal rainfall over the region. The impact of El Niño droughts is severe during the below normal epochs. Extreme climatic events such as floods and droughts in the Philippines have been associated with the El Niño Southern Oscillation phenomenon. That the El Niño events have some influence on cyclone formation and frequency in the Philippines is also true. The numbers of tropical cyclone occurrences in Philippines are generally below average during the El Niño years. This could be due to the displacement of the pool of warm ocean water towards the east of the International Date Line during El Niño events, thus suppressing tropical cyclone formation over its normal breeding region. Another reason is that tropical cyclones, if developed, tend to recurve towards north or northeastward sparing their passages directly over the Philippines. There is no doubt that drought events due to persistently below normal rainfall conditions in many parts of the Philippines are caused by below average number of tropical cyclone occurrences during these periods. El Niño events have caused tremendous hardship to the Southeast Asian region in recent years; the last serious episode lasted from September 1997 to April 1998. Major droughts in the Philippines have been associated with this phenomenon with significant losses in agricultural production especially rice and corn, and other crops such as coconut and sugarcane. Analysis of historical data in the Philippines indicates a possible nine- to eleven-year cycle of severe drought, the last three severe droughts prior to 1997 having occurred in 1972, 1982, and 1991 ( Jose et al. 1992). The 1991–2 northeastern monsoon experienced in Southeast Asia was weak. For the first half of the monsoon, October to December 1991, most places in Malaysia received normal rainfall. Nevertheless, two areas can be clearly identified to have experienced either very much above normal or below normal rainfall. They were northwestern Peninsular Malaysia, which was affected by drier than normal weather condition, and the central and southern states, where very wet weather was experienced. Along the east coast of the peninsula, where heavy rain normally falls during the northeast monsoon, incidents of extremely heavy rainfall were not recorded even though many places, especially in the central and southern regions, recorded above normal rainfall. During the second half of the monsoon, January

to March 1992, most places in Malaysia, particularly Sabah, eastern Sarawak, Perlis, and Kedah, recorded exceptionally below normal rainfall, while a few others experienced near-record lows. In Brunei, the seasonal rainfall from October 1991 to March 1992 was 30 per cent below average. Of all the El Niño phenomena, the last in 1997– 8 was the most intense one with serious consequences on agricultural output, water supply, and smoke haze, the last as a result of uncontrolled forest fires burning in Kalimantan, Sumatra, and parts of Sarawak and Sabah.

The Influence of Southeast Asia on Global Climate Several natural and anthropogenic activities within the Southeast Asian region have the potential to cause global climate change mainly owing to regional geology and human occupation of the land.

Volcanic Eruptions Much of insular Southeast Asia, particularly the island archipelagos of Indonesia and the Philippines, is located on very active tectonic plate margins, which render them susceptible to earthquake and volcanic activities. Large volcanic eruptions spewing voluminous amounts of dust and gases have the potential of causing global climate change. The largest volcanic eruption in the region occurred in the Late Pleistocene, some 75 000 years ago, which produced the Toba Caldera with dimensions of 100 × 30 km (Ninkovich et al. 1978). The extent of tuff deposited as a result of the Toba volcanic eruption covered some 20 000–30 O00 km2 with a thickness of several hundred metres. In Peninsular Malaysia, at a site in Kota Tampan, volcanic ash up to 3 m thick was first reported by Scrivenor (1931), later suggested by van Bemellen (1949) to have originated from that eruption. Obviously, the volume of ash, dust, and gases injected into the atmosphere would have been great, and the impact on global climate as a result of that eruption would have been significant. The Tambora eruption in 1815 caused extensive damage to both the Sumbawa Island on which the volcano is located and the group of nearby islands such as Lombok, Flores, south Sulawesi, and Bali. The amount of gas and material emitted into the atmosphere caused 1816 to be known as the year without summer in Europe with mean temperature about 3°C below average. That year England experienced the lowest June temperature on record, at 12.9°C (Sigurdsson 2000).

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Equally significant was the volcanic eruption of Krakatau in 1883 in the Sunda Strait. In more recent years the eruption of Mount Pinatubo in the Philippines has attracted much attention from the scientific community. Its last eruption was some 600 years ago, when it deposited nearly one km3 ( Jose et al. 1992) of pyroclastic material. The June 1991 eruption resulted in a large stratospheric aerosol cloud that encircled the Earth by the middle of July. Stowe, Carey, and Pellegrino (1992) have shown that volcanic aerosols of Pinatubo circled the Earth in twenty-one days. Much of this aerosol was confined to the tropics. Volcanic dusts that fail to enter the stratosphere have a much shorter residence time of a few days, or at most a few weeks, but for those that enter the stratosphere (particularly those with minute particle size) have a much longer residence time. It was estimated that the Pinatubo explosion at 0555 hours on 15 June injected some 20–22 km3 ash, while comparison of satellite-derived eruption column temperatures with atmospheric temperature profiles from nearby radiosondes yielded an altitude of 25–30 km as the cloud spread west-southwest towards mainland Asia. The eruption column over the volcano reached 30–40 km. Strong sustained activity lasted until early 16 June. Visible-band images showed ejection of very dark-coloured material throughout the day (from 0630 to 1631 hours) in contrast to the light-coloured plumes generated by other phases of the eruption. The west-southwest migration of the stratospheric cloud caused the leading edge to reach Bangkok the following day, more than 2000 km away. Ashfalls were experienced in southern Viet Nam (from Da Nang to the Mekong Delta, 1400 km west to 1800 km westsouthwest), parts of Borneo (Sabah, Sarawak, and Brunei, 1000–2000 km southwest), and Singapore (2500 km southwest). By 23 June a nearly continuous zone of enhanced SO2 as much as 30° wide (Stowe, Carey, and Pellegrino 1992) extended from south of Indochina to central Africa. In the South Atlantic, the effects of this eruption could be observed from the milky appearance of the sky in contrast to the normal dark blue sky at high altitude owing to the significant diffused solar radiation caused by the Mie scattering from the aerosols (Saunders 1993). In the northern latitudes, Blumthaler and Ambach (1994) have detected a reduction due to Mount Pinatubo volcanic aerosols of about 10 per cent in direct solar irradiance, for solar elevations between 30° and 60°, in a Swiss alpine station. Michalsky, Pearson, and LeBaron (1994) detected the effect of Mount Pinatubo eruption in various mid-latitude locations, where they found a reduction of hourly direct solar radiation of

about 15–20 per cent during the 1992 winter, using data within one, two, or three hours of local noon. Its effects were felt in the Mediterranean region, where a sharp change in solar radiation, direct and diffuse components, coinciding with the arrival of the Pinatubo aerosol cloud was noted in the coastal area of Almeria, southeastern Spain (Olmo and Alados-Arboledas 1995). The effects of the Mount Pinatubo eruptions on the stratosphere were the subject of intensive investigations for a few years after the event (Bluth et al. 1994; Grant et al. 1994). While the effects of the Pinatubo eruption were evident on radiation, the effects on SO2 and ozone were also significant. Strong enhancement of SO2 for two months was noted in Brazil (Sahai, Kirchhoff, and Alvala 1997) two weeks after the main eruption. Given that Mount Pinatubo was possibly the most sulphur-rich eruption in this century, contributing about 20 tonnes of SO2 into the stratosphere (Robcock and Mao 1992; Bluth et al. 1993; Lambert et al. 1993), the winter of 1991–2 was extremely warm over North American and Eurasian mid-latitudes and very cold in Western Asia (Robcock and Mao 1992). Aerosols were transported to high latitudes earlier and to higher altitudes in the Southern Hemisphere than in the Northern Hemisphere (Lambert et al. 1993). Volcanic eruptions also exert influence on ozone levels in the atmosphere as referred to by Sahai, Kirchhoff, and Alvala (1997). Following the Pinatubo eruption, there was a 6 per cent decrease in ozone in the tropics (Schoeberl et al. 1993). Huang and Massie (1997) showed that within a few months of the eruptions the total column ozone decreased by 5– 6 per cent in the tropics, 3– 4 per cent at mid-latitudes, and 6–9 per cent at high latitudes in the Northern Hemisphere. The region of ozone loss appeared to spread over an 8–10 km layer between 20 and 30 km altitude. This layer appeared to be roughly coincident with the region of significant aerosol loading of the stratosphere (Chandra 1993). Ozone losses began approximately a month after the eruption, consistent with the time required for the SO2 to convert to sulphuric acid aerosol. Ozone values remained below normal until December 1991 (Schoeberl et al. 1993). Lower than normal ozone below about 25 km and higher than normal ozone above it appears to have been a persistent feature of ozone profiles at 20°N after the eruption of Pinatubo (Hofmann et al. 1993). Volcanic eruptions almost invariably inject large amounts of ash into the atmosphere. Ash injected may not be significant in terms of volume a short time after an eruption. However, a non-negligible amount of very

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tiny particles may remain in the stratosphere three to six months after a major eruption. The radiative implications of such ash particles cannot be unequivocally discounted (Huang and Massie 1997). Almost a decade earlier, a series of sporadic eruptions of Mount Galunggung from April 1982 to January 1983, with greater intensities between April and October, were significantly disruptive. From 5 April to 19 September the Total Ozone Mapping Spectrometer carried on NASA’s Nimbus 7 satellite detected and measured twenty-four different SO2 clouds. An estimated 1739 kilotons of SO2 was outgassed by these explosive eruptions. An additional 300 kilotons of SO2 was estimated to have come from sixty-four smaller explosive eruptions. During the nonexplosive phase some 400 kilotons of SO2 were produced. In total about 2500 kilotons (± 30 per cent) were produced (Bluth et al. 1994). Advanced Very High Resolution Radiometer data showed the ash cloud reaching Australia by 17 July (Hanstrum and Watson 1983). Whether the Mount Pinatubo or the Galunggung eruptions caused a significant drop in temperature over the next several years after the event was difficult to determine, although a general prediction of 0.5°C drop was highly probable. Whether they could effect global climate change is even more difficult to determine. Volcanic events of great magnitude in Southeast Asia, however, do tend to affect the climate of the globe.

Land Use Change and Forest Fires The extent of land use change at the expense of forest areas in Southeast Asia has been a subject of study over the years. For example, extensive land use change on the island of Borneo and the Malay Peninsula from the early colonial period to 1990 and its environmental consequences have been discussed by Brookfield et al. (1990). However, very few studies deal with the probable effects this transformation may have on global climate change (Brookfield, Potter, and Byron 1995). The possible impact of slash-and-burn agriculture and deforestation on climate change has also been investigated (Tinker, Ingram, and Struwe 1996; Angelsen 1995). One major effect of large-scale forestland conversion to other uses is the change in the albedo property of the new land surface. Several authors (Charney et al. 1977, Meehl 1994) have noted that higher land albedo values can reduce seasonal rainfall by reducing the absorbed solar radiation at the surface, leading to cooler land temperatures, decreased land–sea temperature contrast, less rainfall, and a weak monsoon. Associated with land use change is the use of fire in burning of forests. Large-scale clearing of forests by fire is a convenient way of clearing large jungle areas for

plantations, and this is rampant where law enforcement is weak as in parts of Sumatra and Kalimantan. On a much smaller scale is the use of fire by swidden cultivators, who traditionally have been using this method for their seasonal cultivation of subsistence crops. What is significant is the fires that raged out of control, burning thousands of hectares of forests as well as the dry but thick organic peat in parts of Sumatra and Kalimantan for months on end during very dry conditions caused by the El Niño phenomena. As a result of such forest fires, large amounts of carbon are burnt and CO2 and smoke are injected into the atmosphere. Smoke haze blankets the sky over large areas of Southeast Asia over several months reducing the incoming solar radiation to the Earth’s surface. The results of the analysis of climatic and pollution data from Singapore during the 1994 smoke haze episode seem to suggest that within the context of the El Niño phenomenon, and owing to the very large extent of smoke haze coverage, its impact on regional energy and carbon dioxide budgets might have been considerable (Nichol and Goh 1995). Past events of large-scale forest fires included the 1974 fire in Kalimantan that destroyed more than 4 million ha of forest vegetation. Other incidents occurred in 1983, 1991, 1994, August–November 1997, and April 1998. Wirawan (1995) has discussed the 1983 fires in various parts of the world. In Southeast Asia some 950 000 ha of logged and unlogged dryland forests were burned in Sabah (Beaman et al. 1985), while some 2.7 million ha of swamp and dryland forests were destroyed (Schindele, Thoma, and Panzer 1989) in that year. Singapore was affected by smoke haze for a couple of months in 1994 causing a degradation in air quality (Singapore Meteorological Service 1995) and a reduction of global solar radiation of 11 per cent below the mean during the highest haze month (Nichol 1997). The 1997 forest fires in Kalimantan caused many parts in Sarawak, Sabah, and the whole of Brunei to be blanketed with smoke haze, while the fires in eastern parts of Sumatra caused much of Peninsular Malaysia and Singapore to be so affected. The 1997 fires caused the destruction of large areas of forests in Kalimantan and Sumatra. Estimates vary between 800 000 and 4.5 million ha, while the fires of April 1998 burnt 395 000 ha of forest and ground cover in these two Indonesian islands (Tay 1998). While the health effects were obvious from the significant increase in eye, respiratory, bronchial, and asthmarelated complaints, the effects on plant productivity and hence crop production were also significant, because of reduced radiation. The long-term effects of such significant but episodic events on the global climate are at best conjectural at this point.

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Climate Change There is considerable concern about global climate change in Southeast Asia. Most Southeast Asian countries participated in international conventions on climate such as the Montreal Protocol on Substances that Deplete the Ozone Layer, the United Nations Framework Convention on Climate Change, the Rio Summit in 1992, and the Kyoto Summit in 1997. All this serves to heighten this awareness and concern at the highest levels. The global warming associated with sea-level rise may cost the region much in terms of inundation of low-lying coastal areas, the most populated and most developed agricultural land; intrusion of seawater into groundwater aquifers; and the loss of natural habitats. However, given the nature of climate change prediction models, climate change research is still relatively weak in the region. In addition, there is the difficulty of obtaining models that will be able to predict such changes over small areas (Henderson-Sellers 1993). In Malaysia, Chong and Chan (1994) have tried to relate land use change to climate impact in the region, particularly in relation to Malaysia. They concluded from their analysis of annual mean maximum, annual twenty-four-hour mean, and annual mean minimum temperature records in various parts of Malaysia that there are warming trends, with the annual mean maximum temperatures carrying the largest trends in all parts of Malaysia. The General Circulation Model (GCM) developed at the Goddard Institute of Space Studies has been applied to predict climate scenarios for the Malaysian region (Chong and Chan 1994). A 3 or 4°C rise in mean temperature for both a control of (1 × CO2) and (2 × CO2) is predicted, but no change in seasonal pattern of rainfall was obtained. The coastal regions of Sarawak would experience an increase in rainfall during the winter monsoon, while the southwestern parts of Peninsular Malaysia would see increases in rainfall during the inter-monsoon months. For (1 × CO2), the GCM did not show any significant change in the rainfall and wind speed during the winter monsoon in both the Indonesian and Malaysian region (Chong and Chan 1994). The authors noted that a comparison of the pressure between the northern and southern parts of the South China Sea also showed no significant change in gradient values. There is thus no evidence to suggest that there are changes in the winter monsoon intensity over the South China Sea region when the world is warmer. Despite the inadequacy of current models to predict future climate scenarios over a small region, plans must

be drawn and actions taken to anticipate such highly probable phenomena. For example, warmer conditions shorten the growing periods of rice, particularly the main-season crop. Predicted yields could be reduced by as much as 12–22 per cent and for the second-season crop, increased irrigation water would have to be provided by as much as 15 per cent higher for directseeded crop (Said and Yong 1990a,b). The impact on oil palm production would depend on where it is grown. In coastal alluvial locations increase in exceptionally heavy rainfall would increase the water table thus reducing production, while on higher ground with constant solar radiation, increased rainfall would have the reverse effect (Mohamad and Harun 1990). As far as rubber is concerned, wetter months reduce production, interfering with tapping. Ongoing experiments to improve yields and reduce production costs can negate the adverse effects of climate change (Yew and Hassan 1990). Warming of sea-surface temperatures would have a significant effect on tropical storms and weather in the region (Lighthill et al. 1994). Climate models indicate that CO2 doubling will cause sea-surface temperature to increase in the primary tropical cyclone basins such as the northwest Pacific of 1 to 1.5°C (WMO 1995). Others have mentioned the possibility of more violent and more frequent tropical cyclones (Bruenig 1990; CSIRO 1992). It has been shown earlier that typhoons located in the western Pacific do influence the weather pattern in Malaysia. Should their track shift northwards, then drier weather conditions are expected in Malaysia and the neighbouring regions. But should the track shift southwards, more extreme rainfall, stronger winds, and higher waves affecting in particular the coastal areas and other offshore activities (Ooi and Lim 1997) could be expected. Climate warming will certainly affect the whole region to varying degrees, drawing the attention of meteorologists, planners, and governments in the ASEAN region to this problem.

Conclusion The climate of the Southeast Asian region is a complex phenomenon, brought about by an interplay of numerous physical and meteorological factors. Far from having a uniform climate, a wide variety of climate and climate influences are found in this region. These factors are not just internal to the region. A greater understanding of the climate and climatic influences of the region and the mechanics of severe events like El Niño and La Niña would go a long way towards appreciating their future trends. Policies and strategies could be put

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in place to tackle any potential adverse impacts such as those arising from global climate change.

References Angelsen, A. (1995), ‘Shifting Cultivation and “Deforestation”: A Study from Indonesia’, World Development, 23: 1713–29. Beaman, R. S., Beaman, J. H., Marsh, C. W., and Woods, P. V. (1985), ‘Droughts and Forest Fires in Sabah in 1983’, Sabah Society Journal, 8/1: 10–30. Blumthaler, M., and Ambach, W. (1994), ‘Changes in Solar Radiation Fluxes after Pinatubo Eruption’, Tellus, 46b: 76–8. Bluth, G. J. S., Doiron, S. D., Schnetzler, C. S., Krueger, A. J., and Walter, L. S. (1993), ‘Global Tracking of SO2 Clouds from the June 1991 Mount Pinatubo Eruptions’, Geophysical Research Letters, 19: 151–4. —— Casadevall, T. J., Schnezler, C. C., Doiron, S. D., Walter, L. S., Krueger, A. J., and Badruddin, M. (1994), ‘Evaluation of Sulphur Dioxide Emissions from Explosive Volcanism: The 1982–1983 Eruptions of Galunggung, Java, Indonesia’, Journal of Volcanology and Geothermal Research, 63: 243–56. Brookfield, H., Potter, L., and Byron, Y. (1995), In Place of the Forest: Environmental and Socio-economic Transformation in Borneo and the Eastern Malay Peninsula (Tokyo: United Nations University Press). —— Lian, F. J., Low, K. S., and Potter, L. (1990), ‘Borneo and the Malay Peninsula’, in B. L. Turner II, W. C. Clark, R. W. Kates, J. F. Richards, J. T. Matthews, and W. B. Meyer (eds.), The Earth as Transformed by Human Action: Global and Regional Changes in the Biosphere over the Past 300 Years (Cambridge: Cambridge University Press), 492–512. Bruenig, E. F. (1990), ‘Overview: The State of Knowledge of Global Change with respect to the Tropical Zone’, in National Biological Conference of Thailand, including Asia and the Pacific Region, on Global Change: Effects on Tropical Forests, Agricultural, Urban and Industrial Ecosystems Proceedings, Bangkok, 22–24 Oct. 1990 (Bangkok), 11–15. Chandra, S. (1993), ‘Changes in Stratospheric Ozone and Temperature due to the Eruptions of Mt Pinatubo’, Geophysical Research Letters, 20/1: 33–6. Charney, J. W., Quirk, J., Chow, S. H., and Kornfield, J. (1977), ‘A Comparative Study of the Effects of Albedo on Drought in Semi-Arid Regions’, Journal of Atmospheric Science, 34: 1366–85. Cheang, B. K. (1987), ‘Short- and Long-Range Monsoon Prediction in Southeast Asia’, in J. S. Fein and P. L. Stephens (eds.), Monsoons (New York: Wiley), 579–606. Chia, L. S., and Chang, K. K. (1971), ‘The Record Floods of 10 December 1969 in Singapore’, Journal of Tropical Geography, 33: 9–19. Chin, P. C. (1958), Tropical Cyclones in the Western Pacific and China Sea Area from 1884 to 1953 (Hong Kong: Royal Observatory). Chong, A. L., and Chan, A. K. (1994), ‘Climate Change in the East Asian Seas Region’, in L. M. Chou (ed.), Implications of Expected Climate Changes in East Asian Seas Regions: An Overview, UNEP RCU/EAS Technical Reports Series no. 2 (Paris), 29–60. CSIRO Report (1992), Climate Change Scenarios for South and Southeast Asia, Asian Development Bank Regional Study on Global Environmental Issues, 26 (Manila). Das, P. K. (1986), Monsoons, WMO no. 613 (Geneva). Exell, R. H. B. (1976), ‘The Fluctuation of Solar Radiation in Thailand’, Solar Energy, 18: 549–54. Flores, J. F., and Balagot, V. F. (1969), ‘Climate of the Philippines’, in H. Arakawa (ed.), World Survey of Climates of Northern and Eastern Asia, vol. 8 (Amsterdam: Elsevier), 159–204.

Gan, T. L., and Tan, S. F. (1970), ‘An Important Trigger Mechanism for Tropical Cyclone Genesis’, in World Meteorological Organisation, Proceedings of the Regional Training Seminar, Singapore, 433–44. Grant, W. B., Browell, E. V., Fishman, J., Brackett, V. G., Veiga, R. E., Nganga, D., and Minga, A. (1994), ‘Aerosol Associated Changes in Tropical Stratospheric Ozone Following the Eruption of Mt. Pinatubo Eruption’, Journal of Geophysical Research, 99: 8197–211. Hanstrum, B. N., and Watson, A. S. (1983), ‘A Case Study of Two Eruptions of Mount Galunggung and an Investigation of Volcanic Eruption Cloud Characteristics Using Remote Sensing Techniques’, Australian Meteorological Magazine, 31: 171–7. Hastenrath, S. (1991), Climate Dynamics of the Tropics, updated edn. (Dordrecht: Kluwer Academic Publishers). Henderson-Sellers, A. (1993), ‘Climate Model Predictions for the Southeast Asian Region’, in H. Brookfield and Y. Byron (eds.), South-East Asia’s Environmental Future: The Search for Sustainability (Tokyo: United Nations University Press), 133–50. Hofmann, D. J., Oltmans, S. J., Harris, J. M., Komhyr, W. D., Lathrop, J. A., DeFoor, T., and Kuniyuki, D. (1993), ‘Ozonesonde Measurements at Hilo, Hawaii Following the Eruption of Pinatubo’, Geophysical Research Letters, 20: 1555– 8. Houze, R. A., Geotis, S. G., Marks, F. D., and West, A. K. (1981), ‘Winter Monsoon Convection in the Vicinity of North Borneo. Part I: Structure and Time Variation of the Clouds and Precipitation’, Monthly Weather Review, 109: 1595–1614. Huang, T. Y. W., and Massie, S. T. (1997), ‘Effect of Volcanic Particles on the O2 and O3 Photolysis Rates and their Impact on Ozone in the Tropical Stratosphere’, Journal Geophysical Research, 102/D1: 1239– 49. Jackson, I. J. (1989), Climate, Water & Agriculture in the Tropics, 2nd edn. (London: Longman Scientific & Technical). Jose, A. M., Francisco, R. V., Juanillo, E. L., and Hilario, F. L. (1992), ‘Some Implications of Mt Pinatubo Eruption on the Environment’, Brunei Darussalam: Report of the 15th Meeting of the ASEAN Subcommittee on Meteorology and Geophysics (Brunei Darussalam). Kripalani, R. H., and Kulkarni, A. (1997), ‘Rainfall Variability over Southeast Asia—Connections with Indian Monsoon and ENSO Extremes: New Perspectives’, International Journal of Climatology, 17: 1155–68. Lambert, A., Grainger, R. G., Remedios, J. J. Rodgers, C. D., Corney, M., and Taylor, F. W. (1993), ‘Measurements of the Mt. Pinatubo Aerosol Cloud by ISAMS’, Geophysical Research Letters, 20/12: 1287–90. Lighthill, J., Holland, G., Gray, W., Landsea, C., Craig, G., Evans, J., Kurihara, Y., and Guard, C. (1994), ‘Global Climate Change and Tropical Cyclones’, American Meteorological Society Bulletin, 75: 2147–57. Lim, J. T. (1981), Effects of Tropical Cyclones on Malaysian Weather, Research Publication no. 3 (Kuala Lumpur: Malaysian Meteorological Service). McGregor, G. R., and Nieuwolt, S. (1998), Tropical Climatology (Chichester: Wiley). Martyn, D. (1992), Climates of the World (Amsterdam: Elsevier). Meehl, G. A. (1994), ‘Influence of the Land Surface in the Asian Monsoon: External Conditions versus Internal Feedbacks’, Journal of Climate, 7: 1033– 49. Michalsky, J. J., Pearson, E. W., and LeBaron, B. A. (1994), ‘An Assessment of the Impact of Volcanic Eruptions on the Northern Hemisphere’s Aerosols Burden during the Last Decade’, Journal Geophysical Research, 95: 5677– 88. Mohamad, A. T., and Harun, M. H. (1990), ‘The Effect of Rainfall and Soil Drainage on Oil Palm Yield Performance in Coastal Region’, National Study Group Report, Malaysia.

The Climate of Southeast Asia 93 Neumann, C. J. (1993), Global Overview in Global Guide to Tropical Cyclone Forecasting, WMO/TC no. 560, Report no. TCP-31 (Geneva: World Meteorological Organisation). Nichol, J. E. (1997), ‘Bioclimatic Impacts of the 1994 Smoke Haze Event in Southeast Asia’, Atmospheric Environment, 31: 1209–19. —— and Goh, K. C. (1995), ‘The 1994 Smoke Haze Episode, Singapore’, in Commission on Climatology Proceedings, Workshop on Climatology and Air Pollution, IGU, Mendoza, Argentina. Ninkovich, D., Shackleton, N. J., Abdel-Monem, A. A., Obradovich, J. D., and Izett, G. (1978), ‘K–Ar Age of the Late Pleistocene Eruption of Toba, North Sumatra’, Nature, 276: 574–7. Olmo, F. J., and Alados-Arboledas, L. (1995), ‘Pinatubo Eruption Effects on Solar Radiation at Almeria (36.83 N, 2.41 W)’, Tellus, 47b: 602–6. Ooi, S. H., and Lim, J. T. (1997), ‘Global Climate Change: Possible Typhoon Related Impacts on Malaysia’, in Report on the 20th Meeting of the ASEAN Sub-committee on Meteorology and Geophysics, vol. ii, 22–6 July, Singapore (unpaginated). Ramage, C. S. (1968), ‘Role of a Tropical “Maritime Continent” in the Atmospheric Circulation’, Monthly Weather Review, 96: 365. Robcock, A., and Mao, J. (1992), ‘Winter Warming from Large Volcanic Eruptions’, Geophysical Research Letters, 12: 2405–8. Saddler, J. C., and Harris, B. E. (1970), The Mean Tropospheric Circulation and Cloudiness over Southeast Asia and Neighbouring Areas, Scientific Report no. 1. (Honolulu: Hawaii Institute of Geophysics, University of Hawaii). Sahai, Y., Kirchhoff, V. W. J. H., and Alvala, P. C. (1997), ‘Pinatubo Eruptions: Effects on Stratospheric O3 and SO2 over Brazil: Rapid Communication’, Journal of Atmospheric and Solar-Terrestrial Physics, 59: 265–9. Said, S., and Yong, E. F. (1990a), ‘Effect of Climate Change on Rice Production in Malaysia’, National Study Group Report, Malaysia. —— —— (1990b), ‘Environmental Impacts and Socio-economic Consequences of Climate Change on Rice Production in the Muda Area’, National Study Group Report, Malaysia. Saunders, R. (1993), ‘Radioactive Properties of Mount Pinatubo Volcanic Aerosols over the Tropical Atlantic’, Geophysical Research Letters, 20: 137–40.

Schindele, W., Thoma, W., and Panzer, K. (1989), The Kalimantan Forest Fire 1982–3 in East Kalimantan, pt. I: The Fire, the Effects, the Damage and Technical Solutions, FR Report no. 5 ( Jakarta: GTZ/International Timber Organization). Schoeberl, M. R., Bhartia, P. K., Hilsenrath, E., and Torres, O. (1993), ‘Tropical Ozone Loss Following the Eruption of Mt Pinatubo’, Geophysical Research Letters, 20: 29–32. Scrivenor, J. B. (1931), The Geology of Malaya (London: Macmillan Press). Sigurdsson, H. (2000), Encyclopedia of Volcanoes (San Diego: Academic Press). Singapore Meteorological Service (1995), ‘Smoke Haze over Singapore, Malaysia and Indonesia’, World Meteorological Organisation Bulletin, 44: 147–50. Stowe, L. L., Carey, M. R., and Pellegrino, P. P. (1992), ‘Monitoring the Mt. Pinatubo Aerosol Layer with NOAA/11 AVHRR Data’, Geophysical Research Letters, 19: 159– 62. Sukanto, M. (1969), ‘Climate of Indonesia’, in H. Arakawa (ed.), World Survey of Climates of Southern and Western Asia, vol. viii (Amsterdam: Elsevier), 215–29. Tay, S. S. C. (1998), ‘What should be Done about the Haze?’, Indonesian Quarterly, 26/2: 100–17. Tinker, P. B., Ingram, J. S. I., and Struwe, S. (1996), ‘Effects of Slash-and-Burn Agriculture and Deforestation on Climate Change’, Agriculture, Ecosystems and Environment, 58: 13–22. van Bemmelen, R. W. (1949), The Geology of Indonesia, 2 vols. (The Hague: Government Printing Office). Wirawan, N. (1995), ‘The Hazard of Fire’, in H. Brookfield and Y. Byron (eds.), South-East Asia’s Environmental Future: The Search for Sustainability (Tokyo: United Nations University Press), 242–60. WMO (World Meteorological Organization) (1976), The Monsoon Experiment, GARP Publication Series no. 18 (Geneva). —— (1995), Global Perspective on Tropical Cyclones, WMO/TD no. 693 (Geneva). Yew, F. K., and Hassan, J. (1990), ‘The Biophysical and Socio-economic Impact of Climate Change on Rubber (Hevea Brasiliensis) in Peninsular Malaysia’, National Study Group Report, Malaysia.

6

Soils of Southeast Asia R. Dudal

Introduction Towards the end of the nineteenth century, with the advent of soil science, soils of the humid tropics were recognized as a separate entity called ‘tropical forest lateritic soils’. The term ‘lateritic’ was derived from laterite (Latin later, brick), a term coined by Buchanan (1807) to describe an iron-rich clay from south India which, when hardened upon exposure, was used as building material. Originally it was thought that laterite represented soil formations throughout the humid tropics, hence the generalization of the name to all red soils in the region. The great diversity of the tropical soils was realized only around the 1930s along with the limited areal occupation of laterite in the tropics. It was actually in Southeast Asia that Vageler (1930) and Mohr (1944) wrote the first two books on tropical soils, based essentially on their study of soils in Indonesia. The two volumes of Mohr’s book were published in Dutch in 1934–8. The English translation appeared in 1944. They attempted to classify soils of the tropics according to thickness, degree of weathering, parent material, and fertility. The understanding of the morphology, genesis, and distribution of soils in Southeast Asia evolved with the establishment and development of soil surveys in different countries of the region from the 1950s. A first overview was prepared by Dudal and Moormann (1964), using the 1938 and 1960 soil classification systems of the United States Department of Agriculture (USDA) (Baldwin, Kellogg, and Thorp 1938; Soil Survey Staff 1960). A revised version was in place by 1974 (Dudal, Moormann, and Riquier 1974). Preparation of a soil map of the world at a scale of 1:5 million started in 1961 at the initiative of the Food and Agricultural Organization of the United Nations

(FAO), UNESCO, and the International Society of Soil Science (ISSS). In 1974 a unified soil classification was prepared and published (FAO 1974). A volume was specifically devoted to Southeast Asia (FAO 1979). The present chapter is based on this publication, and reference should be made to it and the accompanying map (1:5 million) for detailed information about the soils of the region. In 1998 the FAO–UNESCO soil classification system was merged into the World Reference Base for Soil Resources (WRB) as a generally recognized means for communication and correlation in soil classification. The following overview uses the WRB nomenclature. The units are listed below with a brief definition. A full description of the characteristics of the different units is beyond the scope of the present review, but basic publications on WRB (FAO 1998; ISSS 1998) can be used for a complete account regarding criteria and terminology. Information used for the preparation of the soil map of Southeast Asia (FAO 1979) has been drawn from a diverse set of maps and publications dealing with soils, geology, landforms, vegetation, and land use at different scales. Soil classifications used in different countries vary widely, ranging from systems developed in temperate environment to local legends and lists of unclassified series. The WRB has been used to harmonize all this documentation, not to replace national soil classification systems but rather as a common denominator for correlating different schools of thought. This classification is based on soil properties defined in terms of diagnostic horizons and characteristics which should be measurable or observable. An understanding of soil formation processes contributes to a better soil characterization, but these processes should not as such be used as differentiating criteria. The units used in this chapter are distinguished at a high level of generalization

Soils of Southeast Asia 95 Table 6.1 Reference soil groups Soil groups

Description

Fluvisols (Latin fluvius, river) Arenosols (Latin arena, sand) Vertisols (Latin vertere, to turn) Andosols (Japanese an, black, and do, soil) Cambisols (Latin cambiare, to change) Podzols (Russian pod, under, and zola, ash) Gleysols (Russian gley, muck) Luvisols (Latin luere, to wash out)

Soils from young alluvium Soils with very limited development over sandy material Dark clay soils which show swelling and cracking with changing moisture conditions Soils from volcanic deposits Weakly to moderately developed soils Soils with an iron–aluminium–organic accumulation in the subsurface Soils with temporary or permanent wetness near the surface Soils with subsurface accumulation of high-activity clays and medium to high base saturation Soils with subsurface accumulation of low-activity clays and low base saturation Deep clayey soils having a pronounced nut-shaped structure with shiny ped surfaces Deep, strongly weathered soils with a chemically depleted but physically stable subsoil Soils with an irreversible hardening clayey horizon rich in iron and quartz Shallow soils over hard rock or very gravelly material Soils of which the formation is strongly conditioned by human influence Soils composed of organic materials

Acrisols (Latin acris, acid) Nitisols (Latin nitidus, shiny) Ferralsols (Latin ferrum, iron and aluminium) Plinthosols (Greek plinthos, brick) Leptosols (Greek leptos, thin) Anthrosols (Greek anthropos, man) Histosols (Greek histos, tissue)

(Table 6.1). Although their diagnostic features are significant for purposes of management, they are primarily for providing a general overview of the soil patterns of Southeast Asia. Further subdivisions and more detailed information are required for land evaluation and land use recommendations. The nomenclature adopted by the WRB is gaining international status. While discussing the different soil units of Southeast Asia, names from other classification systems are also mentioned in order to establish equivalents. Special reference is made to the USDA Soil Taxonomy (Soil Survey Staff 1998), which also aims at coverage of the global distribution of soils.

Soil Formation Soils are natural bodies that are formed over time by the interaction between climate, relief, parent material, and living organisms encompassing vegetation, fauna, and human influence. Even though hot and humid conditions in the rainy tropics generally favour the development of deep, highly weathered regoliths, present-day knowledge does not support previous theories concerning a distinctive and exclusive pathway of soil formation peculiar to this climatic zone. Soil and land resources surveys do not uphold the concept of uniformity of soils in the humid tropics. The great diversity of parent material and landform supports a wide range of soil characteristics. Volcanic ashes, which constitute a considerable share of parent materials in Southeast Asia, readily develop into andosols in their initial stages of weathering. Vertisols are mostly associated with poorly drained basic or calcareous rocks or lacustrine marls. Highly weathered basalt

produces nitisols and luvisols. Gneisses carry acrisols and nitisols. Sandstones produce acrisols. Limestones develop into luvisols, or into leptosols when strongly eroded. Sands become arenosols or podzols. Recently deposited alluvium gives rise to fluvisols and gleysols. Coastal marshes produce histosols. Deep clayey soils, characterized by low content of weathered minerals and dominated by kaolinite and oxides of iron and aluminium, ferralsols, are common in the humid tropics. However, they occur mainly on Precambrian crystalline rocks on stable shields. These old and strongly weathered soils are rather scarce in the tectonically and volcanically active areas of Southeast Asia. Although soil management practices are acknowledged as having an impact on soil formation, early soil classifications have not systematically catered for soils, the properties of which have been induced or modified by human activities. In Southeast Asia, anthropogenic influence is particularly marked as a result of long periods of agriculture and high population densities, through deforestation, terracing, irrigation, and manuring. In this region, a group of anthrosols is widely found.

Fluvisols Fluvisols originate from recent fluvial deposits, and are without any diagnostic horizon except an accumulation of organic matter at the surface, a gley horizon, or a thionic horizon. They are also known as alluvial soils or as fluvents in the Soil Taxonomy. In Southeast Asia, fluvisols occur extensively in the flat regions of the Mekong Delta, the Sông Hóng Delta, the Central Plain of Thailand, and in the valleys of the

96 R. Dudal

Irrawaddy and the Sittang in Myanmar. Smaller areas occur on the east coast of Sumatra, the south and west coasts of Kalimantan, and the north coast of Java. In fluvisols, variations in colour, texture, and organic content are linked to the river regime. Coarse material occurs on alluvial fans, levées, and former channels, and the finer material in areas of slow sedimentation such as backswamps, lakes, and basins. Soil composition and mineralogy depend to a large extent on parent material and on the major soils in the river basin. Calcareous or basaltic material tends to develop into heavy clays (commonly dark, eutric fluvisols). Acrisols and ferralsols give rise to a medium-texture soil of poor quality (dystric fluvisols). In mangrove soils (Moormann 1961) of the deltas and coastal plains, reduction of sulphates and the presence of sulphuric acid through oxidation makes these soils highly acidic (thionic fluvisols). Fluvisols are highly variable. Organic matter may vary between 1 and 30 per cent. Many of the soils are acidic with a pH value less than 5.5 (dystric fluvisols). Thionic fluvisols have pH values between 3 and 4.5 and release Al, toxic for plants. Eutric fluvisols with higher pH are from calcareous and basic igneous rocks or young marine deposits. Base saturation ranges between 40 and 70 per cent in acid soils, as low as 30 per cent in places in mangrove areas, and 100 per cent in eutric fluvisols. Neutral to alkaline soils may contain Na and free Ca (calcaric fluvisols). Well-drained soils may carry forest or bamboo; poorly drained soils are commonly grass-covered; highly acid soils carry swamp vegetation, e.g. Cyperacea and Melaleuca leucadendron; and saline soils are usually under mangrove. Fluvisols are intensely farmed and can support a high population density, up to 1900 persons km−2 in Java. Crop patterns, rotations, and management are closely related to hydrologic conditions. Rice is the principal crop, and more than 80 per cent of the wet paddy comes from these soils, with yields per hectare varying from several hundred kg (thionic fluvisols) to more than 5 tons (eutric fluvisols). Coconut, sugarcane, and fruits grow on well-drained levées, and maize, tobacco, cotton, jute, and kenaf are locally important. Productivity is considerably increased by appropriate fertilizer application, improved irrigation and drainage, and flood protection.

Arenosols Arenosols develop from sandy material with little or no profile differentiation and without the characteristics of cambic or oxic horizons. In Soil Taxonomy, they belong to the entisol order. Arenosols form on flat to hilly areas. Those from volcanic material occur in an undulating

to mountainous area. These soils have a wide climatic range, and the natural vegetation is primarily determined by climate. They occur mainly along coasts but locally also on wide river terraces. Tephric arenosols are extensively distributed in the volcanic regions of Java, Bali, and Sumatra. On recent dunes and beach sands arenosols are commonly yellowish-brown to very pale brown in colour and include a considerable amount of weatherable minerals and, locally, calcareous shells (calcaric arenosols). Ancient dunes leached of carbonates form eutric arenosols. Dystric arenosols occur in regions with high rainfall. Strongly bleached quartz-rich sands are albic arenosols. Tephric arenosols evolve from volcanic material, and their texture and composition depend on the nature of the volcanic eruption concerned. Commonly, the coarsetextured tephric arenosols are found near centres of eruptions and the fine-grained ones evolve at a distance. The silica content increases with distance from the eruption centre but that of calcium, magnesium, and iron decrease. Stoniness in soil is possible with boulders derived from scoriaceous or torrential eruptions. Arenosols could be calcareous under a dry climate, and eutric or dystric depending on the annual rainfall and on the pH value of the parent material. Arenosols have a low agricultural potential and are not generally cultivated except for coconut plantations as in Indonesia and the Philippines. With adequate rainfall they are used for pineapple or for afforestation with casuarina trees. Below 1000 m, the soils derived from volcanic ash are terraced and planted with rice. Forests dominate upper heights. In central and east Java, highquality tobacco is grown, elsewhere soybean, groundnut, sweet potatoes, and maize.

Vertisols Vertisols are the dark clay soils of warm climate regionally known as black cotton soils, regurs, or tirs. They are known as regurs in Viet Nam, margalitic soils and black earths in Indonesia, compact dark savanna soils in Myanmar, terres noires basaltiques in Cambodia and Viet Nam, Guadalupe clay in the Philippines, and grumusols in the Dudal and Moormann (1964) classification. Vertisols are found in central and east Java, Madura, the Sunda Islands, the Central Plain of Thailand, Luzon, central Myanmar, on calcareous parent material in Cambodia, and as small areas on basalt and old alluvium in Viet Nam. These soils correspond to the vertisols of Soil Taxonomy. Vertisols are heavy soils with a 30– 80 per cent clay content. The clay is mostly montmorillonitic (with

Soils of Southeast Asia 97

small amounts of illite and kaolinite), making them very plastic and sticky when wet and very hard when dry. Deep fissures form as they swell and shrink, and a characteristic micro-topography, gilgai, may develop at the surface. The structure varies between being highly granular for a depth of 5–10 cm or prismatic, with crusts at the surface. Slickensides develop on the peds, and the washing of the surface material into fissures leads to a cyclic mixing process in the upper horizons. Some vertisols may also have marked CaCO3 concretions or salt accumulation at depth. The depth is highly variable and in basaltic areas vertisols can be very stony. By definition, vertisols have 30 per cent or more clay to a depth of 100 cm. The upper horizons pH values range 6.0–7.5 but increase to ≥ 7.8 with depth. Base saturation is usually over 50 per cent, Ca and Mg are the dominant cations, and Na may be significant. Vertisols have a high base exchange capacity. Organic matter is between 0.5 and 1.5 per cent; higher amounts develop in areas of poor drainage. Vertisols commonly develop in a seasonal climate (four- to seven-month dry season) over basic parent material (basalts, andesites, marls, calcareous alluvium, lake deposits). The natural vegetation is tree savanna or open forest, and the relief tends to be flat to slightly undulating. On higher slopes and dissected plateaux they are strongly associated with basic parent material, particularly basalts. These soils are difficult to work when they are either too wet or too dry. With irrigation rice is grown, frequently in rotation with sugarcane. Shortage of water leads to farming of dry rice, maize, and soybeans. Cotton is common in Java, Myanmar, northeast Thailand, and Viet Nam; other cash crops are tobacco and kapok. Vertisols on sloping land are liable to erosion. Reafforestation with teak is locally possible. Strongly eroded vertisols have been used as rangeland.

Andosols The name ‘andosols’ was used first in Japan and later in Indonesia. Occurring over large areas in Java, Sumatra, Bali, and the southeastern part of Luzon, these soils have also been termed high mountain soils, mountain black earth, humic mountain soils, and black latosols. They form on volcanic ashes up to an elevation of 2500 m, with an annual mean temperature of 14–20°C and an annual rainfall between 1800 and 7400 mm. They belong to the andisol order of Soil Taxonomy. Andosols are medium to light in texture with low bulk density and a high silt–clay ratio. They are porous and very friable. The water-holding capacity is high,

and the andosols characteristically smear when rubbed. When dried, they dehydrate irreversibly at times. A hard pan frequently occurs in the volcanic ashes from which these soils are formed. Allophane dominates the clay fraction. Andosols are subdivided according to their organic content and base saturation. Their clay content is 10– 40 per cent, whereas the silt fraction is abundant and may occur between 30 and 75 per cent. Base saturation and organic matter vary between 20 and 40 per cent and 5 and 20 per cent respectively. The nitrogen and potassium content is significant, but the soils are deficient in phosphorus and often in minor elements like manganese. Natural montane vegetation grows on high-altitude andosols, locally replaced by plantations of Pinus mercusii, Agathis alba, and eucalyptus. Tea is grown up to 1500 m and cinchona between 1500 and 2000 m. In east Java, they grow Arabica coffee. Below 1000 m, irrigated rice is grown in rotation with dry crops (mainly groundnuts and sweet potatoes). Vegetables, fruits, and flowers are grown on andosols in Java, and oil palm and wrapper tobacco in north Sumatra.

Cambisols Cambisols have been called brown forest soils because of their resemblance to soils in the temperate region. In Soil Taxonomy these would be the eutrochrepts, dystrochrepts, and ustochrepts of the inceptisol order. These soils show weak horizon development. The stratification and structure of the parent material is no longer apparent, but the soils contain considerable amounts of weatherable material in their silt and clay fractions. A certain amount of clay formation results from in situ weathering, but without appreciable illuviation or clay coatings. The surface horizons have crumb or weak sub-angular blocky structure, a friable consistency, and low bulk density. The base saturation in regions of low rainfall could be up to 100 per cent. If the parent material is non-calcareous, pH values are commonly above 5. Cambisols are young soils found on parent materials that are subject to continuous erosion or recent accumulation. They occur on steep slopes, colluvium, old alluvium, and rapidly eroded calcareous rocks. They are found across a wide climatic range from hot lowlands to cool mountains and an annual rainfall range between 600 and 3000 mm, but apparently not in continuously wet areas. The natural vegetation ranges from open forest to tropical rainforest. As such, they tend to occur in small patches, although the cumulative area covered is significantly high. In hills, shifting cultivation is

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common. In central Java cambisols are on marls and mostly uncultivated with limited subsistence cropping of maize and rice. Attempts have been made towards reforestation. More intensive land use is restricted by relief and shallow soils.

Podzols The term podzol has been used in Indonesia, west Malaysia, and Sarawak. Locally, podzols are known as padang soil or kerangas soils, the terms groundwater podzols and humus podzols have also been used. In Soil Taxonomy, these soils are classified in the spodosol order, mainly in the aquod and humod suborders, with local variants in the orthods. Podzols of Southeast Asia are sandy. They develop an organic surface layer of matted mor under natural forests. The subsurface horizon is light grey, strongly bleached, grading into a dark brown to reddish illuvial horizon of organic matter alone or with iron and aluminium oxides. In west Borneo, a 40 cm bleached horizon is followed in the subsurface by a dark brown illuvial horizon 15 cm thick, and a reddish-brown accumulation of iron 20 cm thick. These grade diffusely into a yellowish-brown sandy parent material. The illuvial horizon is only slightly hardened, and there is no sign of poor drainage. Their depth could be considerable; 100 cm thick bleached horizons and illuvial horizons 250 cm below the surface are known. The soils are strongly acidic, with surface pH values of ≤ 4.5 tending to increase slightly with depth. Organic matter content is 1.5– 4 per cent in the surface and about 10 per cent in the subsurface. Finely dispersed organic matter colours the seepage a typical dark brown. In general, the sand fraction is over 80 per cent, and the base saturation is usually below 15 per cent. Podzols tend to occur on flat or slightly undulating terrain in regions with an annual rainfall over 2000 mm. The most extensive development is on old sandy coastal terraces, but they are also common on quartzites and sandstones, and on acid volcanic rocks (liparites and dacites). On raised beaches, podzols are well drained, although they probably evolved under conditions of impeded drainage. The natural vegetation of the lowland podzols is the kerangas forest which in Borneo includes Dacrydium elatim, Casuarina sumatrana, Agathis dammara, Agathis alba, and Whiteodendron moultanian. Orchids and mosses are abundant. A degraded kerangas forest is replaced by padang vegetation, which is a collection of scattered groups of stunted trees over patches of ground moss, resembling a heath forest. The natural vegetation changes with altitude; for example, Pinus merkusii forests occur higher in north Sumatra. Podzols with peaty

top layers are seen at about 1500 m in west Malaysia. Podzols occur extensively on the coasts of west and south Borneo and on the islands of Bangka and Belitung. Smaller patches have been recorded along the coasts of south Cambodia, south Thailand, west Malaysia, and east Sumatra. High-altitude podzols have been reported from north Sumatra, west Malaysia, and Papua. These soils do not have great agricultural potential, but near settlements are used to grow subsistence crops and vegetables with manuring. In Borneo they have been used for pig-rearing, and in Bangka and Billiton for growing white pepper. Clearing of natural vegetation is best avoided as regeneration is very slow.

Gleysols Gleysols are hydromorphic soils but do not include those very rich in organic matter (histosols) and those developed on recent alluvial deposits (fluvisols). Earlier names used for gleysols in Southeast Asia were low humic gley or grey hydromorphic soils. In Soil Taxonomy, gleysols fit aquents or aquepts. These soils display gleying (brownish-grey or olive grey matrix with lighter-coloured mottles) throughout the profile or immediately underneath the surface with evidence of prolonged saturation with water at all time. Their pH values may range 4–7 depending on environmental conditions. Gleysols are developed on poorly drained low areas. The parent material is commonly alluvium or colluvium, but acid rock residuals may be present. Climate does not seem to be important apart from the fact that the most desaturated ones tend to occur in regions with high rainfall. The natural vegetation is wet grassland, locally with scattered shrubs and trees. Gleysols are found on all older river terraces and deltas. They are common in low plains of Borneo, Java, Papua, and central and northeast Thailand. Small patches may occur on the lower slopes of hills in association with acrisols and luvisols. Rice is commonly grown on these soils along with tobacco, sugarcane, vegetables, and kenaf. With fertilizers and irrigation, double cropping is possible as in south Viet Nam.

Luvisols These soils have been identified as non-lateritic red loams, non-lateritic grey–brown sandy loams, terra rossa, non-calcic brown, and red-yellow Mediterranean soils in various parts of Southeast Asia. In Soil Taxonomy they belong to the alfisol order. These are soils with a subsurface accumulation of clay, an argic horizon, with a sub-angular blocky structure and

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clay coatings. When dry, they acquire a hard to very hard consistency. The colour of the argic horizon ranges from yellowish brown to reddish brown to dusky red or dark reddish brown. Their base saturation is always high and increasing with depth. Luvisols have a medium to high base exchange capacity. The pH values range between 6 and 7, the subsoil becoming slightly alkaline, especially in depressions. Kaolinite is the dominant clay, and some illitic clays are commonly present; so are micas and feldspars throughout the profile. Luvisols occur in regions with a mean annual temperature over 20°C, a total annual rainfall below 1500 mm, and a marked dry season. The dark brown or yellow soils tend to form acid parent material (granite, gneiss, or quartzite) and the reddish varieties on Feand Mg-rich parent material (basalt, limestone). The latter variety is found over the well-drained karst areas of south central Java. The vegetation on luvisols is usually open forest, scrub (often with spiny scrubs), and anthropogenic savanna. The required climatic conditions control the occurrence of luvisols, and they are found in central Myanmar, southeast Viet Nam, east Java, Madura, south Sulawesi, and the Central Plain of Thailand. The soils are relatively fertile but are usually shallow or stony. Irrigation is necessary for successful farming, and they are used to grow rice in central Myanmar and central Thailand, cotton in east Java, and tobacco. Fruits, vegetables, and a variety of crops can be grown in areas of relatively higher rainfall. Shortage of water in other places makes pasture the optimal land use. In spite of reforestation being difficult, teak plantations have been attempted in Java and Thailand.

Acrisols In Southeast Asia, acrisols have been described as red– yellow podzolic soil, red, yellow, or yellowish-brown lateritic soils, grey podzolic soils, and grey earths. Acrisols belong to the order of ultisols in Soil Taxonomy. The eluvial horizon is pale and shows weak structure. The argic horizon, a subsurface accumulation of clay, displays high chroma, red to yellow with a blocky structure and coatings. Concretions, even a continuous plinthic horizon, may develop in the weathering zone. Soft concretions may occur at depth, especially in high rainfall areas such as southeastern Cambodia, west Java, Sumatra, and west Malaysia. The level of the present or past water table is marked by concretions or a plinthic horizon. Base saturation is under 50 per cent and the base exchange capacity is usually low. Kaolinite is the dominant clay, and the soils are poor in weatherable minerals. The pH is usually below 5.5, dropping to

4.5–5 in the wettest areas. The loose bonding between organic-matter clay and the mineral components leads to bleaching of the surface horizons. Acrisols tend to form on acid to moderately basic parent material, on residuals of igneous, sedimentary, and metamorphic rocks, and on old alluvium. The annual rainfall is more than 1500 mm, and there is no marked dry season. The natural vegetation on acrisols is lowland tropical forest, open dipterocarp forest, and pine forests in highlands or short-grass savanna. Acrisols are found in more than half of Viet Nam and are well developed over the non-volcanic areas of west Java, Sumatra, central Borneo, Papua, and west Malaysia. They are also found in the valleys of the Mekong, Chao Phraya, Ta Chin, and lower Irrawaddy. Acrisols are nutrient-poor and erosion-prone, the latter owing to the low subsoil permeability, sharp transition between surface horizons and heavier subsoils, and low aggregate stability. Land abandoned after clearance acquires alang-alang (Imperata cylindrica). Shifting cultivation of rice and kenaf on these soils is common. In the wetter areas, rubber and oil palm plantations are common, with tea on higher grounds.

Nitisols Nitisols were earlier known as lateritic soils, red earths, Rotlehme, terres rouges, and dark red and reddish-brown latosols. They are currently called nitisols because of their argic horizon and the shiny aggregate surfaces. They partly correspond to paleudults in Soil Taxonomy. These soils are strongly weathered with kaolinite dominating the clay fraction and the presence of sesquioxides. The profile is deep but without distinct horizon differentiation except slight clay accumulation with depth. Clay content is between 50 and 80 per cent, the silt and sand fractions are very low. The surface horizons are deep red or reddish-brown, with granular to sub-angular blocky structures of high porosity and stability. Nitisols are acid to slightly acid with pH values between 4.5 to 6.5. Base saturation ranges between 20 and 60 per cent; the sorptive capacity of the clay complex is low. Essentially these soils develop from basic parent material (basalts, andesites, diorites) or granites and gneisses containing substantial amount of biotite over undulating country or low hills. The annual temperature is above 22°C, the annual rainfall between 1000 and 3000 mm with a dry season that lasts less than four months. The natural vegetation is primary rainforest or forest savanna. Secondary forest takes over after clearance. Nitisols are found extensively on the lower volcanic ranges of Sumatra, Java, Bali, Maluku (the Moluccas),

100 R. Dudal

the Philippines, Viet Nam, and east Cambodia. They also cover the high basaltic plateaux of east and west Myanmar and west Thailand, and less extensively central Lao PDR, southeast Thailand, and central Peninsular Malaysia. Despite a lack of high natural fertility, these soils are very productive because of their depth, physical properties, and resistance to erosion. They are terraced for irrigated rice, and other crops that are grown include groundnuts, sweet potatoes, beans, cassava, varieties of fruits, and plantation crops (rubber, sisal, and kapok). Coffee and cocoa are also grown on these soils. The soils respond well to balanced application of fertilizers.

Ferralsols Ferralsols used to be known as lateritic soils or latosols. They are the equivalent of oxisols in Soil Taxonomy. These soils are very low in weatherable minerals, show little horizon differentiation, and a very low exchange capacity. They differ from nitisols in being more acid, less clayey, having weaker structure, and possessing lower base saturation. No horizons of clay accumulation occur. Mottled horizons and concretions may be found at depth. The colour of the solum is red to reddishyellow, locally to yellow. The actual colour is related to the iron content of the parent material, soil drainage, and climate. The pH values average 4.5, base saturation seldom exceeds 30 per cent. The base exchange capacity and the water-holding capacity are low. These soils generally occur on acidic parent material (granite, gneiss, migmatites) and on undulating land. The natural vegetation is primary rainforest or forest savanna. The forests on ferralsols have been cleared less for shifting cultivation than those on nitisols. They occur extensively in Papua, northeast Sumatra, Borneo, and Mindanao, and on the uplands of south Myanmar, Thailand, Viet Nam, and west Malaysia. They are found in association with nitisols, acrisols, and cambisols. Land use depends on relief and local climate. Large areas of oil palm and rubber plantations are in northeast Sumatra and white pepper in Borneo, Bangka, and Belitung. Subsistence farming (frequently shifting in nature) grows maize, sweet potatoes, and cassava. Ferralsols are low in plant nutrients but respond well to applications of N and P.

Plinthosols Plinthosols (from the Greek plinthos, brick) are an iron-rich clayey material which hardens when drying. The term plinthite is currently used as a substitute for laterite. Plinthosols used to be called lateritic soils or

groundwater laterites. The latter are plinthaquox in Soil Taxonomy. Plinthite is a red mottled clay that can be cut with a spade or a knife but hardens upon repeated wetting and drying. When hardened, the material turns into petroplinthite, also referred to as ironstone or laterite. Plinthosols are characterized by a plinthite or petroplinthite horizon occurring at shallow depth. These soils often show waterlogging at surface. All plinthites have a high iron and aluminium content, up to a total of 80 per cent sesquioxides. Generally they have a low cation exchange capacity and a low base saturation. Easily weatherable minerals are absent. Soft plinthite is dense and obstructs deep percolation of water and penetration of plant roots. The formation of plinthite is associated with level to gently sloping areas and fluctuating groundwater or presence of seepage zones. Iron is segregated by alternating reduction and oxidation. Plinthosols with soft plinthite are indigenous to the rainforest. Soils with petroplinthites are more common in the drier savanna. Where petroplinthite becomes exposed to the surface above the present drainage base, the ironstone cap resists further erosion, which may lead to relief inversion, occupying the higher parts of the landscape. Plinthite, which originally was considered to be the characteristic feature of the soils of the humid tropics, occurs only in ill-drained parts of the landscape or in the form of petroplinthite on top of the exposed landscape. Plinthosols are not extensive in Southeast Asia, being derived from old basalt on plateaux in southern Viet Nam and Cambodia, and on sedimentary rocks in Borneo, Bangka, Belitung, and Thailand. Poor soil fertility and waterlogging problems in bottomlands are serious limitations for productive land use. Shallow and skeletal petroplinthite soils suffer from a limited rootable volume so that the potential for arable farming is limited. Extensive grazing is common. On the other hand, plinthite is a valuable building material for brick-making, or, when hardened, as surface gravel in road construction.

Leptosols Leptosols are very shallow soils over hard rock or a highly calcareous material. They are also known as lithosols or lithic subgroups. Leptosols on calcareous rocks are known as rendzinas; those on acid rocks have also been called rankers. Because of their shallow nature, leptosols have an incomplete solum without clear morphological features. They generally develop a surface horizon with organic

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matter, the composition of which is related to the nature of the parent material and the climate under which they occur. Leptosols in calcareous material may carry organic surface layers with intensive biological activity. Leptosols include a large variety of soils with widely differing chemical and physical properties related to the material from which they are formed. In general, leptosols are free-draining soils. Calcareous leptosols usually have better physical and chemical properties than the non-calcareous varieties. The shallowness of leptosols imply a low water-holding capacity. Leptosols are found at medium to high altitudes and across strongly dissected relief. They are common in mountainous areas where erosion prevents deep weathering and development of soil horizons. In Southeast Asia, leptosols are often linked to the presence of limestone or dolomite with tropical karstic landforms. These soils, therefore, are found in the mountainous areas of Borneo, north Sumatra, Papua, and west Malaysia. Smaller patches occur throughout the region with high relief. They occur in association with soils of more pronounced development, such as luvisols or acrisols. These soils have little agricultural potential. The rugged relief and stoniness do not lend themselves to farming and irrigation. Calcareous leptosols may be more fertile with regard to the availability of plant nutrients, but their low water retention capacity and erosivity limit their use in agriculture. Leptosols naturally are under forests, which is the best type of land use for these soils.

Anthrosols The term anthrosol has been recently coined to describe soils that were formed or profoundly modified through anthropogenic activities. In Southeast Asia, this name applies especially to the so-called paddy soils which have been strongly influenced by long-term irrigation for the rice crop. When not separated as a distinct group, these soils have been recognized as anthropogenic subgroups of other major soils. Paddy soils develop special characteristics owing to irrigated flooding associated with wet rice cultivation. The most striking feature is a surface gley with ferruginous mottling, often concentrated in thin tubes around the rice roots and set in a greyish-brown soil matrix. The surface horizon may turn bluish-grey during inundation owing to the reduction process. This surface gley may extend to a depth of 50–60 cm. In freely drained soils, irrigated over a long period, compounds of iron and manganese migrated from the upper horizons and accumulate at 20–60 cm below the surface, at times forming a hard pan. Ploughpans are also seen in many paddy soils

following repeated puddling. The ploughpans limit the rooting depth. Other type features are accumulation of irrigated silt and terracing. Surface horizons of paddy soils have a near-neutral soil reaction when submerged. Under reducing conditions, Fe and Mn may be present in toxic quantities. Although the features described are most evident in the surface horizons, the soils maintain their essential characteristics in the subsoil including texture, fertility status, internal drainage, and sorptive capacity. Paddy soils are associated with an intensive agricultural environment. The requirement for irrigation limit their occurrence in flat or terraced landscape. The age-old cultivation of irrigated rice in Southeast Asia has had a profound influence on the environment, creating anthropogenic landscapes with a controlled hydrology. These soils, as expected, occur in the rice-producing alluvial plains of Southeast Asia and are extensive in gleysols. In upland regions, especially in Java, the type surface gley and pan are superimposed on other types of soils on which paddy is grown, mostly on terraced areas of nitisols and andosols. The relatively impervious nature of vertisols, however, prevents downward migration, which with widespread fissuring does not allow hard pan formation. As anthrosols cut across different soils, it is difficult to map them as a separate entity. Because of controlled moisture regimes and the great care devoted to irrigated rice farming, paddy soils have a high agricultural potential. With the advent of high-yield rice varieties in the 1960s, the rational use of fertilizers, improved water management, and integrated pest control, the production from paddy soils in Southeast Asia has doubled, even tripled, over large areas.

Histosols Histosols comprise a wide variety of organic soils formed under environments that range from lagoonal marshes to poorly drained mountain depressions. They have been known as bog soils, half-bog soils, peaty soils, marsh soils, or muck soils. They correspond to the histosols of Soil Taxonomy. By definition, these soils contain at least 20–30 per cent organic matter (depending on texture) in a surface layer at least 30 cm thick. They have a low density and are saturated with water for part of the year. They lie deep (1.5–5 m) on the east coast of Sumatra. The organic surface layer contains large quantities of undecomposed roots, branches, even tree boles over a grey to white reduced mineral horizon. The soils could be very acid, with pH values of 4 in the surface increasing only to 4.5 with depth. In Java, however, eutrophic peats occur locally. Nutrient level

102 R. Dudal

is very low; the texture of the mineral horizon ranges between sand and clay. Histosols develop in badly drained areas where the water table is high or even above the ground surface, for several months of the year as in coastal marshes, inland swamps, and mountain depressions. Most of the organic soils occur at or slightly above sea level or river floodplains. The annual rainfall is between 1500 and 2500 mm, and the natural vegetation is peat swamp forest, the nature of which varies with local conditions; Melaleuca leucadendron dominates in certain areas. In coastal areas, histosols may occur in association with podzols formed from sandy parent material; in volcanic uplands with andosols. In floodplains, layers of mineral and organic horizons may alternate. In Southeast Asia, histosols cover large areas: the east coast of Sumatra, west and south coasts of Borneo, both coasts of Peninsular Malaysia, and the south coast of Papua. They have been estimated to cover about 6 million ha in Indonesia and 15 per cent of the total area of Sarawak on the island of Borneo. In Indonesia and west Malaysia, the natural forests on histosols yield timber, whereas in northeastern Sumatra small rubber plantations have been established in drained areas. In general these soils have a low crop yield, although population pressure often requires that they be reclaimed. In central Sumatra and south Borneo rice is planted as the floods recede. The successful utilization of these soils depends on strict water control and balanced fertilizer use.

Conclusion The soils of Southeast Asia are dominated by acrisols. They are estimated to cover 51 per cent of the total land area. With the exception of Java and the Lesser Sunda Islands, where andosols prevail, acrisols may occupy up to 75 per cent of the land in various parts of the region. An overview of the soil distribution in Southeast Asia is shown in Fig. 6.1 (FAO 1979). Acrisols are highly acidic, low in nutrients, high in Al saturation, and erosion-prone. The agricultural potential is limited, and acrisols remain mostly under forest. These soils, however, have been used effectively for acidtolerant crops such as rubber and oil palm. Indonesia, Malaysia, and Thailand together have 7 million ha under rubber. Shifting cultivation with rotation of short-term farming with regrowth of the forest is still common. Population pressure in Indonesia has led to the policy of transmigration, moving people from Java to, among other places, south Sumatra. Where this involves settlement on acrisols, inputs such as lime and fertilizers

are necessary in order to ensure a viable agriculture. Technical solutions for the acidity problems have been developed (von Uexküll 1982; Craswell and Pushparajah 1989), but economic feasibility, required infrastructure, and marketing of inputs and outputs remain major constraints. In south Sumatra, abandoned arable land has changed into anthropogenic Imperata cylindrica savannas. In southern Thailand, high-input management on acid soils appears to have been successful for growing fruit trees. Nitisols cover about 5 per cent of the area. They have a moderate to high agricultural potential, a deep rooting volume, and a relatively stable structure, and are resistant to erosion. A great variety of crops are grown on them including plantation varieties such as rubber. On the lower slopes of volcanoes they are often terraced to grow irrigated rice. Ferralsols cover approximately 4 per cent of the area. Their limited extent in Southeast Asia differs strikingly from the distribution in Africa and South America, where ferralsols are dominant on old stable landforms. Their good physical properties are in contrast to their low natural fertility. They have a moderate agricultural potential under good management. Ferralsols are often associated with plinthosols, which have a very limited production capacity. Plinthosols occupy only a very small area, too small to be depicted on small-scale maps. Luvisols and cambisols each cover about 5 per cent of the total land area of Southeast Asia. They have a moderate to high agricultural potential except in highly dissected areas. These soils respond well to inputs, and under good management satisfactory production can be achieved for a variety of annual or perennial crops. In drier areas, their intensive use is dependent on the availability of irrigation water. Fluvisols and gleysols cover approximately 8 and 5 per cent of the area respectively. These are the main ricegrowing soils of the region. Agriculture on these soils has been considerably intensified in the last three decades, and yields have markedly increased. Myanmar, Thailand, and Viet Nam are rice-exporting countries. Andosols cover about 2 per cent of the region, but they are the dominant soil group in Java and west Sumatra and an important soil component in the Philippines. In general, they have a high agricultural potential and are good rice producers under irrigation. Tea, coffee, and a wide variety of horticultural crops are grown at higher altitudes. Vertisols occupy about 2 per cent of the area and are best for the production of irrigated rice and sugarcane. Podzols and arenosols each cover about 2 per cent of the area. Owing to their coarse texture and poor water

Fig. 6.1. Soils of Southeast Asia (Source: Adapted from Food and Agriculture Organization of the United Nations (FAO 1979)

104 R. Dudal

retention capacities, these soils are not suitable for sustained cultivation. Pepper has grown well with good management on some podzols. Histosols cover about 6 per cent of Southeast Asia. They are particularly extensive in east Sumatra, south Borneo, Papua, and Malaysia. Because of population pressure, attempts have been made to reclaim histosols through drainage. However, subsidence, difficult water control, poor nutrient content, and limited accessibility result in a low production potential. Lithosols cover nearly 4 per cent. Their shallow nature and stoniness generally preclude cultivation. Anthrosols cut across the above soil groups. Of the total arable land in the region 26 per cent is irrigated, half of which has been terraced. Southeast Asia is well endowed with soil and water resources. Of the land area presently used for agriculture, two-thirds are arable and the rest is used for growing permanent crops. An account of the agro-ecological potential of the region has been published by FAO (1980). Large areas are still under forest. Southeast Asia is a major player in the tropical timber trade. Plantation forestry is widely practised. The soils and water resources of Southeast Asia are precious assets to the region. Improved knowledge of their characteristics and distinctive nature should allow for their protection and sustainable development.

References Baldwin, M., Kellogg, C. E., and Thorp, J. (1938), ‘Soil Classification’, in Soils and Men, USDA Yearbook, 979–1001 (Washington).

Buchanan, F. (1807), A Journey from Madras through the Countries of Mysore, Canara and Malabar (St James: W. Bulmer; London: East India Co.). Craswell, E. T., and Pushparajah, E. (1989), Management of Acid Soils in the Humid Tropics of Asia, ACIAR Monograph no. 13 (Canberra). Dudal, R., and Moormann, F. R. (1964), ‘Major Soils of Southeast Asia’, Journal of Tropical Geography, 18: 54– 80. —— —— and Riquier, J. (1974), ‘Soils of Humid Tropical Asia’, in Natural Resources of Humid Tropical Asia (Paris: UNESCO, Natural Resources Council), xii. 159–78. FAO (Food and Agricultural Organization) (1974), FAO–UNESCO Soil Map of the World 1:5 000 000, vol. i: Legend (Paris: UNESCO). —— (1979), FAO–UNESCO Soil Map of the World 1:5 000 000, vol. ix: Southeast Asia (Paris: UNESCO). —— (1980), Report on the Agro-ecological Zones Project: Results for Southeast Asia, World Soil Resoures Report 48/4 (Rome: FAO). —— (1998), World Reference Base for Soil Resources, World Soil Resources Report 84 (Rome: FAO). ISSS (International Society of Soil Science Working Group RB ( J. A. Deckers, F. O. Nachtergaele, and O. C. Spaargaren (eds.)) (1998), World Reference Base for Soil Resources: Introduction (Leuven: Acco, ISSS/ISRIC/FAO). Mohr, E. C. J. (1934– 8), De bodem der tropen in het algemeen en die van Nederlandsch-Indië in het bijzonder, 2 vols. (Amsterdam: Med. Kon. Inst. van de Tropen). —— (1944), The Soils of Equatorial Regions with Special Reference to the Netherlands East Indies (Ann Arbor). Moormann, F. R. (1961), The Soils of the Republic of Viet Nam (Saigon: Ministry of Agriculture). Soil Survey Staff (1960), Soil Classification: A Comprehensive System. 7th Approximation (Washington: USDA). —— (1998), Soil Taxonomy: A Basic System of Soil Classification for Making and Interpreting Soil Surveys, Natural Resources Conservation Service, Agricultural Handbook 436 (Washington: USDA). Vageler, P. W. (1930), Grundriss der Tropischen und Subtropischen Bodenkunde (Berlin: Verlagsgesellschaft für Ackerbau MBH). von Uexküll, H. R. (1982), Efficient Fertilizer Use in Acid Upland Soils of the Humid Tropics, FAO Fertilizer and Plant Nutrition Bulletin, 10 (Rome: FAO).

7

Vegetation Richard T. Corlett

Introduction Southeast Asia is not a natural biogeographical unit: it extends well north out of the tropics in Myanmar, while the eastern boundary bisects the island of New Guinea. It is also divided in two by one of the sharpest zoogeographical boundaries in the world, Wallace’s line (Figure 7.1; Whitmore 1987). There is, however, one important unifying feature that distinguishes it from

most other regions of the tropics: Southeast Asia is a region of forest climates. Only on the highest mountains in Papua and northern Myanmar is the climate too cold for forest and, with the possible exception of some small rain-shadow areas, it is nowhere too dry. Elsewhere the only permanent non-forest vegetation in the region before the human impacts of the last few millennia was on coastal cliffs and beaches, seasonally flooded river plains, active volcanoes, and perhaps some small inland

Fig. 7.1. The location of Wallace’s line, which marks the boundary between the Oriental and Australian faunal regions

106 Richard T. Corlett

areas on soils too poor to support forest. Today, however, as a result of human impacts, forest occupies less than half of the region, with various anthropogenic vegetation types occupying the rest. The recognition of Southeast Asia, as defined here, as a separate political and geographic entity is very recent, so it is not surprising that there has been no previous account of the vegetation of the whole region. Van Steenis (1957) gave a general account of the vegetation of Indonesia, while Whitmore (1984) concentrated on the tropical evergreen forests of the region, with only a brief description of the vegetation of drier climates. Champion (1936) described the principal forest types of Myanmar, while Vidal (1997) covered the vegetation of Thailand, Cambodia, and Lao PDR. Numerous other publications describe smaller areas or specific vegetation types. To a first approximation, the potential natural vegetation of the region (Plate 1) up to about 20°N is controlled by two main environmental gradients: a horizontal gradient of water availability and a vertical, altitudinal gradient. Water availability is determined largely by the amount and distribution of rainfall, with the length of the dry season the most important factor, although the water storage capacity of the soil becomes increasingly significant at the drier end of the gradient. Increasing altitude results in a regular and predictable decline in temperature, but also less regular changes in many other factors. This two-factor model of Southeast Asian vegetation is complicated in the northern part of the region by a significant decline in temperature—particularly annual minimum temperature—with increasing distance from the Equator. Overlaid on this simple model are more complex patterns resulting from extremes in soil and drainage conditions. All systems for classifying and naming vegetation types are to some extent arbitrary, and which is ‘best’ depends on the purpose. The choice is between groupings based on features of the physical environment (climate, soil, and topography), the appearance of the vegetation (structure, physiognomy, and seasonal changes), or the actual plant species present. The last of these—the floristic approach—undoubtedly gives the most precision, but is impractical on the scale considered here. An environment-based classification works well if only natural, climax vegetation types are being considered (as in Plate 1), but human impact has broken the tight link between vegetation and environment on which it relies. I have therefore followed many other regional authors in using a combination of physical environment and appearance to divide the vegetation of the region into major vegetation types (Table 7.1).

Table 7.1 Major vegetation types of Southeast Asia Lowland vegetation Tropical rainforests Tropical seasonal forests Tropical deciduous forests Moist deciduous forests Dry deciduous forests Deciduous dipterocarp forests Forests on extreme soil types Heath forests Forests on ultramafic rocks Limestone forests Secondary forests Logged forests Bamboo forests Savannas and grasslands Shrublands and thickets Beach vegetation Plantations Agroforestry Other dryland crops Montane vegetation Montane forests Lower montane forests Upper montane forests Subalpine forests Alpine vegetation Wetlands Mangrove forests Brackish water swamp forests Freshwater swamp forests Peat swamp forests Herbaceous swamp forests Ricefields

The Vegetation Lowland Vegetation Tropical Rainforests Tropical rainforests—or tropical lowland evergreen rainforests to be more precise—have the highest biomass, structural complexity, and both floristic and faunistic diversity of any vegetation type in the region. The main canopy is generally 30– 40 m high, with scattered emergents rising to 50 m or more. Many of the larger trees have spreading buttresses for support. Below the main canopy is an additional tree layer that includes both small, shade-tolerant understorey species and young individuals of the canopy and emergent species. True shrubs are relatively rare, and the herbaceous ground layer is generally sparse and patchy. Woody lianas are common, and include many species of rattans—spiny, climbing palms which reach their greatest diversity in Southeast Asia. Some canopy trees may be deciduous, but the leafless period is brief and not synchronized between

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species. At least in the western part of the region, the lack of a clear, annual periodicity is most obvious in the reproductive phenology, with many species flowering and fruiting at intervals of less or more than a year, and community-level mass-flowering episodes at irregular, supra-annual intervals (Corlett and LaFrankie 1998). Tropical rainforests are limited to areas without seasonal water or temperature stress, although less regular dry periods may occur. In Southeast Asia, there are two major blocks of rainforest: a western block on the Sunda Shelf (principally, Sumatra, the Malay Peninsula, and Borneo), and an eastern block in New Guinea. Between these blocks there are — or were until recently—tropical rainforests in the wetter parts of the Philippines, Sulawesi, and Java, in much of central and northern Maluku, and the wettest parts of Nusa Tenggara. There are also outlying areas of tropical rainforest in southwest Myanmar, and elsewhere in continental Southeast Asia, but these have not been studied in detail. The most obvious difference between the two major blocks of tropical rainforest concerns the tree family Dipterocarpaceae. With a few exceptions, dipterocarps dominate the emergent and canopy layers of the Sunda Shelf rainforests, while in New Guinea, although three dipterocarp genera are present, they are only patchily significant. The total diversity of tree species, however, seems to be similar in the two areas, with more than 200 species greater than 10 cm in diameter in the richest 1 hectare plots sampled, with most other plots falling in the range 120–200 species (Turner 2001). In contrast, the rainforests on the islands between Borneo and New Guinea, and in continental Southeast Asia, seem to be less diverse.

Tropical Seasonal Forests In areas that experience a regular annual dry period of one to four months—up to six months on deep soils— the aseasonal rainforest described above is replaced by forests which, although still predominantly evergreen, exhibit regular, seasonal changes, synchronized by the annual drought. These changes are most striking in semi-evergreen rainforests, in which up to half the canopy trees are deciduous, although the lower storeys are largely evergreen. However, the proportion of deciduous trees does not show a simple relationship with the length of the dry season, and largely evergreen forests occur on deep soils in sheltered valleys and as a narrow ‘gallery forest’ along watercourses in the same climate as fully deciduous forests. These ‘dry evergreen’ forests exhibit annual cycles of growth and reproduction, and have a wilted and lifeless appearance at the peak of

the dry season. In comparison with rainforests in areas without a dry season, seasonal forests are generally less tall and less diverse, with a tendency to local dominance by one or a few species. Towards the north of the region, winter low temperatures reduce water stress but absolute minima below 10°C probably exclude many tropical species. Despite this, dipterocarp-dominated tropical seasonal forests extend to 27°N in sheltered valleys in Myanmar (Kingdon-Ward 1945). The dry season makes tropical seasonal forests susceptible to fire, and thus replacement by more tolerant vegetation types, while their occurrence on the best soils makes them susceptible to clearance for agriculture. The limited extent of these forests in continental Southeast Asia today is thus probably a result of human impact, rather than the absence of suitable environments. Tropical seasonal forests also occur in eastern Indonesia, but detailed studies are lacking.

Tropical Deciduous Forests Deciduousness in the tropics is a response to seasonal water stress, and one would expect a gradual change from fully evergreen to fully deciduous forest along a gradient of increasing length and severity of the dry season. This is rarely, if ever, observed in modern Southeast Asia, and boundaries between largely evergreen and largely deciduous forests are typically abrupt or through only a narrow ecotone. In most cases, this is because the boundary between tropical seasonal forest and deciduous forest is between a fire-sensitive, fireexcluding vegetation type and one that tolerates and often promotes fire. Although the palaeoecological record and, indeed, the existence of a fire-tolerant flora provide evidence for fires before modern humans arrived in the region (Hope, Chapter 2 in this volume), the fire frequency has undoubtedly increased tremendously— and the intensity of each fire decreased, since fuels do not accumulate—with a huge, but largely unknown, impact on the extent, structure, and floristic composition of fire-tolerant forest types. Much of the existing deciduous forest has probably replaced fire-sensitive tropical seasonal forests in relatively wet areas, while, at the other end of the spectrum, fire has degraded the drier deciduous forests into savanna and grassland. Tropical deciduous forests typically occur in regions with a three- to seven-month dry season. However, as the length of the dry season increases, soil characteristics and topography have an increasingly important influence on the vegetation and, in combination with differences in human impact, can produce a complex mosaic of very different vegetation types in the same regional climate. Deciduous forests themselves are very

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varied in structure and floristic composition, as well as the degree of deciduousness and the length of the leafless period, which may be brief. Foresters often distinguish ‘moist deciduous forest’, with some trees more than 25 m tall and the lower storeys largely evergreen, and ‘dry deciduous forests’, with a lower canopy and almost all species deciduous, but it is possible to find all intermediates between these types. Bamboos are common, but not universal, in the understorey, with grasses increasingly important as drought, fire, or other disturbance opens up the canopy. Other types are distinguished on the dominance of a particular species, such as teak, Tectona grandis, in its native range in Myanmar, Thailand, and Lao PDR, and where naturalized in seasonally dry parts of Indonesia. One important deciduous forest type is recognized by both foresters and local people as distinct throughout its range in continental Southeast Asia (Stott 1990). This is the ‘deciduous dipterocarp forest’, which is also called ‘savanna forest’ in some literature. It varies considerably in stature and the openness of the canopy, but is dominated throughout its range by a characteristic group of tree species, including six deciduous (or semi-deciduous) dipterocarps. Deciduous dipterocarp forest is most widespread in areas with low rainfall (< 1500 mm), a four- to seven-month dry season, and poor sandy or gravelly soils. In some core areas it may be an edaphic climax, but its extent has been greatly increased by near-annual fires and cutting for timber and firewood. The dominants are thick-barked, firetolerant, and coppice well after cutting. The generally low diversity of both the flora and fauna, relative to other forest types, supports the idea that this is largely an anthropogenic formation. In some parts of the central dry zone of Myanmar and on some small, low islands of eastern Indonesia, the total annual rainfall is less than 800 mm and the dry season lasts nine months or longer. The natural vegetation in these areas would probably be a ‘thorn forest’, dominated by low, thorny, deciduous trees, particularly in the genus Acacia. Such vegetation is extremely vulnerable to fire and other human impacts, so little, if any, remains, in anything like its natural state.

Forest on Extreme Soil Types In general, the influence of soil type on vegetation appears to increase with declining rainfall and increasing seasonality. Even in the wettest areas, however, three extreme types of substrate tend to produce distinctive vegetation types. The best studied of these are the ‘heath forests’ that are found on strongly weathered soils, developed on sand or sandstone. They are most

extensive on Borneo, but smaller areas have been described from the Malay Peninsula, Sumatra, and eastern Indonesia. Heath forests are both structurally and floristically distinct from the surrounding forest on other substrates. Typically there is a low, uniform, small-leaved canopy that is easily recognized on aerial photographs. Despite a considerable research effort, it is still not clear if the primary cause of this distinctiveness is a shortage of nutrients, or a low water-holding capacity and thus high incidence of drought (Becker et al. 1999). The soils under heath forest are useless for agriculture, and, although light, selective logging may be sustainable, clear-felling seems to lead to irreversible soil deterioration. Ultramafic rocks are extensive only on Sulawesi, but smaller areas occur throughout the region. Soils derived from these rocks are very variable but tend to be relatively shallow and to have both low concentrations of important plant nutrients and potentially toxic levels of nickel, magnesium, and other metals. On some sites the vegetation is not distinct from the surrounding areas, while in others—probably on the more extreme soils—the forest is sparse or stunted, and contains species that are rare or absent on other substrates. There is evidence that it is the low water-holding capacity of the soil, rather than soil chemistry, which is responsible for the low stature of these forests (Proctor, Bruijnzeel, and Baker 1999). Soils under these forests are infertile and not usually cultivated. Exposed limestone has a very patchy distribution in the region. Irregular weathering produces a wide variety of plant habitats, differing in slope, soil thickness, and soil chemistry. Forest on limestone is very variable but in wet areas tends to have a shorter stature and lower tree diversity than on other substrates. The herbaceous flora, in contrast, may be rich and often includes species found nowhere else. In the seasonal tropics, limestone outcrops sometimes support more impressive forests than the surroundings, although this may simply reflect reduced human impact as a result of their inaccessibility and general unsuitability for agriculture.

Secondary Forests The term ‘secondary forest’ is often applied indiscriminately to all forests that have been disturbed, particularly by human impact. However, the major human impacts —clearance for agriculture and selective logging for timber—have such different effects that it makes sense to distinguish them clearly (Corlett 1995). Secondary forest is therefore defined here as forest that has regrown after clearance. In contrast to logged forest, where recovery is dominated by species that survive on

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the site, secondary forest in this restricted sense consists largely of species that have dispersed to the site from elsewhere. Young secondary forests are usually easily recognized by their low, uniform canopy and a tendency to dominance by one or a few species. Older secondary forests are more variable and increasingly resemble primary forest in structure and species composition. When or if the differences finally disappear will depend on many factors, of which the most important are probably the size of the cleared area—and thus the proximity of primary forest seed sources—and the fertility of the soil. Forest successions after brief cultivation in small clearings are very different from those in deforested landscapes that have been degraded by prolonged cultivation.

Logged Forests Most lowland forests in the region have had some timber removed, but the logging intensity has varied greatly, from the harvesting of scattered individual trees by local people to the mechanized commercial logging of up to 72 trees per hectare, in those exceptional areas where valuable trees occur at high density ( Johns 1997). More typically, the range in tropical rainforests is 8–24 trees per hectare, with only large individuals of commercially valuable species removed. Although the logging is highly selective, the damage caused to the remaining forest by the felling of huge-crowned emergents and their removal along skid trails is not. Recently logged forest has an increased fuel load and is more open, and thus drier, than unlogged forest, making it more susceptible to fire. Moreover, the soil over much of the logged area is compacted, leading to decreased infiltration, increased erosion, and slow regeneration.

Bamboo Forests Bamboos are a component of most forest types in the region, and some species can become abundant when the forest is disturbed by logging or shifting cultivation, particularly in seasonal areas. Although some bamboo stands are probably natural, the extensive, almost monospecific bamboo forests that occur in parts of continental Southeast Asia seem to be largely secondary in origin.

Savannas and Grasslands In Southeast Asia, the term ‘savanna’ is usually applied to vegetation with a discontinuous tree layer over a more or less continuous grass layer. Some areas of lowland savanna in the driest parts of eastern Indonesia may be natural, owing their existence to some combination of drought, soil factors, lightning-caused fires, and/or seasonal inundation. However, all existing savannas are burned more or less regularly by humans, and there

is little doubt that most have replaced forest as a result of human impact. Stable, fire-maintained savanna depends on the occurrence in the local flora of tree species that can not only survive but also regenerate under such conditions. In the region under consideration, many of these species are in genera of Australian origin, such as Casuarina, Eucalyptus, and Melaleuca, which presumably owe their fire-resistance to an ancestry on that dry and fire-prone continent. Melaleuca savannas occur throughout the region on seasonally waterlogged or inundated soils, while Casuarina and Eucalyptus savannas occupy huge areas in seasonal eastern Indonesia. Species of Acacia and the palms Borassus flabellifer and Corypha utan also form extensive savannas in this part of the region. In contrast, savanna is much less extensive in lowland continental Southeast Asia and, where it does occur, usually represents a transitional stage in the degradation or regeneration of forest, rather than a more or less stable, fire-maintained vegetation type. Where the fire regime — or a combination of fire, cutting, and/or grazing by livestock— exceeds the tolerance of the local tree flora, treeless grassland replaces savanna. Grasslands also develop directly on soils exhausted by prolonged cultivation or short fallows, and are maintained by regular burning. Except in the drier parts of the region, the invasive and extremely firetolerant grass Imperata cylindrica (alang alang, lalang, cogon, etc.) is characteristic of such situations.

Shrublands and Thickets Natural evergreen shrublands are found at the altitudinal limit of tree growth, and it is possible that deciduous shrublands and thickets are the natural vegetation in some of the driest lowland areas. The extensive shrublands and thickets in the region today, however, are mostly on sites that have been deforested and abandoned. Deciduous and often thorny thickets are a common and persistent result of clearing forests in seasonally dry areas, while evergreen shrublands are a short-lived successional stage in wetter areas. Naturalized exotic plants, such as the tropical American composite Chromolaena odorata, are often prominent or even dominant in such vegetation types.

Beach Vegetation On accreting sandy beaches, a low community of creeping herbs, grasses, and sedges occupies the zone between high-water mark and the margin of the beach. On undisturbed beaches, there is then a belt of coastal forest, extending 5–50 m inland, sometimes with a seaward fringe of more or less pure Casuarina equisetifolia.

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In most of the region, however, this coastal forest has now been replaced by coconut plantations. Both beach communities are dominated by a characteristic group of plant species with very wide geographical ranges: in some cases throughout the tropics.

Plantations Huge and increasing areas in the region are planted with tree and shrub plantation crops, particularly coconuts, rubber, and oil palm, with smaller areas of coffee, cocoa, cashew nuts, and other species. The area planted with trees for timber, pulp, plywood, or fuel, in contrast, is relatively small, but also rapidly increasing. The most widely planted tree species, including Acacia mangium, Eucalyptus urophylla, Gmelina arborea, Paraserianthes falcataria, Pinus merkusii, P. kesiya, and Tectona grandis, are native somewhere in the region, although now planted far outside their natural ranges. Others, such as Leucaena leucocephala and Swietenia macrophylla (mahogany), are exotics. Almost all commercial plantations are monocultures and thus have a much simpler structure and lower diversity than any natural forest, although both plant and animal diversity increase if a native understorey is allowed to develop.

Agroforestry ‘Agroforestry’ is the term for a wide range of treedominated, multi-species cultivation systems used to produce both food and cash crops. They range from mixed tree plantations to multi-layered systems that also include annual crops. The floristic diversity of agroforestry plots may be high, and their structure is often more similar to secondary forest than to monoculture plantations. Such areas may provide an important habitat for wildlife in an otherwise deforested landscape (Thiollay 1995). Although each patch is usually small, their aggregate area in much of the region is huge. Agroforestry systems cover nearly 10 per cent of Sumatra, where small, family-run Hevea gardens, which have a variable component of native tree species, produce more rubber than the industrial monoculture plantations (Laumonier 1997).

Other Dryland Crops Dryland cultivation of non-woody crops ranges from the polycultures of some shifting cultivators to highly mechanized industrial monocultures. Shared features include the simplified structure and low species diversity compared with natural vegetation, and the incomplete soil coverage for at least part of each cropping cycle. In general—and in apparent contrast to wetland rice and dryland tree crops—this type of agriculture requires

either long fallow periods or massive external inputs in the form of fertilizer and pesticides. Numerous different dryland crops are grown, but it is interesting to note the importance of the American crops maize, cassava, and sweet potato, which were introduced to the region by Europeans in the sixteenth and seventeenth centuries.

Montane Vegetation Highland areas are relatively more extensive in Southeast Asia than in Africa or South America, but, unlike the other two regions, only a few peaks at the two extremes of Southeast Asia exceed the climatic forest limit of 3800– 4000 m. In Papua, Puncak Jaya attains 5030 m, while in northern Myanmar, the highest mountain in the region, Hkakabo Razi, reaches 5881 m. Both these mountains, and a few of their neighbours, have caps of permanent snow and ice. Mount Kinabalu, the highest peak between Irian Jaya and Myanmar, attains 4101 m, but the summit is largely bare rock, scoured by Pleistocene ice, so there is no climatic treeline. The treeline is also depressed on the many active volcanoes in Java and eastern Indonesia, where it occurs at the base of the debris from active cones, with only patchy herbaceous vegetation and scattered shrubs above. Temperature declines with altitude at a rate of around 0.6°C per 100 m. All other environmental factors (except day length) also change as one ascends a mountain, but these changes are not necessarily unidirectional or correlated with each other. After decreasing air temperature, increasing cloudiness is the factor most often invoked to explain changes in vegetation, but the mechanism by which it acts is still uncertain (Bruijnzeel and Veneklaas 1998). Rainfall and relative humidity usually increase, at least to intermediate altitudes. Soils typically become more acid and organic, and there is increasing evidence that the nutrient supply limits plant productivity in montane forests (Tanner, Vitousek, and Cuevas 1998). Vegetation structure, physiognomy, and floristics all change with altitude. These changes may be gradual but, particularly at higher altitudes, are often more or less abrupt, resulting in stepwise pattern of vegetation change along an apparently smooth environmental gradient. With increasing altitude, the forest becomes shorter, tree heights more even, the crowns and leaves smaller, rooting more shallow, and cold-intolerant plant families progressively drop out. The most dramatic differences often coincide with the zone of persistent cloud, where trunks and branches become gnarled and bryophytes cover all surfaces. This vegetation is often referred to as ‘cloud forest’ or ‘mossy forest’, although the bryophytes are mostly liverworts rather than mosses.

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The most widely used nomenclature refers to the forest zones above the lowlands as lower montane, upper montane, and subalpine, but only the tallest mountains have all three. The zonation is much compressed on small or isolated mountains, with each zone occurring at a lower altitude. The reasons for this compression are not fully understood, but one factor seems to be a change in the pattern of persistent cloud. There are also often major floristic differences between adjacent mountains that cannot easily be attributed to environmental differences. Large areas dominated by a single tree species have been widely reported, particularly in New Guinea, where extensive pure stands of Nothofagus species seem to be a legacy of natural disturbance events. In the alpine region above the altitudinal treeline at 3900–4000 m, the vegetation is dominated by grasses, low shrubs, or herbs. Permanent caps of snow and ice are only found on mountains that exceed 4650 m, although tongues from the glaciers on higher peaks extend considerably lower. Montane forests are largely evergreen, even where the lowland forest is deciduous. Where a regular dry season occurs, however, these forests are extremely vulnerable to fire, and in such areas they have often been reduced to relict patches in moist or topographically protected sites, or eliminated altogether. Large areas of seasonal uplands are now covered in open woodlands or savannas, often dominated by fire-resistant pines in continental Southeast Asia, and by Casuarina junghuhniana or Eucalyptus species in eastern Indonesia. Even larger areas are treeless grasslands. Even in the wettest places, unusual dry periods make the higher-altitude forests susceptible to fires that are started in adjacent open areas. Tree-fern savannas occur in such deforested areas above 2500 m in Papua. On an equatorial mountain, frosts occur every night at the treeline and occasionally down to 2000 m or below, in special situations. The diurnal temperature range is much greater than the annual range, so plants cannot adapt by deciduousness or seasonal dormancy. In the north of the region, in contrast, frosts are a predictable, annual event throughout the montane zone, and winter cold is a significant ecological factor. North of about 20° latitude, both winter-deciduous trees and evergreen conifers from temperate genera start to become an important part of the montane flora. On the highest mountains in northern Myanmar (26°–28°N), the broad-leaved evergreen forest of lower altitudes acquires an increasing admixture of deciduous species and conifers up to around 3000 m, from where coniferous forest dominated by fir (Abies) continues to an irregular treeline at about 4000 m (Kingdon-Ward 1945). Snow

carpets the ground below the fir trees for several weeks every year. In the alpine region above the treeline, a Rhododendron-dominated scrub gradually gives way to a herbaceous turf, with stunted shrubs, below the line of permanent snow.

Wetlands Wetlands can be defined as areas where flooding or saturation of the soil occurs with such frequency and duration that the organisms that live there need special adaptations. In the absence of human impacts, the natural vegetation of most wetland areas in Southeast Asia would be forest. The existence of natural, nonforest wetlands is confirmed, however, by the presence of several species in the regional fauna that require such habitats. The forested wetlands of Southeast Asia are extremely varied, and the classification used here is necessarily both simplified and somewhat arbitrary.

Mangrove Forests Mangrove forests occupy the upper half of the intertidal zone on muddy shores. Compared with other forest types in the region, they have a much simpler structure and much lower floristic diversity, although the mangrove forests of Southeast Asia and northern Australia are more species-rich than those elsewhere in the tropics. Typically there is a clear zonation of species, controlled by the frequency and duration of tidal flooding, the amount of freshwater input, and the characteristics of the substrate. About one third of the world’s mangrove forests are in Southeast Asia, with the largest areas in Sumatra, Kalimantan, and Papua. Huge areas have been lost or severely degraded in the last few decades, by conversion to shrimp or brackish fish ponds, reclamation for agriculture, salt ponds, or urban development, and logging for charcoal, woodchips, and pulp.

Brackish Water Swamp Forests Areas subject to tidal flooding by brackish water support a flora distinct from both the mangrove forest and the freshwater swamp forest. The nipa palm Nypa fruticans is characteristic of such sites, forming pure or mixed stands along the tidal section of rivers as well as covering extensive low-lying areas in estuaries.

Freshwater Swamp Forests Forests subject to flooding by freshwater are extremely varied within the region and are lumped together into one category purely for convenience. Much of this variability reflects differences in the periodicity and duration of flooding. Swamp forests near the coast may be flooded daily or a few days a month when river water is backed

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up by the tides. This ‘freshwater mangrove’, as it has been called, shares many features with the true mangrove forest, including stilt roots and peglike pneumatophores (Corner 1978). Further inland, flooding may be semipermanent, irregular, or seasonal, and vary in depth from a few centimetres to several metres. It is difficult to make generalizations about forests growing in such varied environments, but the flora is generally less diverse than in adjacent dryland forests and there is a tendency for dominance by one or a few tree species. In Papua and, to a lesser extent, Maluku, there are large areas of sago palm, Metroxylon sagu, in pure stands or mixed with other species. Swamp forests are not confined to the everwet parts of the region, but those in seasonally dry areas are easily degraded by cutting and fire. Large areas dominated by more or less pure stands of paperbark (Melaleuca species) are one result of this process. Elsewhere, swamp forests have been replaced by woody thickets or by grassdominated vegetation. In addition, vast areas of freshwater swamp forest have been converted to wet-rice cultivation.

Peat Swamp Forests The freshwater swamp forests described above sometimes have a thin layer of peat on the soil surface, but in peat swamp forests this layer is deeper (0.5 m to more than 10 m) and the surface is above the highest limit of wet-season flooding by mineral-rich river water. The water-table is higher than the surrounding area, so the only external input of water and nutrients is from rainfall. The peat generally consists of partly decomposed woody material in an amorphous semi-liquid matrix. The organic matter content is 90–98 per cent, while both pH and nutrient content are low. Unlike freshwater swamp forest, forest on deep peat is confined to areas with high rainfall and without a long dry season, and is most extensive on the islands of Sumatra, Borneo, and New Guinea, where it blankets huge areas (Rieley and Ahmad-Shah 1996). Smaller areas occur in the Malay Peninsula and Southeast Thailand, and on Mindanao, Sulawesi, Halmahera, and Seram. It is found mostly in the coastal and sub-coastal lowlands, close to sea level, but also extends up river valleys and occurs in isolated basins at higher altitudes. The best-developed peat swamps have a characteristic convex surface, and a sequence of forest types occurs from the margin towards the centre. The outer zone, on the thinnest peat, is similar to dryland forest in structure and floristics, while the successive zones on deeper peat have a progressively lower height, smaller tree girth, higher density of stems, and lower species diversity. In some extreme cases, the vegetation at the

centre of the largest swamps is like an open, savanna woodland (Anderson 1983). Peat swamp forests appear to have a lower diversity and density of wildlife than dryland or freshwater swamp forests, presumably because of the relatively low primary productivity. The less extreme types of peat swamp forest are a very important source of commercial timber, particularly ramin (Gonystylus bancanus). With careful management, it may be possible to harvest this timber on a sustained yield basis, although this does not usually happen at present. Swamp forest on shallow peat can be successfully converted to rice, pineapple, coconut, or sago production, but attempts to convert deep peats have usually failed.

Herbaceous Swamps In Southeast Asia, natural non-forest wetlands seem to be confined, in the lowlands, to areas with seasonal rainfall. In these climates, swamp grasses, sedges, herbs, or ferns cover large areas on alluvial plains, where flooding is too deep, frequent, or prolonged, or the substrate too unstable, for the establishment and growth of trees. The major non-forest wetlands of mainland Southeast Asia were in the lower basins of the major rivers, the Irrawaddy, Chao Phraya, Mekong, and Sông Hóng. Over the last century, these have been almost entirely converted to the cultivation of rice, and now support dense human populations. The animals that were dependent on this habitat have become endangered or are extinct (e.g. Schomburgk’s deer in central Thailand). However, vast areas of swamp grassland still survive in Papua. Herbaceous swamps, dominated by sedges, grasses, or herbs, are also common in montane basins throughout the region and, at least at higher altitudes, some of these are apparently natural.

Ricefields Rice was domesticated and first cultivated in natural wetlands (probably in the Yangtze Valley in China), but the area used for wet rice cultivation has been greatly extended by the development of techniques for water retention and irrigation. Today, rice is by far the most important crop in the region, occupying a total of around 400 000 km2. Some areas have probably been in continuous cultivation for millennia, demonstrating a sustainability not shown by any other farming system in the region. With the exception of the relatively small areas of upland rice, ricefields are flooded for part of the year, making them also the most extensive wetland type in the region. These fields, and the associated dykes, ditches, and ponds, provide an important habitat for the more tolerant wetland birds, mammals, and other vertebrate and invertebrate animals.

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Recent Human Impacts on the Vegetation The vegetation of Southeast Asia was by no means pristine a century ago. Modern humans have been in the region for at least 50 000 years (Hope, Chapter 2 in this volume), and by 1900 large areas of forest had been cleared in the main centres of wet-rice agriculture, such as Java and the Sông Hóng Delta, while much larger areas throughout the region had been modified by centuries or millennia of shifting agriculture and, in seasonal areas, anthropogenic fires. However, the great majority of Southeast Asia (around 80 per cent; Flint 1994) was still under forest of some sort, and human population densities outside the main centres were very low. Even in 1950 Thailand and the Philippines— which today are the countries with least forest cover —still had more than half their forests. Vast tracts of lowland forest in Malaysia and western Indonesia have disappeared since the 1970s, and the current rates of forest loss and degradation in Southeast Asia are higher than in any other region of the tropics (FAO 1997).

Forest Loss The principal cause of deforestation in the region is agricultural expansion. Shifting cultivation is still prevalent in many areas where sufficient forest remains, but the traditional long-rotation systems, which may be sustainable at low population densities and with the application of traditional knowledge, have been largely replaced by unsustainable short rotations or continuous encroachment by pioneer farmers. In the region as a whole, however, most clearance is now for permanent agriculture. This includes small-scale clearance for subsistence crops as well as the vast areas, particularly in Indonesia and Malaysia, converted to plantations of commercial tree crops. The decline in the forest area is considerably greater than the expansion in the cultivated area because a proportion of the forest cleared is replaced by grassland and shrubland after a more or less brief period of unsustainable exploitation. In theory, most or all of this could revert to valuable forest in time, but continued disturbance by fire, cutting, grazing, and repeated attempts at cultivation often prevents forest regeneration, and old secondary forests are rare. Large, continuous tracts of more or less undisturbed forest with an intact flora and fauna are now largely confined to Borneo, Sumatra, Sulawesi, and Papua, with smaller areas in Lao PDR and Myanmar (Bryant, Nielsen, and Tangley 1997). In much of the region, forest now survives, if at all, only as small, more or less degraded fragments.

Forest Degradation The most visible cause of forest degradation in the region is now commercial logging, although subsistence uses of wood (for firewood, charcoal, and small timber) are still very significant in some areas, particularly in continental Southeast Asia. There has been a vast increase in the international timber and pulp trade since 1950, and Southeast Asia has been, and continues to be, the major tropical source. The major suppliers have shifted over time as accessible stocks have been exhausted and governments have attempted to protect what remains. In the year 2000 Malaysia (mostly Sabah and Sarawak) dominated the global trade in tropical logs and sawnwood, while Indonesia was the top producer of plywood. Within Indonesia, the focus of exploitation is moving east, from Sumatra and Kalimantan to Maluku and, especially, Papua. The Philippines, which dominated the tropical timber trade in the 1960s, is now a net importer, as is Thailand. Few countries in the region now allow the export of unprocessed logs, and legal logging has been stopped altogether or severely restricted in some places. However, with no decline in demand, logging bans simply shift sources across borders, and illegal logging is a major problem in many parts of the region. Indeed, it is likely that the majority of timber harvested in the region comes from illegal operations. The adverse affects of logging on both future timber production and biodiversity can be greatly reduced, except at high logging intensities, by the strict implementation of reduced-impact logging guidelines (Johns 1997). These include retention of unlogged areas along streams, vine-cutting before felling, directional felling of trees, and careful planning of the skid trails along which logs are dragged out. At present, however, logging is poorly managed throughout the region. Logging techniques tend to maximize adverse impacts, and forests are often logged again within a few years as smaller trees or different species become worth harvesting. In many areas, most logging activity is illegal, but even licensed timber harvests are ineffectively regulated. Management of most tropical forests for a sustained yield of timber appears to be biologically feasible but has usually failed, in practice, for a variety of economic, social, and political reasons. The most damaging impact of commercial logging operations is often by providing access to farmers, who clear and cultivate along the logging roads. Moreover, as forests are opened up and fragmented by roads, logging, and agricultural encroachment, and logging residues provide a combustible fuel load, they become increasingly susceptible to fires. During the El Niño-associated droughts between September 1997 and May 1998, fires

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started to clear land for agriculture and plantations in western Indonesia spread out of control, burning huge areas of forest and blanketing the region in smoke and haze for several months. Recently logged forests were far more likely to burn than those that had not been logged or had been logged long ago (Siegert et al. 2001). Smaller fires occurred throughout the Indonesian archipelago and elsewhere in the region. These fires have now become an annual event, even in years with normal rainfall. Although the damage caused by recent fires in rainforest regions has been most dramatic, the role of fire as a cause of forest degradation in the seasonal tropics has certainly been underestimated, because the fires in these areas are so regular as to appear a natural, seasonal event. Indeed, it is probable that the majority of the deciduous forest in the region burns in a typical year. Almost all these fires are started deliberately by rural people. Fires destroy the understorey, kill the less firetolerant trees, and reduce regeneration of most woody species, leading to a long-term reduction in structural complexity, biomass, and species diversity.

Present Forest Extent and Distribution Estimates for the present-day forest cover of the region vary considerably between sources, particularly for continental Southeast Asia, where all stages of forest clearance and degradation are sometimes mixed in a fine-grained mosaic. The numbers used here come from the FAO’s assessment of forest resources and refer to forest areas in 1995 and the annual rate of change over the period 1990–5 (FAO 1997; Table 7.2). A new assessment by FAO reports forest areas in 2000 and estimates Table 7.2 Human population, total forest area, percentage forest cover, and annual percentage loss of forest area for the countries of Southeast Asia Country

Populationa 2000 (million)

Forest areab 1995 (km2)

Forest coverb 1995 (%)

Annual lossb 1990–5 (%)

Brunei Cambodia Indonesia Lao PDR Malaysia Myanmar Philippines Singapore Thailand Viet Nam

0.3 13.1 212.1 5.3 21.2 47.7 75.7 4.0 62.8 78.1

4340 98 300 1 097 910 124 350 154 710 271 510 67 660 40 116 300 91 170

82.4 55.7 60.6 53.9 47.1 41.3 22.7 6.6 22.8 28.0

0.6 1.6 1.0 1.2 2.4 1.4 3.5 0.0 2.6 1.4

a b

From UN (2000). From FAO (1997).

changes over the period 1990–2000, but the currently available figures are aggregated in a way that makes them difficult to compare. Moreover, all such estimates suffer greatly from uneven data quality. I have omitted largely urban Singapore and largely forested Brunei from this discussion because of their small sizes. The map of the present-day forest cover (Figure 7.2) is based on the best available information for 1999. In 1995 less than half (47 per cent) of the region was still forested, and forest continued to be lost at the rate of about 1.4 per cent per year. The proportion of the remaining forest that has been degraded by logging or other activities is not accurately known, but it is probable that at least half has been damaged to some extent and that this area is increasing by at least 1 per cent per year. Approximately half of the remaining forest was in Indonesia, which also had the highest percentage of total land area covered by forest (61 per cent) and the lowest annual percentage forest loss (1.0 per cent), although this translates into the highest absolute rate of loss. Within Indonesia, there was a wide range of variation, from the largely deforested islands of Java and Bali (c.3 per cent forest) to the largely forested Papua (77 per cent). Malaysia and Myanmar still had substantial total forest cover (47 and 41 per cent, respectively), although the lowlands have been largely cleared in both countries. Cambodia and Lao PDR were also reported to have a high percentage of forest cover (56 and 54 per cent, respectively), but much of the remaining forest is badly degraded. Viet Nam, Thailand, and the Philippines had the least forest (28, 23, and 23 per cent, respectively), and much of this is in small, more or less degraded patches. Other sources report an even lower forest cover for Viet Nam than the estimate of the FAO. The Philippines and Thailand also had the highest annual rates of forest loss between 1990 and 1995 (3.5 and 2.6 per cent, respectively). Both countries have moved in the last few decades from being major exporters to net importers of wood products. The impact has not been equal across forest types, with deforestation concentrated in the fertile lowland areas and logging most active in the more accessible of the remaining forests. Mangrove forests have also suffered massive clearance and degradation over the last few decades. Montane forests, in contrast, have generally received less damage, particularly at higher altitudes.

Forests and Wildlife As a result of the small extent of natural non-forest vegetation in the region, the native non-coastal flora and fauna are largely adapted to and dependent on forest. For instance, in Malaysia, 78 per cent of the mammal

Fig. 7.2. The forest cover of Southeast Asia (Source: World Conservation Monitoring Centre, 1999)

116 Richard T. Corlett

fauna is dependent on forest and another 12 per cent of species prefer forest (Cranbrook 1988). The proportions are similar for birds and, probably, most other major groups of organisms. Despite this overwhelming dependence on forest, the loss of somewhat more than half the region’s total forest cover would not be of such serious conservation concern if the impacts had been evenly spread. They have not. Instead, entire habitats have been lost from large areas of the region while other areas and other habitats still remain relatively intact. The gradual turnover of species with distance means that even very similar habitats in different parts of the region may have very different biotas and cannot substitute for each other. Thus the two major blocks of tropical rainforest, on the Sunda Shelf and on the island of New Guinea, share very few species of plants and animals, and the survival of large tracts of lowland forest in Papua does not compensate for the rapid deforestation of lowland western Indonesia. Differences in species composition between different vegetation types in the same area can be just as great, although they vary between different groups of organisms. Swamp forests, for instance, have a distinctive flora, but their bird and mammal faunas are similar to those of adjacent dryland forests. In view of the disproportionate loss of lowland forests in most parts of the region, the change in species composition with altitude is of particular significance. For most groups of organisms, species diversity declines with altitude, and with birds, at least, there are species that are restricted to the extreme lowlands, below 150– 300 m (Wells 1997). The progressive loss of lowland species with increasing altitude is only part of the story, however, since in most groups this is at least partly compensated by the addition of species that are restricted to higher altitudes. Even where forest exists, much of it has been more or less degraded by logging and other human impacts short of clearance. Selective logging will inevitably change the composition of both the flora and fauna, but current evidence suggests that most species can survive, at least through a single logging episode ( Johns 1997). Exceptions are likely to include forests on deep peat and other extreme soil types, forests on steep slopes, and forests near the climatic limits of tree growth. Extrapolation from short-term studies to the long-term effects of multiple logging cycles is impossible, particularly if recovery times between logging episodes are too short. The addition of hunting pressure, especially in those parts of the region where firearms are freely available, has eliminated all large vertebrate species from large tracts of accessible forest (Robinson and Bennett 2000).

Forests and People Dense tropical forests are one of the least attractive environments for human subsistence, since only a small proportion of the human-edible biomass is accessible from ground level. Thus the first modern humans to enter the region, 50 000– 60 000 years ago, probably favoured coastal and riverine environments. It has even been suggested that humans cannot live in interior rainforests without access to agricultural food via trade (Bailey et al. 1989). Archaeological evidence, however, suggests that they did so in Peninsular Malaysia, although, in contrast, much of the interior of Borneo seems to have been totally uninhabited until very recently (Bellwood 1997). Present-day non-agricultural hunter-gatherers in the region include the Negritos of Peninsular Malaysia and the Philippines, and other groups, in Borneo and Sumatra, who may have had an agricultural ancestry. There are also several groups of hunter-gatherers in Papua. The much larger numbers of shifting cultivators also have a varying degree of dependence on hunting and gathering activities. In addition to their importance in subsistence, numerous non-timber forest products are traded locally, regionally, and, in a few cases, internationally. Rattans —the stems of spiny, climbing palms— are the most important of these non-timber forest products, with most of the global production coming from Southeast Asia. Increasingly, however, rapid population growth, migration, and urbanization have led to a highly simplified view of the forest, as a source of timber or as land to be cleared for cultivation. While it makes sense for countries to liquidate part of their natural resources in order to finance economic and social development, the proceeds have rarely been invested wisely, and the costs have been borne to a disproportionate extent by the region’s rural poor, who are most likely to depend directly on forests for food, fuel, and basic raw materials. Even where the benefits of maintaining forest cover to protect watersheds and vulnerable slopes, and as a sustainable source of forest products, are recognized, achieving these aims has proved very difficult.

Climate Change No discussion of tropical vegetation can ignore the issues raised by current predictions for global warming, driven largely by rising carbon dioxide levels. Since forests hold 20–50 times as much carbon as non-forest vegetation, tropical deforestation and forest degradation are major sources of anthropogenic carbon emissions. Although globally less important than emissions from fossil fuels, emissions from forest damage almost certainly exceed

Vegetation 117

those from fossil fuels for all countries in Southeast Asia, except Singapore and, possibly, Brunei. For the years 1990–5, it has been estimated that Indonesia was third in the world (after Brazil and Congo) in carbon emissions from deforestation, with Malaysia, Myanmar, and Thailand all in the top ten (Potter 1999). Thus forest protection will have a major impact on total carbon emissions from the region. Deforestation also leads to increased emissions of other greenhouse gases, including methane and nitrous oxide. The converse of the above is that reforestation in tropical countries provides one of the cheapest options for removing carbon dioxide from the atmosphere. Half the dry weight of a tree is carbon, and additional carbon continues to be absorbed as long as a plantation or secondary forest continues to grow. This option is particularly attractive to major carbon emitters in developed countries, since growth rates can be much higher and costs much lower in the tropics. Finally, if the predictions are correct, biologically significant climate change will occur in Southeast Asia over the next century and will provide an additional stress for vegetation and wildlife (Corlett and LaFrankie 1998; Hulme and Viner 1998). Current global climate models suggest both a general increase in temperature of at least 1–2°C and changes in the regional pattern of rainfall. The links between climate and forest ecology are too poorly understood to make realistic predictions, but the increasing fragmentation of the tropical forest will greatly reduce the possibility of species adjusting by migration.

Conservation Southeast Asia, as defined here, includes less than 3 per cent of the world’s land area yet supports approximately 23 per cent of the world’s bird species (2240 species; Inskipp, Lindsey, and Duckworth 1996) and 22 per cent of the world’s non-marine mammal species (880 species; Corbet and Hill 1992; Flannery 1995). Estimates for the flowering plant flora vary, but it is probably in the range of 40 000– 60 000 (Turner 2001), some 15–25 per cent of the estimated global total. Information is less complete for other groups of organisms, but it is likely that the region supports between a fifth and a quarter of the world’s total complement of plant and animal species. Most of these species are found only within the region, and most of those which are more widespread are shared only with South or East Asia, where human population densities and threats to wildlife are at least as great. In practice, the long-term survival of the great majority of

Southeast Asia’s native biota, and thus a large fraction of the global biota, will depend on the protection of their natural habitats within the region. For most species this will mean protecting forest. Protected areas of adequate size need to be created in each sub-region that has a distinct biota and must incorporate all major vegetation types present, including the full altitudinal gradient. The ideal protected area would extend from the coast to the summit of the highest peak, but there are very few places where this is still possible. Although the best forest available should be protected, it is inevitable in most of the region that forests suffering various degrees of disturbance will also have to be included to ensure a large enough area to support species that live naturally at low densities. Moreover, it is unlikely that large enough tracts of commercially valuable forests will be set aside if all logging is prevented, or that local support will be forthcoming if all subsistence exploitation is excluded. A reasonable compromise would be a totally protected core or cores for the most disturbance-sensitive species and habitats, surrounded by a much larger area of buffer forest that is sustainably exploited for commercial or subsistence needs. Both total protection and sustainable exploitation, however, require a level of monitoring and enforcement that has so far been achieved at few sites in the region. In many parts of Southeast Asia, it is now too late to save large, continuous tracts of even badly degraded forests. No lowland forest area in Thailand is large and undisturbed enough to retain all its original bird fauna (Round 1988) and, in most of the Philippines, forest survives, if at all, as disturbed ‘islands’ in a ‘sea’ of agriculture and anthropogenic grassland. While these forest fragments cannot maintain the complete biota, they are by no means useless for conservation (Turner and Corlett 1996). Fragments provide the last refuge for plants and animals that cannot survive in a completely deforested landscape and can potentially be the ‘seeds’ from which continuous forest cover is re-established in future. Techniques for the rehabilitation and restoration of tropical forests are currently being investigated in many parts of the region, but it is too early to say how successful they will be (Elliott et al. 2000). The conservation situation in the region today is changing rapidly. Published figures for the proportions of each country or habitat protected are soon out of date and, in any case, unreliable, since an area that is protected on paper may receive little or no effective protection on the ground. On paper, about 8 per cent of the region is in protected areas of some form, including about 16 per cent of the remaining forest (World

118 Richard T. Corlett

Conservation Monitoring Centre protected areas database). The protection of forest is strongly weighted towards hill slopes and montane areas, with relatively little of the most species-rich lowland forests included. Malaysia, Viet Nam, the Philippines, and Myanmar have less than 10 per cent of their land area protected. The situation in the Philippines is particularly critical because so much of the biota is endemic, sometimes to single islands. Indeed, the Philippines may have the most endangered vertebrate fauna in the world. The unprecedented speed and magnitude of the ecological changes now taking place in the region make it impossible to predict the long-term consequences. Although many species have had their ranges drastically reduced in the last few decades, the number of known, recent extinctions in the region is still very low. The real extinction rate is undoubtedly higher, however, since many other species have not been recorded recently and some have not been seen for decades. Extinctions from less-studied groups, such as most invertebrates, would be impossible to detect. Moreover, even in the best-known groups, many more species may be already committed to future extinction by tiny population sizes or reproduction below the replacement rate. The good news, however, is that the great majority of the region’s native species still survive, however precariously. There may still be time to save most of the region’s biological wealth if action is taken now.

References Anderson, J. A. R. (1983), ‘The Tropical Peat Swamps of Western Malesia’, in A. J. P. Gore (ed.), Mires: Swamp, Bog, Fen and Moor, B: Regional Studies (Amsterdam: Elsevier), 181–99. Bailey, R. C., Head, G., Jenike, M., Owen, B., Rechtman, R., and Zechenter, E. (1989), ‘Hunting and Gathering in Tropical Rain Forest: Is it Possible?’, American Anthropologist, 91: 59–82. Becker, P., Davies, S. J., Moksin, M., Mohd Zamri Hj Ismail, and Putri Maharani Simanjuntak (1999), ‘Leaf Size Distributions of Understorey Plants in Mixed Dipterocarp and Heath Forests of Brunei’, Journal of Tropical Ecology, 15: 123–8. Bellwood, P. S. (1997), Prehistory of the Indo-Malayan Archipelago (Honolulu: University of Hawaii Press). Blasco, F., Whitmore, T. C., and Gers, C. (2000), ‘A Framework for the Worldwide Comparison of Tropical Woody Vegetation Types’, Biological Conservation, 95: 175–89. Bruijnzeel, L. A., and Veneklaas, E. J. (1998), ‘Climatic Conditions and Tropical Montane Forest Productivity: The Fog has not Lifted’, Ecology, 79: 3–9. Bryant, D., Nielsen, D., and Tangley, L. (1997), The Last Frontier Forests: Ecosystems and Economies on the Edge (Washington: World Resources Institute). Champion, H. G. (1936), ‘A Preliminary Survey of the Forest Types of India and Burma’, Indian Forest Records, new ser.: Silviculture, 1: 1–286. Corbet, G. B., and Hill, J. E. (1992), The Mammals of the Indomalayan Region: A Systematic Review (Oxford: Oxford University Press).

Corlett, R. T. (1995), ‘Tropical Secondary Forests’, Progress in Physical Geography, 19: 159–72. —— and LaFrankie, J. V. (1998), ‘Potential Impacts of Climate Change on Tropical Asian Forests through an Influence on Phenology’, Climatic Change, 39: 439–53. Corner, E. J. H. (1978), The Freshwater Swamp-Forest of South Johore and Singapore (Singapore: Botanic Gardens). Cranbrook, Earl of (1988), ‘Mammals: Distribution and Ecology’, in Earl of Cranbook (ed.), Malaysia (Oxford: Pergamon Press), 146–66. Elliott, S., Kerby, J., Blakesley, D., Hardwick, K., Woods, K., and Anusarnsunthorn, V. (2000), Forest Restoration for Wildlife Conservation (Chiang Mai: International Tropical Timber Organization and the Forest Restoration Research Unit, Chiang Mai University). FAO (Food and Agriculture Organization) (1997), State of the World’s Forests (Rome: FAO). Flannery, T. (1995), Mammals of New Guinea (Ithaca, NY: Cornell University Press). Flint, E. P. (1994), ‘Changes in Land Use in South and Southeast Asia from 1880 to 1980: A Data Base Prepared as Part of a Coordinated Research Program on Carbon Fluxes in the Tropics’, Chemosphere, 29: 1015–62. Hulme, M., and Viner, D. (1998), ‘A Climate Change Scenario for the Tropics’, Climatic Change, 39: 145–76. Inskipp, T., Lindsey, N., and Duckworth, W. (1996), An Annotated Checklist of the Birds of the Oriental Region (Sandy, Beds.: Oriental Bird Club). Johns, A. G. (1997), Timber Production and Biodiversity Conservation in Tropical Rain Forests (New York: Cambridge University Press). Kingdon-Ward, F. (1945), ‘A Sketch of the Botany and Geography of North Burma’, Journal of the Bombay Natural History Society, 45: 16–30, 133– 48. Laumonier, Y. (1997), The Vegetation and Physiography of Sumatra (Dordrecht: Kluwer Academic Publisher). Potter, C. S. (1999), ‘Terrestrial Biomass and the Effects of Deforestation on the Global Carbon Cycle’, BioScience, 49: 769–78. Proctor, J., Bruijnzeel, L. A., and Baker, A. J. M. (1999), ‘What Causes the Vegetation Types on Mount Broomfield, a Coastal Tropical Mountain of the Western Philippines?’, Global Ecology and Biogeography, 8: 347–54. Rieley, J. O., and Ahmad-Shah, A. A. (1996), ‘The Vegetation of Tropical Peat Swamp Forests’, in E. Maltby, C. P. Immirzi, and R. J. Safford (eds.), Tropical Lowland Peatlands of Southeast Asia (Gland: IUCN), 55–73. Robinson, J. G., and Bennett, E. L. (2000), Hunting for Sustainability in Tropical Forests (New York: Columbia University Press). Round, P. D. (1988), Resident Forest Birds in Thailand: Their Status and Conservation (Cambridge: International Council for Bird Preservation). Santisuk, T. (1988), An Account of the Vegetation of Northern Thailand (Stuttgart: Franz Steiner Verlag). Siegert, F., Ruecker, G., Hinrichs, A., and Hoffman, A. A. (2001), ‘Increased Damage from Fires in Logged Forests during Droughts Caused by El Niño’, Nature, 414: 437– 40. Stott, P. (1990), ‘Stability and Stress in the Savanna Forests of Mainland Southeast Asia’, Journal of Biogeography, 17: 373– 83. Tanner, E. V. J., Vitousek, P. M., and Cuevas, E. (1998), ‘Experimental Investigation of Nutrient Limitation of Forest Growth on Wet Tropical Mountains’, Ecology, 79: 10–22. Thiollay, J. M. (1995), ‘The Role of Traditional Agroforests in the Conservation of Rain Forest Bird Diversity in Sumatra’, Conservation Biology, 9: 335–53. Turner, I. M. (2001), ‘An Overview of the Plant Diversity of SouthEast Asia’, Asian Journal of Tropical Biology, 4: 1–16.

Vegetation 119 —— and Corlett, R. T. (1996), ‘The Conservation Value of Small, Isolated Fragments of Lowland Tropical Rain Forest’, Trends in Ecology and Evolution, 11: 330–3. UN (United Nations) (2000), World Population Prospects: The 2000 Revision (draft) (New York: Population Division, Department of Economic and Social Affairs, UN). van Steenis, C. G. G. J. (1957), ‘Outline of Vegetation Types in Indonesia and Some Adjacent Regions’, Proceedings of the Pacific Science Congress, 8: 61–97.

Vidal, J. E. (1997), Paysages végétaux et plantes de la péninsule Indochinoise (Paris: Karthala). Wells, D. R. (1997), Birds of the Thai–Malay Peninsula (San Diego: Academic Press). Whitmore, T. C. (1984), Tropical Rain Forests of the Far East, 2nd edn. (Oxford: Clarendon Press). —— (1987), Biogeographical Evolution of the Malay Archipelago (Oxford: Clarendon Press).

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II

Specific Environments

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8

Granitic Terrains C. R. Twidale

Introduction

Sources and Previous Work

Granite underlies substantial areas of Southeast Asia (Figure 8.1). It forms the core of many of the major uplands. Yet exposures are scarce. High rainfall, consistently high temperatures, and the naturally abundant vegetation have together caused the granite to be deeply weathered. Most of the land surface is underlain by a more or less thick mantle of weathered rock or regolith. Only where the regolith has been removed by natural agencies, for example on some hill crests and steep midslopes, in river channels, and in coastal areas, is the bedrock naturally exposed, though road cuttings, quarries, and other artificial excavations provide excellent sections. Anthropogenically induced and accelerated soil erosion have also revealed bedrock morphology in places. The granitic terrains consist essentially of high ridges rising abruptly from the valley floors or adjacent plains. In detail, slopes, river channels, and rocky coasts strewn with granite blocks and boulders are characteristic of the region, and the nature of granite weathering has also influenced the character of the sediment load transported to rivers and coasts.

Though the granites of Southeast Asia are well documented geologically and as sources of tin and other minerals, there are few modern accounts of their geomorphological aspects. Early travellers like Logan (1848) made astute observations relevant to the development of granitic forms, and the officers of the geological surveys of Malaya and, later, of Malaysia have, taking their lead from the first director onwards, noted salient features of the granitic terrains they mapped. These observations and interpretations, taken together with the few specifically geomorphological studies of particular features, and analyses of granitic landforms in other countries, permit the granitic terrains of Southeast Asia to be placed in context.

I should like to thank Dato Hassim and Datin Aloyah Hassim of Petaling Jaya; Emeritus Professor K. T. Joseph, of Shah Alam; officers of the Geological Survey of Malaysia (particularly Mr Chen, Mr Loganathan, Mr Yunus, Mr Ibrahim, and Mr Nanjan), for facilitating field work at various times; also Professors Maung Maung Aye (University of Mawlamyne, Myanmar) and Hiroshi Ikeda (Nara University, Japan), who kindly sent me photographs. I am also grateful to Dr Jennie Bourne (Adelaide), Dr Avijit Gupta (Leeds), and Emeritus Professor Ian Douglas (Manchester) for critical readings of the chapter in draft form and for constructive suggestions.

Occurrences of Granite Granitic rocks are widely distributed in Southeast Asia (Figure 8.1), particularly in the mainland states (Hutchison 1989). Those of the Malay Peninsula were emplaced at various depths: shallow epizonal, deep catazonal, but mostly mesozonal emplacement at 5– 11 km depth. In plan, granites are widely distributed (Gobbett and Tjia 1973; Chinese Geosciences Research Institute 1975; UNESCO 1980). In the Malay Peninsula, granites occupy the cores of major regional anticlines, and many plutons are exposed in the breached crests of such structures. They are commonly associated with ultrabasic bodies which are elongated along strike, and there are many hypabyssal sills and dykes, including prominent quartz reefs, such as that exposed in the Klang Gates Ridge near Kuala Lumpur.

Fig. 8.1. Distribution of granitic rocks in Southeast Asia (Source: after Chinese Geosciences Research Institute 1975)

Granitic Terrains 125

Archaean granites are exposed in the Indosinian Massif. Elsewhere, at least four Phanerozoic phases of emplacement have been postulated: Late Carboniferous, Triassic, Late Triassic–Early Jurassic, and Late Cretaceous, with some suggestion of an additional Devonian event (Haile and Bignell 1971; Jones 1978). Stratigraphic evidence shows that Malaysian granites post-date the Triassic, for strata of that age are affected by mineralization associated with granites, but they are older than undisturbed Tertiary basin sediments. Radiometric ages provide similar data (Haile and Bignell 1971; Hutchison 1973). In west Borneo, large granitic batholiths are exposed, but elsewhere in the Indonesian archipelago the granites occur in small plutons and stocks, as for example in the Sula Spur of Sulawesi. In these insular regions plutonic rocks are relatively scarce and are areally subordinate to youthful volcanic and sedimentary terranes.

Tectonism: Regional Setting The region under review is delineated by the junctions or sutures separating the Pacific Ocean, Indian Ocean (Indo–Australian), and Eurasian Plates (Katili and Reinemund 1984; Hutchison, Chapter 1 in this volume). However, the southerly projection of the Eurasian Plate can be regarded as a separate and distinct tectonic unit, the Sunda Plate, which is moving very slowly northwards (McCaffrey 1996) relative to the rest of the Eurasian Plate. The Indo–Australian Plate is thrusting north beneath the Eurasian and the subsidiary Sunda Plates, and continued (if slow) plate migrations and concomitant deformation accompanies this motion. Late –Middle Tertiary (Miocene) and Lower Pleistocene tectonism are evidenced. But apart from the active plate margins, the crust of the region is stable. In Malaysia, for example, seismic activity and intensity are low, except in Sabah and on the west coast of the peninsula, where the effects of severe Sumatran earthquakes are felt (Kong 1993). Within the continental sectors of the Sunda Plate, the Indosinian Massif or Craton underlies Indochina. It is bordered to the west by a zone of Variscan folding. The Malay Peninsula is a zone of Cretaceous folding, with the Andaman–Java zone of Tertiary orogenesis to the west and south. Though the mainland is comparatively stable, the island region is one of the most tectonically active in the world, and is subject to recurrent earth movements as evidenced by frequent earthquakes and volcanic outbursts. Tectonism is responsible for the altitude of the granite ranges, for the structures into which the granites were emplaced have been uplifted. Regional isostatic response to the erosion of great thicknesses (several kilometres) of host rocks implied by the exposure of

the plutons by deeply incised rivers has additionally ensured that the ranges stand high. Many rivers have incised their beds and produced the present impressive relief amplitude. The en echelon wrench faults that traverse the Malay Peninsula trend NNW–SSE and are components of global patterns. Most of them are sinistral, and of post-Triassic age (Gobbett and Tjia 1973). Large areas are in substantial compression (in West Malaysia mostly east–west and NNW–SSE; see Tjia 1978), resulting in rupture and distinctive fracture patterns at various scales (Alexander and Proctor 1955; Tjia 1973, 1978). Fracture systems and sets determine patterns of weathering and erosion and hence of drainage, so that within a given granite outcrop, shearing and fracture density are crucial (Ahmad 1980). For instance, in Pahang, the Bukit Lanchar Massif stands at about 395 m, whereas the Bukit Lamar area, eroded in granite with a lower fracture density (fewer partings per unit area, and therefore decreased susceptibility to attack by water) stands almost 1000 metres higher. Exposure of the granite masses and their subsequent dissection is due to the work of rivers, and especially to intense erosion during brief periods of high discharge (Leopold, Wolman, and Miller 1964; Douglas 1998). The headwater reaches of rivers near divides, however, are zones of very slow surface erosion (Horton 1945). As in other parts of the world, the river systems of Malaysian granitic terrains develop angular and rectangular patterns related to the orthogonal and rhomboidal fracture patterns of the country rock; which are in turn related to regional tectonics and shear. Slope also has a marked influence on drainage patterns. In the granitic Benom Complex, for instance, the regional pattern is determined by slope and is radial, but in detail the effects of northeast–southwest and northwest–southeast fractures in the country rock are manifest (Ahmad 1979).

Granite Weathering General Remarks The granitic terrains of Southeast Asia are noted, first, for the great thickness of regolith (Figure 8.2); secondly, for the common breakdown of the rock to a granite sand or grus; thirdly, for the common sharp transition between weathered and unweathered rock; fourthly, for the widespread survival of corestones or core-boulders; and fifthly, for the capricious distribution of weathering in detail. Intense and deep chemical alteration of rocks is the norm in stable humid tropical regions. In Malaysia, 40 m deep regoliths are commonplace, and weathering to depths of 65 m have been reported (Ingham and Bradford

126 C. R. Twidale

Fig. 8.2. Regolith exposed in quarry face in upland east of Gopeng, Perak

1960). In Singapore weathered mantles up to 30 m thick are reported, though 10–20 m is more typical (Pitts 1992). The weathering is overwhelmingly chemical or biochemical, for the feldspar reacts with water and undergoes argillation to produce amorphous colloids, oxides, and clays, and mica is hydrated and is eventually altered to clays such as vermiculite, chlorite, and, commonly, kaolinite. Quartz is etched in some profiles and is reduced in amount or absent in others, implying that it has been dissolved (Larsen 1948). Some of the products of weathering, including silica, are evacuated in solution (Larsen 1948; Trendall 1962), though fines may be flushed out during heavy rains (Ruxton 1958). In granitic (and sandstone) terrains, siliceous speleothems (Caldcleugh 1829; Vidal Romani et al. 1998) demonstrate that (amorphous) silica goes into solution and is translocated; and though no such speleothems have yet been reported from the area under discussion, quartz and amorphous silica occur in seams in weathered rock (Alexander 1959). Bearing in mind reported occurrences of stalactites and stalagmites from similar environments (e.g. Rio de Janeiro; Caldcleugh 1829) it may be a matter of finding such speleothems rather than their not being developed in Southeast Asian granitic terrains. Despite evidence and arguments to the contrary concerning silica solution and transport (Davis 1964: 888– 9), given the inputs of organic acids and the abundance and activities of bacteria (Douglas 1978), silica solution is a significant aspect of granite weathering in Southeast Asia. Variations in silica content of rivers and ground-

waters (Livingstone 1963) probably reflect lithological controls rather than prevalent high temperatures. Thai rivers sampled averaged 16 ppm, which is not especially high, for the global median figure for silica in groundwaters is 17 ppm, and for rivers 14 ppm (Davis 1964). Douglas (1978) reached similar conclusions. Though the amounts of silica in solution were not unusually high in the tropical rivers he analysed (including that of the Klang above Puchong, in Selangor), they were nonetheless substantial and imply that considerable volumes of silica are evacuated, almost 6 m3 km−2 yr −1 in the Sungai Gombak, Malaysia (Douglas 1978).

Course of Weathering Analysis of many granitic weathered mantles suggests that water reacts with the rock by a combination of solution, hydration, and hydrolysis. Granite is crystalline and of low permeability but it is typically well jointed (pervious), and weathering is concentrated along, and extends from, fractures in the granite (Logan 1849; Scrivenor 1931; Chhibber 1934). Meteoric waters infiltrate from the surface into the rock so that weathering advances downward into the bedrock (Ruxton and Berry 1957). Thus, the most advanced alteration occurs at the top of the profile which has been longest in contact with moisture. The initial and earlier stages of weathering are represented at the base of the regolith, just above the weathering front or lower limit of significant alteration (Mabbutt 1961). The weathering sequence can be identified by examining the physical and

Granitic Terrains 127

chemical variations in the regolith along the section up from the weathering front. Experience elsewhere suggests that in granite (and in some other rock types) the initial change on contact with water is a physical breakdown into flakes or laminae (Larsen 1948; Twidale 1986). Water readily penetrates laminated granite, and the flakes are next broken into fragments. Even a slight disaggregation causes a dramatic increase in the permeability of granite (Kessler, Insley, and Sligh 1940), and the surface area exposed to water contact after physical breakdown is enormous. Alteration of feldspar and mica, initiated as soon as water penetration occurs, increases apace, leading to the production of clay. Alteration continues until most feldpars have disappeared and the regolith consists of a stiff clay with a few large feldspars and quartz crystals (Twidale 1986). These gradually disappear, and eventually the regolith resulting from the alteration of granite consists entirely of clay. Working in Perlis, Jones (1978) referred to granite weathering producing a thick clayey soil. The clay is commonly a reddish colour due to the presence of iron (haematite). In profiles still containing corestones, clays account for more than 50 per cent of volume, and in general, clay content increases from the weathering front upwards, with intensity of weathering, in the completely decomposed rock (Nossin and Levelt 1967). In Singapore, mica and feldspar are converted to kaolinite (Nossin and Levelt 1967). Quartz gradually goes into solution for, as Alexander (1959) observed, the ‘proportion of quartz grains diminished upwards’ and is commonly absent in higher horizons. She also noted that weathering cuts across quartz crystals, and that some surfaces are rough or pitted (see below). She carried out laboratory experiments on the loss of weight of quartz crystals exposed to the elements and found significant changes after only six weeks (cf. Caillère and Henin 1950). Scrivenor (1931: 137) recorded that feldspar phenocrysts protrude from weathered surfaces, so much so that it is possible to hang a hammer on them; but other, apparently similar, crystals are rotten. Roe (1951) made similar observations in the Fraser’s Hill area, where some feldspar phenocrysts are resistant and protrude up to 12–13 mm, giving a rough, knobbly, or pitted surface (see below), whereas others in the same general area are preferentially rotted. In some instances the alteration is most pronounced at the edges of crystals, allowing the central mass of the crystal to fall out, leaving hollows. As further examples of the unpredictability of weathering, Alexander (1959) noted that in Singapore some quartz crystals crumble on touch, whereas others, close by, remain fresh and cohesive. Also in Singapore, Gupta

et al. (1987) found fresh feldspars which have survived not only weathering but also transportation and deposition to form an alluvium of putative latest Tertiary–earlier Pleistocene age.

Core-Boulders Wherever it is exposed in Southeast Asia, whether in coastal sections or in river channels, granite is seen to be divided into cubic or quadrangular blocks by orthogonal fracture systems. This is obvious in stream channels where the local relief demonstrably reflects fracture spacing. On the coast large blocks tend to be upstanding, and give rise to large residual blocks or to whalebacks and domes, depending on scale (Hilde and Engel 1967; Jones 1978). They stand in marked contrast with the clefts and depressions that are coincident with zones of intense fracturing. These partings are avenues of water infiltration and hence weathering, and, as Logan (1849) and others deduced, the angular blocks were converted to rounded masses because of the preferential weathering of the corners and edges. In attributing the rounded boulders to differential subsurface weathering, Logan followed the deductions of Hassenfratz (1791), Humboldt and Bonpland (1852–3, e.g. 482) and several other eminent geologists of the time (Twidale 1978). Scrivenor (1913: 364–5) was also familiar with such spheroidal or ovoid masses of fresh rock set in a matrix of grus or clay and referred to them as ‘core-boulders’ (Figure 8.3), which term is synonymous with the ‘kernels’ and ‘corestones’ of other authors. Ingham and Bradford (1960) reported a core-boulder almost 8 m diameter in the Kinta Valley. Core-boulders develop in two stages, the first involving subsurface fracture-controlled weathering, the second stripping of the grus and the exposure of the core-boulders: they are etch forms. The surface of each core-boulder is a discrete weathering front. When the weathered granite is washed away, they are exposed as boulders, perhaps the most common of all granite landforms. In Singapore, Alexander (1959: 126) observed ‘core-boulders, the shape of which depends on the disposition of the joints, embedded in overlying weathered material’. Nossin and Levelt (1967) considered that corestones had been ‘isolated by concentric inward weathering from joints’, observed that they increase in size in depth, and identified them as remnants of country rock in situ. In some areas of deep weathering, such boulders provide the only means of identifying bedrock. Thus, in Sabah, in the lowlands east of the Kinabalu Massif, the only outcrops of the granite bodies are large, isolated, residual boulders, like those around Kampong Tambatuhan, near Paranchangan (Collenette 1958).

128 C. R. Twidale

(a)

Fig. 8.3. Core-boulders exposed (a) near Gamencheh, Negeri Sembilan, and (b) near Sungai Batang, Padang (b)

In the Fraser’s Hill area of Selangor, Perak, and Pahang the granite is rotted to depths of up to 60 m, with core-boulders, some with concentric outer shells (onion skin weathering) well developed (Roe 1951). Preservation is a matter of location, not rock type,

and the shape and size of core-boulders a function of fracture geometry and duration of weathering. Core-boulders have been concentrated by the removal of grus and the resultant settling and accumulation of the rounded masses. In addition, Scrivenor (1931: 138)

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records that torrential rains cause flash, local, and shortlived floods powerful enough to wash boulders into valley floors, where they are piled on one another, and beneath and between which streams flow (Scrivenor 1913). Such accumulations of boulders are known as compayrés in France (Twidale 1982: 94, 99), and gugup in Malaysia (Scrivenor 1931: 124). Though widespread, the weathering of granite in Southeast Asia is capricious or inconsistent in its operation and distribution. Several workers (e.g. Ingham and Bradford 1960) commented on the contrast between large masses of fresh granite adjacent to masses of deeply rotted rock. Scrivenor (1931: 137– 8) also observed the rapid transition from rotten to fresh rock, that is, in the development of a weathering front. Roe (1951), however, noted that though there is frequently a sharp contact between boulder and matrix (fresh and rotted rock), elsewhere the transition is gradual. Alexander (1959: 126) recorded that ‘hard rock with water-clear feldspar and unweathered amphibole crystals passes, in a distance of a few millimetres, into an orange or red, more or less sandy clay which still carries the textural pattern of the original rock’. Many core-boulders exposed in artificial excavations are stable for many years, but others crumble quickly. For example, near The Gap, Selangor, a core-boulder 50 cm in diameter was blasted in half during road-building. It rotted completely in ten years (Scrivenor 1931: 136–7). Certainly, though fresh coreboulders are widely preserved, many are rotted in situ, i.e. spheroidal masses of rotten rock, frequently of a colour different from that of the surrounding matrix, are exposed in cuttings, for instance at Brinchang, in the Cameron Highlands. In some instances, as for example in quarries on Karimun Island, western Indonesia, they are identified by arcuate remnants of indurated perimeters of the former corestones. They are referred to by some workers as ‘ghost corestones’.

Mass Movements As Scrivenor (1931) and others noted, the widespread development of a thick regolith with core-boulders presents engineering problems. They are especially acute in granitic terrains. The contained masses of fresh rock (core-boulders) impede excavation. Also, given high and intense rainfall, the regolith tends to be unstable, partly as a result of the saturation of the clayey fraction of the regolith, partly as a result of sapping by streams running along the weathering front. Mass movements of debris are common. Weathering of clays illuviated to, or developed near, the impermeable weathering front provides lubrication (cf. Myers 1977, concerning

the weathering of serpentine and pyroxene to produce smectite), as does water. Clearing of slopes for cultivation, and the steepening of slopes during road construction, or for building purposes, aggravates the problem for it causes unbuttressing and failure of the slope above, resulting in the regolith sliding over the weathering front. For instance, a large landslip occurred in the outskirts of Georgetown, Penang (Malaysia), on the afternoon of 28 November 1999. Streams and springs had been previously noted on the slope, and a large mass of grus and core-boulders slipped downslope, coming to rest close to the base of an apartment block and overwhelming several parked cars (Bourne and Twidale 2000; see also Alexander 1968: 84–5). Such events are commonplace: ‘The scars of landslides can be seen on all the granite ranges’ (Scrivenor 1931: 138). So (1971) reported that in Hong Kong heavy rains in June 1966 resulted in the formation of 559 landslides on steep slopes (most of them granitic), only about half of which were ‘anthropogenic’, i.e. modified by human activity. Heavy rains are certainly a prime cause of mass movements, but a strong case can be made for seismic triggering in some areas (Branner 1896; Simonett 1967). Whatever their causation, mass movements of regolithic materials are a significant agency shaping slopes and transporting detritus to lower levels, occasionally with dire consequences.

Humid Tropical Plains and Uplands Topography and Major Landforms In many areas, granite exposures have been weathered and eroded to plains of low relief, as for instance in west Borneo. Elsewhere, however, in the ancient orogens and cratons of Indochina, in uplands, granite gives rise to hills and ranges several of which stand more than 3000 m above sea level. Examples of granitic peaks include, in the north, Van Bau (2913 m) and Pau Tsi Luing (3076 m), and to the south Ngoc Linh (2598 m) and Chu Tong Sin (2405 m). On the coast, too, granitic (including rhyolitic) hills form prominent headlands such as Mui La (Cap Varelle) and Vung Tau (Cap Saint Jacques) (Naval Intelligence Division 1943). The orogenic belts to the west consist of granite ridges together with remnants of older strata into which the granites were intruded. They extend from Myanmar and Thailand through West Malaysia into Sumatra and Java, and thence into New Guinea. The islands of Sinkep, Bangka, and Belitung are the summits of mostly submerged granitic ranges. Granite ranges rise to heights of more than 3000 m above sea level in Myanmar. In southwestern Thailand, Khao Luang rises to 1786 m. In West Malaysia, Gunung

130 C. R. Twidale

(a)

Fig. 8.4. (a) Sketch of bare rock surface exposed in southern Myanmar (Source: drawn from photograph kindly supplied by Professor Maung Maung Aye)

(b) Bare rock slopes exposed in steep face of valley in south-western outskirts of Georgetown, Pulau Pinang (b)

Ledang, climbed by Wallace (1869), stands at 1276 m, and Tahan (2187 m), Camah, and Batu Putih peaks are also prominent. Some, like Gunung Tera (1530 m) in Kelantan, are conical. These granitic terrains consist of ridges, ranges, and isolated conical hills of all-slopes topography, though the slopes tend to convexity. Though typically inclined at some 30–40°, with a relief amplitude of up to several hundreds of metres, most slopes carry a more or less thick regolith, though bare granite is exposed in near-vertical slopes at many sites. For example, in southern Myanmar bornhardts with sheet structure are well developed in the Zingyaik Range (Figure 8.4a), and Chhibber (1934: 337) records the presence of ‘great curving surfaces of bare rock’ in the granitic terrains of the Tavoy district of

Myanmar. Granite bosses occur in several of the granite uplands. Within the overall all-slopes topography of West Malaysia, steep rock walls are exposed in many places. Logan wrote that ‘Amidst the luxuriant forest that always covers granitic hills and mountains, the explorer suddenly finds himself facing a high perpendicular wall of rock’ (Logan 1848: 102). During his ascent of Gunung Ledang Wallace sought a ‘stone field’ (padang-batu) which, it transpired, was a ‘steep slope of even rock, extending along the mountain side farther than we could see’ (Wallace 1869: 24). In Malaysia, Scrivenor (1931: 139) also recorded the occurrence of bare rock slopes, for example, on Bujong Melaka, and on Gunung Stong, near Kuala Pergau, Kelantan. Steep, bare, and faintly convex-outward rock slopes are also

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prominent in the fault-controlled valley on the eastern side of Pulau Pinang (Figure 8.4b) and on the northern slopes of the granite upland that forms the core of Langkawi group of islands. Outcrops in road cuttings and in coastal cliffs and platforms suggest that they are exposed sheeting planes or sheet structures. In East Malaysia also, many hills and ridges have rounded crests with rock exposures (Wall and Wilford 1966). The well-named Bukit Bald is a domical hill developed in microgranite in Sabah (Lim 1981). Some granite outcrops occur in isolated hills or inselbergs, for instance Gunong Melaka in the Kinta Valley (Ingham and Bradford 1960), and the Kuala Selangor and Jugra, also in Selangor (Scrivenor 1913). There are also literal granitic inselbergs off the coast, e.g. in Pulau Ubin and Frog Island at the eastern end of the Johor Strait (Logan 1849, 1851), Penang (Scrivenor 1931: 6–7, 13), Langkawi (Jones 1978), Perhentian and Redang off Trengganu (MacDonald 1967), and Hon Trung Lon (and the rhyolitic Hon Trung Nho) off south Viet Nam (Hilde and Engel 1967). Pulau Tioman off the east coast of Peninsular Malaysia (Bean 1972) is well known for its prominent granitic pinnacles, the Chula Naga, or Dragon’s Spines (Scrivenor 1931: 141). Whether occurring as ranges or as isolated residuals, the granite hills may meet the adjacent plains and valley floors in a sharp break of slope. Scrivenor (1931: 124) remarked on the abrupt reduction in the gradient of streams at the hill base, but the transition from hill to plain is sharp also where there are no streams. Scrivenor (1931: 122–3) attributed such basal steepening to marine erosion, and this may be valid locally for there is evidence of higher sea levels in the recent past (see Jones 1978: 9; Tjia 1975; Tjia et al. 1977). But Swan (1970: 38; see also 1972) also noted basal steepening of hillslopes at high interior sites and invoked what he termed ‘inward weathering and backwearing of piedmont slopes’, which may imply that such piedmont angles or nicks are due to subsurface scarp-foot weathering and subsequent erosion, and basal sapping (Twidale 1962, 1967). In many coastal areas (e.g. of Kedah) and in major valleys (e.g. the Kinta Valley near Ipoh), the sharp break of slope between hill and plain reflects the lapping of sedimentary deposits against the bases of the residuals. But in many places the perceived abrupt transition is relative, for small alluvial and colluvial fans have accumulated at the base of many slopes (A. Gupta, personal communication, November 2001).

Palaeosurface Remnants Summit bevels are evident at many sites. Their origin varies. Some are roof pendants, which are preserved in many places in the Malaysian Main Range (Hutchison

1973). Gunung Panti, for instance, carries a cap of Cretaceous strata (Swan 1972). Many, however, are remnants of planation surfaces. Plateaux and high plains are preserved on quartzite and sandstone, e.g. Gunung Gagau and Gunung Lesong capped by Late Jurassic–Early Cretaceous rocks in West Malaysia, and on limestone, e.g. on the Langkawi Islands (Tjia 1972). Granitic terrains also include quite extensive high plains and other evidence of former surfaces of low relief. Such features are prominent in the Fraser’s Hill area of West Malaysia where Gunung Semangko (1824 m) is an extensive high plain (Roe 1951). The Cameron Highlands provide another example of a high granitic plain of substantial extent. In Kedah, Courtier (1974) noted summit bevels in the southern part of Gunung Bintang. Some prominent hills such as Gunung Pulai (715 m) stand above the general level of the surface, and are at least as old as the summit high plain, but the ages of the granites and associated uplifts clearly show that the all-slopes topography is essentially a Cenozoic development. Assemblages of blocks and boulders (the skyline tors of King 1958) surmount high plains in several places. Examples of such crestal forms include the tower of Batu Gumbar Orang (939 m) on the Benom Granite outcrop in Pahang (Ahmad 1980), the rocky outcrops of Bukit Chemargong (973 m) and Bukit Panbunga (729 m) in Pahang (Ahmad 1976), and the large bevelled boulder and acuminate pinnacle on Gunung Belumut (1010 m) in Johor (Rajah 1986). These crestal remnants are probably etch forms which projected into the base of the regolith and which have been exposed as a result of the evacuation of the cover.

Models of Development Several models have been proposed in explanation of all-slopes topography. Cotton (1958a: 110; 1958b: 199) regarded topography as ‘mature’ and labelled it ‘feral’ in the sense of ‘unsubdued’ (1958a: 110); which sits strangely with an author renowned for his way with words, for ‘feral’ usually means either wild or untamed, or funereal and gloomy. Notwithstanding, Cotton attributed all slopes with sharp crests to the intersection of opposed slopes shaped, in New Zealand, by a combination of periglacial and fluvial activity (possibly accelerated by anthropogenic activities), and rounded crests to subsequent weathering and wash (see also Birot 1949). Louis (1959) offered an interpretation, admittedly based on the study of a single group of inselbergs at Hua Hin, western Thailand, involving scarp recession. The inselbergs described are not very steep-sided, and are more in the nature of forest-covered isolated ranges.

132 C. R. Twidale

Long spurs more gently inclined than the higher slopes extend from the base of the hill. He attributed the residuals to the regressive erosion of shallow valleys, particularly in fissured rocks—a form of scarp recession. He argued that the residuals are monadnocks de position, or remnants of circumdenudation. Some elements of these constructions can be incorporated in an interpretation of the all-slopes topography characteristic of the Southeast Asian lower elevation granitic terrains which involves the development of Richter slopes (straight or rectilinear slopes with a detrital veneer of essentially uniform thickness covering the underlying bedrock slope) and takes account of conditions prevailing in a humid tropical environment. The components of all-slopes topography are formed as a result of the progressive upslope recession of the bluff and the spread of detritus over the resultant inclined slopes (Fisher 1866). Faceted slopes evolve on welljointed granite. Thus it can be postulated that with continued weathering and erosion, and assuming free evacuation of detritus from the base of the slope, the slope will evolve in such a way that the bluff is reduced in size and retreats upslope in time. The so-called debris slope, in reality a Richter slope, or bedrock slope with a thin veneer of detritus, extends upslope and eventually occupies most of the incline, merging at the upslope extremity with the crest rounded by weathering but with bare rock (commonly blocks and boulders) exposed there. In this way an all-slopes topography develops. In the humid tropics of Southeast Asia, however, the detrital or regolithic cover is commonly a few tens of metres thick. Alternatively, the field evidence concerning the character of the bedrock forms can be taken into account and used as the basis of a model of landscape evolution. Both in mainland ranges and in insular settings the basic granite form appears to be the dome-shaped (or domical) hill or bornhardt, many of which are in other places structurally determined and of subsurface or etch origin (Falconer 1911; Twidale 1982; Vidal Romani and Twidale 1998) and which in Southeast Asia appear to be of similar derivation. Whether in upland complexes or isolated hills, the granite masses located at shallow depth beneath the regolith appear to be domical. The steep, smooth, rocky slopes found throughout the region are convex-outward, and exposures in coastal sections and artificial excavations show that the fresh bedrock is subdivided by sheet fractures. Though widely perceived as ‘offloading joints’ (Gilbert 1904; Oen 1965; Selby 1970), field and laboratory evidence suggests that some such fractures, at least, are neither due to offloading nor are they joints, but are shear planes associated with

crustal compression (Vidal Romani et al. 1995; Twidale et al. 1996). Both the abundant core-boulders occurring within the regolith and the skyline or crestal blocks and boulders suggest that the outer layer or layers of the bedrock have been incompletely weathered. In monsoonal areas regions such as northern Australia and Hong Kong which are characterized by seasonal heavy rains and high temperatures, block- and boulder-strewn hills called nubbins or knolls are typical of granitic terrains (Twidale 1981). They are two-stage forms due to, first, aggressive water-related subsurface weathering which results in the disintegration of the outer sheet structures of the domical hills, and, secondly, the washing away of the weathered detritus or grus to reveal blocks and boulders covering an intact domical core. In Southeast Asian granitic areas, rapid and intense weathering has converted domes into block- and boulder-strewn nubbins with convex-upward slopes, the development of which has most commonly stopped at stage one, with the regolith still essentially in situ (Figure 8.5). Comparable elongate granite ridges with numerous core-boulders only partly exposed can be seen, for example, in the Ca Na area of southern Viet Nam (Tjia, Majid, and Mohamad 1998). In many parts of Southeast Asia, block- and

Fig. 8.5. Slope development in the context of bornhardt–nubbin model: (a) stages in valley deepening and regolith development and (b) end result

Granitic Terrains 133

(a)

Fig. 8.6. (a) Boulder-strewn surface cleared for plantation agriculture near Tampin, in Negeri Sembilan (b) Boulder-strewn hill in ranges east of San Diego, southern California (b)

boulder-strewn slopes are exposed following clearance and accelerated soil erosion (Figure 8.6a). Similar features are reported from other regions, as for instance in the Peninsular Ranges of southern California (Figure 8.6b) inland from San Diego ( Jahns 1954),

and also in West Africa. Indeed the mechanism outlined above was described by Falconer in his account of what he termed kopjes, but what are here referred to as nubbins, within the walls of Kano, in Hausaland, northern Nigeria:

134 C. R. Twidale Kogon Dutsi, the larger of the two flat-topped hills of diorite, although deeply decomposed, still preserves in its lower part detached boulders or cores of unweathered rock. If the subsequent erosion had continued until the weathered material had been entirely removed, the flattened hill would have been replaced by a typical kopje of loose boulders resting upon a smooth and rounded surface of rock below. (Falconer 1911: 247)

Minor Landforms Several minor forms well and widely developed on granite surfaces in other parts of the world are also developed in Southeast Asia, but others are either uncommon or absent. Several of those that have been recognized are in other areas known to be of etch or two-stage origin (Twidale and Bourne 1975, 1976; Twidale 1982), corroborating the suggestion that the host hills, whether bornhardts or nubbins, originated at the weathering front; that they are incipient etch forms. The evidence from Southeast Asia is equivocal.

Flutings As Logan noted, in Malaysia steeply inclined rock surfaces are frequently ‘indented . . . by vertical grooves’ (Logan 1848: 102), and such flutings, grooves, karren, or silikatrillen are surely the most obvious of minor forms developed, preserved, exposed, and reported developed on granite surfaces in the humid tropics (Figure 8.7a). (a)

Fig. 8.7. Flutings (a) on boulder near Tampin, Negeri Sembilan, (b) on boulder exposed on southern littoral of Pulau Ubin, Singapore in the Strait of Johor (b)

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In Sarawak, they are up to 2 m deep (Wall and Wilford 1966). Logan (1848, 1849, 1851) examined those exposed on the shore of Pulau Ubin (Figure 8.7b) and nearby islands, where such flutings are of similar depth. He observed that some extended beneath the surface of the soil (which is grus in situ) and concluded that they were initiated beneath the land surface, at the soil–rock interface or weathering front. Flutings developed on core-boulders exposed as a result of soil erosion corroborate this interpretation. On the other hand, Alexander (1959) could not find flutings on covered or buried surfaces in Singapore, and inferred that they develop on exposed surfaces. Tschang (1961, 1962) also concluded that flutings developed in the Tampin area had evolved on exposed surfaces, as did Bean (1972), working on Pulau Tioman, off the east coast of Pahang, Malaysia. Both explanations are correct. Flutings have evolved both on exposed and on covered surfaces. They are present on surfaces which have been freshly exposed either through natural erosion or by excavation, but have also developed, albeit with extraordinary rapidity, on recently exposed steeply inclined faces (Lageat, Sellier, and Twidale 1994). On exposed surfaces run-off in the form of wash develops turbulence on the rough granite surface and divides into subparallel linear flows, causing grooves or flutings. Grus and soil accumulated in the floors of gutters and basins retain moisture and lichens, and mosses colonize such moist sites, and both detritus and biota could contribute to the weathering and hence the enlargment of the bedrock form (Alexander 1959; Bean 1972). Flutings are on slopes that are too steep to retain soil and moisture. Flutings which extend to the very crest of the host block or boulder, such as those described by Wall and Wilford (1966) from Sarawak, may have originated beneath the land surface with the crest close to or at the surface, so that infiltration began at the top of the host block, or there has been simultaneous fluting and reduction of the host mass by subsurface weathering so that the grooves were intercepted by the contracting perimeter.

Rock Basins Rock basins are characteristic of granite outcrops the world over, and Southeast Asia is no exception. They have been referred to as ‘water eyes’ (Alexander 1959; Tschang 1961, 1962), but these are lenticular in plan and develop along fractures whereas most basins take the form of shallow, flat-floored pans. Evidence from other places shows that they were probably initiated by moisture-related exploitation of weaknesses such as

fractures, fracture intersections, or concentrations of susceptible minerals, such as mica, in the country rock. This may have taken place on exposed surfaces, but in some instances, at any rate, has occurred at the weathering front, at the base of the regolith.

Pitting As previously mentioned, pitting (rough surfaces due to the differential weathering of feldspar and quartz at the crystal scale) is reported from several sites in West Malaysia (Roe 1951; Twidale and Bourne 1976) as a result of accelerated soil erosion consequent on land clearance. Under the prevailing lowland climatic conditions it may not persist long enough to become commonplace.

Sinkholes Crowther (1984) described hilltop depressions up to 40 m diameter from the summit of Gunung Layang Layang. Crowther attributed them to differential rates of lowering as between soil-filled depressions, susceptible areas (e.g. fractures) and the ‘normal’ surface. They are more likely due to piping in the regolith like that which has caused depressions in other humid tropical or monsoonal lands (Patz 1965; Feiniger 1969; Trescases 1975; Twidale 1987; Mendonça et al. 1993).

Other, Less Common, Minor Forms Teh (1995) described ‘cerebral’ (presumably brainlike) patterns from a coastal site in West Malaysia. It is what is more generally termed alveolar weathering, or it may be similar to the curious, apparently solutional, wavy patterns of low (1–2 cm high), rounded ridges and intervening swales or linear depressions reported developed on the undersides of diorite blocks in Sabah (Wilford 1966), on granite at Boulder Rock, southeast of Perth, Western Australia, and on dolerite near Umtata, in the Transkei. Flared slopes are not obviously widely developed, though this may be a matter of non-exposure rather than non-development, for blocks and boulders with concave and fluted sidewalls are developed in the Tampin area as a result of soil erosion consequent on vegetation clearance preparatory to plantation development (Figure 8.8). Tafoni and honeycomb (alveolar) weathering due to haloclasty (salt crystallization) are not known inland, probably because salt is flushed through the system by the meteoric waters and does not crystallize. They are, however, found on the coast (Berry and Ruxton 1957; Twidale 1982: 280 ff.; Teh 1995). Granite caves or

136 C. R. Twidale

Fig. 8.8. Flared and fluted side-wall of boulder exposed by accelerated soil erosion near Tampin, in Negeri Sembilan

shelters are fortuitous gaps between piles of blocks and boulders (Scrivenor 1931: 137) or they may be sheet fractures widened by weathering (Kastning 1976; Ikeda 1994). Despite rock stress and the shear dislocations, familiar neotectonic minor forms are rare. Split rocks are present (Tschang 1962) but other tectonic forms are not in evidence. For example, Ikeda (1996) reported that a 7.2 magnitude earthquake struck the Kobe– Nara region of central Honshu on 17 January 1995. He reported marked resultant landscape and landform changes, including landslides, dislodged and split blocks, and squeezed ( jumping) boulders on upper slopes, which are less confined and buttressed than the rocks beneath valleys. Parts of the Australian cratons which are more stable than most of West Malaysia have assemblages of neotectonic features (Twidale and Sved 1978; Twidale and Bourne 2000). It may be that in Southeast Asia similar tectonic forms, as well as Atents and polygonal cracking, are developed in bedrock but remain concealed beneath the all but ubiquitous weathered mantle.

The Climatic Factor The granitic terrains of Southeast Asia basically simulate those of other regions, but the bedrock forms are in most areas hidden beneath a regolithic blanket. Only in a few sites, and those of comparatively limited extent, can the bedrock forms be observed. They suggest that

bornhardts are the basic form and that nubbins and coreboulders have developed from this basic form by deep, and probably rapid, chemical and biochemical weathering. This situation adds to the interest of Kinabalu, a lowlatitude high-altitude granitic massif devoid of vegetation, which, though modified by glacial and nival action, affords an opportunity for comparison and contrast with the true humid tropical terrains.

The Kinabalu Massif, Sabah, East Malaysia Gunung Kinabalu lies within 6° of the Equator but rises to a height of 4101 m, in comparison with other regional peaks such as Semeru, Java (3676 m), Apo, Philippines (2954 m), and Gunung Tahan, West Malaysia (2184 m). At Paka Cave at the base of the massif and at 2984 m a minimum of 5.5°C has been recorded. Low’s Peak, the highest point on the Kinabalu Massif, is below the snowline, but ice forms at times during the nights. The Kinabalu Massif is developed on an outcrop of Late Tertiary granodiorite about 435 km2 in extent and ringed by Late Cretaceous ultrabasic rock ( Jacobson 1970). Why Kinabalu stands in isolation so much higher than the surrounding areas is controversial. Collenette (1958) identified faults around part of the Massif and suggested that it is a horst block. Fitch (1963), on the other hand, accepted that upfaulting is involved, but suggested that in addition, and given the compressive character and active tectonism of the region, the granodioritic mass has risen bodily through a lubricating ultrabasic screen. Wilford (1967) favoured

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this mechanism and correlated the summit plateau of the Kinabalu Massif with the adjacent but lower rolling planation surfaces. He attributed the topographic contrast and abrupt transition between the two to faulting. Tjia (1973) construed the orientation of major fractures, which conform to regional patterns, as indicating some tectonic control (Collenette 1958). The outlines of the massif not only are in some measure determined by faults, but within the upland fractures have been exploited by weathering and erosion to produce cliffs and clefts. In plan form, Kinabalu is shaped like a J with several prominent peaks in addition to Low’s Peak, St John’s (4097 m), and Queen Alexandra (4004 m). At the toe of the J and beyond the 900 m deep Low’s Gully, near the base of the vertical stem of the J, stand King Edward Peak (4086 m) and King George Peak (4068 m). Discounting the peaks and pinnacles, the mountain stands even-topped at 3810–3962 m above sea level. The sedimentary terrains of the surrounding high plains are forested. The ultrabasic rocks exposed around the base of the massif between Kambaranga (2146 m) and Paka Cave (about 2957 m) are also wooded. But above Paka, granodiorite is exposed, bare except for scattered shrubs in crevices. The flanks of the massif are steep, and on the crest are spectacular steep-sided peaks and pinnacles (Figure 8.9; Lee 1996; Ikeda 1999). There is some pitting and also alveolar weathering, but common minor granite forms like basins and gutters are absent. The rock surfaces, whether gently inclined or steep (or even overhanging, as on the western slope of Low’s Peak), are characterized by spall plates. The summit and higher flanks of Kinabalu have been glaciated, with an ice cap of some 5 km2 (Koopmans and Stauffer 1967; Jacobson 1970; Lee 1996). A valley glacier occupied Low’s Gully, and several minor cirques were developed. Glacial features are preserved above 3658 m, though extending to 3200 m in Low’s Gully. The cliffs are 1500 m high on the west side of the massif. Evidence of glaciation includes striations and grooves, polished and plucked surfaces, and friction cracks (Tjia 1973). Periglacial or nival features including solifluction flows are well represented. Some of the lower and smaller hollows may be due to nivation and snow-patch sapping. The concave western slope of Low’s Peak and associated spall plates can be interpreted as due to pressure release (Gilbert 1904), though the origin of the concavity then calls for explanation. Alternatively it could be an arcuate plane of fracture (shearing, faulting) related to compressive stress or, and especially given Fitch’s

(1963) suggested uplift mechanism, intrusive strain. The spalling which is well developed on these upland surfaces could be construed as sheet structure due to tectonic compression, the geometry of the fractures reflecting planes of least principal stress aligned parallel to the surface (Merrill 1897; Dale 1923; Twidale et al. 1996). The thin spalls could be pseudobedding, and an expression of cold climates (freeze – thaw action). Or the concavity could be a flared slope associated with, and formed beneath, the regolith associated with a palaeosurface and exposed as a result of glacial scour and/or nival transport. The Kinabalu Massif is a manifestation of continued and spectacular tectonism. Its morphology contrasts with the granitic terrains of lower areas in the humid tropics of Southeast Asia but demonstrates the presence and significance of fractures in determining bedrock morphology. On it are preserved remnants of a probable planation surface as well as various minor forms which can be interpreted as of etch character.

Conclusion The foregoing observations can be used as the basis of various interpretations of the granitic terrains of the humid tropics. Those of the Malay Peninsula are the best documented, and can reasonably be taken as indicative of the evolution of similar terrains found in other parts of Southeast Asia. Critical evidence includes first, the bevels and plateaux which point to the development of a planation surface of low relief prior to the latest major period of orogenesis and uplift and stream dissection, probably in the latest Cretaceous or Early Palaeogene. This suggests that the contemporary landscapes of the region have developed during the last 60 Ma or so. Secondly, the impacts of structure, including tectonics, are crucial. This is most obvious in the Kinabalu Massif where the country rock is widely exposed, but on the available evidence applies elsewhere also, for only tectonic uplift can explain the high altitude of, and the relief amplitude developed within, the mainland ranges. Fractures have determined the location and pattern of master streams and of tributary systems, and hence the framework, as well as many local aspects, of the topography and landform assemblages, especially the ubiquitous core-boulders. Thirdly, the thick regolith underlying the flanks of the all-slopes terrains suggests that weathering outpaces erosion, which is inhibited by the dense vegetation cover. If erosion had outpaced weathering, the core-boulders that are widely developed would surely in places form stony carapaces. Slopes are eroded, predominantly by

138 C. R. Twidale

(a)

(b) Fig. 8.9. (a) The peak of Kinabalu (b) The Donkey’s Ears and spall plates near the crest of Kinabalu (Photos: Professor H. Ikeda)

rivers and streams but with substantial impacts by mass movements of debris in landslides, earthflows, etc. Such changes can be spectacular and swift. Yet, overall, slopes appear to be stable and to be affected by only slow erosional changes. The widespread development of regolith suggests the likelihood of etch development, i.e. the development of a bedrock topography and morphology at the base of the regolith, and subsequent stripping of the weathered mantle and exposure of the weathering front by rivers and by mass movements. Many bedrock forms may have been initiated in the subsurface by moisture attack or by tectonism at the base of the regolith. There are indications of their presence in the granitic terrains of Southeast Asia beneath the regolith in coastal exposures and where accelerated soil erosion has exposed the country rock; but for the most part they remain hidden. In October 1854 Alfred Russell Wallace ascended Gunung Ledang (Mount Ophir), a granite peak located inland from Malacca (now Melaka), in what is now West Malaysia. He remarked that he ‘would ever look back with pleasure’ (Wallace 1869: 26) on this, his introduction to mountain scenery in the eastern tropics, but he expressed disappointment with the monotonous views of the forested granitic ranges. In one sense Wallace was correct, for most humid tropical granitic terrains are similar, with all-slopes dominant. Outcrops are scarce, making it difficult to ascertain the relationship between bedrock and surface. Yet, the landscapes he observed, and others like it, are susceptible to analysis and rational, albeit speculative, interpretation, and may well have congeners in other lands.

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9

Volcanic Islands Herman Th. Verstappen

The Distribution of Volcanic Islands and Plate Tectonics Volcanism is of widespread occurrence in the tectonically active zones of Southeast Asia. It is a dominant feature in many (particularly smaller) islands where other landform types are absent or scarce. The geographic distribution, major landform types, exogenous and endogenous processes, resources, and hazards of southeast Asian volcanic environments are discussed, first in general terms, and thereafter by using the examples of two typical volcanic islands, Bali and Lombok (Indonesia), which also illustrate the interaction between tectonism and volcanism in this part of the world. The distribution pattern of volcanism in Southeast Asia is related to plate tectonics, as discussed in Chapter 1. Three major plates dominate the region: the Eurasian, Indo-Australian, and Pacific, each of which is composed of several sub-plates. They meet at a triple point situated south of the Bird’s Head of Papua. Volcanism develops where, at some distance from the deep sea trenches that mark subduction zones, the subducting material melts and the magma rises to the surface. Volcanic geanticlinal belts, known as volcanic arcs and stretching parallel to the subduction zones, are thus formed. The arcs are often affected by transcurrent or compartmental faulting, and their roofs may collapse in places. The activity of individual volcanoes comes to an end when the magma chambers concerned are emptied or become inactive otherwise. Volcanism becomes extinct in (part of ) a volcanic arc when subduction abates. It may shift in position with changes in the configurations of the related subduction zone and plates. The plates, subduction zones, and the location of the volcanoes in Southeast Asia are shown in Figure 1.1.

All volcanoes discussed in this chapter are Quaternary volcanoes in the sense that the oldest and most eroded ones ended their activity in the Lower Quaternary. The volcanism is of the intermediate andesite –basaltic Circum-Pacific suite, but locally more acidic rocks (rhyolites, dacites, etc.) occur. Neogene volcanic materials, intercalated with marine strata, are common, particularly in the flanks of the volcanic arcs of the region. Volcanic rocks, dating from Cretaceous and older geological periods and related to Pre-Tertiary subduction patterns, occur in Peninsular Malaysia, Borneo, and other areas outside the present arcs. The landforms of these areas are governed primarily not by volcanism but by denudational processes, and therefore have not been discussed. A long and active volcanic arc that occurs in southern Indonesia is related to and runs parallel to the subduction zone of the northward-moving Indo-Australian Plate. It stretches from the northernmost tip of Sumatra in a southeasterly direction towards the Sunda Strait, and thereafter trends eastward in Java and the Nusa Tenggara Islands situated further east. Volcanism of the arc is extinct between the islands of Alor and Wetar, where the island of Timor, an outpost of the Australian continental part of the Indo-Australian Plate, approaches it from the south. However, a belt of active, mostly submarine volcanoes is developing further to the north. The arc thereafter loops counter-clockwise around the Banda Sea under the influence of the southernmost part of the Pacific Plate system, which forms a westwardmoving transcurrent belt south of the Caroline and Philippine Sea Plates, dragging along northwestern New Guinea, the Sula archipelago, and several other islands. The submarine volcanoes mentioned above are situated where the western end of this belt and the Indo-Australian Plate converge.

Volcanic Islands 143

Two curved volcanic arcs occur in northeastern Indonesia at either side of the narrow, north–southoriented, North Moluccas Plate, which is subducting vertically between the Pacific Plate in the east and the Celebes Sea Plate, forming part of the Southeast Asian Plate in the west. The volcanic arc situated to the east is short and passes over Halmahera and adjacent islands. It is convex towards the west. The other volcanic arc passes over the Minahasa district in the northernmost tip of Sulawesi and is convex towards the east. It continues northward towards the volcanic Sangihe archipelago. The volcanic arc of the Minahasa is linked with the arc passing over northern and southwestern Sulawesi, where volcanism, however, in the absence of active subduction, is extinct. The Una-Una volcano, situated in Gorontalo Bay, has a somewhat anomalous position outside the volcanic arc. It is, in my view, probably related to the transcurrent belt mentioned above (Verstappen 2000). As the North Moluccas Plate tapers out in the north, the Pacific and Eurasian Plates meet in the so-called Philippine Mobile Belt (Aurelio 1989; Javelosa 1994). The (Pacific) Philippine Sea sub-plate subducts at the Philippine Trench to the east and the Celebes Sea, Sulu Sea, and South China Sea sub-plates at the Negros and Manila Trenches in the west. Active volcanism accompanies the process from Mindanao and Jolo in the south via Negros and Leyte to Luzon and the Babuyan Islands in the north. Violent eruptions are on record in the mobile belt, even of volcanoes previously considered extinct, such as Pinatubo, situated east of the subducting South China Sea Plate. The plate contact continues towards Taiwan, where active volcanoes, however, are absent. There are about 90 active volcanoes in Southeast Asia of which eruptions since 1600 are on record. Most of these (71) are in Indonesia; the remaining 19 occur in the Philippines. In addition, 45 volcanoes are reportedly in the fumarolic stage (34 in Indonesia and 11 in the Philippines), while 27 solfatara fields not associated to a particular volcano also exist, almost exclusively (24) in Indonesia (Neumann van Padang 1951, 1953; Kusumadinata 1979). The highest volcano in the region is Mount Kerinci (3800 m) in central Sumatra. Large islands are usually crowned by several volcanoes and often also display non-volcanic landforms. Certain smaller islands, such as Lombok in southern Indonesia, are dominated by a single volcanic body that, with its fluvio-volcanic slopes and adjacent lowlands, makes up most of the island. A number of small and low volcanic islands are only the emerging tops of tall and largely submarine volcanoes.

Major Types of Volcanic Landforms in Southeast Asia Andesite –basaltic stratovolcanoes are among the most prominent volcanic features of Southeast Asia. Their slender cones dominate the landscape of the volcanic arcs and are characterized by distinctly concave slopes. In detail, however, their forms are much more complex owing to mid-slope lava outflows, the formation of lateral eruption points, and spatial differentiation of slope processes. Central eruptions are the commonest among stratovolcanoes, although truly monoconic stratovolcanoes are rare. Many of them are bi-cone because of minor shifts of the centre of activity in the top area. Some stratovolcanoes are located on a fault and show fissure characteristics that influence their geomorphology. Faulting may also result in aligned eruption points of various types ranging from stratovolcanoes to explosion craters, lava domes, etc. Transcurrent faults, more or less parallel to the arcs, compartmental faults perpendicular to them, and faults in diagonal directions have located and developed many stratovolcanoes in the region. Constructive volcanic processes are dominant in the development of stratovolcanoes. Their destruction occurs at a later stage, either suddenly on the occasion of a paroxysmal eruption or gradually when the volcanic activity abates and denudational processes take over (Verstappen 1963, 1964). Calderas are the second main type of volcanic features. Unlike stratovolcanoes built up gradually by constructive (explosive and effusive) processes, calderas basically result from destructive events. They testify the occurrence of paroxysmal Plinian eruptions in the past that were accompanied by collapse of the roof of the magma chamber and the major part of the volcanic body, and/ or blow-out of massive amounts of volcanic material that previously formed part of the cone together with new volcanoclastics. Calderas are usually circular or oblong features, measuring 2–20 kilometres across. Consistent with the petrographic characteristics of the intermediate andesite –basaltic Circum-Pacific suite the calderas of Southeast Asia are all of the siliceous type. Calderas caused by the outflow of basaltic lava, known from other parts of the world, do not occur in Southeast Asia. The smaller, usually circular, calderas are of the crater subtype where blow-out was a major cause. The larger ones frequently have irregular shapes and are of the depression subtype where collapse was dominant. Calderas are almost as common as stratovolcanoes in the region. Their formation is accompanied by the ejection of large quantities of pumice. The largest caldera formation in historical times, worldwide, is the paroxysmal

144 Herman Th. Verstappen

eruption of the Tambora volcano in the island of Sumba, Indonesia, in 1815. It measures 6 km across. The volcano lost 1400 m in height on that occasion and 60 000 people were killed, the majority from the widespread starvation that followed owing to destruction of paddy fields. Another ill-famed case of recent caldera formation is the Krakatau eruption of 1883 in the Sunda Strait. The accompanying tsunamis killed about 36 000 people on the adjacent shores of Java and Sumatra (Stehn 1929; Simkin and Fiske 1983; LIPI 1985). No historical caldera is known from the Philippines, but the Taal caldera, Luzon, recorded a famous prehistoric example followed by recent volcanic activities at intervals. Tectonic depressions situated in the volcanic arcs that have been the scene of volcanic eruptions along the faults that bound them are a third major type of volcanic landforms. The eruptions are essentially of the fissure type, and range from steam eruptions to massive tuff and ignimbrite outbursts. Steam explosions result when groundwater penetrates downward along a fault and is heated to boiling point in the process. The best example in the region is the Suoh pull-apart basin, southern Sumatra, situated in the Semangko transcurrent fault zone, which stretches from north to south in the Barisan Mountains. Much more violent ignimbritic fissure eruptions may occur where major faults reach down to the roof of a magma chamber. The extensive ignimbrite plateaux so formed are extremely striking landforms. The earlier existing volcanic and other types of relief are largely buried by the ignimbrites that are hundreds of metres thick. Almost vertical slopes are maintained in the ignimbrites, and a plateau landscape, dissected by deep and extremely narrow valleys, results after erosion. The best example is the Lake Toba area in northern Sumatra, where extensive ignimbrite deposits originated from a graben parallel to and situated east of the Semangko transcurrent fault mentioned above. Van Bemmelen (1939, 1949) held the view that rising magma was the primary cause of the phenomenon and resulted in a swelling or ‘tumour’, the roof of which subsequently collapsed during the ignimbritic paroxysm. Because even the oldest ignimbrite flows have, in part, entered the deep valley of the Asahan River, which drains Lake Toba, it is evident that the Toba graben existed and was drained by the Asahan River before the ignimbrite eruptions began (Verstappen 1961, 1973). There is no proof for a pre-existing ‘tumour’, although neotectonic movements of volcanic origin do occur. The role of faulting was certainly more important as a primary cause of the events than van Bemmelen assumed. The widespread occurrence of Neogene pumaceous tuff deposits in the Lampung area, southern Sumatra, and

in northern Banten, west Java, are presumably related to the collapse of the Sunda Strait along a number of mostly diagonal faults. Volcanic and tectonic activity in the area continues even at present, but the existence of a former ‘tumour’ cannot be demonstrated. Pull-apart phenomena related to transcurrent faulting in Sumatra may be a leading factor particularly in the broadest southwestern parts of the strait. The Quaternary evolution of a volcanic area in Banten, bordering on the Sunda Strait, is elaborated upon elsewhere in this chapter on volcanic islands. It exemplifies the continuing tectonic and volcanic evolution of the Sunda Strait and its surroundings. In addition to the three main categories of volcanic landforms mentioned above, a great diversity of smaller volcanic features also occurs. This comprises mafic effusions such as basalt plateaux and shield volcanoes, as well as more acid rhyolitic or dacitic lava domes and plugs. The latter may come into existence very quickly, even overnight, such as the one that in 1898 unexpectedly rose in a coffee estate near Mount Lamongan, eastern Java. A variety of forms resulting from explosive eruptions likewise exists: explosion craters (often containing lakes), tuff ring craters, and ash and cinder cones.

Geomorphic Processes and the Development of Volcanic Slopes The stratovolcanoes of Southeast Asia, and the calderas that have developed from them, resulted from the combined activity of explosive and effusive eruptions. There is, however, a marked dominance of loose, tuffaceous material over lavas in many cases. Lava plugs or tholoids may form at the top of a volcano or rise from a central crater, where short flows of comparatively viscous lava may form short, convex flows. The less viscous basaltic and andesite –basaltic lava flows usually penetrate the volcanic body and emerge in the lower part of the slopes. This has a striking effect on their configuration. They become more irregular in outline and elevated in position as a result of these effusive activities reflected in the contour patterns. The aerial photograph of Figure 9.1 shows the northeast slopes of the Ciremai volcano in western Java. The flows emerge from the volcanoclastics at an elevation of approximatley 2000 m. They become broader and form several branches at about 1500 m where the slopes become gentler and the flow velocity lessens. The branching is affected by the pre-existing relief. Flow structures and pressure ridges are clearly visible, and so are the trenches that resulted from the collapse of lava tubes. Such tubes result from the rapid cooling off and solidification of the crust and

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Fig. 9.1. Vertical airphoto (scale 1:24 000) of the lava flows on the northeast slopes of the Ciremai volcano, west Java, Indonesia

continuous downslope flow of the hot interior. The tubes thus are emptied and often collapse. Most of the lava flows come to rest at the lower part of the slope where a gently inclined, irregular, lava plateau with convex rims has been formed. However, where ashes and other fragmentary material dominate, the slopes are smooth and often described as concave, especially for active or dormant stratovolcanoes. This is a generalization, as usually the slopes can be

divided in three parts of essentially different characteristic forms and origin. The upper part is straight and formed under the influence of ‘dry’ downslope transportation of ashes and coarser debris by gravity that results in a maximum angle of repose of about 34°. The middle part of the slope is also rather straight but is much more gentle. Its gradient decreases slightly downslope and is generally in the order of 8–12°. It is usually very extensive and results from ‘wet’ fluvio-volcanic deposition by lahars (volcanic mudflows). The finegrained material travels the greatest distance, and the texture of the material varies with the eruption. The valleys are often filled to capacity, and may even overflow, during the eruptions, and rapid incision follows thereafter. Lahar terraces may develop that relate not to phases of valley formation but to subsequent mudflows. The lowest part of the slope is built up by freshet deposits of the radiating ravines and has a gradient of less than 2°. It gives way to fertile alluvial plains where fluvial and littoral processes dominate. The three parts are usually separated by distinct breaks of slope. The widespread occurrence of fluvio-volcanic slopes indicates the importance of lahars in connection with ash eruptions in the region. The recent eruptions of Mount Pinatubo discussed in Chapter 15 exemplify this. Some lahars are hot eruption lahars, caused by the rains that occur during (and are often generated by) a volcanic event. Others are cold, rain-fed lahars that develop in the rainy season after an eruption. Extensive fluvio-volcanic footslopes thus may ultimately be formed. The importance of fluvio-volcanic phenomena on the volcanic islands of the region is explained by the high annual precipitation and intense rain showers associated with the prevailing humid tropical climate. Volcanic mudflows are much less developed in the islands of southeast Indonesia, where the rainfall is considerably less and the dry season much longer. The characteristic landforms of fluvio-volcanic footslopes are shown in the vertical aerial photographs of western Mindanao, Philippines (Figures 9.2 and 9.3). This is part of a volcanic complex that culminates in the Ragang (2815 m) stratovolcano and also Mount Maranat, Mount Matutun, and the slender Tangkulan cone (1678 m). The Calaya solfatara fields form the eastern limit of it. The fluvio-volcanic slopes are dissected by a number of radiating ravines with concave long profiles that began to develop on the straight and gently sloping surface immediately after the eruption. The smaller ones were largely inactivated when, at a later stage, the major and more deeply incised ones deprived them of groundwater flow. The dissection was facilitated by subsequent uplift of the area as evidenced by

146 Herman Th. Verstappen

Fig. 9.2. Vertical airphoto (scale 1:33 000) of fluviovolcanic slopes, west of Sasangani Bay, Mindanao, Philippines

several raised shorelines that can be traced between the toe of the footslope and the present coastline. The flow presumably dates from the Middle or Upper Pleistocene. Figure 9.3 shows a less dissected, and possibly somewhat younger, remnant situated in the same area. The oval depression that was drained initially by a small, later inactivated, shallow rivulet has a subterraneous drainage now. It is probably the site of a steam explosion that

occurred in the wake of the eruption, when the lahar material was still hot. Destruction of the volcanic slopes by incision of radiating ravines and by various types of mass movements also has an important effect on their geomorphological characteristics. The incision of ravines is particularly rapid on ash-covered slopes and isolated ash cones. Cinder cones are more resistant. Some ravines are associated with

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Fig. 9.3. Vertical airphoto (scale 1:33 000) showing less dissected lahar deposits and details in an area near Figure 9.2

faults that occur in the top areas of certain volcanoes and particularly large radiating ravines then are the result. Locally, part of the crater rim may be eroded and a horseshoe-shaped crater formed. Ultimately this sector of the volcanic body may completely collapse, giving rise to a broad concavity in the volcanic body and a hummocky zone of deposition at its foot. In certain cases, such as at the Raung volcano in eastern Java,

the material has been transported as far as 60 km from the volcano. Because stratovolcanoes result from both explosive and effusive eruptions, inclined bedding of alternating lava beds and ash layers is common. The lava beds may in such cases act as slipping planes for the covering beds, as for example at the Gedeh volcano, west Java (Verstappen 1963). Destructive mass movements may result from gullying only, but it is

148 Herman Th. Verstappen

more likely that most of them have been triggered by volcanic eruptions, earthquakes, or faulting. The weight of a volcano increases with its growth, and this may ultimately cause deformation of its foundation leading to gravitational spreading (van Bemmelen 1937, 1949; Speelman 1979). A well-documented example of this phenomenon is the Jembangan volcano in central Java. The entire volcanic complex broke away from its roots to move laterally as has been established by repeated triangulation. Large portions of the volcano slid down along slip faults and introduced overthrusting in the underlying Miocene and Pliocene beds.

Natural Resources of Southeast Asian Volcanoes Volcanic activity may devastate the surrounding areas in various ways, as evident from the diverse volcanic processes mentioned above (Verstappen 1988, 1992, 1994). Even toxic materals may be deposited. An important factor is the burning of vegetation on the slopes by hot ashes, nuées ardentes, or incandescent lava. This may result in accelerated erosion, especially on ash-covered slopes where gullying, initiated in the fresh ash cover, cuts through the damaged vegetation and soil layer into the underlying tuffaceous material. Figure 9.4 is an example from the 1982–3 eruption of Galunggung, west Java. Apart from rills and gulleys, flow phenomena can also be observed in the ashes covering these formerly largely forested slopes (Katili and Adjat Sudrajat

1984). Gulleys thus formed can reach a depth of several metres within a week. The ashes may also cover paddy fields, disrupt existing irrigation systems, destroy villages, and endanger the population. Volcanic hazards, discussed in Chapter 15, are prevalent, and disaster reduction measures such as volcanic hazard zoning and early warning systems are crucial. The beneficial aspects of active volcanism, however, outweigh by far setbacks caused by incidental disasters. The fertile andosols have led to advanced agricultural practices, ancient civilizations, and high population density. The rate of weathering of fresh volcanic material is high in the humid tropics; pioneer vegetation invades the ashes soon after an eruption; and after a decade the land is often in use again. Mohr and van Baren (1954) elaborate on the chemical composition of the diverse volcanic ashes that occur in the region and draw attention to the fact that their fertilizing effect can sometimes be observed up to 200 km from the source. Leaching of the ashes starts with the first rains, and calcium, magnesium, other bases, and some liberated SiO2 are removed. Ultimately the SiO2 precipitates at depth, cementing the loose ashes, thus forming tuff. This material is pervious and hardly susceptible to mass movements. Near-vertical natural slopes are common, and agricultural terraces, paths, and roads are comparatively easy to maintain. Horticulture is important on the higher parts of the volcanic slopes where vegetables and other products are grown for the urban centres lower down and for the tourist resorts in the cool mountains.

Fig. 9.4. Ash deposits and burnt vegetation on the slopes of the Galunggung volcano, west Java, after the 1982–3 eruption

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The lower slopes are, since the introduction of irrigation more than 1000 years ago, the scene of terraced paddy fields and the production centre of the staple food: rice. The high productivity of these lower volcanic slopes is rooted not only in the fertility of their soils, but particularly in the almost inexhaustible amount of groundwater stored in the large stratovolcanoes of the humid tropical Southeast Asia. The groundwater emerges, usually all round them, in a spring horizon on the lower slopes where the volcanic cones flatten. Faults or old lava flows may complicate the situation in places. Figure 9.5 shows an example from the Slamet volcano, central Java. The false colour SPOT image of Plate 2 shows the spring horizon and the downslope paddy fields on the Karang volcano in Banten, west Java. Characteristically, the slopes of these stratovolcanoes are dissected by two independent sets of radiating ravines: those originating in the top area are fed by rainwater with a very irregular (at times zero) discharge; and those starting at the spring horizon much lower down are fed by groundwater. The former may have a very high

sediment yield when ash eruptions occur. They then become ‘sand rivers’ situated several metres above the paddy fields and disturbing the natural drainage in the plains. Fluvio-volcanic slopes, groundwater resources, paddy fields, and alluvial plains become insignificant where only the top of a volcano emerges and its submarine slopes are steep. Tephra is then deposited mainly in the sea. Rain-dependent agriculture and animal husbandry are practised in these areas of more limited resources, as in southeastern Indonesia, where, in addition, rainfall is lower and the dry season long and variable. A number of volcanoes provide hot water for the households in surrounding villages, and others are used for the production of geothermal energy. Figure 9.6 shows a geothermal station in the Dieng Plateau, central Java. Sulphur, collected at solfatara or from sulphuric mud deposited in craters or calderas, is another resource. Figure 9.7 illustrates sulphuric mud deposits in the Sorikmerapi crater, Sumatra. The economic importance is mainly at the village or regional level. Soil, groundwater,

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Fig. 9.6. A geothermal power-plant in the volcanic Dieng Plateau in central Java, Indonesia

Fig. 9.7. Sulphuric mud deposits in the crater bottom of the Sorikmerapi volcano, Sumatra, Indonesia

and climatic resources are of greater importance for agriculture and tourism.

The Volcanic Landforms of Bali and Lombok: An Example The characteristics and diversity of the landforms developed in the volcanic islands of Southeast Asia are demonstrated by Bali and Lombok, situated east of Java in southern Indonesia. These islands have been selected

because they are both almost exclusively composed of volcanic terrain. They are dominated by one or more high volcanoes surrounded by characteristic fluvio-volcanic slopes and extensive lowlands that form the basis of local agriculture, economy, and civilization. On the bigger island of Java large areas of non-volcanic terrain are found, while the volcanic islands further to the east lack the required geomorphological diversity. The geomorphological sketch map of the islands (Figure 9.8) demonstrates the dominance of volcanic

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GEOMORPHOLOGICAL MAP OF BALI AND LOMBOK (INDONESIA) F

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landforms. Large calderas and high active stratovolcanoes with the related broad fluvio-volcanic slopes cover most of the islands. Strongly eroded volcanic terrain covers an area of extinct Early Quaternary volcanism where Pleistocene footslopes and terraces are common. Dissected denudational mountains, developed in Neogene volcanic material, occur in places. Non-volcanic Neogene rocks, mainly limestone, form the southern tilted plateau zone on both islands. The limit between fluvio-volcanic slopes and alluvial plains is a transition zone and thus arbitrary. The subduction zone is located to the south of the islands and stretches east –west, as in Java. The Sunda Shelf, bordering Java to the north, has given way, however, to the oceanic part of the plate, where the volcanic arc is less elevated. The southern tilted plateau zone, mentioned from Java and composed of Tertiary volcanic material covered by or intercalated with Neogene karstified limestones tapers out and fails east of Lombok. Remnants of it, and also of the escarpment that marks the fault along which the central portion of the geanticline slid down, can be traced in the southern peninsula of Bali on Nusa Penida and along the south coast of Lombok. The remainder of the islands forms part of the central, lowland zone of the geanticline, which is crowned and fertilized by the active stratovolcanoes mentioned above. Strongly eroded, Early Quaternary volcanoes that have lost most of their volcanic landform characteristics can be traced in western Bali. They form an east–weststretching mountain range that rises to an elevation of approximately 1400 m in Mount Merbuk in the west and Mount Patas further east. A similar but

F 11

f Rinjani f 3726

Fig. 9.8. Geomorphic sketch map of the islands of Bali and Lombok, Indoesia 1. young-Quaternary or active volcanoes 2. old-Quaternary volcanoes 3. denudational hills in Neogene volcanic rocks 4. karstified tilted southern zone 5. piedmont or terraces 6. alluvial plain 7. stratovolcanoes 8. fluvio-volcanic slopes 9. calderas 10. fault 11. geomorphological boundaries 12. lakes

12

much smaller area occurs in western Lombok, where Mount Punikan reaches an elevation of almost 1500 m. Volcanic landforms and active volcanism occur in each of these two islands. The main eruption centres are situated at some distance from the north coast, and extensive fluvio-volcanic footslopes give way to the lowlands in the south, which end abruptly at the escarpment of the southern zone mentioned above. These fertile volcanic footslopes, and the adjacent lowlands, benefit from the large quantities of groundwater that emerge where the volcanic bodies flatten towards the plains. They are covered by terraced paddy fields alternating with traditional villages and form the economic and cultural heart of the islands. The volcanic area of Bali comprises, from west to east, the Silanglaya caldera and nearby Batukau (Tambanan) stratovolcano, the Batur caldera, and the Agung stratovolcano, which at 3142 m is the highest peak on the island. The Labuan Amuk semi-circular caldera and the Seraja stratovolcano are much lower features bordering the east coast. Only the Batur and Agung are active volcanoes. A major Batur eruption occurred in 1926 (Stehn 1928), but because of the protective caldera rim around it caused only limited damage. The Agung eruption of 1963 (Zen 1964), after being dormant for 120 years, devastated the easternmost parts of the island. The Silanglaya caldera has an oval shape and measures approximately 13 km from east to west and 8 km from north to south. A volcanic cone rises from it and is surrounded by three lakes. The Batur caldera is a nested caldera, the outer one measuring 14 × 10 km. The inner one is more or less circular and measures 7 km across. It contains the crescent-shaped Batur

152 Herman Th. Verstappen

Fig. 9.9. The nested Batur caldera, Bali, seen from the top of Mount Agung

Lake. A young volcanic cone, Mount Batur, borders on the lake, and recent volcanic activity is situated along a northwest–southeast fault that runs across the cone. The morphology indicates that two paroxysmal eruptions have occurred in the past, each of which resulted in collapse of the volcano. Faulting played a subordinate part in these events. Figure 9.9 shows the configuration of the nested Batur caldera as seen from the top of Mount Agung. The crater of this slender stratovolcano shown in the figure was the site of the 1963 eruption. A lava flow developed on the north slope. Pyroclastic flows and lahars came down along the radiating ravines of the northeast, east, and south slopes, which were devastated also by nuées ardentes. All volcanic landforms of Lombok relate to a single, complex volcano: the east –west-stretching Rinjani (3726 m), which is marked by a large lake at 2000 m above sea level (Milius 1950). The configuration of the volcano is shown in the vertical aerial view of Figure 9.10; the top area and the lake in the ground view of Figure 9.11. In contrast to the situation in Bali, faulting is an important element in the development of this volcano and the collapse of its caldera. The top of the Rinjani volcano can be seen in the lower right corner of the aerial view (Figure 9.10). The radiating ravines on its slopes are shallow and indicate fairly recent origin. The slopes to the west of the lake and the escarpment in the left of the photograph are much older and more eroded. Yet older caldera ridges, not shown, exist further to the west. It is evident that volcanic activity has shifted

eastward in the course of time along an east–west fault that stretches across the crater at the top of Rinjani and the fresh cone of Mount Baru (with some lava flows), which is situated in the lake. This cone is the site of recent activity, and is located at the crossing with a major northeast–southwest fault that is also the cause of the straight and deep Putih Valley north of the lake. A second, smaller cinder cone, with some lava flows, was formed slightly further towards the northwest during a minor eruption that occurred in 1943 (Milius 1950). The oldest eruption phase of the Rinjani volcano was characterized by the effusion of basaltic lavas, which accounts for the occurrence of the lake. Andesitic lavas and extensive clastic deposits (andesitic breccias) testify a gradual change to less mafic and particularly more explosive eruptions. These thick volcanic formations form the body of the volcano and are covered by two deposits of pumice of varying thickness. They are separated from the underlying volcanoclastics and from each other by distinct weathering layers. These pumice formations result from two paroxysmic events, probably fissure eruptions, that caused the collapse of the volcano. The caldera so formed obviously resulted from the interplay of tectonism and volcanism.

The Interaction of Volcanism and Tectonism: A Review The volcanic (and non-volcanic) arcs that characterize the tectogene mobile parts of Southeast Asia are complex

Volcanic Islands 153

Fig. 9.10. Vertical aerial view of the top area of Mount Rinjani, Lombok, and the east–west stretching tectonic depression with the lake and the cinder cones of Mount Baru (I and II)

and diverse. Thick, Lower Miocene volcanic deposits and folded sedimentaries usually form the core of these geanticlinal structures that resulted from mid-Miocene orogenesis. At places, pre-Tertiary rocks are also exposed. Upper Miocene beds, mostly limestones, cover these

older formations unconformably, and at present form conspicuous tilted karstified plateaux at the subduction side of several areas of the volcanic arcs: southern Sumatra, Java, Halmahera. The Plio-Pleistocene represents a phase of renewed orogenic activity. Foldings

154 Herman Th. Verstappen

Fig. 9.11. Ground view of Lake Rinjani looking west towards the top of the volcano

resulted, particularly in the central parts and the sides bordering Neogene geosynclines. The Kendeng and Rembang ridges in east Java exemplify this. Important vertical movements also took place and are reflected in the widespread occurrence of Pleistocene raised coral reefs. Tectonism is a major feature of the volcanic arcs in the Holocene leading to interaction of faulting and volcanism. Long transcurrent faults occur in volcanic arcs where plate tectonics result in tangential stresses. They are very conspicuous in Sumatra and also in the Philippines, where they have a pronounced effect on the location and alignment of the volcanic activity. The dextral transcurrent Semangko Fault zone in Sumatra, for example, gave rise to steam explosions along a fault in the Suoh pull-apart basin and, farther north, to linear tuff extrusions and ignimbrite eruptions. The repeated massive ignimbritic eruptions in the Toba depression in northern Sumatra occurred just outside the Semangko zone, along parallel faults that stretch in the direction of the long axis of the island. In other areas volcanic arcs are predominantly affected by deep-seated compartmentation faults stretching approximately perpendicular to the subduction trenches. These faults have, for example, a major influence on the geomorphological differentiation of west, central, and east Java. They have a north–south orientation and complement the east–west faults that relate to the collapse of the volcanic geanticline. The escarpment that separates the tilted southern plateau zone from the

broad plains of the collapsed central zone is a major geomorphological element of the island. It can also be traced further east, in Bali and Lombok. East–west faulting is a leading element also in the Bandung Basin (Nossin, Damen, and Voskuil 1996). Sunda Strait and its surroundings, where the direction of the volcanic Sunda arc changes from northwest – southeast in Sumatra to east –west, is a special case. It is characterized by the occurrence of numerous oblique faults that govern the sites of volcanic activity and coastal configuration. Faulting is the initiating factor that subsequently triggers volcanic outbursts when the faults reach a magma chamber. Collapse, faulting, and volcanism thereafter are interacting factors, as in Plate 2, an annotated false colour SPOT image covering northwest Java (Banten) and the adjacent parts of the Sunda Strait. The top part shows Banten Bay and the adjacent lowlands of the north coast of west Java, which are separated by the eroded Gedeh volcano from the northern entrance of Sunda Strait along the left margin of the image. A complex volcanic area occupies most of the land area, with the southwest –northeast-stretching Danau depression as a major feature. Numerous faults can be traced, mostly stretching approximately southwest–northeast and northwest –southeast. One of the northwest–southeast faults runs across the top of the extinct Tukung volcano, determining its location as well as the horseshoe shape of the crater. It can be traced northwestward to the eroded volcano of Sangean Island. The partly collapsed crater of this volcano is open to the

Volcanic Islands 155

southwest, and its northwest, northeast, and southeast slopes are cut off by post-eruption faults as demonstrated by the rectilinear coastlines. Parallel to this northwest– southeast fault and north of the Tukung volcano another fault resulted in the andesite –basaltic Meramang dyke (van Bemmelen 1949). It cuts off the southern slope of the elongated Marikangen volcano, which pre-dates the collapse of the Danau depression as the tuffaceous Danau deposits abut against it at the fault. The strongly eroded Mokol and Payung volcanoes probably also pre-date the Danau eruptions, while the fresh-looking Pinang is considered post-Danau by van Bemmelen, based upon observations of Hetzel. The horseshoe-shaped crater of the Parakasak volcano is influenced by a north– south fault. This eroded volcano pre-dates the Danau depression, as is shown by the southwest–northeast fault that cuts off its northern foot. Mount Karang (1778 m) is the highest stratovolcano of this volcanic complex. Its upper part is affected by an east–west fault. This stratovolcano post-dates the Danau depression as is evidenced by the fact that its fluviovolcanic footslope penetrated a gap in the southeastern part of the Danau escarpment. It is an important aquifer. Groundwater emerges where the volcanic beds taper out towards the surrounding plain. The spring horizon is indicated by the change in colour that results from the transition of the natural vegetation (red) to the irrigated paddy fields (blue-green) further downslope. The Pulasari volcano is a rather undissected cone, and post-dates the Danau caldera. The strongly eroded Pulasari and Tompo volcanoes (situated in the lower left corner of Plate 2) are affected by northwest–southeast faults and presumably pre-date the Danau depression, although van Bemmelen considers them to be of postDanau age. The Danau pumice deposits form fluvio-volcanic slopes around the depression, particularly in the west, northwest, and east. They are not related to any former collapsed volcanic cone, but were emanated by fissure eruptions along the faults that created the surrounding escarpment. The terrain configuration suggests that the Danau depression is a graben structure in a volcanic area where major fissure eruptions have occurred. Similar Neogene deposits occurring at depth may be associated with the earlier and much larger collapse of the Sunda Strait. The Gedeh volcano to the north is a separate feature. It is surrounded by an alluvial plain, and was clearly an island before accretion connected it with Java. Its strongly dissected slopes testify a pre-Danau age. The sea invaded its radiating valleys as a result of the postglacial rise in sea level. The island of Panjang, situated

in Banten Bay, is a coral island with well-developed sand cay, shallow moat, and shingle ridges that reestablished itself in the Holocene. It probably has a volcanic foundation. The diversity of volcanic landforms occurring in insular Southeast Asia is demonstrated by the examples given in this chapter. Their distribution pattern and geomorphological evolution are linked with plate subduction and active faulting. The widespread occurrence of fluvio-volcanic slopes is favoured by the humid tropical climate characterizing large parts of the region. Soil and groundwater resources far outweigh the risks incurred by volcanic eruptions and are the basis of the high agricultural productivity and economic strength of the Southeast Asian volcanic islands.

References Aurelio, M. (1989), ‘The Philippine Fault: An Example of a Major Strike-Slip Fault behind a Subduction Zone: A Study on its Central Segment’, Ph.D. thesis, University de Pierre et Marie Curie, Paris. Javelosa, R. S. (1994), ‘Active Quaternary Environments in the Philippine Mobile Belt’, Ph.D. thesis, University of Utrecht/ITC. Katili, J. A., and Adjat Sudradjat (1984), Galunggung, the 1982–1983 eruption (Bandung: Volcanological Survey). Kusumadinata, K. (1979), Catalogue of Indonesian Volcanoes (Bandung: Volcanological Survey). LIPI (Lembaga Ilmu Pengetahuan Indonesia) (1985), Proceedings of the Symposium on 100 Years Development of Krakatau and its Surroundings, Jakarta, 23–27 August 1983, vol. i ( Jakarta: Natural Sciences). Milius, G. (1950), ‘Letusan terachir dalam Pegunungan Rindjani’, Geographical Institute Djakarta Publication, 5 ( Jakarta: Geographical Institute), 3–12. Mohr, E. Ch. J., and van Baren, F. A. (1954), Tropical Soils (The Hague: Van Hoeve Publishers). Neumann van Padang, M. (1951), Catalogue of the Active Volcanoes of the World including Solfatara Fields, pt. 1: Indonesia (Naples: International Volcanological Association). —— (1953), Catalogue of the Active Volcanoes of the World including Solfatara Fields, pt. 2: Philippine Islands and Cochin China (Naples: International Volcanological Association). Nossin, J. J., Damen, M., and Voskuil, R. P. G. A. (1996), ‘Morphostructural Relationships in the Lembang Fault Area, West Java, Indonesia’, Zeitschrift für Geomorphologie, suppl. vol., 84: 185–201. Simkin, T., and Fiske, R. S. (1983), Krakatau 1883: The Volcanic Eruption and its Effects (Washington: Smithsonian Institute Press). Speelman, H. (1979), ‘Geology, Hydrogeology and Engineering Geological Features of the Serayu River Basin’, Ph.D. thesis, Free University Amsterdam. Stehn, Ch. E. (1928), ‘De Batoer op Bali en zijn eruptie in 1926’, Vulkanologische Mededeelingen (Bandung: Volcanological Survey), 9: 67. —— (1929), ‘The Geology and Volcanism of the Krakatau Group’, 4th Pacific Science Congress Proceedings, Batavia/Bandoeng (Batavia: Van Dorp), vol. 1: 2– 55. van Bemmelen, R. W. (1937), ‘Example of Gravitational Tectogenesis from Central Java (Karangkobar Region)’, De Ingenieur in Nederlandsch Indië, 4/3: IV/55– 66.

156 Herman Th. Verstappen van Bemmelen, R. W. (1939), ‘The Origin of Lake Toba (North Sumatra)’, in 4th Pacific Science Congress Proceedings. Batavia/ Bandoeng (Batavia: Van Dorp), vol. 2a, 115–25. —— (1949), The Geology of Indonesia, 2 vols. (The Hague: Government Printing Office). Verstappen, H. Th. (1961), ‘Some “Volcano-Tectonic” Depressions of Sumatra, their Origin and Mode of Development’, Royal Netherlands Academy of Sciences Proceedings, B64/3: 28–443. —— (1963), ‘Geomorphological Observations on Indonesian Volcanoes’, Tijdschrift Koninklijk Nederlandsch Aardrijkskundig Genootschap, 80: 237–51. —— (1964), ‘Some Volcanoes of Halmahera (Moluccas) and their Geomorphological Setting’, Tijdschrift Koninklijk Nederlandsch Aardrijkskundig Genootschap, 80: 297–321. —— (1973), A Geomorphological Reconnaissance of Sumatra and Adjacent Islands (Groningen: Wolters Noordhof).

—— (1988), ‘Geomorphological Surveys and Natural Hazard Zoning, with Special Reference to Volcanic Hazards in Central Java’, Zeitschrift für Geomorphologie, suppl. vol., 80: 81–101. —— (1992), ‘Volcanic Hazards in Colombia and Indonesia: Lahars and Related Pheomena’, in G. McCall, D. Lamming, and D. Scott (eds.), Geohazards Natural and Man-Made (London: Chapman & Hall), 33–42. —— (1994), ‘The Volcanoes of Indonesia and Natural Disaster Reduction’, Indonesian Journal of Geography, 26/68: 27–35. —— (2000), ‘Outline of the Geomorphology of Indonesia, International Institute for Aerospace Survey and Earth Sciences (ITC), Publication 79 (Enschede). Zen, M. T. (1964), ‘The Volcanic Calamity in Bali in 1964’, Tijdschrift Koninklijk Nederlandsch Aardrijkskundig Genootschap, 81: 92–100.

10

Karst in Southeast Asia David Gillieson

Introduction Flying over the patchwork quilt of land uses that comprise Southeast Asia, one often sees extensive tracts of rugged topography with plateaux pitted with depressions, deep gorges, rivers arising at the bases of mountains, and towers arising from alluviated plains. These are the karst lands, formed on limestone bedrock and subject to the solutional erosion of that bedrock above and below ground. With a total area of about 400 000 km2, Southeast Asia contains some of the more extensive karst regions in the world (Figure 10. 1). Many of these karst areas are of high relief with spectacular arrays of tower and cone karst. Many have now been inscribed on the World Heritage list in recognition of their unique geomorphology and biology. They are scattered throughout the islands of the Malay archipelago as well as the adjoining fringe of the Asian mainland. Karst is found in Malaysia, the Philippines, Thailand, Brunei, Indonesia, Cambodia, Viet Nam, Lao PDR, and Papua New Guinea. Geologically the carbonate rocks hosting karst range in age from Cambrian to Quaternary, a span of about 500 million years (Letouzey, Sage, and Muller 1988). Over that time limestone solution and other landscape processes have produced an array of karst landforms including towers, cones, plateaux, and dolines, underlain by extensive cave systems. There have also been strong external influences of tectonism, eustatic, and climatic change. Today human modification of karst processes and landforms is proceeding at a rapid pace. Despite their characterization as the ‘botanical hothouse extreme’ (Jennings 1985) the karstlands of Southeast Asia are most diverse, reflecting the influence of varied geology, uplift history, eustatic change, and climates

past and present. Karst landscapes range in elevation from sea level to nearly 4000 m, and comprise extensive plateaux with dolines, tower karst, cone karst, and lowlying swampy terrain. The carbonate rocks on which they have formed range widely in age, and can be soft and impure or hard and crystalline. Many areas have been wholly or partially blanketed by volcanic ash during their evolution. Thus the region contains an unparalleled array of karst landscapes with some of the longest caves in the world. The tectonic evolution of the Southeast Asian region is complex owing to the interactions between four major crustal plates: the Philippine, Pacific, Australian, and Eurasian Plates. An older, stable region comprising the Asian mainland and Borneo abuts a younger, unstable region associated with widespread neotectonism, coupled with island arc volcanism. The limestones of the Indochina peninsula are dominantly Palaeozoic in age, with the greatest area of karst in Myanmar and Thailand (Verstappen 1960). Other areas lie in northern Viet Nam, Cambodia, Lao PDR, and Malaysia. There are Cambrian limestones in Lao PDR and Viet Nam, as well as Devonian, Carboniferous, and Permian limestones elsewhere in the region. The most extensive limestones in Borneo are the massive shallow-water Eocene to Early Miocene rocks in northern Sarawak. Smaller carbonate outcrops of Cretaceous age are found in northern Kalimantan and Sabah, with younger Miocene uplifted reef limestones along the northwest coast. The Sunda and Banda volcanic arcs form the underlying structure for the karst of Indonesia. The karst is discontinuous and is formed on Mesozoic and Tertiary limestones. In contrast, the karst of Sulawesi is formed on Tertiary limestone in the central and southwestern

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Fig. 10.1. Karst areas of Southeast Asia (Source: Day and Urich 2000)

parts of the island, and has had a distinct geological evolution. The Philippines contain a complex array of karst landscapes formed on rocks ranging from Cretaceous to Tertiary in age and also widespread Quaternary raised reefs. The older rocks represent tectonic blocks accreted to an island arc terrain at the margin between the Philippine and Eurasian Plates. This chapter first provides an overview of the distribution and geology of principal karst areas in countries of the Southeast Asian region, then detailed descriptions of some key karst areas in several countries. A comparison of dominant karst morphologies in the region precedes a final section in which the impacts of human activities and guidelines for sustainable economic development of the karsts are provided. Day and Urich (2000) provide an excellent overview of the protected karst areas of Southeast Asia and their management issues.

Karst and Caves of Malaysia Carbonate rocks are widespread in Peninsular Malaya, distributed in a belt running along the axis of the peninsula and broadening near the Thai border. These areas of Palaeozoic rocks contain spectacular tower karst terrain which at Langkawi have been drowned by sea-level rise. On the island of Borneo, extensive younger limestones are found in the south and in the northern parts of Sarawak and across into neighbouring Brunei and Sabah. In this section the tower karsts of the Kinta Valley, Perak, and the karst plateaux of Gunung Mulu in Sarawak are described in detail.

Tower Karst of the Kinta Valley, Perak State Some of the most spectacular karst landscapes in Peninsular Malaysia are found in the Kinta Valley. This

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Fig. 10.2. Radar image of the Kinta Valley, Malaysia Tin-mining scars show as white patches. Limestone towers visible as dark irregular blocks adjacent to the tin-mining areas. The granitic terrain of the Cameron Highlands lies to the right-hand side of the image.

area of tower karst flanks the extensive alluvial plains of the Sungai Setoh which have been mined for tin for at least 100 years. The limestones of the Kinta Valley have been steeply folded and partially metamorphosed. This has resulted in a diversity of tropical karst landforms, found in a smaller area than comparable karst styles in the classic karst of Guanxi Province, near Guilin, China. Karsts in southern China are widely scattered, are formed on pure, thick, and old limestones, have been subjected to strong Cenozoic uplift, and have formed under a long, warm, humid monsoonal climate, unaffected by glaciations. Three principal karst styles are recognized there, and provide the best terminology for broad-scale karst morphology: Fenglin, or tower karst, characterized by a forest of limestone towers standing above a comparatively flat surface, often alluviated. Fengcong, or peak-cluster depression, where a limestone massif has a large number of closed polygonal depressions united in a common rock base, often elevated above the

surrounding terrain. The depressions are separated by conical hills (hence the alternative name ‘cone karst’) whose shape may depend on local details of structure and lithology. Gufeng, isolated towers in alluvial plains, are the result of extensive dissection of fenglin by meandering rivers which isolate blocks of limestone and then reduce them by undercutting, foot cave development, and cliff spalling. In some areas this terrain has been flooded by the rising sea to produce isolated karst islands with marine notches and navigable caves. The Kinta Valley karst (Figure 10.2) shares similarities with the fenglin and fengcong styles of tropical karst, with the towers on the eastern side of the valley resembling fenglin and the western side fengcong. This juxtaposition is unique and is due to the greater degree of metamorphism on the eastern side, adjacent to the granite upland. The steep cliffs of the eastern side of the valley, the sinking streams, and the dramatic cave entrances resemble the classic tower karst. In contrast

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the western side has relatively gentler relief, with a complex network of karst depressions separated by conical hills, some cliffed. It is nearer to the fengcong style, and has formed on softer rocks. There is a wide range of small-scale solution features on the karst— solution rills, deep runnels, and pinnacles—and the largely intact rainforest vegetation ensures the continuity of limestone solution processes. This is an outstanding massif in which visitors can see a wide range of tropical karst landforms in a small area, accessible by road and by foot. Gunung Tempurung is a 600 m high limestone tower in the Kinta Valley located to the south of Ipoh. It is typical of a large number of tropical karst landforms formed by the dissolution of limestone by percolating rainwater. The tower contains at least one extensive cave system, Gua Tempurung, which has a length of approximately 4800 m and a vertical range of about 200 m. The Gunung Tempurung Tower is an erosional remnant of a thick sequence of Silurian–Permian Kinta Limestones initially formed as a shelf deposit near an ancient coastline. The carbonate rocks lie adjacent to and are laterally bounded by Late Mesozoic granite plutonic rocks (Figure 10.3) emplaced by activity related to Late Triassic uplift from plate boundary stresses along the western edge of the Malay Peninsula. The limestones have been folded and compressed between the granites and have been altered by contact metamorphism to marbles and skarn. Hydrothermal mineralization of the limestone host rock has yielded deposits of tin, with some tungsten minerals and other minor ores. The limestones were subaerially exposed and have been extensively

karstified. In the central part of the karst tower a rivercave system, Gua Tempurung, developed from local damming of the north and south outlets of a small catchment derived from the granite upland area to the east. In several locations inside the dry upper chambers of the cave, vein deposits of tin (cassiterite) are evident in walls and ceilings. Additionally alluvial deposits with round pebbles and coarse stanniferous sands are present in the cave. The limestones are folded and faulted along a north–south axis with the limestone being compressed between two granite plutons. Associated with the east and west compressive forces are two conjugate joint sets that control much of the plan-morphology of the tower. (Gillieson, Holland, and Davies 1995). One joint set is aligned NNE–SSW with another NNW–SSE. Large eroded fissures occur on the sides of the tower aligned with the joint sets. Extensive joints formed from unloading stresses occur on the sides and flanks of the tower and promote spalling. Where the joint spacing is small, pinnacle karst features develop, mostly on the flanks of the tower. There are also many shutteridges which enclose small ponded basins analogous to the poljes of classical karst. Deep ravines score the sides of Gunung Tempurung, and both these and the shutteridges are aligned with dominant joint sets. There is no evidence of major faulting, but joint sets are well developed in the Kinta Limestone. Shear faults due to compression run parallel to the strike or on bedding planes. Minor tensional faults run obliquely across the valley. These two major joint sets control much of the orientation of Gua Tempurung and also have influenced the location

Fig. 10.3. The Gunung Tempurung karst massif in the Kinta Valley, Malaysia The towers are 600 m high and rise from an alluviated plain which has been mined for tin.

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of vein tin deposits. A major change in the orientation of Gua Tempurung Cave may be related to the presence of a minor thrust fault visible as a gangue in the cave walls. The eastern side of the tower with juxtaposed granite has resulted in a massive limestone–marble lithology weathering to rugged tower karst relief. The crests of the towers are rounded with numerous small gullies and some large depressions. One depression near the summit of Gunung Tempurung has two clearly visible cave entrances which may connect to underlying Gua Tempurung. The cliffs that characterize this karst terrain are maintained by regular spalling and the undercutting promoted by ponding of water in old tin workings and in karst depressions. In contrast, the western and southwestern flanks of the tower are characterized by less altered limestone resulting in cone karst terrain of gentler relief. This cone karst has numerous enclosed depressions or dolines, which drain to the alluvial marginal plain to the west. The depressions appear to occur at two levels that may relate to geologic structure. The Old Alluvium which infills the Kinta Valley is estimated to be 3–10 m thick on the northern side of Gunung Tempurung and from 10 to 20 m thick on the southern flank. The likely age of the Old Alluvium, investigated elsewhere in Malaysia, is Pleistocene with some probable Late Pliocene units (Smart, Andres, and Batchelor 1984; Yeap 1995). It overlies the Gopeng Beds, which are comprised of sandy clay and clay with pebbles and boulders. Bedding is indistinct in this formation in which the boulders are matrix supported. It is likely to be a mudflow deposit derived from the steep granitic terrain to the east and west of Gunung Tempurung. The Gopeng Beds may have had a crucial role in the ponding of water which promoted the enlargement of Gua Tempurung. The granite upland areas east and west of the tower result in the limestone forming a laterally confined carbonate aquifer. The east side of the tower is characterized by the limestone–granite contact; the west side is characterized by a broader corrosion plain. Both surface water and groundwater supplies recharge for the limestone aquifer from the east side. Flow is not diverted to the north or south by the tower; rather corrosion of an embayment has allowed the large allogenic (granite upland) catchment to be developed with the entire basin contents flowing through a large conduit system (Gua Tempurung) down the most efficient hydraulic gradient to emerge as a cave spring on the west side of the tower. There are other recharge components that supply the tower, the first being other allogenic streams sinking in the embayment either north or south of the Gua

Tempurung Cave but then becoming tributary to the master drainage system associated with the main conduit. The second component consists of smaller percolation water components that enter the conduit system from the upper reaches of the tower; these can be observed in the cave as drips from the roof or small tributary streams. The percolation water is from autogenic recharge from the upper surfaces of the tower. Both the quality and quantity of this water are important for the maintenance of speleothem growth. Rainwater infiltrating into the limestone is acidified by carbon dioxide from plant-root respiration and by organic acids released by tropical vegetation. This acidified water dissolves limestone, and upon degassing of the carbon dioxide in the cave atmosphere deposits calcium carbonate; thus the quality of the feed water for cave formation growth is totally dependent on the maintenance of healthy forest vegetation on the limestone towers. The allogenic recharge components are chemically different, with a rapid-flowing allogenic source from surface water from the granite upland, and another component of deeper circulation from the granite bedrock and a narrow strip of the limestone, low down near the east flank of the tower. The basin that feeds the Gua Tempurung sink has been filled with sediments from two sources, detritus and alluvium from weathering of the granite, and sediments from mining operations and construction of the main highway one, which runs to the east of the tower, almost parallel to the granite–limestone contact. Discharge from the limestone aquifer occurs either through the cave streams or through a deep fissure system into ponds or springs. Steep bedding suggests deep circulation with steeply rising conduits feeding springs. There are local reports of thermal springs on the west side of the tower suggesting deep circulation. Local informants, some having worked in the mining business, report that occasionally the basin feeding Gua Tempurung sink becomes flooded, with water input exceeding the output capacity down the cave stream. This flooding occurs at a frequency of about four times a year. Preliminary dye tracing experiments and dyedilution gauging under low flow conditions have shown that flood pulses move rapidly through the cave with a velocity of at least 1.1 ms−1. The alluvium and detrital deposits in the basin have allowed a few relief springs to rise through the sediments near the Gua Tempurung sink and then flow down the cave conduit. The unvegetated tailing deposits from the tin-mining operations, being relatively impermeable, permit relatively large run-off components also to contribute to the Gua Tempurung sink. The road batters of the highway

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Fig. 10.4. A well-preserved phreatic tube with solutional scallops and an incised canyon at the highest levels of Gua Tempurung

are rapidly eroding and also contribute large amounts of coarse granite colluvium to the stream feeding Gua Tempurung. This excessive sediment supply reduces channel capacity of the cave conduit. Effective erosion control may involve terracing, bund construction, and revegetation of the steep highway batters. Gua Tempurung is a classic example of a through-cave which breaches a strike ridge of limestone (Figure 10.4). As such its passages may be divided into two types, those initiated by dissolution of autogenic water percolating down from the upper tower surface and those related to allogenic water draining from the marginal granite into the stream sink. Early interpretation of cave development in this part of Malaya (Walker 1960; Paton 1962) suggested that higher sea levels (up to 100 m above mean sea level) may have produced the higher-level passages and horizontal niches visible in the landscape. Higher global sea levels have been reported throughout much of the Tertiary. There is evidence for

a sea level at least 90 m above present in the Middle Miocene, and also in the Pliocene. During the last interglacial (c. 125 000 bp) sea level attained an altitude of 1.5–9 m. There is widespread evidence for sea levels 2–3 m above present mean sea level in the Malay Peninsula, but although much higher sea levels have been postulated, no unequivocal evidence has yet been obtained for these in the Malay Peninsula. In the absence of such evidence, the high-level passages and niches may relate to local ponding behind mudflow deposits and valley aggradation in the Sungai Kampar. In Sarawak, Smart et al. (1985) have invoked a similar mechanism for the formation of some passages in Clearwater Cave, Mulu. However, there is good evidence (Walker 1960) to suggest that the original cavities were formed by hydrothermal action, that is through the dissolution of limestone by rising hot mineralized water. The ore occurs in sulphidic stanniferous ‘pipes’ which in many cases are infilled karst cavities, some clearly caves (Figure 10.5). All the deposits of tin are fissure fillings or metasomatic replacement of the limestone by warm fluids containing sulphur, arsenic, tin, copper, and iron. Thus the principal minerals in the tin veins are cassiterite, arsenopyrite, chalcopyrite, bornite, and pyrite, often found in a calcite matrix. The oxidation of the pyritic minerals due to water-table fluctuations (both shortand long-term) has probably produced sulphuric acid, which has a key role in cave development. Modern deep circulation of acidified water to 100–130 m is evidenced by pumping tests. On the southern side of Gunung Tempurung the Jehosophat Mine (Figure 10.5) was developed along a vein which followed a nearvertical fault plane in the limestone. This ore body had a length of 440 m, a width of 1 m, and an excavated depth of 40 m. The deposit comprised cemented stanniferous alluvium of sand size, associated minerals being cassiterite, quartz, tourmaline, topaz, and ferric oxides in a calcitic cement. The emplacement of tin ores as veins in the limestone may therefore have been accompanied by corrosive action due to sulphuric and other acids. This early phase of cavern development may be tentatively assigned to the late Tertiary, possibly Late Miocene, or Pliocene prior to the deposition of the Old Alluvium. Subsequently, karst water invaded these cavities as zones of higher secondary porosity in the limestone mass. This may have occurred when the land surface was much higher than today and with less relief. Stratigraphically many of the caves are older than the Old Alluvium. In many places excavation of the alluvial tin deposits has revealed buried stalagmitic columns

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Fig. 10.5. Tin-bearing solution pipes in the alluviated karst of the Kinta Valley, Malaysia. (a) General context (b) Situation at Jehosophat Mine

which link epiphreatic roof sections with lower bedrock. In several chambers there are extensive deposits of well-rounded tin-bearing alluvium which are clearly derived from the Old Alluvium of the Kinta Valley. Fragments of these early cave conduits are visible as phreatic tubes or phreatic roof sections (Figure 10.4) at high levels, 100–140 m above present stream level. One example is preserved as a tube, 20 m in diameter, with scalloped walls indicating substantial water flow, and a hydraulic jump where it leaves the main stream canyon above Gergasi Cavern. A flat, scalloped roof

section is visible at the highest levels of the cave and has been formed under conditions of passage-full discharge. In most locations this has been entrenched to a depth of 80 m to form a vadose canyon. Large incuts on the walls of this canyon suggest that downcutting has been episodic, with phases of sediment infilling and lateral channel migration of the sediment fill. This has been followed by downcutting leaving wall niches with either partial alluvial sediment fills or bare rock surfaces. These downcutting phases may be related to increased catchment run-off and a lowering of regional

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base level. Conversely, phases of sediment infilling may be related to increased sediment supply consequent on tectonic uplift, seismic events, or destabilization of vegetation due to climatic change. It is very likely that these episodes of sediment infilling and incision are related to glacial–interglacial cycles over the last 2 million years. Finely laminated sediments in these deposits would be amenable to palaeomagnetic dating, and suggest that there have been long periods of relative stability in the catchment. Today the cave stream is incising older sediments and is carrying a bedload of poorly sorted angular colluvium derived from erosion of the road batters and remobilization of fine clays from tailings. Present suspended sediment yields from rivers in Perak State range between 88 and 144 m3 km−2 y−1 (Douglas and Spencer 1985). These are high by world standards.

Gunung Mulu National Park, Sarawak One of the world’s great karst areas lies near the source of the Sungai Tutoh in northern Sarawak. The recently listed World Heritage area of Gunung Mulu National Park has been the subject of scientific study since the pioneering work of Wilford (1964) and gained momentum with the Royal Geographical Society expedition in 1978. Three major Tertiary rock formations are evident in the park (Osmaston and Sweeting 1982). In order, Palaeocene and Eocene shales and sandstones comprise the Mulu Formation; these outcrop to the east as the summit of Gunung Mulu, reaching an altitude of 2377 m. The Melinau Limestone Formation of Upper Eocene, Oligocene, and Lower Miocene ages outcrops as a spectacular line of karst hills up to 1500 m in thickness and reaches a maximum altitude of 1682 m at the summit of Gunung Api. The limestone formation has been dissected by deeply incised gorges dividing the limestones into four distinct blocks (Figure 10.6). From south to north, the hilly areas of the south have been separated from Gunung Api by the Melinau Paku Valley. The spectacular Melinau Gorge divides Gunung Api from Gunung Benarat, while the Medalam Gorge divides Gunung Benarat from Gunung Buda. The limestones are pure, massively bedded, dipping 80° to the northwest while fault and joints run northeasterly. The upper surface of the limestone massifs is spectacular and extremely rugged with pinnacle and arête morphology, deep gullies, and cliffs. There is a very strong control of jointing on these landforms. The Miocene Setap Shale Formation outcrops as a gentle line of hills to the west. Between the limestone outcrop and the Setap Shales is a broad alluvial plain overlying the limestones, with Pleistocene terraces covered in

kerangas forest (a type of heath forest). Major uplift occurred during the late Pliocene to Pleistocene, 2 to 5 Ma bp. This uplift is reflected in the wide range of cave passage levels in the karst. The area today is tectonically stable. The surface geomorphology and hydrology have been studied extensively (Day 1980; Osmaston and Sweeting 1982; Walsh 1983; Rose 1984; Farrant et al. 1995). The Mulu karst has provided very significant information on the tectonic and climatic evolution of the island of Borneo and the humid tropics in general. Large alluvial fans are evident emerging from the Melinau Gorge and the Melinau Paku Valley, with remnants of early Pleistocene fans preserved as terrace gravels. Fan aggradation is due to climatic control rather than tectonic influences. Increased rainfall during interglacials produced higher sediment loads, while relatively drier glacials with less sediment transport led to incision of the fans. This sequence of alluvial deposits thus provides an important record of glacial–interglacial cycles with which to interpret landform evolution. It is the best-preserved sequence in Southeast Asia, rivalling that of the Kinta Valley in Malaysia. At the south end of the park aggradation is also occurring on the Tutoh River, causing backing up of the Melinau River with increased sediment deposition. The present pattern of relief indicates that the Melinau and Medalam Rivers flowed to the northwest along the line of the Belait River, the Melinau later to be diverted south towards the Tutoh River via stream capture and the Medalam north to the Limbang River. Predictions for the future indicate periods of diversion of the Melinau drainage system from the Melinau Gorge area towards the Medalam and Limbang Rivers due to rapid sediment deposition. This will probably divert more surface water underground into evolving cave conduits, creating more karst hydrologic systems running along the strike to new springs in the upper Medalam Valley. The alluvial sequences and drainage patterns are thus still evolving, and provide a unique opportunity to study the dynamics of major tropical rivers in relatively undisturbed natural systems. This is especially relevant to enhanced understanding of the effects of increased sediment load, base level changes, and climatic variability under conditions of global warming. The karst mountains exhibit classical tropical karst features, pockmarked with dolines, closed depressions, valleys, and caves. The scale of the karst features is impressive, and a large variety is present in a relatively small area. In the southern hills, the Garden of Eden is one of the world’s largest collapsed dolines, being over 1 kilometre in diameter. Hidden Valley, on the east side

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Fig. 10.6. Distribution of limestone massifs in the Gunung Mulu National Park, Sarawak The four principal blocks of limestone are separated by deep gorges and karstic valleys. Over 295 km of cave passages have been mapped in this area, which is now inscribed on the World Heritage list.

of Gunung Api, is a deeply incised closed valley with a misfit stream sinking in its bed (Figure 10.7). Dye tracing proved a connection through to Cobra and Good Luck Caves and the Melinau Paku Valley. The Melinau, Melinau Paku, and Medalam Rivers have truncated the limestones, forming deeply incised gorges with towering 300 m high cliffs and remnants of high-level caves. Some of the world’s finest examples of pinnacle karst can be found on the karst mountains of Gunung Benarat and Api. This rich diversity of karst landforms is of outstanding scientific and educational value as much of it is relatively accessible to visitors. The caves of the Gunung Mulu National Park are now world-famous and have been systematically explored since the first expedition in 1978. Over 100 caves are

now known with a total mapped length of 295 km, ranging in altitude from local base level to 600 m asl (Brook, Eavis, and Lyon 1982). The caves are some of the largest to be found in the world, with 40 × 100 m strike-oriented passages and fine examples of all types of cave formation or speleothem. They are among the finest examples of tropical river caves known, with well-developed flood incuts, extensive clastic sediment deposits, and circular or elliptical tubes linking different cave levels. Four caves have been developed with pathways and lighting for visitors. Deer Cave is the world’s largest natural cave passage, measuring 120– 150 m in diameter (Figure 10.8). This easily accessible passage is perhaps the finest example of a large tropical river cave.

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Fig. 10.7. Hidden Valley, a major karstic valley bisecting the Gunung Api massif The limestone cliffs are about 300 m high.

Fig. 10.8. The entrance of Deer Cave (Gunung Mulu National Park), one of the largest natural tunnels at over 1500 m long and 150 m wide

The Good Luck Cave (Gua Nasib Bagus) contains the Sarawak Chamber, the world’s largest natural single chamber within a cave. The chamber is 600 m long by 415 m wide and 80 m high, with a floor area of 162 700 m2 and a volume of 12 million m3, dwarfing any other chamber in the world so far discovered. The

maximum unsupported roof span is 300 m. The role of faults in the evolution of the chamber are easily seen (Gilli 1993). The Clearwater Cave System is presently 116 km in length, the longest cave so far discovered in Asia and the eleventh longest cave system in the world at the present

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time. Exploration is not yet complete. It displays outstanding passages developed at many levels owing to the complex alluvial history of the Melinau River and its tributaries. Dye tracing from the Melinau Gorge proved a connection through to the Clearwater Cave Resurgence, a distance of 12 km. Clearwater Cave and Green Cave contain the largest examples of phytokarst to be found in the world, oriented pinnacles close to the entrances pointing towards the light, the result of cyano-bacterial growth and control over speleothem orientation. Clearwater Cave also contains the world’s longest wind-blown stalactites, each over 1 metre long. The cave sediment deposits are still in situ at all altitude levels, affording the opportunity for important scientific investigations of climatic change (Farrant et al. 1995). These clays and gravels are therefore not subject to normal surface erosion processes but have been preserved in situ for millennia. Palaeomagnetic dating of these deposits has revealed a rapid uplift rate of the mountains of 19 cm per 1000 years. Reversal of the earth’s magnetic field recorded in the sediments at 1.8 million years before present indicates that the caves are at least 2 million years old, possibly as much as 3 million years. Notches in the walls of the caves at various levels can be correlated with interglacial periods. The caves are predominantly phreatic in origin but exhibit vadose, phreatic, and perhaps paragenetic profiles with some of the world’s finest examples of phreatic pendants. The caves are therefore of outstanding significance in recording major changes in earth history (Gill 1999). The wide range of soil types and altitude in Gunung Mulu National Park account for a diverse range of vegetation, all inland vegetation formations being representative, the only exception being those on igneous soil types. On the Gunung Mulu massif, multi-storeyed mixed dipterocarp lowland forest can be found up to an altitude of 800 m; 284 species of trees were recorded in three plots totalling 1.2 ha. This forest is among the most diverse in Malaysia. Between 800 and 1200 m, lower montane forest vegetation predominates, again rich in species. Upper montane forest from 1200 to 1900 m marks the lower limits of mossy forest with 155 species of trees identified in five plots totalling 0.365 ha. Upper montane forest between 1600 and 2177 m comprises a dense mass of stunted trees with four species of Rhododendron and many species of orchids and pitcher plants, Nepenthes muluensis being endemic to the Gunung Mulu National Park. The upper montane forest on the summit rises to 2177 m with four species of endemic terrestrial orchids. The lowland alluvial forest, tropical heath forest, peat swamp, and riparian forest all contain numerous endemic

species, being the most complex vegetation formation in the park. Emergents attain a height of 40 m with a maximum girth of 250 cm. The Setap Shales in the Sungai Mentawai drainage exhibit lowland mixed dipterocarp and kerangas forest; these formations are rich in palm flora. The range of soil, rock, and altitude zones has ensured a high diversity of the flora in the 17 vegetation types represented. On average, 780 different species of trees can be found in a typical 10 ha plot. The park is one of the richest sites in the world for palms: 111 species have so far been identified. A total of 3500 species of plants have been identified, which include 1326 dicotyledons, 115 Dipterocarpaceae, and 63 Lauraceae. This includes 1500 species of flowering plants, and 1700 mosses and liverworts, which have been catalogued along with 4000 species of fungi. Vegetation on the limestone is distinctive, containing numerous calcicolous species that are endemic. The forest on the generally steep scree slopes is predominantly open with few widely spaced trees. This vegetation is dominated by massive emergents that may exceed 5 m in girth and 50 m in height. The scree itself, with sharply eroded rocks and boulders and damp calcareous soils, provides a wide range of sub-habitats and niches that support a diverse shrub and herbaceous flora. In contrast, the immense limestone cliffs are exposed to direct sunlight and generally lack deep soil or effective soil moisture. As a result the habitat in these areas is particularly harsh and difficult for plant growth. Some shrubs and small trees take root in the ledges and sills and crevices of the limestone. A common genus in these areas is Boea, a plant in Family Gesneriaceae with at least five species represented in the region. At lower levels where the cliffs are less exposed, there is often a prolific cover of herbs containing a number of calcicolous Gesneriaceae. Among the endemics found on the cliffs is the palm Salacca rupicola. On the steep limestone slopes, a dense forest of small shrubs and trees occurs, but on the more moderate slopes high forest is found. At low altitude, up to about 800 m, the forest is reasonably dense, dominated by large emergents that may be as tall as 40 m with a girth of more than 250 cm. The principal species are dipterocarps. The shrub layer is sparse and the deep litter layer mainly bare with very sparse herbaceous flora, except on exposed limestone rocks. Lower montane forest is considered to begin at about 800 m, though there is no distinct boundary between this and forest at the lower levels. The terrain in these areas is generally steep and rugged with a dense forest composed of small trees. Dipterocarps are mainly absent in this area, and the forest is dominated by non-calcicolous

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Fig. 10.9. Pinnacle karst at 1200 m altitude on the flanks of Gunung Api, Gunung Mulu National Park

species. Trees tend to have a poor growth habit and are often bent and leaning. As the canopy is somewhat broken, there is a prolific amount of growth at ground level. Epiphytes such as orchids, ferns, and aroids are abundant on the lower stems of trees in these areas. The upper montane limestone forest starts at about 1200 m and is abundant in bryopyhtes that cover the deep humus layer and drape from the forest. The terrain in these areas is extremely broken and does not allow the development of a consistent forest canopy (Figure 10.9). Small trees of stunted habit are interspersed with shrubs, and in places the bare limestone rock is exposed. On the deep humus layers, calcicolous plants are replaced by calcifuges that include two conifers. In areas that may have been cleared by fire, vegetation is dominated by plants of the genus Pandanus. In the deep rock clefts, a sparse but diverse moss flora is evident. The flat plateau that forms the summit of Gunung Api at 1580 m is covered by a low growth of vegetation that seldom exceeds 1 m in height. The vegetation of this area is dominated by orchids and pandans including three species of Rhododendron. The caves themselves contain highly specialized flora of non-vascular plants, particularly algae. These are often oriented to the light, producing a distinctive needle formation known as phytokarst. Mulu has the best examples of this in the world. The caves contain a diverse range of troglobitic species, many as yet unidentified, and large colonies of bats and swiftlets. Many of these troglobitic species are endemic. Deer

Cave alone contains five endemic troglobitic species; one of the world’s largest colonies of free-tailed bats, Tadarida plicata, numbering approximately 3 million; and the largest number of different species of bats to be found in one single cave. The evening flight of between 2 to 3 million bats from the huge portals of Deer Cave is a natural wonder.

Karst of the Philippines The St Paul Subterranean River National Park is located on the northwestern coast of Palawan, which formed part of the land bridge between Borneo and Luzon in glacial epochs. As such, the biological affinities with Borneo are of great interest, and the potential exists for studies of long-term environmental change on the edge of the Sunda Shelf. The limestone itself is of Middle Miocene age, quarried locally as marble, and the fossil fauna and sediments of the caves can be expected to span much of the last 25 million years. The karst is therefore of regional significance and may provide valuable evidence for long-term climatic, eustatic, and faunal change. The karst landscape includes extensive limestone towers and polygonal karst as well as a large polje (enclosed basin with subterranean outlet) at Culiatan. The towers attain altitudes of 600 to 1000 m asl. The tops of the towers have extensive spitzkarren (rough sharp pinnacles), while the flanks have deep solution runnels. The limestone massif is drained by the St Paul

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Underground River, more than 6 km long. Given that the entire catchment is reserved, this site offers significant potential for ongoing studies of tropical karst landform processes, as well as interactions between terrestrial and marine processes on a limestone coast. The island of Bohol, in the south central Philippines, is world-famous for a distinctive karst known as the chocolate hills. The limestone covers an area of about 600 km2. This landscape is characterized by smooth, cone-shaped, isolated residuals of limestone, typical mogotes, which are locally metamorphosed and have elongated interfluvial residuals. Three sets of summit elevations have been discerned, while local mogote form reflects the gentle folding of the area. The karst depressions are usually elongated as well, and sometimes closed. These narrow valleys open into flat valleys which resemble poljes. Individual karst landforms include extensive caves, stream sinks and springs, and tufa dams on the watercourses. Over 50 per cent of the karst landscape is flat and supports irrigated agriculture, principally rice. In addition, crops such as maize and taro are grown, as well as vegetables and tree crops such as mangoes and jackfruit. Steeper slopes are terraced and may be planted to rice or support plantations. According to Urich (1991), there has been a profound decline of about 40 per cent in the outflow of springs in the karst over the last twenty years. This is not explained by climatic change; rather shifts in land use and resource allocation seem to be implicated. Deforestation preceding shifting cultivation has been one cause; this has been driven by a rapidly expanding population and a limited base of flat arable land. An increase in the use of water for domestic consumption and irrigation is also involved. Finally, quarrying of the mogotes for roadbuilding materials has accelerated and has reduced and disrupted the flow of underground water. Associated with this is a decline in water quality due to the combined effects of intensified use of agricultural chemicals, especially pesticides, domestic chemicals such as detergents, and human or pig sewage. These have resulted in the decline of useful bird populations, such as the cattle egrets. In addition, local water-borne diseases such as gastroenteritis, typhoid, and internal parasites have become more common in the population.

Karst and Caves of Thailand Along the western border of the country from the Malay border to the Shan States of Myanmar, the regional geology consists of parallel ranges developed on sedimentary rocks folded longitudinally and intruded by granites. Approximately 15–20 per cent of Thailand is

underlain by limestone bedrock of variable purity and age. Three types of carbonate rocks are found in Thailand. The Ordovician limestones of the Phattalung region are thinly bedded, sandy, or argillaceous and are thus relatively impure. The more pure and massive Permian limestones outcrop in the south of the country, where they attain a thickness of 1000 m. The Triassic limestones are found in the north, in Lampang Province. In the south of the country, extensive alluvial plains separate residual tower karst, some of which has been drowned by sea-level rise. To the north and west of the large Chao Praya Basin, large areas of folded limestone form residual plateaux with some marginal tower karst (Troll 1973). In the Chiang Mai area, Deharveng and Gouze (1983) described several types of karst terrain including karst ridges and crevice karst. Extensive doline fields occur near Satun on the Malaysian border ( Jennings 1972). Dunkley (1985) has described the extensive karst (1000 km2) in the Nam Lang–Nam Khong area of the Mae Hong Son province of northern Thailand. The regional geology consists of severely folded belts of Palaeozoic sediments varying in age from Ordovician to Permian. The region appears to have been above sea level throughout the Mesozoic and Tertiary, resulting in an erosion surface evidenced by remarkably accordant summit ridges. Drainage is along deep structural troughs running north–south and separating limestone plateaux and ridges with rugged terrain including large cliffs, dolines, springs, and extensive caves. Local relief is up to 1000 m on the limestone. Some polygonal karst is present, within which grikes and minor solution features are rare. In contrast, closed depressions or dolines are numerous and some attain a large size. Several can be considered to be poljes with alluviated and often swampy floors, e.g. Nam Lang depression with an area of 40 km2. Depression slopes are steep with near-vertical cliffs and thin soil mantles. The towers of southern Thailand have been described by Kiernan (1988), who concluded that the landscape results from a complex interplay of lithological, structural, and climatic influences, further moulded by both marine and fluvial processes. In the Phang-Nga–Krabi region the karst landscape is dominated by poljes draining through strike ridges or isolated limestone towers. Caves have formed where allogenic streams run across alluvium before sinking into the bases of the ridges and towers. These towers continue into the sea at Phang-Nga Bay, where drowned caves can be traversed by boats. Internal lake depressions, or hongs, have the appearance of cenotes, though solution rather than collapse is the dominant process. Recently Smart (forthcoming) has provided details of a calcite speleothem recovered

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by cave divers from a depth of 59 m and dated by radiocarbon at 34.2 + 4.5 ka bp. Speleothems occur in the drowned caves of Krabi at depths between 15 and 90 m, and provide an opportunity to reconstruct sea-level curves from this tectonically stable area. The seasonally heavy rainfall regime results in large quantities of water draining undergound, especially where concentration of flow occurs on non-carbonate rocks marginal to the limestone. Stream sinks at the contact carry aggressive water underground, resulting in probable high rates of cave passage enlargement. As well as limestone solution, corrasional enlargement by entrained sediment is a potent process. Gravel trains in the caves are common, while cave walls are frequently coated in organic mud. Within the caves are very extensive calcite speleothem deposits which may entirely block passages. Collapses of large blocks are common near entrances and may be aided by seasonal stream undercutting. In southern Thailand, the Thungyai–Huai Kha Khaeng Wildlife Sanctuaries contain significant areas of lowland riverine forest and other forest types more typical of strongly seasonal tropical climates (dry evergreen forest, dry dipterocarp forest, and savannah formations). This area includes low-relief limestone terrain with important karst wetlands in dolines and poljes.

Karst of Indonesia The classic karst of the Gunung Sewu area of southern Java has been studied extensively since the pioneering work of Lehmann (1936). This limestone area presents the classic cone karst, or fengcong, developed on Tertiary limestones of Miocene age. Of note is the juxtaposition of karst and volcanic deposits, including thick ash which has partially blanketed and infilled depressions. Cones and their intervening cockpits are aligned with faults and major lineaments (Waltham et al. 1983). The density of cones is estimated at 40 000 in an area of 1300 km2 (Urushibara-Yoshino and Yoshino 1997). The Gunung Sewu has been uplifted during the Quaternary, resulting in extensive marine terraces which penetrate the valleys and are cut in sandstones as well as limestones. On the lowest terraces the cones are very small, while the oldest terraces at 80 m asl have high cones and deep dolines. Some of the deeper dolines are used as reservoirs, or telaga, implying volcanic infills. The population density in this area is extremely high and ranges from 359 persons km−2 for the Tepus district to 936 persons km−2 in the alluvial Wonosari district. Since a very high proportion of the area is used for agriculture, there is virtually no intact primary or even secondary forest. The rainfall is highly seasonal and variable, thus droughts are common.

Karst of Viet Nam Viet Nam has exposed carbonate karst over about 20 per cent or 60 000 km2 of its total territory, distributed mainly in its north and northwest. The latter area (including Son La and Lai Chau) is one of the most important karst areas, where carbonate rock extends for 400 km from the border with South China to the east coastal zone in a belt ranging from 5 to 40 km wide. In general Viet Nam has a humid tropical climate heavily influenced by the Southeast Asia monsoon. As a consequence, the climate in northwest Viet Nam is characterized by high precipitation (annual rainfall 1413 mm), higher annual mean temperature (21°C), and high average air humidity (80 per cent). Geologically, widespread carbonate rocks have been deposited since the Precambrian up to the Holocene. Some pure limestone strata are locally 2000 m thick and are strongly folded and faulted. The dominant karst-forming rock is the Middle Triassic limestone (including the Dong Giao and Muong Trai Formations), mainly found in northwest Viet Nam. Karstification in the northwest region is enhanced by thick limestone strata, a more humid climate, denser vegetation, and thicker soil cover (Tran Kong Tau 1991). Northern Viet Nam is characterized by successive phases of rifting and collision, with most carbonate rocks subjected to folding, faulting, and the influence of differential neotectonic movements. Some hot springs outflow in active tectonic belts, such as Muong Tai–Pi Toong in Son La Province and Kenh Ga in Ninh Binh Province. There is a diversity of karst landscapes, which are due to both allogenic and autogenic drainage. The main karst morphological types in northern Viet Nam also change from northwest to southeast along a gradient (Do Tuyet 1997). There is a sequence from karst plateaux at Moc Chau and Son La (altitude 620–940 m) to fengcong (between Moc Chau and Muong Khen, or between Son La and Da River, alt. 650–1100 m) and fenglin (from Muong Khen to Ngo Quan, alt. 160–70 m). This sequence of karst landscapes is quite similar to those recognized from the Guizhou Plateau to the Guangxi Plain of southern China (Figure 10.10). Owing to the local setting, Viet Nam has a broader gufeng (isolated peak plain) along its coastal zone, more arable karst land on the karst plateau, and reduced rocky desertification in the karst areas. The karst plateaux of Viet Nam exhibit a wide range of morphologies including dolines of varying size, blind valleys which drain non-carbonate rocks through open cave systems, and in particular poljes. Poljes are widely developed in Viet Nam, and many are occupied and

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Fig. 10.10. Sequence of karst landforms in the Guilin area of China This sequence is also seen in Viet Nam. (Source: Salomon 2000)

used for agriculture. Most of them are smaller than 100 km2, ranging from 3–5 km2 to 40–50 km2, and ranging in elevation from less than 10 m in the coastal zone to over 1500 m at Sin Ho. The genetic types of poljes (Gams 1978) are mainly border poljes, structural poljes, and base-level poljes. In terms of karst microforms, sharp rillenkarren and solution runnels have been recorded in every karst area, especially in the Mai Chau district, where karren develops along vertical bedding planes. Coastal karren is well displayed in Ha Long Bay, Quang Ninh Province, where spitzkarren and solution runnels combine with sea-water corrosion notches to produce a spectacular landscape. Karrenfields are often found in open valleys where thin soil cover exists and there has been local stripping of this cover. In northern Viet Nam an unusual landscape formed in freshwater tufa can be seen along National Highway 6 from Hanoi. A continuous old tufa tableland can be seen from Yen Chau village to Du Lang village. Fed by karst springs, an apron of freshwater tufa has formed in the recent geological past, and then regional neotectonic uplift has produced a dissected tableland with relict waterfalls and rimstone pools.

Phong Nha Karst Region In the central part of Viet Nam, the thick Carboniferous and Permian limestones of the Phong Nha area are very extensive and have been folded and dissected since the Jurassic. There are extensive interbedded shales and sandstones, which may act to locally perch karst watertables and create poljes. These massive formations are well exposed and present an important biostratigraphic record for understanding the geological evolution, uplift

history, and long-term environmental changes on the Sunda Shelf and surrounding lands. In terms of the preservation of an intact karst hydrological system, the Phong Nha area presents one of the best examples of an extensive tropical karst with natural vegetation in a good, largely undisturbed condition. From a scientific viewpoint the opportunity to understand the workings of such a system is unique. This would be an ideal site in which to establish multidisciplinary studies including geomorphology, cave biology and ecology, terrestrial ecology, and cultural ecology. Underlying the karst is an extensive network of caves whose development is probably related to long-term (Mesozoic to Tertiary) tectonic history and consequent stream incision, as well as Pleistocene sea-level change. Given their location, these caves can be expected to preserve calcite formations and sediments (with included fauna) which could be used to elucidate long terrestrial environmental histories on the margins of the Sunda Shelf. The value of this for international programmes such as PAGES (Past Global Changes) is outstanding. The area of 400 000 ha exhibits a range of tropical karst landforms including very extensive caves. The British expedition reports (Limbert 1997) show that the caves are clearly in world class with large tubular passages, developed under the water-table but now drained, as well as canyon passages carrying large streams. The caves and karst of Phong Nha have clearly attracted considerable attention from past visitors and, more recently, organized tourism. This has been recognized by the Vietnamese government through its declaration of the Phong Nha Nature Reserve and its ongoing management planning, which is of a high standard. In 1997 some 28 000 visitors came to Phong Nha, and

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this can be expected to increase substantially. The area offers considerable potential for a wide range of recreational opportunities, from true wilderness experiences to developed tourist caves. The existing infrastructure near Phong Nha Cave does not impinge on the conservation values of the core wilderness area of the karst, and the proposed buffer zone on its periphery will further aid the maintenance of those values. Twelve to fifteen separate vegetation formations have been recognized. Most of the designated area is under primary lowland rainforest, and this is diverse with 735 species of vascular plants in 140 families. Twentyfive species of plants are listed as having threatened status by the Vietnamese government. Significantly, the area is on the overlap between northern (SinoTibetan) and southern (Malaysian) floras. Thus species of Dipterocarpus coexist with genera such as Platanus and Burretiodendron. Over 75 per cent of the reserve is relatively undisturbed primary rainforest on limestone and other substrates, while human-modified vegetation (swidden, shrubland, and secondary forest) accounts for about 22 per cent, and is mostly confined to the margins of the area. The varied fauna also complements the diversity of vegetation formations. Limestone areas, especially extensive ones, are usually significant refuges for animals. There are seventy-three endangered or threatened animal species in the Phong Nha area. The recently discovered giant muntjac (Megamuntiacus vuquangensis) has been recorded in the primary evergreen dense forest on limestone. The complex terrain and dense vegetation create isolated niches, and often physical barriers such as escarpments or major rivers traversing the karst promote allopatric speciation. The endangered black leaf monkey or langur (Trachypithecus francoisi) has also been recorded from this karst. In the west of the limestone area in Lao PDR, the subspecies T. f. laotum, which has a very distinctive white band around the head, has been found to be locally common. However, in the eastern portion extending into Viet Nam, an all-black form has been found, apparently bordering the range of the subspecies T. f. hatinhensis in Viet Nam. Thus there are at least three subspecies of these threatened leaf monkeys in the area. Douc langurs (Pygathrix nemaeus) appear to have declined or been lost from several areas of Viet Nam and Lao PDR in recent years, particularly from degraded forest areas and smaller isolated forests. They appear to be more susceptible to human pressures owing either to hunting or to habitat degradation than several of the macaque species also present. The status of cave invertebrates, both aquatic and terrestrial species, is virtually unknown but has great

potential given the ecological context. Given the integrity of the karst, the range of micro-habitats, and its biogeographic setting, we might expect to find species of both Pangean–Tethyan and Gondwanan origins. In particular, the orders Syncarida and Remipeda of aquatic crustaceans may well be present. Research into the cave faunas and their ecology should be a matter of high priority, as these will be the first organisms impacted by development for cave tourism. In summary the assemblage of plants and animals shows very high endemism and biodiversity in a relatively unaltered setting. Phong Nha is therefore of outstanding natural value for baseline studies of ecosystem processes in the humid tropics. The combination of alluvial and limestone ecosystems provides opportunities to understand long-term ecosystem dynamics, landscape ecology, and the impacts of visitors on tropical protected areas in a range of recreational settings. The World Heritage listed property of Ha Long Bay in northern Viet Nam also contains significant karst topography and caves, but in a coastal setting. The caves are mostly small in comparison, but provide key evidence of changing sea levels. Biodiversity values are also important, but the emphasis is on marine resources.

Dominant Karst Landforms in Southeast Asia Pinnacle Karst Pinnacle karst is a spectacular small area landform found principally in the humid tropics and subtropics, the best known examples coming from the islands of Borneo (Waltham 1995) and New Guinea (Williams 1972; Gillieson and Spate 1997). Typically the pinnacles are razor-edged limestone blades, closely packed, which can attain heights of 30 to 50 m. The intervening depressions frequently contain thin organic soils, dense root mats, and deep fissures. Hundreds of pinnacles may be found in an area of a few hectares: on Gunung Api, Sarawak, they occur over an area of 2 ha in a shallow bowl at an altitude of 1100 m. The geological context in which they are set provides some information about their mode of formation. The best examples are formed in massive limestones with well-developed conjugate joint sets in both horizontal and steeply inclined sets (Osmaston 1980; Waltham 1995). These sets facilitate the removal of solutional products into the karst, and collapses along the inclined joints further aid the dissection. Subaerial solution aided by intense rainfall etches the pinnacle edges and flanks into sharp flutings. Shafts in the depressions can be

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descended to depths of 30 or 40 m and frequently terminate in soil blockages or narrow crevices, often blocked by speleothem growth. On Gunung Api and elsewhere, pinnacles of various sizes are widespread but usually masked by rainforest vegetation. Most outcrops have pinnacles up to 5 m high with dense root mats and some shallow soils. Where forest has been cleared or denuded by fires, these become apparent and are frequently modified by fire spalling of their surfaces. Only in certain areas, where the geological structure is favourable, do they achieve massive proportions. Pinnacle karst can be distinguished from shilin, or stone forest, by the presence of subsoil solution notches and flutes in the latter. Shilin is typically formed by the widespread partial or total stripping of a deep soil mantle from massive dissected limestones on karst plateaux. The type example, from Yunnan Province in China, contains pinnacles 10 to 20 m high, formed in massive limestones well dissected by vertical joints (Knez 1997).

Karst Towers Wilford and Wall (1965) have described the geological setting of karst towers in Sarawak. Four main genetic types can be identified: 1. Residual hills protruding from a planed carbonate surface veneered by alluvium. This is a very common karst style throughout Southeast Asia where alluviation has occurred marginal to the limestone. Examples include the tower karst of the Kinta Valley, Malaysia, and the towers of south Sulawesi, Indonesia (McDonald 1976). 2. Residual hills emerging from carbonate inliers in a planed surface cut mainly across non-carbonate rocks. This style is more common where older Palaeozoic limestones occur as strike belts in volcanic or other sedimentary rock types. Examples include the karst of northern Thailand and the Southern Highlands, Papua New Guinea. 3. Residual carbonate hills protruding through an aggraded surface of clastic sediments that partially buries the underlying karst topography. 4. Isolated carbonate towers rising from steeply sloping pedestal bases of varying lithologies. In many areas limestone massifs have been dissected by meandering rivers to produce isolated karst towers in an alluvial plain. In the Kinta Valley of Malaysia, alluvial tin deposits have been mined, leaving a clay plain with numerous circular pits filled with thixotropic muds. Excavation of the alluvium reveals corroded limestone

pinnacles beneath coarse river gravels; a combination of solution and corrasion is implicated.

Dolines and Dry Valleys The dolines of tropical karst often differ in form from those of temperate climates. In Java (Lehmann 1936; Jennings 1985) closed depressions have convex hillslopes which produce star-shaped depressions with steep gullies carrying water to a stream sink in the depression base. They are more often set among steep-sided hills, though there are areas of plateaux karst with more conventional bowl shaped dolines, often on less pure limestone. Poljes, flat-floored depressions kilometres in extent, are relatively common in plateaux karst either where alluviation has been dominant or where impermeable rocks such as sandstone or shale underlie the limestone. In Malaysia, such landforms are known as wangs. At Phong Nha in Viet Nam, poljes punctuate the plateaux and are traversed by spring-fed streams. In New Guinea poljes are also widespread and create opportunities for settlement and agriculture in otherwise inhospitable karst terrain.

Marine Karst Variations in the relative levels of sea and land throughout the Quaternary have produced several examples of marine karst in Southeast Asia. The tower karst of Ha Long Bay, Viet Nam, rise from a shallow sea (less than 20 m deep) with the steep sides maintained by a combination of undercutting and collapse to produce an archipelago of limestone islands recently listed as World Heritage. Cave passages lead to flooded depressions within the towers ( Jennings 1985). These distinctive karst landforms have become world-famous and provide new focuses for tourism in the region.

Minor Surface Solution Forms As well as solution rills and runnels, solution ripples form on underhangs and consist of sharp ribs separating near-horizontal hollows about 2 cm across and tens of centimetres long (Wilford and Wall 1965; Jennings 1985). The water passing over them has a high content of organic acids from vegetation and soil. Solution runnels, separated from each other by sharp ribs of limestone, may be tens of metres long and run down many limestone rock faces. Swamp slots are usually associated with present or past water levels at the margins of alluvium; they may be several metres deep and contain fine-grained alluvium. Where they exist at several levels owing to valley incision, such sites may provide opportunities for dwellings and cave temples throughout the region.

174 David Gillieson

Environmental and Resource Issues in Karsts of Southeast Asia Agriculture, forestry, water management, limestone extraction, and tourism are usually the most important forms of economic activity in karst areas. Most of the world’s population is dependent upon agriculture, and agriculture is ultimately dependent upon the thin skin of soil on the Earth’s surface. Some karsts offer rich and highly productive soils that are utilized for both general and specialized agriculture. Millions of people live in karst areas, but karst soils are often particularly vulnerable owing to degradation by a variety of karst-specific processes that add to the usual pressures on soil. The increased pace of settlement of karst areas in Southeast Asia has been driven by population growth and movements of people due to economic pressure and, in some cases, warfare. Once lands exploited only for forest resources, the karsts are being cleared and settled in many countries. In general, the soils are thin and friable. Where shifting cultivation is practised, and areas left fallow for a decade or more, then secondary forest can grow on the karst and erosion is minimized. But under increased pressure, the soil resource is liable to degradation with high recorded rates of erosion (Uhlig 1980; Gillieson 1996). Loss of organic matter and other macronutrients may then become a serious problem, as these are concentrated in the top few centimetres of most karst soils. Coupled with this is the problem of reduced water retention. During extended dry seasons there is insufficient depth of soil to retain moisture, and crop failure may become widespread. Tourism is a major economic activity in some karsts of the region, including the use of both developed and undeveloped caves, and surface scenery, thereby generating local employment. The Batu Caves near Kuala Lumpur combine tourism and religious observances. Every year around 500 000 people visit the caves over a few days. There are now many tourist caves with lighting systems and pathways, not counting caves used for ‘adventure’ cave tours where visitors carry their own lights. Throughout Southeast Asia limestone landscapes contain important resources for agriculture, water supply, forestry, extractive industry, and tourism. In addition, karst areas contain significant cultural resources such as archaeological sites, cave temples, and monuments. The exploitation of limestone resources can have serious impacts on biodiversity and the integrity of the karst system unless careful planning is carried out. Limestone biodiversity is extremely vulnerable to disturbance because

• Species highly adapted to extreme habitats cannot survive outside that habitat after disturbance. • Species with a small range can easily become totally extinct. • Cave-dwelling species with a small range and small numbers at a site are very prone to local extinction, which may represent a significant proportion of the total population. The integrity of limestone areas is threatened at a large scale by • quarrying of limestone for agricultural lime, cement, building stone, and industrial fluxes; • changes in limestone hydrology due to abstraction of irrigation water, dam construction, deforestation, and increased stream sedimentation; • pollution by dust, silt, industrial effluents, and the impacts of acid rain; • uncontrolled harvesting of birds’ nests and guano in caves; • uncontrolled and inappropriate use of caves for tourism; • hunting using non-traditional methods; • over-collecting of commercially valuable plant species for medicine and food. Hunting is an important cultural and economic activity for the Penan people of the Gunung Mulu National Park of Sarawak, whose customary rights are enshrined in legislation. But other riparian groups also wish to hunt and do so periodically despite park regulations. Thus there can be local depletion of animals such as wild pigs, monkeys, and hornbills. Elsewhere in the region wild meat and live animals are often sold in village markets. Leaf-eater monkeys, Trachypithecus spp., are seen for sale in north Thailand and Lao PDR, while several species of cave-dwelling bats such as the hairless bat and the black-bearded tomb bat can be found in markets in Sulawesi and Thailand. The collection of birds’ nests and guano from caves is usually regulated by adat, or customary law, with rights being allocated to families and often restricted during the breeding season. Unfortunately the high international price for birds’ nests can tempt illegal collection, which may be hard to police in remote karst areas. This has already resulted in depletion of populations at several sites in Thailand, Sarawak, and Kalimantan. Massive excavations for guano in caves disrupts or destroys arthropod populations dependent on the guano as a food source, and may also change the directions of flow of percolation water resulting in localized flooding and erosion. Collection of plants is a widespread activity in limestone areas. In Hanoi cycad palms from the Ha Long

Karst in Southeast Asia 175

Bay World Heritage area adorn hotel foyers and gardens. Slipper orchids from karst areas in Indonesia, Viet Nam, and other places sell for high prices in the United States and Europe, resulting in local extinctions. The increased use of herbal medicines in the urban areas of Singapore, Bangkok, and Jakarta places increased pressure on the floral resources of karst areas. Tourism is expanding throughout the region and especially in its nature-based and adventure forms. Visitors have ecological impacts within caves, while the necessary infrastructure of pathways and lighting can alter water flow and desiccate calcite formations. Vandalism and graffiti are also widespread and difficult to remove or disguise after the event. The continued presence of visitors can adversely affect wildlife populations such as bats and swiftlets, interfering with breeding and in some cases forcing them to move. The International Union for Conservation of Nature and Natural Resources has produced a set of guidelines which address some of these issues (Watson et al. 1997). Clearly there will be ongoing and increasing demand for limestone resources throughout the region. Guidelines for the selection of sites need to follow the guidelines of Vermeulen and Whitten (2000). Specialists in karst geomorphology and biodiversity need to be engaged, and in general, • limestone areas in which no karst features or caves occur, or limestone bedrock below alluvium, or dolomitic limestones should be preferentially exploited; • selection of isolated limestone hills likely to contain endemic species or small populations of plants and animals should be avoided; • sites should be located in the largest limestone areas, but should leave a significant proportion untouched. The impact of extractive industry should be contained and effectively monitored, with provision for rehabilitation; • sites in areas with underground streams and springs should be avoided. This rational approach to the use of limestone resources will ensure that choices among candidate sites are made strategically and in a fully informed manner. It parallels the environmental and social impact studies being carried out for other resource allocation issues such as hydroelectricity. Thus it is necessary that countries in Southeast Asia with extensive limestone resources make systematic and complete inventories and evaluations of the karst resource for the benefit of present and future generations. This may necessitate the development of specialized legislation to take account

of the particular conditions of karst landscapes. Such legislation should include a requirement for strategic management plans, environmental impact plans, socioeconomic impact plans, and provisions for mitigations and rehabilitation. Non-governmental organizations active in the field could make very valuable contributions here. These reports should be produced in a timely fashion so that planning is proactive rather than reactive, so that stakeholders can take good notice of the advice and act responsibly.

References Brook, D. B., Eavis, A. J., and Lyon, M. K. (1982), ‘Caves of the Limestone: Gunung Mulu National Park, Sarawak’, Sarawak Museum Journal, 51/1: 95–120. Day, M. J. (1980), ‘Landslides in the Gunung Mulu National Park’, Geographical Journal, 146/1: 7–13. —— and Urich, P. (2000), ‘An Assessment of Protected Karst Landscapes in Southeast Asia’, Cave and Karst Science, 27/2: 61–70. Deharveng, L., and Gouze, B. (1983), ‘Grottes et karsts des environs de Chiang Mai (Thailande)’, Karstologia, 2/2: 55– 60. Do Tuyet (1997), ‘Overview on Karst of Viet Nam’, in Yuan Daoxian (ed.), Global Karst Correlation (Utrecht: VSP Press), 179– 92. Douglas, I., and Spencer, T. (1985), Environmental Change and Tropical Geomorphology (London: George Allen & Unwin). Dunkley, J. R. (1985), ‘Karst and Caves of the Nam Lang–Nam Khong Area, North Thailand’, Helictite, 23/1: 3–22. —— (1995), The Caves of Thailand (Sydney: Speleological Research Council). Farrant, A. R., Smart, P. L., Whitaker, F. F., and Tarling, D. H. (1995), ‘Long-Term Quaternary Uplift Rates Inferred from Limestone Caves in Sarawak, Malaysia’, Geology, 23: 357– 60. Gams, I. (1978), ‘The Polje: The Problem of Definition’, Zeitschrift für Geomorphologie, 22: 170– 81. Gill, D. (1999), Nomination of the Gunung Mulu National Park, Sarawak, Malaysia for World Heritage Listing, Report to UNESCO World Heritage Committee (Kuching: Sarawak Forestry Department). Gilli, E. (1993), ‘Les Grandes Volumes souterrains du massif de Mulu (Borneo, Sarawak, Malaisie)’, Karstologia, 22: 1–14. Gillieson, D. (1996), Caves: Processes, Development and Management (Oxford: Blackwell). —— (1998), ‘Evaluating Hillslope Stability in Tropical Karst’, Acta Carsologica, 6: 99–118. —— and Spate, A. (1997), ‘Karst and Caves in Australia and New Guinea’, in Yuan Daoxian (ed.), Global Karst Correlation (Utrecht: VSP Press), 229–54. —— Holland, E., and Davies, G. (1995), ‘Karst Geomorphology and Hydrology of Gunung Tempurung, Perak, Malaysia’, Helictite, 33/1: 35–42. Jennings, J. N. (1972), Karst (Canberra: Australian National University Press). —— (1985), Karst Geomorphology (Oxford: Blackwell). Kiernan, K. (1988), ‘Mangroves, Mountains and Munching Molluscs: The Evolution of a Tropical Coastline’, Helictite, 26/1: 16–31. Knez, M. (1997), ‘Lithologic Properties of the Three Lunan Stone Forests (Shilin, Naigu and Lao Hei Gin)’, in X. Chen (ed.), South China Karst (Postojna, Slovenia: ZRC SAZU), 30– 43. Lehmann, H. (1936), ‘Morphologische studien auf Java’, Geographische Abhandlungen, 9: 15– 67.

176 David Gillieson Leichti, P., Roe, F. N., Haile, N. S., and Kirk, H. J. V. (1960), ‘The Geology of Sarawak, Brunei and the Western Part of North Borneo’, Bulletin of the British Borneo Geological Survey, 3: 1–360. Letouzey, J., Sage, L., and Muller, C. (1988), Geological and Structural Map of East Asia (Tulsa, Okla.: American Association of Petroleum Geologists). Limbert, H. (1997), ‘Viet Nam 1997’, International Caver, 12: 3–10. McDonald, R. C. (1976), ‘Limestone Geomorphology in South Sulawesi’, Zeitschrift für Geomorphologie, suppl. vol., 26: 98–103. Osmaston, H. A. (1980), ‘Patterns in Trees, Rivers and Rocks in the Mulu Park, Sarawak’, Geographical Journal, 146/1: 33–50. —— and Sweeting, M. M. (1982), ‘Geomorphology: Gunung Mulu National Park, Sarawak’, Sarawak Museum Journal, 51/1: 75–94. Paton, J. R. (1962), The Origin of the Limestone Hills of Malaya, Professional Paper E-58-1T (Kuala Lumpur: Geological Survey of Malaya). Rose, J. (1984), ‘Alluvial Terraces of an Equatorial River, Melinau Drainage Basin, Sarawak’, Zeitschrift für Geomorphologie, 28: 155– 77. Salomon, P. Y. (2000), Précis de Karstologie (Pessac: Presses Universitaires de Bordeaux). Smart, D. (forthcoming), ‘A Preliminary Date (Radiocarbon) for a Sub-Sea Level Speleothem from Krabi Province, Southern Thailand’, Cave and Karst Science. Smart, P. L., Andres, J. N., and Batchelor, B. (1984), ‘Implications of Uranium Series Dates from Speleothems for the Age of Landforms in Northwest Perlis, Malaysia: A Preliminary Study’, Malaysian Journal of Tropical Geography, 9: 59–68. —— Bull, P. A., Rose, J., Laverty, M., and Noel, M. (1985), ‘Surface and Underground Fluvial Activity in the Gunung Mulu National Park, Sarawak: A Palaeoclimatic Interpretation’, in I. Douglas and T. Spencer (eds.), Tropical Geomorphology and Environmental Change (London: Allen & Unwin), 123–48. Tran Kong Tau (1991), ‘Karst and Soils Developed on Limestone in Viet Nam’, in Proceedings of the International Conference on Environmental Changes in Karst Areas—I.G.U.–U.I.S., Italy, 15–27 Sept. 1991, Quaderni del Dipartimento di Geografia, no. 13 (Padua: Università di Padova), 165–71. Troll, C. (1973), ‘Beobachtungen von Tropenkarst in Thailand und Malaya’, Geographische Zeitschrift Beihefte, 32: 1–16. Uhlig, H. (1980), ‘Man and Tropical Karst in Southeast Asia’, GeoJournal, 4/1: 31–44. Urich, P. B. (1991), ‘Exploitation of Tropical Karst Resources for the Cultivation of Wet Rice’, Proceedings of the International Conference on Environmental Changes in Karst Areas—I.G.U.–U.I.S., Italy,

15–27 Sept. 1991, Quaderni del Dipartimento di Geografia, no. 13 (Padua: Università di Padova), 39– 48. —— (1996), ‘Deforestation and Declining Irrigation in Southeast Asia: A Philippine Case’, International Journal of Water Resource Development, 12/1: 49– 63. —— and Reeder, P. P. (1999), ‘Plantation Forestry in Tropical Limestone Uplands: Environmental Constraints and Opportunities’, Professional Geographer, 51/4: 493–506. Urushibara-Yoshino, K., and Yoshino, M. (1997), ‘Palaeoenvironmental Change in Java Island and its Surrounding Areas’, Journal Quaternary Science, 12/5: 435– 42. Vermeulen, J., and Whitten, A. J. (2000), Impacts of Industrial Use of Limestone Resources on Biodiversity and Cultural Heritage (New York: World Bank). Verstappen, H. (1960), ‘Some Observations on Karst Development in the Malay Archipelago’, Journal of Tropical Geography, 14: 1–10. Walker, D. (1960), ‘The Alluvium, and Changes in the Relative Levels of Land and Sea’, in F. T. Ingham and E. F. Bradford (eds.), The Geology and Mineral Resources of the Kinta Valley, Perak, Geological Survey of Malaya District Memoir 9 (Kuala Lumpur). Walsh, R. P. D. (1983), ‘Hydrology and Water Chemistry of the Gunung Mulu National Park’, Sarawak Museum Journal, 30: 121–80. Waltham, A. C. (1995), ‘The Pinnacle Karst of Gunung Api, Mulu’, Cave and Karst Science, 22/3: 123–26. —— Smart, P. L., Friederich, H., Eavis, A. J., and Atkinson, T. C. (1983), ‘The Caves of Gunung Sewu’, Cave Science, 10/2: 55–96. Watson, J., Hamilton-Smith, E., Gillieson, D., and Kiernan, K. (1997), Guidelines for Cave and Karst Protected Areas (Gland: World Conservation Union (IUCN)). Wilford, G. E. (1961), Geology and Mineral Resources of Brunei and Adjacent Parts of Sarawak with Descriptions of Seria and Miri Oilfields, Memoir of the Geological Survey British Territory, Borneo, 10. —— (1964), The Geology of Sarawak and Sabah Caves, Geological Survey, Borneo Region, Malaysia, Bulletin 6. —— and Wall, J. R. D. (1965), ‘Karst Topography in Sarawak’, Journal of Tropical Geography, 21: 44–70. Williams, P. W. (1972), ‘Morphometric Analysis of Polygonal Karsts in New Guinea’, Geological Society of America Bulletin, 83: 761–96. Yeap Ee Beng (1995), ‘Origin of the Subsurface Limestone Karst and Development of Placer Tin Deposits of Peninsular Malaysia’, Paper presented to the International Association for Geomorphology– SE Asia Regional Conference, Singapore, 18–23 June 1995, in Program with Abstracts, 83.

11

The Coastal Environment of Southeast Asia P. P. Wong

Introduction

Geological Background

Several physical features combine to make Southeast Asia one of the most distinct and unique coastal regions in the world. The mainland or continental part of Southeast Asia consists of a number of peninsulas extending south and southeast from the Asian continent and separated by gulfs and bays. The world’s two largest archipelagos form the islands of Southeast Asia. During much of the Pleistocene, a large part of the South China Sea was dry land, and the islands of Sumatra, Java, and Borneo were linked to the mainland by the exposed shallow Sunda Shelf. Southeast Asia comes under the influence of the monsoons, or seasonal winds, which have an important impact on its coasts. The region is also a high biodiversity zone, characterized by its rich coral reefs and mangroves. This chapter examines the coastal environments of Southeast Asia in three stages. First, the major elements that make the coastal environments of Southeast Asia distinctive are discussed. The focus is on the coastal processes, as the geological framework and Quaternary have been covered in earlier chapters. Secondly, the various coastal environments in the region (excluding estuaries and deltas discussed in Chapter 13) are described next in terms of their extent, characteristics, and significance, with sufficient examples given to show their variability. Finally, the chapter ends with an assessment of the major environmental problems facing the region’s coastal environments— coastal erosion and rising sea level associated with climate change. Overall, this chapter provides the physical basis for a better appreciation of coastal development in Southeast Asia.

The coastal environments of Southeast Asia bear the impact of significant geological and climatic factors (Figure 11.1). Geologically, the core of the region is an extension of the Eurasian Plate meeting the IndoAustralian and the Pacific Plates and two lesser ones (Philippines and Molucca Sea) with mountain chains trending in a general north–south direction. The island of New Guinea is part of the Indo-Australia Plate. Island arcs have developed along the convergent margins, and many are volcanically active and also associated with shallow to deep earthquakes. Seismicity is concentrated in a broad belt along the Indonesian island arc and the Philippine islands which forms part of the CircumPacific zone of earthquake activity. Most of the region’s sea floor is characterized by the Sunda Shelf, a relatively flat continental shelf extending from 40 to 200 m below the present sea level. In the last 2 million years, worldwide colder periods (glacials) were associated with lower sea levels and warmer periods (interglacials) with higher sea levels. During this period, the lowest sea level was about 180 m lower than present, almost outlining the Sunda Shelf and exposing three times more land in Southeast Asia at the last glacial period than is visible now (Figure 11.1). The shelf is cut by two large submarine valley systems. One is a continuation of present rivers in Sumatra and west Kalimantan draining into the South China Sea and the other drains Java and south Kalimantan south towards of the Makassar Strait. The Java Trench, in the Indian Ocean south of Java and Sumatra, and the Mindanao Trench, east of the Philippines, form two natural bathymetric boundaries for the Southeast

Fig. 11.1. Major geological and climatic elements influencing the coastal environments of Southeast Asia (Sources: data from Tjia 1980; Morgan and Valencia 1983)

Coastal Environment of Southeast Asia 179

Asian marine region, separating it from the Indian and Pacific Oceans (Morgan and Fryer 1985). The coasts of Southeast Asia have been formed by highmagnitude episodic events as well as common coastal processes. Vulcanicity and associated tsunamis spawned by undersea earthquakes and volcanic eruptions have directly affected the coasts of Indonesia and Philippines. The most spectacular example of coastline changes in recorded history was caused by the eruption of Krakatau in 1883 (Simkin and Fiske 1983). A large volcanic island was reduced to a small one surrounded by three ash-mantled islands with retreating coastlines. The associated tsunamis, up to 40 m high, resulted in cliff and beach recession on the west Java coast. In 1927 volcanic activity revived and initiated the core of Anak Krakatau, which has grown to several hundred metres above sea level within a large caldera flanked by the three islands mentioned above. More recently, tsunamis generated by earthquakes have altered the north coast of Flores in 1979 and in 1993 and caused landslides in the coastal areas of Lombok (Monk, de Fretes, and Reksodiharjo-Lilley 1997). On 12 December 1992 an earthquake in the Flores Sea generated a tsunami that was responsible for the neardestruction of Babi Island, e.g. 263 casualties out of a population of 1093. Apparently, the most destructive wave caused by the tsunami swept over the back of the island relative to the tsunami source (Minoura et al. 1997). On 3 June 1994 an earthquake 200 km south of Java created tsunami waves which severely damaged villages on the south coast of Java (Synolakis et al. 1995; Maramai and Tinti 1997). On a smaller scale, submarine volcanic activity is responsible for mud volcanoes in various parts of Southeast Asia, building temporary islands up to several metres high off the Arakan coast in Cheduba Strait (Bird 1985a) and mud islands off the Sabah coast. The impacts of sea-level changes on coastal morphology and deposition can be seen in southern Thailand. Here, the west coast is continuously emerging, resulting in the formation of numerous islands, drowned valleys, and narrow beaches between truncated headlands and backed by crescent-shaped beach ridges. In contrast, the east coast is tectonically subsiding with thick sequences of Quaternary fluvial and deltaic aggradational depositions, and Holocene progradational beaches, large sand spits, and lagoons, assisted by a coastal current transporting sediments into the region (Dheeradilok 1995). The last eustatic change of sea level in Southeast Asia occurred 11 000 ± years ago. Two, or possibly three, older Pleistocene events of sea-level drops have been recorded (Biswas 1973). While detailed chronology

has yet to be worked out, it is possible that Southeast Asia saw the sea level rising from below the present msl after the last glacial. The Holocene sea-level changes are indicated by a variety of evidences, such as abrasional, biogenic, and depositional shorelines found at various levels (commonly at 0.5–1 m, about 2 m, and 5– 6 m above msl). These pose the possibility of not accepting a gradual decrease in the sea level but also to consider the possibility of a stepwise descent with long periods of standstills, storm waves, or local uplift being responsible for some of the ‘raised shorelines’ (Tjia 1992). For example, along the entire coast of Viet Nam and outlying islands, the evidences of a sea-level rise at 1–2 and 4–5 m above present level are in the form of deposits of shells, corals, oysters still attached to rocks, old reefs, beach rock, notches, and sea caves. All are less than 5000 years bp, although the higher levels are more suspect (Eisma 1985).

Coastal Processes Prevailing coastal processes in Southeast Asia are strongly influenced by the monsoons. The northeast monsoon is from December to February and the southwest monsoon from June to August. The transition from north to south monsoons occurs from March to May and the reversal is from September to November. The coasts are exposed to seasonal high waves during the monsoons (Figure 11.1). The northeast monsoon influences the eastern margin of the Philippines, Viet Nam, and the east coast of Peninsular Malaysia. The southwest monsoon influences the coast of Myanmar, the west coast of southern Thailand, and Indonesia from Sumatra through the south coast of Bali, Lombok, and Sumbawa up to Sumba. From July to November tropical cyclones or typhoons originate from the Pacific Ocean and they affect the northeastern region, i.e. central and northern Philippines and Viet Nam (Morgan and Valencia 1983). The cyclones from the Indian Ocean affect the Arakan coast of Myanmar. The cyclones bring high-energy waves to the coasts. In general, the coastal currents in Southeast Asia are driven by prevailing monsoon winds. During the northeast monsoon, currents from the north along the mainland coast of Asia flow into the region (the Mindanao Current, a branch of North Equatorial Current), and during the southwest monsoon the current direction is reversed. Strong circulation patterns result from the orientation of the principal seas, i.e. South China, Banda, Flores, and Java Seas. In other parts of the region the currents are less affected by the monsoons. In the eastern part of the region the currents are more freely

180 P. P. Wong

connected with Pacific Ocean waters. Despite the relatively strong currents, flushing action tends to be weak owing to reversing directions. Upwelling and sinking of surface waters occur alternately in the Banda and Arafura Seas (Morgan and Fryer 1985). The coasts experience low to moderate tidal ranges and a variety of tidal types influenced by conditions in the Pacific and Indian Oceans. Characteristic semi-diurnal tides of the Indian Ocean prevail in the west, affecting the Andaman Sea and the Strait of Malacca. Mixed tides of the Pacific occur in the eastern Indonesian archipelago and the Philippine waters. Almost pure diurnal tides predominate in the Gulf of Thailand and the Java Sea (UNEP/IUCN 1988). Within the region, the tidal range does not exceed 3 m except near the heads of exposed gulfs, such as the Gulf of Tonkin and the Gulf of Martaban (Figure 11.1). Water temperature and salinity pattern are strongly influenced by climatic factors, particularly the monsoons. For example, during the northeast monsoon, the water temperature off the north coast of Viet Nam is 18–20°C owing to the inflow of colder water masses from the high latitudes. This also results in lower water temperatures in the South China Sea. Generally, high surface temperatures of 28–30°C prevail on the west coast of Sumatra and the eastern waters of the Indonesian archipelago (Bleakley and Wells 1995). The annual temperature variations in the surface waters are small. The average annual range of sea surface temperature in the equatorial region is less than 2°C but is slightly higher (3–4°C) in the Banda Sea, the Arafura Sea, and Timor Sea, as well as in the waters south of Java (Bleakley and Wells 1995) and a maximum of 8–10°C in the Gulf of Tonkin for reasons mentioned above. Salinity is extremely variable, the result of the effects of high rainfall, run-off from many large rivers, and the geographical subdivision of the seas. The distribution of discharges from land, presence of large bays, and channels with little water exchange contribute to the general lowering of the salinity. The large excess of rainfall over evaporation results in an average salinity of less than 34 ppt within most parts. Annual salinity variations are affected by the monsoon cycle of rainy and dry seasons (Bleakley and Wells 1995).

Coastal Diversity As a result of various geological and geomorphic factors, a variety of humid tropical coasts (Bird 1982) are found over the region. The approximately 17 000 islands in Indonesia, 7000 islands of the Philippines, and numerous islands off the coast of mainland Asia provide many

island types ranging from coral cays to raised limestone, volcanic, and continental islands. The coastal environments of Southeast Asia are also characterized by high ecological diversity. Within the Indo-West Pacific biogeographical region, the communities of the mangroves and coral reefs of Southeast Asia attain their greatest diversity. Coastal diversity is even possible within a small area with a low-energy coast. For example, Singapore, within its 300 km of coastline, has the following different coastal forms (Swan 1971): mangroves, tidal flats, beaches, and fringing platforms, as well as cliffs, caves, shore platforms, and beach conglomerate (ironstone). This is due to intense and continuous chemical weathering and differences in the rock type and exposure. Table 11.1 provides a summary of countrywise coastal characteristics, and Figure 11.2 shows the general distribution of the region’s beaches, mangroves, and coral reefs. The basic coastal features are generally related to geology and operating processes. Mountain ranges subjected to structural disturbances produce the Dalmatian-type coast, e.g. Arakan. Headlands are widely associated with granitic rocks, quartzites, and shales. Their distinctive profiles are often associated with welljointed granite that has been subjected to intense tropical weathering and subsequently exposed. Limestone gives rise to dramatic karst coasts, e.g. the west coast of Thailand. Mangrove coasts prevail in sheltered and shallow areas and take advantage of materials brought down by the rivers, e.g. on both sides of the Malacca Strait. Coastal dunes can be found where strong winds and sediments are available, e.g. the Vietnamese coast. Corals flourish very well beyond the shallow areas and along the tectonically active margins of Southeast Asia. Ejecta from volcanoes provide sediment for beaches, e.g. on Bali and Lombok. In eastern Indonesia, atolls and barrier reefs build on subsiding ridges and platforms, and corals have produced some of the best beaches in this region.

Rock Coasts Rock coasts are formed where resistant rocks face the sea and the weathered products have been swept away. They are associated with mountain ranges that come near to the coast of the Sunda Shelf and along the island arcs. Rocky shores occur on the coasts of many islands in Southeast Asia. Observation of rock coasts in this region indicates three possible morphological types. The first is the continuous rocky coast as high cliffs. The southwest coast of Sumatra and the Pacific coastline of the Philippines and Sulawesi have such extensive rocky topographies

Fig. 11.2. Beaches, mangroves, and coral reefs in Southeast Asia (Sources: data from Morgan and Valencia 1983; WCMC 1997; BAKOSURTANAL 1998)

182 P. P. Wong Table 11.1 Synopsis of coasts of Southeast Asian countries Country

Length of coastline (km)

Basic coastal features

Brunei

130

Sandy towards the west and mangroves to the east and around estuaries

Cambodia

400

Mainly mangroves with rocky promontories separating sandy beaches, many backed by dunes

Indonesia

81 000

More than 17 500 islands. Extensive mangroves on east coast of Sumatra, east and south Kalimantan, and southern Papua. Rocky coast dominates Sulawesi, Lesser Sunda Islands, south Sumatra, south Java. Largest sand dunes are west of Parangtritis. Coral reefs are widespread; barrier reefs, many fringing reefs and atolls, particularly in Flores and Banda Seas

Malaysia

4800

Peninsular Malaysia: mangrove on west coast; 70% of east coast is sandy coast backed by beach ridges, distinct J-shaped bays between rocky headlands East Malaysia: mangroves, except for sandy and rocky coasts in northeast and northwest Sabah. Sarawak: sandy coast on east and mangroves on west

Myanmar

2300

Three distinct parts: Arakan coast of Dalmatian type with many emergent features; Irrawaddy Delta prograding up to 60 m yr−1; Tenasserim coast of rocky promontories, bays, and mangroves

13 000

More than 7000 islands, generally with steep, locally cliffed coast and sectors of sandy coasts. Mangroves in sheltered areas. Extensive coral formations on islands and as outlying reefs

Philippines Singapore

300

Most of natural coasts reclaimed; artificial beaches between series of breakwaters and sea walls

Thailand

3000

Western coast along peninsula characterized by mangroves; beaches on islands. Eastern coast predominated by sand beach ridges, spits, and lagoons. Head of gulf dominated by Mae Nam–Chao Phraya Delta. Southeast coast of wide, sandy bays between rocky promontories

Viet Nam

3800

Sông Hóng Delta in north and Mekong Delta in south. Between them, rocky headlands, beaches, small estuaries, and some dunes. Lagoons from 11 to 17°N

Sources: Bird (1985a, b, c); Eisma (1985); Pitman (1985); field data.

(MacIntosh 1982). In particular, wave erosion of limestone creates sheer or fissured cliffs with little or no beach formation. The second type refers to promontories. They include the small rocky outcrops and boulder formations commonly found above coral reef flats to larger headlands between sandy beaches. Rocky islands constitute the third category. The profile of rock coasts varies according to lithology. The typical profile is usually steep with a rocky face that often continues below the sea surface. An occasional narrow sand or shingle beach is found in the upper tidal zone. Granites (Bangka, Belitung, and Bintan Islands, and Riau–Lingga archipelago), sandstones (Bako in Sarawak), old limestones (Kaloatoa Island in the Flores Sea), and volcanic rocks (Lembeh Island, south of Padang) give rise to steep cliffs (MacKinnon et al. 1996; Whitten et al. 1987). On coasts exposed to waves, granite boulders are often precariously balanced, resulting in rockfalls (Figure 11.3). Joint systems give rise to more variation, like the steep coralline limestone cliffs on the Bukit Peninsula in Bali. Salt weathering and biochemical processes also aid in the break-up of rocks. In particular, profiles of limestone and sandstone cliffs show wave-cut platforms and marine notches. Series of caves and clefts have produced dramatic karstic landscape in Ha Long Bay (Viet Nam) and Phang-Nga Bay

(Thailand) and at El Nido, Palawan. At Phang-Nga Bay, limestone ridges formed marine tower karst rising 300 m (Pitman 1985) which have been the location of a number of films, including The Man with the Golden Gun. Karstic features are also found on low limestone coasts such as on Mactan Island, Philippines. Rock-strewn slopes plunge down into the sea in the Anambas, Natuna, and Tembelan Archipelagos (MacKinnon et al. 1996). The zonation of organisms on rocky shores in Southeast Asia follows the typical pattern with three major zones (supra-littoral, mid-littoral, and sub-littoral), characterized by three key groups of organisms: littorinid snails, barnacles, and algae, respectively (MacIntosh 1982; Tomascik et al. 1997b). High surface temperatures and desiccation greatly limit the tropical fauna and flora in comparison to those of temperate rocky shores. A rich assemblage of organisms occurs at the lowest tidal level and in crevices, where the environment is less extreme. Tropical rock pools are subject to extreme heating and wide fluctuations in salinity and consequently support a minimal biota. Contrary to expectation, rock coasts are not bare of vegetation. The overlying regolith has provided sufficient foothold for vegetation except where boulders occur. Where the slope lessens and salt spray is less, some form of lowland forest can be expected (Whitten

Coastal Environment of Southeast Asia 183

Fig. 11.3. Granite coast with boulders precariously balanced and subject to rockfall on the exposed side of Pulau Perhentian, Malaysia

et al. 2000). In Kalimantan and Sumatra, under the influence of salt spray, no one type of vegetation dominates. Species such as Barringtonia, Casuarina, and Calophyllum, and figs (Ficus) and pandans, cling to the rocks (MacKinnon et al. 1996).

Sandy Coasts Sandy coasts include various depositional forms such as beaches, spits, beach barriers, and dunes. Apart from mangroves and coral reefs, beaches are fairly common in Southeast Asia and predominate on the more exposed coasts, e.g. the Vietnamese coast, east coast of the Malay peninsula, south Java, west coast of Sumatra, and around the Philippines archipelago. They occur extensively on the shores of coral islands and are interspersed among other shore formations in the region. They vary from steep beaches of coarse sand built up on ocean-facing coasts or exposed to strong surf, to intertidal flats of mixed sediments, with a narrow sandy fringe at the high-water mark on more protected shores, e.g. north Sulawesi (MacIntosh 1982). In Southeast Asia, the sandy coasts are a reflection of past conditions, geological structure, and exposure. The general direction of beaches, spits, and barriers is influenced by the direction of swell. Those under the swell of the northeast monsoon face east-northeast and those under the Indian Ocean swell face mainly southeast– southwest (Eisma 1982). The availability of sediment can be an important factor, particularly for barriers, sand spits,

and beach ridges. In southern Thailand, the eastern peninsular coast is protected from the northeast monsoon, and benefits from longshore drift. This has resulted in the development of a complex of coastal sand barriers and spits, lagoons and mangroves, including the remnants of former beach ridges further inland (Pitman 1985). The sandy coast of central Viet Nam has a dozen large brackish water lagoons, reflecting interactions between lithology, littoral processes, seasonal climate, flooding, and human intervention (Zeidler and Nhuan 1998). On a more local scale are various coastal forms, e.g. trailing spits and tombolos, resulting from the protection offered by small islands (Figure 11.4). The origin of sediments has an important bearing on the character of sandy coasts. Rivers provide sediments of different composition and grain size. Other sediments can vary from black volcanic to white coral sand. Volcanic black sediments are fairly common in Indonesia and the Philippines, given the degree of volcanism, e.g. the coasts of Bali, the Senggigi coast of Lombok, Davao Gulf (from materials of Mount Apo), and Camiguin Island (from materials from the Hibok–Hibok volcano) (Bird 1985b). Near coral reefs, beach materials are derived from the break-up of corals, e.g. Pulau Seribu (Thousand Islands) northwest of Jakarta Bay. Other limited beach sediments include the forams for Sanur Beach (Tomascik et al. 1997a) and puka shells for the beach on the north coast of Boracay. Gravel beaches are limited and associated with rocky headlands, e.g. north Bintan.

184 P. P. Wong

Fig. 11.4. A double tombolo formed as a result of the monsoons from opposite directions affecting three rocky islets on a reef flat, Ko Nang Yuan, Gulf of Thailand

Beaches are dynamic systems, and in Southeast Asia they are dominated by the seasonal monsoons, which have an effect on the beach profiles, beach gradient, and grain size (Wong 1981). The monsoons also have geomorphic implications for coastal drainage. Over the short term, sand bars close up river mouths and aggravate flooding, e.g. on the east coast of Peninsular Malaysia. Over geologic time, a distinctive parallel drainage emerges as coastal streams cut through the beach ridges and seek to flow into the more sheltered area. Such streams are found on the sheltered gulf coast of Thailand (Pitman 1985) and the east coast of Peninsular Malaysia with stream outlets in the sheltered bays of zeta-form bays. Coastal dunes are limited in occurrence in Southeast Asia, and this may be explained by the prevalent wet conditions, which do not give rise to aeolian transport easily over wet sand. Sand dunes are found on more exposed coasts, e.g. along Arakan and Tenasserim of Myanmar (MacIntosh 1982), the Viet Nam coast, north coast of Madura, south Java at Parangtritis, southeast Java near Puger, and Aparri on northern Luzon (Bird 1985b). With active dunes up to 30 m in height, the dunes at Parangtritis are the best-developed coastal dunes in Southeast Asia (Figure 11.5). Sandy coasts in Southeast Asia have a distinctive beach vegetation which can be divided into two formations. On the seaward side is the pes-caprae formation (named after Ipomoea pes-caprae) which consists of various legumes, grasses, and sedges with spreading roots

that bind sand. Other species include legume Canavalia, sedges Cyperus pedunculatus and C. stoloniferus, and grasses Thuarea involuta and prickly Spinifex littoreus (Whitten et al. 2000; Wong 1978). The she-oak Casuarina equisetifolia is one of the first plants on the landward edge of the pes-caprae formation. It occurs frequently in pure stands and represents a late stage in plant succession. Unless the shore advances or provides fresh habitat, the she-oak will be replaced by other species (Whitten et al. 1987). Behind the pes-caprae formation is the Barringtonia formation with trees such as Barringtonia asiatica, Calophyllum inophyllum, and Terminalia cattapa. Other types include the coconut palm (Cocos nucifera), large bushes (Ardisia elliptica, Heritiera littoralis, Excoecaria agallocha), pandans (Pandanus), Scaevola taccada, Hibiscus tillaceus, and Thespesia populnea (Whitten et al. 2000). The Barringtonia formation becomes narrower where the coast is increasingly steep and rocky (Whitten et al. 1987). On some coasts of Southeast Asia, the pes-caprae formation has been modified by other natural conditions. In drier coastal areas, e.g. on the Gili Islands off Lombok, the cactus dominates the pes-caprae formation (Figure 11.6). Where the soil conditions are unsuitable, such as black, volcanic sand which absorbs more heat and thus becomes extremely hot, the pes-caprae vegetation becomes very poor, e.g. in north Sulawesi (Whitten et al. 1987). A wide and well-developed belt of beach vegetation helps to stabilize the sandy shores. The pes-caprae

Coastal Environment of Southeast Asia 185

Fig. 11.5. Coastal dunes at Parangtritis, south coast of Java

Fig. 11.6. Owing to drier conditions, cactus has replaced the more typical Ipomoea formation on the north coast of Gili Trawangan, Lombok, Indonesia

formation can form a thick carpet to stabilize the backshore but is often cut back by the monsoon waves. On the north coast of Pulau Bintan (Riau archipelago), a thick belt of larger bushes in the Barringtonia formation is more effective than large trees and palms to lessen the impact of erosion, at least by episodic events, such as the occurrence of high tides in December 1999.

Mangroves In comparison with the mangrove flora of equivalent latitudes on the Atlantic shores of Africa and the Americas, the mangroves of Southeast Asia are extremely diverse. Mangrove is the dominant coastal community in tropical Asia, with the Malay–Indonesian region as its

186 P. P. Wong

Fig. 11.7. Along various sectors of Bunaken Island, north Sulawesi, Indonesia, mangroves have established on the reef flat and protect the sandy beach

centre of distribution (Bleakley and Wells 1995). Conditions in the region are favourable for the mangroves to develop fully. These include the regular tidal inundations, which have a strong influence on the zonation and the structure of the vegetation. The presence of coral reefs, sand bars, and deltas leads to greatly reduced wave energy, another favourable condition. The mangroves in Southeast Asia occur typically along deltaic or more sheltered coasts that receive sediments from inland (Figure 11.2). The most extensive mangrove areas are along the east coast of the Malay Peninsula, Sumatra, Borneo, Thailand, and in areas associated with the deltas of rivers entering the Strait of Malacca. Within the region, Indonesia has the largest area of mangroves with 4.25 million ha, of which about 2.9 million ha are in Papua. Malaysia, with 650 000 ha, has the second-largest area of mangroves, while Thailand and Viet Nam have about 200 000 ha, the Philippines 100 000 ha, Brunei 7000 ha, and Cambodia 10 000 ha. In Viet Nam the mangrove cover has decreased by about 50 per cent since 1943 (Bleakley and Wells 1995). The coastal environment of Southeast Asia has changed significantly as a result of the severe decrease in the area under mangrove. According to Bleakley and Wells (1995) about 91 000 ha (46 per cent) of the mangroves in Thailand are under some form of use (farming, mining, salt-farming, and infrastructure activities), and there was a 25 per cent decrease in mangrove cover between 1979 and 1987. In the

Philippines, mangroves are estimated to cover about 20 per cent of that present in the 1920s, with the best stands remaining on the islands of Palawan and Mindanao. In Indonesia, the mangroves in the western parts of the country, particularly Java, have suffered heavily from human impacts, which include illegal cutting and conversion to other uses. In Sumatra, out of 1.37 million ha of original mangroves, 55 per cent remained in 1987 and 29 per cent in 1993, largely as a result of proliferation of tambak (brackish water ponds) (Chua, Ross, and Yu 1997). The mangroves in the eastern part of Indonesia are less affected, but signs of degradation have been recorded in some locations (e.g. Ambon and Halmahera). The three major mangrove ecosystems associated with coasts and deltas, lagoons, and islands can be found in Southeast Asia. They are well represented in Borneo, the region’s largest island, where the most extensive mangroves are at the mouths of the Kapuas, Mahakam, and Sebuku. These mangroves can penetrate far inland along the rivers, e.g. 240 km up the Kapuas (MacKinnon et al. 1996). Mangroves are less common outside deltas and river mouths. Protected by fringing reefs, mangroves can sometimes establish on some reef flats (Tomascik et al. 1997b). On Bunaken Island, north Sulawesi, mangroves have established on the reef flat to protect a sandy beach (Figure 11.7). Mangroves reach their maximum development on the Andaman Sea coast, where individual trees can be 40 m in height. On the Malaysian west coast, the most

Coastal Environment of Southeast Asia 187

Fig. 11.8. Major zones in a mangrove forest. The numbers refer to annual days with tidal wetting. (Source: adapted from Whitten et al. 2000)

biodiverse mangroves are in Sungai Merbok in Kedah (Ong and Gong 1996). Despite overlapping vegetation types, it is generally recognized that the mangroves in Southeast Asia have several major ecological–succession zones with associated geomorphic significance (Whitten et al. 2000). The zones are related to the frequency and depth of tidal flooding and in a seaward direction are (Figure 11.8): 1. the terrestrial margin, not affected by wave action; 2. the true mangrove forest (Bruguiera), morphologically separated by rivers, streams, and gullies; 3. the eroded bank or the seaward edge of the mangrove forest (Rhizophora), marked by a nearvertical bank 1–1.5 m high where the soil is slowly removed by wave action. Erosion also takes place on the inner bends of streams; 4. the pioneer zone (Avicennia or Sonneratia), sloping seaward; 5. the foreshore of soft mud and no vegetation. The mangrove plants indicate several adaptations to survive in saline and waterlogged conditions. For example, distinctive root systems in various forms, such as breathing roots or pneumatophores, stilt roots (Rhizophora), spike roots (Sonneratia and Avicennia), and loops or knee roots (Bruguiera) have developed for gaseous exchange above the waterlogged and anoxic soil. Also, the trunks of Ceriops have openings (Whitten et al. 1987).

Mangroves play an important geomorphological role in the coastal environment of Southeast Asia. Previously, it was thought that mangroves cause aggradation or build up land directly, but their role is better understood today. Mangroves may accelerate land extension but do not have any influence on the initial development of landforms. It is the shape, topography, and history of the coastal zone that determine the resulting mangrove habitat, Tidal levels and soils are the most important factors (Whitten et al. 1987, 2000). Shore protection starts with the process of sediments building up on the landward side of the mangroves as the root systems of established stands slow down silt-laden water. Plant succession allows mangrove pioneers to move seaward and thus ensures coastal stability. Prograding coastlines are most distinct at mouths of large rivers as silt deposited by rivers is trapped by mangroves, as in Sulawesi or Papua. In Sumatra, Palembang, a thriving port when visited by Marco Polo in 1292, is now 50 km inland (MacKinnon et al. 1996). The mangrove plants themselves have a limited capacity to stop erosion. Replanting is effective on low-energy eroding coastlines. An exception is Rhizophora stylosa, which can occur on wave-swept coasts with stable coralline substrates (Whitten et al. 1996). The value and importance of mangroves in providing a wide range of environmental functions have been well documented (Hamilton and Snedaker 1984; BAKOSURTANAL 1998; Gilbert and Janssen 1998; Primavera 2000). Mangroves form an important habitat

188 P. P. Wong

for breeding fish and as a nursery for many species of fish, shrimp, and other biotas. Their direct benefits include wood for building, firewood, charcoal, and pulp. The intertidal flats trap sediments, absorb flocculated clays, and at low tide, a variety of small crustaceans and molluscs can be found. Mangroves also provide services such as storm protection, flood abatement, erosion control, and waste treatment. Brackish water forests characterized by the nipa palm (Nypa fruticans), which forms pure, often extensive stands, are found at the inner boundary of mangrove forests, at the upper tidal limit of rivers, and behind sandbars or barriers thrown up by waves. The nipa palm is one of the most versatile plants in the coastal zone, from which a number of food and household products (kernels for food, sap for alcohol, wine, and vinegar, leaves for wrapping food and for roofing and walls, and midribs of leaves for brooms) are derived (Chew 1996).

Coral Reefs Southeast Asia occupies only 2.5 per cent of the ocean surface, but contains 30 per cent of the world’s coral reefs (Chou 1998). The high diversity occurs at the generic and, especially, at the species level. Geologic, oceanographic, and climatic conditions in the region are favourable for reef growth, and the tropical storms are restricted to the higher latitudes in the Philippines, Thailand, and Viet Nam. The distribution of corals is indicative of certain specific environmental conditions affecting coral growth (Figure 11.2). For example, corals flourish where water movement is moderate and in situations close to the open sea. Harsh exposure, together with the presence of high amounts of sediment, discourages coral growth. Some corals have considerable regenerative capabilities following damage by large waves; for example, Acropora coral actually benefits from fragmentation. Indonesia and the Philippines support the most extensive areas of coral reef in the region— some 20 000 islands and a combined coastline of 100 000 km. Welldeveloped reefs are also found off the southern coasts of Myanmar and Thailand, on the offshore islands of Viet Nam, on the east coast of Peninsular Malaysia, and off Sabah (UNEP/IUCN 1988). Reefs are less common on the mainland coasts and on larger islands, and absent around large river estuaries. All major reef types— fringing, barrier, platform, atoll— can be found in Southeast Asia. Fringing reefs are most common and are present around most small to medium-sized islands. The three major barrier reefs in Indonesia occur southeast of Kalimantan, south of Sulawesi, and curving outward towards the islands

of Batu and Banjak off Sumatra (Bird 1985c). A large number of atolls and isolated reefs of various shapes (often referred to as shoal, platform, or patch reefs) are found in the region and located on navigation charts. There are also thousands of subsurface reefs, many of which are uncharted. One such area of subsurface atolls and shoal reefs is in the Palawan and Camarines reef banks in the Philippines (McManus 1988). Taka Bone Rata in the Flores Sea is the third-largest atoll in the world. Corals typically grow on a variety of hard substrates. They can grow along rocky shores with no substantial limestone reef development and these are referred to as ‘non-reef coral communities’ to distinguish them from structural fringing reefs. Such examples can be found in parts of Ambon Bay (Indonesia), the Si Chang Islands (Thailand), Sombrero Island (Philippines), and various rocky shores in Malaysia and Singapore (McManus 1988). The typical profile of a fringing reef in Southeast Asia shows several distinct zones: a shelf of limestone bordering the coast, an intertidal or slightly subtidal reef flat, a wave-breaking reef crest, and an outer reef slope broken into series of ridges and rifts (McManus 1988) (Figure 11.9). Some fringing reefs can extend considerable distances offshore. For example, the fringing reefs at Sanur– Nusa Dua, Bali, extend a considerable distance offshore, up to 700 m, but separated by a deep-water channel, often called the ‘boat channel’ as boats used it for access across the reefs to reach the sea. Depending on tidal conditions, this channel can be of varying depth. Within the channel the water level remains high where the connection with the sea is relatively restricted (Tomascik et al. 1997a). Coral reefs have a protective role for the coast, since waves break at fringing reefs and much of the energy is dispelled. Over the years, damage to corals has been caused by natural and human factors. Damage to reefs by natural events include climate, tides, and tectonic events, coral predators, and disease (UNEP/ IUCN 1988). The worldwide coral bleaching event of mid-1997 to late 1998 affected the reefs of Southeast Asia (Wilkinson 1998). Massive mortality (often near 95 per cent of shallow corals) occurred in Singapore. ‘Severe’ bleaching with around 50–70 per cent mortality, and also coral recovery, was reported in Thailand and Viet Nam. ‘Moderate and patchy’ bleaching on some reefs in large areas, with a mix of coral recovery and around 20–50 per cent mortality, but no effects in other parts, occurred in parts of Indonesia and the Philippines. ‘Insignificant’ or no bleaching was reported for most of Indonesia. Bleaching and mortality were most pronounced in shallow water (less than 15 m)

Coastal Environment of Southeast Asia 189

Fig. 11.9. Typical profile of a fringing reef in Southeast Asia (Source: adapted from White 1987)

and particularly affected staghorn and plate Acropora and other fast-growing corals. Many of the massive, slow-growing species bleached, but recovered within one or two months. More observations and monitoring are required to determine whether bleached corals will recover (or die), and whether damaged reefs have the potential to recover. More importantly, there is a need for continued observations to determine whether this is a rare, severe event, or part of a pattern of increasing disturbance associated with global climate change. Within Southeast Asia, more damage to the coral reefs has come from human activities. These activities result in sedimentation, water pollution, coastal development, deleterious fishing methods, intensive recreational use, and overexploitation of reef resources. The destruction of protective coral reefs has caused severe erosion in several locations in Indonesia as in Sengdiku, near Candidasa and Sanur Beach in Bali, the west coast of Java near Serang, Cilegon, and Pandeglang (Whitten et al. 1996), and the east coast of Java. Constituting a common physiographic and ecological zone of many reefs in the Indonesian and Philippines archipelagos are coral cays or low islands constructed entirely of biogenic materials. A wide variety of them corresponding to those in the Great Barrier Reef are found: varieties of unvegetated solitary island, vegetated solitary islands, multiple islands, and complex low wooded islands. Also included is one category not discussed so far, fringing reef islands, similar to cays, and found on the reef flats of many large fringing reefs (Tomascik et al. 1997b). On a much smaller scale, reef tombolos form across the reef flats and on the lee of patches of reefs found on exposed coasts, e.g. in north Bali

and north Bintan. The materials forming the tombolos are coral debris that have been well compacted by refracted and diffracted waves. A significant but often forgotten habitat closely associated with coral reefs is sea grass. Southeast Asia has the most highly diverse sea grass flora in the world with about twenty species from seven genera. Sea grasses are found in shallow coastal waters or in lagoons between the coral reefs and the shore (Fortes 1988). Few species extend into the intertidal or littoral zone and can tolerate high temperatures during the days at spring low tides (Whitten et al. 1987). Generally, they grow gregariously and large enough to form meadows or dense beds covering large areas of coastal waters. A number of factors influence the distribution of sea grasses (Whitten et al. 1996). While the distance from shore affects general change of species, the nature of the substrate and its depth determine the presence of species. Sea grasses are best developed around calm and sandy beaches. Their destruction comes from a change in substrate composition and suspended sediment concentration. Sea grasses perform a significant geomorphic role along the coast. They are fast-growing and can bind shallow sediments against erosion. For example, at Sanur (Bali), the stability of the beach system is indirectly dependent on the sea grass. The beach sediments are produced from corals, forams, and tetrahedral tests or shells of species of benthic foraminifera. The quality of the beach depends on forams, which, in turn, depend on the distribution of the sea grasses. In addition, the sea grasses serve to stabilize lagoon sediments and reduce turbidity (Whitten et al. 1996).

190 P. P. Wong

Sea grasses also perform a wide spectrum of biological and physical functions, serving as habitat and nursery areas for fish, many invertebrates, turtles, and dugong. They are a source of food for the dugong and the green turtle and they also provide alternative feeding sites for commercial and forage organisms. Sea grass meadows in some areas have been converted to commercial algae farms. They can provide a buffering effect from oil pollution (Fortes 1988).

Coastal Erosion While progradation has been widely reported for deltaic coasts of Southeast Asia, e.g. the Irrawaddy, Mekong, Sông Hóng, etc., many other coastal sections are affected by erosion. Bird (1985d) provides an overview of the severity of erosion, the more significant erosional sectors, and associated factors. Myanmar’s coasts do not suffer from much erosion. Much of Thailand’s coastal erosion is on the western coast of the Thai Peninsula. Malaysia’s coastal erosion is on the east coast of the peninsula. In the Philippines, erosion occurs on coasts exposed to high wave energy. Within protected coasts, e.g. the Lingayen Gulf, erosion is related to the passage of tropical cyclones. In Indonesia and Peninsular Malaysia, erosion takes place on many beach ridge plains, arising from the reduction of sediment load to the river or diversion of the channel due to river capture. Within the region, coastal erosion has in many places been associated with human activities, such as the removal of mangroves and coral reefs. For example, in Bali, severe erosion resulted from the destruction of the protecting reefs at Sanur and Candidasa (Wong 1998) and from sediment starvation arising from extension of the airport runway at Denpasar, which affects Kuta. Coastal erosion can result from the force of tides; for example, in the Musi Banyuasin Estuary, south Sumatra, the mangroves are uprooted by violent water interaction between river discharge and tidal influx (Fortes 1988). Less frequent events, such as unusually high tides, have an impact on erosion (Wong 1991). The extreme spring tides of December 1999 also resulted in erosion reported in Malaysia, Sabah, Bintan, and Singapore.

Climate Change and Implications for Coastal Environments In the light of the possible impact of climate change and a rise in sea level, Paw and Chua (1991) provided a preliminary picture for Southeast Asia. The physical impact of a sea-level rise would include coastal erosion

and inundation of low-lying areas, salt intrusion, flooding due to storm surges and high tides, and habitat loss. The economic impacts would include destruction of coastal properties and changes in land use systems, water management systems, navigation, and waste management. The Regional Workshop on Climate Change Vulnerability and Adaptation in Asia and Pacific concluded that, apart from other impacts of climate change, sealevel rise is of the greatest concern to the islands and coasts in this region (Amadore et al. 1996). Indonesia, Malaysia, the Philippines, and Viet Nam have formed national committees to examine the potential impacts of sea-level rise based on the scenarios generated by the Intergovernmental Panel on Climate Change. Such assessments have been useful in showing the problems of coastal development, which would sometimes be compounded by the potential sea-level rise. Using the scenarios of 0.3, 1, and 2 m rise in sea level, Perez et al. (1996) discussed the potential impact of a sea-level rise on the Manila Bay coastal area, which is representative of many economically important coastal areas in Southeast Asia. The study showed the areas of vulnerability, particularly to flooding, and also where the bay itself is affected by passing cyclones at a frequency of three cyclones per five years and the resulting storm surges. This example also points out the long wait for mitigation measures to be implemented. As with most of Southeast Asia, more information and education on the topic, concern at all government levels, the development of a responsive strategy, long-term effective coastal zone management, and financial support are required. The study on Viet Nam by Zeidler (1997) provides some idea of the vulnerability at the national level. A 1 m sea-level rise would cause annual flooding impacting on 17 million people, of which 14 million are in the Mekong Delta, resulting in a loss of $US17 billion per year. If no measures are taken, about 40 000 km2 will be flooded annually. A protection strategy against such a rise in sea level would cost a further $2.4 billion, bringing the total protection strategy cost to about $9 billion. Tri, Adger, and Kelly (1998) examined the benefits of a strategy of rehabilitation of the mangrove coast in three coastal districts of northern Viet Nam to meet the risk of tropical storms.

Conclusion Southeast Asian coasts are geologically young if measured from the time when the shallow Sunda Shelf was exposed in the Pleistocene. The barriers and beach

Coastal Environment of Southeast Asia 191

ridge complexes date largely from the fall in sea level in the last 5000–6000 years. Physical conditions have provided the region with a wide range of humid coasts and a rich diversity of mangroves and coral reefs. In part, the coastal diversity can be attributed to the large number of islands in the region. Geologically, many coasts are affected by vulcanicity, earthquakes, and the resulting tsunamis. The climatic impact is particularly strong during the monsoons, which increase the wave energy and result in coastal erosion and river mouth changes. Human activities in the coastal zone have also accelerated the coastal erosion problem. Sea-level rise is probably the single most important physical factor to affect Southeast Asian coasts in the near future. Climate change may also increase the frequency of storms and is probably related to the coral bleaching. In the long term, climate change and the associated rise in sea level will have a profound impact on Southeast Asian coasts.

References Amadore, L., Bolhofer, W. C., Cruz, R. V., Feir, R. B., Freysinger, C. A., Guill, S., Jalal, K. F., Iglesias, A., Jose, A., Leatherman, S., Lenhart, S., Mukherjee, S., Smith, J. B., and Wisniewski, J. (1996), ‘Climate Change Vulnerability and Adaptation in Asia and the Pacific: Workshop Summary’, Water, Air, and Soil Pollution, 92: 1–12. BAKOSURTANAL (National Coordination Agency for Surveys and Mapping) (1998), Indonesia: Atlas Sumber Daya Kelautan (Marine Resources Atlas of Indonesia) (Jakarta: National Coordination Agency for Surveys and Mapping). Bird, E. C. (1982), ‘Coastal Landforms of the Asian Humid Tropics’, in C. H. Soysa, L. S. Chia, and W. L. Collier (eds.), Man Land and Sea: Coastal Resource Use and Management in Asia and the Pacific (Bangkok: Agricultural Development Council), 3–13. —— (1985a), ‘Burma’, in E. C. Bird and M. L. Schwartz (eds.), The World’s Coastline (New York: Van Nostrand Reinhold), 767–9. —— (1985b), ‘Philippines’, in E. C. Bird and M. L. Schwartz (eds.), The World’s Coastline (New York: Van Nostrand Reinhold), 873–7. —— (1985c), ‘Indonesia’, in E. C. Bird and M. L. Schwartz (eds.), The World’s Coastline (New York: Van Nostrand Reinhold), 879–88. —— (1985d), Coastline Changes: A Global Review (Chichester: Wiley). Biswas, B. (1973), ‘Quaternary Changes in Sea-Level in the South China Sea’, Geological Society of Malaysia Bulletin, 6: 229–56. Bleakley, C., and Wells, S. (1995), ‘Marine Region 13, East Asian Seas’, in G. Kelleher, C. Bleakley, and S. Wells (eds.), A Global Representative System of Marine Protected Areas, vol. iii: Central Indian Ocean, Arabian Seas, East Africa and East Asian Seas (Canberra: Great Barrier Reef Marine Park Authority; Washington: World Bank; Gland: World Conservation Union), 107–36. Chew, Y. F. (1996), ‘Wetland Resources in Malaysia’, in State of the Environment in Malaysia (Penang: Consumers’ Association of Penang), 111–16. Chou, L. M. (1998), ‘Status of Southeast Asian Coral Reefs’, in C. Wilkinson (ed.), Status of Coral Reefs of the World: 1998 (Townsville: Australian Institute of Marine Science); www.aims.gov.au/pages/ research/coral-bleaching/scr1998/scr-06.html (25 Oct. 2001).

Chua, T. E., Ross, S. A., and Yu, H. (eds.) (1997), Malacca Straits Environmental Profile (Quezon City: GEF/UNDP/IMO Regional Programme for the Prevention and Management of Marine Pollution in the East Asian Seas). Dheeradilok, P. (1995), ‘Quaternary Coastal Morphology and Deposition in Thailand’, Quaternary International, 26: 49–54. Eisma, D. (1982), ‘Asia, Eastern, Coastal Morphology’, in M. L. Schwartz (ed.), The Encyclopedia of Beaches and Coastal Environments (Stroudsburg: Hutchinson Ross), 76– 82. —— (1985), ‘Viet Nam’, in E. C. Bird and M. L. Schwartz (eds.), The World’s Coastline (New York: Van Nostrand Reinhold), 805–11. Fortes, M. D. (1988), ‘Mangrove and Seagrass Beds of East Asia: Habitats under Stress’, Ambio, 17: 207–13. Gilbert, A. J., and Janssen, R. (1998), ‘Use of Environmental Functions to Communicate the Values of a Mangrove Ecosystem under Different Management Regimes’, Ecological Economics, 25: 323– 46. Hamilton, L. S., and Snedaker, S. C. (eds.) (1984), Handbook for Mangrove Area Management (Hawaii: UNEP and East-West Center). MacIntosh, D. J. (1982), ‘Asia, Eastern, Coastal Ecology’, in M. L. Schwartz (ed.), The Encyclopedia of Beaches and Coastal Environments (Stroudsburg: Hutchinson Ross), 67–76. MacKinnon, K., Hatta, G., Halim, H., and Mangalik, A. (1996), The Ecology of Kalimantan (Hong Kong: Periplus Editions). McManus, J. W. (1988), ‘Coral Reefs of the ASEAN Region: Status and Management’, Ambio, 17: 189– 93. Maramai, A., and Tinti, S. (1997), ‘The 3 June 1994 Java Tsunami: A Post-Event Survey of the Coastal Effects’, Natural Hazards, 15: 31–49. Minoura, K., Imamura, F., Takahashi, T., and Shuto, N. (1997), ‘Sequence of Sedimentation Processes Caused by the 1992 Flores Tsunami: Evidence from Babi Island’, Geology, 25: 523– 6. Monk, K. A., de Fretes, Y., and Reksodiharjo-Lilley, G. (1997), The Ecology of Nusa Tenggara and Maluku (Hong Kong: Periplus Editions). Morgan, J. R., and Fryer, D. W. (1985), ‘The Marine Geography of Southeast Asia’, in G. Kent and M. J. Valencia (eds.), Marine Policy in Southeast Asia (Berkeley: University of California Press), 9–32. —— and Valencia, M. J. (eds.) (1983), Atlas for Marine Policy in Southeast Asian Seas (Berkeley: University of California Press). Ong, J. E., and Gong, W. K. (1996), ‘Mangroves, Fish and Chips’, in State of the Environment in Malaysia (Penang: Consumers’ Association of Penang), 121– 4. Paw, J. N., and Chua, T. E. (1991), ‘Climate Changes and Sea Level Rise: Implications on Coastal Area Utilization and Management in South-East Asia’, Ocean and Shoreline Management, 15: 205–32. Perez, R. T., Feir, R. B., Carandang, E., and Gonzalez, E. B. (1996), ‘Potential Impacts of Sea Level Rise on the Coastal Resources of Manila Bay: A Preliminary Vulnerability Assessment’, Water, Air, and Soil Pollution, 92: 137– 47. Pitman, J. I. (1985), ‘Thailand’, in E. C. Bird and M. L. Schwartz (eds.), The World’s Coastline (New York: Van Nostrand Reinhold), 771–87. Primavera, J. H. (2000), ‘Development and Conservation of Philippine Mangroves: Institutional Issues’, Ecological Economics, 35: 91–106. Simkin, T., and Fiske, R. S. (1983), Krakatau 1883: The Volcanic Eruption and its Effects (Washington: Smithsonian Institution Press). Swan, S. B. St C. (1971), ‘Coastal Geomorphology in a Humid Tropical Low Energy Environment: The Islands of Singapore’, Journal of Tropical Geography, 33: 43– 61. Synolakis, C., Imamura, F., Tsuji, Y., Matsutomi, H., Tinti, S., Cook, B., Chandra, Y. P., and Usman, M. (1995), ‘Damage Conditions of East Java Tsunami of 1994 Analyzed’, EOS, 76/26: 257, 261–2.

192 P. P. Wong Tjia, H. D. (1980), ‘The Sunda Shelf, Southeast Asia’, Zeitschrift für Geomorphologie, 24: 405–27. —— (1992), ‘Holocene Sea-Level Changes in the Malay–Thai Peninsula, a Tectonically Stable Environment’, Bulletin of the Geological Society of Malaysia, 31: 157–76. Tomascik, T., Mah, A. J., Nontji, A., and Moosa, M. K. (1997a), The Ecology of the Indonesian Seas, pt. 1 (Hong Kong: Periplus Editions). —— —— —— —— (1997b), The Ecology of the Indonesian Seas, pt. 2 (Hong Kong: Periplus Editions). Tri, N. H., Adger, W. N., and Kelly, P. M. (1998), ‘Natural Resource Management in Mitigating Climate Impacts: The Example of Mangrove Restoration in Viet Nam’, Global Environmental Change —Human and Policy Dimensions, 8: 49–61. UNEP/IUCN (United Nations Environmental Programme/International Union for Conservation of Nature and Natural Resources) (1988), Coral Reefs of the World, vol. ii: Indian Ocean, Red Sea and Gulf (Nairobi and Gland: UNEP/IUCN). WCMC (World Conservation Monitoring Centre) (1997), Coral Reefs and Mangroves of the World, www.wcmc.org./uk/marine/data/ coral_mangrove/marine.maps.main.html (19 July 2001). White, A. (1987), Coral Reefs: Valuable Resources of Southeast Asia (Manila: ICLARM). Whitten, A. J., Mustafa, M., and Henderson, G. S. (1987), The Ecology of Sulawesi (Yogyakarta: Gajah Mada University Press). —— Soeriaatmadja, R. E., and Afiff, S. A. (1996), The Ecology of Java and Bali (Hong Kong: Periplus Editions).

—— Damanik, S. J., Anwar, J., and Hisyam, N. (2000), The Ecology of Sumatra (Hong Kong: Periplus Editions). Wilkinson, C. (1998), ‘The 1997–1998 Mass Bleaching Event around the World’, in C. Wilkinson (ed.), Status of Coral Reefs of the World: 1998 (Townsville: Australian Institute of Marine Science); www.aims.gov.au/pages/research/coral-bleaching/scr1998/scr01.html (25 Oct. 2001). Wong, P. P. (1978), ‘The Herbaceous Formation and its Geomorphic Role, East Coast, Peninsular Malaysia’, Malayan Nature Journal, 32/2: 129– 41. —— (1981), ‘Beach Changes on a Monsoon Coast, Peninsular Malaysia’, Geological Society of Malaysia Bulletin, 14: 59–74. —— (1991), ‘Impact of a Sea Level Rise on the Coasts of Singapore: Preliminary Observation’, Journal of Southeast Asian Earth Sciences, 7: 65–70. —— (1998), ‘Coastal Tourism Development in Southeast Asia: Relevance and Lessons for Coastal Zone Management’, Ocean and Coastal Management, 38: 89–109. Zeidler, R. B. (1997), ‘Continental Shorelines: Climate Change and Integrated Coastal Management’, Ocean and Coastal Management, 37: 41–62. —— and Nhuan, H. X. (1998), ‘Littoral Processes, Sediment Budget and Coast Evolution in Viet Nam’, in E. B. Thornton (ed.), Coastal Dynamics ’97: Proceedings of the International Conference (Reston, Va.: ASCE), 566–75.

12

The Mekong River Basin Ian Douglas

The Basin The 4800 km Mekong (known as the Lan Tsan Chiang or Lancang in its upper reaches in Yunnan Province, China) rises at 5100 m elevation on the eastern edge of the Tibetan (Xizang) Plateau where the Yangtze (Chang Jiang) and Salween also rise (Fig. 12.1). With a drainage basin covering 795 000 km2, the river ranks as the ninth largest and twelfth longest in the world and discharges some 475 billion m3 of water to the South China Sea annually. The mean annual flow at Kratié in Cambodia (where the catchment area upstream is 646 000 km2) is 14 700 m3 s−1 with a maximum of 67 000 m3 s−1 and a minimum of 1250 m3 s−1 (Committee for Coordination of Investigations of the Lower Mekong Basin 1966; Volker 1983). The river flows from the Tibetan Himalayas southward through China receiving tributaries from a small part of Myanmar. The drainage basin also encompasses nearly all of Lao PDR, northeast Thailand, most of Cambodia, and part of the Central Highland and the delta of south Viet Nam. In the heart of Cambodia, where the river is joined by the Tonlé Sap or Great Lake River, it rises from 1 or 2 m above sea level in May to 8 or 10 m above sea level in August. The Mekong Basin embraces some of the most diverse scenery in the world, with landforms ranging from deep gorges, to spectacular karst features, great lakes, and a huge delta. These varied landscapes support one of the most biologically diverse river systems in the world, surpassed only by the Amazon and possibly the Nile. The high biodiversity varies greatly across the following distinct landform and biogeographic provinces: 1. the eastern edge of the Tibetan Plateau (here termed the Chinese upper reaches);

2. the highlands of Myanmar, northern Thailand, and the northern Lao PDR; 3. the Annamite Mountains of eastern Lao PDR and western Viet Nam; 4. the plains around the central Mekong in Lao PDR, Thailand, and Cambodia; 5. the Tonlé Sap Basin; 6. the Mekong Delta and coastal mangroves (MacKinnon and MacKinnon 1986).

The Climatic Diversity and Rainfall Seasonality The area has a great diversity of rainfall, both seasonally and spatially (Fig. 12.2). The high, cold, and sometimes snowy headwaters are relatively dry compared to the other parts of the basin. The annual rainfall at Denchin on the edge of the Tibetan Plateau is around 650 mm a year, while on the Lao–Vietnamese border at the head of the Nam Theun River the yearly rainfall totals over 3000 mm. The latter well-watered highlands provide a large part of the total flow of the Mekong, as a comparison of the Vientiane and Mukdahan monthly discharge graphs indicates (Fig. 12.3). The Khorat Plateau of eastern Thailand is relatively dry. Annual rainfalls average less than 1500 mm a year at stations such as Khorat and Ubol (Committee for Coordination of Investigations of the Lower Mekong Basin 1966). The cold winters of the upper Mekong exert their influence as far south as the Xieng Khuang Plateau in the Lao PDR where at 1150 m above sea level, the water in irrigation channels sometimes freezes (Robequain 1952). The run-off regime of the Mekong is akin to that of its neighbouring rivers, the Salween and the Yangtze. In all three rivers, the bulk of their water is supplied by

TPGC12 14/12/2004 12:38 Page 194

Fig. 12.1. Major rivers and locations in the Mekong Basin

Fig. 12.2. Mean annual rainfall over the Mekong below China

196 Ian Douglas

Fig. 12.3. Mean monthly discharges at key gauging stations on the main river and tributaries in the lower Mekong Basin (Source: Based on data in the Water Year Books of the Mekong Committee for 1965 and 1966)

The Mekong River Basin 197

tributaries entering their middle courses. The summer southwest monsoon rains provide the water for the most spectacular feature of the Mekong, its annual flooding. The seasonal run-off pattern (as indicated by the Stung Treng station in Fig. 12.3) reflects the monsoon rainfalls, with high flows occurring from June to October and low water from February to April (Volker 1983). At Phnom Penh, flood peaks exceeding 50 000 m3 s−1 tend to occur towards the end of September or in early October. The rainfall seasonality dominates life in the Mekong Basin. Wet-season rice is the principal product: upland rice from swidden (shifting cultivation systems) in the hills and paddy rice in the lowlands and valley floors. Hill-rice-cultivating peoples are often living at densities of less than 20 per km2, whereas the intense paddy cultivation in the delta supports more than 300 per km2. Fisheries are strikingly important for the people of the Mekong. In the Lower Mekong Basin (south of the Chinese border) 1.5 million tonnes of fish and other aquatic animals are caught each year (2 per cent of the world’s capture fishery) with another 0.5 million tonnes from reservoir fisheries and aquaculture. About 40 million people, two-thirds of all those living in the Lower Mekong Basin, are actively involved, at least part-time or seasonally, in the fisheries (Mekong River Commission 2002). Navigation and drainage play a key role in the commercial life of the lower reaches of the river. However, since the late 1950s efforts have been made to coordinate international activities to exploit the hydropower and irrigation potential of the river’s water (Schaff and Fifield 1963; White 1963; Mitchell 1998; Jacobs 2002).

Plans to Develop the Basin’s Water Resources Internationally the Mekong Basin is probably most renowned for the plans to develop more than fifty giant hydroelectric dams and diversion schemes. These great waterworks were the vision of Raymond Wheeler, a retired general of the United States Army Corps of Engineers. Like most water engineers, Wheeler thought that a major obstacle to economic development in the Mekong Basin was annual flooding. He believed that the rivers should be dammed, and the flood-dependent agriculture replaced with modern irrigated cash crops. Wheeler’s recommendations for damming the Mekong led the United Nations to set up the Mekong Committee in 1957 to ‘promote, coordinate, supervise and control the planning and investigation of water resources development projects in the Lower Mekong Basin’ (Transboundary Freshwater Dispute Database 2002).

Seven impressive dams and reservoirs were planned for the Mekong mainstream. The generating capacity would total 23 300 MW while the reservoirs would be capable of storing more than one-third of the river’s annual flow (Fig. 12.4). The High Pa Mong Dam alone would have required the resettlement of 250 000 people, flooded 3700 km2 of land, and cost $US10 billion. None of these mainstream dams has been built. During the 1960s several ‘multi-purpose’ dams were completed on Mekong tributaries in northeast Thailand, and in 1968, the 150 MW Nam Ngum Dam in Lao PDR began to export electricity to the Electricity Generating Authority of Thailand. Throughout the 1970s, wars in Cambodia, Lao PDR, and Viet Nam prevented further planning for the Mekong. The Mekong Secretariat wrote in retrospect: ‘The Committee by force of circumstance was obliged to concentrate only on national-level and tributary development and even that in only some of the riparian states. The much more important and ambitious mainstream development projects had to be postponed to when all four countries could again sit at the same table’ (Ryder 1994). By the late 1980s peace and the lifting of trade embargoes on Viet Nam led to renewed support for the Mekong Committee, especially from Australia, the Netherlands, and Sweden. Consultants and companies from these countries developed plans for hydropower schemes that would serve peak demand for power in rapidly industrializing Thailand. New privatization attitudes, fostered by the major donor agencies, suggested that the planned dams be built on a build-own-operatetransfer basis. The arrangement called for foreign contractors or groups of investors to capitalize, build, and operate projects for twenty or thirty years until accruing a predetermined profit, after which the government would take over. Strong pressures by campaigning non-governmental organizations in Thailand led national leaders to look to the unharnessed rivers of Lao PDR for potential dam sites. Lao PDR government leaders saw their country as a future Switzerland. Major international transport and communications routes linking Thailand to China and Viet Nam would be developed. Great hydropower stations would supply electricity to these rapidly industrializing neighbours. Such advances would deliver the country from years of isolation and economic stagnation. The idea of mainstream dams was revived in the late 1980s. China began building dams on the Yunnan reaches. Plans were prepared for a scaled-down, Low Pa Mong Dam with a crest at 210 m above sea level, instead of the original 250 m elevation. The low dam

198 Ian Douglas

Fig. 12.4. The original plans for hydropower development in the Lower Mekong Basin

The Mekong River Basin 199

would require the resettlement of 60 000 people, less than a quarter of the displacement involved in the High Dam proposal. Cambodia and Lao PDR expressed concerns about the impact of the Pa Mong development on flood flows downstream. Locally and internationally vociferous criticism of both mainstream and tributary dam proposals is heard. While the water resources of the basin are an immense asset, their value for different purposes, such as irrigation, fisheries, hydropower, and irrigation, changes as international economic situations and energy resource strategies alter. The rapid discovery and exploitation of natural gas resources in the South China Sea and Gulf of Thailand dramatically affected electricity generation strategies in the Mekong countries in the 1990s. By 1999 more than a quarter of Thailand’s electricity was being generated from gas-fired power stations, while less than a tenth of the country’s needs were being met by hydropower sources either within the country or across the Mekong in Lao PDR. Future development and management of the water resources of the Mekong will be affected by these economic situations. Even so, all dam and reservoir projects will have to consider their impacts on communities and wildlife. Basin-wide attention to the general condition of soils and land degradation is needed as the problems of the eroded mountainsides are linked to effects downstream right to the areas of acid sulphate soil problems in the paddy fields of the delta. The Mekong River Commission, while retaining a hydropower and navigation development programme, is now placing much more emphasis on sustainable development, improving land and water management within the catchment areas, and emphasizing the importance of aquatic ecosystem health and downstream fisheries ( Jacobs 2002; Mekong River Commission 2002).

The Chinese Upper Reaches In Xizang Province (Tibet) and Yunnan, the Lancang Jiang flows for more than 1000 km through a succession of steep-walled gorges, some more than 1000 m deep. In these upper reaches the Mekong parallels the Salween to the west and the Yangtze to the east. Although these gorges are no more than 80 km apart in places, they are separated by rugged, high mountain ranges rising to 6000 m. The Chinese sector of the catchment has a population of nearly 5 million people. Yunnan Province alone has about 18 000 species of plant and plays a highly important role in the biodiversity of both China and the Mekong Basin. Four hundred and sixty-two of the 821 species of

Chinese medicinal materials recorded in the China Medicinal Dictionary are found in the province. More than thirty kinds of animal in Yunnan, such as the wild elephant, golden-haired monkey, gibbon, and wild buffalo, have been given the highest level of protection in China. In addition, the province also contains 766 species of birds, 370 species of freshwater fishes, and more than 13 000 species of insects. Since 1949 about 1200 species of tropical plants have been brought into the province. Most of them grow well in the new environment. For example, Yunnan is now the second most important province in China, after Hainan Island, for rubber plantations. Since the 1960s, slash-and-burn agriculture, and burning, logging, and clearing of forests, have reduced and continuously threatened the forest coverage of the province. Some logging has been stopped, but slashand-burn agriculture continues due to increased pressure of growing population on forest land. Although deforestation remains a serious problem in the area, the provincial government is vigorously implementing afforestation projects. In the fertile Lancang Valley, known as Sipsongpanna, the surrounding steep hills still have about 75 per cent forest cover supporting over 5000 known plant species and 400 species of animals.

Chinese Water Resource Projects In the 1970s planning began for a system of eight (now seven) major dams and associated power stations on the lower Lancang Jiang (Table 12.1). After delays through lack of capital, the first of them, the 1500 MW capacity Manwan, began to supply the Kunming– Chuxiong urban–industrial axis and southwest Yunnan in mid-1994. The second barrage, the 1350 MW capacity Dachaoshan Hydropower Station, was completed in 2000. The third, Xiaowan, is due for completion in 2012. The fourth, Jinghong, further south, intended primarily for the export of power to Thailand, should be completed by 2010. Lao PDR, Myanmar, Thailand, and Cambodia have all protested about the plans as the Yunnan reservoirs will reduce the water available for storage in other projects downstream.

Improving Access and Trade The Yunnan area has long been relatively isolated from the countries to the south, except by the river. Yet there are ambitious plans for road links between Thailand and China either across Myanmar from Mae Sai on the Thai border to Daluo on the Chinese border, or through Lao PDR from Chiang Khong in Thailand through Muang Sing to Mangman in Yunnan. Thailand and Yunnan are both experiencing rapid economic growth,

Table 12.1 Existing and proposed hydropower dams in the Mekong River Basin Dam

Active storage (109 m3)

Installed capacity (MW)

Main stem dams, China (Upper Mekong (Lancang) River) Manwan 250 1500 Dachaoshan 370 1350 Jinghong 250 1500 Xiaowan 990 4200 Nuozhadu 1220 5500 Mensong — 600 Gongguoqiao 120 750 Ganlanba 150 Planned main stem dams, Lower Mekong (construction unlikely 2003) Stung Treng 980 Sambor 10 3300 Don Sahong 240 Ban Koum 2330 Pa Mong ‘A’ 2030 Pak Lay 1010 Sayabouri 1260 Luang Prabang 970 Pak Beng 1230 Completed and planned dams, Lao PDR Nam Mouan Phase 1 Nam Ngum 2/3 Nam Ou 2 Nam Hai (Pump storage) Nam Theun 2 3.91 Nam Ngiep 2 Nam Ngiep 1 Nam Ngum 3 Nam Theun 1 Nam Ou 1 Sekong 4 Nam Ngum 2 Sekaman 3 0.47 Sekong 3 Nam Ngum 4 Sebanghieng 2 Sekaman 1 Sekong 5 Nam Tha 1 Nam Theun Hinboun Nam Theun 3 Nam Suang 2 Senamnoy Houay Ho Nam Khan 2 Dak-E-Meule M Nam Khan 1 Nam Cha 1 Sekaman 4 Nam Kong 1 Huay Lamphanh Nam Lik 2 Sepone Nam Khan 3 Nam Sane Nam Ting Nam Theun 4 Nam Cha 2 Nam Theun 5

Persons displaced/ resettled (no.)

Construction status

Sources

3513 6054 32 737 2264 23 826

Completed 1993 Completed 2000 Completion end 2003 Completion due 2012 Possibly 2017 p p p

a a,b a a,c a,d a a d

9160 5000–60 000 0 2573 23 260 8710 1720 5200 1670

p p p p p p p p p

1080

p

f

1047 950 800 681 495 440 400 400 399 346 320 300 298 290 285 255 253 230 210

p p p Due to start late 2003 p p p p p p p Starting 2004 p p p p p p Completed 1998

f f f f f f f f f f f f f f f f f f f

p p p Completed 1999 p p p p p p p p p p p p p p p

f f f b f f f f f f f f f f f f f f f

200 195 192 150 145 115 115 115 115 105 103 100 100 95 90 80 80 70 65

4500

Est. zero, but > 1000 in fact

3000

Table 12.1 Continued Dam

Active storage (109 m3)

Sebanghieng Senamnoy D Nam Leuk Sebangfai 1 Sedone Sekaman 2 Sebangnouane Sebanghieng 3 Senamnoy 2 Xeset Nam Beng 1 Nam Ngum Extension Sexou Nam Kong 2 Nam Mang 3 Nam Mang 1 Khone Waterfall Dak-E-Meule U Namkong 3 Nam Ko Keng Wek Nam Noua Completed dams in Thailand Sirindhorn Chulabhorn Ubolratana Pak Mun

Installed capacity (MW)

Persons displaced/ resettled (no.)

65 63 60 60 54 53 50 50 48 40 45 40 35 30 30 30 29.12 23 21 1.5 1.5 1

0.225

36 15 25 136

Completed and proposed dams in Viet Nam Dray H’Linh Drayhlinh Yali Falls Thuong Kontum Pleikrong Se San 3 Se San 3a Se San 4 Buon Kuop Srepok 3 Srepok 4 Buon Srha Duc Xuyen

12 28 720 260 123 270 100 330 280 137 195 85 58

Completed and planned dams in Cambodia O Chum 1 Prek Thnot Prek Liang 1 Prek Liang 2 Lower Se San 1 Lower Srepok Kamchay Stung Mnum Stung Sen 3.7

1 18 40 45 900 480 120 439 40

Construction status

Sources

p p Completed 2000 p p p p p p Completed 1991 p Completed 1999

f f b,f f f f f f f b f f f f f f f f f f f f

p p p p p p p p p p Completed Completed Completed Completed

1500

7500 2500

15 000

1968 1971 1966 1997

Completed 1995 Feasibility study Completed 2001 Pre-feasibility Starting late 2003 Started 2002 Started 2003 Pre-feasibility Feasibility study Pre-feasibility Pre-feasibility Pre-feasibility Pre-feasibility

b,k k b g,h k,l g,k g,k g,k k i,k k k k

Operational in 2001 p p p p p Pre-feasibility p p

m j g g g m j j m

Note: p = planned. Sources: a Chapman and He (1994). b Mekong River Commission (2001). c Chen Liang (2003). d International Rivers Network (2002a). e Hirsch. and Cheong (1996).

f Australian Mekong Resource Centre (1999). g International Rivers Network (2002b). h Miller et al. (1999).

i j k l m

b b b b

Hydro in Vietnam (2003). Kajander (2001). Nguyen Anh Tuan (2003). Vietnam News Agency (2003). Hydro in Cambodia (2003).

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but the physical gap which separates them (between Daluo and Mae Sai) is approximately 250 km of bad roads. In 1990 travel between Mae Sai and Kengtung was said to require eight and a half hours’ non-stop on a motorcycle, but improvements are now under way in Myanmar. With some 95 km southwest of Kunming due for completion as a four-lane highway by 2003 and the remainder of the Chinese section scheduled to be completed by 2006, the prospect of an ‘Asian Highway’ through the Mekong corridor linking Kunming with Bangkok is coming closer to fruition (Chapman and He 1994). The Asian Development Bank has allocated resources for the necessary work through the difficult terrain of Lao PDR. Such a link would transform Chinese cross-border trade with Myanmar and Lao PDR, in both volume and diversity. The notorious illegal movements of heroin from border areas of Myanmar to Kunming and beyond, and the unofficial exports of logs to China, notably from Lao PDR, have received wide publicity. However, these items are merely two components of a trade flow that has grown quickly from a trickle to a torrent, extending well beyond the areas near the border. And as Yunnan’s trans-border trading area expands, it is increasingly enmeshed with Thailand’s trans-border trade reaching northwards from Mae Sai through Kengtung to the Yunnan border at Daluo, and from Chiang Khong-Ban Houei Sai on the Mekong to Mengla County.

The Mountains of the Myanmar, northern Thailand, and northern Lao PDR Areas The rugged highlands of the Golden Triangle area are geologically complex, with a series of features produced by the collision of the Indosinian block with the Shan–Thai block in Upper Carboniferous to Triassic times (Barr et al. 1990) resulting in volcanic arcs, backarc rifts, and complex thrust features. The Mekong and its tributaries cut through these ranges in a series of gorges and defiles. In the 845 km stretch from the Myanmar border down to Vientiane, rock reaches of river alternate with smooth water. The river bed and banks are mainly rocky. Small plains occur at the mouths of tributaries, but much of the valley is narrow and steep-sided with side-walls rising to more than 1000 m in places. The lowest of these gorges, 24 km upstream of Vientiane, is the site of the proposed Pa Mong Dam, originally planned as the largest main river dam in the whole Mekong development scheme. Within these sectors, mass movements on the walls of gorges and the steep

slopes of tributary valleys are relatively frequent and may cause small diversions through new gravel bar development in the channel (Gupta and Chen 2002; Gupta et al. 2002). Typical of these areas is the Nam Mae Kok catchment of northern Thailand, where population pressure on the land is intense. The land use pattern is complex, with forests altered by shifting cultivation (swidden agriculture) on the mountain slopes and terraced ricefields in the valley floors and flatter parts of the lower basin. Shortage of land means that individual swidden plots are cultivated for more than a year and that fields are often rested for only two or three years. Soil nutrients are not replaced under such intensive use and thus the forest does not regenerate, but tough Imperata cylindrica grass invades, turning whole hillsides into grasslands (Anderson 1993). Many farmers now spend the bulk of the dry season digging out the resistant Imperata cylindrica root mat to prevent it from colonizing croplands and making weeding excessively labour intensive. Much of the pressure on land, and thus the change from forest to grassland, arises from movements of people from remote mountain villages into lower catchment areas. Sometimes such movements have been government-directed, sometimes refugees are involved, and in Lao PDR the changes are a legacy of the bombing and upheavals associated with the Vietnam War of the 1960s and early 1970s. The latter effects are well illustrated by the Nam Ngum catchment in Lao PDR. Here the key environmental issues are: • the resource use conflicts arising from the differing patterns of natural resource use of the ethnic groups of the area, often affected by the legacy of war and forced resettlement; • unsustainable forms of swidden agriculture which are reducing forest cover and degrading soils, causing erosion and thus leading to alternations of flood and drought with consequent food and water shortages; • unregulated land use and degradation, arising from the lack of a system for land allocation and security of tenure; • limited staff capacity for resolving resource issues; • pressures on the catchment area from external agricultural, hydropower, and logging interests all capable of providing Lao PDR with substantial export income. The result is a landscape in transformation. Often hillsides of grass have replaced forest. Fires for land clearance blaze out of control, burning away far more regrowth vegetation than is required for clearing new

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plots. The small plains are too restricted to provide all the irrigated wet ricefields the people would like. The effort required by farmers to sustain their food supply is increasing year by year, yet externally funded projects to assist them have often been too short-term to be sustainable and to achieve the results needed.

The Tributaries of the Annamite Ranges The uplands and ranges on the eastern margin of the Lower Mekong Basin comprise a variety of folded and fractured sedimentary rocks with areas of granitic intrusions and volcanic rocks. Considerable areas of limestone form impressive, but little-known, karst scenery, especially in the upper Nam Theun catchment. Great sandstone escarpments dominate some of the Laotian river valleys. Large areas of basalt have given rise to fertile plateaux, particularly in the Central Highlands of Viet Nam around Pleiku. These formerly densely forested areas contain great biodiversity. The Lao PDR part of the area includes the 1600 km2 Nakai Plateau and a forested region along the Lao–Vietnamese border of particular biological significance. The plateau and the upper Theun catchments provide one of Southeast Asia’s largest remaining areas of undisturbed rivers and forest, particularly lowland forested rivers, old-growth pine and cypress forest, and wet evergreen and montane forests. The areas where more plants and animals not found elsewhere in Viet Nam occur are the Hoang Lien Son range in the north, the Central Highlands, and Da Lat Plateau. The traditional swidden and hunting and gathering subsistence economies of the area have depended heavily on this biodiversity. The people have relied in the first instance on the stability of the forest environment. However, increasing population, migration from other areas, and the loss of some land due to the legacies of war, especially unexploded bombs and landmines, have given poor local people no choice except to clear more forest for agriculture and so reduce the diversity and stability of local ecosystems. The major tributaries draining this area are the Nam Kading (Nam Ca Dinh), with its major affluent the Nam Theun; the Banghiang (Se Bang Hiang); the Se Kong; the Se San; and the Sre Pok. All these rivers have mountainous headwaters, but extensive flat plains around their lower reaches. The Nam Theun Basin with its tributaries has a potential hydropowergenerating capability of about 17 000 GWh/year. Two projects, the Nam Theun 2 (600 MW) and the Nam Theun Hinboun (210 MW), were under negotiation for

implementation in 1998. The Nam Theun Hinboun dam was completed in 1998, but work on the Nam Theun 2 dam had still not been started in mid-2003. The Se Kong and its tributaries have a combined hydropowergenerating capacity of about 16 000 GWh/year. On the Upper Se San Basin in Viet Nam six potential hydropower projects with a combined installed capacity of about 1600 MW have been identified with a combined average generating capacity of about 7700 GWh/year. The first of these projects, the 700 MW capacity Yali Falls Dam, began operating in 2000, not without controversy over sudden releases of water that caused havoc downstream in Cambodia. The Pleikrong (120 MW) run of river plant was due for completion in 2002. Work started on the 220 MW Se San 3 hydropower project in 2002 with the aim of coming on stream in 2006. The Se San 4 project was scheduled to begin later. These projects are key elements in Viet Nam’s ‘Power Development Strategy to 2005’.

The Thai, Lao, and Cambodian Middle Reaches and Plains From the plain around Vientiane and the adjacent Thai lowlands, the Mekong flows to Savannakhet and Mukdahan for 470 km through a broad flat valley varying from 30 to 130 km wide. Apart from a set of minor rapids 25 km upstream of Savannakhet, the stream is gentle in slope, meandering with occasional rocky islands and sandbars. Ninety-five kilometres downstream of Savannakhet, the river enters a reach of some 150 km which begins with the Khemmarat Rapids and then has a rocky bed through several steep-sided gorges until it emerges from the hills some 24 km above Paksé. Here, at 869 km from the sea and only 86.5 m above sea level, it assumes the full character of a major lowland river, with a channel up to 1.5 km wide and many sandbars. A little further downstream the river has anabranches and many sandy islands. Many of the plains southeast of Muang Khannouan are developed on dry sandstone surfaces which take on a particularly arid aspect in the long dry season. Scrubby vegetation is interspersed with the yellowish cones of termite mounds. Much of the Khorat Plateau in Thailand, a great series of Mesozoic sedimentary strata with outward-facing escarpments to the west and south, takes on a similar appearance. The gently undulating surface of the Khorat Plateau slopes towards the southeast. The Phu Phan Ridge separates a series of relatively short rivers draining to the north directly to the Mekong from the tributaries of the Nam Chi–Nam

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Mun system. The main rivers are incised below the general level of the plateau. In the dry season the channel of the Chi is often 10 to 12 m below the level of the floodplain. Widespread water shortage is often felt during the dry season in these plateaux. The shallow, sandy soils are low in fertility and have rapid infiltration rates and low soil moisture storage capacities (Dixon 1978). These areas are highly fragile once the land is developed for agriculture. Soil erosion can be severe and may lead to rapid siltation of any reservoirs further downstream. Use of sloping land for maize cultivation has led to ploughed soils being exposed to the first rains of the wet season. Much of the tractor ploughing is up and down slope and leads to so much soil erosion that formerly incised streams have been transformed by siltation into broad shallow swales. Salinity may be a problem as a result of drainage from the salt strata in the Mahasahakhan Formation in the eastern part of the Khorat Plateau and adjacent areas of Lao PDR (Wilander 1990). The riverine floodplains support large areas of paddy rice (sawah). Along the Mekong itself, fisheries are equally important, often involving every family, particularly in southern Lao PDR, where the Mekong spreads up to 14 km wide in the rainy season (Baird 1996). The lowland rice-growing areas characteristically comprise three broad landform types: colluvial–alluvial low-angle coalescing fans; old alluvial plains; and river and lake floodplains. The coalescing fans are developed on material derived from adjacent hill slopes, largely that carried by local rivers and streams, but including some colluvial material derived from surface slope movements. The old alluvial plains are the remnants of old former floodplains. The older terraces are often dissected and have an undulating topography, while the younger terraces still have flat surfaces and support soils similar to those on the modern floodplain (White and Oberthür 1997). Three categories of modern floodplain are found: meander floodplains, expansive floodplains, and lacustrine floodplains. The meander floodplains characterize the middle reaches below Vientiane, comprising the river channel, natural levées, back slopes, and flood basins. Flood waters that overtop the levées deposit fine sand and silt on the levées and clay in the flood basins. Many flood basin hydromorphic soils are used for rice production, while the levées are used for housing, vegetables, and cash crops. Lateral migration of the river channel results in breaching of levées and the formation of cut-offs that in turn become filled with sediment. The expansive floodplains occur in the lower

reaches of the river through central Cambodia and into the delta. In these areas the river channel and its levées give way to wide, extensive basins with few prominent features. The basin may extend for several kilometres and is often crossed by shallow minor channels, without levées that carry water in the dry season. During the wet season, however, these basins, as well as many of the levées, are covered with 2 m or more of water for two or more months (White and Oberthür 1997). The flat and featureless lacustrine floodplains have much in common with the expansive floodplains and are covered with fine-grained sediments whose nature varies with the lithology of the surrounding area and the catchments supplying the rivers entering the lake. The most extensive of these lacustrine floodplains are those around the Tonlé Sap.

The Tonlé Sap In the heart of Cambodia, the river is joined by the Tonlé Sap or Great Lake River (Fig. 12.5), it rises from 1 or 2 metres above sea level in May to 8 or 10 metres above sea level in August. During the high-water period, the flow of the Tonlé Sap River reverses, spreading the Great Lake over 10 000 km2 (Fig. 12.6). By October water levels in the Mekong drop, and the major tributaries, including the Tonlé Sap, begin to flow back to the Mekong as the Great Lake recedes. At low water the lake covers some 2400 km2. The flooded forests and plain provide habitat for a multitude of fish searching for food and spawning grounds. Teeming with aquatic life and decomposing organic matter, the Great Lake has been described as ‘a veritable vegetable and animal broth’. To the south of the great basin surrounding the lake are the Cardamom Mountains (Phnom Kravantes), a crystalline basement complex overlain by Permian limestones and Triassic sandstones, whose highest point is Phnom Aural at 1813 m above sea level. To the north, the basin is closed off by the rectilinear Liassic and Jurassic sandstone escarpment of the Dong Rek Range, whose highest point is only 756 m, being effectively the southern edge of the sedimentary syncline of the Khorat Plateau. The east and north of the basin are occupied by ancient basement rocks outcropping as faulted, generally low relief forms (Carbonnel and Guiscafré 1965). The Tonlé Sap depression is a large area of Quaternary sediments. The plain around the lake has largely been cleared of its natural vegetation. On the Cardamom and Dong Rek Hills and escarpments areas of dense forest remain, although many of them were being illegally exploited

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Fig. 12.5. The Tonlé Sap Basin

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Fig. 12.6. Water balance of the Tonlé Sap (Source: Based on data from Carbonel and Guiscafré 1965)

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during the 1990s. Towards the east of the basin much of the area is covered with an open forest formation with grass between the trees. Around the lake itself is the flooded forest, much degraded by fuelwood cutting. The forest, with important Barringtonia, Terminalia, and Hydnocarpus species, is often flooded to a depth of over 5 m in the wet season. The flooded forest is central to the overall ecology of the Tonlé Sap system and to its biological productivity. The current pressures on the flooded forest for the production of fuelwood and charcoal and the conversion to agricultural land have created concern and speculation as to the likely implications. Yet the specific ecological interactions and the precise relationships that support this productivity are relatively unknown. Remote sensing data indicates that forest once covered approximately 1 million ha around the lake. This has been reduced to 361 700 ha of flooded forest and 157 200 ha of degraded forest and associated vegetation types. The clearing of the flooded forest appears to have intensified in the 1990s but the rate is not quantified.

The Tonlé Sap fishery The Tonlé Sap Lake is one of the most important economic and natural resources of Cambodia with an annual fish production estimated by the Cambodian Fisheries Department at some 50 000 tons or 71 kg ha−1 y−1 for an average water surface of 7000 km2. The fish catch from the Tonlé Sap, one of the most productive freshwater fisheries in the world, represents roughly 60 per cent of the animal protein intake of the population of Cambodia. The fish of the flooded forests (called ‘white fish’), including species such as carp, herring, and threadfin, live in the open waters and migrate long distances to spawn in more sheltered inland spots. The ‘black fish’ group, which includes catfish, murrels, and snakehead species, stays in the shallow, muddy waters. Some fish move with the flood waters back into the rivers and mainstream; others move back to the estuary. However, fisheries’ catch figures are likely to be grossly underestimated and there is evidence of severe problems in the Tonlé Sap fisheries. Even if the overall size of the fish catches is not diminishing, the average size of the fish is becoming progressively smaller. Fisheries specialists report a strong downward pressure on the larger species and several, including the large carp Catlacarpio siamensis, not having been recorded for a number of years. Given the highly efficient harvesting methods on the lake, some species are likely to be disappearing and there is a general problem with maintaining the reproductive stock for a number of species.

Most of the 8 million Cambodians eat fish year round. In the Great Lake and surrounding areas, the commercial harvest is permitted only in October and February. Should plans to dam and divert the Mekong and its tributaries be realized, the fisheries will be severely affected. Normal migration paths will be blocked, resulting in a steep decline in the harvest of fish available to local fishermen.

Sedimentation and Changes in the Depth of the Tonlé Sap The seasonal flow of water from the Mekong to the Tonlé Sap brings sediment to the lake and lacustrine floodplain. The many small to medium-sized rivers entering the lake also bring sediment whose characteristics differ from that derived from the main river. The resulting small deltas are affected by the seasonal changes in lake levels. These varied sediment types and changes in lake levels result in a diversity of deposits that give rise to local soils often having alternating layers of coarse and fine material (White and Oberthür 1997). Several reports since 1960 indicate that the physical conditions in the Tonlé Sap are changing rapidly. Historical data on hydrological variation in the lake clearly shows a precipitous decline in the minimum level of the lake, from 3 metres to 2 metres between 1925 and 1956, and from 2 metres to 1 metre in 1957. Much uncertainty remains over the rate of infilling of the lake. Severe sediment problems arise from alluvial gemmining in the Battembang area. Many changes in river dynamics and morphology on the southern tributaries occurred as a result of huge canals dug during the Pol Pot regime. Logging activity since 1990 has aggravated the sediment problem. Clearing of the forest in the inundated areas is also implicated in the siltation of the lake. Estimates for the period of time at the current rate of influx of sediment before the lake becomes so full of sediment that it no longer holds any water in the dry season range from 100 to 150 years.

The delta The lower reaches of the Mekong River have a gradient of only 2.5 cm km−1. The river empties into the South China Sea, where the waves rework the delta sediments creating the prominent Ca Mau Peninsula (Fig. 12.7). From the Bassac westwards mangroves fringe the delta, while sands occur between the mouths of the Mekong and the Bassac. The tidal range at the mouth of the Mekong is 3.2 m, with the tidal influence extending 388 km upstream to Phnom Penh.

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Fig. 12.7. The Mekong Delta showing the approximate limit of brackish water penetration

The delta is bounded to the northwest by preQuaternary surfaces. Pleistocene terraces border the river and delta. In the lower delta, these terraces gradually dip below the modern deltaic plain. The Trans Bassac depression, a lowland along the Bassac, is one of the natural overflow basins of the Mekong system.

Sediment carried southwestwards along the coast from the river mouths and overbank flood deposits have gradually built up this relatively featureless flat plain. Much of the area consists primarily of brackish water marshes and small tidal channels bordered by mangrove vegetation. Some of the area has been reclaimed

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for agricultural purposes, with prominent, straight irrigation canals. The largest basin in the delta is the 4560 km2 primarily uncultivated Plain of Reeds (Fig. 12.1). Cultivation is restricted mainly to the small channel banks of distributary streams and along the long man-made canal banks that are used for drainage. The vegetation of this plain consists mainly of numerous species of Juncus, a fresh- to brackish-water marsh plant. The major distributaries of the delta plain usually have many minor channels divided by temporary sandbars forming a braided pattern. These unstable channels are frequently eroded and shift laterally. Most of the mid-channel islands are protected by dykes and are cultivated. Broad natural levées border the active channels and surround the inter-distributary basins (called bengs in Viet Nam). These levees are usually several metres above the adjacent marshy inter-distributary surface. The lower deltaic plain has alternating stranded beach ridges and mudflats extending up to 60 km inland. These stranded ridges of sand and shells parallel the coastline and rise up to 5 m above the adjacent swampy depressions, called swales. Small tidal channels and salt-tolerant vegetation such as mangrove and Nipa palm usually occupy the swales. The ridges are ideal sites for habitation and cultivation of oil palms. Broad tidal mudflats exposed at low tide normally front the delta shoreline. The large volumes of organic debris that accumulate on these mudflats are extremely rich biologically (Short and Blair 1986).

Rice Cultivation on the Deltaic Plain Rice cultivation in the Mekong River Delta is largely governed by hydrology, rainfall pattern, and the availability of irrigation. The annual rainfall over the delta area varies from 1500 to 3000 mm (Fig. 12.2). The massive freshwater flows which inundate much of the Mekong Delta during the wet season nourish the delta’s highly productive and essential rice crops (Vo-tong Xuan 1993; Tran Thanh Be 1994; Vo Quang Minh 1995). The delta has been called Viet Nam’s rice-basket, which emphasizes its economic and social importance to Viet Nam (Tran Thanh Be 1994). Two of the three main rice seasons in the delta, the He-Thu (HT) and the Mua (M) seasons, coincide with the rainy season, which typically starts in May and lasts until November. The major ricecropping systems in the area are the single, double, and triple systems. The single rice crop is invariably the Mua (M), the long-established rain-fed rice-cropping system using local, traditional varieties with a longer growth period. It is practised mainly in the tidally inundated

coastal area subjected to salinity intrusion prior to the rainy season. The double cropping system may be the Dong Xuan–He Thu (DX–HT) or the He Thu–Mua (HT–M) system. The DX crop, planted at the end of the rainy season, needs to be irrigated. Triple cropping involves all three (DX–HT–M) rice seasons and occurs only in limited areas along rivers. In areas affected by deep flooding during the rainy season, the second crop (HT) is planted earlier in April, depends on pump-irrigation for its water, and is harvested before the onset of the flood. In areas where flooding is not severe or commences later, the HT crop is planted in May–June and relies on rainwater. The HT–M system is typically practised in the salinity-affected areas and in areas where irrigation is not available. Both crops are rain-fed. Hence the crop calendar varies each year, depending on the onset of the rainy season. The rice crops may be planted using the transplanting or the direct seeding methods. In the direct seeding method, the seeds are either sown onto dry fields prior to the start of the rainy season (dry direct seeding) or pre-germinated seeds may be sown onto wet fields (wet direct seeding). The HT crop is generally wet or dry direct seeded while the M crop may be transplanted or wet direct seeded.

Saline Water Intrusion in Viet Nam During the dry season, the Mekong River flows are so low that sea water intrudes into the lower reaches of the river, producing brackish water conditions that are unsuitable for rice growth (Fig. 12.7). At present approximately 2 Mha of land are subject to dry-season salinity with saline water extending 50 km inland (Vo Quang Minh 1995). Increasing diversions of upstream Mekong flows for dry-season irrigation, both in Viet Nam and in countries upstream, threaten to exacerbate saltwater intrusion into productive lands. Water use in the dry season has increased sharply since 1980, mostly due to land development, increase in the pumping capacity, and new rice varieties. In the coastal areas of the Mekong Delta saline intrusion in the dry season often limits rice production to one crop per year. As a means for increasing income some farmers have adopted the practice of allowing saline water onto the ricefields for the rearing of prawns. However, a number of environmental problems have arisen which have raised concern over the long-term environmental and economic sustainability of this integrated farming system in the delta. In an effort to limit salt-water intrusion into agricultural areas, several saline water intrusion floodgates have been installed in the delta. However, their impacts on soil acidification, the transport and storage of acid, and the impacts of stored acid

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on agricultural and aquatic productivity generally have been ignored. Floodgates on estuarine, acid sulphate floodplains have been found to promote soil acidification and lower plant production, act as large acid reservoirs, form barriers to fish migration, reduce fish breeding, damage fish feeding areas, diminish tidally driven acid neutralization, and release hundreds of tonnes of acidity into estuarine reaches. White et al. (1996) put forward the hypotheses that: 1. installation of floodgates in Viet Nam will result in extremely poor upstream water quality during the dry season; 2. the resulting water quality will cause major decreases in upstream dry-season irrigated crop production and fish and aquatic production. Their work in Australia suggests the following specific changes may occur after installation of saline intrusion floodgates in Viet Nam’s Lower Mekong Delta: • increase in upstream acidification in the dry season due to prevention of brackish water neutralization; • decrease in rice production in those areas using impounded acidified waters for dry-season irrigation; • decrease in area available for fish migration, feeding, and recruitment; • large acid fluxes through floodgates on the break of the wet season; • increase in downstream fish mortalities and diseases. Floodgates already installed at My Phuoc in Soc Trang Province, servicing an area of 13 400 ha containing 3400 ha of severe acid sulphate soils with the remainder being moderate acid sulphate soils, are producing acidification and stagnation of upstream water and rising water-tables in the project area.

Sulphidic Sediments or Acid Sulphate Soils The post-1980 expansion of agriculture in the Mekong Delta has led to severe acid sulphate soil problems affecting 1.6 million ha in the delta (Brinkman 1982; Vo-tong Xuan 1993). Iron sulphides in the marine clays of the delta react with the atmosphere when drained, releasing sulphate into solution in the drainage water and bringing the pH up to 4 or even 2.5. Acid soil solutions can be extremely toxic to plants. Extremely low crop production can occur for lengthy periods when this acid soil solution is within the root zone of the crop (Dent 1986; Moore et al. 1993). Although Viet Nam has had the reclamation of 150 000 ha of highly acid sulphate soils as a national

priority since 1990, progress is slow because the high variability of the affected soils makes it difficult to develop guidelines for farmers in specific locations (Thammongkol 1991). Flooding the land by pumping or gravity flow at high tide and drainage by gravity through the soil has been the leaching treatment to remove acidity, but it tends to be required at the end of the dry season, when water is in short supply. The leaching treatment thus competes with the needs of other water users, and especially with maintaining a minimum river discharge to mitigate salinity intrusions (Minh et al. 1997).

The Environmental Issues Forests and Land The great biodiversity of the Mekong Basin provides good arguments for the preservation of large tracts of undisturbed forest wildlife habitat. For example, the extremely rare wild ox (Bos sauveli) is found in the forest area at the triangle of the Vietnamese, Cambodian, and Lao borders. Equally the forests are the home of many peoples who have lived by swidden agriculture and by hunting and gathering for many generations, but whose environment has been changed by many recent developments. Throughout the basin, growth in the numbers of shifting cultivators, in the resettlement of people, and in inter-regional migration has increased the pressures on land. To meet the growing urban demands for forest, food, and recreational resources of expanding mining, and of mineral resource and hydropower developments, change in land cover and land use is inevitable, but may lead to severe degradation of land and biotic resources. The Central Highlands of Viet Nam illustrate these impacts well. The region has a high potential for erosion and flash floods. During the six-month dry season, drought and wildfire destroy the surface soil structure and deplete the vegetation cover, creating the preconditions for severe erosion during the first wet-season storms. Many reservoirs built to conserve water resources have lost much of their capacity through sedimentation (Hydrometeorological Service of SRV 1995). Rapid population growth and economic development have changed the ground cover, accelerating the erosive effects of heavy rain and aggravating the flash flood hazard. Since 1985 the high prices for coffee grown in the Central Highlands have led people from the coastal regions to move into the highlands and purchase land from ethnic minority groups in order to grow coffee. The ethnic groups then move further

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up into the mountains, clearing more forest for swidden agriculture so aggravating the situation. Fifty per cent of the forest of Lâm Hà district has been removed to make way for coffee cultivation. Efforts to restrain illegal timber cutting and to reforest the areas of poor vegetation are hampered by lack of resources. In the four provinces of the Central Highlands, only 60 000 ha have been replanted with trees. New hydroelectric and irrigation reservoirs, such as the Yaly Reservoir, are displacing people from the areas to be inundated and creating a demand for new areas for permanent agriculture. If the resettlement schemes are not successful, the displaced peoples are likely to leave the new sites and to revert to shifting cultivation (Douglas 1997). Three broad categories of tropical deforestation are occurring in the region: natural deforestation, subsistence deforestation, and deforestation caused by profit maximization (Nophea Kim Phat et al. 1999). Natural deforestation is deforestation caused by climatic changes. These changes typically happen over a long period (thousands of years). Subsistence deforestation can be found in all the countries of the region. Hill tribe peoples dependent on swidden agriculture are being encouraged and strongly persuaded to settle in permanent villages. For many authorities, the curtailing of swidden agriculture, as practised by tribespeople such as the Hmong, is important to the preservation of the dwindling natural habitat of the region (Gu and Clarke 1999). However, the accusations over responsibility for forest degradation levelled against swidden farmers are often unfair. During the 1980s, for example, although mountain minorities’ activities were the proximate cause of perhaps 5 per cent of annual deforestation in northern Thailand, they were regularly named by officials and technocrats as the primary cause of the entire country’s deforestation problem. Such attitudes may reflect historic Thai views of ethnic minorities more than the outcomes of scientific research on environmental processes (Lohmann 1999). In Cambodia, similar problems arise among the ethnic minorities in the upland forested plateau areas of Ratanakiri Province, Cambodia. These shifting cultivators derive a significant proportion of their natural resources from the forest with which they live in close harmony. Development pressures in Ratanakiri Province are straining both the local communities and their environment, especially in communes close to Bun Lung, the commercial and administrative centre (Nhem Sovanna 1999). Deforestation for profit maximization is occurring in every country. Interlinked commercial, political, and military forces are the drivers of the major changes in

forests in Cambodia and elsewhere. Currently, there are twenty-four forest concessions covering 6.33 million ha in Cambodia, almost three times the 2.2 million ha experts say should be allocated if logging is to be sustainable. In 1989 Thailand imposed a national logging ban and cancelled some 300 concessions. This and rapid economic growth has greatly increased demand for Cambodian wood and thus has led to deforestation or forest degradation. The government of Cambodia has been unable to prevent illegal exports of about 1 million m3 of timber each year across the Thai– Cambodian border. In addition, Cambodian logs are being exported from Stung Treng, Ratanakiri, Mondulkiri, and Kratié to Vietnam and Lao PDR on a massive scale (Nophea Kim Phat and Yuji Uozumi 1999). In both Myanmar (Bryant 1998) and Cambodia, money to support rebel and guerrilla movements has come from forest exploitation. According to Talbott and Brown (1999) ‘Mining [removal of trees at an unsustainable rate] of Cambodia’s forests has been key to the power of the military and political leaders in Cambodia. Timber sales have been a primary source of income not only for the reigning governments, but also for the guerrilla armies that have challenged them.’ In other countries also the military has tight links with the forest industries, and, as elsewhere in the tropical world, the granting of forestry concessions by governments may be part of the patronage system. All this means that there are powerful profit motives pushing for timber exploitation and few incentives for sustainable forest management and protection of logged-over areas to permit forest regeneration. The Asian Development Bank project ‘Poverty Reduction and Environmental Improvement in Remote Watersheds in the Greater Mekong Sub-region’ has set out to contribute to sustainable development in remote watershed areas, while addressing the serious environmental problems of deforestation, erosion, siltation, flooding, habitat destruction, and biodiversity losses. People in the project areas will gain improved access to employment alternatives in order to reduce shifting cultivation, illegal logging, and trade in endangered species. Others have introduced local hill farmers to the concepts of hillside ditches, vegetation strips, contour farming, and intercropping with trees to stabilize the soil. The Mekong River Commission’s work programme on catchment management provides for small grants for community projects to support improved farming methods to protect land and water resources (Mekong River Commission 2002). While action to support local farmers of this type is needed, attention should also be given to introducing more

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sustainable forest management, better control of land clearance for agriculture, and the reduction of the environmental impact of mining, industrial, and urban development. Much of the environmental protection and land degradation reduction effort may be addressed at the wrong target.

Water Resources (Power and Navigation) The rapids on the Mekong at Kratié limit the upstream passage of vessels, but are the potential Sambor Dam site (Fig. 12.4). Today, they symbolize the dilemma over the future of the river: should there be a massive development of the power and navigation potential of the main river, or should there be a nation-by-nation exploitation of tributary resources? Is the vision of Lao PDR as a Southeast Asian Switzerland, exploiting the renewable energy of tributaries and providing services to its larger neighbours, realistic and viable? Development of the water power and navigation potential of the Mekong is thus an opportunity for conflict as well as for cooperation. Already, closure of dams upstream is beginning to have an impact on the delta. Flows into the Tonlé Sap have changed. The diverse interests and needs of the countries in the Mekong Basin have the potential to create and exacerbate existing intraregional tensions. Conflicts of interest are developing over use of the river. It cannot supply the water demanded by human users and the ecological functions it provides. At present, the Mekong is being used unsustainably. In the lower Mekong countries, the annual population growth rate averages 2.29 per cent. Around 2.5 million people are added to the regional population each year. Prior to the economic downturn of 1997 GDP was growing at over 8 per cent a year, with the industrial sector expanding by over 11 per cent. Provision of safe water and reliable electricity supplies was not able to meet the new demands. Now water scarcity and its adverse impact on the people, economies, and ecology of the Mekong River Basin have the potential to generate or exacerbate an international security issue. Many of the abundant energy resources are underused. Only 2 per cent of the hydropower potential has been harnessed to date. Annual per capita electricity consumption in the countries of the Lower Mekong ranges from 55–60 kWh in Cambodia, Lao PDR, and Myanmar to about 900 kWh in Thailand. These levels are just a fraction of the consumption in the Republic of Korea of over 3000 kWh, in Japan 6400 kWh, and in the United States 10 500 kWh. Present growth means that the energy supply in the Mekong Basin will increase more than six times in the period up to

2020, with Thailand accounting for two-thirds of the total. One future scenario is a shift to closer integration of hydropower and natural gas sources of electricity, through transmission grid interconnection and crossborder gas trade. Many benefits will result from the complementarity of energy resources, load diversity, hydrological diversity, exchanges of base energy for peak energy, increased supply reliability, reduced reserve capacity requirements, reduced system losses, and reduced environmental impact. Emissions of carbon dioxide, sulphur dioxide, and nitrogen oxides will be lower than under a national self-sufficiency approach. Lao PDR has about 75 per cent of the hydropower potential of the Lower Mekong, an estimated 505 000 × 106 Kwh (Lesaca 1983). The Lao government looks to hydropower development as the primary source of income for the country in the future. In relation to natural resource management, the government appears cautious about negative impacts of large-scale developments, mainly hydropower projects. On the other hand, the programme for hydropower development is ambitious, and approved projects will have widespread and negative impacts while achieving some economic gains. The Lao PDR electricity authority, Electricity du Lao, has a non-binding Memorandum of Understanding with its Thai counterpart the Electricity Generating Authority of Thailand to supply 3000 MW by the year 2006. The Lao government is aware that large-scale hydropower developments will have severe impacts on the country’s ecosystems and rural people, with potentially large decreases in biodiversity. Nevertheless, the consensus view within the Lao government is that the country needs to make the sacrifices in order to develop a crucial export industry. The Lao PDR Ministry of Industry and Handicraft hopes to raise $US2.5 billion in foreign capital, over twice the national GDP, for investment in up to fifty-eight big dams by 2030. The Lao government has set a target of completing twentythree dams by 2010. The potential competition with hydropower from natural gas, particularly in view of the slowdown in the Southeast Asian economies at the end of the 1990s, has placed regional hydroelectric development at a crossroads. In 1998 the Asian Development Bank observed that ‘the prospects for hydro power export projects in the region depend largely on the availability of cheap gas for Thailand’. The implication is that if low-cost gas is available (and it is), even a very modest hydro programme becomes non-viable. The great hydropower projects are thus facing both economic threats and environmental protests. Only

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China’s cascade of seven dams along the Upper Mekong in Yunnan Province seems likely to be unaffected (Table 12.1). However, these main river dams in Yunnan will have the following impacts: 1. On site in Yunnan: • farmland would be submerged by dam reservoirs; • fish migration would be obstructed by dams; • fish habitat would be destroyed by dam construction and regulation of the river for hydropower production; • communities would be displaced leading to environmental degradation in the uplands of Yunnan; • increased pressure on, and competition for, upland resources would lead to conflicts between settlers and other highland communities. 2. Downstream in the Mekong Delta: • annual floods and sediment deposition which rejuvenate soils and trigger fish productivity would be reduced; • construction and operation of upstream dams would reduce water, sediment, and nutrient flow to the delta, which threatens the delta’s agriculture and fisheries-dependent economy. On the tributaries in Lao PDR, the greatest concerns are over the loss of the forested river valleys where many distinct ethnic communities now live. As part of a new agreement for Lao PDR to provide 1500 MW to Thailand, plans call for three projects on the Nam Theun River, the Mekong’s fourth largest tributary: Nam Theun 1, Nam Theun 2, and Nam Theun-Hinboun (Table 12.1). Combined, the projects will significantly alter the river and the Theun Valley, which is home to 70 000 people, many of whom are from the Thai Men, Thai Meuy, Thai Pao, Thai Yuang, Thai Senkap, Thai Oh, Thai Khang, and Lao Kaleung ethnic groups. Approximately 130 species of fish have been identified in Nam Hinboun and Nam Theun Rivers. Productive fisheries such as at Ban Kangvit, just upstream of the Nam Theun-Hinboun Dam site, provide several tons of fish each year for direct consumption and for local fish traders. The reservoir behind the 600 MW Nam Theun 2 Dam will flood about one-fourth of the 1600 km2 Nakai Plateau, an area designated by the Lao government in 1993 as a national biodiversity conservation area. The plateau is unique for its mosaic of pine forests, grasslands, and swamps which serve as important grazing areas for livestock and breeding ground for fowl. With its large, shallow reservoir, Nam Theun 2 will reduce the flow of the river throughout the year, cutting it to a trickle in the dry season. At the same time it will drastically alter the flow regime of the Xe Bang Fai, affecting

the livelihoods of over 100 000 people (International Rivers Network 2002c). Government-sanctioned logging and preparation for dam sites continues wherever Memoranda of Understanding for future development have been signed. Downstream of the protected areas lies one such project, the Nam Theun-Hinboun Dam, which is a joint venture primarily between the Lao government, the Norwegian firm Norpower, and the MDX Group from Thailand. At the Nam Theun-Hinboun site, project roads brought in loggers, poachers, and migrant settlers. Logs from the site went to sawmills across the Mekong in Thailand and then on to Japan, Taiwan, and Singapore. Development planning for the Nam Theun has been so dominated by large-scale hydropower schemes that such projects are being pursued in the national interest to the exclusion of other possible land and water uses more suited to the local communities. The resettlement of 4400 people following the establishment of the 3500 km2 Nahai Nam Theun National Biodiversity Conservation Area in the catchment of the planned Nam Theun 2 Dam aimed to raise their living standards by, for example, providing them with different and improved crop and animal farming opportunities. There will be a specific indigenous people’s plan as they are among the resettled community. Such actions have been criticized as being unrealistic and likely to add to the degradation of the catchment outside the conservation area. Particular doubts have been raised about the way the Bholisat Phattana Khed Phou Doi (BPKP, or Mountain Region Development Company), a military logging company, has accelerated logging of the surrounding Nakai Plateau since 1993. The BPKP sought to have the reservoir area cleared of timber by the year 2000, when the Nam Theun 2 Dam was originally expected to start supplying electricity to Thailand. To process the large volumes of timber extracted from the reservoir area, the BPKP has expanded its logging and wood processing facilities considerably. The BPKP currently operates five sawmills and one plywood mill, runs a joint venture chipboard factory with a Hong Kongbased company, and has authorized operation of nine privately owned sawmills near the provincial capital of Takhek. Since 1995 there has been a 60 per cent increase in the number of people employed by the BPKP and privately owned mills in Khammouane Province, with 600 people employed by BPKP and another 2000 people working in the privately owned mills. Logs are exported across the Mekong to Thailand and by road to the port of Vinh in central Viet Nam, where the BPKP exports logs and other wood products to Japan,

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Korea, and Hong Kong. It is expected that once BPKP has exhausted its supply of timber from the dam’s reservoir area, it will want to continue logging in the remainder of the dam’s watershed area including in the area allocated for conservation. Such problems are likely to arise again and again as the Lao PDR develops its water resources. The Mekong and its tributaries are a major engineering challenge, involving the overcoming of dry-season water shortages by careful river regulation combined with power generation. Controversy will heighten as local populations grow and the pressures on land increase. Potentially, hydropower could benefit the entire population of the region, but virtually all governments throughout the world have failed to distribute the benefits of such renewable resource exploitation equitably. The challenge is to see that such equity is essential for the sustainable management of the environment and resources of the Mekong. Without this, people will have to continue to clear forests, extend swidden agriculture, and exploit resources illegally in order to make a living and survive. Ultimately both the people and the environment will suffer. The situation is not always helped by external opposition to dam-building and emotional championing of minority and wildlife causes. Much of this activity is narrowly focused and misses the point that the secure future of humanity depends on both global and local environmental security. Part of this security requires the rehabilitation of hydropower’s unjustifiably battered environmental reputation. The International Solar Energy Society (ISES) compared fourteen energy source environmental impacts across thirty-four categories of impact within the total energy cycle, from raw materials extraction and plant construction, to energy production, consumption, and waste disposal. Coal produced the most impact (ISES score 256), then oil (235), and then gas (221). Nuclear came next (217), very close to but marginally better than gas. Hydro scored 158, almost 60 points below its nearest rival and nearly 100 points better than coal. As a benchmark, energy efficiency scored 121. Gas is highly overrated from an environmental standpoint by authorities such as the World Bank while hydro and nuclear are highly underrated.

Aquatic Resources (Fisheries) Most people in the Mekong Basin depend on freshwater fish. According to international statistics, in Lao PDR, 6.2 kg of freshwater fish per capita were caught in 1993, in Cambodia, 7.6 kg per capita. These catches grew by 34 and 21 per cent respectively in 109 years. In Thailand and Viet Nam, where the coastlines are much

longer, the freshwater fish catches were 4.6 and 3.6 kg per capita respectively. Management of river, reservoir, and lake fisheries is thus an important aspect of the total environmental management of the Mekong Basin. Developments upstream are significant for all downstream fisheries. Since 1970 the people and environment of the Mekong Delta have been subjected to rapid and extreme changes, affecting urban and agricultural areas and natural ecosystems. Poverty and landlessness are common outcomes. To survive, many turn to the already diminished natural resource base, leading to environmental degradation, increased pressure on natural ecosystems, and further poverty. Among the fishery activities affected by changes upstream on the Mekong is the extensive cultivation in the Mekong Delta in Viet Nam of two species of river catfish Pangasius micronemus and Pengasius larnaudii (which do not breed in ponds). The industry depends almost entirely on harvesting of young wild fish (fingerlings) of these species in the section of Mekong River near the Vietnamese – Cambodian border (An Giang Province). These young fish are then raised in ponds and tanks in simple, traditional ways. The fish so raised constitute an important source of low-cost but high-quality animal protein for people in rural areas. Any reduction in the supply of fingerlings could cause hardship and nutritional deficiencies to many Vietnamese Mekong Delta residents, who probably could not afford to breed or raise the fish by more advanced artificial methods.

Deltaic Forest Resources Dense forests of Melaleuca cajuputi once occupied most of the seasonally inundated, acid sulphate soils of the delta, principally on Ca Mau Peninsula, in the Long Xuyen Quadrangle, and on the Plain of Reeds. They were an important source of wood (e.g. firewood, posts, poles, and piling) and non-wood (e.g. wild game, fish, honey, and essential oils) products for local communities and had significant environmental benefits such as flood mitigation and maintenance of water quality and wildlife habitat. It is estimated that today only 121 000 ha of natural Melaleuca forests remain in the delta and about 5000 ha are lost annually through illicit cutting and burning (Nguyen Duy Chuyen 1995). The wastelands created by this process have high priority for reforestation. Melaleuca provides many direct benefits such as wood. It can help to improve the quality of water in acid sulphate soil areas, by raising its pH and reducing the concentrations of toxic metal ions. Rain that has passed through the Melaleuca canopy or flowed down

The Mekong River Basin 215

the trunks of Melaleuca trees does not affect the pH of surface water. However, leaf litter of the Melaleuca forest appears to play a vital role in improving water quality. Trials have indicated that a Melaleuca-and-rice farming system designed to capitalize on the multiple benefits of Melaleuca can be practicable, economical, and sustainable. On the low-fertility coastal sands, casuarina-growing represents one of the few technical options for profitable and sustainable land use. In addition to the direct contributions to household income via sale of fuelwood and poles, the casuarinas provide protection from typhoons and desiccating westerly winds and provide valuable organic matter for farming. They provide the framework for a very successful agroforestry system. Vinh Hoa village, for example, illustrates how institutional reform (doi moi), infrastructure development (construction of an access road), and market access have transformed the local economy. Saleable farm produce (casuarina wood) and ready access to local markets underpin a sense of optimism in the community at Vinh Hoa on the coastal sands. The adoption of rice-growing technology in the raising of casuarina seedlings meant that all households had access to as many plants as they needed. Farmers reported that the technology was so simple that it was now difficult to sell seedlings in the market. Commercial fuelwood now provides 40– 45 per cent of household income, which almost doubled after the emergence of the commercial fuel market and the expansion of casuarina tree farming. All households reported that they can sell as much fuelwood as they can produce at present. There is also a local demand for casuarina wood for roofing poles, boat masts, and oars and furniture. Viet Nam’s mangrove forests have also decreased drastically, both in area and quality. Whereas 400 000 ha of coastal area were covered with mangrove forests in 1950, only 252 000 ha of mangrove forest were left in 1983, despite substantial replanting of the warravaged mangrove forests after 1975. Since 1983 areas have succumbed to the onslaught of uncontrolled wood extraction, paddy expansion, and, most importantly, extensive shrimp farming. The integrity of old-growth mangrove ecosystems has been severely compromised. This has been particularly true for the vast mangrove areas in the Southern Mekong Delta. In the peninsula province of Ca Mau (Fig. 12.7) (which accounts for half of all mangrove forests in Viet Nam), 60 000 ha (55 per cent) of mangrove forest were lost to aquaculture development from 1983 to 1992. The productivity of coastal fisheries in the Southern Mekong Delta has also worsened, particularly in the past three years.

Improving Irrigation in the Delta The delta’s water delivery system differs from conventional irrigation systems in upland areas. No centrally controlled pumps or large regulators exist. All that is needed in the Mekong Delta is a ditch branching off from secondaries and connecting to the field. Irrigation, drainage, and local water transport systems share the same canals and waterways in the delta, which are hydraulically operated by tides from the South China Sea and the Gulf of Thailand. At the field level, low-lift pumps are commonly used by farmers in the delta to control water in and out of their fields according to crop needs. They pump water for irrigation during the dry season, when the water level in the canals is lower than the field level, and, in some places, for drainage during the rainy season when the tidal drain is inadequate. The availability of water resources in the Mekong Delta alternates from surplus to shortage every six months, affected by a tropical monsoon climate. Between July and December heavy rainfall and run-off occur, causing flood waters to cover over 25 per cent of the area for long periods as water spills out from rivers in the upstream part of the delta and has difficulty draining away nearer the sea. There is a great need for protecting human life and property, and improving the poor living conditions. Short periods of inundation provide the benefits of flushing of acid soils and deposition of sediments on floodplains, so bringing nutrients to inland fisheries. During the first and last parts of the January– June dry season, the low level of discharges in the river is sufficient to meet water requirements for in-stream and off-stream uses. But during the end of March–April, sea-water intrusion becomes severe and limits the availability of freshwater for irrigation. Rural water supply for domestic use is also affected adversely. Sustainable development of the Mekong Delta is central to achieving the country’s targets for sustained growth. However, there are pocket areas in the delta where the farmers are among the country’s poorest people. Salinity- and acidity-related poor drinkingwater supply conditions (access to clean water is about 30 per cent), hard living conditions caused by flooding and inundation, and inadequate rural transport compare unfavourably with other areas in Viet Nam.

Conclusions The Mekong is one of the world’s great rivers, still relatively untamed and unaltered. The closure of the Yunnan dams has not had much effect on the total regime as such a large proportion of the total flow

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comes from other parts of the basin. However, the forest resources of the lowlands and the majority of the hills are virtually depleted and only a few of the upland forests, largely in Lao PDR, with small pockets in the other countries, remain intact. The scenic resources are unknown, but as the late 1990s fashion to visit Viet Nam becomes twenty-first-century mass tourism, the wild landscapes of Lao PDR and Cambodia will come under increasing threat. Sadly, the Mekong Basin also contains the legacies of war, from the huge concrete airstrips built by the Americans in the Viet Nam Central Highlands near Pleiku to the bomb craters on the Plain of Jars in which soil conservation project officers plant trees. The civil conflict in Cambodia has left about a third of the country peppered with landmines that make farming impossible. Pressure on the remaining area is thus increased. Many of the forests in Viet Nam have few trees over 20 years old due to the effects of napalm and agent orange and the deforestation in the postwar period as displaced people cleared land to grow rice to survive. Continuing mistrust among communities and fear of insurgency have produced resettlement programmes that, coupled with those related to resource developments, lead to further forest depletions and overuse of land. These population movements have a ripple effect. As people move to large-scale agricultural schemes or are displaced by industry, mining, or reservoir-building, the other inhabitants of the area tend to move further up into the hills. The steeplands they occupy are often less stable and less fertile, and so they use more land for the same amount of food production, thus accelerating the land degradation process. The unnerving contrast between the stable, but perhaps over-fertilizer-dependent, wet ricefields and the degraded secondary forests and the grass-covered steep hillsides suggests that the environment of the Mekong Basin is appoaching a threshold similar to that of its people. As television reaches the tiniest settlements, bringing the global media into traditional homes, so the global economy brings its environmental impact into the tributaries of a great river. Energy hunger, tourist appetites, and urban food and raw material demands are about to drive the Mekong environment into an unstable state, which will end up costing the people and nations of the region far more than the application of sustainable management now would involve. It would be good to think that the lessons learned from the mismanagement of other great river basins throughout the world could be applied to avoid the worst degradation of this varied, fascinating, and wonderful landscape.

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Nguyen Duy Chuyen (1995), ‘Sustainable Uses of Acid Sulphate Soils by Reforestation in the Mekong Delta and Long Xuyen Quadrangle’, Proceedings of the National Technical Workshop, Forest Based Development of Long Xuyen Quadrangle, 3–5 Aug., Long Xuyen, An Giang, Vietnam. Nhem Sovanna (1997), Resource Management Policy in Ratanakiri Province, Cambodia (Ottawa: International Development Research Centre). Nophea Kim Phat, Yuji Uozumi, Ouk Syphan, and Tatsuhito Ueki (1999), ‘Forest Management Problem in Cambodia: A Case Study of Forest Management of F Company’ [ Japanese], Journal of Forest Planning, 5/2: 65–71. Piper, B. S., Gustard, A., Green, C. S., and Sridurongkatum, P. (1991), ‘Water Resource Developments and Flow Regimes on the Mekong River Hydrology for the Water Management of Large River Basins’, International Association of Hydrological Sciences Publication (Wallingford), 201: 45–56. Pons, L. J. (1973), ‘Outline of the Genesis, Characteristics, Classification and Improvement of Acid Sulphate Soils’, in H. Dost (ed.), Proceedings of the International Symposium on Acid Sulphate Soils, 13–29 Aug. 1972, ILRI pub. no. 18 (Wageningen: International Institute for Land Reclamation and Improvement), i. 3–27. Robequain, C. (1953), L’Indochine (Paris: Armand Colin). Ryder, G. (1994), ‘Overview of Regional Plans’, World Rivers Review, 9/4, www.nextcity.com/probeinternational/Mekong/articles/9412-1-WorldRivers.htm (accessed 25 July 2003). Schaaf, C. H., and Fifield, R. H. (1963), The Lower Mekong: Challenge to Cooperation in Southeast Asia (Princeton: Van Nostrand). Short, N. M., and Blair, R. W. (eds.) (1986), Geomorphology from Space: A Global Overview of Regional Landforms, National Aeronautics and Space Administration, Scientific and Technical Information Branch Publication SP-486 (Washington). Talbott, K., and Brown, M. (1998), ‘Forest Plunder in Southeast Asia: An Environmental Security Nexus in Burma and Cambodia’, Environmental Change and Security Project Report, Issue 4 (Washington: Woodrow Wilson Center, Spring), 53– 60. Thammongkol, T. (1991), ‘Groundwater Quality Monitoring in the Lower Mekong Basin’, UN Economic Commission for Asia and the Pacific Water Resources Series, 70: 117– 21. Transboundary Freshwater Dispute Database (2002), Mekong Committee, www.transboundarywaters.orst.edu/projects/casestudies/ mekong.html (accessed 25 July 2003). Tran Thanh Be (1994), ‘Sustainability of Rice – Shrimp Farming System in a Brackish Water Area in the Mekong Delta of Vietnam’, M. Sc. thesis, University of Western Sydney–Hawkesbury. Tuong, T. P., Hoanh, C. T., and Khiem, N. T. (1991), ‘AgroHydrological Factors as Land Qualities in Land Evaluation for Rice Cropping Patterns in the Mekong Delta of Vietnam’, in P. Deturck and F. N. Ponnamperuma (eds.), Rice Production on Acid Soils of the Tropics (Kandy: Institute of Fundamental Studies). van Breeman, N. (1973), ‘Soil Forming Processes in Acid Sulphate Soils’, in H. Dost (ed.), Proceedings of the International Symposium on Acid Sulphate Soils, 13–29 Aug. 1972, ILRI pub. no. 128 (Wageningen: International Institute for Land Reclamation and Improvement), i. 66–130. Vietnam News Agency (2003), Another Power Plant to be Built for Central Highlands 28 May 2003, www.vnagency.com.vn/newsA.asp? LANGUAGE_ID=2&CATEGORY_ID=30&NEWS_ID=10508 (accessed 25 July 2003). Volker, A. (1983), ‘Rivers of Southeast Asia: Their Regime, Utilization and Regulation’, International Association of Hydrological Sciences Publication (Wallingford), 140: 127–38.

218 Ian Douglas Vo Quang Minh (1995), ‘Use of Soil and Agrohydrological Characteristics in Developing Technology Extrapolation Methodology: A Case Study of the Mekong Delta, Vietnam’, Msc thesis, University of the Philippines, Los Banos. Vo-tong Xuan (1993), ‘Recent Advances in Integrated Land Uses on Acid Sulphate Soils’, in D. L. Dent and M. E. F. van Mensvoort (eds.), Selected Papers of the Ho Chi Minh City Symposium on Acid Sulphate Soils, Mar. 1992, ILRI Pub. No. 53 (Wageningen: International Institute for Land Reclamation and Improvement), 129–36. Vu Ngoc Thanh (1996), ‘Biodiversity and Biodiversity Loss’, Seminar paper, Australian National University. White, I., Melville, M., and Sammut, J. (1996), ‘Possible Impacts of Salinewater Intrusion Floodgates in Vietnam’s Lower Mekong Delta’, Seminar paper, Australian National University. White, P. F., and Oberthür, T. (1997), ‘Geomorphology and Hydrology of Cambodian Ricelands’, in P. F. White, T. Oberthür, and P. Sovuthy (eds.), The Soils Used for Rice Production in Cambodia: A

Manual for their Identification and Management, Cambodia–IRRI– Australia Project (Manila: IRRI), 3– 8. Wilander, A. (1990), ‘Water Quality Monitoring Network in the Lower Mekong Basin’, UN Economic Commission for Asia and the Pacific Water Resources Series (Bangkok: ESCAP), 67: 227–39. Willett, I. R., Crockford, R. H., and Milnes, A. R. (1992), ‘Transformation of Iron, Manganese and Aluminium during Oxidation of a Sulphidic Material from an Acid Sulphate Soil’, in H. C. W. Skinner and R. W. Fitzpatrick (eds.), Biomineralization Processes of Iron and Manganese: Modern and Ancient Environments (CremlingenDestedt: Catena Verlag), 287–301. —— Melville, M. D., and White, I. (1993), ‘Acid Drainwaters from Potential Acid Sulphate Soils and their Impact on Estuarine Ecosystems’, in D. L. Dent and M. E. F. van Mensvoort (eds.), Selected Papers from the Ho Chi Minh City Symposium on Acid Sulphate Soils, Mar. 1992, ILRI pub. no. 53 (Wageningen: International Institute for Land Reclamation and Improvement), 419–25.

13

Southeast Asian Deltas Colin D. Woodroffe

Introduction Deltas and estuaries are actively evolving suites of landforms formed where rivers meet the sea. Deltas are characteristically subaerial (and subaqueous) sediment wedges that protrude from the shoreline, whereas estuaries are typically tidally influenced lower parts of rivers in which the shoreline recedes inland. However, the individual distributaries of deltas, which may themselves be cuspate, exhibit estuarine characteristics, and it is convenient to use the term ‘deltaic – estuarine’ to describe river mouth tidal and alluvial plains. There are extensive low-lying coastal and deltaic– estuarine plains throughout Southeast Asia. These represent productive and relatively easily settled land, which has led to clearance of the natural vegetation of many of these plains for agriculture, silviculture, or settlement. Deltaic–estuarine plains are geologically young, responding to Late Quaternary sea-level and climatic fluctuations, and actively undergoing change in the modern landscape. Most have adopted their present form only in the past few thousand years, and are still active centres of deposition. Worldwide expansion of deltas occurred in the early to mid-Holocene as a result of deceleration of postglacial sea-level rise and the coincidence of sea level with extensive low-gradient shorelines (Stanley and Warne 1994). The formation of deltaic–estuarine plains in semi-arid areas may have been a catalyst for the appearance of civilizations based upon cultivation (Stanley and Warne 1993). Deltas in Southeast Asia, however, presented major challenges to pre-technical societies, as a result of their propensity to flood, poor access across the many bifurcating channels, and malaria, and were slower to be colonized (Büdel 1966). However, they have subsequently become important areas

supporting large populations, particularly as a result of successful management of inundation for the cultivation of rice (van de Goor 1966). Overbank flooding is a prominent feature of most deltas and assures nutrient re-enrichment of fertile, but immature, soils supporting intensive farming. On the other hand, such flooding can also represent a major hazard, damaging property and in some cases resulting in loss of life. It is often controlled, or control over the extent of flooding is sought through engineering works. In the Southeast Asian region there are marked geological contrasts between the tectonically active, volcanic landscapes of the Indonesian island arc, the Philippines, and New Guinea (largely outside the area considered by this review) on the plate boundary, and the Tertiary and Quaternary sedimentary basins flanking the marginal South China Sea. Much of the area considered here lies adjacent to the extensive low-gradient continental shelves of the Sunda Platform, which were exposed during periods of low sea level such as the last glacial maximum (Tjia 1980). The continental area of Southeast Asia and bordering regions, with highly tectonic hinterland and prominent monsoon, are known to deliver large volumes of sediment to the oceans (Milliman and Meade 1983). Recently it has been demonstrated that steep, tectonically active island catchments, such as those through the Indonesian island arc, also contribute disproportionately large sediment volumes to the ocean (Milliman and Syvitski 1992). For instance, the rate of sediment removed from the catchment in the Solo and Brantas Rivers in Indonesia, or the Sepik and Ramu Rivers in New Guinea (1200–1600 t km−2 y −1) is among the highest anywhere (Hoekstra 1993a; Chappell 1993). These rivers are directly comparable to the Ganga–Brahmaputra– Meghna system in terms of rates of sediment removal.

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The Indo-West Pacific is also a centre of diversity for shallow marine biota, and contains some of the most diverse coastal habitats, such as mangrove forests. Thus, with large sediment supply, broad shelves across which coastlines have migrated, and a diverse and productive biota, deltaic–estuarine plains in Southeast Asian and adjacent regions provide examples of some of the most dynamic coastal environments in the world. Several deltas in the region are of significance in terms of oil and gas reserves, and deep drilling indicates sedimentary accumulation through the Tertiary and Quaternary with alternating transgressive and regressive phases controlled partly by sediment supply and subsidence history, but especially by fluctuations in sea level. An insight into how these thick sedimentary sequences have evolved can be gained from examination of the modern Holocene deltas and the development of these deltas during the Late Pleistocene, as a result of the last sea-level cycle. This period of development is reviewed in this chapter for several Southeast Asian deltas in order to understand the variability in processes and dynamics. Intricately interwoven within the interpretation of natural environmental change is the need to decipher the impact that humans have had upon the landscape. Within Southeast Asia, clearing of forest areas at heights of around 1400 m was well advanced by 7000 years bp (Maloney 1980), and many mountain environments between 1000 and 2500 m had been cleared by 4000 years bp (Walker and Flenley 1979; Flenley 1985, 1988). Most Southeast Asian deltas bear a prominent imprint of human influence. This chapter reviews the morphology and Holocene environmental history of some of the major deltaic–estuarine plains of Southeast Asia.

Delta Morphology and Processes Deltas and estuaries span a range of morphodynamic (changing form and physical processes) types, ranging between extremes that are river-dominated, tidedominated, or wave-dominated (Wright and Coleman 1973; Wright 1985). Deltas develop at the mouth of rivers that have a large drainage basin with a substantial discharge and sediment load. Their form is influenced by structural control, and most deltas show some degree of geological inheritance. Those within temperate areas have been characterized within a continuum of coastal depositional landforms (Boyd, Dalrymple, and Zaitlin 1992); while tropical deltas are generally less well understood (Wright 1989). The geometry of the receiving basin is important, particularly in terms of determining the frictional interaction with the basin floor, while waves

and tides within the basin have great significance in shaping the delta shoreline (Wright 1985; Suter 1994).

Morphological Components Deltas can be divided into several major morphological components. In many cases the extensive studies of the river-dominated Mississippi River Delta have influenced the recognition of these components. However, in the case of Southeast Asian deltas, it is perhaps more appropriate to devise an idealized delta within which there are river-, tide-, and wave-dominated sectors. Figure 13.1 is a schematic model of such a delta, comprising components identified in many of the deltas of the world. The model is described; many of its components can be recognized in large deltaic systems like that of the Ganga–Brahmaputra–Meghna system (discussed below). A series of case studies of deltas in the Southeast Asian region (Figure 13.2) is described, and subsequently several generalizations about delta form and process are drawn. A typical delta can be divided into subaerial and subaqueous components. The subaerial delta comprises an upper deltaic plain, generally clearly showing the imprint of fluvial processes, and a lower deltaic plain that is influenced by both fluvial and marine processes. In the case of Southeast Asia the lower deltaic plain is usually dominated by mangroves which indicate brackish water conditions, and its landward extent is broadly defined by the tidal limit. It is also generally possible to identify an active delta, the conduit through which the river channels reach the sea, and an abandoned section. The abandoned part of the deltaic plain becomes dominated by tidal or wave processes as both take on greater importance relative to river processes. Tidal creeks may be numerous through such areas, distinguished from river distributaries by their rapidly tapering widths landwards, and their tightly meandering patterns. In many cases it is apparent that the course and broad form of major tidal creeks is controlled by formerly active distributaries, preserved as palaeochannels, and carrying only a residual flow from the main river.

Process Operation over Time Deltas represent areas of active sediment deposition and have often been depocentres for long periods of geological time, and thus are generally subsiding. Within the active delta, sediment deposition more than offsets the gradual rate of subsidence, and the river mouth progrades (builds seaward). In the abandoned deltaic plain, however, subsidence is not countered by sediment

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Fig. 13.1. Schematic model of typical Southeast Asian delta, showing the main morphological components on the delta, and typical landforms associated with river-, tide-, and wave-dominated sectors

deposition, and the coast tends to recede. Where wave energy is relatively high, sandy sediments may be concentrated into shore-parallel beach ridges. Sediment may be carried by longshore transport and deposited on a marginal plain. There are often equally extensive subaqueous delta deposits. Fluvial levées typically flank the more active distributaries and may continue under water as subaqueous levées. A range of bars, ephemeral shoals, and islands can form at the distributary mouth. Coarsergrained sediment settles first as the fluvial discharge is carried into the receiving basin, dependent upon relative salinity and turbidity (Wright 1985). A relatively Fig. 13.2. Southeast Asia showing the Sunda Shelf and the probable extension of river systems across it at times of lower sea level, and deltas discussed in the text 1. 2. 3. 4.

Irrawaddy Delta Mekong Delta Mahakam Delta Rajang Delta

5. Baram Delta 6. Chao Phraya 7. Solo Delta

222 Colin D. Woodroffe

steep delta front of coarser sediment occurs, with finer sediments carried further seaward, and forming a prodelta in deeper water (Figure 13.1). A typical delta may go through a cyclical pattern of distributary build-out and abandonment through time. The gradual seaward extension of the active delta leads to a situation where it may become hydraulically more efficient to divert flow through a shorter route to the sea, with overspill through a breach in the levée (termed a crevasse splay), or through a switching of channels (avulsion). This part of the delta, deprived of sediment supply, is now likely to be subject to erosion through tidal or wave processes, and this may be accelerated through the gradual subsidence of the delta. These cyclic patterns occur over a timescale of hundreds to thousands of years, and thus can rarely be observed, but may be inferred from stratigraphic and radiometric dating studies of deltaic– estuarine plains. Over longer timescales, changes in sea level have pronounced effects upon delta dynamics. A gradual rise of sea level will result in the gradual landward retreat of all those depositional environments of a delta in which sedimentation rate is not fast enough to keep up with sea-level rise. It is apparent that deltas have gone through a series of transgressive and regressive cycles during the fluctuations of sea level associated with glacial and interglacial cycles. Vegetation is an important element of the natural dynamics of unaltered deltas (Fosberg 1966). The shoreline of deltas in Southeast Asia is fringed by mangroves. Mangrove patterning can be extremely complex, but a broad zonation occurs throughout the Indo-West Pacific. Avicennia and Sonneratia occur at the seaward margin, with Rhizophora in a distinct zone rooted above mean sea level, replaced to landwards by Bruguiera and in places Ceriops, and with a landward zone of Avicennia (Macnae 1968).

Irrawaddy Delta The Irrawaddy River is tectonically constrained. Sediments from the Irrawaddy have largely filled the Trough of Burma, between the Arakan mountain range to the west and the Shan Plateau to the east, and are continuing to prograde into the Andaman Sea. A considerable sediment thickness accumulated through the Tertiary, containing alternations of marine transgressive and freshwater sediments (Stamp 1940). The wedge-shaped delta covers an area of 35 000 km2, of which the southern half is lower deltaic plain (Figure 13.3). The intertidal zone in the coastal strip is

dominated by mangroves. Behind this zone there are extensive areas covered by irregularly tidally flooded Heritiera fomes (Stamp 1925). The gradient of the deltaic plains is gentle, and most are low-lying. Only a slow rate of vertical accretion occurs on the plains, because the early ‘burst of the monsoon’, preceding the main discharge peak of the river, ensures only precipitation-fed flooding of the deltaic plains. The sediment-laden flood waters generally remain within the river channel, and are carried through the delta and out to sea, rather than being deposited overbank. Embankments further constrain the degree of flooding experienced along major distributaries (Volker 1966). The central portion of the delta is the active delta plain, with much of the flow down the Eya distributary and adjacent creeks that form a lobate delta front. The easternmost distributary, the Rangoon River, receives little of the main river’s flow, despite interconnecting channels, and drains largely from the slopes of Pegu Yoma, and the westernmost distributary, the Bassein River, also receives little of the flow. Penetration of salt water upstream in the dry season, December to April, is most marked in the Bassein and Rangoon Rivers (Aung 1993). The shoreline of the delta is often a ridge of low dunes, about 1 m above the hinterland, covered with Casuarina. Build-out occurs through the development of offshore sandbanks, which are in turn colonized by grasses, then mangroves, and subsequently silt up completely (Stamp 1940). Rapid progradation averaging 5 km in 100 years was suggested by Chhibber (1934) from comparison of early surveys, but this rate seems to occur only locally, and about half this rate is more typical across the entire delta. It is apparent that a large part of the sediment load (estimated as 0.2– 0.3 × 109 t yr −1), including a large solution load, is carried beyond the delta (Stamp 1940). Strong currents associated with the southwestern monsoon contrive to carry this sediment eastwards into the Gulf of Martaban, where modern muds are encroaching over relict outer-delta calcareous sands, with turbidity currents taking some of the sediment down the Martaban submarine canyon (Rodolfo 1975).

Mekong Delta The Mekong is a large river draining a long, narrow catchment of around 795 000 km2. A delta has built up at its mouth, where sediments of more than 2000 m thickness have accumulated during the Cenozoic, recording a series of transgressive–regressive cycles (Nghi et al. 1991).

Southeast Asian Deltas 223

Fig. 13.3. Irrawaddy Delta, showing major distributaries and extent of salt water (Sources: based on Volker 1966; Aung 1993)

Although the river is pronouncedly seasonal, its flow is moderated by diversion of flood waters at Phnom Penh into a subsidiary basin in Cambodia, the Grand Lac–Tonlé Sap, connected via the Tonlé Sap River (Figure 13.4). At the apex of the delta at Phnom Penh the river bifurcates into two major distributaries, the Bassac and Mekong Rivers, with minor levées. These are relatively straight,

sandy channels, which meander slightly as a result of bank-cutting and slumping, with channel shoals, bars, and lateral accretion deposits (Gagliano and McIntire 1968). The thalweg is 10 m deep for much of the delta, but shallows to around 5 m deep near the mouth. Tidal range is 2.5–3.8 m at springs in the South China Sea. The delta is characterized by complex asymmetric tidal

224 Colin D. Woodroffe

Fig. 13.4. Mekong Delta, incorporating data on pollen cores and radiocarbon dates on coastal progradation (Sources: based on Chiem 1993; Nguyen, Ta, and Tateishi 2000)

currents with amplitude decreasing up distributaries as far as Phnom Penh. On the other hand, tides of only 0.5–1.0 m at springs are experienced in the Gulf of Thailand along the western margin of the delta. During the wet season a salt wedge forms at the mouth of the distributaries, and sediment (predominantly fine silt) is deposited on the delta front, but during the dry season the distributaries are well mixed with a turbidity maximum, and sediment movement back upstream (Wolanski et al. 1996; Wolanski, Nhan, and Spagnol 1998). The delta front is reworked in the dry season, forming shore-parallel sandy ridges.

The Holocene sediments overlie an oxidized Pleistocene alluvial surface (Kolb and Dornbusch 1975). Abandoned areas of the upper deltaic plain consist of the Plain of Reeds and the Long Xuyen Quadrangle (Figure 13.4). Both of these are underlain by mangrove muds, and pollen analysis has indicated gradual replacement of mangroves and brackish water conditions by grasses and sedges (Chiem 1993). Radiocarbon dates indicate that mangrove forests flourished in these areas at 5680 years bp (Nguyen, Ta, and Tateishi 2000), and both areas contain well-developed acid sulphate soils (Brinkman et al. 1993). The active delta plain consists

Southeast Asian Deltas 225

of the areas between the active distributaries; it contains numerous beach ridges (locally termed ‘giong’), marking former positions of the shoreline, commencing around 4550 years ago (Nguyen, Ta, and Tateishi 2000). The Camau Peninsula, on the other hand, is a mangrove-dominated (Rhizophora, Avicennia, and Sonneratia, with landward Bruguiera) marginal plain. It does not show evidence of distributaries through it but has rapidly prograded at up to 80 m y−1 (Gagliano and McIntire 1968; An and Luong 1993). Much of the flooded areas adjacent to the distributaries were previously vegetated with Melaleuca, reeds (Eleocharis spp.), and aquatics (Brown 1968), although many of these areas have now been converted to rice cultivation, or more recently to shrimp ponds.

Mahakam Delta The Mahakam Delta, in eastern Kalimantan, has been the subject of detailed sedimentological exploration, and is considered an example of a tidally dominated delta. Unlike most deltas in Southeast Asia, it is prograding into deep water, rather than onto the former Sunda Platform (Allen, Laurier, and Thouvenin 1979). It drains a catchment of more than 75 000 km2. The area experiences a humid equatorial climate; mean annual temperatures are 26–30°C, and average annual rainfall is around 2300 mm. Winds are from the northeast or west. The Makassar Sea, into which it empties, has a very low wave energy, with a median significant wave height of less than 60 cm. Spring tidal range is up to 3 m. Mean discharge is probably of the order of 1000–3O00 m3 s−1, and sediment load probably 8 × 106 m3 yr−1. Within the delta, a continuous sedimentary sequence goes back to the Miocene; it is up to 6000– 8000 m thick and contains evidence for a number of transgressive–regressive cycles (Combaz and de Matharel 1978; Carbonel and Moyes 1987). The delta plain can be divided into an upper fluvial delta plain; a lower tidal delta plain; a delta front, 8–10 km in width; and a prodelta in water depths of greater than 5 m to around 60–70 m (Allen, Laurier, and Thouvenin 1979). The active deltaic plain comprises the sandy distributaries to the north and those to the south of the fan-shaped delta. Those to the east receive little river discharge, and are tidally dominated (Figure 13.5). The seawardmost parts of the deltaic plain are covered by the mangroves, Sonneratia caseolaris, Avicennia marina, and Rhizophora spp. Behind these mangroves are extensive areas covered by the salt-tolerant palm Nypa fruticans, Heritiera littoralis, and also in places by Oncosperma tigillarum and Acrostichum aureum (Caratini

and Tissot 1988; Dutrieux, Denis, and Populus 1990). Landward parts of the delta plain are covered by swamp forest. Although much of the delta is muddy, the active distributaries, 5–15 m deep, carry sand, and comprise channels of more or less constant width, generally straight but with meandering thalwegs and sandy channel bars. These distributaries form sandy lateral accretion lenses, and periodically avulse, whereupon abandoned channels are filled with clay plugs. At their mouths, subaqueous levées build out seaward, and distributary mouth bars form offshore. By contrast, in the abandoned delta, channels are meandering, tapering, and tidally dominated. The delta front is generally muddier in these tidally dominated parts of the delta, and numerous shore-perpendicular shoals occur, extending the tidal channels offshore. Erosion is typical of the delta shoreline in these areas no longer actively receiving fluvial sediment. Sand or shell winnowed from the eroding delta front, or more typically organic fragments, particularly of lignite, are concentrated into small shore-parallel ridges, 1–2 m high and 2–3 m thick, which can be seen extending back up to 3 km into the deltaic– estuarine plain (Gastaldo and Huc 1992). The stratigraphy of the delta consists of prodelta clays, which are compacted as the delta progrades over them; they are overlain by delta-front shelly sands, which in turn are overlain by tidal flat muds (clayey silts), and delta plain organic-rich clays. In generalized long section, the muddy delta sediments are interrupted by sandy delta-front lenses associated with the distributary mouth progradation; they also contain lenses of channel sands formed as lateral accretion deposits (Figure 13.5). The deeper stratigraphy and Late Quaternary history of the Mahakam Delta is known as a result of the exploratory boreholes of Allen, Laurier, and Thouvenin (1979), which demonstrate up to 50 m of Holocene sediments, and one deeper MISEDOR (Milieux Sédimentaires Organiques) core to 639 m, within which the Holocene sediments are 80 m thick. Postglacial reoccupation of the delta is recorded by transgressive sediments, with a radiocarbon age of 10 200 years bp at 71 m in the MISEDOR core, and similar ages on prodelta muds at shallower depths elsewhere in the delta (Allen, Laurier, and Thouvenin 1979; Gastaldo and Huc 1992). Much of the delta appears to have prograded along the southern distributary by around 5000 years bp (Allen, Laurier, and Thouvenin 1979). Further dates of around 2000 years bp indicate a second later Holocene phase of build-out in this area.

226 Colin D. Woodroffe

Fig. 13.5. Mahakam Delta in east Kalimantan: ecological zonation (Sources: after Caratini and Tissot 1988; Dutrieux, Denis, and Populus 1990)

and chronostratigraphy (Sources: based upon Allen, Laurier, and Thouvenin 1979; Gastaldo and Huc 1992)

Rajang and Baram Deltas Peat swamp forest is extensive around the island of Borneo (Rieley, Sieffermann, and Page 1993), and the stratigraphy of the Holocene deltaic– estuarine plains in Sarawak consists of a basal marine clay deposited beneath mangrove forests, overlain by woody peat

formed beneath peat swamp forest (Anderson 1964). Stratigraphic and chronological data are available for the Baram and Rajang Rivers (Figure 13.6), which drain the folded Cretaceous and Eocene sedimentary rocks of central Borneo, with catchments experiencing mean annual precipitation of 3000 mm or above. Tidal range is around 2 m at the mouth of the Baram

Fig. 13.6. Rajang and Baram Deltas in Sarawak, showing distribution of peat swamp and idealized long section (Sources: based upon Anderson and Müller 1975; Staub and Esterle 1993, 1994)

228 Colin D. Woodroffe

River, and similar, although locally variable and up to 6 m at the mouth of the Rajang, up which tidal influence extends at least 150 km upstream. Mangroves are replaced upstream on each river with Nypa and Saccharum sp. Peat swamp forms ombrogenous domed peat accumulations over much of the plains flanking both rivers. The peat swamp has been shown to demonstrate a zonation with six phasic communities forming a continuum. The communities are: Gonystylus–Dactylocladus –Neoscortechinia association; Shorea albida–Gonystylus –Stemonurus association; Shorea albida consociation; Shorea albida–Litsea–Parastemon association; Tristania– Parastemon–Palaquium association; and Combretocarpus– Dactylocladus association (Anderson 1964; Hewitt 1967; Staub and Esterle 1994). The first, or early, swamp community can be inundated by river floodwaters, but the subsequent communities develop a peaty substrate that is elevated above flood levels, and fed by rainwater. Shorea albida is clearly a dominant element of much of this peat swamp forest, reaching up to 40 m tall. The later swamp communities are increasingly stunted. Pollen analysis demonstrates that these peats formed over a mangrove mud (silty clay). West of Marudi in the Baram Delta this mud is encountered more than 11 m below the surface, and records change from early swamp communities, in which Gonystylus, Dactylocladus, and Campnosperma are found, through Shorea albida-dominated phases, to late swamp with increased Combretocarpus and renewed Dactylocladus (Anderson and Müller 1975). The temporal sequence thus reflects the spatial zonation, and indicates the gradual replacement of communities as the peat dome has been built up and as nutrient supply has altered as the swamps become more ombrogenous. Radiocarbon dates indicate basal mangrove at 4270 years bp, replaced by peat swamp by at least 3850 years bp, and dominated by Shorea by 2255 years bp (Anderson and Müller 1975). The Baram River has infilled a prior valley, and oil industry investigations have indicated accumulation of sediments representing at least eight transgressive– regressive cycles since the Late Eocene (Rijks 1981). In the Holocene, the river has developed a lobate delta protruding seaward from the valley (Figure 13.6). The best-developed peat swamp forests are to the south in areas which infilled before 4000 years ago (Caline and Huong 1992). The Rajang River shows a series of deltaic distributaries with intervening mangrove-covered islands; the delta covers 6500 km2, of which peat covers 50 per cent, while the adjacent coastal plain accounts for 4500 km2,

of which 80 per cent is peat-covered (Staub and Esterle 1993). The actively accreting rectilinear delta-coastal plain and the abandoned tidally flushed part of the plain can be distinguished (Figure 13.6). An annual sediment load of around 8 × 107 t enters the system, and the plains extend by an additional 1 km2 each year. Mangroves are extensive over the seaward islands and adjacent intertidal areas, but are replaced by Nypa landwards. The active distributaries are floored by mud (silty clay and clayey silt), whereas the tidally flushed distributaries are floored by sands and gravels, in contrast to other deltaic systems in the region. The peat swamps have developed over mangrove sediments. Based on radiocarbon dates, Staub and Esterle (1993) suggest that an average rate of progradation has been 8 m y −1 over the late Holocene. However, aerial photography indicates seaward buildout of as much as 50 m y −1. This is because present river sediment loads may be as much as three times the natural pre-development loads. Progradation occurs by the expansion of distributary mouth bars and subsequent vegetation colonization by mangroves, with welding of offshore bars onto the shoreline. All distributaries are relatively deep (more than 15 m); those carrying the largest loads of discharge and sediment are relatively straight and have salt wedges at their mouths (e.g. Igan and Lassa). The obtuse bifurcation angles of distributaries suggest structural control. Acidic waters emanate from the peats; these give rise to pH values of around 4 and account for the absence of calcareous material in the sediments. The fully mixed but tidally dominated and sediment-starved Belawai and Rajang distributaries are associated with meandering channels. Staub and Esterle (1993) conclude that much of the sediment brought down by the Rajang River is successfully transported through the delta by the active channels and does not lead to extensive overbank sedimentation.

Deltaic-Estuarine Plains of the Central Plain of Thailand The most extensive accumulation of deltaic – estuarine sediments in Thailand occurs at the north of the Gulf of Thailand, forming the low-lying plains of the lower central plain, flanking the Chao Phraya River, and extending from the Mae Klong River to the Bang Pakong River (Figure 13.7). Tertiary and Quaternary sediments reach more than 2000 m thick beneath these plains, comprising a sequence of superimposed aquifers (Sinsakul 2000). The main rivers flowing across the plains are

Southeast Asian Deltas 229

Fig. 13.7. Chao Phraya, Thailand, with stratigraphical long section (Sources: after Sinsakul 2000; long section based on Somboon 1988; Somboon and Thiramongkol 1992)

230 Colin D. Woodroffe

constrained by levées; while the plains are seasonally inundated and are used for rice cultivation. The modern coastline of the gulf is fringed by mangrove, much of which has been converted for prawn aquaculture or salt ponds. A Holocene sequence of sediments overlies either a basal unit of Late Pleistocene oxidized fluvial sediments or weathered bedrock. An organic-rich mangrove mud records the transgression across this subaerially exposed surface, between 7800 and 5600 years bp (Somboon 1988; Somboon and Thiramongkol 1992). The maximum height reached by the transgression occurred around 6000 years ago, and the shoreline at that time was north of Ayutthaya (Sinsakul 1992). At the Senanivate housing estate, just north of Bangkok, the basal transgressive mangrove facies, dated at 7800– 7500 years bp, is overlain by marine sediments, which are in turn overlain by a regressive intertidal unit, dated at around 4600 years bp (Somboon and Thiramongkol 1992). The Bangkok Clay that underlies most of the present plains has been deposited since 5000 years bp. Further radiocarbon dating is revealing the details of late Holocene regression and variation in sedimentation rates (M. Umitsu, personal communication).

Deltas on the Thai–Malay Peninsula There are numerous smaller deltaic–estuarine plains along the margin of the Thai–Malay Peninsula. These have developed in a similar way to the larger deltas already described. Microfossil studies show a similar pattern of change from mangrove to freshwater swamp forest during the early to mid-Holocene (Webber 1954; Haseldonckx 1977; Bosch 1986; Kamaludin 1989a,b). The delta of the Klang River contains a stratigraphy in which peat swamp has formed over marine sediments, very similar to that beneath the peat swamps of Borneo, and radiocarbon dates also indicate replacement of mangroves soon after 4900– 4500 years bp (Coleman, Gagliano, and Smith 1970). Rapid deltaic progradation in the mid- and late Holocene has been demonstrated for much of the coast. Reconstructions of the Merbok Estuary provide a good example (Khoo 1996). Avulsion of estuarine channels is indicated by a sequence of palaeochannels associated with the mouth of the Perak River (Koopmans 1964). The east coast of the peninsula has a Holocene history that has been much more constrained by the development of sand barriers, which have influenced the development of individual rivers in different ways (Nossin 1964; Bosch 1988). The Sungai Pahang has a cuspate delta with a complex pattern of

ridges; the Pattani River has built a cuspate delta in the lee of a sand spit, while the Mae Nam Ta Pi has switched its birdsfoot delta lobes (Pitman 1985).

Javanese and Sumatran Deltas Java, in common with other islands in the Indonesian island arc, is tectonically active, with volcanism and active seismic activity. Rapid uplift along the arc is documented in the sequence of coral reef terraces on islands such as Atauro and Timor (Chappell and Veeh 1978) and Sumba (Pirazzoli et al. 1991). In common with the island of New Guinea, which lies on the Sahul rather than the Sunda Shelf (and therefore outside the principal area of this review), these islands contribute large annual sediment loads to the ocean. The central parts of Java contain young volcanics; the high sediment yields are further enhanced by humaninduced land use changes that may have increased sediment supply threefold (Bird 1985). There are numerous active rivers draining rapidly eroding island landscapes. Java is typical, with rapid progradation of deltas on its north coast. Extension of delta areas has been observed to have ranged from 0.12–1.11 km2 y −1 for deltas along this coast (Hollerwöger 1966). The Peusangan Delta has moved eastward as a result of river capture, leaving an eroded shoreline to the Djuli River. Rapid progradation along the Sumatran east coast is indicated; Palembang was an active port in the fifteenth century, but is now well upstream amid a broad coastal plain, with extensive peat swamp development. One of the most studied deltas is the Solo, a ‘lowenergy, mud-dominated, rapidly-extending “single finger” delta’ (Terwindt, Augustinus, and Hoekstra 1987). This delta has prograded rapidly since diversion last century to prevent silting of the Straits of Surabaya (Figure 13.8). It drains a basin of 16 000 km2, which receives an average rainfall of 2100 mm a year. The tidal range reaches 2.1 m at springs, and though this is a river-dominated delta, the form of the delta progradation is influenced by tidal complexities (Hoekstra 1989, 1993b). The sediment load is 95 per cent mud (largely silt) which is carried in a buoyant plume, over a salt wedge that can extend up to 100 km upstream in the dry season. The annual sediment load is 19 × 106 t, and, with few crevasses, extension of the delta can be up to 70 m a year. Sand is deposited in fingers marking channel floors; mud supply exceeds the current’s ability to remove it. Hoekstra has documented the fluvial sediment discharge from the Solo River (Hoekstra 1989), but, despite the dominance of river discharge in northern Java, the

Southeast Asian Deltas 231

Fig. 13.8. Solo River, reconstruction of the progradation of the delta, northeastern Java (Source: after Hoekstra and Tiktanata 1988)

Solo Delta has developed in a way that is tide-induced (Hoekstra 1993b). An interesting contrast with these actively prograding north coast deltas is the Porong Delta, emptying into the Strait of Madura. This is a lobate delta with multiple distributaries; this steep-course river, presently controlled by human intervention, is now almost dry (Hoekstra and Tiktanata 1988). Other deltas along the northern Java coast, such as the Citarum and Cimanuk, have also demonstrated rapid progradation over the past 100 years.

Discussion Delta Morphology The delta case studies examined in this review demonstrate that the morphological components identified in Figure 13.1 can be seen in different forms in different deltas. The Ganga–Brahmaputra–Meghna Delta, though to the west of the area of interest, shows many of those components that are typical of deltas in the

Southeast Asian region. It comprises an active deltaic plain, called the Meghna Deltaic Plain, downstream of the confluence of the Ganga and Brahmaputra Rivers. This can be subdivided into a fluvially dominated western section, a central fluvio-tidal section, and a tidally dominated section in the east around the Sandwip Channel (Coleman 1969; Barua 1990; Barua et al. 1994). The lower deltaic plain to the west of the active river, termed the Ganga Tidal Plain, is tide-dominated. It consists of the extensive Sundarbans wetlands (in eastern India and western Bangladesh), dissected by numerous meandering tidal creeks. There are conspicuous former distributaries, including the tidally dominated Hooghly, which flows through Calcutta, which between them carry only around 4 per cent of the Ganga flow. Erosion by fluvially enhanced ebb-tide currents is characteristic of islands on the subsiding tidal plain, in contrast to the net deposition of sandbanks, termed chars, in the active distributaries of the Meghna (Paul and Bandyopadhyay 1987). Recent nearshore observations, determined from historical charts, indicate that subaerial extension of the Ganga–Brahmaputra Delta accounts for only 1–2 per cent of the sediment budget of the rivers (Allison 1998; Allison, Kuehl, and Martin 1998). Large volumes must be stored on the floodplain and delta front, which is prograding at rates of up to 12–15 m per year (Kuehl et al. 1997; Michels et al. 1998). About a third of the sediment load reaches the delta front, with some contemporary sedimentation via the Swatch of No Ground, a submarine canyon, onto the deep-water Bengal Fan (Weber et al. 1997). The long-term evolution of the Ganga–Brahmaputra– Meghna has been controlled by plate tectonics and eustasy (Lindsay, Holliday, and Hulbert 1991). Transgression is recorded by Holocene marine silts and sands (dated 9000–7500 years bp) overlying an oxidized gravel layer (Umitsu 1993, 1996). Progradation has occurred during subsequent regression (Morgan and McIntire 1959), with a transition through mangroves to Heritiera fomes and Nypa fruticans reported around 5800–5000 years bp in Calcutta (Vishnu-Mittre and Gupta 1972). The delta of the Sông Hóng River, in northern Viet Nam, also clearly shows areas that are dominated by river, tide, and wave processes. The apex of the delta contains sequences of fluvial channels showing meander scroll plains and evidence of their avulsion; the western flank of the delta contains parallel series of recurved beach ridges that have been constructed by wave processes; and the eastern flank of the delta consists of tapering tidal distributaries (Mathers and Zalasiewicz 1999).

232 Colin D. Woodroffe

The deltas of Southeast Asia, draining onto the Sunda Shelf, show the influence of river, tidal, and wave processes in differing degrees. Deltas associated with rivers draining catchments which receive a large rainfall, and which are in tectonically active areas such as Java, are river-dominated. They are characterized by rapid channel extension, though also influenced by tidal flows as in the case of the Solo, and with frequent channelswitching. Fluvial processes dominate the channel in the active delta. The abandoned deltaic plain, receiving little if any fluvial sediment input, becomes dominated by tidal or wave processes. It can be extensive in deltas on the Southeast Asian mainland, or on large islands like Borneo. Tidal creeks, exponentially tapering landwards, and often tightly meandering, such as those of the central Irrawaddy Delta or the eastern Mahakam Delta, characterize tide-dominated sectors of the abandoned deltaic plain. Shore-parallel sand or lignite ridges are indicative of the wave-dominated sectors of deltas, such as the central Mekong Delta (especially flanking both the Mekong and Bassac River distributaries), or the eastern margin of the Mahakam Delta. While morphology provides clues to the processes operating and the controls upon delta formation, sedimentation and erosion occur non-linearly and over timescales which do not permit direct observation. Stratigraphy and chronology, particularly radiometric dating of Late Quaternary depositional sequences, provide further insights into the natural dynamics and the way in which these systems have changed, especially in response to subtle forcing functions such as sea-level change. Despite the scarcity of such studies for many deltas in Southeast Asia, there are some general trends that appear to relate to several deltas in the region, which are discussed below. It is also clear that vegetation has played an important role in the natural dynamics of Southeast Asian deltas. Thus mangroves dominate the intertidal shorelines of the region. Mangroves are replaced in areas that are only slightly brackish or freshwater by a range of other wetland species. Extensive Heritiera stands occur in the Irrawaddy Delta, while Melaleuca, reeds, and aquatics dominate the drier Mekong Delta. Oncosperma and Acrostichum also occur in regularly flooded areas throughout the region; chenier or beach ridges are often covered by Casuarina, and on high-ground areas, such as the levées, there may be broad-leaved trees. Within the wettest deltaic plains of Malaysia and Indonesia, peat swamp forest, in which Shorea and other trees are prominent, builds up remarkable ombrogenous, domed woody peats (Anderson 1964). These impressive

wetlands cover the interdistributary basins and serve to ensure that minimal overbank deposition of fine sediment occurs on the former floodplains. Preliminary pollen analysis in several deltaic–estuarine plain settings indicates that there have been broad changes in the vegetation on the plains during the Holocene as these depocentres have adjusted to the relatively stable sea level, after the culmination of the postglacial transgression. In fact the natural vegetation of many deltas has now been irreparably cleared, and there may be little or nothing to indicate what would have been growing there. The human impact is directly related to population pressure. Most of the deltas of continental Southeast Asia have been modified for rice cultivation, or cash crops, and extensive peat swamp areas in southern Sumatra and Kalimantan have been modified as a part of the transmigration programme (NEDECO 1978).

Delta Response to Sea-Level Change The deltaic–estuarine plains of Southeast Asia show a broadly similar trend of postglacial transgression, followed by mid- and late Holocene regression, controlled by the pattern of sea-level change (Woodroffe 1993). The broad pattern of Quaternary sea-level and environmental history in the region provides an important background to the study of delta morphology, evolution, and development issues in Southeast Asia. Quaternary climate is still poorly understood for much of Southeast Asia, but extensive alluvial deposits have been interpreted to indicate a drier, more seasonal climate during glacial times (Verstappen 1975, 1980, 1997; Heaney 1991). Quaternary sea-level changes in the Indo-Pacific region have most effectively been determined from flights of coral reef terraces on the rapidly uplifting Huon Peninsula, Papua New Guinea (Chappell 1974, 1996; Chappell and Shackleton 1986). Based upon uraniumseries dating of the Huon reefs, sea-level events have been correlated with the deep-sea oxygen isotope record indicating two cycles during which sea level has fluctuated through a series of progressively lower oscillations over the last 240 000 years. For those deltaic – estuarine plains for which stratigraphic records are available, it is clear that sea-level rise since the last glacial maximum (the postglacial transgression) has seen the re-establishment of intertidal and marine conditions over the subaerially exposed Sunda Shelf (Hanebuth, Stattegger, and Grootes 2000). Sea level reached its lowest at the glacial maximum, around 21 000–22 000 years bp (18 000 radiocarbon years bp). The depth at which this lowest shoreline

Southeast Asian Deltas 233

occurred is widely taken as 120 m; a drowned delta in the Strait of Malacca recorded about 146 m below present may represent this lowstand (Emmel and Curray 1982). This lowering of the sea level resulted in exposure of much of the Sunda Platform (Biswas 1973; Batchelor 1979; Tjia 1980). Young sedimentary cover over an older alluvial complex on the Sunda Platform appears to represent this transgression (Aleva 1973; Aleva et al. 1973). Throughout much of the region there is evidence that the sea rose above present level in the mid-Holocene, and then fell back to its present position (Geyh, Kudrass, and Streif 1979; Chappell and Polach 1991; Tjia 1996). There has been widespread recognition that the sea was higher than present in the mid-Holocene throughout Southeast Asia (Fontaine and Delibrias 1974; Haile 1975; Tjia 1977; Thommeret and Thommeret 1978; Rimbaman 1992; Scoffin and Le Tissier 1998). The timing, magnitude, and occurrence of this Holocene high sea level was not, however, the same throughout the region, as a result of hydro-isostatic adjustments to global ice and ocean water volumes. Detailed observations from the Thai–Malay Peninsula indicate a sea level at least at its present level by 6000 years bp, reaching a peak of 5 m above present by 5000 years bp, regressing, and then reaching a new peak at around 2.5 m at 4000 years bp, then dropping below present before rising again to around 2 m at 2500 years bp, and reaching its present level around 1500 years bp (Tjia 1996; Fujimotu et al. 1999). The pattern of sea-level and environmental change during the Quaternary has thus controlled the evolution of the coastal and lowland riverine plains of Southeast Asia. It is clear that individual coastal systems have prograded considerably since 6000–5000 years ago, when sea level initially reached its present level. The pattern of transgression is recorded for several of the deltas in the region, and, controlled largely by the rate of eustatic sea-level rise, has been similar for each system. It is recorded by marine sediments overlying an oxidized alluvial surface. The pattern of change over the past few thousand years, on the other hand, has differed between deltaic–estuarine systems. For several deltas, there is evidence that much of the overall build-out had been achieved by 4000 years bp. This appears to mark the time at which extensive peat swamp forests developed over the former mangrove flats in the wetter areas. Thus many of the systems, such as the Rajang, Baram, and Klang Deltas, had prograded at least to the extent of the best-developed peat swamp forests by that time. In the case of the Mekong Delta it appears that this progradational phase marks the initiation of a series of

beach ridges that have been formed intermittently over the past 4000 years. In other systems, deltaic history over the past 4000 years is less clear. Thus for the Chao Phraya, although the transgressive sediments have been well dated, there is a less extensive record of the regressive Bangkok Clay; progradation appears to have reached the present site of Bangkok, and then to have decelerated in the past few millennia. Thus there is a need for many more studies of morphodynamic change of these depositional systems in order to determine the complexity of the coastal and lowland riverine plains and their response to environmental changes at a local scale.

Present-Day Dynamics In recent centuries human influence has begun to assume a far larger role in relation to the natural dynamics of these systems. The natural vegetation of the majority of deltaic – estuarine plains in Southeast Asia has been cleared, or considerably modified. Timber, such as the trees of the peat swamp forests, has been removed, and in the areas of higher population density, much of the delta plain has been converted to rice cultivation, cash crop production, or for settlement. In Sumatra and Kalimantan, peat swamp forests have been removed for the development of large settlements as a part of a transmigration programme. Many of the lower-lying areas have been utilized for prawn aquaculture, though ponds often have only a short-term viability. Large metropolitan areas have also grown up in these lowlying plains, bringing their own pressures. Groundwater extraction from the aquifers beneath Bangkok, especially from within the Holocene Bangkok Clay, has led to rapid subsidence, and Bangkok is subject to extensive flooding (Nutalaya and Rau 1981). Natural flooding, so characteristic of deltaic–estuarine areas, and the source of rich alluvial sediments and fertile soil, has been increasingly controlled, through embankments and artificial levées, and in some cases by damming rivers. Where such soil replenishment is absent, soils deteriorate; elsewhere drainage of Holocene deltaic sediments has resulted in oxidation of pyrite in acid sulphate soils, releasing acid waters and toxic ions (Brinkman et al. 1993). Where sediments do not reach the coast, erosion has become prevalent, particularly in the abandoned deltaic plain where gradual subsidence and the absence of direct fluvial input may lead to shoreline recession. The classic example of this is the Nile Delta, which has experienced enhanced delta-front erosion since construction of the Aswan Dam, a scenario which may be anticipated on other large deltas if dam construction goes ahead, depriving

234 Colin D. Woodroffe

the delta front of its sediment supply (Wolanski, Nhan, and Spagnol 1998). Human influence has overprinted the natural dynamics of these deltaic and estuarine areas. These are areas subject to enormous loss of life where higher than usual flood levels are experienced, as was the case in late 1998 over much of Bangladesh outside the region and in 2000 in Cambodia and Viet Nam inside it. Where more extensive infrastructure is built on low-lying plains, there is more to lose when natural and artificial embankments and levées are overtopped. These problems are exacerbated where groundwater extraction leads to enhanced subsidence or if the sea rises at an increased rate, as anticipated as a result of the greenhouse effect. Deltas and estuaries will continue to be areas of rapid change, in terms of both erosion and deposition, but natural and long-term patterns of geomorphological evolution are likely to become even more inextricably linked with human-engineered changes in the near future.

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III

Environment and People

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14

Accelerated Erosion and Sedimentation in Southeast Asia Avijit Gupta

Introduction Periodic attempts to plot global distribution of erosion and sedimentation usually attribute most of Southeast Asia with a very high sediment yield (Milliman and Meade 1983). The erosion rates and sediment yield figures are especially high for maritime Southeast Asia. Milliman and Syvitski (1992), for example, listed 3000 t km−2 yr −1 for the archipelagos and peninsulas of Southeast Asia. They provided a number of natural explanations for the high erosion rate: location near active plate margins, pyroclastic eruptions, steep slopes, and mass movements. This is also a region with considerable annual rainfall, a very substantial percentage

of which tends to be concentrated in a few months and falls with high intensity. Part of Southeast Asia (the Philippines, Viet Nam, Timor) is visited by tropical cyclones with heavy, intense rainfall and possible associated wind damage to existing vegetation. The fans at the foot of slopes, the large volume of sediment stored in the channel and floodplain of the rivers, and the size of deltas all indicate a high rate of erosion and episodic sediment transfer. This episodic erosion and sediment transfer used to be controlled for most of the region by the thick cover of vegetation that once masked the slopes. When vegetation is removed (Figure 14.1), soil and regolith de-structured, and natural slopes altered, the

Fig. 14.1. Land clearance on steep slopes, in the valley of the Nam Ou, tributary to the Mekong, northern Lao PDR

240 Avijit Gupta

erosion rates and sediment yield reach high figures. Parts of Southeast Asia display striking anthropogenic alteration of the landscape, although the resulting accelerated erosion may be only temporary, operating on a scale of several years. Over time the affected zones shift, and slugs of sediment continue to arrive in a river but from different parts of its drainage basin. The combination of anthropogenic alteration and fragile landforms may give rise to very high local yields. Sediment yields of more than 15 000 t km−2 yr−1 have been estimated from such areas (Ruslan and Menam, cited in Lal 1987). This is undoubtedly towards the upper extreme, but current destruction of the vegetation cover due to deforestation, expansion of agriculture, mining, urbanization, and implementation of large-scale resettlement schemes has increased the sediment yield from < 102 to > 103 t km−2 yr−1. This impressive increase has prompted research into the process and rates of erosion and sediment transfer in Southeast Asia, the effects of such sediment on channel form and behaviour, the resulting changes affecting deltas, beaches, mangroves, and coral reefs, and the general lowering of environmental quality. The quality of the field data is variable, and it is necessary at times to extrapolate numbers measured carefully from an experimental plot to a large region, but enough information is now available to construct a regional account. The changes are large enough to be visually mapped from satellite images (Gupta and Krishnan 1994; Gupta 1996; Gupta et al. 2002). This chapter, a summary account from a range of case studies, (1) generalizes a set of numbers to represent sediment yield from different types of land use changes, (2) identifies the areas prone to high erosion and sediment yield, (3 ) describes the seasonal and temporal nature of the processes involved, and (4) attempts to deduce the effects of accelerated erosion and increased sediment yield on slopes, rivers, and coasts. The present rate of erosion and sedimentation in Southeast Asia does not appear to reflect the natural environment any more in many places.

Land Use Changes and Sediment Yield Where a vegetation cover and a thick regolith exist in southeast Asia, the sediment yield is usually low and erosion limited. This has been shown by extensive studies in the rainforest (Anderson and Spencer 1991). The destruction of this protective cover either naturally (volcanic eruption on the plate margin islands or slope failures from various causes) or by anthropogenic

activities (deforestation, agricultural expansion, mining, resettlement in areas of low population density, urbanization) releases large volumes of sediment. All the anthropogenic activities increased rapidly in the second half of the twentieth century, and such increases in the areas of thick weathered material, steep slopes, short swift streams, and intense and heavy rainfall are reflected in the current high sediment yield. As the description of the vegetation of Southeast Asia in this book by Corlett (Chapter 7) indicates, the present distribution of vegetation is to a large extent not natural but secondary or anthropogenic. Collins, Sayer, and Whitmore (1991) estimated that in the 1980s most of the countries of Southeast Asia had an annual deforestation rate in thousands of km2. Any account of erosion and sedimentation of slopes, rivers, and coasts of Southeast Asia needs to take into account the intensity and extensiveness of this change.

Measures of Sediment Yield Erosion and sediment loss have been estimated for various locations using diverse methods such as calculation of ground-lowering in small erosion plots, sedimentsampling in streams, or rate of extension of a delta in the sea over a number of years (Shallow 1956; Leigh and Low 1973; Peh 1981; Wiersum 1984; Ruangpanit 1985; Weera and Kittipong 1987; Law et al. 1989; Malmer 1990; Anderson and Spencer 1991; Douglas et al. 1992, 1993; Sinun, Wong, and Spencer 1992; Lai 1993; Murtedza and Ti 1993; Greer, Bidin, and Douglas 1994). The majority of the published figures come from Malaysia and part of Indonesia ( Java). The published data that are available have two constraints. First, rock types, slopes, rainfall, and land use vary tremendously in these studies. Secondly, some of the measurements are in t km−2 yr −1, whereas others are in m3 km−2 yr −1. Even when converted into comparable units, no single figure can be used to represent a given set of land use. It is therefore safer to see the representative figures as a range rather than an average. For example, one of the more reliable sets of information comes from measurements off small plots undergoing urbanization. Such measurements range from 1050 to 1500 t km−2 yr −1. Table 14.1 summarizes the data available, given that the frequencies of observation vary between different land use types and the reliability of information varies from type to type. The sediment yield from forests range from nil to 460 t km−2 yr −1, indicating a low annual figure from forested catchments, usually in tens of t km−2. A low figure like this does not indicate the possibility

Erosion and Sedimentation 241 Table 14.1 A summary set of sediment measurement Land use type

Sediment yield (t km−2 yr −1)

Forest Large river basins (mixed) Urbanization Shifting cultivation

100–102 102–104, mostly 103 103 103 for plot measurements 102 for entire basins 102–103, depending on conservation measures used 103, maximum recorded 15 000

Agriculture Logging, early stage

Note: Range of sediment yield compiled from large number of published measurements in Southeast Asia.

of the occurrence of extensive depositional features; rather it indicates a pattern of pulsating transfer of sediment downstream, being stored intermittently as smallscale channel or floodplain features. Destruction of the forest cover, either naturally or due to anthropogenic causes, will lead to high sediment yield, dramatically at times, as illustrated by the landslide devastation of the logged slopes and lower valleys in southern Thailand in November 1988 following about 1000 mm of rainfall in five days and the incidence of soil erosion, huge landslides, and flooded rivers following deforestation in the Bengkulu Province, Sumatra, in the same year. The limited amount of published reliable information regarding soil loss under shifting cultivation or continuous agriculture makes similar representative figures difficult to ascertain. Douglas et al. (1993) have opined that the only valid soil erosion data under shifting cultivation in Malaysia are measurements by Hatch in Sarawak on 40 m2 plots (Hatch 1981). It is, however, difficult to extend such observations to a regional scale. Weera and Kittipong (1987) working in Thailand computed such figures and it seems likely that sediment yields of up to a few thousand t km−2 yr −1 can be expected when the fields are open and no proper conservation measures are adopted. Murtedza and Ti (1993) have pointed out that these figures may drop to hundreds of tonnes when the entire river basin is considered, part of which could be under forest. The figures that exist show a wide range depending on local conditions and application or not of conservation measures. Such figures may vary from 102 to 103 t km−2 yr −1. One characteristic of Southeast Asia is the extensive rubber and oil palm plantations which by the 1980s had replaced 12 per cent of the forests of West Malaysia (Repetto 1988). Shallow (1956) estimated that replacing woodland with tea plantations or vegetable plots in West Malaysian hills increases sediment production by 20 and 30 times respectively. In northeast

Thailand, sediment yield from abandoned fields is more than twice the sediment loss from dry forests (Weera and Kittipong 1987). In Malaysia and Indonesia, largescale land clearance has taken place to resettle a large number of farmers. In Indonesia, nearly 3 million people were resettled from the congested islands of Java, Madura, Bali, and Lombok to the low-density areas of Sumatra, Kalimantan, and Papua under the transmigrasi project. The examples of associated forest degradation include among others the forests and slopes of Lampung Province, south Sumatra, and the peat forest of southern Kalimantan. The FELDA (Federal Land Development Authority) project of Malaysia, in which young rural people are trained as farmers and settled as landholders in virgin areas, have affected the rainforests of Peninsular Malaysia (Collins, Sayer, and Whitmore 1991). Sediment yield following logging in the rainforest could be very high (103 t km−2 yr −1) in the first two or three years and then tail off. The highest figures on record are more than 7000 from Sabah (Greer, Bidin, and Douglas 1994) and 15 000 from Java (Ruslan and Menan, in Lal 1987). Again, it is difficult to choose a representative figure, but somewhere around 1500 t km−2 yr −1 is probably a reasonable estimate. Lai, Ahmad, and Zaki (1996) opined that sediment yield from a logged catchment could be as much as 50 times that of the yield from undisturbed basins. The logging tracks, cleared of vegetation and stamped hard by heavy machinery, often function as run-off channels following the usual intense rainstorms thereby increasing both the flood potential and sediment load of local streams. The sediment will progressively move downstream via a series of transfer and storage. If the sediment is from small coastal streams, it may reach the sea in a few years. One hopes that it would take longer for large basins. The measured sediment yield from mining areas varies, although the discharge from mining may carry a suspended sediment load as high as several thousand mg l−1 and for a period of time due to slow establishment of vegetation over the mined land (Lai, Ahmad, and Zaki 1996). Balamurugan (1991) referred to 5900 t km−2 yr −1 from the 27 km2 basin of the Sungai Jinjang in Malaysia, which includes large abandoned mines. Sediment produced by rapid urbanization in Southeast Asia is comparable to the post-logging figures (Douglas 1978; Leigh 1982; Balamurugan 1991). The storage and transfer of such sediment, however, is temporary, being significant only until urbanization is complete. A number of case studies are available from Singapore (Chatterjea 1989; Gupta and Pitts 1992)

242 Avijit Gupta

Fig. 14.2. Construction in Singapore, exposed sediment

and Kuala Lumpur (Douglas 1978; Leigh 1982). In Singapore, construction with heavy machinery involves removal of the vegetation cover, grading of the surface, and piling up of earth in mounds for later removal (Figure 14.2). Given the exposed condition existing for a number of months (Poon 1984), enough time is available for a large amount of sediment to be eroded and transported to drainage channels. With construction the sediment load of channels decreases rapidly and the material tends to have a finer texture. In sum, the limitation in sediment yield data makes it difficult to work out how much extra sediment will be generated and transported downstream once the natural vegetation is removed. According to the erosionplot-based studies, the change could be extremely high. Anderson and Spencer (1991) refer to an increase of 53 times in Java, from 30 to 1590 t km−2 yr −1. Ruangpanit (1985) measured soil erosion under a range of vegetation cover in Thailand from 41 storms, the total rainfall from which summed to 1128 mm. Where the crown cover in the forest was 80–90 per cent, the total sediment loss was 28.5 kg km−2. Where the crown cover was reduced to 20–30 per cent, the figure increased to 65.3 kg km−2. Perhaps the best estimate of change comes from figures available for rivers draining large basins with drainage areas in 102–103 km2 with a mixed pattern of land use in the catchment area. Estimates of sediment load in streams are available from Indonesia, Malaysia, Thailand, and Lao PDR (Aitken 1981; Hardjowitjitro 1981; Balamurgan 1991; Mekong River Commission various dates). Aitken (1981) provided details of sediment load carried by the Cimanuk

River in Java, which flows through a densely populated basin. Extensive ricefields occur in the lower plains, but the upper slopes are in upland crops, rice, and forest or scrub with volcanoes along the divide. Based on delta growth, Aitken estimated a basin-wide sediment yield of 7800 t km−2 yr −1. Given that the sediment yield will vary across the basin, the range estimated stretches from 5200 t km−2 yr −1 in the more stable areas to 12 000 for the rapidly eroding Cilutung sub-basin. Most of the load carried is less than 0.05 mm in texture, probably due to the preponderance of weathered volcanic material. Estimated sediment yield from other river basins in Java is comparable (Hardjowitjitro 1981). Fagi and Mackie (1988) found similar sediment yield figures, but indicated that sediment yield in several intensely farmed Javanese basins drops remarkably with good land use practice.

Temporal and Seasonal Pattern of Sediment Transport As Douglas et al. (1992) have described, sediment yield from a disturbed site changes with time. In their example of the impact of deforestation in the Sabah forest, sediment yield rose to 18 times the pre-disturbance figure but after two to three years began to tail off and with rapid regrowth of secondary vegetation dropped to figures representative of undisturbed areas (Greer et al. 1996). This is a common pattern, and unless the disturbance is continuous, sediment yield decreases and secondary vegetation covers up the rapidly eroded bare ground. As

Erosion and Sedimentation 243

data from their 102 m plot showed, the 1989 figure of sediment yield from one area was 19 050 t km−2 yr −1, followed by 1050 in 1990. The figures from another, similar plot were 3200 and 1994 for 1989 and 1990 respectively (Greer et al. 1996). If logging occurs in small selected plots, this indicates a continuous supply of sediment to the main stream but with changing source areas over time. Geomorphic processes associated with erosion and sediment transfer also change over time. In areas undergoing urban development in Singapore, a general sequence of events can be discerned for graded bare slopes (Gupta 1992). Within a few minutes of the beginning of rainfall, a sheet of water laden with sediment starts to flow on the surface. Within a few metres, the sheet flow is concentrated into rills and small gullies that erode into the surface material. In urbanized Singapore, this sediment of medium to coarse sand ends up in a lined canal or a perimeter drain. In unurbanized areas, sediment from bare slopes will flow into streams. Unless the bare surface is regularly graded, the gullies enlarge rapidly. In Singapore, 2 × 0.5 m parallel gullies have been seen to develop across construction sites in days. Given sufficient time, an intricate network of channels develops on steep bare slopes, a general pattern which is followed wherever vegetation is removed and land kept bare in the region. The gullies survive until an impervious cover comes into place or, in case of the land being abandoned, secondary vegetation rapidly establishes itself. On many steep slopes of Southeast Asia drained by existing first- or second-order tributaries, debris flows may happen along with slope wash, gully development, and slope failures.

Temporal and seasonal changes in erosion and sediment transfer were described from a 225 km2 area of steep terrain in northern Lao PDR drained by the Mekong (Gupta and Chen 2002). This is an area of steep-sided ridges and valleys, with the ridge crests rising to over 1000 m and the Mekong, within 5 km, flowing at an elevation of 250 m. The local annual rainfall is around 1500 mm. The rain, associated with the southwestern monsoon, is seasonal, 85–90 per cent falling between May and October. The steep slopes are mostly under either forest (a substantial part in the degraded state) or shifting cultivation (usually hill rice with other crops such as long beans and cucumber). Plots are cleared at the beginning of the dry season (December–January), and the biomass on the ground is burnt towards its end (April–May). The rural population is low (tens per km2), but the cultivated slopes are steep. Chen et al. (2000) showed that nearly a quarter of land cleared between 1996 and 1998 was from slopes steeper than 25°. Areas under vegetation or cleared were measured for seventeen nearly cloud-free multispectral images between 1996 and 2000. By multiplying these areas with a measure of average sediment yield, sediment removed from slopes in a particular month was estimated. Of these eight were for 1998, enabling monitoring of changes in land use and sediment transfer through a calendar year. The following conclusions were reached. 1. A seasonal pattern of erosion and sediment transport and storage happens annually, as described in Table 14.2. 2. The amount of sediment loss and transfer is directly related to rainfall received; a drier year (as

Table 14.2 Middle Mekong Basin: seasonal erosion and sediment transfer Period

Slopes

Tributaries

The Mekong

Dec.–Apr. Dry, a few showers

Increasing clearance; very little erosion or sediment transfer due to lack of rainfall

Low flow; past sediment stored mainly on bed, as insets, and at confluences

Sediment stored on bed, as insets, around rock protrusions, and at confluences with major tributaries; gradually falling stage and diminishing sediment transport

May Rains start, rising discharge

Slopewash, gullies, small debris flows; sediment transfer to channels

Sediment pulses transferred following showers; sediment from slopes

Increased velocity and transport of channel sediment; increased competence

June–July(?) Early wet season

Considerable erosion and sediment transfer to channels via processes described above

Transfer of channel sediment to the Mekong; more sediment from slopes; confluence accumulation

Erosion of bed material, insets, and bars; increased competence; bedrock erosion; critical and supercritical flow in places; water tens of m deep in inner channel

Aug.–Nov. Late wet season

Vegetation growth; little erosion; very little sediment transfer

Continuous transfer of channel sediment; confluence accumulation

Continued erosion of bed material, insets, and bars; local critical and supercritical flows; bedrock erosion; deep water in inner channels; sediment accumulation at channel confluences

Source: Gupta and Chen (2002).

244 Avijit Gupta

associated with the El Niño phenomenon) releases significantly less sediment. 3. In areas of shifting cultivation on steep slopes, sediment accumulation, as expected, shows a fluctuating trend but not a progressive one. This trend will change with continuous cultivation without proper conservation measures. A seasonal and temporal pattern in the long-term erosion and sediment yield prevails in Southeast Asia. In the Philippines, White (1995) attributed erosion, shortdistance sediment transfer, and its storage to frequent thundershowers, and its efficient long-distance transfer to much less frequent tropical cyclones.

Spatial Transfer of Sediment Under vegetated natural conditions, river channels are expected to be full of sediment arising from pyroclastic flows followed by lahars only on the volcanic islands of Southeast Asia. Lavigne and Thouret (1995) reported twenty rain-triggered occurrences of lahars in the Boyong River draining the southern slopes of the Merapi volcano in Java. About 106 m3 of material was delivered in six months, completely filling the channel (Figure 14.3). A detailed account of pyroclastic flows and lahars coming down the channels from the Merapi is provided by the same authors in Chapter 16 of this volume. Takanashi (1981) described similar deposition of volcanic material in the channel of the Brantas River from the Kelud volcano in Java. The river channel downstream

carries large amounts of both bed and suspended load, as do other rivers such as the Progo (Voskuil and Zuidam 1982) and, unless anthropogenically removed, become flood-prone (Takanashi 1981) and may provide material for large-scale coastal features such as beaches or dunes near their mouths. Nossin describes laharchoked channels associated with the eruption of Mount Pinatubo on the island of Luzon in Chapter 15 of this book. Sediment transport and deposition following the 1991 eruption of Pinatubo have been studied in detail by Hayes, Montgomery, and Newhall (2002). They discovered negligible critical shear stress and a very high rate of sediment transport related to an unlimited supply of material and enhanced mobility on top of a smooth, fine-grained bed. Anthropogenic modification of the vegetation cover increases erosion and sediment load across Southeast Asia, especially in areas which are either steep or underlain by erodable rock formation. Mapping of the slopes near the Mekong River from SPOT satellite images (Gupta et al. 2002) indicated vegetation clearance of structure-guided tributary valleys in Lao PDR leading to the pulsatory passage of sediment as described in Table 14.2 to the large river, and eventually after a long time period to the coastal waters. Such transfer to the coastal waters is much faster when small, steep rivers flow directly down to the coast through partially or largely cleared basins (Gupta and Krishnan 1994). Koopmans (1972) described the early deposition of coarse bed load in rivers flowing out of the granitic Main Range of West Malaysia. Their extensive floodplains

Fig. 14.3. Volcanic-materialchoked river channel draining the slopes of the Merapi volcano, Java

Erosion and Sedimentation 245

were built primarily by deposition of finer suspended load. Destruction of vegetation in their basins will lead to additional supply of fine suspended material to the streams, increasing the local flood potential. Furthermore, small rivers draining coastal ranges will contribute this sediment to the coastal waters. Such sediment plumes are visible in satellite imagery across Southeast Asia (Gupta and Krishnan 1994). The huge increase in sediment load is reflected in bar formation, raising of channel bed, increased flooding, accelerated floodplain build-up, and changes in channel pattern. For large rivers, Douglas et al. (1993) indicated that unlike the case of small rivers, sediment transport following deforestation may have only local manifestation, except during large floods. This episodic storage and transfer of sediment probably happens in many large rivers of Southeast Asia, although it has not been properly documented. In the Mekong, fresh yellow sand is seen mostly as insets, around rock protrusions in the channel, and forming river mouth fans where tributaries join the main river. Such accumulations are visible both in the field and on 20 m resolution SPOT images (Gupta et al. 2002). A very large volume of sediment is making its way to the coastal waters, although a large part of it may remain stored in the alluvial lowlands of Cambodia and the delta in Viet Nam. An exceptional case has been made for the Citunduy River in Java (Stevens 1994). The Citunduy aggrades owing to both anthropogenic changes of the land use in its catchment area and volcanic eruptions in its upper basin. In spite of both, the river remains stable because of its tough clay bed and banks. The influx of sand is transported above the clay bed.

The Coastal Waters Considerable amount of sediment currently reaches the coastal waters of Southeast Asia. Various fieldbased case studies have reported this (Koopmans 1972; Hadisumarno 1979; Aitken 1981). Sediment plumes are clearly visible on satellite images, even on AVHRR with 1.1 km resolution (Gupta and Krishnan 1994). The AVHRR image used was a mosaic of thirty-eight afternoon passes between 12 November 1990 and 3 March 1991. In the original study, the area covered was from 81° to 120°E and from 29°N to 10°S. A part of it is shown in Figure 14.4. The area of the sediment plumes can be used as an indicator of the amount of sediment reaching the coastal waters. Plumes have also been mapped for Bangkok Bay and off the Mekong Delta (Tanchotikul 1987), and recently off the east coast of Sumatra.

Figure 14.4 is a map of coastal waters where the sediment concentration at the surface or near surface is high enough to produce a bright reflectance. The plumes do not continue indefinitely. The well-known example of it in Southeast Asia is the increase of sediment production near Bengkulu (Sumatra) that lasted only for a number of years (Collins, Sayer, and Whitmore 1991; J. R. Giardino, personal communication). It is likely that Figure 14.4 indicates the situation at the time the images were taken. The large plumes are expected to continue, but the occurrence of smaller ones could change with time. Similarly, one has to consider the lag effect in a long stream between deforestation upstream and plume-building at its mouth. The general picture of a large volume of sediment reaching the sea in this part of the world, however, is valid. Large plumes are expected and are present off the deltas of the major rivers such as the Irrawaddy, Salween, Mekong, and Mahakam. The Chao Phraya is an exception, but its drainage basin is one of less relief, and it is likely that the dams upstream (see Chapter 4) block a considerable amount of sediment. Large plumes merging and stretching for hundreds of kilometres along the coast occur in a number of places: the Tenasserim coast, east of Phuket, between Penang and the mainland, near Melaka and south Johor, near Surat Thani, along the Viet Nam coast near Hué, the east coast of Sumatra, the north coast of Java (near Jakarta, Cirebon, and Tanjong Bugel), between Java and Madura, and around the island of Borneo except the extreme northwest. The position of the small plumes, of course, may have changed subsequent to the preparation of the map. Two conclusions can be reached beyond the obvious one of a large amount of sediment being brought to the coastal waters by regional rivers. First, a number of plumes are difficult to explain strictly according to their natural environment. The most striking example is probably the 800 km plume along the Sarawak coast. This is a summation of sediment plumes individually contributed by a number of rivers, 100–200 km long, draining out of a mountain range parallel to the coast. It seems likely that so much sediment can only be generated following deforestation. Secondly, as Figure 14.5 shows, a number of small river basins tend to plot strikingly above or below the regional average. Those that plot high (Cilutung, Cimanuk, etc.) are small basins in the hills of Indonesia and the Philippines which have been considerably cleared of their natural vegetation cover. On the other hand, the cluster plotting low (Labuk, Padas, etc.) are Malaysian basins which were still mostly under forest in 1990 when the diagram was constructed. Their sediment yield will rise rapidly with

246 Avijit Gupta

Fig. 14.4. Distinct sediment plumes in the coastal waters of Southeast Asia (Source: Gupta 1996)

deforestation. The Mekong incidentally plots very close to the regional trend. Its basin still has a considerable vegetation cover on the upper slopes. Not much work has been carried out on the impact of this extra sediment resulting from anthropogenic

alteration of the natural environment on the coasts and coastal waters of Southeast Asia. This may be expected to increase the dimension of deltas and beaches but on the other hand mangroves (Figure 14.6) and coral reefs may be degraded. The last two chapters in this

Erosion and Sedimentation 247 Cilutung 104

Angat

Cimanuk Agno

Cimuntur

Citanduy

Porong

Cijolang

Damodar

Kali Brantas

YIELD (t km–2 yr–1)

103

Pamanga

Sugut

Solo

Hungho

Kinabatangan

Godavari Mahanadi

Segama

GangaBrahmaputra Irrawaddy

Y = 9458X–.266 Labuk

Krishna

Padas

Mekong Kelantan 102

101 2 10

Trengganu

103

Perak

Chao Phraya

104

105

106

2

AREA (km ) Fig. 14.5. Deviation of sediment yield from the regional expectation, South and Southeast Asia (Sources: data from Milliman and Meade 1983; Hoekstra 1989; Milliman and Syvitski 1992; and Murtedza and Ti 1993; from Gupta and Krishnan 1994)

Fig. 14.6. Mangroves affected by excessive sandy sediment

248 Avijit Gupta

book briefly discuss this possibility. Pollutants riding piggyback on sediment grains are another possibility. A global-warming-forced rise in sea level may further accelerate such effects.

References Aitken, A. P. (1981), ‘Aspects of Erosion and Sediment Transport in Java, Indonesia’, in T. Tingsanchali and H. Eggers (eds.), Southeast Asian Regional Symposium on Problems of Soil Erosion and Sedimentation Proceedings (Bangkok: Asian Institute of Technology), 81–91. Anderson, J. M., and Spencer, T. (1991), Carbon, Nutrient and Water Balances of Tropical Rain Forest Ecosystems Subject to Disturbance: Management Implications and Research Proposals, MAB Digest 7 (Paris: UNESCO). Balamurugan, G. (1991), ‘Sediment Balance and Delivery in a Tropical Urban River Basin: The Klang River, Malaysia’, Catena, 18: 271–87. Chatterjea, K. (1989), ‘Observations on the Fluvial and Slope Processes in Singapore and their Impact on the Urban Environment’, Ph.D. thesis, National University of Singapore. Chen, P., Lim, H., Huang, X., Gupta, A., and Liew, S. C. (2000), ‘Environmental Study of the Middle Mekong Basin Using Multispectral SPOT Imagery’, 2000 International Geoscience and Remote Sensing Symposium (Honolulu) Proceedings (Piscataway, NJ: IEEE Publications), 3237–9. Collins, N. M., Sayer, J. A., and Whitmore, T. C. (1991), The Conservation Atlas of Tropical Forests, Asia and the Pacific (London: Macmillan). Douglas, I. (1978), ‘The Impact of Urbanization on Fluvial Geomorphology in the Humid Tropics’, Geo-Eco-Trop, 2: 228–42. —— Spencer, T., Greer, T., Bidin, K., Sinun, W., and Wong, W. M. (1992), ‘The Impact of Selective Commercial Logging on Stream Hydrology, Chemistry and Sediment Loads in the Ulu Segama Rain Forest, Sabah, Malaysia’, Royal Society of London Philosophical Transactions, B335: 397–406. —— Greer, T., Bidin, K., and Spilsbury, M. (1993), ‘Impacts of Rainforest Logging on River Systems and Communities in Malaysia and Kalimantan’, Global Ecology and Biogeography Letters, 3: 245–52. Fagi, A. M., and Mackie, C. (1988), ‘Watershed Management in Java’s Uplands: Past Experience and Future Directions’, in W. C. Moldenhauer and N. W. Hudson (eds.), Conservation Farming on Steep Lands (Ankeny, Ia.: Soil and Water Conservation Society), 254–64. Greer, T., Bidin, K., and Douglas, I. (1994), ‘Tropical Rain Forest Disturbance and Suspended Sediment Discharging Variation’, in L. J. Olive and J. A. Kesby (eds.), Variability in Stream Erosion and Sediment Transport: Poster Contributions (Canberra: Australian Defence Force Academy), 34–8. —— Sinun, W., Douglas, I., and Bidin, K. (1996), ‘Long Term Natural Forest Management and Land-Use Changes in a Developing Tropical Catchment, Sabah, Malaysia’, in D. E. Walling and B. W. Webb (eds.), Erosion and Sediment Yield: Global and Regional Perspectives, IAHS Publication 236 (Wallingford), 453–61. Gupta, A. (1992), ‘Floods and Sediment Production in Singapore’, in A. Gupta and J. Pitts (eds.), Physical Adjustments in a Changing Landscape: The Singapore Story (Singapore: Singapore University Press), 301–26. —— (1996), ‘Erosion and Sediment Yield in Southeast Asia: A Regional Perspective’, in D. E. Walling and B. W. Webb (eds.), Erosion and Sediment Yield: Global and Regional Perspectives, IAHS Publication 236 (Wallingford), 215–22.

—— and Chen, P. (2002), ‘Sediment Movement on Steep Slopes to the Mekong River: An Application of Remote Sensing’, in F. J. Dyer, M. C. Thoms, and J. M. Olley (eds.), The Structure, Function and Management Implications of Fluvial Sedimentary Systems, IAHS Publication 276 (Wallingford), 399– 406. —— and Krishnan, P. (1994), ‘Spatial Distribution of Sediment Discharge to the Coastal Waters of South and Southeast Asia’, in L. J. Olive, R. J. Loughran, and J. A. Kesby (eds.), Variability in Stream Erosion and Sediment Transport, IAHS Publication 224 (Wallingford), 457– 63. —— and Pitts, J. (1992), Physical Adjustments in a Changing Landscape: The Singapore Story (Singapore: Singapore University Press). —— Lim, H., Huang, X., and Chen, P. (2002), ‘Evaluation of Part of the Mekong River Using Satellite Imagery’, Geomorphology, 44: 221–39. Hadisumarno, S. (1979), ‘Coastline Accretion in Segara Anakan, Central Java, Indonesia’, Indonesian Journal of Geography, 9: 45–52. Hardjowitjitro, H. (1981), ‘Soil Erosion as a Result of Upland Traditional Cultivation in Java Island’, in T. Tingsanchali and H. Eggers (eds.), Southeast Asian Regional Symposium on Problems of Soil Erosion and Sedimentation Proceedings (Bangkok: Asian Institute of Technology), 173– 9. Hatch, T. (1981), ‘Preliminary Results of Soil Erosion and Conservation Trials under Pepper (Piper nigrum) in Sarawak, Malaysia’, in R. P. C. Morgan (ed.), Soil Conservation: Problems and Prospects (Chichester: Wiley), 255– 62. Hayes, S. K., Montgomery, D. R., and Newhall, C. G. (2002), ‘Fluvial Sediment Transport and Deposition Following the 1991 Eruption of Mount Pinatubo’, Geomorphology, 45: 211–24. Koopmans, B. N. (1972), ‘Sedimentation in the Kelantan Delta (Malaysia)’, Sedimentary Geology, 7: 65– 84. Lai, F. S. (1993), ‘Sediment Yield from Logged, Steep Upland Catchments in Peninsular Malaysia’, in J. Gladwell (ed.), Hydrology of Warm Humid Regions, IAHS Publication 216 (Wallingford), 219–29. —— Ahmad, J. S., and Zaki, A. M. (1996), ‘Sediment Yields from Selected Catchments in Peninsular Malaysia’, in D. E. Walling and B. W. Webb (eds.), Erosion and Sediment Yield: Global and Regional Perspectives, IAHS Publication 236 (Wallingford), 223–31. Lal, R. (1987), Tropical Ecology and Physical Edaphology (Chichester: Wiley). Lavigne, F., and Thouret, J. C. (1995), ‘Rain Triggered Lahars in Boyong River after the 22 November 1994 Eruption of Merapi Volcano (Central Java, Indonesia)’, Abstract of paper presented to the International Association of Geomorphologists Southeast Asia Conference, Singapore, June 1995. Law, K. F., Cheong, C. W., Ong, T. S., Mustafa Kamal, B., Tengku Bakry Shah, T. J., Md. Nizum, M. N., Abdul Rahim Nik, Zulfikli, Y., Low, K. S., and Lai, F. S. (1989), Sungai Tekam Experimental Basin Final Report, July 1977 to June 1986, Water Resources Publication 20 (Kuala Lumpur: Bahagian Pengairan dan Saliran Kemetarian Pertanian, Malaysia). Leigh, C. H. (1982), ‘Urban Development and Soil Erosion in Kuala Lumpur’, Journal Environmental Management, 15: 35– 45. —— and Low, K. S. (1973), ‘An Appraisal of the Flood Situation in West Malaysia’, in Symposium on Biological Resources and National Development, Proceedings (Kuala Lumpur: Malayan Nature Society), 57–72. Malmer, A. (1990), ‘Stream Suspended Sediment Load after Clear-Felling and Different Forestry Treatments in Tropical Rainforest, Sabah, Malaysia’, in R. R. Zeimer, C. L. O’Loughlin, and L. S. Hamilton (eds.), Research Needs and Applications to Reduce Erosion and Sedimentation in Tropical Steeplands, IAHS Publication 192 (Wallingford), 62–71.

Erosion and Sedimentation 249 Mekong River Commission (various dates), Lower Mekong Hydrological Yearbook (Bangkok and Phnom Penh: Mekong River Commission). Milliman, J. D., and Meade, R. H. (1983), ‘World-Wide Delivery of River Sediment to Oceans’, Journal of Geology, 91: 1–21. Milliman, J. D., and Syvitski, J. P. M. (1992), ‘Geomorphic/Tectonic Control of Sediment Discharge to the Oceans: The Importance of Small Mountain Rivers’, Journal of Geology, 100: 525–44. Murtedza, M., and Ti, T. C. (1993), ‘Managing ASEAN’s Forests: Deforestation in Sabah’, in M. Seda (ed.), Environmental Management in ASEAN (Singapore: Institute of Southeastern Asian Studies), 111–40. Peh, C. H. (1981), ‘The Suspended and Dissolved Sediment Load of Three Small Forested Drainage Basins in Peninsular Malaysia’, Malayan Forester, 44: 438–52. Poon, P. H. J. (1984), ‘Sediment Production in the Kent Ridge Area’, academic exercise, National University of Singapore. Repetto, R. (1988), The Forest for the Trees? Government Policies and the Misuse of Forest Resources (Washington, D.C.: World Resources Institute). Ruangpanit, N. (1985), ‘Percent Crown Cover Related to Water and Soil Losses in Thailand’, in S. A. El-Swaify, W. C. Moldenhauer, and A. Lo (eds.), Soil Erosion and Conservation (Ankeny, Ia.: Soil Conservation Society of America), 462–71. Shallow, P. G. (1956), River Flow in the Cameron Highlands, HydroElectricity Technical Memoir 3 (Kuala Lumpur), unpub. Sinun, W., Wong, W. M., and Spencer, T. (1992), ‘Throughfall, Stemflow, Overland Flow and Throughflow in the Ulu Segama Rain Forest, Sabah’, Royal Society of London Philosophical Transactions, B335: 389–95.

Stevens, M. A. (1994), ‘The Citunduy, Indonesia—One Tough River’, in S. A. Schumm and B. R. Winkley (eds.), The Variability of Large Alluvial Rivers (New York: ASCE Press), 201–19. Takanashi, K. (1981), ‘Basic Concepts for Debris Control Work in Mt. Kelud, East Java, Indonesia’, in T. Tingsanchali and H. Eggers (eds.), Southeast Asian Regional Symposium on Problems of Soil Erosion and Sedimentation Proceedings (Bangkok: Asian Institute of Technology), 477– 89. Tanchotikul, A. (1987), ‘Investigation of the Physical Condition of the Sea Using NOAA-Satellite AVHRR Data in the Gulf of Thailand’, in F. W. Wezel and J. L. Rau (eds.), Progress in Quaternary Geology of East and Southeast Asia (Bangkok: CCOP Technical Secretariat), 219–26. Voskuil, R., and Zuidam, R. van (1982), ‘Examples of Geomorphologic and Applied Geomorphologic Mapping in Central Java’, ITC Journal (1982–3), 290–302. Weera, P., and Kittipong, P. (1987), ‘Amount of Runoff and Soil Losses from Various Land-Use Sampling Plots in Sakolnakorn Province, Thailand’, in R. H. Swanson, P. Y. Bernier, and P. D. Woodward (eds.), Forest Hydrology and Watershed Management, IAHS Publication 167 (Wallingford), 231–8. White, S. (1995), ‘Soil Erosion and Sediment Yield in the Philippines’, in I. D. L. Foster, A. M. Gurnell, and B. W. Webb (eds.), Sediment and Water Quality in River Catchments (Chichester: Wiley), 391– 406. Wiersum, K. F. (1984), ‘Surface Erosion under Various Tropical Agro Forestry Systems’, in C. L. O’Loughlin and A. J. Pearce (eds.), Symposium on Effects of Forest Land Use on Erosion and Slope Stability, Proceedings (Vienna: IUFRO; Honolulu: East–West Center), 231–9.

15

Volcanic Hazards in Southeast Asia Jan J. Nossin

Active Volcanic Zones in Southeast Asia Active volcanism in Southeast Asia is associated with marked zones of activity in the Earth’s crust that run through south and east Indonesia and the Philippines (Figures 15.1 and 15.2). These zones are also characterized by frequent earthquakes and a measurable movement of tectonic plates, often in the order of 5 cm yr −1. The underlying mechanism is that of subduction of oceanic plates below continental plates; the rigidity of the moving plates causes ruptures and shockwise adjustments (earthquakes). The oceanic plate, while being under thrust, sinks down to great depths below the continental plate and in the process loses its rigidity owing to heating and part assimilation into the underlying magma. Earthquakes are caused in the zone where the subducted plate is still rigid (Figure 15.3). Chapter 1 in this book puts this phenomenon in the regional context. Volcanism in this zone is marked by frequent eruptions, mostly violent and of an explosive nature. It is manifest in distinct belts that comprise all (or nearly all) of the Philippines, and large parts of Indonesia with the exception of, roughly speaking, Kalimantan and Papua. The violence of the eruptions poses threats to human settlements in the surroundings of the volcanoes, to the cultivated lands, and the infrastructure. These threats may occur during and after the actual eruption, and they may indirectly cause other hazards as well. Moreover, volcanoes in apparent dormancy The author gratefully acknowledges assistance in cartographic matters from Dr Koert Sijmons, and comments on the manuscript by Dr Robert Voskuil, both of the Division of Applied Geomorphology, ITC.

that have not erupted in historical times may still come to life as the interval between eruptions may be very long. In the present chapter these hazards will be discussed.

Natural Hazards and Risks Natural hazards have been defined in four ways, of which the 1982 definition of the United Nations Disaster Relief Co-ordinator (UNDRO) seems appropriate to follow in the context of volcanic hazards (Alexander 1993). UNDRO defines natural hazards as ‘the probability of occurrence within a specific period of time and within a given area of a potentially damaging phenomenon’. A hazard therefore may represent a situation with the possibility of a disaster that may affect the population and the environment which are in some degree of vulnerability. Vulnerability (V) is defined as ‘the degree of loss to a given element or set of elements at risk resulting from the occurrence of a natural phenomenon of a given magnitude. It is expressed on a scale from 0 (no damage) to 1 (total loss)’ (UNDRO 1982). A threat of disaster arises when the risk becomes tangible and impending. The concept of risk is considered by UNDRO in the light of three components: • Elements at risk (E): population, properties, infrastructure, economic activities, etc. • Specific risk (Rs): the degree of losses that may be expected as a result of a particular natural phenomenon. Specific risk is considered as the product of the natural hazard (H) and the vulnerability (V). Rs = H × V.

Fig. 15.1. Volcanoes of Indonesia Volcanoes indicated by name are mentioned in text. (Source: Verstappen 2000)

252 Jan J. Nossin

Fig. 15.2. Volcanoes of the Philippines (Source: Phivolcs n.d.)

Volcanic Hazards in Southeast Asia 253

Fig. 15.3. Scheme to illustrate subduction (Source: Selby 1985)

254 Jan J. Nossin

• Total risk (Rt) comprises the number of lives likely to be lost, injuries, damage to properties, and disruption of activities as a result of the particular phenomenon. It is considered the product of the specific risk Rs and the elements at risk E. Rt = E × Rs = E × (H × V). The duration of the phenomenon plays a role in the development of the hazard. This chapter deals with volcanic hazards, hazards that arise out of volcanic eruptions owing to both the eruption itself and situations arising as a result of the eruption. Eruptions may have durations varying from hours to years, and show various phases of intensity and character.

Volcanic Hazards Volcanic eruptions are usually violent affairs which pose serious hazards and risks to the population in the area affected. Depending on the nature of the magma, eruptions can be in the form of a rather quiet outflow of lava (effusive eruptions) or in the form of large explosions caused by release of gases from the magma. The former occurs with more mafic magmas which have a higher fluidity and allow a continuous escape of gases. The latter occur when the lava is of a more silicic composition and a higher viscosity, which may cause the trapping and accumulation of gases. The explosive escape of gas is accompanied by fragmented masses of incandescent lava, which descend upon the volcano and its surroundings as pyroclasts. Composite volcanoes show an alternation of lavas and pyroclasts in the cone’s profile. Volcanicity in Southeast Asia is mostly of the second (explosive) type; lavas are often of andesitic–dacitic composition. The large number of volcanic landforms in this part of Southeast Asia indicates its present and recent past volcanic activities. But the impressive deposits of volcanic sediments (tuff, pyroclastic flow deposits, lahar deposits, fluvio-volcanics) point to a much greater activity in geologically young past periods, which embrace the Pleistocene and the Upper Tertiary. There is also plenty of evidence for older volcanism, but in the context of current volcanic hazards we can leave that out of consideration. The hazards posed by volcanic eruptions can be summarized in two broad categories: syneruptive and post-eruption hazards.

Syneruptive Hazards Magma is normally under enormous pressure which keeps the gassy components dissolved in the hyperviscous molten rock. When pressure is lessened (as in the precursor to an eruption), the gas separates into ‘bubbles’,

which reduce the density of the magma and allow it to ascend further and thus eventually propel itself to the surface, where pressure is lowest. The enclosed gas keeps fluid lava mobile longer after extrusion, and from fluid lava the gas may escape relatively quietly. In viscous lavas, the release of gas occurs with explosions. Explosions also mark the extrusion of the lava, often with great violence. These explosions propel clouds of gas-laden fragments of lava (and sometimes also of the pre-existing volcanic body) out of the crater and into the atmosphere if pressure is high enough and the column hot enough to continue rising with its own heat and release of gas trapped in the ejected rock. If not, then the cloud rolls down the slopes of the volcano. The general term for the ejected incandescent rock fragments is pyroclasts. Volcanic gas is a gaseous mixture including, among other gases, water vapour, carbon dioxide, sulphuric and nitrous oxides, and hydrogen sulphide. The origin of the gas is not fully clear, and often it is considered to originate out of the primary magmatic mass. Gases released from volcanic products are not limited to the actual eruption. In a post-eruption phase, fumaroles may continue to release lesser quantities of gas. These, however, pose their own risk in that they may be poisonous or asphyxiating. Eruption hazards and eruption phenomenology in general are discussed by Tazieff in Tazieff and Sabroux (1983). Syneruptive hazards are related to the type of eruption. Eruption types are described in many textbooks on volcanology (e.g. Bullard 1980; Francis 1993).

Lava Flows Lava outflow as a relatively quiet stream of molten rock is mostly associated with (basaltic) lava of high temperature. Sheets of basaltic lava may thus form, and they are found in several places in Southeast Asia, although the majority of present eruptions are not of this kind owing to the convergent plate margin situation. In principle, lava flows are a major hazard, as there is no way of stopping them and they destroy everything in the way. However, lava flows usually do not move so fast that humans cannot escape them. Many Southeast Asian volcanoes are of the composite type, in which layers of lava alternate with layers of pyroclasts. Blocky lava (cooler or more silicic, or both) may be extruded with or without explosions. In the latter case the erupted material will advance as a lava avalanche if the slopes are steep enough. Release of gases may cause ‘nuées ardentes d’avalanche’, pyroclastic flows caused by the lava avalanche. Mount Semeru ( Java) exhibits this phenomenon. This mountain has been in continuous eruption for over twenty years, exhibiting

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vulcanian explosions along with minor extrusions, and other forms of explosivity and ash emission. The movement of magma in the direction of the vent goes accompanied with earthquakes. These are normally low in intensity. Their occurrence may alert inhabitants and authorities to an impending eruption. The pattern of the earthquakes, and the location of their foci, as they are registered by seismographs, are used to monitor the magmatic movement, as a precursor signal for predicting when the eruption may occur.

Base Surges Plinian eruptions are the most paroxysmal type of explosive eruptions, in which a column of incandescent ejectamenta and hot gases is sent up (up to tens of kilometres) into the atmosphere over the volcano. The outrush of hot gas (laden with glowing fragments of lava and of the volcanic body) can sustain this column, but when the gas pressure from below slackens, the support weakens and the column will sink or collapse under its own weight. This results in the hot gas cloud with its load of glowing eruption products sinking back onto the volcano, causing a surge of hot (800–1000°C) pyroclastics. These base surges rush down the slope of the volcano at hurricane speed and are absolutely deadly. Whatever is in their way is blown down and burnt swiftly. During short and powerful explosions, base surges are also reported to emanate from the vent directly down the slope surrounding the tephra column, which is shot upward. Base surges were observed during the 1991 eruption of Mount Pinatubo (Philippines).

Pyroclastic Flows When the gas pressure from the vent is directed sideways, or is not sufficient to sustain a high column, the pyroclastic material will rush down the slopes directly from the crater causing similar devastation. Only its speed my be somewhat lower, in the order of 60 km h−1. This is the pyroclastic flow. Large volumes of material are thus deposited; their emplacement temperature may be typically in the order of 800°C. It may take years for these pyroclastic flow deposits to cool down. As long as they remain hot, the danger of secondary phreatic explosions exists. There is not much difference, if any, between deposits from base surges and from pyroclastic flows. In the Peléean type of eruption, the magma is so viscous that a plug of magma is formed in the conduit. It is slowly pushed up through the crater by the pressure of the volcanic gas trapped below to form a volcanic crater dome. The gas that has accumulated beneath the plug, which is responsible for the upward movement, sometimes finds cracks in the plug or in the volcano

body, and part of this gas escapes. It streaks, laden with incandescent tephra, down the slope of the volcano, as a blast or a pyroclastic flow. This type was first observed at Mount Pelée on the Caribbean island of Martinique and described as a nuée ardente (glowing cloud). A variant of this type is shown by the Merapi volcano on central Java, where a plug also forms in the conduit, but the pyroclastic flows are also caused by the collapse of parts of the dome. This volcano has been studied by several investigators; Zen (1983) gives an account of Merapi eruptions. Verstappen (1988) studied lahar hazards on the southwest slope. The southwest slope is where most pyroclastic flows and associated lahars come down; this sector does not directly threaten the nearby city of Yogyakarta. Lavigne (1998) discussed the recent lahars on the south slope and their originating factors. Yogyakarta lies directly in their path. More details on the Merapi volcano and its threats are discussed by Thouret and Lavigne in the following chapter.

Pyroclastic Surges Pyroclastic surges are considered a variety of a pyroclastic flow, but of lower density and high velocity, thus producing turbulent flow which is not so directed by the topography as are pyroclastic flows. Because of their lesser density, they travel shorter distances (Francis 1993).

Ash Fall and Airfalls Explosive activity of a volcano hurls pyroclastic materials, often of considerable size and always glowing hot, through the air to land around the crater— and often quite far away. Volcanic ash is in the sand range of grain sizes: its name is misleading. Ordinary ash is usually light, but volcanic ash is just sand and has the same weight. Roofs of houses and buildings collapse under the weight of accumulating ash, especially when wet. It is a major cause of casualties with volcanic eruptions (Figure 15.4). The thickest ash layers fall close to the volcano and cause casualties. The distance from the volcano in which ash fall occurs is often considerable and influenced by the prevailing wind direction. Ash from Mount Pinatubo fell as far as Viet Nam, and ash from the Tambora eruption of 1815 fell as far as south Sumatra and Bangka (Petroeschevsky 1949).

Syneruptive Lahars Lahars are volcanic mudflows triggered off by intensive rainfall mobilizing eruption products deposited around the crater; they will be treated in more detail in the following section on post-eruption lahars. Where a crater lake is present, syneruptive lahars occur as the eruption suddenly empties the crater lake. The water mixes with the solids it picks up on the way downslope to form a

256 Jan J. Nossin

Fig. 15.4. Collapsed roof of San Marcelino market, 25 km southwest of Mount Pinatubo, February 1992

lahar. These lahars are particularly violent, as at Mount Kelud (east Java). Mount Kelud is a highly active volcano with a long record of eruptions at intervals between three and thirty-seven years (Neumann van Padang 1950). The 1919 eruption of this volcano lasted less than an hour. Thirty-eight million m3 of water were ejected from the crater lake, which mixed with eruption products and loose debris existing in the ravines, formed deadly hot lahars with speeds of about 60 km h−1. More than 5100 persons were killed in less than forty-five minutes and 104 villages were destroyed either fully or partly. Measures to alleviate the danger had been taken even before this eruption, but to no avail. A first attempt to drain off the crater lake by a tunnel shortly after the eruption was foiled by a lava plug rising up just in front of the projected outlet in the crater. Eventually a tunnel with a syphon system was constructed in 1923 whereby the content of the crater lake was reduced to 5 per cent of the original volume. The Kelud catastrophe of 1919 was the direct cause of the establishment of the forerunner of the Volcanological Survey of Indonesia in 1920. Mount Pinatubo on Luzon (Philippines) erupted catastrophically in 1991; this eruption is discussed in a following section. As a result of this eruption a large caldera was formed, which filled rapidly with (monsoon and typhoon) rainwater as there are no natural outlets or drainage possibilities. Thus the hazard of syneruptive lahars arose, and currently (2002) plans are under execution to drain off this crater lake.

Heavy rain at the time of the eruption (typhoons, monsoon rains) may mobilize pyroclastic deposits and ash while the eruption is still in progress, and also cause syneruptive lahars.

Tsunamis Tsunamis are long-period sea waves caused by submarine events of considerable violence that displace the overlying and surrounding water. Earthquakes are the most-cited cause, but volcanic eruptions, including caldera collapse, may also generate tsunamis. This includes submarine eruptions, eruptions on volcanic islands, or near-coast eruptions. Tsunamis as a consequence of volcanic events pose a hazard associated with nearshore and island volcanoes. The wavelengths are in the order of 150–250 km, with a maximum up to 1000 km, the amplitudes in the order of a few metres, and the frequency in the order of tens of minutes. This makes them difficult to detect in the open sea, and only when they start breaking near the shore do they transform into a destructive wall of water. A precise account of the Krakatau eruption has been compiled from countless sources by Simkin and Fiske (1983), which also includes an account of the violently destructive tsunamis of the cataclysmic explosions of Krakatau of 26 and 27 August 1883. The eruption started in the month of May that year, preceded by earthquakes in 1880 and 1882. Several heavy explosions were followed by lesser tsunamis in the intervening months. The big tsunami—which came in at least two waves, the second one being the worst— completely

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destroyed the coastal towns Anjer and Merak on the Java shore of the Sunda Strait, Telok Betong on the Sumatra coast, and numerous other towns and smaller kampongs on both shores. The behaviour of the wave when hitting the shore was influenced strongly by the local coastal configuration, the water being reported to have reached places 20 m above sea level, locally higher (over 30 m at Telok Betong, about 40 at Merak). There is no defence against such a wall of water; everything in its path was completely destroyed. People who did not make it to high ground in time had no chance of survival. The number of victims of the Krakatau eruption is given as 36 417 (Kusumadinata 1979), most of them killed by the tsunami. The wave travelled outward and was noticed after the corresponding time interval and with diminishing amplitude in places such as Benkoelen (Benkulu), Batavia ( Jakarta), Bombay, Port Elizabeth (South Africa), and Le Havre (France).

Post-Eruption Hazards Lahars Lahars, as stipulated above, are volcanic mudflows usually triggered off by intensive rainfall mobilizing pyroclasts deposited in a wide zone around the crater. The word is of Javanese origin, first applied to mudflows down the Merapi volcano in central Java, and now widely accepted for this phenomenon. In principle, every volcanic eruption producing pyroclastic flows carries with it the associated risk of lahars. Lahars may be hot, when derived from freshly emplaced pyroclastic deposits, or cold, when remobilization of cooled pyroclastic deposits occurs. The type locality for lahars is Mount Merapi, one of the most active and dangerous volcanoes on Earth. The volcano is 2911 m high with steep upper and middle slopes. The radial drainage system is incised, especially in the midslope area of the volcano. The lahar channels are thus steep, and the material supplied from the source area moves down swiftly—increasing the danger when there is overspill from the incised valleys onto the adjoining arable land. Source material for the lahars is continually supplied by the lava dome of the volcano, where, at irregular but frequent intervals, collapse of parts of the dome occurs, sending pyroclastic flows down the slope. Other forms of magmatic and gas extrusion may also give rise to pyroclastic flows. The lava dome supplies relatively small amounts of source material, but at frequent intervals. Many pyroclastic flows and incandescent rockfalls reach about 2.5 km from the summit, the accumulated products also forming the source for lahars. Lahars are usually associated with the tropical rainfall which may

reach high intensity throughout the year, especially during the rainy season between November and April in Java. A warning system is activated when rainfall intensity in the source area reaches critical values. Many Hindu temples surrounding Merapi had been buried under successive lahar and tephra deposits. In recent times, several of these have been unearthed and are currently being restored. Lavigne (1998) has studied extensively the lahar occurrences at the volcano, and Lavigne et al. (2000a,b) and Thouret et al. (2000) have also reported on them. Lahars are possible in all situations where heavy and intensive rainfall affects recent pyroclastic or ash deposits. Mount Semeru (east Java) has been continually active since 1967 and shows rainfall-related mudflows, like the one in May 1981 that killed over 250 people. The first historically reported eruption took place in 1818, but that does not mean that there was no activity before that date. At 3676 m Mount Semeru is the highest volcano in Java. Mount Pinatubo is located in western Luzon, the Philippines. The mountain, 1745 m high before the eruption, was not considered an active volcano when it began showing signs of unrest in April 1991, leading to one of the world’s largest explosive eruptions of the twentieth century, in June 1991. After the eruption, the caldera rim stands at 1486 m asl. The eruption of between 3.7 and 5.3 km3 of magma deposited an estimated 8.4 to 10.4 km3 of tephra, most of it loose, sand-sized fragments of andesite – dacite composition. This quickly filled the existing valleys and overtopped the divides in the upper catchments, which created new and different watersheds. This loose deposit is easily mobilized after each rainfall exceeding an umbral value of intensity and duration, and then coming down the valleys as lahars. Lahars may be syneruptive but most are post-eruption, continuing for years after the eruption itself until the source material is depleted. The highdensity mass can carry blocks of large volume, which may include parts of bridges, embankments, buildings, vehicles, etc. The speed of lahars depends on the gradient of the valley. On steep upper slopes, lahars may attain speeds in the order of 10 m s−1. In wide open lowland valleys further downstream, this becomes much less. Phivolcs quotes figures in the order of 10–3.5 km h−1 (Punongbayan et al. 1992). Their power is so great that they may wreak havoc even at long distances from the volcano, sweeping away structures which themselves may ram into other objects. The mass of displaced material is so voluminous that it can be contained only by the strongest of dykes. In the first years after the 1991 eruption, most efforts to contain the lahars by

Fig. 15.5. Volcanic deposits around Mount Pinatubo (Source: after Daag and van Westen 1996)

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Fig. 15.6. Pasig-Potrero, effect of lahar after upstream breach of dyke

dykes in the foreland were in vain; most interventions were of an emergency character and had only one dry season for instalment. Initial efforts consisted of using the lahar deposits from the bed, and pushing them up to form dykes. The loose lahar material was often swept away by the successive lahars of the next rainy season. As time elapsed, stronger dykes with concrete linings were emplaced. These may retain the sediment mass, but vigilance for overtopping remains necessary. In places like the Pasig-Potrero lahar bed southeast of Mount Pinatubo (Figures 15.5 and 15.6), the lahar deposits stand locally some 30 m above the surrounding land. Not only does this pose a new hazard in the case of a local rupture or overtopping, but a graver danger lies in rupture or overtopping of the dyke further upstream. Many settlements and buildings in the endangered valley have been relocated behind new dykes, and in some cases have been engulfed by lahar flows that had cut the dyke upstream of their location. Similar situations are found in the other valleys emanating from the Pinatubo edifice, as in the Santo Tomas Valley (Figure 15.7). The Pinatubo lahars have been extensively studied and monitored. Daag (1994) and Daag and van Westen (1996) studied the erosion of the pyroclastic flow source material for the lahars; Daag is currently examining it in detail. The Pinatubo lahars are destructive but offer an open-air life-size laboratory. Figure 15.5 shows the extent of lahars and other volcanic deposits around Pinatubo. The hazard of lahars will continue to exist for years after the eruption, depending on the amount of pyro-

clastic flow deposits and ash, the local relief, and the hydrology. With time, their property changes from that of a debris flow (≥ 80 per cent solids) to hyperconcentrated streamflow (about 40– 80 per cent solids). In Figure 15.7 the changes wrought to the Santo Tomas plain by the Pinatubo lahars can be followed through time. Note that in the 1995 image, the influence of the dykes is evident, as they now effectively contain the lahar deposits. But for the dykes, the whole Santo Tomas plain would be covered with lahar deposits. This also sheds light on the origin of this plain, which is mainly due to infilling with lahar deposits and fluvio-volcanics of an embayment of tectonic origin. Javelosa (1994) introduced a system of risk assessment for the Pinatubo area based on a risk perception in three domains: a geomorphic hazard domain, a geo-resources domain, and a vulnerability domain (Figure 15.8), which applies a numeric score to each domain without actually attaching a monetary value.

Hazards Associated with Lahars The geomorphology of a lahar-affected area changes with the deposition of so much primary material and new deposits. The resulting secondary hazards associated with lahars are discussed below.

Formation of Impounded Lakes Lahar deposits may block the outlet of tributary valleys, impounding the drainage to form lakes. These lakes may be temporary, but pose a danger. When they overtop the natural dam of the lahar material, the outrushing

(a)

(b)

Fig. 15.7. Santo Tomas plain, western Zambales (a) Before the Pinatubo eruption, April 1988 (b) Shortly after the Pinatubo eruption, December 1991 (c) April 1995 Images are Band 2 SPOT XS. (Copyright CNES-SPOT Image)

(c)

Fig. 15.8. Risk zonation around Mount Pinatubo (Source: Javelosa 1994)

262 Jan J. Nossin

Fig. 15.9. Mapanuepe Lake, formed by blocking of the valley by Pinatubo lahars

water may erode away part or all of the remaining dam, creating a flash flood or secondary lahar that may affect areas where it is not expected. Emptying of such a lake occurred in the upstream Pasig catchment in September 1994, and the ensuing lahars engulfed the settlements on the east side of the channel, causing a large number of deaths and destruction of settlements (Atienza 1995). Impounded lakes are also present along the Bucao Valley to the northwest of Pinatubo. The biggest impounded lake is the Mapanuepe Lake to the southwest of Pinatubo, where lahars down the Marella Valley have blocked the outlet of the Mapanuepe River (Umbal and Rodolfo 1996). A sizeable lake has resulted (Figure 15.9), which has been studied in detail by Calomarde (1997). Because of the threat this large lake posed, an artificial spillway has been cut and maintained since 1992 (Janda et al. 1996) maintaining a maximum size and level for the lake’s expansion.

Changes in Upper Catchments Causing Changes in Lahars The emplacement of pyroclastic flow deposits, as they bury existing divides, may create a new drainage distribution system. In most cases, as the loose deposits erode away, the pre-existing drainage system may be expected to resume its function. Daag (1994) reported substantial changes due to phreatic explosions in the pyroclastic flow deposits of Pinatubo. In April 1992 a secondary explosion led to the capturing of the upper Abacan channel by the Sacobia River. This directed lahars into the Sacobia channel,

which previously had threatened the city of Angeles through the Acaban channel. The downstream Sacobia channel received, unexpectedly, large destructive lahars after the capture. Another large secondary explosion occurred in October 1993, cutting deep into the Pasig deposits and leading to a stream capture of the upper Sacobia channel by the Pasig River, adding about 20 km2 to the latter catchment. The major lahars from that area had been directed through the upper Sacobia Valley towards the northeast. After the secondary explosion, the lahars emanating from the upper catchment were redirected into the Pasig-Potrero channel, catching inhabitants of the downstream valley and authorities by surprise, causing numerous casualties and destroying villages and houses.

Raising the Lahar Bed above the Surrounding Land The piling up of lahar material between containing dykes may raise the stream bed considerably above the surrounding terrain. This is especially dangerous where the (incised) stream bed leaves the mountains and enters the lowland. Two threats exist: overtopping of the dykes as lahar deposits continue to accumulate; and dyke rupture or overtopping upstream of settlements behind the dykes. Such events may be triggered by the emptying of impounded lakes or by secondary explosions in the hot lahar deposits. The effect of a lahar flowing behind the dyke can be devastating, as was experienced on various occasions in the Pasig-Potrero Basin southeast of Pinatubo. A phreatic explosion caused

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a stream capture in the upper catchment, referred to above, followed by the emptying of an impounded lake, destroying entire villages (Figure 15.6). At the downstream end, the lahars as flood events continue to bring down material in every rainy season so long as the supply of material in the upstream areas continues (Figure 15.10). In some cases, residents refuse to leave and prefer to raise their houses after each new lahar inundation.

Other Hazards Phreatic Explosions Phreatic explosions are caused where surface water percolates and infiltrates into the hot pyroclastic flow deposits. At high temperature it is instantly converted into steam, which may be trapped until pressure gets high and a steam explosion results. These phreatic explosions can be quite powerful, sending columns of

(a)

Fig. 15.10. Bacolor village in Pasig-Potrero Valley, buried in lahar deposits (a) 1992 (b) 1994 (b)

264 Jan J. Nossin

Fig. 15.10. Continued (c) 1996 (c)

steam and tephra high (over 10 km) into the atmosphere. Steam explosions are not limited to pyroclastic flow deposits; they may occur in all situations where percolating water gets into contact with hot rocks including cooling lava.

Volcano-Tectonic Events: Disruption or Blocking of Channels The removal of large quantities of magma from the volcanic structure in major eruptions may give rise to tectonic movements usually causing earthquakes. The inward collapse of the volcanic superstructure creates calderas. Faulting as a result of magmatic displacement is another feature, well illustrated on the south flank of the Tangkuban Perahu–Sunda complex, west Java, Indonesia (Figure 15.11). This fault has two expressions; the eastern and highest cliff is far older than the western, younger scarp. The movement of the latter has been dated at around 11 000 years bp (Dam 1994; Nossin, Voskuil, and Dam 1996). The effect of the latter on the drainage system of the volcano has been dramatic. All drainage channels were interrupted by the scarp, and the area above it filled with a thick layer of sediments derived from the volcano and intercalated with peat as a result of the blocked drainage. Only recently have connections of the drainage system with the part below the fault started to be re-established across the fault, accompanied by the erosive clearing out of the alluvial deposits (Figure 15.11).

Emission of Poisonous Gas Volcanic activity commonly involves the emission of gas, even when only fumarole activity is taking place. An illustrative case is the Dieng volcanic complex in central Java, an eroded volcanic plateau about 6 by 14 km in size. Small cones and numerous fumaroles characterize the area. Emissions of C02, H2S, and other toxic gases, sometimes taking the shape of small explosions, or phreatic eruptions, account for numerous victims (nearly 200 in one single event in 1979). The plateau houses numerous temples and other monuments and is of great cultural value. It attracts numerous tourists for the rare combination of volcanic and cultural features. Gas emission from fumaroles may lead to toxic gas accumulation in valleys on the slope of the volcano. These ‘death valleys’ are known from a number of volcanoes, such as Tangkuban Perahu in west Java.

The Hazard of Apparent Quiescence or Extinction Periods of quiescence in a volcano’s lifespan harbour danger. Volcano lifetimes are far longer than human memory, and many volcanic complexes have an activity timespan in the order of 10 million years (Simkin and Fiske 1983)—much longer than the human presence on Earth. Long periods of apparent quiescence may mask the slow build-up to an eruption; the longer the repose period, the more violent the eruption is likely to be.

Fig. 15.11. Tangkuban Perahu volcano in the Sunda caldera, with the nearby city of Bandung, and Lembang Ashri on the south flank below the crater (west Java, Indonesia) The straight feature is the Lembang fault. SPOT XS image, 1986. (Copyright CNES-SPOT Image)

266 Jan J. Nossin

Mount Pinatubo’s eruption illustrates this. The volcano had not erupted for over 400 years and was widely considered inactive. Krakatau’s eruption of 1883 was even bigger (18 km3 of tephra), though the volcano was widely regarded as extinct. After the cataclysmic eruption of 1883, its successor (Anak Krakatau) has shown continuous signs of activity up to the present day. The eruption of Mount Tambora (Sumbawa) produced at least five times as much tephra as Krakatau but is less well documented. Mount Tambora was considered extinct or even non-volcanic when it began showing signs of unrest three years before the cataclysmic eruptions of 1815. Its original height, which must have been over 4300 m, surely made it the highest point in the Indonesian islands. In comparison, after the explosion the caldera rim stands at 2850 m around a caldera 6 km across and 600 to 700 m deep (Petroeschevsky 1949; Stothers 1984). The eruption caused an estimated 90 000 deaths. The explosions were noted in Jakarta and on Sumatra, and ash fall was reported from Java. A nitrous smell was observed in Yogyakarta. Lake Toba resulted from an event dated 75 000 years bp, which produced an estimated volume of at least 1000 km3 (possibly twice that amount) of tephra. Mount Pinatubo’s 8–10 km3 of tephra deposit is presently being studied in detail (Newhall and Punongbayan 1996). As a comparison, it helps towards an understanding of the effect of the Toba event. The tuff deposits hundreds of metres thick around some of Sumatra’s volcanoes near Ranau Lake convey a menacing message.

Settlements in Hazard Zones The hazards posed by volcanoes in the densely populated island of Java are not to be underestimated. Some major cities, such as Yogyakarta (Merapi) and Bandung (Tangkuban Perahu), are located close to active volcanoes. The volcanic chronology of the Sunda volcanic complex, of which Mount Tangkuban Perahu is the currently active component, has been studied by Nossin, Voskuil, and Dam (1996) and Dam et al. (1996). The city of Bandung (approximately 2 million inhabitants) is for the greater part built on a fan of pyroclastic flow deposits of this volcanic complex, showing the reach of major cataclysmic events. Dam (1994) studied and dated the sediments in the former lake (the present Bandung plain). The volcanic events in the surrounding volcanic upland were then dated and a chronology of volcanic activity constructed. North of Bandung, near the town of Lembang, luxury settlement estates are being developed close to the active Tangkuban Perahu volcano (too close in my opinion: within the reach of the sul-

phuric smells from the craters). Since 1826 the volcano has reportedly erupted seventeen times, the last activity being a minor phreatic eruption in 1983. A major eruption of Tangkuban Perahu would place the city of Bandung in peril, and the new estates near the crater would be affected even by minor eruptions. Figure 15.11 provides an overview of the situation at Bandung. Merapi is a notorious killer volcano, the most active volcano in Indonesia. Its eruptions so far have been marked by dome collapse as a source for pyroclastic flows and associated lahars. Lahars on the southwest flank reach the Progo River, some 25 km from the summit. Until recently lahars were mainly confined to that sector. The city of Yogyakarta, with some 500 000 inhabitants, lies almost due south of the volcano at a distance of about 25 km. In 1994 lahar activity was redirected southward, as reported by Lavigne (1998). Potentially, Yogyakarta lies in a hazardous position with respect to the volcano— a widely recognized fact. Several other cities in Southeast Asia are located within the reach of potentially dangerous volcanoes. In many cases no actual estimation of the hazards as yet has been carried out. Metro Manila, which felt the effects of the Pinatubo eruption, is also relatively close to the Taal volcano, which is active and closely monitored. Taal has a history of destructive eruptions.

Atmospheric Effects A major eruption could cause a ‘volcanic winter’. For such a long-term worldwide effect, an eruption need to be orders of magnitude bigger than the ones we now witness, sending volumes of volcanic dust into the atmosphere many times larger than happened, for example, in the Pinatubo eruption of 1991. The ash cloud has a short-lived effect on the atmosphere, but the liberated sulphuric oxides form aerosols of H2SO4, which, mixed with fine volcanic ash, strikingly reduces the incoming solar radiation reaching the surface of the Earth (Stothers et al. 1989). A large part of smoke and ash will be removed from the atmosphere in a matter of a few weeks, but stratospheric aerosols take several years to be removed. The year 1816, ‘the year without a summer’, saw the worldwide atmospheric effects of the Tambora eruption, considered possibly the world’s greatest ash eruption since the end of the last glacial. A persistent dry fog that must have been located above the troposphere (Stothers 1984), and that is ascribed to volcanic dust from the Tambora eruption, reduced incoming solar radiation and caused a mean temperature drop in the Northern Hemisphere calculated at 0.4– 0.7°C, affecting agricultural production in a significant way. The effect of volcanic dust on incoming

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solar radiation and on global temperatures has been reviewed by Bullard (1980). The Pinatubo eruption (much smaller than the Tambora event of 1815) resulted in a cooling effect of 0.4–0.5°C in 1992–3 (Self et al. 1996). The effect of the much larger eruptions of the Toba volcano in the Pleistocene must have caused marked global cooling and related climatic change.

Jet Aircraft Safety Several incidents of jet aircraft passing through volcanic eruption plumes are on record. Gourgaud et al. (1988) report at least twenty-four eruptions which (up to 1988) resulted in air saftety incidents with jet planes. Loss of power in jet engines due to stalled compressors is ascribed to blockage of compressor-operating lines caused by accumulation of ‘ceramic’ on the first-stage turbines. This was observed only in three cases, and caused the stalling of all four engines. Two of these cases occurred during the Galunggung ( Java) eruption of 1982–3. The stalling is attributed to melting of ash as it passed through the engine combustion chamber, and then was deposited on the cooler vanes as a glassy (ceramic) substance. The melting point of the ash particles is influenced by their mineralogical composition, and this seems to have an influence on whether the glassy redeposition on the turbine vanes takes place. The Pinatubo eruption caused sixteen in-flight encounters with the volcanic cloud, some at a large distance from the volcano. Two of these involved engine failure (Casadevall, Delos Reyes, and Schneider 1996). Some eruption plumes caused no problems to jet aircraft, and this could be ascribed to a different melting point of the ash particles; the composition may change, however, in the course of an eruption as a result of water interacting with magma.

Hazard Zonation Hazard zonation is a way of protecting the population from the dangers of an eruption. Volcanic hazard zonation can be based on knowledge of the behaviour of the volcano when it erupts, or on the extent and nature of deposits from previous eruptions if data on present eruptions are not available because the volcano has not recently erupted. For the first, observations during an eruption are needed; the second requires detailed investigation in the field. On active volcanoes, a zonation map often shows a zone of permanent danger (sometimes mapped as ‘forbidden zone’), and zones of high, moderate, and low danger, with probable pathways of lava, lahars, or pyroclastic flows (Phivolcs n.d.). The most dangerous

eruptions are those of volcanoes considered dormant or inactive. When they come to life, often centuries after the previous eruption, their eruptions are often much more violent than those of volcanoes showing regular activity. Furthermore, they may catch the settlers around the volcano unawares, as in the case of Pinatubo in 1991. The study of volcanic deposits around suspect dormant volcanoes may throw light on the possible extension of eruption products and the kind of hazard that this poses. There is an increasing awareness of the importance of such studies, though their number is still rather limited. The methodology involves mapping and detailed analysis of the nature and origin of volcanic deposits, which may shed light on the path and strength of earlier pyroclastic flows, surges, lava flows, and lahars. These techniques are of vital importance for the Philippines and Indonesia.

Hazard Zonation and Monitoring of Active Volcanoes in the Philippines Phivolcs (the Philippine Institute for Volcanology and Seismology) has nine observatories at active volcanoes: Taal, Mayon, Canlaon, Mambucal, Cabagnaan, Bulusan, and Hibok-Hibok. Hazard zonation has been done by Phivolcs for the five most active volcanoes: Mayon, Taal, Bulusan, Canlaon, and Hibok-Hibok (Figure 15.2). Volcanoes with a history of known eruption are considered active, but unfortunately the historic record is rather short. Many volcanoes considered extinct can be equally active and dangerous. The Philippine archipelago includes more than 200 volcanoes in five zones, all associated with processes of subduction and/or plate convergence. Twenty-one of these are considered active (Table 15.1). Since the 1991 eruption Pinatubo has been studied extensively. When the volcano started showing signs of activity in April 1991, the volcanologists of Phivolcs and the United States Geological Survey formed a team that closely watched and monitored the eruption’s progress. A hazard zonation map for Pinatubo (about which only limited information was available at that time) was quickly prepared and distributed. Alert levels were adequately disseminated. When the climactic eruption occurred on 14 and 15 June 1991, a catastrophe was averted through accurate forecasting and timely evacuation of tens of thousand of inhabitants. For the ensuing lahars, aggravated by the passing of a tropical storm over the area, there was less preparedness, and more deaths and damage were caused by this post-eruption phenomenon than by the eruption itself. Lahar hazard and risk zonation for the Pinatubo area has, since then, been attempted by various authors and investigators.

268 Jan J. Nossin Table 15.1 Volcanic eruptions in the Philippines Volcano

No. of known eruptions

Last eruption

Location

Mayon Taal Canlaon Bulusan Ragang Smith Hibok-Hibok Didicas Babuyan Claro Camiguin de Babuyanes Cagua Banahaw Calayo Pinatubo Iraya Iriga Biliran Bud Dajo Matutum Kalatungan Makaturing

44 33 24 12 9 8 6 5 1 1 1 1 1 1 ? ? ? ? ? ? ?

1984 1977 1988 1988 1915 1924 1953 1978 1913 1957 1860 1780 1886 1991 1464 1641 1939 1897 1911

Legaspi City, Albay Talisay, Batangas Negros Oriental Sorsogon Cotobato Babuyan Island Mambajao, Camiguin Island Babuyan Island Group Babuyan Island Babuyan Island Group Cagayan Lucena City Valencia, Bukidnon Zambales Batanes Iriga, Caramines Sur Biliran Jolo Island Cotobato Bukidnon Lanao, Mindanao

Note: The locations of volcanoes are shown in Fig. 15.2. There are eighty-nine volcanoes listed as inactive; others do not even have a name. The volcanoes which erupt frequently and are considered dangerous (e.g. Mayon and Taal) are listed towards the top of the table. Since the compilation of the table, at least Mayon has had additional eruptions. Source: Phivolcs.

Hazard Zonation and Monitoring of Active Volcanoes in Indonesia Indonesia has 128 volcanoes considered active, of which about seventy are known to have erupted in the last 400 years. Figure 15.1 shows the volcanoes of Indonesia. Of the total Indonesian territory of 1 907 000 km2, 333 450 km2 consist of volcanic terrain. Of this, 16 620 km2 are considered to be affected by volcanic hazard. About 0.66 per cent of the population of Indonesia lives in volcanic danger areas (Kusumadinata 1979). Continuous monitoring is carried out for thirty-one volcanoes from thirty-five observatories. Visual monitoring on the active volcanoes of Indonesia includes observations on volcano shape and size, the sound of emanating gases, temperature measurements, and lava dome observations. Seismic observation, using mechanical, electromagnetic, and radio-teleseismic equipment, is carried out on the following volcanoes: • Sumatra: Merapi, Talang, Krakatau • Java: Dieng, Lamongan, Ijen, Raung, Merapi, Semeru, Tangkuban Perahu, Galunggung, Kelud • Bali: Agung • Nusa Tenggara Islands: Ebulobo, Ia, Kelimutu, Ili Boleng

• Sulawesi: Lokon, Mahawu, Soputan, Karangetang • Maluku islands: Bandaapi, Gamalama.

Awu,

Electronic distance measurement is periodically carried out at Tangkuban Perahu, Galunggung, and Merapi volcanoes, which also have tiltmeters installed. These are also installed at Kelud volcano. These volcanoes, located close to densely populated areas, are potentially extremely dangerous. Volcanic hazard mapping is still limited to Galunggung, Merapi, Kelud, and Semeru volcanoes in Java, hazard maps also exist for the Agung and Batur volcanoes in Bali, and for Sangeang Api in west Nusa Tenggara. For fifty-nine other volcanoes, preliminary hazard maps have been prepared (Kusumadinata 1979). The source of such data is the Volcanological Survey of Indonesia. A review of active volcanism in Indonesia was carried out by van Bemmelen (1949), and then by Neumann van Padang (1951). The topic is discussed also by Verstappen in Chapter 9 in this volume.

Post-Volcanic Geomorphologic Hazards Volcanic activity emplaces new material (often in substantial amounts) on the Earth’s surface, creating a

Volcanic Hazards in Southeast Asia 269

disequilibrium in the exogene processes active at that time and place. Lava flows may disrupt the drainage system, causing floods in unexpected locations, or disrupting water supply in other places. Pyroclastic flow deposits will, as outlined above, constitute source material for lahars, in the same way as ash deposits. In recent years these geomorphic hazards have been studied by scientists in the Philippines and Indonesia. Examples of such studies are listed in the References section at the end of this chapter, but several of these studies are summarized below. In the Philippines, studies by Daag and van Westen (1996) deal with the erosion of pyroclastic flow deposits as a source for Pinatubo lahars. Dayao (1994) studied these hazards for the Mayon volcano. Figure 15.12 shows unpublished maps by C. van Westen, K. Sijmons, and A. Dayao presenting volcanic deposits around the Mayon volcano on Luzon, Philippines. Pinatubo lahars were studied by Atienza (1995), Javelosa (1994), and Calomarde (1997), while risk assessment was carried out by Nossin and Javelosa (1996). All studies make ample use of remote sensing data. Radar data analysis has been used for Pinatubo lahars by Lopez et al. (1996). Indirectly and inter alia the study of lahars receives attention in numerous publications by Phivolcs and in Fire and Mud, the account of the Pinatubo eruption edited by Newhall and Punongbayan (1996). In Indonesia, a study on volcanic hazards in central Java with special reference to the Dieng Plateau and the Merapi volcano was published by Verstappen (1988). Situmorang (1986) completed geomorphic studies of Mount Ciremai (west Java), the Tengger-Jambangang System, and Mount Semeru (east Java). For the latter he has also identified hazard zones for pyroclastic flows and lahars. Volcanic–geomorphic hazard studies have been undertaken in the Sunda–Tangkuban Perahu area of west Java by Nossin, Voskuil, and Dam (1996), and Noor (1992). Bacharudin (1990) studied volcanic– geomorphic hazards for Mount Gede (west Java), and Mount Agung (Bali). A study by Lavigne (1998) deals in depth with the lahars at Mount Merapi (central Java); Thouret et al. (1998) studied lahars of the Kelud volcano after the 1990 eruption. Indirect references and studies of lahars are found in the publications of the Volcanological Survey of Indonesia.

Quantification Problems: Victims, Damage, Mitigation Risk assessment involves quantification and evaluation. Elements at risk, like infrastructure, buildings, or real estate, can be evaluated using their market, insurance,

tax values, or any other monetary attribute. One problem, which is difficult to solve, is that when such an object appears within a certain hazard zone on a hazard zonation map, its trade value is affected. One might imagine that owners of such a property would not be too pleased with such a situation. It is possible that, in some cases, efforts would be made to suppress the information. Javelosa (1994) has circumvented the problem by attaching a numerical score to the various vulnerability domains. When larger communities like cities fall within a hazard zone, quantification is even more difficult. Moreover, people living within sight of a danger tend to get used to it and develop a sort of acceptance or even indifference. People who are not aware of the potential danger could develop different attitudes, or may be caught unawares. This is a reaction similar to the presence of other natural hazards such as floods or slope failure. For example, farmers and inhabitants of the slopes of the Merapi volcano in central Java are familiar enough with the volcano to heed warnings promptly. This volcano erupts so frequently that inhabitants are fully aware of what may happen. In spite of this, Merapi is Indonesia’s most deadly volcano. On the Dieng Plateau, some 150 people were killed by a poisonous gas explosion in February 1979, in spite of the fact that fumaroles on the plateau emit signs of activity continuously. Semeru, in evident eruption for the past twenty-one years, continues to kill, mountain climbers being among the victims. Catastrophic events, by their nature, attract public attention. In recent times, public media bring in instant worldwide coverage. Events from the more remote past are less well documented. At present, monitoring of hazardous volcanoes also leads to better warning systems with sufficient time for evacuation. Disaster mitigation is improving, though there is still a long way to go. Authorities in general seem more ready to act after a disaster than before its occurrence. For Indonesia, Zen (1983) estimates that over the last 200 years volcanic disasters have taken more than 175 000 lives, 75 per cent of which is ascribed to the eruptions of Tambora (1815: 92 000 casualties), Krakatau (1883: 36 000), Kelud (1919: 5100), Merapi (1993: 1300), and Mount Agung (1963: 1200). In case of volcanic eruptions, geophysical methods of monitoring impending magmatic movements and extrusions may lead to timely prediction of dangerous eruptions. The post-eruption hazards can be estimated, but they are known only after the eruption. For example, it may be necessary to determine how much pyroclastic flow material is newly deposited to act as a feeder for

270 Jan J. Nossin

(a) Fig. 15.12. Volcanic deposits of Mayon volcano (Luzon, Philippines) (Unpublished maps by Sijmons, van Westen, and Dayao)

Volcanic Hazards in Southeast Asia 271

(b) Fig. 15.12. Continued

272 Jan J. Nossin

(c) Fig. 15.12. Continued

Volcanic Hazards in Southeast Asia 273

future lahars. In the International Decade of Natural Disaster Reduction, Taal in the Philippines and Merapi in Indonesia have been assigned as Decade Volcanoes, and are subjects of special studies. An estimate of damage and victims of volcanic activity in Southeast Asia as a whole is difficult to undertake. The big and recent events are well enough documented, but volcanic eruptions occur on remote islands where actual data about damage and victims are difficult to obtain, even now. A good and complete record of volcanic events taking place at any given moment is accessible through the Internet and is also hyperlinked to a number of other websites on volcanism. From the examples cited in this chapter it will be clear that the number of victims and the damage caused by volcanic events in Southeast Asia is high. Mitigation is best achieved by accurate prediction. Systems differ from one country to another, but the general trends that can be discerned are: Hazard zonation. High, intermediate, and low hazard zones are commonly discerned and delineated on volcanoes, based on available data including morphology and drainage systems, predominant wind direction, precipitation patterns, etc. From this data a prediction is made on the effect of events like blasts, surges, and pyroclastic flows, lava flows, and tephra falls, and zones are designated in accordance with the danger. Alert levels. From 1 (low) to 5 (eruption in progress). Timely warning and wide dissemination of the alert level may save (and have saved) numerous lives. The United Nations Disaster Relief Coordinator and UNESCO recognize three phases in a quick-response plan for volcanic emergency: 1. alert: mobilization of civil protection services 5– 15 days before the expected eruption; 2. readiness: evacuation of aged, sick, and very young persons; standby of emergency services; 2–5 days before the event; 3. evacuation: general evacuation 1–2 days before the emergency. Alert stages used by Phivolcs and the Volcanological Survey of Indonesia, however, are more differentiated. In general, risk is also a function of the nearness of sizeable cities, the presence of valuable infrastructure, and the value and priority of geo-resources. The level of knowledge of (potential) hazards, and the level of preparedness of authorities and population, play a fundamental role in the mitigation of hazard and risk. Volcanic hazard in Indonesia and the Philippines continues to be a real threat.

References Alexander, D. (1993), Natural Disasters (London: UCL Press). Atienza, G. T. (1995), ‘Lahar Mapping Using Remote Sensing and GIS Techniques: A Case Study of Pasig-Potrero River Basin, Mt. Pinatubo, Philippines’, M.Sc. thesis, ITC, Enschede. Bacharudin, R. (1990), ‘Geomorphological Approach to Volcanic Hazard Zonation, Using Remote Sensing Images: Two Case Studies from Indonesia: Mt. Gede, West Java, and Mt. Agung, Bali’, M.Sc. thesis, ITC, Enschede. Bullard, F. M. (1980), Volcanoes of the Earth (Austin: University of Texas Press). Calomarde, R. I. (1997), ‘Lahar: An Attribute of Change in the Morphology of Sto Tomas–Marella River, Mt. Pinatubo, Zambales, Luzon, Philippines’, M.Sc. thesis, ITC, Enschede. Casadevall, Tomas J., Delos Reyes, P. J., and Schneider, D. J. (1996), ‘The 1991 Pinatubo Eruptions and their Effect on Aircraft Operations’, in C. G. Newhall and R. S. Punongbayan (eds.), Fire and Mud: Eruptions and Lahars of Mount Pinatubo, Philippines (Quezon City: Phivolcs Press; Seattle: University of Washington Press), 1071–88. Daag, A. S. (1994), ‘Geomorphic Development and Erosion of the Mt. Pinatubo 1991 Pyroclastic Flows in the Sacobia Watershed, Philippines’, M.Sc. thesis, ITC, Enschede. —— and van Westen, C. J. (1996), ‘Cartographic Modelling of Erosion in Pyroclastic Flow Deposits of Mount Pinatubo, Philippines’, ITC Journal (1996), no. 2: 110–24. Dam, M. A. C. (1994), The ‘Late Quaternary Evolution of the Bandung Basin, West Java, Indonesia’, Ph.D. thesis, Vrije Universiteit Amsterdam. —— Suparan, P., Nossin, J. J., Voskuil, R. P. G. A., and GTL Group (1996), ‘A Chronology for Geomorphological Developments in the Greater Bandung Area, West Java, Indonesia’, Southeast Asian Earth Sciences Journal, 14/1–2: 101–15. Dayao, A. A. (1994), ‘Morphostructure and Hazard Implications in Mt. Mayon, Philippines: A GIS-Assisted Volcanic Hazards Study’, M.Sc. thesis, ITC, Enschede. Francis, P. (1993), Volcanoes: A Planetary Perspective (Oxford: Oxford University Press). Gourgaud, A., Camus, G., Gerbe, M.-C., Morel, J.-M., Sudradjat, A., and Vincent, P. M. (1988), ‘The 1982–83 Eruption of Galunggung (Indonesia): A Case Study of Volcanic Hazards with Particular Relevance to Air Navigation’, in J. H. Latter (ed.), Volcanic Hazards (Berlin: Springer), 151– 63. Janda, R. J., Daag, A. S., Delos Reyes, P. J., Newhall, C. G., Pierson, T. C., Punongbayan, R. S., Rodolfo, K. S., Solidum, R. U., and Umbal, J. V. (1996), ‘Assessment and Response to Lahar Hazard around Mt. Pinatubo, 1991 to 1993’, in C. G. Newhall and R. S. Punongbayan (eds.), Fire and Mud (Quezon City: Phivolcs Press; Seattle: University of Washington Press), 107– 41. Javelosa, R. S. (1994), Active Quaternary Environments in the Philippine Mobile Belt, Ph.D. thesis, Utrecht University, ITC Publication no. 20 (Netherlands: ITC Enschede). Kusumadinata, K. (1979), Data Dasar Gunungapi Indonesia (Bandung: Volcanological Survey of Indonesia). Lavigne, F. (1998), ‘Les Lahars du volcan Merapi, Java central, Indonésie’, doctoral thesis, Université Blaise Pascal, ClermontFerrand. —— Thouret, J.-C., Voight, B., Suwa, H., and Sumaryono, A. (2000a), ‘Lahars at Merapi Volcano, Central Java: An Overview’, Volcanology and Geothermal Research Journal, 100: 423–56. —— —— —— Young, K., LaHusen, R., Marso, J., Suwa, H., Sumaryono, A., Sayudi, D. S., and Dejean, M. (2000b), ‘Instrumental

274 Jan J. Nossin Lahar Monitoring at Merapi Volcano, Central Java, Indonesia’, Volcanology and Geothermal Research Journal, 100: 457–78. Lopez, E., Vinluan, R. J., Chorowitz, J., Parrot, J. F., Garcia, F., and Corpuz, E. (1996), ‘Mount Pinatubo Lahar Damage Assessment Using ERS-1 Synthetic Aperture Radar Data’, in Proceedings of the UN–IAF Workshop, Education and Awareness, Space Technology Application in the Developing World (Vienna: UN Office of Outer Space Affairs). Neumann van Padang, M. (1950), ‘Dertig jaren vulkanologisch onderzoek in Indonesie’, Tijdschrift K.N.A.G. ( Journal of the Royal Netherlands Geographical Society), 2, pt. 67, no. 5: 541–66 (with English summary). —— (1951), Catalogue of the Active Volcanoes of the World, pt. 1: Indonesia (Naples: International. Volcanological Association). Newhall, C. G., and Punongbayan, R. S. (eds.) (1996), Fire and Mud: Eruptions and Lahars of Mount Pinatubo, Philippines (Quezon City: Phivolcs Press; Seattle: University of Washington Press). Noor, D. (1992), ‘The Applications of Aerospace Imagery to the Study of Morphostructural Landforms and Volcanism in the Sunda Volcanic Complex, West Java, Indonesia’, M.Sc. thesis, ITC, Enschede. Nossin, J. J., and Javelosa, R. S. (1996), ‘Geomorphic Risk Zonation Related to June 1991 Eruptions of Mt. Pinatubo, Luzon, Philippines’, in O. Slaymaker (ed.), Geomorphic Hazards (Chichester: Wiley), 69–94. —— Voskuil, R. P. G. A., and Dam, R. M. C. (1996), ‘Geomorphologic Development of the Sunda Volcanic Complex, West Java, Indonesia’, ITC Journal (1996), no. 2: 157–65. Petroeschevsky, W. A. (1949), ‘A Contribution to the Knowledge of the Gunung Tambora (Sumbawa)’, Tijdschrift K.N.A.G. ( Journal of the Royal Netherlands Geographical Society), 2, pt. 66, no. 6: 688–704. Phivolcs (n.d.), Volcanoes of the Philippines (Manila: Phivolcs Press). Punongbayan, R. S., Umbal, J., Torres, R., Daag, A. S., Solidum, R., Delos Reyes, P. J., Rodolfo. K. S., and Newhall, C. G. (1992), A Technical Primer on Pinatubo Lahars (Manila: Phivolcs Press). Selby, M. J. (1985), Earth’s Changing Surface (Oxford: Clarendon Press). Self, S., Zhao, J.-X., Holasek, R. E., Torres, R. C., and King, A. J. (1996), ‘The Atmospheric Impact of the 1991 Mount Pinatubo Eruption’, in C. G. Newhall and R. S. Punongbayan (eds.), Fire and Mud (Quezon City: Phivolcs Press; Seattle: University of Washington Press), 1089–1117.

Simkin, T., and Fiske, R. S. (1983), Krakatau 1883: The Volcanic Eruption and its Effect (Washington: Smithsonian Institution Press). Situmorang, T. (1986), ‘A Geomorphological Approach to the Study of Quaternary Volcanic Stratigraphy and to the Assessment of Volcanic Hazards, Using Aerospace Imagery’, M.Sc. thesis, ITC, Enschede. Stothers, R. B. (1984), ‘The Great Tambora Eruption of 1815 and its Aftermath’, Science, 224: 1191– 8. —— Rampino, M. R., Self, S., Wolff, J. A. (1989), ‘Volcanic Winter? Climatic Effects of the Largest Volcanic Eruptions’, in J. H. Latter (ed.), Volcanic Hazards, IAVCEI Proceedings in Volcanology 1 (Berlin: Springer), 3– 9. Tazieff, H., and Sabroux, J. C. (eds.) (1983), Forecasting Volcanic Events (Amsterdam: Elsevier). Thouret, J.-C., Abdurachman, K. E., Bourdier, J.-L., and Bronto, S. (1998), ‘Origin, Characteristics and Behaviour of Lahars Following the 1990 Eruption of Kelud Volcano, eastern Java, (Indonesia)’, Bulletin of Volcanology, 59: 460– 80. —— Lavigne, F., Kelfoun, K., and Bronto, S. (2000), ‘Toward a Revised Hazard Assessment at Merapi Volcano, Central Java’, Journal of Volcanology and Geothermal Research, 100: 479–502. Umbal, J. V., and Rodolfo, K. S. (1996), ‘The 1991 Lahars of Southwestern Mount Pinatubo and Evolution of the Lahar-Dammed Mapanuepe Lake’, in C. G. Newhall and R. S. Punongbayan (eds.), Fire and Mud (Quezon City: Phivolcs Press; Seattle: University of Washington Press), 951–71. UNDRO (1982), Natural Disasters and Vulnerability Analysis (Geneva: Office of the United Nations Disaster Relief Co-ordinator). van Bemmelen, R. W. (1949), The Geology of Indonesia, 3 vols. (The Hague: Government Printing Office). Verstappen, H. Th. (1988), ‘Geomorphological Surveys and Natural Hazard Zoning, with Special Reference to Volcanic Hazards in Central Java’, Zeitschrift für Geomorphologie, ns, suppl. vol., 68: 81–101. —— (2000), Outline of the Geomorphology of Indonesia, ITC Publication 79 (Enschede: ITC). Zen, M. T. (1983), ‘Mitigating Volcanic Disasters in Indonesia’, in H. Tazieff, H. Sabroux, and J. C. Sabroux (eds.), Forecasting Volcanic Events (Amsterdam: Elsevier), 219–36.

16

Hazards and Risks at Gunung Merapi, Central Java: A Case Study Jean-Claude Thouret and Franck Lavigne

Introduction Of the 1.1 million people living on the flanks of the active Merapi volcano in Java (average population density: 1140 inhabitants per km2), 440 000 live in relatively high-risk areas prone to pyroclastic flows, surges, and lahars (Table 16.1). The sixty-one reported eruptions since the mid-1500s killed about 7000 people. For the last two centuries the activity of Merapi has alternated regularly between long periods of lava dome extrusion and brief explosive episodes with dome collapse pyroclastic flows at eight- to fifteen-year intervals. Violent explosive episodes on an average recurrence of twenty-six to fifty-four years have generated pyroclastic flows, surges, tephra falls, and subsequent lahars. The current hazard zone map of Merapi (Pardyanto et al. 1978) portrays three areas, termed the forbidden zone, first danger zone, and second danger zone, based on progressively declining hazard intensity. Revision of the

hazard map has been carried out because it lacked the details necessary to outline hazard zones with accuracy (in particular the valleys likely to be swept by lahars), and excluded some areas likely to be devastated by pyroclastic density currents, such as the 22 November 1994 surge. In addition, risk maps were developed in order to incorporate social, technical, and economic elements of vulnerability (Lavigne 1998, 2000) in the decisionmaking progress. Eruptive hazard assessment at Merapi is based on reconstructed eruptive history, based on eruptive behaviour and scenarios combined with existing models and preliminary numerical modelling (Thouret et al. 2000). The reconstructed past eruptive activity and related damage define the extent and frequency of pyroclastic flows, the most hazardous phenomenon (Camus et al. 2000; Newhall et al. 2000). Pyroclastic flows travelled as far as 9–15 km from the source, pyroclastic surges swept the flanks as far as 9–20 km away from

Table 16.1 Population at risk: population density and growth around Merapi, 1976–1995 Zone of interest

No. of villages

Area (km2)

Population (1976)a

Population (1995)b

Population (density/km2)

Population growth (%) (1990–5)

Elevation > 200 m asl Elevation > 500 m asl Forbidden zone First danger zone Second danger zone

296 89 32 37 —

949.0 374.5 186.4 100.8 —

206 600 — 40 800 72 600 93 200

1 083 400 258 200 79 100 114 800 —

1399 690 424 1139 —

3.6 3.0 3.9 3.6 —

a b

After Suryo and Clarke (1989); 1985 data as in 1976. After Lavigne, unpub. interviews, 1998.

276 Jean-Claude Thouret and Franck Lavigne

the vent, thick tephra fall buried temples in the vicinity of Yogyakarta 25 km to the south, and subsequent lahars spilled down radial valleys as far as 30 km to the west and south. At least one large edifice collapse has occurred in the past 7000 years (Camus et al. 2000; Newhall et al. 2000). Four eruption scenarios have been derived from: (1) the past eruptive behaviour of Merapi; (2) existing models for tephra falls; and (3) numerical simulations of pyroclastic and lahar flowage, based on a digital elevation model, SPOT stereo-pair satellite images, and one 2D ortho-image. The scenarios have been used to map hazardous zones. Three major threats are identified: (1) a collapse of the summit dome in the short to mid-term that can release large-volume pyroclastic flows and high-energy surges towards the south-southwest sector of the volcano; (2) an explosive eruption much larger than any event since 1930, expected to sweep all the flanks of Merapi at least once every century; (3) a potential collapse of the summit area, involving the fumarolic field of Gendol and part of the southern flank, which can contribute to trigger moderate-scale debris avalanches and lahars. Vulnerability and risk elements within the threatened areas of Merapi were assessed and mapped using two techniques: risk zonation based on a topologic and vector-mode Geographic Information System (GIS) (Lavigne 1998), and micro-zonation of lahar-prone areas (Lavigne 1998, 2000).

The Merapi Volcano Geomorphological and Structural Features Gunung Merapi (2965 m) is a stratovolcano located in the highly populated area of central Java (Figure 16.1). The cone has a circular shape above 1000 m asl and is 13 km in diameter. At 500 m asl, the cone is ellipsoidal, and its main axis from west to east measures 29 km. Four main geomorphological zones can be demarcated: 1. The summit of the volcano (> 2600 m asl) corresponds to a recent cone, which has been built over historical time (Figure 16.2). This east side of the cone is truncated by a plateau at 2800 m asl with an extinct crater (Kawah Mati) and the two active hydrothermal fields of Gendol and Woro (Figure 16.1). Analysis of one SPOT2 XS (multiscanner) satellite image (1990) allows us to record the consistent pattern of N45- and 135–165-trending fractures, as shown on Figure 16.2 (e.g. Kukusan Fault), and helps to locate the fumarolic field to the southeast of the summit dome, where northeast-trending and southeast-trending

fractures intersect. New N135-trending fractures as long as 90 m appeared after 1991 in the fumarolic field of Gendol (Vincent et al. 1992). The cluster of N120–165 fractures, which parallels the measured trends in the Gendol area, corresponds to one of the two fracture groups on the SPOT image of 1990 (Figure 16.2). Through infiltration and circulation of fluids at high temperature, the fractures contribute to weaken the rocks that host the dome. The west side of the summit shows a 200 m wide crater, filled by a composite, viscous lava dome (Figure 16.3), which resembles a dome-coulée (after the terminology of Blake 1990). The present activity of Merapi consists of alternating periods of dome growth for several years and brief episodes of dome collapse. 2. From 2600 to 1400 m asl, the Merapi cone shows two geomorphological features. On the north and east sides, the Pasarbubar atrio is a crescentshaped depression at 2600 m asl, surrounded by several peaks (Mount Gadjahmunkur, 2654 m asl; Mount Pusonglondon, 2693 m asl; Figure 16.4) that resulted from the destruction of the Old Merapi caldera wall (Newhall et al. 2000). The slopes are made up of deeply eroded andesitic lava flows (Wirakusumah, Juwarna, and Loebis 1989). Remote sensing has enabled us to outline the distribution of the pyroclastic flow deposits on the south and southwest flanks (whitish areas on Figure 16.2). The delineation has allowed us to estimate the average extent (7–10 km2) and travel distance (7 km) of historical pyroclastic flows. These deposits are confined within a large, horseshoe-shaped amphitheatre identified as from a flank failure, and limited to the east by the Kukusan Fault along the Woro River (Figure 16.2). 3. The lower cone (1400– 600 m asl) has an average gradient of ~16 per cent. It consists of a mixture of cinder tuff, andesitic pumice, scoria and pyroclastic flow deposits, and lahar deposits (Wirakusumah, Juwarna, and Loebis 1989). The south slopes form the base of the hills of Turgo (1285 m asl) and Plawangan (1275 m asl), which are remnants of old andesitic lava flows. The rivers draining the upper part of Merapi have eroded deep valleys (called kali) such as the Boyong and the Kuning Valleys. Break-in slopes due to lithological contrasts between cohesive lava flows and non-cohesive pyroclastic deposits are a common feature of the longitudinal profile of these rivers. 4. The foreland of the volcano (< 600 m asl) is covered by lahar deposits for ~286 km2 (JICA 1980). Thirteen channels, from the Apu River in the northwest to the Woro River in the southeast (Figure 16.1), feed fans of lahar deposits down-valley, below 450 m asl in

Hazards and Risks at Gunung Merapi 277 106° E

110° E

6° S

JAVA MERAPI 100 200 km YOGYAKARTA

1000 m

MERBABU 20 00 m

8° S 0

MAGELANG

K. Apu

7°30' S Gandul BOYOLALI

MERAPI

Trising

Tele ng

Senowo

MUNTILANG Putih

g tan Ba

Opak

KALIURANG

Kuning

Sleman

Kr asa k

Boyong

Ri ve r

Kr as ak

Gendol

oro W

PR OG O

Blongkeng

ol Gend

Pa be lan

t Lama

KLATEN

r ive OR G O PR

Co de

7°45' S

Sambisari YOGYAKARTA 0 110°20' E

Summit domes and andesite lava flows of Recent–Modern Merapi or New Merapi periods

110°30' E

Volcano’s flanks mantled by volcaniclastic deposits and radial valleys that convey debris flows to the west and the south

Approximate boundary (large dots) of Gumuk thick ashfall and surge deposits (2200–1000 yr BP), and boundary (small dots) of Sambisari ashfall deposits (600–470 yr BP). Newhall et al. (2000) attribute these deposits to New Merapi and found them distributed on all sides of the volcano

4

Gendol Hills made up of debris avalanche deposits (Berthommier 1990; Camus et al. 2000, doubtful to Newhall et al. 2000) Probable scar of the caldera of avalanche, with N45-trending fractures

Fig. 16.1. Sketch map of the basic geographic context and geologic features of Merapi volcano and its region (Sources: after Bahar 1984; Berthommier 1990; VSI 1981)

6 km

scale

278 Jean-Claude Thouret and Franck Lavigne 110°25' E

110°30' E

Merbabu

7°30' S

ancient Merapi lava flows

N

lava flow cliffs 7°30' S

middle and recent Merapi deposits middle Merapi deposits k. Senowo Pa

l

h

uti

ng

ata

ng

k. B

k.

Tu

k. Plw Boyong

k. Kuning

ult k.

pak

new Merapi deposits k do e B k.

k. O

Be

be

7°35' S

Gf

e

Older Merapi Young and lava flows Middle Merapi deposits

g

P k.

ld

ancient Merapi lava flows

Kukusan fa

d de w rar y flo Cont e mpo lastic a nd p y ro c

en onk

k. B

Rlf is p

k. Lamat

2965 m br

new Merapi deposits

7°35' S

G

en

do

l

110°25' E 0

2

4

110°30' E 6

8

10

12

14 km

SPOT2 - XS satellite image 8 September 1990 Fig. 16.2. Sketch map of structures and deposits of the Merapi stratocone, interpreted on one satellite SPOT2 XS (multiscanner) image of 8 September 1990 The black, arcuate line that parallels the Kukusan Fault is probably the scar of the horseshoe-shaped avalanche caldera (dashed where inferred). Black, short, and straight lines are N45 and N135–65-trending fractures. Areas of recent volcanoclastic deposits are outlined. Dark grey areas enclosed in dashed lines: block-and-ash flow deposits, airfall deposits, and removed pyroclastic debris of the Recent and Modern Merapi or New Merapi. Light grey areas enclosed in dotted lines: present-day (post-1961) pyroclastic debris and debris flow deposits that remove pyroclastic flow deposits in channels and on volcaniclastic fans. Tu = Turgo Hill, Plw = Plawangan Hill, Pa = Pasarbubar crater (somma rim), Gf = Gendol fumarolic field, ld = lava dome, Rlf = historical lava flow; K., kali = river

the Boyong and Kuning Valleys, and below 600 m asl in the Gendol and Woro Valleys. Most channelled lahar deposits mantle 15–20 km2 in area and have travelled at least 14 km. Average slope is under 3.5 per cent, but a few hills still emerge on the southwest side, such as the Gendol Hills. The origin of the Gendol Hills was interpreted by van Bemmelen (1949) as hummocks from a debris avalanche, but this interpretation has recently been debated (Newhall et al. 2000).

Growth and Eruptive History Geological studies identify discrepancies in interpretations of Merapi’s eruptive history between two research teams. Berthommier (1990), Berthommier et al. (1992), and Camus et al. (2000) have suggested that the eruptive centre probably became active c.40 000 years ago. They divided the eruptive history of Merapi into four periods: Ancient Merapi (40 000–14 000 years bp);

Hazards and Risks at Gunung Merapi 279

Fig. 16.3. Dome-coulée of Merapi volcano in August 1994 This oblique view from the southwest side of the volcano shows a stubby lava flow transitional between a true flow and a low lava dome. This dome collapsed on 22 November 1994 (Photo: Agung Pribadi)

Area destroyed on 18 and 19 December 1930 Block-and-ash flow deposits on 18 and 19 December 1930

2 km

1930 lava flow Babadan

1931 lava flow Totally destroyed village

o

ow

Lamat

Kembang

Kalibeningngisor Blongkeng

Demo Aglik

Sisir

Keningar Terus Boedjong Deles Gemersabrang Semeri

2500

Kroya

2000

Mt Patukalap-alap

Koci Bendo

15

00

Maron

Baturngisor

0 50

Poele Garung Puntuk Gejugan Gentong Pagerjurang Podjok Soko Genting Medjing Klampean

Ku nin g

Sen

VOLCANO MERAPI

Gondangredjo

10

00

Mt Turgo Mt Plawangan

Wates

Turgo

7°35' S

ng

P

g do

Be

Boyo

h uti

Gendol

Partly destroyed village

110°25' E

Fig. 16.4. Map of the destruction area and block-and-ash flow deposits of the December 1930 eruption of Merapi, adapted from the original by Neumann van Padang (1933, pl. 1) Locations of entirely and partially destroyed villages as cited by Neuman van Padang (1933: 31)

280 Jean-Claude Thouret and Franck Lavigne

Middle Merapi (14 000–2200 years bp); Recent Merapi (2200 years bp–ad 1786); Modern Merapi (after ad 1786). Newhall et al. (2000) are of the opinion that the eruptive history of Merapi is essentially Holocene in age, and recognize only three phases: the Proto-Merapi (≥ 5000 bc); the Old Merapi (5000 bc –ad 0); and the New Merapi (ad 0 to the present day). Discrepancies between the two interpretations involve periods of volcanic growth, events, and deposits (Thouret et al. 2000). Periods of growth and destruction that preceded the Merapi stratocone, i.e. before ~7000 years. For example, Camus et al. (2000) attribute the Plawangan and Turgo Hills (Figure 16.2) to the Ancient Merapi, whereas Newhall et al. (2000) suggest that they are remnants of a ProtoMerapi (7000–5000 bc), a large age difference. Flank failure, debris avalanche, and related explosive eruption. Berthommier (1990), Berthommier et al. (1992), and Camus et al. (2000) claimed that a debris avalanche deposit to the west resulted from a flank failure and attributed what was thought to be a ‘blast’ deposit to a laterally directed eruption between 6700 and 2200 years bp. The arcuate Kukusan Fault is identified as parallel to the scar of the flank failure, and the mound-shaped hills of Gendol (Figure 16.1) are thought to have resulted from a debris avalanche deposit (Berthommier 1990; Berthommier et al. 1992). Newhall et al. (2000) now opine that the flank failure of Old Merapi (whose debris avalanche deposits were first seen on the south-southwest flank along the banks of the Boyong River washed away by the 1994 eruption) occurred between 1600 and 1100 years bp. This collapse left a somma rim high on Merapi’s eastern slope that may correspond to the Pasarbubar crater rim. Since then no pyroclastic flow is known to have surmounted the somma rim and spilled down the east slope. Despite the disagreement on the event’s age and on the Gendol Hills’ nature, both groups claim that a flank failure did occur at least once in the past 6700 years. This requires a reassessment of flank failure hazard posed to the west, south, and southwest flanks. Extent of pyroclastic flows, surges and blasts. A series of deposits records the Recent Merapi growth, such as ashand-scoria pyroclastic flow deposits and thick Plinian and/or phreato-Plinian tephra fall deposits that mantle an area in excess of 800 km2, particularly to the south. Additional pyroclastic surge deposits are possibly related to phreatomagmatic eruptions that delivered Gumuk ash (2200–1470 years bp) and Sambisari ash (600–470 years bp) as far as 30 km from Merapi’s summit (Berthommier 1990; Camus et al. 2000). They

argue that Sambisari ash and lahar deposits 8 m thick at the start of the fifteenth century buried the Sambisari temple located 20 km from the source (Figure 16.1). Based on the occurrence of pyroclastic flows to the south and west, in the Yogyakarta plain, and in the Kaliurang vicinity, Newhall et al. (2000) opine that large explosive eruptions followed shortly after the Old Merapi collapse. They suspect, without any direct evidence, that large explosive eruptions followed the c.ad 928 culture change and may have led to the displacement of the Mataram civilization in central Java.

Previous Hazard Zone Mapping The hazard zone map (1:100 000) published by the Volcanological Survey of Indonesia (VSI) (Pardyanto et al. 1978; VSI-MVO 1989) is based on the areal extent of the pyroclastic and lahar deposits from the 1930 and 1969 eruptions only. In 1930–1 channelled blockand-ash pyroclastic flows reached 10–15 km (Neumann van Padang 1931), and pyroclastic surges devastated a surface area more than 60 km2 on the west and southwest flanks (Figure 16.4). The 1978 hazard zone map has divided the volcano’s slopes and surrounding foreland into three zones: the forbidden zone, the first danger zone, and the second danger zone (Figure 16.5; Table 16.1). The forbidden zone, above 1500 m asl on the upper part of the volcano, is frequently affected by rockfalls, pyroclastic flows, and tephra falls including ballistic ejecta. In 1995 about 80 000 people were living or working in that zone, demonstrating that the term ‘forbidden zone’ does not imply exclusion of people by the civil authorities. The first danger zone can be affected by tephra falls or lahars should violent explosive eruptions occur. This area was thought to lie beyond the reach of most pyroclastic flows and lava flows. In 1995, 114 000 people were living in the first danger zone (VSI 1995; Table 16.1). The second danger zone corresponds to the radial valleys draining the volcano’s flanks, particularly towards the west and the south. Lahars and floods can devastate the second zone as far as 30 km down-valley from the summit. The second danger zone supports much more than 100 000 people above 200 m asl (VSI 1995). The map, which outlines areas likely to be affected by destructive volcanic processes according to the distribution of deposits from the last significant eruptions (1930–1, 1961, and 1969), has served a useful purpose for two decades. However, it is no longer adequate or accurate, owing to four main shortcomings:

Hazards and Risks at Gunung Merapi 281 110°20' E

110°25' E

110°30' E

MERBABU area 7°30' S

7°30' S

ul K.Gand

K.Trising K.Senowo t K.Lama

D

K.K ras ak

K.Blongskeng

yke dd s an m da

g tan Ba . K

K.Putih

D

110°20' E

ro Wo K.

arta yak Yog

7°40' S

ndol K.Ge

GH

7°35' S

ak K.Op K.Kuning

k rasa K.K

o dt roa

Gendol Hills

KALIURANG

K.B oyo ng

MUNTILAN 7°35' S

K.Te leng K.Ti ogo

0

1

2

3

4 km

Scale 110°25' E

110°30' E

7°40' S

D = Historical and present summit domes (andesite)

Area quasi-continuously covered by rockfall and gravity-driven collapse of the unstable dome flanks

Area regularly covered by Merapi type dome collapse and block-and-ash flows (since 1961)

Area covered by large pyroclastic flow deposits and high-energy pyroclastic surges (e.g. 1930, 1969)

Area devastated by channelled block-and-ash flow deposits and unconfined pyroclastic surge deposits (1994–7)

Main pathways of channelled debris flows; thicker channels convey lahars at present (i.e. since 1961)

Approximate reach of ‘blast’ from laterally directed explosions (based on Berthommier 1990)

Maximum extent of the avalanche related to flank failure of Bezymianny-type; coinciding with the probable scar of the Old Merapi collapse

VSI ‘forbidden zone’

VSI ‘first danger zone’

VSI ‘second danger zone’

Rocks of Old Merapi and of Merbabu volcanoes

Fig. 16.5. Sketch map of areas affected by Recent and Modern Merapi or New Merapi volcaniclastic debris and limits of the three VSI danger zones (K., kali = river)

1. It indicates boundaries of the field of influence for a given phenomenon without taking into account its frequency or its variable extent and impact in the event of a distinct eruptive behaviour of the volcano. 2. Specific hazards on the map are almost exclusively associated with a specific danger zone (i.e. pyroclastic flows with the forbidden zone, tephra fallout and lahars with the first danger zone, and large-scale lahars with the second danger zone). We now know from the geological record (Camus et al. 2000; Newhall et al. 2000) that people and property are obviously at risk not only in the first and second danger zones, but also beyond their boundaries. Therefore, the forbidden zone should

be extended beyond its mapped limit well into the first danger zone. 3. The 1978 map is out of date, and the hazard zones do not account for the present morphology of the summit and the channels. Hazard evaluation and mapping need to recognize that eruption sites have changed frequently at Merapi (1930–1, 1942–3, 1967, 1984, 1992, 1994). One of the most recent domes overlapped the 1984– 8 dome on the northwest flank in 1990–2, and the focus shifted towards the southwest flank in 1994, and again towards the west-northwest flank in 1997– 8 and in 2000–1 (GVN 1998, 2000, 2001). Thus, the pyroclastic flows from the unstable

282 Jean-Claude Thouret and Franck Lavigne North

Apu 2

Apu 1 1954

1954

Trising 1934

1939 1957,2001 2001 1954 2000-1

Blongkeng 1930

MERAPI SUMMIT

1930 1979 Sat 1969 1967 2000-1 1968 1984 2001

1961

Batang

1972,2001

1984,94 1967 1973

Putih 1930

East

1939, 1961 1942

Gendol 1942

1904 1994

Boyong

Woro

Bebeng

12 km

West

8k m

Senowo Lamat

4k m

1930

Fig. 16.6. Azimuths and travel distances for pyroclastic flows released at Merapi over the period 1904–2000 (Source: after Purbo-Hadiwijoyo and Suryo 1980: 282, modified and updated after 1980)

South

dome threatened the west-northwest flank from February to August 1992, when 2–3 million m3 of block-andash flow deposits were channelled into the uppermost Senowo, Sat, Blongkeng, and Putih Valleys (Figure 16.6). Then, the southwest flank was affected on 22 November 1994, when 3 million m3 block-and-ash flow deposits were channelled as far as 7 km into the Boyong, Bedok, and Bebeng Valleys (Figures 16.1 and 16.2; GVN 1994; Abdurachman, Bourdier, and Voight 2000). Pyroclastic debris were expected on the south flank between the Krasak and Kuning Valleys, as exemplified by the pyroclastic flows that swept down the upper valley of the Boyong in October 1996 and January 1997 (GVN 1996, 1997). However, the latest explosive events shed pyroclastic flows to the west and west-northwest (GVN 1998, 2000, 2001). One of the difficulties of hazard zone mapping is rapid changes in the volcanic morphology, which may happen between the preparation of a given map and the date of the next eruption (see the hazard map of Pinatubo prepared before the June 1991 eruption; Pinatubo Volcano Observatory 1992). For example, the morphology of the channels through which lahars were usually conveyed has changed dramatically since the 1978 hazard map was produced (e.g. the Putih River; Sumaryono 1992). Should a failure occur on the south flank, a large volume of weathered rocks could generate a debris avalanche because the material in the upper

valley of the Gendol has been hydrothermally altered. Hence, debris flows associated with pyroclastic flows and debris avalanches may be channelled in the near future in valleys which have not been lahar-prone over the past thirty years. 4. Although valleys most prone to lahars are identified, the scale used (1:100 000) makes it impossible to map lahar flooding accurately in specific channels.

Hazard and Risk Assessment Based on Extent, Recurrence, and Damage Thouret et al. (2000) identified six main areas affected by past and present hazardous phenomena at Merapi, and ranked them according to their recurrence: 1. An area approximately 3 km2 on the southwest flank is affected almost daily by rockfalls from the dome, to a distance as far as 2 km and as low as 1800 m asl (Figure 16.5). The annual volume of rockfall debris may represent 2–3 per cent of the 11 million m3 of the 1998 dome (Lavigne 1998). 2. The valleys to the west and southwest are frequently swept at two- to four-year intervals by secondary lahars that commonly travel more than 10 km (Figure 16.5). Alluvial plains in this area were submerged by floods following very large eruptions (e.g. 1930–1, 1961, 1969)

Hazards and Risks at Gunung Merapi 283 Table 16.2 Eruptive events and reported damage at Merapi, 1672, and 1822–2001 Date of eruptive event

Type of eruption and secondary debris flows

Life loss

1672a 1822–3a 1832–5a 1837–8 1846–7 1849a

Ex, Ex, Ex, Ex, Ex, Ex,

3000 100 32

1862–4 1871–2a

Ex, PF, LF, D Ex, TF, PF, LF

1887–9 1902–4a 1908–13 1920–1a 1922

Ex, Ex, Ex, Ex, Ex,

1930–1a

Ex,a PF,a LF,a D, ps, Df a

PF, PF, PF, PF, PF, PF,

DF DF, D LF, D LF, D LF, D LF

No. of known affected villages

No. of known syneruptive lahars

Hundreds

200

a

Feb. 1932 1933–5 1939–40 1942–3 1948 1953–4a 1955–8 1961a 1967–8 Jan. 1969a 1970 1972–5 Nov.–Dec. 1976a 1977–9, Jan. 1979 1980–3 13–16 June 1984 1986–91 31 Jan.–14 Mar. 1992 22 Nov.–7 Dec. 1994a 31 Oct. 1996 14–18 Jan. 1997 11–19 July 1998 26 Dec. 2000–10 Feb. 2001a

PF, LF, D D, LF, PF D, LF, PF PF,a D, Df a D, LF

a

Ex, sec. Df Ex, PF, LF, D Ex, PF, D, LF Ex, PF, LF, D Ex, LF, PF Ex, PF,a Ph, LF, D Ex, PF, LF, D Ex, PF,a LF,a Ps, Df a Ex, PF, D Ex, PF,a D, ps, sec. Df a Ex, PF Ex, PF, LF, D, sec. Df a LF, PF, sec. Df a D, DF, PF, sec. DF D, LF, Ex Ex, PF, Ph, D, sec. DF D, Ex, LF Ex, PF, D, DF Ex, PF, ps, DF Ex, PF, D, DF Ex, PF, D Ex, PF, TF, D Ex, PF, TF, LF, D

16 (PF)

3

35 (PF)

1

1369 (PF + DF)

42

(DF)

3 (12 Oct. 1920) — Several tens (19 Dec. 1930, 2 Jan., 27 Apr. 1931) 5 (17 Feb.)

— 64 (PF)

6

6 (PF + DF)

10

3 (PF + DF)

26

9 (DF) 29 (DF)

Several tens Several tens

— — 3 (27–8 Nov.) — 4 (7–8 Jan.) — 2 (25 Nov.) — — —

66 (PF, ps)

Damage to villages and crops

6 missing, several injured

Damage to crops

2 unconfirmed deaths

Damage to crops

—

Notes: Ex = explosions; TF = tephra fall; PF = pyroclastic flows (PF when large-volume > 1 km3); ps = small-volume pyroclastic surge; Df = secondary (= sec.) debris flows (DF when large-scale > 1 million m3); D = summit dome growth; LF = stubby lava flow in summit area; Ph = phreatomagmatic activity. a

Large-scale (> 1 km3 volume) or damaging eruptions.

Sources: After Neumann van Padang (1933); Hartmann (1935); van Bemmelen (1949); Siswowidjoyo, Suryo, and Yokohama (1995); Lavigne (1998); GVN (1998, 2000, 2001). Detailed account of damage to property is given in Lavigne et al. (2000a).

or even moderate-volume eruptions (e.g. 1973– 6). In the Batang River, thirty-three subsequent post-eruptive lahars occurred during the first rainy season following the 1930 eruption (Schmidt 1934). Syneruptive lahars occurred on 19 December 1930 and on 7– 8 January 1969 along nine rivers between the Pabelan and Woro Rivers, the largest travelling on the western flank of the

volcano (Schmidt 1934; Asmanu 1969; Siswowidjoyo 1971; Lavigne et al. 2000a). Between 1822 and 1990 at least twelve out of thirtythree eruptions at Merapi triggered syneruptive lahars (Table 16.2; Lavigne et al. 2000a), which caused death and created havoc in 1849, 1871–3, and 1930–1. Damage caused by seventeen lahars related to the

284 Jean-Claude Thouret and Franck Lavigne

twentieth-century eruptions (Table 16.2) includes at least 100 victims, part or total destruction of about eighty villages and 1500 houses, and the loss of several thousands of hectares of tilled land (Simkin and Siebert 1994). In addition, lahars represent a persistent problem for civil authorities because they can occur without an eruption, as happened in 1975– 6. 3. The 1 km2 upper cone is made of historical and present stubby lava flows. An additional 3 km2 area can be buried by lava-flow-forming eruptions, as happened in 1930–1 (Figure 16.4) and 1975– 6. 4. The western flank has been swept by pyroclastic flows every eight to fifteen years (e.g. 1930, 1969, 1973, 1984, 1994). Two hazard zones could be related to two different types of pyroclastic flow (Figure 16.5): (a) Common, small-volume dome collapse pyroclastic flows (Merapi-type block-and-ash flow) are usually channelled before entering the lower volcano’s flanks. This pyroclastic flow is deadly, as illustrated by the 22 November 1994 eruption (sixty-six victims). The danger is from three phenomena: (i) Gravity-driven dome collapse pyroclastic flows can be channelled as far as 7 km as blockand-ash flows resulting from moderate explosive activity. (ii) Ash cloud surges often overtop block-and-ash flows and can destroy ridge-top villages. The 22 November 1994 pyroclastic surge, whose deposits covered 9.5 km2 (Abdurachman, Bourdier, and Voight 2000), destroyed twenty houses in Turgo village (Figure 16.2). (iii) Gravitydriven dome collapse may occur without being preceded by a seismic or deformation event. Thus, a volcanic disaster can occur at any time in the Merapi area. (b) The less common, but more mobile, scoria flows (St. Vincent-type) sweep all the flanks of Merapi and become channelled in the drainage system as far as 8–11 km (1872) and 10–15 km (1930–1) down-valley. Such scoria flows have caused most of the casualties and damage resulting from Merapi eruptions (Table 16.2), e.g. 3000 killed in 1672 and 1369 in 1930 (Thouret et al. 2000). 5. Areas within a radius of 6–15 km from the vent are covered at least once every twenty-six to fifty-four years by tephra fall, as inferred from the distribution of tephra fall deposits in 1969 (Siswowidjoyo 1971) and wind directions and velocities at 3300 m asl (16– 32 m s−1 from the west and southwest during the rainy season, and 16–24 m s−1 from the east or southeast during the dry season). 6. An area ranging from 10 to 60 km2 can be devastated by small-scale pyroclastic surges travelling as far as 9 km from the vent at approximately a twenty-fiveyear interval, as suggested by the 1930, 1969, and

1994 events (Figure 16.5). Ash cloud surges commonly leave a thin ash deposit that mantles an area exceeding by far that of the companion pyroclastic flow deposits (e.g. in 1930–1). However, Newhall et al. (2000) state that high-energy pyroclastic surges or blasts occurred on all flanks of the volcano well over 20 km from the summit (e.g. beyond Sleman, on the south-southwest flank; Figure 16.1) during the large explosive eruptions of New Merapi (between ad 0 and 1000). Therefore, a zone of potential devastation needs to be shown as far as 20 km towards the south to account for high-energy pyroclastic surges or blasts (Figure 16.5). Additional hazard zones might be outlined, although the nature, extent, and effects of the responsible eruptive phenomena are still controversial among the research teams (Camus et al. 2000; Newhall et al. 2000). To the west an extensive area of approximately ≥ 200 km2 and as far as 20 km distant from the vent was probably buried once in the past 7000 years by a debris avalanche that resulted from flank failure (Figure 16.1). Another extensive area is thought to have been devastated once by a lateral blast during the past 7000 years. Finally, although there is no published account of the effect of volcanic gases and volcanic earthquakes in the past, they cannot be ruled out, as suggested by the 1979 gas eruption at the Dieng Plateau (Zen 1983).

Hazard-Zone Mapping Based on Eruption Scenarios Thouret et al. (2000) selected four scenarios based on hazard types and zones, descriptions of recent eruptions, and eruption magnitudes recorded in the past centuries (Figures 16.7 and 16.8). 1. Small eruptions (Volcanic Explosive Index (VEI) 2, volume of tephra 1– 4 million m3), termed the most common Merapi-type scenario, occurred in 1984, 1992, 1994, 1996, 1997, 1998, and 2000–1 (VS1– MVO 1989; VSI 1990; Simkin and Siebert 1994; GVN 1994, 1996, 1997, 1998, 2000, 2001). 2. Moderate eruptions (VEI 2–3, volume of tephra ≥ 4 million m3) such as the 1953– 4 and 1969 events, represent the second most likely scenario to be expected at Merapi at a twenty- to fifty-year interval. 3. Large eruptions (VEI 3– 4, volume of tephra 4 × 106–1 × 107 m3) comparable to the 1930–1 and 1872 events, can be expected once or twice a century. 4. Catastrophic eruptions (VEI 4–6, volume of tephra > 107 m3) are unknown through the reliable historical record since 1672, but probably occurred in prehistoric time.

Hazards and Risks at Gunung Merapi 285 110°20' E

7°30' S

110°30' E

110°25' E

7°30' S

Trising an

bel

Pa

MERAPI

▲

2000

m

t

ma

La

Muntilan

00

0m

K. Woro

6 km

dol K. Gen

K. B

4

K. Kuning

2

pak

ak

ras

K K.

0

Kaliurang

K. O

ng

K.

80

te Be

oyo

0m

40

K.

7°35' S 0m

ng

ng

ta Ba

m

10

60

7°35' S

utih

K. P

Scale

7°40' S

7°40' S

1

2 Area affected by lava dome rocksliding and non-explosive avalanching

5

3 Area affected by frequent small- to moderate-scale eruptive dome avalanching, blockand-ash flows, and companion ash cloud surges

6 Area of the tephra fall enclosed in the 1 cm isopleth

110°30' E

110°25' E

110°20' E

4 Area affected by moderately large, less common, and more common pyroclastic flows and surges

7 Valleys likely to be swept off by debris flows in case of largevolume pyroclastic flows

Channels that usually convey small to moderate-scale, mostly rain-triggered lahar event

8 Area that could be swept off by voluminous pyroclastic flows from high eruptive columns, that would overspill the Pasarbubar rim

Approximate boundary of the populated zones at risk in case of a violent ‘mixed’ eruption

Fig. 16.7. Hazard zone map for the Merapi-type eruption scenario based on the 1961–98 scenario (boxes 1– 4). Also shown are the approximate hazard zones in the event of the mixed effusive-Peléean eruption scenario based on the 1930–1 events (boxes 5 –8)

The eruption scenarios are portrayed on two hazard zone maps (Figures 16.7 and 16.8). We shall describe the four scenarios as follows: 1. The Merapi-type scenario of dome growth encompasses two types of eruptive activity (Figure 16.7). First,

a quasi-continuous eruptive activity involves two processes: long-lasting periods of viscous lava activity that build domes and stubby lava flows, accompanied by rockfall from the unstable summit dome, which affect only the upper 2–3 km2. A dense network of channels removes the slope deposits and fosters small-volume,

286 Jean-Claude Thouret and Franck Lavigne 110°10' E

110°20'

110°30'

110°40' E BORNEO

400 km BALI

JAVA

0

1200

SALATIGA 80

SECANG

00

after Hartmann 1935

7°25' S

0

Mt MERBABU

60

MAGELANG

SIMA

500 300

40

SELO

200

7°25' S

10

Progo river

SUMATRA

0

BOYOLALI

▲

KARASURA Feleng

7°35'

7°35'

Borobudur Krasak

800

er

riv

Sambisari

o 200

Kuning

400 00 3

Prog

600

ol Gend

ISLEMAN

7°45'

KLATEN Prambanan

Wor o 7°45'

YOGYAKARTA 200

Code

SONTOLO

600 400

BANTUL

0

7°55' S JAVA SEA

110°10' E

1

5

10

15 km

7°55' S

scale 110°20'

Elliptical area enclosed by the 1 cm isopleth, likely to be covered by thick tephra fall from a Plinian eruptive column; smaller circle of 5 km radius = area covered by ballistic ejecta

110°30'

2

110°40' E

3 Summit area prone to lava flowage and dome avalanching

Upper flanks of the volcano that can be swept off by large-volume pyroclastic flows channellized down-valley

5

4

The two arrows indicate the prevailing seasonal winds

Valleys likely to convey largescale debris flows

6

7 Valleys conveying large-scale lahars in case of large-volume pyroclastic flows from high eruptive columns

10

9 Alluvial piedmonts that could be flooded in case of very large-scale debris avalanches and debris flows

Volcano’s flanks that could be devastated by high-energy pyroclastic surges or ‘blast’

8

Maximum reach of the potential flank failure (based on the old Merapi collapse) that would trigger large-volume debris avalanches

Coastal lowlands that could be flooded in case of very large-scale lahars

Fig. 16.8. Hazard zone map for the Plinian eruption scenario (based on the 1872 eruption: boxes 1–5) and for the worstcase eruption scenario based on historical or new Merapi eruptions (boxes 6–10)

Hazards and Risks at Gunung Merapi 287

rain-triggered lahars every two years on average (Lavigne et al. 2000a). Secondly, the typical Merapi-type scenario involves dome collapse, as shown by the fact that historical eruptions on Merapi were usually acompanied by more pyroclastic flows than on any other volcano in the world. The scenario includes a spectrum of gravity-driven pyroclastic flows, i.e. small-volume block-and-ash flows (≤ 3 million m3 of deposit) on a two-year basis (e.g. 1992, 1994, 1996, 1998, and 2000–1), and moderatesized block-and-ash flows (≥ 3 million m3) every eight to fifteen years on average (e.g. 1969, 1984). At the same time ash cloud surges mantle slopes (e.g. 1984) and occasionally destroy the forest in nearby radial valleys (e.g. Bebeng in 1969, Boyong in 1994). Over one to five years after a given eruption, pyroclastic debris are remobilized into small to moderate-volume debris flows by heavy rains from November to March. Numerical simulation helps to explain gravity-driven pyroclastic flows. Since 1992 the summit dome has increased in volume (11 million m3 in 1995) and become prone to failure. A question that arises is whether the frequent dome collapses (1992, 1994, 1996, 1997, 1998, and 2000–1) counterbalance the dome growth as magma is stored high in the summit cone. Kelfoun (1999) has simulated a partial collapse of the summit dome involving block-and-ash flows (Figure 16.9) and compared its run-out distance with that of similar flows at Unzen (Figure 16.10), after Yamamoto Takarada, and Suto (1993). Even though the delineation is rough, the distance dome debris is expected to flow could be shown using the energy line principle (Malin and Sheridan 1982) and a conservative ratio of height versus run-out length of 0.33 (exceeding the average 0.22 ratio computed by Hayashi and Self 1992). Travel distance of the dome collapse flow is 5 km. Figure 16.9 shows the area on the west flank likely to be buried by as much as 3 million m3 of debris at a rate of 15 000 m3 a minute: this represents about 90 per cent of the active part of the dome (3.3 million m3) and about 33 per cent of the total dome volume (11 million m3 in 1995) (Ratdomopurbo 1995; Young et al. 2000). 2. The second eruption scenario, termed mixed effusive–Peléean, is based on the large 1930–1 eruption (Figure 16.7; Neumann van Padang 1933). First, hazards are related to repeated dome collapse pyroclastic flows and to lava flows (e.g. the area ≥ 1 km2 buried by lava flows on the west-northwest flank in 1930 and 1931). Secondly, the mixed eruption scenario also leads to two additional hazards. Relatively severe hazards of tephra fall can affect all the densely populated areas on Merapi’s flanks, as shown by the tephra fall

on Yogyakarta in December 1930 and the large-scale lahars (several million m3) which will be triggered during and after the eruption, as in 1931. Severer hazards are expected from Peléean-type eruptions, i.e. voluminous pyroclastic flows and high-energy surges resulting from the explosive destruction of a large part of the summit dome. Based on the events of December 1930 and January 1931, voluminous pyroclastic flows resulting from the collapse of large domes and over-steepened lava flows can travel as far as 10–15 km in radial valleys that drain the west and south flanks of Merapi (Figure 16.7). High-energy surges can devastate a fanshaped area in the order of 102 km2 to the west and southwest. This is based on our interpretation of the account from Neumann van Padang (1933) of the deadly ‘ash-laden surge’ of 18–19 December 1930, which apparently decoupled from the pyroclastic flow, deposited 30– 40 cm of ash, and devastated a fan-shaped area of 25 km2 to the west. The angle for the open sector affected by high-energy surges or blasts, with respect to the summit dome, can vary (Figure 16.7) from as little as 15° (1994) to as much as 60° (1969). 3. The third eruption scenario, termed sub-Plinian and based on the large April 1872 eruption (Figure 16.8; Hartmann 1935), can recur at least once every century. Eruptive columns 6–14 km in height can emplace thick tephra fall deposits on the whole area, including Merapi and the plain of Yogyakarta. On 16–17 April 1872 tephra fell mostly to the east of Merapi (blocks at 5 km and lapilli at 10–16 km of the vent), while fine ash reached more than 400 km to the northeast. In addition to the geological data, pre-existing models help to forecast the areas affected by tephra fall. Thouret et al. (2000) determined isopachs for tephra fallout based on estimates of the height of the eruptive column (km), the average magma discharge (kg s−1), the eruption duration (minutes), and the thickness of the tephra fall deposits near the crater, derived from the 1969 isopach map of Siswowidjoyo (1971). Isopleths for tephra fallout are derived from studies by Carey and Sparks (1986), based on a 10–14 km high eruptive column and a wind velocity of 16–32 m s−1. The 1 cm isopleth is assumed to outline the most hazardous zone. Figure 16.8 shows the 1 cm isopleth zone for tephra falling out from a 14 km high eruptive column, which corresponds to a sub-Plinian eruption with relatively moderate magma volume (about 106 –7 m3) and discharge (about 103– 4 kg s−1). As the data from the Meteorological and Geophysical Centre, Yogyakarta Airport, indicate, tephra distribution at Merapi depends on two opposite meteorological situations. During the dry season (April– October) winds blow from east-southeast,

288 Jean-Claude Thouret and Franck Lavigne

N summit dome area covered by a simulated dome collapse

ta n Ba

iv gr

er

d ere d v o ac late Are simu pse a by e colla dom

1994 block-and-ash-flow deposits 1994 surge deposits

iv e

er

r

ri

ing

v

K un

B oyong r

ak ri v e r K r as

adjacent areas likely to be affected by the potential dome collapse of 3 million m3 volume

r

e do

B

Turgo

Boundary of the area effected by the 1994 pyroclastic surge

0

ive r

k

2

Kaliurang

4

6 km

Fig. 16.9. Part of an ortho-image which covers a digital elevation model (based on a stereo pair of SPOT satellite images) illustrating the simulation of gravity-driven pyroclastic currents The white zone depicts the area likely to be buried if a 2 million m3 volume of pyroclastic debris collapsed from the summit dome and was shed on the southwest flank of Merapi. The light dashed line indicates the boundary of the 1994 pyroclastic flow deposits, unconfined on the upper flank but channelled down-valley in the Bedok and Boyong Rivers. The heavy line outlines the boundary of the area devastated in part or totally by the 1994 pyroclastic surge. The heavy dashed line indicates the boundary of adjacent areas likely to be affected by the potential dome collapse of 3 million m3 volume, i.e. about 90 per cent of the active part of Merapi’s dome and about 33 per cent of the total volume of the dome. (Source: Kelfoun 1999)

whereas during the wet season (November–March) north-northwest winds prevail. Wind velocity remains constant at 3300 m asl (average 16–24 m s−1), but increases during the dry season at higher altitude (19– 40 m s−1 at 5000 m asl), instead of 16–32 m s−1 during the wet season. Tephra fallout from a relatively high

column occurring during the dry season is likely to reach the Yogyakarta plain. Two areas are likely to be affected by tephra fallout. The first (Figure 16.8) has a northwest–southeast axis, according to that of the tephra fallout from the moderate 7– 8 January 1969 eruption (Siswowidjoyo 1971). The second area (Figure 16.8),

Hazards and Risks at Gunung Merapi 289 10 Merapi, 1994 pyroclastic flows

✦

✦

Unzen, 1991 pyroclastic flows

Merapi 1930: pyroclastic flows

Small-volume pyroclastic flow Large-volume pyroclastic flow

0. 02

0.1

0. 1

Unzen, 1991 debris flows

0. 5

Height (km)

1

Debris flow BLACK SYMBOLS:

✦

1

10

MERAPI

100

Length (km) Fig. 16.10. Plot of height (vertical drop) and length (run-out distance) for the simulated dome collapse, the longest 1930 pyroclastic flow, and the 1994 channelled pyroclastic flows at Merapi For purposes of comparison, large- and small-volume pyroclastic flows, debris flows, and cold rock avalanches at Unzen are also shown (Source: after Yamamoto, Takarada, and Suto 1993, modified)

based on a sub-Plinian eruption scenario, can be much wider and roughly circular, although one of the prevailing wind directions (shown by two arrows in Figure 16.8) can elongate the shape. In 1872 pyroclastic flows were channelled into all the radial valleys from the Senowo–Trising Rivers (on the northwest flank) to the Gendol–Woro Rivers (on the south flank), except the northeastern and eastern valleys. However, pyroclastic flows can gain enough momentum from the collapse of ≥ 14 km high eruptive columns to overrun the somma rim. They can also sweep the north and east flanks of Merapi, and travel as far as 15 km away from the vent in all directions (Figure 16.8). The scenario may also involve large explosive eruptions similar to the historic–prehistoric eruptive episodes. The thick Gumuk and Sambisari ash fall and surge deposits buried temples between 2000 and 1000 years bp and c.600– 500 years bp (Berthommier 1990; Camus et al. 2000; Newhall et al. 2000). The pyroclastic surges can devastate Merapi’s flanks as far as 20 km away from the vent on all sides of the volcano. 4. The worst-case scenario, perhaps as scarce as once in 7000 years, encompasses two eruptive events (Figure 16.8). First, a flank failure of the south and southwest flanks and a collapse of the whole summit dome could induce a voluminous debris avalanche and subsequently trigger large debris flows. Flooding down-

valley could extend to the lowest parts of the Progo and the Code –Kuning– Gendol catchments (Figure 16.8). Secondly, according to the interpretation given by Berthommier (1990) and Berthommier et al. (1992), a blast surge may generate a high-energy laterally directed explosion of the Bezymianny-type event (Siebert 1984). The maximum 100° angle for the open sector could parallel the shape of the avalanche caldera (Figures 16.1 and 16.2), although areas likely to be devastated cannot be outlined on a reliable basis.

Risk Assessment and Mapping We assessed and mapped volcanic risk at Merapi at two scales (Lavigne 1998): (1) a small-scale risk zonation using GIS for the whole volcano; (2) a large-scale micro-zonation focused on lahar hazard, notably within Yogyakarta city (Lavigne 2000).

Small-Scale Risk Zonation with Vector-Mode GIS and Vulnerability Estimation This method is based on a topologic and vector-mode GIS. Risk zonation consists of four steps: (1) hazard mapping from four eruption scenarios recognized in the past at Merapi (Figures 16.7 and 16.8); (2) semi-quantitative appraisal of population vulnerability; (3) assessment of properties for estimating the amount of potential loss for each scenario; (4) computerized drawing of volcanic risk maps. Vulnerability is a complex concept, commonly studied quantitatively. It is usually defined as the degree of loss to a given element at risk, expressed on a scale from 0 (no damage) to 1 (total loss) (UNDRO 1979; Smith 1992). However, a more recent approach also estimates the social and cultural factors which reduce or amplify the effects of a natural phenomenon (Drabek 1986; Thouret and d’Ercole 1996). At Merapi, we identified factors of vulnerability through a data base (Lavigne 1998), created by enquiries carried out in the 296 villages (desa) of the volcano slopes (> 200 m asl). To assess people’s vulnerability, we ranked and weighted fourteen parameters (Table 16.3) through published case studies, taking into account the Javanese way of life (Lavigne 1998). After computing the fourteen parameters, we ranked the vulnerability of each village in five classes, from very low (< 180) to very high (> 240). We also assessed potential loss of property and real estates, according to the average value of the elements at risk in 1995. The localities, when mapped for population vulnerability (Figure 16.11), reveal differences among Merapi’s flanks. People most vulnerable to volcanic hazards are living within three areas: (1) remote villages

290 Jean-Claude Thouret and Franck Lavigne Table 16.3 Vulnerability parameters for the people living on the Merapi slopes Factor type

Vulnerability parameter

Coefficient

Classes

Level of vulnerability

Demographic and social factors that increase vulnerability

Population density (per km2)

10

> 4000 2000–4000 1000–2000 500–1000 < 500

5 4 3 2 1

Technical and functional factors that reduce vulnerability

Percentage of young people < 15 years old

7

> 50 40–50 30–40 25–30 < 25

5 4 3 2 1

Percentage of people > 50 years old

7

< 25 20–25 15–20 10–15 < 10

5 4 3 2 1

Percentage of illiterate people

1

> 40 30–40 20–30 10–20 < 10

5 4 3 2 1

Telephones (per 1000 inhabitants)

2

> 20 20–5 1–5 0–1 0

1 2 3 4 5

Buses and lorries > 10 passengers (per 10 000 inhabitants)

7

> 50 20–50 10–20 0–10 0

1 2 3 4 5

Minibus and cars < 10 passengers (per 10 000 inhabitants)

5

> 50 20–50 10–20 0–10 0

1 2 3 4 5

Asphalt roads (length per village, km per km2)

4

>2 1–2 0.5–1 0–0.5 0

1 2 3 4 5

Schools (per 1000 inhabitants)

1

>4 3–4 2–3 1–2 80 60–80 40–60 20–40 < 20

1 2 3 4 5

Hospitals (per 10 000 inhabitants)

6

>3 2–3 1–2 0–1 0

1 2 3 4 5

Hazards and Risks at Gunung Merapi 291 Table 16.3 Continued Factor type

Vulnerability parameter

Coefficient

Classes

Level of vulnerability

Clinics (per 10 000 inhabitants)

4

>5 3–5 1–3 0–1 0

1 2 3 4 5

First aid centres (per 10 000 inhabitants)

2

>4 2–4 1–2 0–1 0

1 2 3 4 5

Doctors (per 10 000 inhabitants)

3

> 20 10–20 5–10 0–5 0

1 2 3 4 5

Source: After Lavigne (1998).

Fig. 16.11. Population vulnerability zonation on the Merapi flanks, based on fourteen weighted parameters

on the upper slopes (> 600 m), where population densities remain high (typically > 500 inhabitants per km2) and bamboo houses, which are easily burned by pyroclastic flows, ash falls, or hot lahars, are common; (2) villages on the southeastern flank with large families and

a high illiteracy rate; (3) along the Progo River, on the southwestern flank, where a very high population density (> 1000 inhabitants per km2) combines with dirt roads and lack of adequate transportation, rendering evacuation difficult.

292 Jean-Claude Thouret and Franck Lavigne Table 16.4 Estimate of potential damage for four eruption scenarios of the Merapi volcano Eruption scenario

Recurrence (years)

Magnitude (× 106 m3 of deposits)

Hazards

Estimated potential damage People loss (no.)

Dome collapse

2–8

< 10

Peléean eruption

25–50

10–100

Plinian eruption

100–200

100–1000

Cataclysmal eruption

2000–3000

> 1000

Pyroclastic flows (Merapi-type nuées ardentes), lahars Pyroclastic flows (Peléean-type nuées ardentes), lahars Plinian column collapse (St Vincent-type nuées ardentes), ash fall, lahars Blast and/or debris avalanche, Plinian column collapse (St Vincent-type nuées ardentes), ash fall, lahars

Financial loss ($US million)

342 000

100

430 000

300

950 000

c.3000

1 000 000

> 3000

Source: After Lavigne (1998).

In contrast, people are less vulnerable on the southern flank of Merapi, because the transport infrastructure is better and there are many medical centres. However, potential loss of property and tilled land would be higher on Merapi’s southern flank than elsewhere, as this area has been extensively built up and supports the essential goods and infrastructure for the region, estimated to be in the range of $US20– 30 million. Risk relates to the expected number of lives lost, people injured, damage to property, and disruption of economic activity due to a natural phenomenon (UNDRO 1979; Nossin and Javelosa 1996; Slaymaker 1996; Blaikie et al. 1997). Lavigne (1998) used a topologic and vector-mode GIS to draw computerized (function topologic crossing) volcanic risk maps, resulting from the combination of the hazard zone maps and the vulnerability maps for the four eruptive scenarios of Merapi (Figures 16.7 and 16.8). GIS is a powerful tool for risk zonation because it allows (1) integration of a large amount of data from several sources, (2) regular updating of data, (3) modulation and simulation of different scenarios for prevention planning management, and (4) automatic interrogation of the database. On the south, southwest, and west flanks of Merapi, a small or medium-size dome collapse threatens at least 342 000 people; over 430 000 in case of a Peléean eruption (Table 16.4, Figure 16.12a,b). On the upper slopes of the volcano, about 80 000 people are still living in the forbidden zone delineated by the VSI (Figure 16.5). The value of property and cultivated land likely to be affected by small or medium-sized pyroclastic

flows, which is estimated to be $US100 million, rises to three times that figure for large Peléean-eruptionrelated pyroclastic flow hazards (15 km in length; Table 16.4). About 950 000 people (in 1995) were directly threatened by pyroclastic flows, lahars, and ash falls in the event of a Plinian eruption; over 1 million people, should a voluminous debris avalanche (and/or a blast) of a cataclysmic eruption occur on the south and southwest flanks of Merapi (Table 16.4; Figure 16.12c,d ). Such severe eruptions would be disastrous for two main reasons: Lack of transport. In case of a Plinian eruption, evacuation of the population living within the hazardous area could not be achieved within a few hours or even within a day. Only 80 000 people (8.5 per cent of the population at risk) could be rapidly evacuated using the 1450 local buses and lorries and the 2750 minibuses and cars that are available. The southwest flank, which represents the most hazardous area, is the area where lorries and buses are fewest. Inadequate location of the evacuation centres. Evacuation orders apply only within the hazardous zone delimited by the VSI (Figure 16.5), which does not consider a Plinian eruption scenario. Therefore, even if an evacuation order were given in time, people would take refuge on the lower slopes of the volcano, which are also threatened in a severe eruption. Direct losses in the event of total destruction of property on the volcano, without considering secondary damage from lahars and floods, would amount to $US3 billion (Table 16.4; Figure 16.8).

Fig. 16.12. Risk zonation for people living on the Merapi slopes, based on four eruptive scenarios (Source: after Lavigne 1998)

294 Jean-Claude Thouret and Franck Lavigne

Fig. 16.13. Flowchart of vulnerability assessment study within lahar-related hazard zones (Source: after Lavigne 2000)

Large-Scale Micro-Zonation Focused on Lahars and Risk Assessment The first method deals with lahar. Until 1995 no accurate hazard map was available for lahar and flooding around the Merapi volcano, even for Yogyakarta, and the existing maps were too small-scale to outline the lahar hazard zones with accuracy. The previous warning system therefore relied on assumptions and past experience (e.g. lahars which occurred in the Code River on 8 January 1969). Since 1995 six detailed maps for hazard zones prone to floods and lahars have been available at a 1:10 000 scale for the rural areas: Boyong Valley, which was affected in 1994, and the Bebeng–Krasak Valleys (Lavigne, Thouret, and Bacharudin 1995a,b), and at a more detailed scale (1:2000) for the lower Code Valley, across the suburbs of Yogyakarta city (Lavigne, Thouret, and Bacharudin 1995c). The mapping of potentially affected areas (Figure 16.13) was based on geomorphological investigation, lahar flow simulation, and field enquiries (Lavigne 2000). The elements at risk in thirty suburbs along the Code River in Yogyakarta city were assessed in order to complete the hazard zonation, using a set of SPOT ortho-images (1987) at a scale of 1:2000 and from field enquiries (Fig. 16.14). However, indirect and long-term losses produced by the protracted disruption of economic activity and/or cost of reconstruction were not assessed.

Field interviews provided information on the three groups of factors (Table 16.5) that render people and property vulnerable (Lavigne 1998). 1. The main factor is population density (7000 inhabitants per km2 in the Code Valley, 2000 in the Boyong Valley, and 1000 in the Krasak Valley) and growth. In the Boyong Valley, people at risk from potential lahar paths did not live in areas of hazard level 1 and 2, but 3250 people lived in area of hazard level 3 in 1995, and as many as 10 560 people in area of hazard level 4. In addition, 16 750 people lived in the lowest part of the Code Valley, within area 4. However, the map should be revised periodically because of the population growth of about 0.5 per cent a year in the specific area most prone to lahars, and as much as 3 per cent in the whole Merapi area. Migration to nearby zones or outside the flanks of Merapi, although encouraged by the civil authorities, is very limited, and people who migrate commonly settle in urban areas also prone to lahars. About 13 000 people live at risk in the suburbs of Yogyakarta along the Code River with the population density exceeding 5600 inhabitants per km2. Population growth amounts to 2 per cent a year, partly because of urban migration from the Merapi countryside. The districts at highest risk are in the city centre, part of which is threatened by small-scale floods or lahars (300 m3 s−1). In these districts, population density is at its highest (43 000 inhabitants per km2 in Prawirodirjan;

Fig. 16.14. Example of hazard and risk assessment in Prawirodirjan, a suburb of Yogyakarta city Satellite SPOT ortho-image, 1987 (A), sketch maps showing population density (B), micro-zonation of lahar and flood hazard (C), and risk (D) (Source: after Lavigne 1998)

Table 16.5 Nomenclature and classification of vulnerability types, based on thirty-eight interviews in the lahar-prone areas of the Boyong, Code, and Krasak Rivers Valley and lahar-prone area

Boyong

Level of hazard Lahar maximum discharge (m3/s) Area (ha)

1 < 200 1

2 200–300 18

3 300–500 240

4 500–700 480

1 < 200 7

2 200–300 38

3 300–500 144

4 500–700 674

1 < 1000 36

2 1000–1500 172

3 1500–2000 326

4 2000–2500 581

0 0

31 10 563 2.4 1977

36 12 462 2.5 2019

4

12

23

0

8 3252 2.2 1600

0 0 0 0

555 136 350 1041

2142 391 484 3017

0 0 0 0

2 9 1 7

0 0 0 0 0 0 0 0

Physical vulnerability of people Threatened villages Population (1995) Growth rate (1990–5) (%) Density (people per km2)a Technical vulnerability Houses Stone Wood Bamboo Total Public buildings Schools Mosques Prayer houses Stores Infrastructure, equipment Market Warehouses Economic vulnerability Asphalt roads Bridges Rice fields Tilled land (ha), dry fields Total Approx. cost of likely lossb ($US million) a b

—

Code

— — —

— — —

— — —

30 12 828 10.7 5685

4 1142 15.8 1734

23 7494 8.9 1146

37 11 960 6.4 970

47 16 905 5.2 1032

2409 475 597 3481

— — — —

— — — —

— — — —

3110 55 40 3205

252 0 21 273

1170 278 379 1827

1839 291 681 2811

2700 319 973 3992

13 26 15 23

21 29 26 30

— — — —

— — — —

— — — —

27 23 19 65

1 1 4 2

7 18 23 21

14 26 38 57

24 36 47 138

1 0

1 1

2 1

— —

— —

— —

4 59

0 1

0 9

1 14

3 21

2 2 128 16 144 10

5 29 328 73 401 36

6 32 384 77 461 42

— — — — — —

— — — — — —

— — — — — —

23 28 87 0.5 87.5 52

2 11 34 11 45 0.5

15 29 422 33 455 23

27 56 698 155 853 36

37 67 929 213 1142 51

Population density of the whole threatened villages, not especially within the lahar-prone areas. Based on the average value per unit of each element at risk.

Source: After Lavigne (1998).

Krasak

Hazards and Risks at Gunung Merapi 297

15 000 inhabitants per km2 in Ngupasan); population growth has been between 2 and 3 per cent per year (1990–5), and civil defence protective measures are absent or poor (Figure 16.14). 2. Technical factors that increase or decrease the vulnerability of people and property include the location and quality of construction within lahar-prone areas (Table 16.5). Additional information covered local disaster relief organizations and specific counter-measures for civil defence at the village scale (observation posts, transportation, hospitals, and first aid stations in each village or district). 3. Factors that render the economy vulnerable include the infrastructure, equipment, facilities, and businesses in the threatened area, including shops, markets, warehouses, roads, bridges, and tilled land (Table 16.5). This allowed an estimate of the value of potential loss and short-term consequences on the economy of the region (Lavigne 2000). Hence, all elements at risk in thirty suburbs along the Code River were assessed using field enquiries, which helped to complete the large-scale micro-zonation in the city of Yogyakarta. The information on the factors of vulnerability were included in a database that can be used by local urban planners.

Conclusion: Hazard and Risk Assessment, a Challenge at Merapi Volcano A revised assessment of volcanic hazards and risks at Merapi requires at least the following: 1. Appraisal of the recent eruptive behaviour of Merapi in order to solve the discrepancies that arise from studies of eruptive deposits. Our understanding of its past eruptive behaviour remains incomplete as it lacks precise and full information on the extent, behaviour, and characteristics of past eruption deposits. 2. Determination of the likely eruption scenarios, and locating and measuring the weak structural features of the cone using remote sensing and ground monitoring. 3. Mapping areas prone to pyroclastic and debris flows by numerical modelling based on a digital elevation model and 2D ortho-image, and supported by field measurements. However, current use of models is restricted by our fragmentary knowledge of the eruption parameters, flow behaviour, and triggering processes. Furthermore, geophysical constraints on the evolution of dome-building, the magma system, and deformation of the summit area are still not completely understood, despite significant progress in monitoring (Young et al. 2000).

4. Incorporation of factors of vulnerability of people and properties at risk, in order to achieve micro-zonation of pyroclastic-flow and lahar-prone areas. A risk map must consider the demographic, economic, and technical factors that render people and properties vulnerable (d’Ercole 1991; Blong 1996; Thouret and d’Ercole 1996). The tasks require a holistic approach and the joint efforts of earth and social scientists. A few examples are available from studies of the response of people coping with earthquakes in California (Mileti 1993) and with eruptions of the Pinatubo and Taal volcanoes in the Philippines (Punongbayan and Tuñgol 1994). Volcanic hazards and risks at Merapi are among the highest in the world owing to the persistent eruptive activity, the very high population density (over 1400 inhabitants per km2 on the west and south flanks of Merapi), and the growth rate of the population at risk, which is as much as 3 per cent per year on the volcano’s flanks (Lavigne et al. 2000a). In order to minimize volcanic and lahar-related hazards and risks, the government has provided three counter-measures: Construction and development of Sabo dam structures (Suryo and Dahlan 1987; Sukardi 1988). The Ministry of Public Works started the Mount Merapi Project in 1969, with technical assistance from the Japan International Cooperation Agency from 1977. The project is designed to protect and shelter inhabitants and food production areas from disaster caused by lahars; to investigate, survey, design, and implement the Sabo structures; and to repair and rehabilitate the irrigation structures damaged by lahars (Mount Merapi Project 1980). Improvement of the warning system (Ministry of Construction of Japan 1985; Harjosuwarno and Sakatani 1993). Significant progress has been made over the past ten years on understanding lahars, mainly through the surveillance network of the Balai Sungai dan Sabo (formerly known as the Sabo Technical Centre). Rainfall and lahar sensors, video cameras, and additional sensors have been installed (Lavigne et al. 2000b). A hightechnology warning system for volcanic eruptions and lahars has gradually replaced the traditional kentongan (log drum) system (for example sirens were installed in Kaliurang and other villages during summer 1995). Interestingly, the Merapi Volcano Observatory, the local branch of the VSI, is officially responsible for the warning system in the event of ‘primary’ lahars and pyroclastic flows, whereas the responsibility for rain-triggered lahar remains with the Balai Sungai dan Sabo. Community preparedness and the dissemination of knowledge of volcanic and lahar-related disasters (Whitcomb 1988; Harjosuwarno and Koga 1988; Sawai, Sairin,

298 Jean-Claude Thouret and Franck Lavigne

and Pangesti 1994). This task, as well as evacuation, is the responsibility of the Indonesian organization Satuan Koordinasi Pelaksanaan Penanggulangan Bencana Alam (Coordination Centre for Natural Hazard Mitigation). In sum, eruptive hazard assessment at Merapi is based on a reconstruction of its eruption history and behaviour, and on scenarios combined with existing models and preliminary numerical modelling. Hazard and risk assessment is also based on the extent, recurrence, and damage that eruptive phenomena and lahars have generated at Merapi over the past two centuries. We have assessed and mapped the volcanic risk at Merapi on two scales (Lavigne 1998): (1) a small-scale risk zonation using GIS for the whole volcano; (2) a large-scale micro-zonation focused on lahar hazard, notably within Yogyakarta city. Additional interviews in the field provided information on the three groups of factors, namely population density, technical factors, and potential loss and short-term consequences for the economy of the region, that render people and property vulnerable.

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Ph.D. thesis, Université Joseph Fournier, Institut de Géographie Alpine, Grenoble. Drabek, T. E. (1986), Human Response to Disaster: An Inventory of Sociological Finding (New York: Springer). Global Volcanism Network (1994), ‘Merapi’, Bulletin of the Global Volcanism Network (Smithsonian Institution, Washington), 19/10: 2, 3, 11. —— (l996), ‘Merapi’, Bulletin of the Global Volcanism Network (Smithsonian Institution, Washington), 21/10: 7. —— (1997), ‘Merapi 1997 Eruption’, Bulletin of the Global Volcanism Network (Smithsonian Institution, Washington), 22/6: 8. —— (1998), ‘Merapi (Indonesia): Increasing Activity Culminates in Mid-July Pyroclastic Flows’, Bulletin of the Global Volcanism Network (Smithsonian Institution, Washington), 23/7: 2–3. —— (2000), ‘Merapi (Indonesia): Dome Failure and Growth during January 2001; Over 30 Pyroclastic Flows’, Bulletin of the Global Volcanism Network (Smithsonian Institution, Washington), 25/12: 14. —— (2001), ‘Merapi (Indonesia): Failure of 1998 Lava Dome on 10 February Causes Major Eruption’, Bulletin of the Global Volcanism Network (Smithsonian Institution, Washington), 26/1: 2. Harjosuwarno, S., and Koga, S. (1988), ‘How to Propagate Knowledge of Mudflow Disaster’, Proceedings: Debris Flow Forecasting and Warning System at Mt Merapi (Yogyakarta: Volcanic Sabo Technical Centre). —— and Sakatani, Y. (1993), ‘Present Condition of Mudflow Forecasting and Warning System in the Area of Mt Merapi, Central Java’, in Proceedings of the Seminar on Sabo Engineering (Yogyakarta: Volcanic Sabo Technical Centre), 1–17. Hartmann, M. A. (1935), ‘Die Ausbrüche des G. Merapi (Mittel Java) bis zum jahre 1883’, Mineralogie, Geologie und Paläontologie, 75/B: 127–62. Hayashi, J. N., and Self, S. (1992), ‘A Comparison of Pyroclastic Flow and Debris Avalanche Mobility’, Journal of Geophysical Research, 97/B6: 9063–71. JICA ( Japan International Cooperation Agency) (1980), Master Plan for Land Conservation and Volcanic Debris Control in the Area of Mt Merapi ( Jakarta). Kelfoun, K. (1999), ‘Processus de Croissance et de Déstabilisation des Dômes de Lave du Volcan Merapi ( Java Central, Indonésie): Modélisations numériques des dômes, dynamique des écoulements pyroclastiques associés et surveillance par stéréo-photogrammétrie’, Ph.D. thesis, Université Blaise Pascal, Clermont-Ferrand. Lavigne, F. (1998), ‘Les lahars du volcan Merapi, Java central: Dynamique, budget sédimentaire et risques associés’, Ph.D. thesis, Université Blaise Pascal, Clermont-Ferrand. —— (2000), ‘Lahars Hazard Micro-Zonation and Risk Assessment in Yogyakarta City, Indonesia’, GeoJournal (Dordrecht: Kluwer Academic Publishers), 49/2: 131– 8. —— Thouret, J.-C., and Bacharudin, R. (1995a), Lahar-Related Hazard Zones along K. Boyong (Bandung: VSI), map 1:10 000. —— —— —— (1995b), Lahar-Related Hazard Zones along K. Bebeng Krasak (Bandung: VSI), map 1:10 000. —— —— —— (1995c), Lahar-Related Hazard Zones along K. Code in Yogyakarta City (Bandung: VSI), map 1:2000. —— —— Voight, B., Suwa, H., and Sumaryono, A. (2000a), ‘Lahars at Merapi Volcano: An Overview’, Journal of Volcanology and Geothermal Research, 100: 423–56. —— —— Suwa, H., Voight, B., Young, K., Lahusen, R., Marso, J., Sumaryono, A., Dejean, M., Sayudi, D. S., and Moch (2000b), ‘Instrumental Lahar Monitoring at Merapi Volcano, Central Java, Indonesia’, Journal of Volcanology and Geothermal Research, 100: 457–78.

Hazards and Risks at Gunung Merapi 299 Malin, M. C., and Sheridan, M. F. (1982), ‘Computer-Assisted Mapping of Pyroclastic Surges’, Science, 217: 637–40. Mileti, D. S. (1993), The Loma Prieta, California, Earthquake of October 17, 1989: Public Response, US Geological Survey Professional Paper (Washington), 1553B: ‘Societal Response’. Ministry of Construction of Japan (1985), Report of Overseas Development Project of Forecasting and Warning on Flood and Mudflow (Tokyo). Mount Merapi Project (1980), Volcanic Debris Control in the Area of Mt Merapi (Jakarta: Ministry of Public Works). Neumann van Padang, M. (1931), ‘Der Ausbruch des Merapi (Mittel Java) im Jahre 1930’, Zeitschrift für Vulkanologie, 14: 135–48. —— (1933), ‘De Uitbarsting van des Merapi (Midden Java) in den Jahren 1930–31’, Vulkanologische en Seismologische Mededelingen, 12. —— (1953), Catalogue of the Active Volcanoes of the World Including Solfatara Fields, 1: Indonesia (Bandung: VSI). Newhall, C., et al. (2000), ‘Reconnaissance Pyroclastic Stratigraphy of Merapi Volcano, Central Java: 7000 Years of Explosive Eruptions’, Journal of Volcanology and Geothermal Research, 100: 9–50. Nossin, J. J., and Javelosa, R. S. (1996), ‘Geomorphic Risk Zonation Related to June 1991 Eruptions of Mt Pinatubo, Luzon, Philippines’, in O. Slaymaker (ed.), Geomorphic Hazards (Chichester: Wiley), 69–95. Pardyanto, L., Reksowirogo, L. D., Mitrohartono, F. X. S., and Hardjowarsito, S. H. (1978), Volcanic Hazard Map, Merapi Volcano, Central Java (l/100 000) (Bandung: Geological Survey of Indonesia), II/14. Pinatubo Volcano Observatory (1992), ‘Lessons from a Major Eruption: Mount Pinatubo, Philippines’, Earth in Space, 4/6: 5–10. Punongbayan, R. S., and Tuñgol, N. M. (1994), Impacts of the 1993 Lahars and Long-Term Lahar Hazards and Risks around Pinatubo Volcano (Manila: Phivolcs Press). Purbo-Hadiwijoyo, M. M., and Suryo, I. (1980), ‘Distribution Pattern of the Merapi Volcanic Debris, South Central Java’, in Volcanological Survey of Indonesia (ed.), Volcanology Merapi (Bandung), 276–90. Ratdomopurbo, A. (1995), ‘Étude sismologique du volcan Merapi et formation du dôme de 1994’, Ph.D. thesis, Université J. Fourier, Grenoble 1. Sawai, K., Sairin, S., and Pangesti, D. R. (1994), ‘Behavior of Inhabitants at Recent Disasters in Central/East Java’, in Japan–Indonesia Joint Research on Natural Hazard Prediction and Mitigation (Kyoto: Disaster Prevention Research Institute, Kyoto University), 261–75. Schmidt, K. J. (1934), ‘Die Schuttströme am Merapi auf Java nach dem ausbruch von 1930’, De Ingenieur in Nederland Indies, 7–9: 1–69. Siebert, L. (1984), ‘Large Volcanic Debris Avalanches: Characteristics of Source Areas, Deposits, and Associated Eruptions’, Journal of Volcanological and Geothermal Research, 22: 163–97. Simkin, T., and Siebert, L. (1994), Volcanoes of the World, 2nd edn. (Tucson: Geosciences Press). Siswowidjoyo, S. (1971), Laporan letusan G. Merapi, Report on the 7–8 January 1969 Eruption of Merapi (Yogyakarta: MVO). —— Suryo, I., and Yokohama, I. (1995), ‘Magma Eruption Rates of Merapi Volcano, Central Java, Indonesia during One Century (1890–1992)’, Bulletin of Volcanology, 57: 111–16. Slaymaker, O. (1996), ‘Introduction to Geomorphic Hazards’, in O. Slaymaker (ed.), Geomorphic Hazards (Chichester: Wiley), 1–7.

Smith, K. (1992), Environmental Hazards: Assessing Risk and Reducing Disaster (London: Routledge). Sukardi, S. (1988), ‘Towards Appropriate Technique of Sabo Works’, in Proceedings: Debris Flow Forecasting and Warning System at Mt Merapi (Yogyakarta: Volcanic Sabo Technical Centre). Sumaryono, A. (1992), ‘Nuées ardentes Deposit and its Subsequent Runoff through Channel’, in Proceedings of the International Conference on Geography in the ASEAN Region, 31 August–3 September 1992 (Yogyakarta). Suryo, I., and Clarke, M. C. G. (1985), ‘The Occurrence and Mitigation of Volcanic Hazards in Indonesia as Exemplified at the Mt Merapi, Mt Kelut and Mt Galunggung Volcanoes’, Quarterly Journal of Engineering Geology, GBR, 18(1): 79–98. Suryo, I., and Dahlan, Z. (1987), ‘Lahars in Indonesia and Counter Measures Minimize its Hazards’, in Proceedings of the Symposium on Mudflows in Japan and Indonesia (Tokyo: Public Works Research Institute, Ministry of Construction), 171–18. Thouret, J.-C., and d’Ercole, R. (1996), ‘Vulnérabilité et risques naturels en milieu urbain: Effets, facteurs et réponses sociales’, Cahiers des Sciences Humaines, 32/2: 407–22. —— Lavigne, F., Kelfoun, K., and Bronto, S. (2000), ‘Toward a Revised Hazard Assessment at Merapi Volcano, Central Java’, Journal of Volcanological and Geothermal Research, 100: 457–78. UNDRO (United Nation Disaster Relief Organization) (1979), Prévention et atténuation des catastrophes, vol. vii: Aspects économiques (Geneva: UNDRO). van Bemmelen, R. W. (1949), The Geology of Indonesia, vol. 1a (The Hague: Government Printing). Vincent, P. M., Camus, G., Gourgaud, A., and Berthommier, P. C. (1992), ‘Le Merapi: Scénarios éruptifs dans la situation actuelle (Octobre 1992), d’après les enseignements des éruptions historiques’, in Y. Lageat and J.-C. Thouret (eds.), Actes du Colloque de l’Association de Géographes Français: Rythmes morphogéniques en domaine volcanisé (Clermont-Ferrand), 5: 237– 46. VSI (Volcanological Survey of Indonesia) (1990), Gunung Merapi (Bandung: Direktorat Vulkanologi). —— (1995), A Guide Book for Merapi Volcano, Merapi Decade Volcano International Workshop (Yogyakarta: VSI-IAVCEI). VSI–MVO (Merapi Volcano Observatory) (1989), Peta daerah bahaya Gunung Merapi, modifikasi peta daerah bahaya 1978 (Yogyakarta), 1:100 000. Whitcomb, G. (1988), Disaster Management Information and Communication System in Indonesia ( Jakarta: Department of Public Works). Wirakusumah, A. D., Juwarna, H., and Loebis, H. (1989), Geologic Map of Merapi Volcano, Central Java (1/50 000) (Bandung: Direktorat Vulkanologi, VSI, Ministry of Mines and Energy). Yamamoto, T., Takarada, S., and Suto, S. (1993), ‘Pyroclastic Flows from the 1991 Eruption of Unzen Volcano, Japan’, Bulletin of Volcanology, 55: 166–75. Young, K. D., Voight, B., Subandriyo, Sajiman, Miswanto, and Casadevall, T. J. (2000), ‘Ground Deformation Measurements at Merapi Volcano: Electronic Distance Measurements, 1988–1995’, Journal of Volcanology and Geothermal Research, 100: 233–59. Zen, M. T. (1983), ‘Mitigating Volcanic Disasters in Indonesia’, in H. Tazieff and J.-C. Sabroux (eds.), Forecasting Volcanic Events (Amsterdam: Elsevier), 219–36.

17

Hydrology and Rural Water Supply in Southeast Asia Goh Kim Chuan

Introduction The East Asian economic turmoil of 1997 and its lingering effects belie the decade of unprecedented economic growth in the Southeast Asian region. This economic boom saw a significant increase in the per capita income of the population of the respective countries and a corresponding rise in the standards of living. The decade also saw increased government spending on infrastructural development of basic amenities, including irrigation extension and rural water supply. The demand for and consumption of water increased significantly in both cities and the rural areas. In contrast to the escalating demand for water by the economies of the Southeast Asian countries, available resources remain limited despite the fact that the region generally receives more rainfall than it loses through evaporation annually. Annual, seasonal, and spatial variations in the rainfall within and between countries on the one hand, and accelerated demands for water from the various sectors of the economy on the other, put a severe strain on the available water resource base. In addition, natural resources in the form of rivers, groundwater storage, and lakes are rapidly diminishing in quality as a result of domestic, agricultural, and industrial waste discharges. In the coastal plains, excessive groundwater abstraction resulting in salt-water intrusion has affected groundwater resources. Inland, and in the watershed areas, rapid and extensive development has been at the expense of forested land, which has given way to new urban centres and residential and industrial complexes, while uncontrolled logging and shifting agriculture have caused the deterioration of the remaining forested ecosystem and natural watersheds. Given these factors, the future water resources

scenario of the region seems bleak unless urgent steps are taken to manage seriously the resources in a judicious and sustainable way. Water will certainly feature as an important issue of development in the region in the decades ahead, given that large population concentrations and economic development are located in the lower parts of river basins. This chapter describes the hydrological conditions of the Southeast Asian region and examines the nature and extent of water resources that have been put to use for rural and agricultural development. The thrust of the discussion is on rural water supply and irrigation water use, as issues dealing with urban water are dealt with in Chapter 19.

The Hydrological Cycle An understanding of the water resources of the region must start with the hydrological cycle. The source of the relatively abundant surface water in the region is essentially rainfall. However, in the transition between its fall as rain and its abstraction from rivers and/or from aquifers for utilization, natural processes on the ground surface, within the soil, and in the ground modify its quantity and quality. It is the clear understanding of the hydrological processes within these zones that is of vital importance in the appraisal of the potential water resources that can be put to various uses.

Precipitation The annual rainfall varies from place to place in the region, but even for a particular location, annual fluctuations may be as high as 20–30 per cent above or below the long-term average. Seasonal effects of the monsoon wind systems combined with the influence

Hydrology and Rural Water Supply 301

of topography ensure that heavy rainfalls are experienced in areas more exposed to the prevailing winds while lower amounts fall in sheltered locations. For example, annual rainfall may be as high as 5000 mm in some exposed western slopes of the Barisan Range in Sumatra, the Chin and Rakhine Hills and the Taninthayi Range in Myanmar, as well as in the upland parts of Borneo and Papua, but as low as 500 to 1000 mm in places like the sheltered Dry Zone of Myanmar or the northeast of Thailand. The annual totals for much of the rest of Southeast Asia lie somewhere in between. The lowland Malay Peninsula, for example, generally receives annual rainfall varying between 1800 and 2500 mm. Figure 5.1 gives an idea of the variation of annual rainfall in Southeast Asia. Upland areas act as barriers to rain-bearing winds (see Chapter 5). Forced ascent of winds triggers condensation, leading to significant amounts of orographic rainfall. A direct relationship exists between rainfall distribution and elevation for Southeast Asia. In Malaysia, places like Kedah Peak (about 1000 m above msl) and Maxwell Hill near Taiping (609 m elevation) receive 30–40 per cent more rainfall than coastal locations. On the highlands of Java the belt of maximum rainfall is found on mountain slopes but not at the highest altitudes (de Boer 1950). The same pattern is seen in western Sumatra, the hills of Myanmar, and the mountain ranges of Viet Nam. While increased rainfall on highlands is often attributed to orographic effects, the influence of forest vegetation cannot be totally ignored. Forest vegetation that still occupies much of the mountain ranges in Southeast Asia helps the condensation process. If not in enhancing cooling and precipitation, vegetation on highlands certainly helps to trap clouds and fog, producing cloud drips. Although increases in total precipitation through cloud-stripping may be sizeable for isolated trees but less in closed forests due to mutual sheltering of trees, in the wet tropics such rainfall may constitute between 4 and 18 per cent of total rainfall, an amount that is not insignificant. In persistent wind-driven clouds, the amount of clouds stripped by forest vegetation may be in the order of hundreds of millimetres of water per year (Stadmuller 1987). This partly explains the more than 4000 mm of annual precipitation in the upland areas of the Temburong district of Brunei Darussalam and in parts of Papua. Given the greater amounts of rainfall received on mountain ranges and slopes, the upland areas constitute the most significant source of much of the water resources harnessed for use in Southeast Asia. The water produced is clean, and because of its good

quality, treatment costs are significantly lower. Additionally, the upland areas guarantee a constant flow of water in rivers and streams.

Interception Raindrops travelling to the ground will be intercepted first by the forest canopy. Vegetation types and canopy density together are important parameters that influence interception loss, as does rainfall characteristics such as intensity and duration. The forests of Southeast Asia, while generally evergreen, show much variation in terms of canopy structure, shape of the crown, the density of leaves, and the total leaf area. In the humid tropical rainforest, annual interception loss (Ei) is significant. Estimates of Ei in tropical forests vary considerably from 4.5 to 45 per cent of gross rainfall ( Jordan and Heuveldop 1981). For example, case studies on annual interception loss in Malaysia indicate figures of 36 per cent from the forested Sungai Lui catchment, Selangor (Low and Goh 1972), 33 per cent from the Waterfall Forest Reserve on the island of Penang (G. H. Teoh 1983), and 26.6 per cent from the dipterocarp forest at Bukit Berembun, Kuala Pilah, Negeri Sembilan (Baharuddin 1989). Manokaran (1979) in his study of the Pasoh Forest Reserve, Malaysia, showed that monthly interception varied from 5.04 to 34.3 per cent of rainfall, with an annual rate of 21.8 per cent. Based on the more reliable studies of throughfall, interception losses average 13 per cent of annual rainfall in lowland rainforests and 18 per cent in montane forests (Bruijnzeel 1990). Interception varies from as low as 0.15 to as high as 100 per cent of precipitation between individual storms. A strong relationship was found between interception and rainfall amounts of storms where r = 0.68 was obtained for the Pasoh Forest Reserve. The relationship between interception and rainfall intensity, however, shows a different pattern. A curvilinear relationship between the two variables was obtained for the natural forest in Penang based on forty-eight storms, with rainfall intensities varying from 0.1 mm to 50.8 mm per hour and the interception expressed in per cent of total rainfall (G. H. Teoh 1983). Over the past three decades rapid economic development in most Southeast Asian countries has seen the conversion of large tracts of forested lands for different purposes, but most significantly for agriculture. Large areas of forest have given way to cash crops, particularly rubber and oil palm, with a different canopy structure and different interception loss. For example, T. S. Teoh (1977) found that the interception loss for rubber (Hevea brasiliensis) was 23 per cent of the annual rainfall. Parts

302 Goh Kim Chuan

of Malaysia and Indonesia are planted with cocoa, and according to Opakunle’s (1989) finding in Africa, interception loss from cocoa plantation can be as high as 24.3 per cent.

Stemflow and Throughfall Studies on stemflow and throughfall in the region are few. Manokaran (1979) carried out a thorough investigation of stemflow and throughfall at Pasoh Forest Reserve under the Malaysian International Biological Programme for the year 1973. Eight trees within a 100 m2 plot were gauged. The results showed that monthly stemflow values varied from 0.23 to 0.92 per cent and the annual rate was 0.64 per cent. On per storm basis, it varied from zero to 2.65 per cent. Rainfall intensity exerts an important influence on stemflow. Other influencing factors on stemflow include antecedent atmospheric conditions, temperature, humidity, wind conditions during a storm, the degree of wetness of the canopy and tree trunks and branches, the position of the individual tree branches in relation to those of the neighbouring trees, and bark texture. The importance of the individual canopy structure was evident in that more stemflow was recorded for small trees of the Knema malayana than for larger-girth trees, implying that the size of trees does not necessarily produce greater stemflow (Bruijnzeel 1990). As far as stemflow in rainforests is concerned, it can be taken to be about 1–2 per cent of the incident rainfall. This amount does not vary with different types of tree crops. Md. Kamil (1991) found similar amounts of nutrients from stemflow of rubber trees in Malaysia. Stemflow carries nutrients such as nitrates, phosphorus, magnesium, and calcium to the ground, which is significant in influencing tree growth, particularly in natural vegetation. While the overall contributions of stemflow to soil water may be small, it is nevertheless significant in influencing river water quality (Herwitz 1986). Throughfall is a significant component of rainfall that reaches the ground. Bruijnzeel (1990) showed that, where the methodology employed was more reliable, the general average annual throughfall was 85 per cent of the incident rainfall for lowland forests and 81 per cent for montane forests. Similar results were obtained by Waidi Sinun et al. (1992) for the forest of Danum Valley in Sabah (Figure 17.1). In that study for the period September 1986 to September 1990 an annual mean of 80.7 per cent of the incident rainfall was recorded. In Peninsular Malaysia, throughfall and stemflow accounted for 73 per cent and 0.4 per cent of annual rainfall respectively (Baharuddin 1989). Throughfall varies from 0 to

99.01 per cent of rainfall in storms. A high correlation exists between precipitation amount and throughfall (r = 0.96) on the basis of storm events (Manokaran 1979), but much lower correlation with rainfall intensities (r = 0.35). Stemflow and throughfall are important sources of rain that reaches the ground.

Evapotranspiration Potential evapotranspiration (Et) is essentially a function of the potential energy available in a situation of unlimited water supply in the soil. Actual evapotranspiration (Eo), on the other hand, is governed not so much by the energy available as by the actual amount of water available in the soil at any particular moment. It is the wetness or dryness of the soils as governed by present or past rainfalls that is the main limiting factor. If the soil is saturated, then the actual evapotranspiration in a forested environment is close to the potential value. Thus the Et–Eo ratio could be close to 1, as many studies in Southeast Asia have shown (Kenworthy 1969; Low and Goh 1972; Manokaran 1979; Abdul Rahim 1988). Evapotranspiration values can be derived either from direct measurements or by indirect methods, mainly through calculations or through water balance estimates. In the Sungai Tekam Basin study (Drainage and Irrigation Department 1989) in Malaysia, the forest evaporation as derived from pan evaporation was estimated to average 1251 mm per year. For the same area, using Penman’s equation, and based on climatic data for six years and an albedo value of 0.18, the estimated monthly evapotranspiration tends to be consistently higher than that of pan Et (128 mm as compared to 108 mm). Estimates based on catchment studies may be erroneous owing to invisible and ungauged gains or losses from subsurface flows of water. Thus, overestimation of Et will result from basin leakage and conversely for basins receiving additional unaccounted water. For small basins the problem of determining spatial rainfall is not a major one, but for larger ones, the problem of determining this parameter causes additional difficulty in the estimation of basin areal evapotranspiration. Bruijnzeel (1993) has indicated that a precision of 15 per cent for Et is difficult to achieve. As far as higher montane forests are concerned, values are definitely lower as a result of lower transpiration rates in cooler environment, high humidity, and increased precipitation due to cloud-stripping. Annual Et values for lower montane forest in general converge around 1225 mm (Bruijnzeel 1990). In west Java, at Ciwidey, it was 1170 mm at an elevation of 1740 m (Gonggrijp 1941).

Hydrology and Rural Water Supply 303 100°

110°

120°

130°

140°

Chin Hills DRY ZONE

20°

N

20°

Rakhine Hills R.

Mae Chaem

Chao Phraya

g on ek M i Ch R.

Baguio

R. Mun

Andaman Sea

Manila

South China Sea

Angkor Tonlé Sap

Philippine Sea

Laguna de Bay

PACIFIC OCEAN

10°

10° Surat Thani Nakhon Si Thammarat

Penang Is

Maxwell Hill

B

Danum Valley

a

Kuala Lumpur

R.Segama

r

T.Chini T.Bera

i

P.Ketam

Ulu Langat

Lake Toba

S a

n

0°

0° R a n g e

Ambon

Java Sea Cidurian Ciwidey

10°

Pompengan

Banda Sea

Gubug Penggaron Timur Yogyakarta

10° 0

500 100°

110°

120°

130°

140°

Fig. 17.1. Location map

Run-off Run-off from forested catchment comprises a combination of various components: channel precipitation, quickflow, and baseflow. In the Malaysian context, annual run-off varies with the type of catchment, land use, and topography. In a tropical rainforest the amount of surface run-off is relatively small compared to the baseflow. In the six years of study in Sungai Tekam Basin, Malaysia, direct run-off accounted for 25 per cent of total run-off. The reason for the large baseflow contribution is the high infiltration capacities of the soil within the forest ecosystem. This observation was true of many other studies. Goh (1972) for example, compared the annual run-off between a forested Ulu Langat catchment with that of the disturbed Damansara Basin. The former was of higher relief than the latter. From the hydrological data observed in 1969 it was seen that not only was the annual run-off less than half

of the annual rainfall, the run-off in the forested basin was considerably lower than the disturbed catchment despite its greater elevation. The significance of such forested catchments for water supply purposes cannot be overemphasized.

Water Balance From the above discussion of the various components of the hydrological cycle that pertain to Southeast Asia, one can get an idea of the water balance of the various parts of the region. Annual water balance would be less profitable than a consideration of the monthly or weekly water balance, as this will be more useful for determining the available moisture for agricultural needs. In general, the monthly water balance nearer the Equator in Southeast Asia shows a surplus for most months of the year compared to areas further north or south. Areas further away from the Equator,

304 Goh Kim Chuan

such as northern Thailand, show greater variations between months and a longer and distinct period of deficit in the first six months of the year. Water balance varies between a forested catchment and a disturbed or developed basin. It is thus important to understand the water balance characteristics of river basins in the region so that better planning and utilization of water resources can be instituted.

Water Quality of Forested Catchments Nutrients in tropical forest soils come from precipitation, fixation of nitrogen by micro-organisms, decomposition of organic matter, and deep weathering of underlying rocks. Organic decomposition and chemical weathering under the humid, warm tropical environment are effective, and changes in the nutrient loading in river water due to these processes are easily detectable. Thus the amount of nutrients carried by streams can be regarded as a useful indicator of an area’s nutrient status both before and after a change in land use (Nye and Greenland 1960; Finck 1973). Several studies on the nutrient status of forests in Southeast Asia have been conducted, but results obtained vary considerably. Crowther (1987) found much higher potassium concentrations in soil water at the shallow soil–bedrock (limestone) contact in Perak than in deep weathering groundwater seeping into the underlying caves. In Sabah, Bruijnzeel (1990) found that potassium concentrations in the soil solution decreased rapidly with depth in montane forests on ultrabasic rocks, with very low amounts present in stream water. Thirty times as much potassium is washed down from the vegetation by throughfall and stemflow than is carried out by overland flow and throughflow (Waidi Sinun et al. 1992). Whatever may be the variations in individual water quality components, streams flowing in forested catchments are generally low in solute content and in individual chemical constituents. Baharuddin (1989) showed that in the Bukit Berembun Basin potassium, sodium, calcium, magnesium, phosphate, ammonium nitrogen, and nitrate nitrogen contributed by throughfall and stemflow were 43, 11, 30, 13, 1, 6, and 8 kg ha−1 yr −1 respectively. In the same catchment, however, Zulkifli et al. (1989) found extremely high calcium and magnesium concentrations, a feature they found difficult to explain. The most commonly occurring element, silica, which reflects the weathering status of rocks, is not high in stream waters of forested catchments. In the above study, the mean value was not more than 17 mg l−1,

comparable to results obtained by Douglas (1967), who found a mean of 25 mg l−1. Iron concentrations, which are normally not included in nutrient transport studies of forested catchments, may be significant. Mean weighted values of 0.3, 0.35, 0.56, and 0.81 ppm were found for rainwater, throughfall, stemflow, and streamflow respectively (Leigh 1978) in the lowland Pasoh Forest Reserve, Peninsular Malaysia. Apart from chemical nutrients carried out by streams from a forested catchment, streams also carry with them mineral sediments in the form of suspension. This sediment transport reflects the mechanical erosion of the soil surface by the act of surface run-off enhanced by tree fall and uprooting, burrowing activities of animals and organisms in the soils, episodic events of landslides and slope failures, and within-channel processes. Over the long term, however, the sediment yield from a stable tropical rainforest ecosystem is low. In Peninsular Malaysia suspended sediment studies have shown values ranging 28–112 ppm (Peh 1981), 2–76 ppm in Sungai Marong Kanan in Pasoh Forest Reserve (Leigh 1978), and 2–1609 ppm in the Sungai Gombak under a wide range of flow conditions (Douglas 1968). Sediment transport tends to be very significant in large episodic storm events owing to high run-off from intense rainfall. Sabry Mohamad (1997) showed that in the Sungai Tekala catchment, Ulu Langat, Selangor, five storms of high intensity registered during a thirteenmonth study period accounted for 25 per cent of total rainfall and 49 per cent of suspended load. He referred to a study by the Forest Research Institute of Malaysia where it was reported that 15 per cent of sediment yield in one year was due to one storm event with 121 mm rainfall. Douglas et al. (1992) also confirmed this pattern where 51 per cent of sediment load was carried out of the Danum catchment in Sabah by eleven storm events during the period of observation.

The Hydrological Effects of Changing Land Use The pace of forest conversion and depletion in Southeast Asia has been a matter of serious concern for some time. The introduction of large plantations in the late nineteenth century, resettlement and land development schemes from the mid-1960s to tackle rural poverty, urban expansion, and general economic development since the 1970s have drastically reduced forest acreage. In Thailand, for example, forest coverage declined from 70 per cent in 1945 to 53 per cent in 1961 and to 18 per cent in 1989 (Charoenphong 1991). Damage

Hydrology and Rural Water Supply 305

due to logging, forest fires, shifting cultivation, and illegal occupation of forest lands contributed to the deterioration of much of what remained of the natural ecosystem in the region. During the Viet Nam War in the 1970s the use of the defoliant ‘agent orange’ caused the destruction of large areas of natural forests in rural Viet Nam, the impacts of which are still felt today. The impact of such large-scale forest clearance on river systems in Southeast Asia is evident. A more subtle form of change is not in the actual depletion of forest acreage itself, but in the deterioration and degradation of the once pristine forest ecosystem chiefly through logging activities. In Malaysia, logging affected some 300 000 ha of forested lands each year throughout the 1970s and 200 000 ha yearly in the early 1980s but was limited to 149 000 ha per year in the 1990s. The rate of replanting is very slow and has little chance of compensating the damage due to logging (Hurst 1990). Pressure on land is greater in Thailand, where lack of enforcement has seen hillslopes deforested and illegal logging rampant. Deforestation due to illegal and excessive logging was blamed for the disaster that occurred in late November 1988 when, following heavy rains, flooding and landslides occurred in fourteen southern provinces with devastating effects. Large areas of the mountainous regions of Nakhon Sri Thammarat and Surat Thani provinces suffered extensive erosion and vegetation destruction. As a result of this experience the Royal Thai government on 11 July 1989 instituted a general ban on all commercial logging in the country (Charoenphong 1991). In Viet Nam, concern for watershed management in light of deforestation of catchment areas has been expressed for the Hoa Binh Reservoir on the Da River, the largest tributary of the Sông Hóng system of Viet Nam (Vu 1993). Changes in the forest ecosystem bring about changes in the factors controlling the surface run-off and infiltration capacity; hence the storage and subsequent flow characteristics. Increase in run-off is inevitable and with it the rate of sediment removal into streams. Transformation of forests into agricultural land uses in the Sungai Tekam catchments in Pahang showed large increases in annual run-off. Those two forest catchments were converted to cacao and oil palm respectively. Significant increases in run-off occurred in the second and fourth years of planting, 157 per cent and 470 per cent of the normal amount respectively (Abdul Rahim 1988). In the case of logging, however, carefully executed selective logging has little effect on streamflow, but water yield increases with greater amounts of timber removed, a conclusion reached in

the study at Bukit Berembun catchments in Peninsular Malaysia (Abdul Rahim 1989). Natural regrowth of forests that have been logged even more than a decade ago may not be able to restore the natural infiltration capabilities of soils, especially on former tractor tracks as found by van der Plas and Bruijnzeel (1993) in Upper Segama, Sabah, East Malaysia. The effects of changing land use on water yield should not be seen only on an annual basis. The flow regime throughout the year should also be considered. The high flow and dry-season low flows are equally important from the point of view of water resource utilization. Low flows during the dry season are more a result of reduced infiltration due to compaction of soils that enhances surface run-off during the wet spells. Bruijnzeel (1990) argued that when infiltration opportunities are reduced after forest clearance, surface run-off would be efficient and baseflow affected despite reduced evapotranspiration. He further argued that if infiltration characteristics were maintained through good land husbandry and conservation practices despite forest extraction, the effect of reduced evapotranspiration after clearing would show up as increased baseflow. From the point of view of water use, the commonly observed deterioration in river regimes following tropical forest clearance is not so much the result of clearing itself but rather a reflection of the lack of good land husbandry during and after the operation.

Potential Water Resources The amount of water as a resource varies from country to country and from region to region within Southeast Asia and within each country. The climatic characteristics of rainfall and the seasons, as influenced by the monsoon winds, govern the availability of water resources to a large extent. Countries nearer the Equator tend to have higher annual rainfalls which are well distributed, although the notion of a uniformly wet climate throughout the year has proven to be a fallacy on many occasions when dry spells of several months affected parts of Peninsular Malaysia, Singapore, Indonesia, and the Philippines. Regions further north like the north and northeast of Thailand, Lao PDR, Cambodia, and Viet Nam experience highly seasonal rainfall; highest amounts are received in the southwest monsoon from June to August, while the winter months of the northeast monsoon bring very little rain. This variability in the rainfall is also governed by topographic characteristics and the distance from the coast, both of which affect the prevailing winds, as well as seasonal effects of extreme weather phenomenon of the

306 Goh Kim Chuan

typhoons. In the case of topographic factors, the western slopes of the mountain range of western Myanmar receive high rainfall amounts from the southwest monsoon, because of orographic lifting. On the lee side of the mountain barrier, the lowlands of the Dry Zone of Myanmar have a distinct dry rainfall regime. Distance from the coast certainly affects the northern and northeastern parts of continental Southeast Asia. As far as the typhoons are concerned, the effects are more marked in the Philippines, where most of the heavy rainfall in eastern Luzon and the Visayas comes from this source. However, the path of the typhoons also leads to the eastern shores of the northern parts of Viet Nam with associated strong winds and heavy rainfall.

Main Sources of Water

mental impacts arising from dam construction will be significant if the cumulative effects of many projects are assessed. There are also other large rivers (see Chapter 4) within the Southeast Asian region. The sizes of their basin and discharge obviously determine their importance as major sources of water supply. For a small country like Singapore, rivers are very small, and the same applies to many of the smaller islands of the Philippines and Indonesia. But, unlike the small islands in the latter two countries, Singapore’s water demand is high, and cannot be met by surface sources from within its territory. In contrast, larger regions and even islands like Sumatra, Luzon, and Mindanao, and even Brunei Darussalam on the large island of Borneo, have big rivers whose natural water supply far exceeds the local demand. Regulation of the total flow is minimal.

Rivers

Lakes

As in most countries with relatively wet climate, rivers in Southeast Asia form an important source of water supply for various uses. In a wet environment, the drainage density is higher, there is a wider distribution of stream network, and the flow is more reliable throughout the year. The cost of abstraction is also lower for obvious reasons of ready accessibility and continuity of supply. Southeast Asia has some large and important rivers. The Mekong is one of the longest rivers in the world, flowing through several countries of Southeast Asia. The potential contribution of the Mekong, described as ‘the greatest single resource of mainland Southeast Asia’ (quoted by Ojendal 1995), to the development of one of the poorest regions of the world, containing almost 50 million people in its riparian states, is enormous. Currently cooperation among the riparian states is still at its formative stage and many moral and legal issues of who has the right to the water between upstream and downstream countries remain unresolved. As economic development of individual states and the use of the water of the Mekong and its tributaries is a vital ingredient in this process, conflict resolution needs to be well managed (Chaippat 1992). The Mekong River Commission, formed in 1995, provided such a mechanism for understanding between the riparian states (Ojendal 1995). The Mekong River has a large hydropower potential estimated at 285 400 gWh capacity per year. Rising demand for hydropower and the need for increased foreign exchange that sale of electricity could give to countries like Lao PDR, Thailand, and Viet Nam may result in constructions of dams in the future. Environ-

Several large and naturally formed lakes are found in Southeast Asia. Some of the notable ones are the Tonlé Sap in Cambodia, Tasik Bera and Tasik Chini in Peninsular Malaysia, Lake Toba in the highlands of north Sumatra, and Laguna de Bay in Luzon, Philippines. While many of these lakes contain substantial amounts of fresh water that could be utilized for different purposes, many are located too far away from human settlements to play a major role in supplying water or providing a means of livelihood to the surrounding population. In Malaysia, lakes like Tasik Bera and Tasik Chini in Pahang have yet to be developed because of distance from major towns. However, for lakes that are close to population centres, their resources have been exploited, and the quality of the aquatic ecosystem is suffering from the effects of pollution as in the case of Laguna de Bay. The problems facing these lakes are quite different from one lake to another. Lakes that are located in remote areas are slowly being used for ecotourism. Examples of such lakes are Lake Toba in north Sumatra and Tasik Bera in Malaysia. But for lakes which face serious pollution problems and overexploitation of resources such as Laguna de Bay (Goh 2000) and Tonlé Sap, the urgent task is one of rehabilitation and management of the water resources as well as that of the overall lake ecosystem. Mining activities have resulted in artificial lakes and ponds. Tin-mining in Peninsular Malaysia, parts of Thailand, and Viet Nam has created a pockmarked landscape of numerous mining pools of various sizes, which contain fresh water that can be used. In most cases, the water in these ponds is not tapped, apart from some ponds which have been used for purposes

Hydrology and Rural Water Supply 307

of combined duck-rearing, fish-farming, and fruit and vegetable growing.

Groundwater Traditionally, shallow groundwater has been the source of water supply for communities in river valleys and lowland plains. However, because shallow groundwater is not reliable, communities wholly dependent on this source have been facing problems of water supply. During times of flood, wells are inundated, while in the dry seasons they do not contain much water. Even in normal circumstances, because of their location near to domestic waste-water discharge, the well water can easily become contaminated and unsafe for drinking. Nearer the coast, salt water affects the quality of well water if extraction far exceeds natural recharge. In water-stressed areas, it is thus necessary that deep groundwater sources be tapped. Many reasons can be put forward to justify the use of deep groundwater. Taking Peninsular Malaysia as an example, particularly in parts of Kedah and Perlis, surface water dries up during the prolonged dry season in the first few months of each year. In areas where demand has been increasing and supply is lacking, or where pollution of surface water has increased treatment costs drastically, deep groundwater sources should be explored. In many parts of Southeast Asia where sedimentary rocks and lowland plains are extensive, deep groundwater from aquifers has been harnessed for industrial and domestic uses. To a small extent, deep groundwater is tapped for use in some parts of Malaysia, Indonesia, and Viet Nam. Groundwater studies in the early 1980s showed that groundwater potential in Malaysia was not extensive (Pang 1981). In general, when compared to the total water used in any country in Southeast Asia, groundwater accounts for a small proportion of the overall water resources (Table 17.1).

Rural Irrigation Development Rice has always been the main staple diet of the region, and rural populations have generally been involved in its cultivation. Grown extensively, wet rice in river valleys and coastal plains or hill rice on the slopes, rice cultivation has supported human populations from early times. Rice cultivation rituals, songs, myths, and festivals such as the bun bang fai (skyrocket) of northeast Thailand and Lao PDR all point to the central role water plays in the life of farming communities in the region (Rigg 1992), water that is subject to the vagaries of the monsoon. Christie (1992) has shown that inscriptions from the ninth to thirteenth centuries indirectly

Table 17.1 Water resources and their use in Southeast Asia Country Cambodia Indonesia Lao PDR Malaysia Myanmar Philippines Singapore Thailand Viet Nam

1 88.1 2530.0 270.0 566.0 605.0 292.0 0.6 198.8 324.0

2

3

410.0 0.0 0.0 0.0 476.4 0.0 n.a. 210.0 556.0

n.a. 456.0 0.0 23.2 n.a. 33.0 n.a. 0.0 n.a.

4 0.52 48.80a 0.99b 11.60 3.96b 44.10 395.00 33.13 54.30

5

6

7

94.0 89.0 82.0 77.0 90.0 84.0 0.2 91.0 86.0

5.0 9.0 8.0 10.0 7.0 12.0 54.7 5.0 4.0

1.0 2.0 10.0 13.0 3.0 4.0 45.1 4.0 10.0

Notes: n.a. = not available. Column headings: (1) Annual internal renewable water resource (km3); (2) Annual river flow from other countries (km3); (3) Exploitable groundwater (km3); (4) Annual water withdrawal (km3); (5) Percentage of withdrawal for agricultural consumption; (6) Percentage of withdrawal for domestic consumption; (7) Percentage of withdrawal for industrial and commercial consumption. a b

1975. 1987.

Source: Compiled from UN (1995).

point to the early water management systems of the subak society in Java and Bali, particularly regarding the former. Frequent references to officials connected with water, to technicians connected with water control, and to taxes associated with irrigated agriculture were found on these inscriptions. In Cambodia, the success of the ancient Angkor empire was its ability to perfect the floodwater retreat agriculture. By careful selection of indigenous, short-day rice varieties with an ecological understanding of the retreating water in relation to the Tonlé Sap, the great lake of Cambodia, and by employing bunding techniques, the society was able to produce three or four crops a year (Stott 1992). In Malaysia, the Muda plain has been sporadically planted in earlier periods but more widespread rice cultivation in the northern part of the Malay Peninsula was introduced from Siam in the fifteenth century (Taylor 1981). In Java, communal irrigation systems have existed for many centuries. Evidence of control of water for rice cultivation dates back to the Hindu period (ad 700–1500). The earliest inscription referring to irrigation dates from ad 804 (Schaik 1986). However, with the increase in population and rapid economic development, traditional methods of rice production had to be improved. Essential to the efforts to increase production of rice to meet growing demands was the need to control and reduce consumption of irrigation water, if the ricefields of Southeast Asia were to continue to supply the expected increases in aggregate demand for rice under relatively water-scarce conditions ( Johnson 2000).

308 Goh Kim Chuan Table 17.2 Irrigated acreage in Southeast Asia, 1961–1997 (103 ha)

Table 17.3 Small-scale surface irrigation: Indonesia, Philippines, and Myanmar

Country

1961

1970

1980

1990

1997

Country

Irrigation type

Indonesia Malaysia Myanmar Philippines Thailand Viet Nam

3900 228 536 690 1621 1000

3900 262 839 826 1960 980

4301 320 999 1219 3015 1542

4410 335 1005 1560 4238 1840

4815 340 1556 1550 5010 2300

Indonesia Myanmar Philippines

Small-scale < 500 Small-scale < 400 Communal < 1000 Private systems

Area (ha)

Total area (ha)

Small-scale/total irrigated sector (%)

2 × 106 423 000 715 000 220 000

35 40 44 15

Source: Gleick (2000).

Source: Ambler (1994).

Water is an indispensable factor in rice production, and it requires 5000 l of water to produce 1 kg of rice (Hossain and Fischer 1995). Because of the high water requirement, irrigation schemes need to ensure a reliable supply of water for rice cultivation. However, large and successful irrigation schemes in Southeast Asia are few, as they require large capital investments in infrastructure development and maintenance. Nevertheless, irrigated areas in seven main rice-growing countries in the region have been expanding over the past four decades (Table 17.2). The largest irrigation scheme in Malaysia is the Muda Irrigation Scheme in the northwest coastal plain of Kedah, including parts of Perlis and Province Wellesley, covering an area of 92 700 ha, which in 1993 produced 285 281 t of rice. The success of this irrigation project depends largely on the clean and high-quality water from upland catchments of the Muda and its tributary the Pedu. The Muda and Pedu supply good-quality water from behind their respective dams for the irrigation needs of the off-season paddy crop. In most cases the water for the main-season crop comes from rainfall. But the second crop is usually heavily supported by irrigation water. One characteristic of paddy cultivation is the high water consumption for each season’s crop. The consumption of water in irrigated fields of paddy is a wasteful use of water as large quantities are lost through direct evaporation and canal leakage. Given that rice is most sensitive to drought during the reproductive phase, a continuous flooded condition should be maintained in the fields during this period. Owing to evapotranspiration, seepage, and deep percolation during land preparation, some 900 to 2500 mm of water may be required per cropping season. Roger and Bhuiyan (1990), referring to Kung (1971), commented that the average consumption in irrigated ricefields in forty-three locations in seven rice-producing countries was 1240 mm of water per crop, a high value by any standard.

Other attempts to develop irrigation projects include the Mekong-Chi-Mun transbasin transfer of water in northeast Thailand. As a result the net value of the dryseason crop is substantially higher than that of the wet-season production, and the ability to grow crops year-round has made agro-industrial development possible, resulting in value-added production to the region (Tawatchai and Singh 1996). This attempt at improving crop production and also the overall livelihood of the people has proven quite successful. However, in many parts of Southeast Asia, and in fact in most parts of northeast Thailand where pronounced seasonality of rainfall is the norm, rural poverty is very much tied to the scarcity of water. The northeast is the poorest region of Thailand, where more than 38 per cent of the population lived below the poverty line in the 1990s, compared to 24 per cent for the entire country (Hussain and Doane 1995). Large areas of paddy cultivation are concentrated in the deltas of large rivers in Southeast Asia such as the Irrawaddy in Myanmar, the central plains of the Chao Praya, the lower Mekong in Cambodia and Viet Nam, and the Hanoi coastal plain. In most parts of rural Southeast Asia, small-scale irrigation schemes predominate, and though they vary in size by definition (Table 17.3), they are tremendously important (Ambler 1994). However, small-scale irrigation schemes are difficult to run, and many factors determine their success or failure, including, among other factors, weather, soil conditions, water control measures, government or communal control, and size of operations. Northeast Thailand illustrates this point. Only 6 per cent of the land in northeast Thailand (roughly the Khorat Upland) is under irrigation, mainly served by semi-circular ponds. This region experiences a long, cool, dry season stretching from November to April with the driest period in December and January. Although May to October is regarded as the wet season, most rainfall is in fact concentrated during August and September. Its location between mountain borders

Hydrology and Rural Water Supply 309

shelters it from maritime influences that affect the rest of the country. The land is flat, with a subsoil that allows seepage of flood waters during the rainy season, but for the rest of the year high evaporation encourages rapid water loss. While supplementary water is available underground that is less susceptible to drought, it is quite inaccessible as it lies at between 30 and 60 m, often in isolated reservoirs and beyond the reach of simple technology of the rural communities. Another problem with irrigation systems in northeast Thailand involves the headworks for the control of irrigation water. These structures are very often damaged by floods and bank erosion during the rainy season. The use of wood and bamboo stakes to repair headworks has led to the problem of scarcity of wood. The impermanence of these structures often works against improving irrigation systems and the livelihood of the population, even when there is help from the government. Salinization is another major problem in northeast Thailand. For 17 per cent of the area thick beds of rock salts occur in the subsurface, evaporation surpasses rainfall during the dry season, water is utilized inappropriately, irrigated farming is confined in dry seasons, deforestation is common, and irrigation with saline water occurs. It has become evident that the provision of irrigation alone does not necessarily guarantee results. The case of northeast Thailand indicates that despite investment in irrigation facilities by the Royal Irrigation Department of Thailand, lack of water, a poor understanding of the biophysical factors, and the traditional socioecological patterns have resulted in less than 10 per cent increase in family income as a result of these schemes, the Mekong-Chi-Mun project notwithstanding (Surareks 1986). Irrigation schemes in Indonesia cover a total of 5 million ha, but the majority are small compared with the Muda irrigation scheme in Malaysia; 20 000 ha of gross irrigable land is considered large in Indonesia. On Java, the largest area is the Cidurian (west Java) with a gross area of 20 000 ha and an irrigable area of 12 000. The Pompengan (south Sulawesi) has a gross area of 5500 ha and an irrigable area of 4,680. The Gubug and Penggaron Timur in central Java have irrigable areas of 5000 and 4500 ha respectively (Varley 1989). The generally small size of government or communal irrigation schemes is due to the relatively small catchment areas, low river discharges, and the limited area of the coastal plains to be irrigated. Java, which has more than 70 per cent of all irrigated land and 82 per cent of the government irrigation schemes in Indonesia, has the highest concentration of such

schemes. It is the rich, fertile volcanic soils and topography and other environmental factors that are exceptionally favourable to irrigation (Geertz 1963) even though these schemes are small. As much as 95 per cent of Indonesia’s total annual production of rice is grown on irrigated land (Rose 1982). This has been possible through the construction of tertiary canal systems in the 1970s. By the mid-1980s about 1.8 million ha of tertiary canal systems had been constructed, of which 1.5 million ha comprised technical irrigation systems. For further improvement, water users in tertiary units were organized into water users’ associations responsible for the operation and maintenance of the newly constructed tertiary canal systems. Indonesia has been a net importer of rice since at least 1911, and for years has been the world’s largest importer. In the peak period of 1977 as much as 14 per cent of the total domestic rice demand was imported. The largest quantity (2 012 000 t) ever imported was in 1980 (Tumari Jatileksono 1987). The demand for rice increases yearly at a rate equal to the increase in population. Over the last decades, however, Indonesia has had a policy of self-sufficiency in rice, and hence independence from rice imports. The effects of the Green-Revolution technology that was introduced in Indonesia became evident in the late 1970s and early 1980s. Together with the introduction of improved rice varieties, state intervention (Bromley, Taylor, and Parker 1980) in the 1970s was seen through the construction of tertiary canal systems and establishment of water users’ associations. By the mid-1980s about 1.8 million ha of tertiary canal systems had already been constructed, of which 1.5 million ha were technical irrigation systems. Water users’ associations were responsible for the operation and maintenance of the newly constructed tertiary canal systems. In spite of all these efforts, the costeffectiveness of irrigation technologies and the success of these associations have been questioned (World Bank 1987; Perkins 1994). Nevertheless, increases in production in 1969– 83, at 5–10 per cent annually, were impressive, and in 1985 the country became independent of imports. However, in 1991, for the first time since 1985, a decrease in production occurred. Rice imports (600 000 t) were again necessary owing to a prolonged and extensive dry season and a further decrease in acreage planted. Many government irrigation schemes in Southeast Asia have not been performing as well as planners had hoped for owing to poor management despite large initial investments. Unless concerted efforts are made to use the water more effectively through better farm

310 Goh Kim Chuan

water management, this underperformance of irrigation schemes will continue. While lowland irrigation schemes have attracted investments by governments, hill rice cultivation, due to its small scale, does not draw much attention in this respect. Hill rice cultivation is important in northern Thailand and Lao PDR, Shan State of Myanmar, large parts of Indonesia, and the Philippines. The cultivation of hill rice in these regions has employed technology which farmers have developed for centuries through the building of canals along steep slopes. While diversion of water from mountain streams for irrigation is relatively easy, the main problem is maintenance of these canals in working order. Given the steep topography and area served, the length to area ratio in some areas can be as low as 1 km ha−1 of irrigated area, although the range tends to be between 5 and 40 km ha−1 of canal (Bacdayan 1980). The difficulty of managing the small hill irrigation systems in Indonesia made it necessary for the government to turn such areas over to the farmers. As of mid-1993 the government had turned over to the farmers 800 systems totalling nearly 63 000 ha. In certain countries of Southeast Asia hill agriculture, supported by irrigation facilities, have proved to be productive. In others, indiscriminate use of fire, destruction of forest cover, and improper land use in the upper reaches of rivers have threatened the very system that is supposed to support the livelihood of the rural people. In the region between Baguio and the Mountain Province on the Western Cordilleras, dams built some twenty-five years ago are almost inoperable owing to siltation. The irrigation systems that support the agriculture of the highlands are being threatened (Bingham 1990). In north Thailand, expansion of cultivated areas and attendant water use have become issues that have generated conflicts between the hill tribes and the lowland Thai farmers. This issue has been studied by Punyawadee (1998) in the Mae Chaem catchment, part of the Chao Praya Basin, where the increased use of water has significantly impacted on the quantity, quality, and reliability of water supply in the lowlands and the central basin.

Rural Potable Water Supply Development While large quantities of water, sometimes of good quality originating from mountain catchments, have been harnessed for agriculture in irrigation schemes in Southeast Asia, the dire need for potable water by rural populations has not been adequately met. Government

investment in water supply projects has tended to be urban-biased, while the rural areas are very much left to themselves to solve their water supply needs. In Indonesia, for example, some two-thirds of the people living in rural communities were reported not to have access to sufficient and reliable potable water and sanitation facilities several years ago (O’Brien 1992). In Myanmar, water supply coverage in the rural areas is barely 15 per cent, and 76 per cent of its population of 40 million in 1989 live outside large towns and cities (Opie 1989). In Malaysia, a concerted effort was made by the government, starting in November 1986, to provide rural water supply under the Malaysian Rural Water Supply Scheme. The approach taken was that of a turnkey contract involving the construction of 137 projects in thirteen states under the British–Malaysian joint venture (Dumbleton 1989). As a result of this scheme, almost 85 per cent of Malaysia’s rural population is served with a piped water supply, the remaining 15 per cent for reason of their remoteness having to look for other alternatives. For isolated villages in Sarawak, small-scale efforts at providing potable water include the use of shallow groundwater via inclined tubewells instead of vertical wells dug in shallow aquifer or rainwater cistern systems from roof run-off (Goh 1992). The use of ponds to collect rainwater and surface and groundwater run-off is also practised (Goh 1991). Communities in small islands may have to rely solely on rainwater for their potable supply. Until recently, Pulau Ketam, an island off the Selangor coast (Malaysia), relied heavily on this source of natural supply (Teh and Bird 2000). In Ambon, deep groundwater wells have supplemented the spring water from the hills to supply the needs of the towns and rural areas. Because of demand, utilization of deep groundwater, especially in the narrow plain between Ambon Bay and the hills, has been overextended, and it is feared that groundwater abstraction has already exceeded the natural recharge of the aquifer by rainfall infiltration (de Bruijn, Hoogsteen, and Bos 1999). Around Yogyakarta, groundwater supplies the water requirements of many villages (Sudarmadi 1994, 1995). Thailand implemented the Masterplan for Rural Water Supply and Sanitation in the 1980s, soon after the declaration of the UN International Drinking Water Supply and Sanitation Decade. Rural areas within the provincial jurisdiction of the Department of Health were the beneficiaries of the Community Potable Water Project. Large-scale and small-scale projects were implemented, but the success of this entire project was mixed.

Hydrology and Rural Water Supply 311

In northeast Thailand, small-scale water projects have been implemented to provide potable water to villages. Some 76.2 per cent of all water supply sources are government-funded, reflecting low self-reliance, as in Malaysia, where rural water supply has the mark of government efforts. For example, the Master Plan (1985–91) envisaged: • upgrading 49 500 shallow and deep wells and pipe water systems; • rehabilitating 4900 shallow and deep wells and pipe water systems; • constructing 2450 pipe water systems, 17 350 shallow and deep wells, and 4.4 million water jars. No less than twelve government agencies under five ministries are involved in the provision of drinking and domestic water; the figure rising to sixteen when agricultural and industrial water is taken into account, and when non-governmental organizations are counted, there are some thirty-three agencies in the water sector. Given this scenario it is not surprising that water has become as much, if not more, a political question as it is an ecological, agricultural, or socio-economic one. It is widely perceived to be the cause of economic stagnation in regions such as the northeast, and hence water is often simplistically perceived as the panacea for economic ills (Weiland, Oberndorfer, and Braham 1993).

Conclusion The water balance characteristics of Southeast Asian countries vary considerably within the region. While many parts receive high rainfall annually, there are others where the natural supplies are insufficient to support development. For the Southeast Asian region as a whole, which is experiencing rapid economic development, water resource use has been rather inequitable —much more is consumed by the urban areas than the rural, and, in the rural context, much more is allocated to agriculture than for drinking. It would appear that unless a drastic and holistic approach in water resources development and management is adopted, the future development of Southeast Asia will perpetuate the imbalance in resource allocation in favour of agriculture at the expense of supply to rural communities. The appropriate use of available water resources to meet demands from many sectors must be urgently designed to avoid a water crisis in the near future. Already Thailand has experienced one of its worst water crises, in 1994 (Rigg 1995), and parts of Malaysia did so in 1998 (Hamirdin 1998). The assumption that the Southeast Asian region is humid, and, by implication, its water

supply is unlimited, is a fallacy. In places like Java, Peninsular Malaysia, and Thailand, there is growing competition between various users. It is thus important for the countries in the region to protect their catchment areas, as these source areas would ensure that rivers will be able to bring the much-needed water for use in the more developed downstream sections. Upland watershed management through reafforestation of private and state lands has started in Java (McCauley 1991) by provincial and district governments through funds provided by the central government. The time has come for such countries to review the use of water by irrigation relative to other urgent needs. There is a growing pressure to limit the wasteful use of water by irrigation by charging farmers for the water used. The current practice of allowing farmers to enjoy highly subsidized rates for irrigation water used should be curtailed (Svendsen and Rosegrant 1994). Many rivers in Southeast Asia have been affected by human activities either through abstraction of water or by deterioration in quality due to pollution. Of all the rivers in Southeast Asia, the Mekong is the least affected at the moment, which promises great potential for the development of the countries through which it flows. The water resources of the Mekong are vast, and the potential for irrigation, power, and navigation is immense. The development of the river could benefit greatly the various riparian states whose economies are lagging behind other countries in the region. In the past, cooperative efforts through the United Nations, multilateral banks, and industrialized countries (Kirmani 1990) have not produced tangible results. The present climate of amity is different, and in the context of ASEAN cooperation, the countries within the Mekong Basin, as well as those outside it, can hope for greater development and prosperity in the years ahead.

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18

The Urban Environment in Southeast Asia Ian Douglas

Introduction As elsewhere, the major cities of Southeast Asia suffer from traffic congestion, air pollution, water supply shortages, garbage disposal inefficiencies, and sewage treatment inadequacies (Barrow 1981). Such problems are not confined to the capital cities and other centres of over a million population. They are prevalent, and often worse, in hundreds of smaller towns of a thousand to a million inhabitants. Most such urban centres have a large proportion of poor, ill-housed people who have difficulty in doing anything to improve their environment. At the same time, the bigger cities will also have some select, well-managed, often walled and gated, suburbs where the quality of housing and water and sanitation services is excellent. However, all social groups may be vulnerable to the air pollution and disease risks associated with a generally poor urban environment. Floods, landslides, and subsidence also do not distinguish the wealth or social status of their victims. These multiple, overlapping urban environmental problems are a response to a complex set of causes or drivers.

The Biophysical and Socio-economic Drivers of the Urban Environmental Situation The character of cities and towns in tropical Southeast Asia is driven in part by the types of human activity within and around them and in part by the environment in which they are situated. The hot and often humid climate has increasingly led to changes in house design from buildings with verandas and arcades designed to be cooled by natural air flows, to more boxlike structures dependent on air conditioning. The exhausts from the air conditioners inevitably add heat to the outside air, warming the immediate urban

environment, often making the narrow streets of many cities hotter and more uncomfortable than they otherwise would be. The design and character of buildings are governed by environmental, aesthetic, functional, and cost considerations. In part building styles reflect the type of shelter needed and in part they make statements about their owners and the activities which go on inside them. It is the same with the settlement as a whole. A town or village has features that help it to cope with the natural environment around it, especially heavy rains and strong winds. However, these settlements also defy nature and protect their inhabitants against natural extremes, so that cultural and economic activities can come together. Part of Southeast Asia is affected by tropical cyclones (or typhoons) that cause extreme winds, sometimes with tidal waves, and prolonged, heavy regional rainfalls over many hundreds of square kilometres, particularly in Viet Nam and the northern Philippines. For example, in October 1996 typhoons, tropical storms, and flash floods killed more than 400 people and seriously affected more than 26 000 households in northern Viet Nam. One of the most extreme such storms brought 1000 mm of rain to Baguio (Figure 18.1) in Luzon, Philippines, in just twenty-four hours. Even lands generally regarded as ‘below the wind’, such as Sabah in northern Borneo, can experience rare cyclones, for example, Cyclone Greg, which badly damaged the town of Keningau (Figure 18.1) in December 1996. Everywhere, much of the rain is concentrated in short-duration, high-intensity thunderstorms which may bring 50 mm or more in thirty minutes, with the power to erode bare ground and cause rivers to rise a metre or more in half an hour. The most severe problems occur when rainfalls of over 100 mm occur in a short period.

The Urban Environment 315

Fig. 18.1. Location map

When 283 mm fell in twenty-four hours at Penang International Airport (Figure 18.1) on 6 September 1999, widespread flooding and landsliding occurred over most of the island. Other areas suffer occasional droughts, especially in El Niño years. For example, early in 1998 it was reported that drought in Sabah had led to critical shortages of both food and water. Streams dried up and agricultural fields became barren, or were burnt, around the towns of Matunggong, Kota Marudi, Sipitang, and Keningau, with food shortages affecting 12 000. Water shortages threatened many parts of Peninsular Malaysia in the 1997–8 El Niño as water distribution infrastructure development had not kept pace with the rate of urban population and water consumption growth.

These extremes are superimposed on the seasonal rhythm of monsoonal influences which create sevenmonth wet seasons in the northern and southern areas of Southeast Asia and provide alternations of sources of moist air streams in the equatorial regions. Few areas never experience a succession of two or more months with less than 100 mm rainfall. When the rain occurs, it is often extremely intense, naturally causing rivers to rise several metres in one or two hours.

Highly Productive, Diverse Biotic Systems With high temperatures throughout the year and throughout the equatorial part of the region there is sufficient moisture for continuous vegetation growth. This creates a huge variety of life forms such that under

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natural rainforest conditions growth and decay, powered by a high mass of decomposers, turn over plant nutrients at a rate not surpassed elsewhere. As soon as this natural biological hothouse is disturbed, organisms seize opportunities to reoccupy the newly opened spaces in the ecosystem. No artificial structure is free from the intervention of organisms that, unless preventive action is taken, will invade every jar of water, every wooden building, and every concrete structure. The combination of high temperature, high humidity, and intense biotic activity is a recipe for weathering and decay. While the biotic activity allows market gardeners to grow four to six crops of vegetables a year to feed urban populations, it also encourages a great variety of disease vectors that can threaten the health of people in human settlements.

A Dynamic and Varied Hydrology and Geomorphology The nature of tropical rainfall, characterized by highintensity, short-duration storms, makes good urban drainage essential, but that rapid drainage sends water to rivers more quickly than before, so increasing storm run-off peaks and possibly raising the frequency of flooding. Urban concrete channels fed by water from roofs and roads can cause storm run-off volumes to increase fourfold compared to those from natural forested catchment areas. The high humidity, constant warm temperatures, and biotic activity favour the breakdown of rocks and the development of thick mantles of weathered material on many hill slopes. The diversity of rocks within the region means that while some areas have weathering profiles of 30 m or more deep, in others the weathered rock and soil profile is less than 2 m thick. However, only the most resistant rocks, such as quartz dykes and quartzites, outcrop at the surface. Generally, the surface soils and weathered material are readily eroded when the vegetation is removed and they are exposed to the intense rain. Thus erosion on construction sites and consequent downstream channel changes cause major problems.

Active Population Growth and Migration While the growth of Southeast Asia’s three megacities, Bangkok, Jakarta, and Manila (Table 18.1), is rapid, many smaller towns and cities are growing at an even faster rate. Viet Nam illustrates the relationship between large cities and smaller urban centres, even though its population is still largely rural. It is the second most populous country in Southeast Asia with a 1995 population of 74 million, then projected to reach

Table 18.1 Population size and growth in some of Southeast Asia’s largest cities, 1960–2015 City (metropolitan area), by country

Brunei Bandar Seri Begawan Cambodia Phnom Penh Indonesia Bandung Jakartaa Medan Palembang Semarang Surabaya Ujung Pandung Lao PDR Vientiane Malaysia Ipoh Kota Kinabalu Kuala Lumpur Kuching Myanmar Yangon Philippines Davao Metro Cebu Metro Manila Zamboanga Singapore Thailand Bangkok Chiang Mai Viet Nam Haiphong Hanoi Ho Chi Minh City

Population (m)

Forecast population (m) 2015

1960

2000

0.02

0.33

n.a.

0.20

1.20

n.a.

1.00 1.45 0.284 n.a. 0.37 1.00 n.a.

3.40 11.20 1.70 1.14 0.80 3.20 0.94

5.09 15.30 2.75 n.a. 1.12 3.56 n.a.

0.15

0.50

n.a.

n.a. 0.22 0.40 0.56

0.38 n.a. 3.80 n.a.

n.a. n.a. n.a. n.a.

1.00

4.50

n.a.

0.12 n.a. 2.40 1.00 1.65

1.01 1.00 11.50 4.50 3.47

1.70 n.a. 14.66 n.a. n.a.

2.30 0.25

10.30 1.00

12.26 n.a.

n.a. 0.90 2.30

0.45 3.40 5.00

n.a. 5.00 n.a.

Note: n.a. = not available. a In 1990 the total population of the entire mega-urban region (EMR) around Jakarta was 17.1 million. Sources: World Resources Institute (1998); www.singstat.gov.sg/FACT/HIST/hist1.html: Encarta World Atlas.

80 million by 2000. About 15 million (20 per cent) of the population is urban. The four major cities, Ho Chi Minh City (formerly Saigon), Hanoi, Da Nang, and Haiphong, account for 37 per cent of the urban population with the remainder living in about 500 locations. About 80 per cent of the urban population is concentrated in seventy-seven other towns with populations of over 15 000 that form the country’s most important urban and economic centres and, at the same time, are the sources of pressing environmental issues for the country.

The Urban Environment 317 Table 18.2 Indicators of the urban infrastructure provision in the largest cities of Viet Nam Urban infrastructure indicator

Haiphong

Hanoi

Ho Chi Minh City

Total untreated waste discharge (million m3 y−1) Population served with sewage systems (%) Sewage receiving treatment (%) WHO drinking-water quality standards met by piped supply? Solid waste collected (%) No. of motor vehicles Population (million) 2000

70 20–35 0 No 70 235 000 0.45

120 20–35 0 No < 50 42 000 3.40

240–300 60 0 Yes 80 775 000 5.00

Source: Fink (2001).

Cities of the most populous countries, like Indonesia and the Philippines, are beginning to exhibit more complex environmental problems than the chief, primate cities of smaller countries. By 2010 the population of the Greater Bandung area in Indonesia is expected to be approximately 7 million. The metropolitan of Bandung already has a population density of about 120 people per ha. DKI Jakarta has about 11 million people at a density of about 170 per ha. Migration to cities adds to their problems. In-migrants formed 41.1 per cent of the population of Jakarta in 1971, while the Rizal Province of the Philippines which contains the expanding outer suburbs of Manila, received 67 000 in-migrants from other parts of the country every year in the 1970s (Lightfoot 1999).

Expanding Economies The great expansion of the Southeast Asian economies since 1970 has been a foremost driver of urban change. Between 1965 and 1990 manufacturing as a percentage of GDP grew from 8 to 21 per cent in Indonesia, 20 to 26 per cent in the Philippines, and 14 to 27 per cent in Thailand (Drakakis-Smith 1993). In Thailand the percentage share of GDP accounted for by industry has grown from 24 per cent in 1970 to 36 per cent in 1990. Only 211 factories were polluting the air and releasing waste-water discharges in 1969. By 1989, 26 235 factories were doing so. The number of factories producing hazardous waste also increased dramatically over this period. Economic growth in Viet Nam was greatly accelerated by the introduction of reforms in 1986. Since then it has averaged around 7– 8 per cent p.a. leading to increased urbanization (estimated to grow by 70 per cent in the next ten years) and rising urban incomes. Foreign investment in Viet Nam sometimes has been limited by infrastructure constraints in urban areas (Table 18.2). Water and sanitation systems established in cities and towns between 1890 and 1910 have not

been adequately maintained or upgraded because of resource constraints.

Interventionist Governments Much of the economic growth in Southeast Asia comes from state-led development (Rigg 1991). Many resourcerich countries have used a combination of increased local processing of indigenous raw materials, importsubstituting industrialization, and export-oriented industrialization (Dicken 1990). Most Southeast Asian governments have ambitious plans for the development of their capital cities, including large prestige projects, such as new airports, new urban fixed-rail transport systems, and new industrial zones. These plans can drive the direction and rate of urban development. Investment in the capital city, especially in public hospitals and educational facilities, often attracts people from rural areas. These migrants from the country to the city believe that their children will have far greater opportunities of achieving good health and gaining a high-quality education than in a village served by a one-room school and a clinic in a neighbouring centre 15 km away. Nevertheless, with overwhelming urban problems such as traffic congestion and its associated air pollution, housing shortages, and inadequate water supply, governments find it extremely difficult to pay close attention to the growing problem of the disposal of hazardous waste from the expanding industrial sector. Many governments have made special efforts to develop industry and commerce around their cities. Strategies such as the Johor–Riau–Singapore growth triangle centred on Singapore, or Viet Nam’s northern economic development triangle linking Hanoi, Haiphong, and Ha Long, deliberately encourage investment and industrial and urban growth. Indonesia made Batam Island, 21 km south of Singapore, the country’s first free trade zone. The island now has three of the forty-two industrial estates in the whole of Indonesia (Royle 1997). A huge environmental

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and social transformation is occurring as the island’s population grows from 150 000 in 1995 to a planned 700 000 in 2006. The Vietnamese government has designated three areas of the country as industrial development zones: (1) Hanoi–Haiphong–Quang Ninh, (2) Quang Nam–Da Nang, and (3) Ho Chi Minh City–Bien Hoa-Vung Tau– Dong Nai. These areas are receiving the bulk of foreign investment in the country, currently the dominant form of investment, mainly into the industrial sector. The resulting high rates of migration to the country’s largest cities is increasing pressure on the government to provide gainful employment in the cities. These events and trends have led to a series of environmental problems. Most Southeast Asian countries have been successful in promoting economic growth, but that has also produced urban growth at differing rates. Among the capital cities, Singapore and Phnom Penh represent the extremes. While per capita incomes in Singapore exceed those of many west European countries, their equivalent in Phnom Penh is akin to the levels prevalent in most of sub-Saharan Africa. The income levels are reflected in the investment in urban infrastructure and environmental management. Thus the government of Viet Nam, through its ‘Doi Moi’, or ‘renovation’, policies has provided for macro-economic stabilization, market liberalization, and institutional and legal reforms that have had a tremendous impact on economic performance. From 1989 to 1994 average GDP grew at 7.3 per cent p.a. and exports increased by nearly 30 per cent p.a. The annual inflation rate has been reduced from 400 per cent p.a. in 1988 to single-digit levels since 1993. Viet Nam, however, remains a densely populated and poor country largely dependent on natural resources and primary products for its economic well-being.

The Urban Atmosphere A brown haze often exists over large Southeast Asian cities. Sometimes this is almost entirely due to particulate emissions within the urban area, but at others it is the result of volcanic eruptions or of distant fires. Emitted gases have the capacity to be transported over large distances, sometimes many hundreds of kilometres, and may give rise to deposition in another country. The potential for such transboundary air pollution was evident in the Indonesian forest fires in 1990–1 and 1997. In Kuala Lumpur, Malaysia, the severe haze episodes of August 1990 and September–October 1991 occurred during prolonged dry periods in which a diurnal land– sea breeze effect built up and levels of total suspended

particulates rose. A Landsat image obtained during the August 1990 haze event showed that the haze was associated with heavy traffic along Kuala Lumpur’s main highways and with agricultural burning. The breeze from the land was carrying the pollutant plume out towards the sea. During August 1991 the suspended particulate content of the air increased by 13 per cent per day from 120 µgm−3 on 16 August to over 400 µgm−3 on 27 August. A change in weather conditions saw the particulate content then drop rapidly from 28 to 31 August (Samah 1992). Vehicular emissions account for more than 75 per cent of the air pollutants in the Klang Valley. Probably some 53 per cent of the air pollutants are due to hydrocarbons associated with the combustion of fossil fuels and with agricultural burning. The second major source is sulphur dioxide, contributing about 25 per cent, with nitrous oxides making up another 10 per cent. Others, such as particulates and carbon monoxide, contribute about 5 per cent each of the total air pollutants. The main health problems linked to these haze episodes were respiratory and eye problems. People susceptible to related diseases were warned by the Health Department to stay indoors. The area affected by the air pollutants from the 1997 fire spread for more than 3200 kilometres, east to west, covering six Asian countries and affecting around 70 million people. In the Malaysian sate of Sarawak, the air pollution index hit record levels of 839 (levels of 300 are equivalent to smoking eighty cigarettes a day and are officially designated as ‘hazardous’).

The Most Polluted Megacities One consequence of rapid economic growth in Southeast Asia is the increased demand for energy, primarily for industrial production and transportation. The countries of Southeast Asia are generally not as dependent on coal and non-market fuels as India and China. However, the rapid growth in energy consumption and resulting increase in emission of air pollutants have caused a general degradation in air quality, primarily in urban areas. Suspended particulate matter, lead, sulphur dioxide, carbon monoxide, and volatile organic compounds variously cause air quality and human health problems in major cities. Bangkok, Jakarta, Manila, and Kuala Lumpur have all experienced a noticeable deterioration in their air quality in recent years. (See Table 18.3.) Energy demand forecasts indicate that air pollution is likely to increase during the coming decade, although stricter policies, such as requirements that catalytic converters be installed in new vehicles, will help to some extent.

The Urban Environment 319 Table 18.3 Air quality in major Southeast Asian cities (annual means) City

Population (thousands)

Total suspended particulates (mg m−3)

Sulphur dioxide (mg m−3)

NO2 (mg m−3)

Bangkok Jakarta Kuala Lumpur Manila Singapore Surabaya WHO Guideline values for annual means

6547 8621 1238 9286 2848 3200

223 271 85 200 n.a. 330 90

11 n.a. 24 33 20 43 50

23 n.a. n.a. n.a. 30 15 50

Note: n.a. = not available. Sources: Based on data for 1995 from the WHO’s Healthy Cities Air Management System and the World Resources compared with the WHO guideline values. World Bank (1999); Santosa (2000).

Air pollution in Jakarta is severe. The city has had three or more pollutants which exceeded World Health Organization (WHO) health protection guidelines (Kretzchmar 1995). Ambient levels of particulate matter exceed health standards at least 173 days per year. Vehicle emissions constitute the most important source of harmful pollutants (44 per cent of particulates, 89 per cent of hydrocarbons, 73 per cent of nitrogen oxides, and 100 per cent of lead). As the demand for motor vehicles rises with economic growth, attendant pollution is likely to worsen (World Bank 1994). The residential sector also contributes about 41 per cent of particulate matter, largely from the burning of solid waste by households and by refuse recyclers; industry contributes the greatest share of sulphur oxides (63 per cent) (World Bank 1994). Jakarta and other cities in Indonesia experience acid rain. Monitoring from 1995 to 1997 revealed the average pH of rain in Jakarta to be 5.2. In Pakanbaru, Sumatra, and Manado, Sulawesi, the pH of rainwater in 1995 averaged 4.9 (data from the Indonesian Conservation Database List). In Thailand industrial energy consumption accounts for 20 per cent of primary energy demand. Fuel oil and biomass are the major energy sources for the industrial sector. Although lignite and coal currently constitute a small share of the energy consumption, use of these fuels is growing rapidly. From 1986 to 1988 lignite and coal usage increased by 75 per cent, whereas fuel oil usage increased by only 20 per cent. The growth of lignite and coal consumption in the industrial sector is having a significant impact on air quality. In 1991 Thai industries (including manufacturing, construction, and mining) emitted some 13.2 million tons of carbon dioxide (15 per cent of the country’s total emissions), 208 500 tons of sulphur dioxide (22 per cent), 70 000 tons of nitrogen dioxides (12 per cent), and 351 000 tons of suspended particulate matter (56 per cent). Four industrial sectors

are responsible for a large proportion of the industrial air pollution: the non-metal industry, food processing, pulp and paper, and the textile industry. Motor vehicle emissions are characterized by compounds of carbon and nitrogen, including methane. Bi-weekly measurements of atmospheric methane concentration by a grab-sampling/GC analysis method at Chulalongkorn University, Bangkok, revealed annual average CH4 concentrations (August 1991–July 1992) of 2.11 ± 0.20 ppm (Boonjawat et al. 1995). At Klong Aum, Nonthaburi, north of the city, the mean was 2.4 ± 0.55 ppm. In 1993 the CH4 annual mean at Chulalongkorn University was 2.12 ppm, and for seven months in 1994 it was 1.94 ppm. During August in 1991– 4 the natural variability of atmospheric CH4 concentration showed consistent seasonal variations, characterized by concentrations above the mean during November–February, when Bangkok was subject to northerly winds, and lower concentrations under the influence of the southwest monsoon from May to October. The continuous monitoring showed that peak methane and carbon dioxide concentrations coincided with traffic congestion time, the early morning (6–10 a.m.) on working days, and with some late afternoon traffic peaks. During long holidays and Sundays concentrations of these gases were close to baseline levels. Methane peaks observed at night were not related to transport and may be related to biotic activity, perhaps from the decomposition of municipal wastes and waste water by micro-organisms. The air pollution problems in Bangkok are intimately related to the way the city has grown, the provision of public transport, and the road system. The awkwardly arranged road system, with a few wide through roads bounding large blocks of the city, only covers 11 per cent of the urban land surface, a smaller percentage than in most international cities. The antiquated, but cheap,

320 Ian Douglas

bus service is slow, crowded, and uncomfortable. Until the 1997 economic crisis those who could do so bought cars and added to the traffic congestion. To alleviate the latter the police used various strategies to remove obstacles to the free flow of traffic. This multi-nodal city has many within- and across-town commuter movements, most of which take less than 40 minutes, but the density of traffic remains a problem that is not easily solved when mass transit, road construction, and urban planning are tackled in a piecemeal, disaggregated manner (Ross et al. 2000). Many of the urban air pollution problems are intimately related to the state of national economies and to environmental legislation. For example, in 1990 no unleaded petrol was available in the Philippines. In that year the 1.5 million tonnes of leaded petrol used in the Philippines contained 0.8 g l−1 Pb. By 1998 the lead content of petrol had been reduced to 0.15 g l−1. However, the cities still have many old vehicles, without catalytic converters, unable to operate on zero lead, and therefore some lead emissions still occur. Most people in Southeast Asia aspire to own a motor vehicle, be it a motor scooter in Viet Nam or a car in Malaysia or Thailand. Inevitably, many treasure old vehicles, but cannot always afford to keep them in first-class condition, or to replace them with new models with better emission controls. In addition, thousands of battered diesel trucks and buses emit large amounts of particulates every day.

Urban Water Problems Towns and cities in the region rely on diverse water sources. Some of the largest depend on inter-basin transfers, such as the supplies from Johor to Singapore, or from the Langat and Selangor Rivers to Kuala Lumpur. Many depend primarily on groundwater, but have to augment it by supplies from reservoirs. On large rivers, deteriorating water quality through pollutant discharges means that the towns and cities along the lower reaches that depend on the river for their water supplies will face increasing costs for water purification.

Surface Water Supplies Roof water collection is an important source of supply for many urban households in Southeast Asia. An average annual rainfall of 2000 mm yields a large catch, but air pollution creates a risk of rainwater contamination. The traditional main water source has been direct pumping from the nearest river. Such sources are now inadequate for many cities and alternatives have to be sought.

The Bangkok Metropolitan Water Works Authority draws water from the Chao Phraya River at a rate of 42 m3 s−1, which is not far below the minimum flow of the river at the end of the dry season. The river basin receives an average annual rainfall of 2000 mm. However, even at the water intake, the dissolved oxygen content of the river water is 50 per cent below the natural level. Faced with these problems of both quantity and quality of water, the Authority is planning to take new supplies from the Mae Klong River, 60 km away. Despite all these measures, only 60 per cent of Bangkok’s inhabitants receive direct supplies from the Authority. Piped water is estimated to reach about 50 per cent of the urban population in Viet Nam. In the largest cities access ranges between 60 and 80 per cent, while in small towns it is under 30 per cent. The rest use shallow wells, rainwater, rivers, and ponds. In the larger cities residents frequently carry water to their homes from public taps. Owing to past resource constraints, the quality of service has deteriorated. Low pressure and intermittent supply are common. Treatment plants are often ineffective, suffering from design and construction faults; in many cases, they simply do not exist and water is supplied untreated. Groundwater is a source for only 30 per cent of urban water systems, while 70 per cent use surface water. The result of these developments is that drinkingwater supplies in urban areas have been compromised as a result of pollution discharges and increasing water demand. The quality of urban water supplies is generally poor. Only Ho Chi Minh City largely meets drinkingwater standards set by the WHO for its piped water supply. The Hanoi system may also soon meet WHO criteria, assuming successful implementation of plans for disinfectant services and training of operational staff. Of the 436 urban centres with populations exceeding 5000 in Viet Nam, only 100 have piped water systems. Together, these systems serve about 6 million people, or about 47 per cent of the urban population. Of the remaining urban inhabitants, about 46 per cent use water from shallow wells (in most cases unprotected and polluted), rainwater collection tanks, streams, and ponds. With about 80 per cent of the water supply not meeting drinking-water standards and poor access to sanitation, water-borne diseases account for much of the ill health in the country.

Groundwater Abstraction Many cities rely in part on groundwater supplies for domestic and industrial needs. Those on coastal plains frequently suffer problems of subsidence and salinity

The Urban Environment 321

owing to groundwater abstraction. The problems of Bangkok are well known (Buapeng 1987; Ramnasong and Buapeng 1991) and are dealt with in more detail elsewhere in Chapters 19 and 21. The eight groundwater aquifers beneath the city are extensively exploited. That at 500 m depth is already exhausted. The high rate of groundwater removal is leading to increasing subsidence. New land development projects in the new areas on the east bank of the Chao Phraya River involve opening new wells, and are causing new areas of subsidence. Even though only one-third of the total water demand of DKI Jakarta of about 750 million m3 y −1 is met by groundwater (Figure 18.2), it is of considerable economic and social importance, because about 70 per cent of the population and many industries depend upon it.

Most groundwater is abstracted from tens of thousands of shallow wells (80 per cent) and more than 3000 deep wells (20 per cent). From less than 10 million m3 y −1 in 1950 groundwater abstraction was steadily increased to 250 million m3 y −1 in 1995, following the growth in population and industrial development. In the latter year about 53 million m3 y −1 were being drawn from deep aquifers, 50 per cent more than that from registered wells (33.8 million m3 y −1). The rapid increase in groundwater abstraction after 1970 (Figure 18.2) resulted in a drop in the level of the groundwater table from between 10 and 20 m below sea level to between 15 and 39 m below sea level in twenty years (Budi Tjahjadi 1991). Water quality deteriorated and saline water intrusion became such a problem in the older part of the city that over a large

Fig. 18.2. Trends in the water supply of DKI Jakarta, Indonesia, showing increase in ground-water abstraction since 1980 (Source: After Rismianto and Mak 1994)

322 Ian Douglas

Fig. 18.3. Penetration of saline water beneath Jakarta, Indonesia, showing the inland movement of the boundary between the saline and fresh groundwater in (a) the first deep aquifer system at 40–140 m depth, and (b) the second deep aquifer system at more than 140 m below the surface (Source: Based on data in Budi Tjahjadi 1991)

area groundwater abstraction is no longer allowed, save exceptionally where boreholes must be more than 250 m deep (Figure 18.3). The marked increase of saltwater penetration in the first deep aquifer (40–140 m below sea level) under metropolitan Jakarta over the years 1987 to 1990 was quite marked (Figure 18.3a) and is likely to have continued. Shallow groundwater with over 500 mg l−1 chloride and specific conductance levels exceeding 1500 µmhos cm−1 extends as far as 9 km inland under Jakarta. Saline water in the deeper aquifer system has progressively moved southward, reaching 3–7 km inland. Abstraction has also led to land subsidence. In Bandung, surface water and shallow groundwater exploitation poses a serious threat to the sustainability of urban development. Ground- and surface-water pollution affects the poorer urban and rural people. Since 1980 intensive exploitation of the aquifers in the Bandung city area has lowered groundwater levels dramatically (IWACO–WASECO 1991). Contamination of the shallow groundwater (at depths from the surface

down to 40 m below), which is extracted for local water supplies from dug wells or shallow boreholes, is widespread in many parts of the basin. The deeper groundwater in the confined aquifers remains relatively uncontaminated, but the fall in the hydrostatic heads in the aquifers and increasing contaminant loads suggest that this quality may be threatened in the future. The contamination of the shallow aquifers is mainly due to the infiltration of untreated domestic sewage as only a small part of Bandung is connected to a sewer system (Wagner 1991). Industrial waste water discharged directly to canals and streams, poorly managed urban waste dumps, and increasing fertilizer residues from the surrounding vegetable farms add to the contamination hazards. In Viet Nam, groundwater is used to meet Hanoi’s entire drinking-water demand and more than 500 000 m3 is abstracted daily. In some areas, ammonium concentrations above 25 mg l−1 have been measured, which is well in excess of today’s drinking-water standards. This is a threat against the future utilization of groundwater in

The Urban Environment 323

Hanoi. In order to limit, and if possible prevent, a further degradation of the groundwater quality, it is necessary to identify the pollution sources (Andersson and Norrman 1998).

Waste-Water Disposal Provision of sewerage has lagged behind urban growth in much of Southeast Asia. In Thailand, facilities have insufficient capacity to handle the volume of domestic and industrial waste water now being generated. In Bangkok, much such waste is discharged directly into rivers and canals (klongs) without treatment. Many watercourses in urban and industrial areas are therefore seriously polluted. In addition, groundwater contamination, especially with coliforms and heavy metals, is becoming increasingly severe in Thailand. This is particularly the case in Bangkok, where it has implications for future availability and quality of drinking water. The canals of Bangkok in a crude way serve as a kind of natural oxidation pond for the sewage discharged into them. Organic matter will be diluted and oxidized by natural water bacteria to produce soluble oxides and bacterial growth which will flocculate and settle to the bottom as in a natural pond. The heat and sunlight accelerate the process. Several community canal clean-up schemes have removed the silt from the channels and have reduced the waste deposited in them (Boonyabancha 1999). One study calculated that the 191 km of canals could cope with the sewage from 7.9 million people in Bangkok. The city’s population exceeds that number. In the Philippines only 15 per cent of the Manila Metropolitan Region is sewered. Disposal of sewage sludge from existing septic tanks in the region (over 48 000) is uncontrolled. In Metro Manila it is estimated that 25 million m3 of acid and alkaline liquid waste is disposed annually, primarily from the electronics industry. In addition, almost 2000 m3 of solvents and 22 000 tonnes of heavy metals, infectious wastes, biological sludges, lubricants, and intractable wastes are disposed on land or into watercourses. In Viet Nam, access to sewerage services is available to 60 per cent of the population in Ho Chi Minh City, but in general coverage is much lower in other cities, including Hanoi (30 per cent). The combined drainage– sewerage collection network is in need of extensive repair, and treatment of waste water, including servicing of septic tanks, is non-existent, leading to pollution of inner-city waterways and groundwater. The main waste-water problems in Viet Nam are: (a) extreme deterioration of physical infrastructure due to shortage of investment and deferred maintenance resulting

from the exigencies of a war economy and subsequent resource constraints, and (b) institutional weakness of the implementing agencies, which lack a secure and predictable revenue base. In addition, none of the cities in Viet Nam has operating treatment systems for domestic sewage. In Ho Chi Minh City, for example, the total waste-water discharge from households and commercial establishments is estimated at 550 000 m3 day −1. About 46 per cent of the household blackwater is treated by septic tanks, the remaining 54 per cent, and 100 per cent of greywater, is discharged untreated.

Solid-Waste Management Urban growth and economic prosperity leads to rapid expansion in solid-waste generation. In Bangkok, per capita solid-waste production in the city is around 320 kg y −1 (World Resources Institute 1996). In 1987 an estimated 5100 tonnes of solid waste were generated daily by Bangkok’s population. Of this 3900 tonnes were collected and some 110 tonnes recycled. The remaining 24 per cent was dumped, mostly onto vacant areas or directly into canals or rivers. Most of the collected waste goes to landfill sites, such as those at On-Nuj, Nong Khaem, and Ram Intra, which were operating in 1988 (Phantumvanit and Liengcharernsit 1989). The industries in Thai cities currently generate approximately 2 million tonnes of hazardous waste each year. Over 72 per cent of hazardous waste comprises heavymetal sludge and solids, mostly produced by the basic metal industry. However, waste management is relatively well advanced in the metals industry, a large part of these heavy-metal-contaminated wastes being chemically stabilized on site and stored in containers. The volume of hazardous waste generated in Thailand is expected to grow at a rate of 8.6 per cent p.a., in response to continuing growth in the metals industry, transport equipment and machinery manufacture, and the chemicals, textiles, rubber, and pulp and paper industries. This growth produced a threefold increase in the current volumes of hazardous waste, to 6 million tonnes per year, by 2001. Solid-waste collection and disposal in Viet Nam is inadequate in terms of both population coverage and effectiveness. Some 20 per cent and 50 per cent of the quantity of solid waste generated in Ho Chi Minh City and Hanoi respectively is not collected. The uncollected wastes are instead dumped in vacant grounds, backyards, or lakes and streams. In addition, the current disposal sites are not properly designed, are inadequately sited and poorly managed, and will all be filled to capacity in a few years. No facilities exist for the controlled,

324 Ian Douglas

safe disposal of hazardous and industrial waste. Existing plans for new sites generally underestimate the amount of solid waste that will be generated from a growing and increasingly wealthy urban population. In the Philippines, industrial wastes (hazardous and non-hazardous) are generally poorly handled— either dumped on land within the factory area or discharged with waste water. About 6000 t of solid wastes are generated daily in Metro Manila; and of these, only about 3400 t are collected and transported to landfill sites. Large landfill sites become major geomorphic features of large Southeast Asian cities. The biggest waste dump in Manila, at Balut, Tondo (Figure 18.4), used to receive approximately 650 t of solid waste per day. This dump occupied 34 ha of Manila Bay and created a hill of garbage rising 40 m above sea level. At one time some 25 000 ‘rag-pickers’ obtained wastes from this dump, with perhaps 60 000 depending on recycling waste for their basic needs. Landslides and fires constantly put these people at risk. Public concern led to the closure of several dumps like Balut, and now just one landfill site for Manila remains, the 73 ha San Mateo landfill in nearby Rizal Province, to which 6000 t of waste are transported by truck every day (Wallerstein 1999). Nearby groundwater and streams are contaminated with high levels of coliform bacteria and other pollutants. Improvements are in hand in the Philippines. The Industrial Waste Exchange Philippines project started in 1987, promoting waste transfer and utilization between industrial firms. The Environmental Management Bureau acts as a clearing-house, providing linkages and information to industry. Recycling of waste oil to produce lowgrade fuel is also undertaken in several companies. The Toxic Substances and Hazardous Waste Control Act of 1990 prohibits the entry, even in transit, of hazardous wastes and their disposal within the Philippine territorial limits. The Act provides for the proper management of hazardous wastes by specifying the responsibilities of waste-generators, waste-transporters, and wasteprocessors. It requires registration of all waste-generators in a prescribed form; establishes a manifest system to be maintained, which will include waste transport records, specification of waste storage and labelling, and the issue of permits for new waste treatment and disposal facilities. Singapore illustrates the growth and changes in waste production and disposal. Solid-waste production in Singapore in 1985 was about 1.5 Mt per year or 600 kg per capita (Vickeridge 1992). By 1998 the total amount of refuse collected had increased to 2.84 Mt y −1 or 1.1 t per capita (Singapore, National Environment Agency 2002). In 1985 about half the domestic waste was

taken to the Ulu Pandan refuse incinerator plant and the remainder was dumped at controlled landfills at Lim Chu Kang and Lorong Halus. In 1998 three incineration plants, at Ulu Pandan, Tuas, and Senoko, processed a total of 1.88 million tonnes or 66.3 per cent of the total refuse generated in Singapore. The rest of the refuse was disposed of at the Lorong Halus dumping ground. The landfill space of Lorong Halus dumping ground was completely used up by 31 March 1999 and Pulau Semakau, the first offshore landfill, is now the only landfill in Singapore. It started operation on 1 April 1999. Everyday non-incinerable waste and incineration ash is taken by barge to Pulau Semakau via the Yuas Marine Transfer Station. Singapore’s efficient system provides a model for the wider region.

Urban Rivers Land clearance for building construction occurs around all tropical cities, often cutting tens of metres into deeply weathered hillsides and temporarily exposing large areas to the erosive power of the rain. All the processes previously described with reference to forestry operations apply with even more effectiveness in this situation. Around the city agricultural land may be used for extremely intensive market gardening, or may be less well tended and used for cattle grazing while owners wait for land prices to rise to a point when it is propitious for them to sell to developers. Other land on the urban periphery is alienated for waste disposal and landfill operations, and is occupied illegally by impoverished squatter settlers or is converted to recreational uses. All these pressures reduce the vegetation cover and accelerate erosion. Some mining activities involve moving huge quantities of overburden and working alluvial material. Around the alluvial tin mines of Southeast Asia are huge areas of mine waste and piles of sand that has passed through the separators used to sort out the grains of tin. Such land is both degraded and derelict, and has special problems of rehabilitation. Other mining operations such as copper-mining degrade land both through their urban-type infrastructure problems of roads and through their need to dispose of tailings.

Water Pollution In urban areas in Southeast Asia, rivers are seldom regarded as scenic assets. Exceptions are the tidal Kuching River in Kuching Sarawak, which is very much a focal feature of the city, and the Singapore River, which has been restored from the highly polluted state it was in in the 1960s. Generally, urban rivers are unattractive, discoloured, often sediment-laden, and low in oxygen.

The Urban Environment 325

Fig. 18.4. The Manila area, Philippines

Approximately 20 000 of the factories in Thailand registered with the Department of Industrial Works are classified as water-polluting industries. The total load of Biochemical Oxygen Demand (BOD) in a river

reflects the organic matter coming from domestic and industrial waste-water discharges. Prior to treatment Thai industries discharged about half a million tons of BOD in 1991, of which the sugar industry accounted

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for 29 per cent, the pulp and paper industry produced 19 per cent, and the rubber industry discharged 18 per cent. However, although the BOD load of industrial waste-water discharges is high, some 70 per cent of industrial effluents are now treated prior to discharge into watercourses. Discharge of untreated waste water degrades the quality of the rivers and canals. The contamination of the rivers is not caused solely by industrial waste water; domestic and agricultural waste waters are also significant sources of water pollution. Due to inadequate sewerage infrastructure and a severe shortage of sewage treatment facilities, residential waste-water discharges are now posing a serious threat to river quality in Thailand, particularly in urban areas. The BOD load from residential sources accounted for 93 per cent of the total BOD load to the Mae Klong River in 1990 and for 75 per cent of the total BOD load to the Chao Phraya River in 1988. In Viet Nam, there is no treatment of waste water in Hanoi, either from households or from the expanding industrial sector. Furthermore, large amounts of fertilizers are spread within agricultural activities, which traditionally represents the largest sector in the area. The historic To Lich River, which flows through Hanoi, has become entirely septic over its length in suburban and urban settlements, as it accepts waste water and solid wastes from households and commercial and industrial establishments. Several countries and cities have embarked on river clean-up programmes, often involving partnerships with community groups, such as those working on the canals in Bangkok. The Prokasih (Clean River Programme) was introduced by the Indonesian central government to reduce water pollution in eight priority areas. Under Prokasih, cooperative action between local communities and the government of Jakarta has reduced the pollution of the Ciliwung River since 1989. In Surabaya, the provincial government and local communities have helped to clean up the Kali Mas through removal of debris from the stream and through building toilets, achieving local management of sectors of the river, tree-planting, banning waste disposal into the river, and preventing bathing. Water pollution episodes are now newsworthy, and the river is used for rowing, waterskiing, and other leisure activities (Santosa 2000). In Manila, in the early 1950s, the Pasig River (Figure 18.4) could still provide water for homes, fish for dinner, and irrigation for farms. It was a channel for ferrying goods between Manila Bay and Laguna de Bay. But even then the river was slowly but steadily

dying as people and industry used and abused it without much thought for the future. By the 1970s all the fish and aquatic plants had disappeared. People could no longer swim in it or even use it to wash clothes. The Pasig River has high levels of metals like lead and manganese from the huge quantities of industrial and domestic wastes discharged into it. Manila Bay receives the untreated sewage outfalls from some 8 million people and has high mean copper, lead, and cadmium levels (Vicente-Beckett 1992). Decades of siltation have reduced the depth of the river to only 3– 4 metres in places. Nevertheless, a major improvement programme has been undertaken since 1995 and the BOD has decreased by 30 per cent.

Sedimentation Most rivers in cities in Southeast Asia suffer from the build-up of silt and debris on their beds. These river channel deposits reduce the depth of the channel, which means that it will overflow at a lower river stage than previously. The clay, silt, sand, and gravel in the river thus can result in more frequent flooding, with sometimes major overflows of smelly, muddy water into houses and other buildings close to the river. Most such sedimentation problems are created by human activities, especially mining and land clearance for construction. Mining wastes are a problem in many towns and cities throughout the region. Often the mine waste dumps are on hillsides above settlements. Extreme rainfalls can make the dumps unstable and cause debris to be washed into nearby rivers. During the passage of Typhoon Babs over the Philippines in January 1999, 3500 people on Marinduque Island had to be evacuated from the banks of a river that became filled with silt washed down from a copper mine. Debris from tin-mining frequently escapes from tailings mounds and washes downstream. Many rivers in Peninsular Malaysia, such as the Kinta and Selangor, have had periods of such severe siltation that they have had to be channellized. On the Stung Sangkor above Battembang, Cambodia, unregulated gemstone-mining releases so much sediment that the river channel has been silted up. The bed of the river is much higher than previously, and the town now suffers much more frequent flooding than before. Urban and industrial construction are perhaps now the most significant contributors of sediment to urban rivers. When the rotted rock and soil beneath the tropical forest is exposed by land clearance, raindrop splash erodes the soil. The rainwater running over the ground at first creates little rills that carry the eroded

The Urban Environment 327 Table 18.4 Results of urban catchment studies in Pulau Pinang, Malaysia Catchment

Land use

S. Air Hitam S. Air Hitam

Tropical rainforest 4.750 Tropical rainforest in upper part, stable urban area in lower 8.870 Disturbed forest and semi-urban 0.553 Rapidly urbanizing with quarrying and construction 11.523

S. Relau S. Relau

Area (km2)

Rainfall (mm)

Sediment yield (t km−2 y−1)

2580

74.49

2580

376.59

1830

911.09

1830

3102.73

Source: After Wan Ruslan (1995).

sediment. Some of these enlarge into deep gullies, which widen and deepen as they collect more and more water. Eventually huge volumes of mud and water enter nearby rivers. Good examples are to be found on Pulau Pinang, Malaysia, where newly urbanizing areas on deeply weathered granites (Table 18.4) yield forty times the mass of sediment derived from similar areas of natural rainforest vegetation (Wan Ruslan 1995, 1997; Wan Ruslan and Rahaman 1994). Aggradation of these small rivers increases the frequency of nuisance flooding on the urbanized coastal plain of the eastern part of the island.

Flooding Urbanization changes the rainfall–run-off ratio. In a tropical rainforest as little as 5 per cent of the rain may run over the surface of the ground. In a fully built-up area as much as 95 per cent of the rain may do so. Thus, as the built-up area grows, peak discharges in the urban rivers increase. For example, in the Upper Bukit Timah Canal in Singapore the expected five-yearly peak discharge increased from about 74 m3 s−1 in 1950 to nearly 92 m3 s−1 by 1986. The concrete-lined river channel has had to be enlarged, and two diversions have been constructed to take excess storm run-off away from the main channel through the city centre (Gupta 1992). Metropolitan Manila has long had a severe flooding problem, and Manila City itself is particularly vulnerable. A few hours of moderately intense rainfall can inundate significant portions of this densely populated city. The worst and most destructive floods may occur when the Pasig River overflows its banks during intense local rainfall. To reduce overbank flows the Mangahan

Floodway Project has been developed. The project limits the discharge of the Pasig River when it reaches bankfull stage by diverting the excess flood water through a man-made channel to Laguna de Bay. It protects the Greater Manila area from peak flows up to a 100-year recurrence interval. In January 1999, when Typhoon Babs crossed central Luzon, about 15 700 people were evacuated from lowlying slum areas close to rivers and a further 4500 in suburban Marikina had to be moved away from the local river. These low-lying areas of Metropolitan Manila are still threatened by flooding, because the Pasig and other river channels and drainage facilities have insufficient flow capacity. To cope with this problem, a detailed design of a north Laguna lake flood control and drainage project has been completed. A large part of Naga City in the Bicol region of southeastern Luzon lies about a metre below sea level. The city is vulnerable to flooding during high tides and storm surges, as well as by overflow from the Bicol River, whose catchment area includes much steepland on the slopes of Mounts Isarog and Iriga. Land clearance on the slopes increases the sediment load of the river, reducing channel capacity and aggravating the seasonal flood problem. Some natural flood basin areas, such as around Baao, have been occupied by agricultural settlements, and greater use of the natural flood detention storage has been prevented by community pressure. However, major embankments around Minabalac are being developed to alleviate flooding in Naga City itself. Like many other coastal cities in Southeast Asia, Kota Kinabalu, Sabah, has gained new housing land by clearing steep slopes, draining and filling former mangrove swamps, and reclaiming land from the sea. Together these processes have increased run-off, reduced the capacity of natural flood storage areas, and lengthened the seaward ends of river channels. Consequently, large investments have been needed to improve drainage and provide for flood mitigation schemes, but because of poor planning and implementation, the people continue to suffer floods and traffic jams. The piecemeal nature of some of the work leaves, at least temporarily, bottlenecks where channels or culverts are inadequate, causing flash floods every time it rains, as along Jalan Damai, especially outside Tshung Tsin School. Similar urban flood problems can be found throughout Southeast Asia. Many arise as a result of land use change upstream. Others occur because floodplains have been constrained by development, river channel

328 Ian Douglas

capacity has been reduced by siltation, or bridges and culverts are inadequate to cope with increased peak discharge. Often the poorest people in the community are the victims of floods as they cannot obtain land elsewhere and have no choice but to live in hazardous areas.

Subsidence Subsidence Due to Withdrawal of Fluids Any removal of liquids underground can leave empty spaces between the mineral grains in the rocks or sediments. The weight of the overlying material may then cause the grains to be compressed and pushed closer together allowing the land surface above to be lowered. Such land subsidence is now a critical problem in many Southeast Asian cities, especially in Bangkok. It occurs on a large scale in northern Jakarta. Groundwater abstraction is clearly the main cause of the observed land subsidence in the city (10 to 90 cm in the period 1978–90, more than 50 cm in a zone of approximately 150 km2). Experiments suggest that, if nothing is done, the total subsidence may eventually be five times more than that already observed. This means a total final subsidence of 4 to 6 metres in the most subsidenceprone zones in northern Jakarta. The water supply in Hanoi is mainly based on groundwater. Water exploitation began in 1901. Before 1985 the groundwater extraction rate was about 150 000 m3 day −1. By 1991 this had risen to 300 000 m3 day −1. In 1997 it was 400 000 m3 day −1. By 2003 it was expected to be 800 000, and by 2005 over 1 million m3 day −1 (Nguyen Truong Tien et al. 2001). This pumping has produced a fall in groundwater levels in the central and southern areas of central Hanoi. Groundwater now tends to flow towards the centre of the city, where the level of water in the second aquifer has declined (Shibasaki and Ngo Ngoc Cat 2001). This, in turn, has led to some 25 mm of annual surface lowering, mainly due to the compression of the weak near-surface organic muddy clay layer due to reduced pore water pressure. Land surface lowering around pumping stations could be as much as 35 mm y −1 (Nguyen Truong Tien et al. 2001). Lowering of the land surface can also arise through dewatering caused by tunnelling and deep foundation excavations. In Singapore, tunnelling in soft marine clays has sometimes produced collapses. Tight precautions have to be taken when large excavations are made. The walls of deep pits may be deformed. In addition, adjacent ground may be lowered (Broms and Wong 1992). As other cities expand, they are encountering similar problems.

Subsidence Linked to Karstic Processes Subsidence of another form occurs in the limestone areas of Southeast Asia. Around many of the prominent tower karst hills of northern Viet Nam, southern Thailand, Malaysia, and Indonesia, extensive alluvial deposits bury karst surfaces that are often pinnacled, encumbered with limestone blocks, and pitted with sinkholes which developed during Pleistocene low sea levels. The buried karst now poses serious problems for civil engineering works (Bergado and Sebanayagan 1987). New high-rise buildings require deeper foundations than the low-rise buildings that sufficed until the 1970s. In Kuala Lumpur, the low-rise structures had their foundations on the stiff clay layer within the alluvium. Taller, multi-storey structures require piling into the underlying limestone. However, the irregularity of both the karst surface and the cavities within the buried karst means that foundation investigations have to be particularly circumspect (Pelli et al. 1997). Drill holes may strike limestone, unaware as to whether it is buried rockfall material or a pinnacle, while a neighbouring hole might pass through several more metres of alluvium before hitting limestone. Analogous karst terrain in northern Viet Nam exhibits a whole range of buried karst engineering problems that can only be overcome by careful geophysical investigation of subsurface geomorphology.

Other Urban Geomorphological Hazards Landsliding Any hill, or steepland area, is difficult to develop without the risk of causing environmental damage. The granite of many Southeast Asian areas, such as much of the Thai–Malay Peninsula–western Borneo belt, is deeply weathered and can be liable to mass movement, through soil slips or landslides, if vegetation is removed and if excavations are made into steep slopes. The depth of unconsolidated material on such slopes may be 30 m or more. While the bulk of this profile may be undisturbed weathered rock, some of it is likely to be colluvial material, weathered rock, and soil material that has been washed down from higher upslope. The presence of colluvium above weathered rock creates a discontinuity between the two types of material and thus the possibility that the rate of infiltration and seepage of water in the ‘overburden’ can change, creating a zone of lateral water flow at the discontinuity. This lateral movement may facilitate soil slips and landslides.

The Urban Environment 329

Fig. 18.5. Sediments of the Bandung area, Indonesia, showing the inter-fingering of volcanic fan debris with lacustrine deposits (Source: After Dam 1994)

The colluvial material usually found only on the lower parts of slopes is a problem when construction involves cut-and-fill techniques. Bandung provides a good example of a Southeast Asian city with complex environmental problems. The city municipality and district cover some 2200 km2, with nearly 6 million people living at a density of over 17 500 per km2, overlying important aquifers of volcanic deposits or fluvial sediments (Dam 1994). The city essentially is built on a fan of volcanic material from the craters of Mount Tangkuban Perahu to the north that becomes inter-fingered with alluvial material and is eventually overlain with lacustrine deposits which formed in the Pleistocene Bandung Lake (Figure 18.5). This sequence of deposits creates a complex of aquifer materials, but generally the quality of water in the volcanic materials is good. The Bandung– Chimahi alluvial fan in particular consists of favourable deposits for groundwater abstraction; artesian wells with yields of up to 20 l s−1 used to be common. The continuing growth of the city has put pressure on the surrounding land. Agriculture on terraced steep slopes and deforestation contribute to severe erosion. In the areas subject to tropical cyclones (typhoons), landslides triggered by extreme rainfalls are always a threat to urban areas. Throughout the Philippines, landslides, whether caused by typhoons or earthquakes,

often render roads, railways, and other key transport systems unusable, hampering rescue and relief operations and isolating disaster areas from the rest of the community. In January 1999 at least 200 000 people were made homeless by landslides in and around the towns of Viga, Bagamanoc, Payo, Caramoran, and San Miguel, on the island of Catanduanes. In August 1999 the Manila area experienced the heaviest rainfalls for twenty-five years. A torrent of mud engulfed the Cherry Hills housing estate in Antipolo, thirtysix people being lost. Elsewhere in the area forty-six people were killed in other floods and landslides. The most vulnerable areas were the shanty towns which have grown up along rivers and on steep hillsides (Gittings 1999).

Volcanic Activity The Philippines and Indonesia lie on the active volcanic island arc of Southeast Asia. Major eruptions, such as those of Mounts Pinatubo and Merapi, have had severe effects on nearby urban areas. Such eruptions can leave enormous quantities of debris, which get entrained by subsequent run-off from typhoons, creating lahars which can bury large areas of land, as happened on the flanks of Mount Pinatubo in 1995, when the entire town of Bacolor was buried under 3 m of volcanic debris. Ash falls and other damage from the active Mount Canlaon

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volcano 38 km away affect San Carlos City on Negros, Philippines. Canlaon City itself is closer to the volcano, and in the event of a major eruption, 40 per cent of its people would be moved to San Carlos City. Even large cities like Bandung are vulnerable to earthquakes and volcanic eruptions. Seismically, Bandung is located in the third highest category of earthquake risk (Zone III) in Indonesia. The well-known active Lembang Fault, located about 20 km north of Bandung, and several other faults in the area, show that the Bandung area is tectonically unstable. Although it is not in the highest earthquake risk zones (I or II), its high population density, and the character of the underlying ancient lake bed sediments that cover a large part of the Bandung Plateau (Figure 18.5), make Bandung one of the most vulnerable Indonesian cities. Following an earthquake, these sediments are likely to vibrate with an oscillatory wavelength that would shake high-rise buildings dangerously. Buildings in the city should be designed to cope with a peak ground acceleration of 150 to 180 cm2 s−1, which might be expected once in 200 years. During such events the varied soil conditions in the area would cause Bandung to experience severe ground-shaking, liquefaction, landslides, and rock avalanches. The vulnerability of the city of Bandung is worse than it need be because lack of guidance and controls during the planning and implementation of urban development has meant that many public and private buildings have been built without measures to protect them against earthquakes. Special attention should be paid to critical facilities such as hospitals, health centres, and state schools, which are often built with limited public funds and inadequate attention to construction quality and earthquake damage mitigation. As land use planning and development regulation in Bandung ran out of control, urban construction expanded onto nearby steep slopes, where the risk of earthquaketriggered landslides is extremely high. In the most densely populated areas of the city, risk of fire as a secondary impact of damaging earthquake is very high. The Fire Departments of Bandung Municipality as well as of the surrounding satellites are not well prepared to cope with such events, especially in the dense shanty areas, where access by fire engines is impossible and water supply is scarce. The whole city would be vulnerable to breaks in gas, water, and electricity supplies in the event of an earthquake.

Quaternary Substrates Many Southeast Asian cities owe their origins to estuarine or island port activities and are built on complex

sequences of Quaternary deposits, laid down in the last 2.5 million years, such as beach ridges, river terraces, deltaic and estuarine sediments, fluvial materials, coastal peat swamps, and rotted rock materials resulting from the weathering of underlying rocks. As they were formed through periods of changing sea levels, the sequences are both horizontally and vertically complex. Sometimes they are of great economic significance, such as the tin-bearing sands contained within many of the fluvial deposits of the western Malay Peninsula and the important groundwater aquifers already discussed. However, they have another economic significance as foundation materials, especially as cities expand with multi-storey buildings. Kuching, located on the Kuching River in Sarawak, provides a good example of the variety of foundation conditions experienced in a low-lying coastal city (Figure 18.6). Four broad categories of foundation conditions can be recognized (Lam 1992): • Very suitable: areas underlain by residual deposits and terrace deposits or both; generally hilly or undulating terrain; tens of metres above flood level; less than 10 m above bedrock; no subsidence risk; little or no filling required (surface stripping usually required to level the ground). • Suitable: areas underlain by shallow estuarine – deltaic deposits and riverine deposits less than 5 m thick; peat deposits less than 1 m thick; normally occurring as narrow belts around the residual deposits; at or below flood or highest-tide levels; 10 to 15 m to bedrock; minor subsidence risk; some filling required. • Less suitable: areas underlain by estuarine or deltaic deposits more than 5 m thick; below flood or high-tide level; 15 to 30 m above bedrock; moderate amount of prolonged subsidence; large amount of fill required. • Least suitable: areas underlain by deep peat of 1 m or more thickness; above flood level but always waterlogged; very poor surface foundation (extremely soft and wet); 12–20 m to bedrock; severe subsidence risk; large amount of fill required. Ideally, the city should be confined to the very suitable and suitable areas and expansion on the less and least suitable areas should be prevented. Nevertheless, shortage of suitable land has meant that a few of the unfavourable areas have already begun to be used for development. Among the problems arising in the peat areas in the least suitable category is excessive settlement. Any increase in the load on the peat expels pore water and leads to compaction of the peat and

The Urban Environment 331

Fig. 18.6. Urban suitability, Kuching, Sarawak (Source: After Lam 1992)

lowering of the ground surface. Any attempt to drain the peat leads to further water expulsion and more settlement, as happened in the Petra Jaya housing estate close to Kuching, where poorly supported fences collapsed

(Lam 1992). Houses built on piles on the peat tend to remain stable, while those with concrete floor slabs laid directly on the peat generally find that eventually the concrete cracks and breaks.

332 Ian Douglas

Urban Construction and Landform Modification Construction on Hill Slopes Many of the most disastrous landslides in Southeast Asian urban areas occur in fill slopes. Construction on fill greatly increases landslide risk. Too little is known about the stability of fill. Often water moves into the fill as interflow or macropore flow through the weathered mantle and colluvial material further upslope. Concentration of such water movement in places within the fill can lead to changes in saturation, which make the fill more prone to slip when heavy rain occurs. Around Kuching, Sarawak, risk of landslides and debris flows occurs once cut-and-fill operations commence on slopes on residual deposits exceeding 15 per cent. On Pulau Pinang, Malaysia, a mudslide in August 1996 at Teluk Bahang was caused by hill clearing by developers. About 200 villagers were affected. On 17 November that year three further landslides occurred within a 7 kilometre stretch from Teluk Bahang to Balik Pulau, Pinang. In Singapore, a slip in March 1984 of some 50 000 m3, 100 m long with a maximum width of 100 m, damaged a road and several houses (Pitts 1992). The material involved was largely fill, but also carried some colluvium-containing boulders.

Land Reclamation in Coastal Cities Land shortages have led many coastal cities to win extra land from the sea. At Kota Kinabalu, Sabah, successive reclamation has seen the waterfront move nearly 100 m seaward. Elsewhere in Malaysia, the Pulau Indah area (Port Klang) is being extended by direct infill and the Bintulu inner port area is being extended. Reclamation has increased the land area of Singapore from 581.5 km2 in 1960 to 620.5 km2 in 1986, a rate of 1.5 km2 per year. The process is a major landform change. Nearly all the fill material is derived from either the land or the seabed. For example, when the Jurong industrial estate was reclaimed, sedimentary rocks from the nearby hills and ridges were used. Hills were levelled and a hilly and swampy terrain was transformed into an almost level platform in which the geomorphology of over 1200 ha had been transformed with over 22.6 million m3 of material being moved (Wong 1969). In reclaiming the land on which Changi Airport is built, more than 36 million m3 of sand were extracted from two seabed areas by cutter-section dredgers and were pumped ashore along 4 km of pipelines (Wong 1992). The new shorelines in Singapore are protected either by wharves or sea walls when used for port or industrial

purposes, or by bunds, or sometimes with a masonry revetment with a strip of fill in front to allow beaches to form. The 8 km shoreline around the Changi Airport site was not specifically protected as little littoral drift was expected. Between 1978 and 1983 little change in the coastline occurred, save at the ends of the reclaimed land where 1:25 slopes were reduced to 1:7 slopes and more than 150 000 m3 of sand were moved landward (Wong 1992).

Urban Biota, Health, and Environment These rapidly expanding cities of Southeast Asia show marked contrasts in lifestyles, quality of life, and health. In the poor areas of Jakarta, infant mortality is from four to five times higher than in middle-income areas. In Manila, the infant mortality rate is 210 per 1000 in squatter areas compared with 76 per 1000 elsewhere. Demographic survey data are generally disaggregated between urban and rural; only rarely are data on urban-poor areas available. The relatively favourable infant mortality rates for urban areas mask the plight of the urban poor. The greatest environmental threats to Jakarta’s 1.4 million poor arise at the household and neighbourhood level. In the poorest wealth quintile, 31 per cent of households have neither a piped water supply nor access to a private well, compared with 12 per cent for the city as a whole (Surjadi et al. 1994). In addition, the poorest households were less likely to have neighbourhood waste collection and more likely to share toilets. Jakarta’s air pollution is associated with high levels of respiratory disease. Respiratory tract infections, for example, account for 12.6 per cent of mortality in Jakarta—more than twice the national average (World Bank 1994). Ambient lead levels, which regularly exceed health standards by a factor of 3 or 4, are associated with increased incidence of hypertension, coronary heart disease, and IQ losses in children (Ostro 1994). Water pollution has severe impacts on human health. Diarrhoea is responsible for 20 per cent of deaths for children under age 5 in Jakarta (Sivaramakrishnan 1986). In the Angke estuary in Jakarta Bay, the mercury content in commercial fish species far exceeds WHO guidelines for human consumption (Firman and Ida Ayu Indira Dharmapatni 1994).

Insect Life and Vector-Borne Diseases Water is an excellent vector for disease. Many bacterial diseases such as typhoid fever, amoeboid dysentery,

The Urban Environment 333

cholera, enteritis, infectious hepatitis, poliomyelitis, schistosomiasis, and anchylostomiasis can be found in natural waters (Berg et al. 1993). However, these organisms grow and spread rapidly in overcrowded conditions where sanitation is inadequate and where drinking water is often stored in open containers. Poor water management is also associated with the spread of the vectors of dengue, malaria, and filariasis. Malaria remains a key world health problem, which, despite many successful campaigns of mosquito eradication, still occurs in many urban areas. There are three broad groups of mosquitos with differing environmental requirements and associated problems in urban areas: Anopheles, Aedes, and Culex. The malaria-carrier Anopheles includes a variety that bites at night, resting in people’s houses during the day. Breeding grounds in urban areas can include pools of fresh water from leaking water pipes or taps, rainwater-filled depressions, or hollows in the ground surface. The Aedes mosquito transmits yellow fever and dengue, which can lead to dengue haemorrhagic fever. In urban areas it breeds in domestic water storage pots, discarded cans, tyres, plastic bags, or coconut shells, flying during the day and biting most severely around dusk. The Culex mosquito, which bites at night and transmits filariasis, is particularly annoying to urban dwellers. The poor spend much money on mosquito coils to try to obtain relief from their bites (Kolsky 1999). They prefer to breed in the organically polluted water of septic tanks, flooded latrines, blocked drains, and contaminated stagnant pools. Urban construction creates many opportunities for such stagnant water bodies to develop, but simple precautions can help overcome the problems (Table 18.5).

Table 18.5 Opportunities for disease vectors created by urban construction and possible solutions Site problem

Solution

Laying flat concrete slabs

Change curing water regularly; where possible use jute sacks for curing Fill after initial construction; dig out just before completion Fill after initial construction; dig out just before completion Either cover, or turn upside down, when empty Drain each week Reduce the surface area by digging deeper Check regularly and design to evacuate debris

Bottom of lift wells WC and bathroom sumps Upright uncovered bins Basement excavations Pits for material extraction Blocked drains

Source: After Kolsky (1999).

Conclusions The urban environment is a complex of habitats for living organisms of all kinds. While usually considered in terms of the human beings it houses, a city or town provides opportunities for all kinds of micro-organisms, bacteria, insects, fungi, plants, rodents, pets, and feral animals. The opportunities for these living things are largely a reflection of how people are managing the urban environment. Southeast Asia provides examples both of good practice in improving human settlements and of the severe environmental problems associated with urban poverty and overcrowding. Many of these problems are the outcome of social issues and the inequity of investment in housing and infrastructure in different parts of the city and across different sectors of society. However, some of them stem from the failure to manage water and soil erosion during land development, and are a major cause of misery in many parts of cities. Failure to prevent the collapse of mine waste tips or garbage dumps is another. Subsidence can affect the most prestigious and the most humble buildings. Earthquakes, however, damage those that are poorly built and inappropriately sited on soft, vibration-prone sediments. Much more can be done to make the urban environment healthier and safer. Cities have grown up in a wide variety of geomorphic situations in Southeast Asia. From the slopes of volcanoes to river deltas and mangrove swamps, they have all had to cope with various issues arising from the modifications of earth surface processes. Some of the geomorphic problems they have to deal with are inherent in the nature of the site; others result directly from human intervention, such as the modification of river channels, cut-and-fill operations on slopes, and drainage of low-lying peat swamps. A thorough understanding of the ground being developed must eventually be built into a full geomorphology and surficial geology of the Quaternary, which will help development in cities and towns throughout Southeast Asia.

References Andersson, L., and Norrman, J. (1998), Ammonium Contamination of Groundwater in the Hanoi Area of Vietnam, Examensarbete med MFS-bidrag, Chalmers tekniska högskola Geologiska Institutionen Publication B 454 (Goteborg: Chalmers Technical University, Geological Institute). Barrow, C. J. (1981), ‘Urbanisation and Growth: Growth and Environmental Degradation in Penang, Georgetown, Malaysia’, Third World Planning Review, 3: 407–18. Berg, L., Johnson, G., and Raven, R. (1993), Environment (Toronto: Saunders College Publishing).

334 Ian Douglas Bergado, D. T., and Sebanayagan, A. N. (1987), ‘Pile Foundation Problems in Kuala Lumpur Limestone, Malaysia’, Quarterly Journal of Engineering Geology, 20: 159–75. Boonjawat, J., Harncharoen, K., Chinanonwait, N., Sirithanapipat, P., Aikawa, M., and Haraguchi, H. (1995), ‘Variability of Atmospheric Methane, Non-methane Hydrocarbons in Urban Atmosphere of Bangkok’, in H. Haraguchi and J. Boonjawat (eds.), Kinetic Behaviors of Greenhouse Gases in Terrestrial Ecosystem of Asian Region, Reports of a New Program for Creative Basic Research Studies, Studies of Global Environmental Change with Special Reference to Asia and Pacific Regions, vol. II-5 (no publisher), 11–15, 9–23. Boonyabancha, S. (1999), ‘The Urban Community Environmental Activities Project and its Environment Fund in Thailand’, Environment and Urbanization, 11: 101–15. Broms, B. B., and Wong, I. H. (1992), ‘Ground Settlement from Construction in Singapore’, in A. Gupta and J. Pitts (eds.), Physical Adjustments in a Changing Landscape: The Singapore Story (Singapore: Singapore University Press), 329–73. Buapeng, S. (1987), ‘Saltwater Intrusion in Bangkok Metropolis’, Journal of the Geological Society of Thailand, 9: 1–2. Budi Tjahjadi (1991), Indonesia: The Impact of Abstraction on Groundwater Quality and Monitoring in the Jakarta Region, UN Economic Commission for Asia and the Pacific Water Resources Series, 70 (Bangkok), 177–86. Dam, M. A. C. (1994), ‘The Late Quaternary Evolution of the Bandung Basin, West-Java, Indonesia’, Ph.D. thesis, Vrije Universiteit, Amsterdam. Dicken, P. (1990), ‘Mining and Manufacturing’, in D. J. Dwyer (ed.), South-East Asian Development: Geographical Perspectives (London: Longman), 192–224. Drakakis-Smith, D. (1993), ‘That Was Then, This Is Now: Forty Years of Social and Economic Change in the Tropical Third World’, Singapore Journal of Tropical Geography, 14: 81–102. Fink, S. (2001), ‘The Sustainability of Toilets in Hanoi, Vietnam’, International Journal of Economic Development, 3/3, http://spaef.com/ IJED_PUB/v3n3_fink.html (accessed 25 July 2003). Firman, T., and Ida Ayu Indira Dharmapatni (1994), ‘The Challenges to Sustainable Development in Jakarta Metropolitan Region’, Habitat International, 18: 88–97. Gittings, J. (1999), ‘Floods Engulf East Asia’, The Guardian, 5 Aug. 1999, 13. Gupta, A. (1992), ‘Floods and Sediment Production in Singapore’, in A. Gupta and J. Pitts (eds.), The Singapore Story: Physical Adjustments in a Changing Landscape (Singapore: Singapore University Press), 389–414. IWACO–WASECO (1991), Bandung Hydrological Study: West Java Provincial Water Sources Master Plan for Water Supply, Main Report (Jakarta: Water Resources Directorate). Kolsky, P. (1999), ‘Engineers and Urban Malaria: Part of the Solution or Part of the Problem?’, Environment and Urbanization, 11/1: 159–63. Kretzchmar, J. G. (1995), ‘Energy Related Air Pollution Problems in the World’s Ten Largest Megacities’, in H. Power, N. Moussiopoulos, and C. A. Brebbia (eds.), Engineering and Management: Air Pollution III, vol. ii (Southampton: Computational Mechanics Publications), 235–53. Lam, S. K. (1992), ‘Progress Report: Quaternary Geological Mapping of the Kuching City Area, Sarawak’, Proceedings of the 23rd Geological Conference: Technical Papers (Kuala Lumpur: Geological Survey of Malaysia), 96–107. Lightfoot, P. (1990), ‘Population Mobility’, in D. J. Dwyer (ed.), South-East Asian Development: Geographical Perspectives (London: Longman), 256–77.

Nguyen Truong Tien, Le Thu Hanh, Vu Quang Hung, and Pham Quang Hao (2001), ‘Land Subsidence Due to Groundwater Lowering in Hanoi, Vietnam’, in D. G. Freedland and Nguyen Truong Tien (eds.), Proceedings of the International Conference on Management of the Land and Water Resources, Hanoi, Vietnam, October 20–22, 2001 (Hanoi: Vietnamese Geotechnical Institute), 21–7. Ostro, B. (1994), Estimating the Health Effects of Air Pollutants: A Method with an Application to Jakarta, Policy Research Working Paper no. 1301 (Washington: World Bank). Pelli, C., Thornton, C., and Joseph, L. (1997), ‘The World’s Tallest Buildings’, Scientific American, 277/6: 64–73. Phantumvanit, D., and Liengcharernsit, W. (1989), ‘Coming to Terms with Bangkok’s Environmental Problems’, Environment and Urbanization, 1: 31–9. Pitts, J. (1992), ‘Landforms and Geomorphic Evolution of the Islands During the Quaternary’, in A. Gupta and J. Pitts (eds.), The Singapore Story: Physical Adjustments in a Changing Landscape (Singapore: Singapore University Press), 83–143. Ramnasong, V., and Buapeng, S. (1991), ‘Mitigation of Groundwater Crisis and Land Subsidence in Bangkok’, Paper presented at the 4th International Symposium on Land Subsidence, Houston, Texas. Rigg, J. (1991), Southeast Asia: A Region in Transition (London: Unwin-Hyman). Rismianto, D., and Mak, W. (1994), ‘Groundwater Problems in Jakarta and their Implications for Piped Water Supply’, Makalah Simposium Air Jakarta 1994 ( Jakarta: Pusat Pengkajian Perkotaan Universitas Tarumanegara), 114–36. Ross, H., Poungsomlee, A., Punpuing, S., and Archavanitkul, K. (2000), ‘Integrative Analysis of City Systems: Bangkok “Man and Biosphere” Programme Study’, Environment and Urbanization, 12/2: 151– 61. Royle, S. A. (1997), ‘Industrialisation in Indonesia: The Example of Batam Island’, Singapore Journal of Tropical Geography, 18: 89–98. Samah, A. A. (1992), ‘Investigation into the Haze Episodes in the Klang Valley, Malaysia’, in A. J. Hedley, I. J. Hodgkiss, N. W. M. Ko, T. L. Mottershead, J. Peter, and W. W.-S. Yim (eds.), Proceedings of the Seminar on the Role of the ASAIHL in Combating Health Hazards of Environmental Pollution (Hong Kong: University of Hong Kong), 221–7. Santosa, H. (2000), ‘Environmental Management in Surabaya with Reference to National Agenda 21 and the Social Safety Net Programme’, Environment and Urbanization, 12/2: 175– 84. Shibasaki, N., and Ngo Ngoc Cat (2001), ‘Using Groundwater Modeling of Hanoi Area for Predicting Groundwater Lowering’, in D. G. Freedland and Nguyen Truong Tien (eds.), Proceedings of the International Conference on Management of the Land and Water Resources, Hanoi, Vietnam, October 20–22, 2001 (Hanoi: Vietnamese Geotechnical Institute), 37– 40. Singapore, National Environment Agency (2002), The Need for Waste Minimisation, http://app10.internet.gov.sg/scripts/nea/cms/htdocs/ article.asp?pid=1459 (accessed 2 May 2003). Sivaramakrishnan, K. C. (1986), Metropolitan Management (Washington: World Bank). Surjadi, C., Padhmasutra, L., Wahyuningsih, D., McGranahan, G., and Kjellén, M. (1994), Household Environmental Problems in Jakarta (Stockholm: Stockholm Environment Institute). Vicente-Beckett, V. A. (1992), ‘Trace Metal Levels and Speciation in Sediments of Some Philippine Natural Waters’, Science of the Total Environment, 125: 345–57. Vickeridge, I. (1992), ‘The Metabolism of Singapore: The Disposal of Wastes’, in A. Gupta and J. Pitts (eds.), The Singapore Story: Physical Adjustments in a Changing Landscape (Singapore: Singapore University Press), 389– 414.

The Urban Environment 335 Wagner, W. (1991), ‘Germany: Investigation and Monitoring of Groundwater Quality in Germany and in the German Technical Co-operation Projects of the ESCAP Region’, UN Economic Commission for Asia and the Pacific Water Resources Series, 70 (Bangkok), 161–4. Wallerstein, C. (1999), ‘Blockade of Landfill Site Raises Spectre of New Smoky Mountain’, The Guardian, 21 July 1999, 15. Wan Ruslan (1995), ‘Urban Erosion on Pulau Pinang, Malaysia’, Ph.D. thesis, Manchester University. —— (1997), ‘The Impact of Hill Land Clearance and Urbanization on Runoff and Sediment Yield of Small Catchments in Pulau Pinang, Malaysia’, in D. E. Walling and J.-L. Probst (eds.), Human Impact on Erosion and Sedimentation, International Association of Hydrological Sciences, 245 (Wallingford), 91–100. Wan Ruslan and Rahaman, Z. A. (1994), ‘The Impact of Quarrying Activity on Suspended Sediment Concentration and Sediment

Load of Sungai Relau, Pulau Pinang, Malaysia’, Malaysian Journal of Tropical Geography, 25/1: 45–57. Wong, P. P. (1969), ‘The Changing Landscapes of Singapore Island’, in Ooi Jin-Bee and Chiang Hai Din (eds.), Modern Singapore (Singapore: University of Singapore), 20–51. —— (1992), ‘The Newly Reclaimed Land’, in A. Gupta and J. Pitts (eds.), The Singapore Story: Physical Adjustments in a Changing Landscape (Singapore: Singapore University Press), 243–58. World Bank (1994), Indonesia Environment and Development: Challenges for the Future (Washington: World Bank). —— (1999), World Development Indicators (Washington: World Bank). World Resources Institute (1996), World Resources, 1996–7 (Oxford: Oxford University Press). —— (1998), World Resources, 1998–9 (Oxford: Oxford University Press).

19

Water in Cities Goh Kim Chuan and Avijit Gupta

Introduction Southeast Asia, with most of its area receiving an annual rainfall of more than 2000 mm, is a region of positive water balance. It is also an area where unfulfilled demand for water is not unknown. Such a contradiction happens at times in its towns and cities. Several Malaysian urban settlements, for example, suffer occasionally from water shortage in a country with an average annual rainfall of about 3000 mm. Kuala Lumpur went through a prolonged period of water shortage in 1998 (Hamirdin 1998) in spite of large allocations made earlier in various five-year plans towards developing water supply infrastructure. Such shortages are common during long dry periods associated with El Niño. Regional water shortages may become more common in future, especially with the rising population and economic expansion. The shortages are the result of an inability to meet the rising demand of water in cities driven by both increasing population and progressive prosperity (Table 19.1). Serious shortage occurs in large cities such as Jakarta, Bangkok, and Manila where a significant proportion of their population has no immediate access to municipal potable water. Even where piped connection exists, supplies are not available round the clock and often do not meet the required water quality standards. In many cities the local sources are inadequate and water has to be brought in from rural areas. The demand for water in a city has to be met on both quantitative and qualitative terms. For example, drinking water supplied to households by a municipal administration has to meet a given standard (WHO 1993). Ideally a city should have enough water to drink, to meet industrial demand, and to be able to store

an adequate volume under pressure for firefighting and street cleansing. Supplying a city with water requires water sources, a treatment system, a distribution system, and arrangements for treating waste water and its disposal. In this chapter we review the current status of water supply in urban Southeast Asia and the sources that are available, concentrating on the major cities. We indicate the success stories as well as the shortcomings. Even among the major cities the water supply situation and techniques vary widely, for example between technologically advanced and prosperous Singapore and the sprawling mega-cities of Manila and Jakarta. In a number of cities both extraction of water and disposal of waste water create significant environmental problems.

Table 19.1 Estimated level of urbanization in Southeast Asia Country

Cambodia Indonesia Lao PDR Malaysia Myanmar Philippines Singapore Thailand Viet Nam

Population (m)

Percentage urban

1998

2015 (est.)

2000

2025 (est.)

Per capita GDP, 2001 ($)

10.7 206.5 5.3 21.5 47.6 72.1 3.5 59.6 77.9

17.0 275.2 10.2 31.6 67.6 105.2 4.2 69.1 110.1

23 40 23 57 28 59 100 22 20

36 55 36 68 40 70 100 33 27

275 680 317 3678 n.a. 928 22 500 1875 414

Note: n.a. = not available. Source: World Resources Institute (1998). Per capita GDP computed from data in World Bank (2003).

Water in Cities 337 100°

110°

120°

130°

140°

0

20°

Hanoi

500 km

N

20°

Vientiane Chao Phraya

Yangon

Manila

Bangkok

Laguna de Bay

Phnom Penh

10°

10°

Ho Chi Minh

Penang Kuala Lumpur Melaka Singapore

0°

0°

Jakarta Bandung

Surabaya

10°

10°

100°

110°

120°

130°

140°

Fig. 19.1. Location map

Sources and Sinks Almost all the major cities of Southeast Asia (Figure 19.1) are less than 300 years old and associated with either the establishment of a colonial power (Kuala Lumpur, Singapore, Manila) or the shifting of capitals by indigenous rulers downriver and away from troublesome invaders (Bangkok, Phnom Penh). As such, they are located on floodplains (Kuala Lumpur, Vientiane, Phnom Penh), deltas (Bangkok), and coastal plains ( Jakarta, Singapore). As cities grew, many stretched beyond flat lands on to hillslopes (Jakarta, Kuala Lumpur, Singapore). These cities also function as primary cities and a disproportionate amount of resources, products, and services is either funnelled into or distributed out of them. They are still growing. The populations of Bangkok, Jakarta, and Manila are expected to double in 18.5, 20.3, and 22.6 years respectively (Giles and Brown 1997). Keeping the cities supplied with water and disposing of their waste products

therefore become nationally important. Wherever a second large group of cities develop in a populous country (Surabaya, Bandung), they follow the same pattern. The original sources of supply (a large proximal river, neighbouring highlands, shallow groundwater) have currently been supplemented by new sources such as deep groundwater or surface water piped in from a distance. In the extreme case of Singapore, where water has become a security issue, this has even led to the building of costly desalination plants and water recycling. The extended and multi-source supply has caused environmental problems such as subsidence in Bangkok and dubious water quality in Jakarta. The subsidence problem may seriously affect more cities on coastal plains, deltas, and intermontane basins. In future, water from traditional sources, as from a major river or a group of upland streams, may have to be shared with other users, the farmers and the expanding small towns. Demand from such users is rising.

338 Goh Kim Chuan and Avijit Gupta

Waste water is treated fully in Singapore before release, but not so in many towns and cities, leading to pollution of waterways, bays, and shallow straits. Part of the Malacca Strait off Melaka is a striking example. The Klang River, which flows through Kuala Lumpur, not only is polluted by metals such as lead, zinc, copper, and cadmium but it also carries a significant proportion of the city sewage (Sham Sani 1993). This type of concentrated pollution via industrial effluents or city sewage impacts both human health and aquatic life. The first is seen in Bangkok and Jakarta, where water from the canals flowing through the city is used for domestic purposes. The second type of impact also happens around Jakarta, where organic pollution has contributed to the decline of coral reefs in the Jakarta Bay. A sample of fish and shellfish from the bay exceeded the WHO limits for lead, mercury, and cadmium by 44, 38, and 76 per cent respectively (World Bank 1992). Between 1975 and 1988, fish yields declined drastically in the Manila Bay (WRI 1996). Similar examples are found across the region. These three issues (rising demand for more water of acceptable quality, geo-engineering problems from largescale water extraction, and environmental degradation due to release of untreated waste water) are likely to persist and would require considerable application of financial, technical, and management resources. The availability of such resources varies across the region. For example, 90 per cent of the residents of Phnom Penh have access to municipal water either via a piped connection or delivered by tankers (Lyonnaise des eaux n.d.). Not all cities are so fortunate. Among the Southeast Asian countries only Singapore, Malaysia, and Brunei provide 100 per cent of urban piped water to house connections. The water supplied meets WHO standards and is available round the clock. However, these are the rich countries of the region, although piped water in Ho Chi Minh City largely meets WHO standards and that of Hanoi may attain those standards in the near future (see Chapter 18). These are common problems, but their prevalence in a Southeast Asian context is perhaps best understood by looking at individual case studies.

Case Studies Jakarta The city of Jakarta is located on the northern coastal plain of Java. Although the original settlement is 500 years old, its recent growth began with the Dutch occupation of the islands of modern Indonesia. In 1945, when Indonesia came into existence as an independent state,

about 600 000 people lived in the capital. By 1995 its population was approaching 9 million. It is projected to rise to nearly 14 million by 2015 (UN 1998). The administrative unit which manages this agglomeration is called Daerah Khusus Ibukota Jakarta (DKI Jakarta). The total agglomeration, which includes the neighbouring prefectures of Bogor, Tengerang, and Bekasi, has a population of about 20 million and is known as JABOTABEK. This remarkable rise in city population has led to a matching areal expansion. The early city was based on a coastal plain flanked by a set of alluvial fans. From its original site, it has expanded to the previously unoccupied coastal wetlands on one side and to the foothills beyond the alluvial fans on the other. A number of rivers flow through the city. The largest, the Ciliwung, has been used as a major water source for the city. Currently, however, groundwater has become essential. According to Doppenberg (1992) groundwater in some form meets 80 per cent of the requirements of the population in DKI Jakarta. Perusahaan Daerah Air Minum (PAM Jaya) is the organization responsible for supplying DKI Jakarta with water. Originally, the municipal water supply was planned for about half a million. About fifteen years ago a survey listed the sources of water available to the citizens of Jakarta, who by then numbered several million: (1) private standpipes and wells extracting almost entirely shallow groundwater, used by about half of the residents; (2) water bought from private vendors, used by about a third; (3) piped water, used by 14 per cent; and (4) rivers and canals flowing through the city, used by a small fraction (World Resources Institute 1996). Several years later, in 1995, water connection had reached 2.4 million in DKI Jakarta, a large number but only 30 per cent of the population (Lyonnaise des eaux n.d.). Several problems are immediately evident. First, the water supply is not dependable for the large number of people who live in Jakarta. Secondly, the quality of the available water generally is poor. This is particularly true for water collected from rivers and canals and bought from private vendors. Moreover, the unit price of water supplied by the vendors is often higher than the unit price of piped water. The poor who live in slums and squatter settlements therefore pay more for worse water. This is especially true in north Jakarta. Crane (1994) showed from a sample household survey that the price for 1 m3 of water was $2.62 if bought from vendors, $1.26 for water from standpipes, $1.08 if sold to household resale customers, and only $0.18 for connected households. Water is allowed to be resold from households connected to the municipal system as it is expected to provide access to more people and generate higher revenue. But

Water in Cities 339

according to Crane only 10 per cent of households bought water in this way. Thirdly, many of the wells tap shallow subsurface water and are susceptible to pollution from septic tanks and industrial discharge and landfills. The aquifers in the subsurface of Jakarta, which is on a coastal plain, vary in depth from less than 40 m to more than 250 m (Apandi and Wiriosudarmo unpub.). Deeper wells have been used to augment the supply, but the extensive groundwater withdrawal has resulted in saline incursion near the coast. Saline contamination has been observed several kilometres inland, and the level in the coastal plain aquifer has dropped below sea level, a potentially dangerous situation. Away from the coastal zone, groundwater is found in sandy aquifers in the old alluvial material that underlies Jakarta. Such storage is easily polluted, and furthermore its withdrawal leads to subsidence of the ground surface, currently measured in several tens of centimetres. The expansion of Jakarta in the southern range of hills (Figure 18.3) interferes with the proper recharging of the aquifers and therefore less water is available in the subsurface. (The problems of subsurface water withdrawal in Jakarta have been discussed in Chapter 18.) Even piped water has to be highly treated because of the low quality of surface water, and then about half is lost in transmission. A large volume of untreated waste water is disposed off in the waterways that run through the city. This has an impact on human health, especially that of young children, and also contaminates the coastal waters of Jakarta Bay (World Resources Institute 1996). Even the supply reservoirs near the city are becoming polluted (Lyonnaise des eaux n.d.). The recent Kampung Improvement Project of Indonesia, however, has improved water quality for several million people across about 200 cities, including Jakarta. Improvements include, among other priority actions, the provision of a standpipe for each 25–35 families. Although the funding comes from a range of sources, the communities themselves are responsible for running and maintaining such facilities (World Resources Institute 1996). Some neighbourhoods in Jakarta now have access to a community water tap and washing area as part of the programme (Lyonnaise des eaux n.d.).

Bangkok Bangkok lies on the Chao Phraya about 25 km inland from the Gulf of Thailand, only slightly above sea level, the elevation ranging between 0.5 and 2 m. The old city of Bangkok was located next to the river and partly on its levée, but its rapid expansion in the second half of the twentieth century has caused it to spread in

all directions. To the east and the south, it is now built over swampy low ground. In 1950 the population of Bangkok was less than 1.5 million. Fifty years later it approached 10 million, which explains the rapid areal expansion and the rising domestic and industrial demand for water. Water supplied to the city by the Bangkok Metropolitan Water Authority comes from the Chao Phraya, as described in Chapter 18. The quantity of this supply is insufficient and even its quality causes concern. Bangkok depends on subsurface water to supplement the volume collected from the river. About a third of Bangkok’s water is pumped out of sand and gravel aquifers. The lower part of the Central Lowland of Thailand is associated with a north–southtrending structural depression filled with at least 500 m of poorly consolidated Quaternary sediment that overlies igneous and metamorphic rocks (Khantaprab and Boonop 1988). The Quaternary deposits essentially consist of deltaic sediment, mainly clay, with several beds of sand and gravel. These sand and gravel layers act as aquifers, which are the source of about a third of Bangkok’s groundwater supply. As Bangkok expanded in the 1960s and 1970s, many private houses and industrial establishments were supported by groundwater. The quality of the water was good and this water was free of city water rates. Its use, therefore, was economical for industries such as breweries and paper manufacturing. Towards the end of the 1980s the daily extraction rate reached almost 1.5 million m3. The piezometric level in the wells was lowered from at the surface to about 9 m depth by 1959, and locally to about 50 m by 1983. Saline water began to leak out of the marine clay beds separating the sandy aquifers and contaminated the groundwater (ESCAP Secretariat 1988). The major environmental impact of this withdrawal was subsidence, which locally approached 10 cm per year. This has led to structural damage in Bangkok and also increased flooding during the rainy season (Nutalaya et al. 1996; Chapter 21, this volume). Stricter control is now imposed on groundwater withdrawal, and the area supplied by Bangkok’s water authority has increased from about 300 km2 in 1985 to about 800 km2 by the year 2000. This, however, is a clear example of degradation associated with overuse of available resources to meet the rising water demand of a major Southeast Asian city. A similar sequence has started to happen in Jakarta and other major cities where groundwater withdrawal is rising. Such cities may have to deal with this problem in future. At present, however, Bangkok remains the striking example in the region of this kind of environmental impact from an urban area of progressively increasing water demand.

340 Goh Kim Chuan and Avijit Gupta

The poor in Bangkok also tend to collect water from the large number of canals and waterways (klongs) found throughout the city and next to which they live. These waterways also function as carriers of polluted waste water of the city leading to unfortunate consequences. Solid waste is also dumped into klongs, and the quality of water, according to the residents, has declined over the years to an alarming level at which it is unsafe to use; however, the poorest people may not have a choice (Ross and Poungsomlee 1995).

Manila Potable water from the Metropolitan Waterworks and Sewerage System (MWSS) of Manila comes essentially from rivers, the groundwater contribution being negligible. The main surface source is the Angat River, whose catchment in Bulacan Province is outside the MWSS service area. While surface water diverted from the Angat, Ipo, and Novaliches catchments is of good quality, other sources are threatened with contamination. The demand in Metro Manila has led to the utilization of the waters of the Umiray River and that of Laguna de Bay. Laguna de Bay is seriously polluted due to discharges of domestic and industrial waste water, expanding human settlements around the lake, increased agriculture and industrial activities, and extensive fish-farming in the lake itself (Goh 2000a). Toxic heavy metals and organic compounds need to be eliminated from the lake water. The Pasig River, which flows through the city, is similarly contaminated. MWSS provides water mostly to middle-class households on the southern side of the metropolis. Miniquis, writing in the mid-1990s, indicated that water was available only for a few hours on alternate days (Miniquis 1996). Lower-income families purchase water from pushcart vendors at higher cost than the piped water, a pattern already described for Jakarta. Only 11 per cent of the population in Metro Manila is served with piped sewerage. The rest is discharged into open ditches and canals, thereby not only increasing general pollution but specifically contaminating natural sources of water as well. Smith (1999) commented that the piped-water distribution system in Manila lacks backflow preventers, and when pressure is extremely low, as during El Niño periods, backflow containing water-borne pathogens and E. coli exposes the population to potential cholera and other epidemics. The pipes in general are old and with cracks at the joints which allow waste water to contaminate the distribution system (Bruestle 1993). The shortfall between the demand and supply from surface water is met by both shallow and deep groundwater extraction, mainly by private

wells although MWSS wells do exist. About 40 per cent of domestic water supply comes from the subsurface, whereas over 80 per cent of the industrial demand is met by deep pumping. This has resulted in the expected large drop in the piezometric level and a much reduced rate of discharge (Hinrichsen and Tacio n.d.). This in turn has led to salinization near the coast, upconing of fossilized water in the inland aquifers, contamination by leachates from dump sites, and pollution from industries and underground storage tanks of petrol stations and bus terminals (Pascual 1992).

Singapore The situation in Singapore is somewhat different from the rest of the major cities as prosperous Singapore has to cope with a limited national supply and a high demand simultaneously. Geology has deprived Singapore of a subsurface supply. The city provides an excellent case study on meeting the rising demand for water using economic and technical abilities. This justifies treating the story of water supply, treatment, distribution, and waste water discharge in Singapore in detail. The present settlement of Singapore started in 1819 as the entrepôt for the Malay Peninsula and most of Southeast Asia. The peninsula then was a single country, Malaya, under various degrees of British occupation. Supply of municipal water started in 1857 from MacRitchie Reservoir (Figure 19.2) to a limited number of users. The primary sources of supply were three impounding reservoirs (MacRitchie, Peirce, and Lower Seletar) surrounded by a protected central catchment area mostly under forest. Starting in 1927, this water was augmented by water piped in from Johor, across the causeway. The practice continued on the basis of a series of agreements (1961, 1962) by which Singapore paid Malaysia for raw water and some of the water after treatment was sold back to Malaysia after the independence and subsequent separation of Malaysia and Singapore. Currently Singapore has a population of nearly 4 million and a per capita gross domestic product (GDP) of about $US22 500, which far surpasses the rest of Southeast Asia, being comparable to a number of European countries. A rapid rise in water demand has been in place since the 1960s. Total annual water consumption rose from about 120 million m3 in the mid-1960s to over 300 million m3 in the mid-1980s with a steep rise in per capita GDP, the demand rising for both domestic and industrial consumptions (Appan 1992). The rising demand for water led to a need to extend the existing reservoirs and build new ones after 1969. Both Seletar and Peirce were extended, and a number of estuaries were dammed to create reservoirs with usable

Water in Cities 341

Fig. 19.2. Singapore: impounding reservoirs and waterworks (Source: Appan 1992)

water over time. The first of these was Kranji in 1972. Figure 19.2 shows the location of the present reservoirs. These are generally shallow, and one of them, Bedok, is a lake created by storing water from nearby urban catchments in a huge quarry originally excavated for extracting Quaternary Old Alluvium for reclamation and construction work. As nearly half of Singapore is a catchment area of some kind, the nature of the physical environments in each catchment means that the primary quality of the water varies between reservoirs. The quality of raw water in the original central catchments in forest areas is good, but even there the demand for land in the small island of Singapore has led to certain types of land use being permitted within these catchments that are expected to produce minimal pollution of water. In other urban catchments various sources of pollution, which in the early days were identified mostly as pig-farming, unsatisfactory human waste disposal practices, and industrial effluents, needed to be dealt with. Various steps have since been taken to deal with such polluting agents. Pig-farming has been banned for years, and better treatment facilities are available for domestic and industrial wastes. Water collected in certain reservoirs has to be treated carefully in order to maintain the potability of Singapore’s tapwater, which is of a high standard (Appan 1992).

The treatment in six plants of the raw water of variable quality from different catchments has been detailed by Appan (1992). The treatment plants are now computerized and the general order of treatment is coagulation, flocculation, sedimentation, and rapid sand infiltration. This is followed by the addition of fluoride and disinfection. Certain qualities of water may require pre-chlorination and aeration and the use of a more elaborate treatment process including ozonation. The quality of water in the distribution system is thereby maintained. Waste water disposal is not a problem as Singapore is well served by a number of sewage treatment plants which process the industrial and domestic waste water. Some of the treated water is purified and recirculated for specific uses such as in industry and cooling towers, but part of the final effluent is released into the sea. The treated sludge, if available, may be used as a soil conditioner. In spite of all these attempts, the local water supply is still inadequate. About five years ago Singapore was consuming over 1.2 million m3 of water a day, about half of which came from Malaysia (Straits Times, weekly edition, 22 Aug. 1998). Over time the arrangement to import raw water from Malaysia has required new agreements (1990) and has generated debate, and Singapore has looked for other sources. So far there

342 Goh Kim Chuan and Avijit Gupta

have been three: imported water from Batam and other neighbouring Indonesian islands, water from desalination plants, and reused water after proper recycling treatment (locally bottled as Newater). All three are expensive procedures which only a technically and economically advanced state like Singapore can afford, but the story also illustrates how water can be a security issue even in a wet tropical environment. The first desalination plant is expected to start operating in 2005, with a daily production of about 100 000 m3, about 10 per cent of Singapore’s current consumption (Straits Times, weekly edition, 25 Jan. 2003). On completion three desalination plants are expected to produce a daily supply of 400 000 m3 of water. In addition, it is envisaged that by 2011 reclaimed water in Singapore will constitute about 2.5 per cent of its total daily water consumption.

Conclusions These four case studies bring out a number of common factors regarding supply of water to meet the urban demand in Southeast Asia. The cities are faced with a rapidly increasing demand for water which in most cases is impossible to meet with surface water as practised earlier. Cities such as Phnom Penh still depend almost entirely on surface water, but most cities are serviced by a variety of sources, subsurface water increasingly becoming an important component. Certain large cities which are almost entirely supplied by groundwater, as in Hanoi, are also in a minority in the region. Although there are success stories such as Singapore and Malaysian cities, most other cities fall short of supplying its residents with quality water. There is also the possibility of conflict over river water among the cities or between a city and the countryside. Dependence on river water may also require using distant sources, which may not necessarily be economically feasible. Although shallow private wells have been a traditional source in this high-rainfall region, use of groundwater on a large scale brings its own problems. Shallow wells are easily contaminated by organic and industrial seepages, whereas extraction from deeper aquifers eventually leads to subsidence and increased flooding. Bangkok is the most striking example, but, with increased extraction, subsidence problems are now present in many other urban areas. This is happening in Jakarta, Bandung, Hanoi, and other cities. The geomorphology and subsurface geology of these cities control the nature and rate of the problem. For example, saline intrusion from the coast is more worrisome in Jakarta at this stage than subsidence, although probably it is only a matter of time before geo-engineering problems associated

with ground subsidence begin to duplicate the situation in Bangkok. Singapore provides the extreme example of high demand driven by the size and prosperity of its population. This has forced Singapore to adapt innovative and expensive procedures for keeping the city supplied with enough water of high quality. Other cities have more flexible options except during prolonged droughts. It is, however, becoming evident that most cities as they grow will require integrated, multi-sourced, innovative, and large-scale planning for water. Some of the techniques such as desalination or imported water, as used in Singapore, may be unnecessary and too expensive for other cities, but a number of them may need to build treatment plants and recycle water at least for specific purposes. Recycled water is now bottled in Singapore. Bottled water of different types is fast becoming a popular source for drinking across Southeast Asia. Part of this demand developed out of the expectation of safe drinking water produced locally, which in a way is an indictment on the city administration. To illustrate, the bottled-water industry is growing at an annual rate of 20 per cent in Indonesia. Innovative methods, including harvesting rainwater off roofs on a private basis, are seen in many small towns of the region. For example, suburban houses have their own roof run-off cistern system in Miri, Sarawak, East Malaysia, but this technique probably only works in high-rainfall areas. Purchasing water from vendors is a traditional practice in a number of urban settlements in Southeast Asia. Their prevalence and continued importance reflect the inability of the local government to provide water for the citizens. The poorer residents end up paying higher prices for low-quality water. In Surabaya, for example, the price differential between vendor-supplied water and the piped variety was reported to range between 20:1 and 60:1 (World Bank 1988). To a large extent, Singapore’s success with water is a result of an efficient technical and administrative structure and management. The administration in many cities in Southeast Asia is not comparable, and this makes a difficult job impossible. A better management strategy improves supply despite physical shortcomings, as in Penang (Goh 2000b). Bangkok has managed to arrest the rate of subsurface water withdrawal to a considerable extent, but the situation in Indonesia has not reached a comparable level (Braadbart and Braadbart 1997). A huge amount of water is lost in most cities as it is transmitted through the urban reticulated system. This amount reaches 6 per cent in Singapore, 20 per cent in Penang, and 21 per cent in Johor Baru, but jumps to 58 per cent in Manila and 63 per cent in Hanoi. This

Water in Cities 343

is due not only to leaks and meter errors but also to illegal connections (McIntosh and Yniguez 1997). Better administrative and technical management would solve part of the current problem. Ensuring a dependable water supply of required quality is not only a case of finding a source but also of management. In 1998 Kuala Lumpur suffered several months of acute water shortage when the municipal piped supply became irregular and alternative arrangements had to be made. It was reported in the newspapers that a prolonged period of low rain (it was an El Niño year) might have been the trigger, but the situation was created also by problems in management and a change of land use in the upper catchment areas. If urbanization of Southeast Asia continues at its current pace, the countries may have to put in expensive technology, efficient infrastructure, and correct land management to ensure proper supply of water to its cities. The task may become even more difficult in the future with further city expansion, degradation of forest cover on upper slopes of water catchments, and a global-warming-driven rise in sea level.

References Apandi, T., and Wiriosudarmo, S. (unpub.), ‘Some Aspects of Environmental Geology for Future Landuse Development in the Jakarta City’. Appan, A. (1992), ‘The Control of Water Quality in Singapore’, in A. Gupta and J. Pitts (eds.), Physical Adjustments in a Changing Landscape: The Singapore Story (Singapore: Singapore University Press), 374–88. Braadbart, O., and Braadbart, F. (1997), ‘Policing the Urban Pumping Race: Industrial Groundwater Overexploitation in Indonesia’, World Development, 25/2: 199–210. Bruestle, A. E. (1993), ‘East Asia’s Urban Environment’, Environmental Science and Technology, 27: 2280–4. Crane, R. (1994), ‘Water Markets, Water Reform, and the Urban Poor: Results from Jakarta, Indonesia’, World Development, 22/1: 71–83. Doppenberg, A. F. J. (1992), ‘Indonesian Water Management towards an Integrated Approach: Water Resources Management JABOTABEK’, Land and Water International, 75: 7–10. ESCAP Secretariat (1988), ‘Geological Information for Planning in Bangkok, Thailand’, in UN–ESCAP (ed.), Geology and Urban Development: Atlas of Urban Geology, vol. i (Bangkok: United Nations Economic and Social Commission for Asia and the Pacific), 24–60. Giles, H., and Brown, B. (1997), ‘And Not a Drop to Drink: Water and Sanitation Services to the Urban Poor in the Developing World’, Geography, 82: 97–109. Goh, K. C. (2000a), ‘Lake Water Management: A Comparative Study of the Biwa and Kasumiguara Lakes in Japan with Laguna de Bay

in the Philippines and its Implications on Other Lakes in Southeast Asia’, Social Science (Osaka: Hannan University), 35/4: 79–94. —— (2000b), ‘Penang Island: Managing Water Supply for Sustainable Development’, in T. S. Teh (ed.), Islands of Malaysia: Issues and Challenges (Kuala Lumpur: University of Malaya), 237–58. Hamirdin, Ithnin (1998), ‘An Analysis of the Water Crisis in the Klang Valley, Malaysia’, Paper presented at the 5th International Southeast Asian Geography Conference, 30 Nov.– 4 Dec., Singapore. Hinrichsen, D., and Tacio, T. (n.d.), ‘The Coming Freshwater Crisis is Already Here’, wwics.si.edu/topics/pubs/popwawa2.pdf. Khantaprab, C., and Boonop, N. (1988), ‘Urban Geology of Bangkok Metropolis: A Preliminary Assessment’, in UN–ESCAP (ed.), Geology and Urban Development: Atlas of Urban Geology, vol. i (Bangkok: United Nations Economic and Social Commission for Asia and the Pacific), 107–35. Lyonnaise des eaux (n.d.), Alternative Solutions for Water Supply and Sanitation in Areas with Limited Financial Resources (Nanterre: Suez Lyonnaise des eaux). McIntosh, A. C., and Yniguez, C. E. (eds.) (1997), Second Water Utilities Data Book—Asian and Pacific Region (Manila: Asian Development Bank). Miniquis, E. (1996), Water Policy in Manila, IDRC Reports (Ottawa: International Development Research Center). Nutalaya, P., Yong, R. N., Chumnankit, T., and Buapeng, S. (1996), ‘Land subsidence in Bangkok during 1978–1988’, in J. D. Milliman and B. U. Haq (eds.), Sea-Level Rise and Coastal Subsidence (Dordrecht: Kluwer Academic), 105–30. Pascual, A. Y. (1992), ‘Determination of Groundwater Pollution Potential in Metro Manila, Using DR Approach’, MS thesis, University of the Philippines. Ross, H., and Poungsomlee, Anuchat (1995), ‘Environmental and Social Impact of Urbanization in Bangkok’, in J. Riggs (ed.), Counting the Costs: Economic Growth and Environmental Change in Thailand (Singapore: Institute of Southeast Asian Studies), 131–51. Sham Sani (1993), ‘Urban Environmental Issues in South-East Asian Cities: An Overview’, in H. Brookfield and Y. Byron (eds.), South-East Asia’s Environmental Future: The Search for Sustainability (Tokyo: United Nations University Press), 341– 60. Smith, W. J. (1999), ‘Drinking Water Issues and Management in the Republic of the Philippines’, Geographical Bulletin, 41/1: 8–24. UN (United Nations) (1998), World Urbanization Prospects: The 1996 Revision (New York: United Nations Secretariat, Department of Economic and Social Affairs, Population Division, ST/ESA/ SER.A/170). WHO (World Health Organization) (1993), Guidelines for Drinking Water Quality, 2nd edn. (Geneva: WHO). World Bank (1988), World Development Report 1988 (Oxford: Oxford University Press). —— (1992), World Development Report 1992: Development and Environment (Oxford: Oxford University Press). —— (2003), World Development Report, 2003: Sustainable Development in a Dynamic World (Washington: World Bank; New York: Oxford University Press). World Resources Institute (1996), World Resources 1996–97: The Urban Environment (New York: Oxford University Press). —— (1998), World Resources 1998–99: A Guide to the Global Environment (New York: Oxford University Press).

20

The Urban Geomorphology of Kuala Lumpur Ian Douglas

Introduction The city of Kuala Lumpur, lying at the junction of the hills of the Main Range (Banjaran Titiwangsa) of Peninsular Malaysia and the coastal plain (Figure 20.1), has many of the environmental problems that beset the urban areas of Southeast Asia. It has to cope with heavy, intense rainfalls, frequent local nuisance flooding, unstable hillsides, complex foundation conditions, and the impacts of mining and construction activities. The citizens, engineers, and planners of Kuala Lumpur have had to find ingenious solutions in order to live in harmony with their environment. While careful investigation and skilful applications of science and technology has overcome many of the problems, others remain unresolved. The persistent problems arise because the links, and thus responsibilities, associated with changes in one place and impacts elsewhere are not acknowledged and the available understanding of hydrologic and geomorphic systems is not applied. Founded by Kapitan China Yap Ah Loy at the confluence of the Gombak and Klang Rivers in 1857 as a tin-mining settlement (Gullick 1983), Kuala Lumpur quickly outgrew its floodplain and fluvial terrace site to spread onto the adjacent hills. The British resident, Captain Bloomfield Douglas, moved his headquarters to Kuala Lumpur from Klang in 1880 and soon after built his official residence on the hill to the west of the Gombak River, where the prime minister’s residence now stands. So began a tradition of the elite living on the hills which has persisted to the present day. In December 1881 the new township and the surrounding tin mines were hit by floodwaters (Gullick 1983), so establishing the problem of living with fluvial extremes which still besets the city.

Virtually every wet season in the first eighty years of Kuala Lumpur’s existence brought some flooding to the town. The river channels became choked with silt carried down from the mines upstream (Gullick 1983). Record rainfall in December 1926 led to a flood 1 m deep in the town centre. After the floods, a new, wider channel, with a double trapezoidal cross-section was built through the town centre. These works enabled a major flood in 1930 to pass through the town without causing any damage (Gullick 1983). Since Malaysia gained independence in 1963, the urban development of Kuala Lumpur and the Klang Valley has been extremely rapid. Kuala Lumpur city itself has grown from 240 000 inhabitants in 1950 to over 1.7 million in 1999. However, the city is part of the larger Klang Valley conurbation extending from the foothills of the Main Range westward to the Malacca Strait and from Rawang, 30 km north of the city, south to the border with Negeri Sembilan. The newer north– south axis (Figure 20.2) has grown rapidly since 1990 with the development of the ‘high-tech’ corridor and the new government centre and international airport. By 2000 the population of the Klang Valley conurbation was around 3.4 million. Between the main axes of growth remain agricultural areas that are being integrated into the urban economy through off-farm employment and the growing of vegetables (Airriess 2000). New developments have taken place on the forested hillsides and over the former rubber plantations and tin-mining areas that once dominated the surroundings of Yap Ah Loy’s town. The muddy confluence of Kuala Lumpur has become muddier owing to the frequent occurrence of excessive soil losses from construction sites and from sites cleared of vegetation but awaiting development. The complex nature of the soils

Urban Geomorphology of Kuala Lumpur 345

Fig. 20.1. Kuala Lumpur: geology and place names

346 Ian Douglas

Fig. 20.2. Growth of Kuala Lumpur, initially along the Klang Valley and now along a north–south axis (Source: After Airriess 2000)

and weathered rock on the newly developed slopes has sometimes proved a major problem when landslides have occurred. The geological history of the site has become significant when deep foundations hit buried topography. Knowledge of geomorphology becomes a significant tool for urban environmental management in this dynamic, expanding city.

Geology The alluvial plains around the river confluence where Kuala Lumpur began is overlooked by Bukit Nanas and the Kenny Hills. Not far to the north are the limestone, tower karst outcrops of the Batu Caves Hills and the

quartz dyke of the Klang Gates Ridge. Further to the north and east are the granites of the Main Range, which were intruded into the Palaeozoic sediments of which the limestones are a part. The Kenny Hills are another sedimentary formation, which has a complex lithology, but which is easily weathered in the humid tropical climate. Usually the granites and the sedimentary rocks are well decomposed, with weathering profiles often 20 to 30 m deep. The limestones and the quartz dykes are the only hard rocks to outcrop at the surface. These outcrops are significant economically, biologically, and culturally. The limestones of the Batu Caves were quarried for building materials, but they contain caves

Urban Geomorphology of Kuala Lumpur 347

with significant bat populations, support a particular type of forest, and house an important Hindu temple. The quartz dyke supports a rare, relatively dry, nutrientpoor habitat in a humid tropical region. The lithological diversity gives rise to a varied set of geomorphic conditions, each of which produces particular problems for urban development and environmental management. Many of the problems are related to on-site ground stability and foundation engineering. Others are the downstream consequences of development upstream, extending from the hillsides in the upper catchment to the coastal mangroves in the delta. The granite of the Main Range varies in jointing and microfissuring, but has a tendency to weather into kaolinitic clays and quartzitic sands, with large residual corestone boulders. Depths of weathering vary considerably, with jointing often controlling the depth of rotted rock more than the topography (Gerrard 1988), although depth of weathering may be high on broad ridge tops. Many ridge crests are sharp, but may have a combination of exposed corestones and rock outcrops from which weathered material has been eroded and sectors of weathered material covered by dense forest under natural conditions. Engineering weathering classifications recognize five major weathering grades (Table 20.1). Most of the terrain in the Main Range and foothill areas falls into weathering grades IV and V. Corestones are present in much of the area, both close to streams and near ridge tops. These corestones are large boulders of unweathered granite surrounded by virtually completely weathered rock. Their presence is almost impossible to detect from the surface, except where they have been exposed by erosion, and thus they are an unpredictable hazard during the construction process. Their distribution is likely to be irregular and some places may have few. At some places in the Main Range, corestones are virtually absent as the rock has so many fine fissures and cracks that percolating water can penetrate throughout the rock mass and thoroughly decompose its constituent minerals (Newbery 1970). Other than the corestones, weathered granitic rock is composed of kaolinitic clays and sands, with many partially decomposed feldspar fragments up to 20 mm in diameter (West and Dumbleton 1970). This gives two dominant sizes of mineral grain in the rotted rock, clay particles less than 0.1 mm diameter and sand and pebble grains 1 to 4 mm diameter. This grain-size contrast is a source of potential instability, as infiltrating water is able to carry the clay-size material through spaces between sand grains, so creating voids that may become filled with water during heavy rains. Under natural

Table 20.1 Rock weathering grades Grade

Term

Description

I

Fresh

II

Slightly weathered

III

Moderately weathered

IV

Highly weathered

V

Completely weathered

No visible sign of rock material weathering; perhaps slight discolouration on major discontinuity surfaces Discolouration indicates weathering of weathered rock material and discontinuity surfaces. All the rock material may be discoloured by weathering Less than half the rock material weathered decomposed or disintegrated into a soil. Fresh or discoloured rock present as a discontinuous framework or as corestones More than half the rock material weathered decomposed or disintegrated into a soil. Fresh or discoloured rock present either as a discontinuous framework or as corestones All rock material decomposed and/or weathered disintegrated into a soil. The original mass structure still largely intact

Sources: After BS 5930 and Fookes and Weltman (1989).

forest conditions, probably some 95 per cent of the rainfall passing through the canopy to the forest floor penetrates through the shallow layer of leaves and other rotting vegetation on the forest floor and infiltrates into the soil. As the permeability of the completely weathered surface material is high, much of the water joins the deeper circulation more than 2 m below the surface. This water is responsible for the chemical decomposition of the original rock minerals and for the downslope transfer of fine clay particles by water moving laterally through the voids in the rotted rock. Such lateral water movement through the weathered rock is termed ‘throughflow’. It plays a major role in regulating river flows and in the stability of hill slopes. Water movement is also likely to occur along the usually irregular weathering front that forms the contact between grades I and II of the weathering profile (Table 20.1). When slopes are disturbed by earthmoving operations or forest clearance, water movement here may lead to slumping of the rotted rock above, and even to large landslides. In places, the granites are overlain by metasediments, thought to have been part of the sedimentary sequence that includes the Kuala Lumpur limestone. Bukit Tahun in Templer Park to the northwest of the city is a good example of limestone resting on granite. These sediments are older than the granite. The granitic magma heated up some of the sandstone and clay beds in the sediments, transforming them into schists. The schists become extremely friable on weathering and

348 Ian Douglas

have less resistance to deforming pressures and poorer drainage characteristics than the weathered granite. Water easily seeps through these schists. In times of heavy rainfall these weak, often saturated, strata form the planes along which slope failure occurs.

Slopes The growth of the city, and the tradition of the wealthy to build prestigious homes on the hills, led to the relatively rapid development of the hills on the Kenny Hill formation to the west of the town centre. Here the sediments are well weathered and decomposed and are easily eroded by running water. On exposed areas, dense rill and gully networks develop quickly, extending drainage networks and readily supplying sediment from gully wall erosion. Any slope with a deeply weathered mantle is potentially unstable (Ibrahim Komoo 1998). Even in the natural forests of these environments, irregular, smallscale landsliding occurs. The slipped material is gradually carried down the surface of the slope. Other soil and plant debris is splashed and washed downslope during heavy rainstorms. Over centuries these small movements led to the formation of a colluvial layer (termed ‘colluvium’) overlying the weathered granite, creating a discontinuity in the rotted rock material covering the slope (known as the regolith). Under high rainfall conditions, water can penetrate rapidly through the colluvial material, but enters the weathered rock less easily. The colluvium can thus move over the top of the weathered rock and slide downslope. Discontinuities in slope deposits, such as those between colluvium and weathered rock, may lead to changes in permeability and thus the build-up of high pore water pressures which can trigger slumping. Small landslips are commonly associated with wind-throw of trees in steep tropical rainforest terrain. Tree fall often involves uprooting, thereby creating a small step or miniature cliff in the slope, the wall of which may subsequently be subject to mass movement. The bare area created by the tree fall allows more rain to infiltrate, and may thus alter pore water pressures and so influence slope stability. However, under the forest, the plant cover protects and stabilizes most of the slopes, and mass movements are infrequent in both space and time. Any form of land development alters the form of the slopes and the passage of water over the ground and into the weathering profile. Exposure and compaction of the ground surface increases the amount of surface run-off. Paving of roads and construction of buildings and parking areas renders large surfaces impermeable and

redirects water to other places. The weight of artificial structures changes the stresses within the soil, altering slope stability. Earth-moving results in cut slopes whose internal pore water pressures may change and fill areas with less compact material than natural weathered slopes and a higher degree of erodability. Developments on steep granitic terrain in and around Kuala Lumpur have led to two stages of geomorphic impacts: those during project construction and those in the period after completion of building work.

Impacts during Construction During construction the original vegetation is disturbed and the soil and weathering profile is exposed to the erosive agents of rain and wind. Road construction leads to rapid erosion by soil-slipping if cuts are too steep, and roadside erosion if cross-drains are not established. Temporary access roads and building yards are particularly liable to erosion. Already the access roads to the development site are affected by road-cut instability and roadside erosion. As cuts are made into the slopes of the steeper parts of the site, slope stability problems will increase. Exposure of partially weathered or solid corestones in cuts may result in severe damage if erosion of the surrounding weathered material causes the corestone to overhang and eventually become unstable. Rotational slumping of steep temporary cuts occurs frequently, the displaced material adding to the loose sediment available for erosion by the next intense rainstorm. Road surfaces provide another source of sediment, frequently also involving the enlargement of roadside drains, especially on the upslope side of a road. Cut-and-fill areas are both liable to erosion, but the problems are most severe on the fills. The relatively loose structure of the fills allows water to detach mineral particles more easily and thus gullies develop rapidly. If the fill lies on weathered rock or colluvium, the permeability of the fill is likely to be greater than that of the material below, and thus water will tend to move laterally at the base of the fill and may induce slipping or sliding. Gullying in fill is likely to create wide, deep gullies whose sides evolve by slumping and undercutting. In addition the gully may intersect subsurface throughflow paths. Where it does so, the emerging throughflow creates overhanging rounded hollows, or shallow cavities in the gully walls. Eventually these overhangs collapse and the gully widens. In cut areas the weathered rock is more cohesive and gully-widening is relatively slow. Instead, depending on the character of the rotted rock material, deep narrow vertical gullies develop allowing water to penetrate quickly several

Urban Geomorphology of Kuala Lumpur 349

metres down into the slope material, perhaps raising pore water pressures and increasing the risk of landslipping.

Urban Soil Erosion and Sedimentation

Post-construction Impacts

Urbanization causes a temporary and extreme rise in sediment production from a drainage basin. Urban construction in Kuala Lumpur, and over much of Southeast Asia, involves bulldozing large areas of land, with much earth-moving and landfilling. Almost all the time there are many hectares of bare construction land around Kuala Lumpur. Over the years their location has shifted further and further from the city centre (Figure 20.3). The soils and weathering profiles exposed during this process are easily eroded and high sediment yields occur (Pushparajah 1985). On cleared and graded slopes, water running over the surface of the ground carries particles of clay, silt, and fine sands to tiny channels on the bare surface. In these channels, a centimetre or two wide, the water flows faster, carrying soil particles with it at concentrations of 10 000– 15 000 mg l−1 at the beginning of a storm. Although the concentration of this sediment in suspension (suspended sediment) falls as the storm continues, the rills usually carry large volumes of this muddy material into drains and natural channels close to the construction site. Such bare eroding slopes and drains choked with sediment are common features of building sites in and around Kuala Lumpur. Good site management and water pollution control suggests that retention ponds should be constructed at the lower end of such building sites to retain water until the worst of the storm has passed, and then to release the water slowly into the adjacent drains. The ponds should be constructed so that the bulk of muddy sediment settles to the bottom and does not add to the debris and silt already in the local drains and channels. However, off-site measures involving increased channel capacity or diverting the increased run-off to a new outlet are often necessary (Gupta 1984). The massive amounts of sediment washed off construction sites are indicated by the high suspended sediment concentrations of 15 343 mg l−1, 19 000 mg l−1, and 81 230 mg l−1 recorded in three streams draining catchments undergoing development in Kuala Lumpur (Douglas 1978; Leigh 1982). Total soil losses by erosion from two construction sites in Kuala Lumpur, one at Bukit Kiara and the other in Damansara, have been estimated at around 100 000 t y−1 and 332 000 t y −1 respectively. Sediment chokes urban waterways, exacerbating flooding and often necessitating the expensive removal of silt from rivers and the reconstruction of stream channels. Areas undergoing construction usually experience annual sediment yields 100 to 1000 times

Expansion of building in the great post-1970 expansion of the city took construction on to slopes of more than 25°, to the northeast of the city, in the vicinity of Ampang and Ulu Klang, where the weathered granite is often 20 m or more thick. Here sliding of colluvium over weathered rock may occur where the vegetation has been disturbed and the hill slope has been modified by cut-and-fill operations to create level building sites. The most tragic and spectacular landsliding in Kuala Lumpur has been in the Ukay Heights areas between Ampang and Ulu Klang in the granite hills to the east of the Klang River. Here multi-storey condominiums (apartment blocks) were built on cut-and-fill platforms on the steep hillside. While the views from the buildings are spectacular, the ground conditions were far from stable. Excavation and subsequent loading by the structures changed the pore spaces in the underlying weathered rock and colluvial slope mantles. Each heavy rain event contributed to changes in the stability of the slope, gradually enlarging voids between mineral grains until an extremely heavy downpour occurred. Water pressures in the enlarged pores upslope of the condominium buildings increased to such an extent that the colluvial material began to slide over the underlying weathered rock. The sliding material pushed against the foundations of the buildings, one of which eventually toppled over. Essentially this tragic slope failure was little different from many that have occurred in and around Kuala Lumpur. While this was on granite, other disasters have been in the weaker metasediments, particularly those in the Ulu Gombak near the Genting Sempah Tunnel. Here a major landslide blocked the road to the Genting Highlands and killed several people, whose vehicles were buried by the debris. While the metasediments are inherently weak and generally unsuitable for any construction, the problems of the granite hillsides can usually be managed if attention is given to zoning them according to their suitability for particular types of structure. Generally, multi-storey buildings have to be located on excavated areas near ridge crests and not on mid-slope colluvium and never on fill. Many Kuala Lumpur residents have houses on cut-and-fill slopes and experience annoying, but manageable, failures of retaining walls and fences. Only a well-publicized few suffer the dreadful damage of major landslides. However, they, and the developers and city planners, should recognize that firm foundations are far more important in the long term than good views.

Fig. 20.3. Distribution of bare areas in the Kuala Lumpur area based on aerial photograph and satellite image interpretation

Urban Geomorphology of Kuala Lumpur 351 Table 20.2 Results of urban catchment studies in Kuala Lumpur Catchment

Land use

Area (km2)

Rainfall (mm)

Sediment yield (t km−2 y−1)

S. S. S. S. S. S. S.

Newly urbanizing Tin-mining and urbanizing Newly urbanizing Newly urbanizing and mature urban Urban and industrial Forest and urban Forest and urban

10.3 27.1 14.2 29.0 35.9 145.0 140.0

2400 2300 2400 2300 2200 2400 2400

1056 2283 1480 1372 1759 1265 1157

Jinjang (1) Jinjang (2) Klang (1) Klang (2) Keroh Batu Gombak at Jalan Pekililing

Sources: Balamurugan (1991c) for all rows except the last: Douglas (1978).

Table 20.3 Erosion on a bare construction site at Mengkuang Heights in Ulu Klang near Kuala Lumpur, Malaysia Type

Erosion rate (t y −1)

Sediment yield (t km−2 y−1)

Slope surface erosion Rill erosion Gully erosion

2544 5291 26 868

37 970 78 985 401 014

TOTAL

34 703

517 955

Source: After Mykura (1989).

greater than those under natural forest (Table 20.2). A large part of those yields may be washed away in just a few major rainstorms, with between 35 and 80 per cent of the annual load often being carried in a single month. On the Mengkuang construction site (Table 20.3) in the Sungai Sering catchment in Ulu Klang in the late 1980s, Mykura (1989) made detailed studies of soil loss from bare slope surfaces, rills, and gullies that had been left exposed to raindrop splash for three years. The actual soil loss related to the erosive energy of the rain and to the sizes of the mineral grains in the soil, the stability of blocks of soil, and the ability of the soil to resist forces trying to break it apart. The one single measure providing to best indication of slope surface erosion was the resistance force, known as shear strength, which is easily measured by a small hand shear vane. Soil surface micro-relief, the presence of small stones on the ground surface, and the development of a hard clay crust sealing the soil surface all affect raindrop impact and infiltration and therefore influence soil losses. Soil loss through rill development is largely controlled by slope angle. Soil shear strength plays a minor, but significant, role. Rills develop as local drainage routes and their presence or absence, position, and

density are largely dependent on surface roughness and microtopography, with features such as cracks caused by shrink-swell in clay soils (particularly where the illite content is high) exploited by concentrated overland flow. Gullies are the major sediment source on exposed construction sites. Gullies increase in size more rapidly on fill material than on cut slopes. In an attempt to control erosion and siltation problems, the federal government introduced legislation enabling local authorities to exert greater control over the layout and management of construction sites, and in 1974 published an urban drainage design standards and procedures manual for Peninsular Malaysian conditions (Lewis et al. 1975). This encouraged developers to take a more responsible approach to building site layout and management (Leigh 1982). However, twenty years later examples of detention ponds that were poorly operated and widespread erosion of cutand-fill slopes could readily be found in and around Kuala Lumpur. Even when construction has been completed, erosion and slope instability may continue. Studies of major highways leading out of Kuala Lumpur (Bayfield et al. 1992) indicated that slope failure is mainly related to failure to provide crest drains or substantial horizontal drains. Erosion can result from cutting slopes too steep for the soil type present, but is mainly related to inadequate vegetation cover. The predominant type of erosion varies with slope steepness. On 83° slopes, slumps were the only type of erosion recorded, with collapses often occurring at discontinuities between variously weathered materials. Slumps increase in frequency with slope angle. Natural vegetation regrowth is most effective on gentle slopes. Although plant cover tends to decline with the size of the cut slope, most cuttings that are more than 15 years old are reasonably well vegetated. Turfing and seeding tended to be more successful on east- than west-facing slopes owing to the former drying out more

352 Ian Douglas

rapidly. However, the spraying of cut-and-fill slopes with a viscous tar-based liquid containing grass seeds (hydroseeding) has often failed to provide an adequate grass cover to prevent further erosion. Among the reasons for this are soil erosion of the bare surfaces prior to application; inadequate soil moisture; uneven spraying; soil acidity; infertile seed; and too much, too little, or inappropriate fertilizer.

Fluvial Geomorphology The fluvial geomorphology of the Sungai Klang and its tributaries has undergone major transformations since 1857. Mining methods in the nineteenth century were crude and much sand and silt escaped into the rivers. The first rubber plantations on the edges of the town in the early years of the twentieth century were clean-weeded to prevent competition for soil nutrients, but the rill and gully erosion of the bare soil led to an increase in drainage density in many Klang River tributaries. Although clean-weeding was quickly abandoned, the gullies remained traceable in the rubber plantations until they were cleared for new suburban development in the post-1970 urban expansion. These changes altered the dynamics of river channels. The direct discharge of enormous loads of silt and sediment from the mining effluents into river systems raises the riverbeds through the accumulation of sediment. Even with the provision of settling ponds and statutory requirements for mine effluent quality standards, some tributaries continue to carry high sediment loads owing to erosion of unvegetated old mining areas and continuing removal of sediments derived from past mining activities stored in river channels. The sediment loads of tributaries passing through former mining areas increase by three to six times as they entrain available sediment (Balamurugan 1991a,b). Higher peak discharges were constrained by small bridges and culverts. Forced through confined spaces at high velocity, these larger flows began to scour around bridge piers and to develop wider channel sections immediately downstream of any bridge. The combination of channel bed aggradation and widening led to flatter, broader cross-sections. While mining is the long-established old cause of high fluvial sediment loads in Kuala Lumpur streams, clearance of land for urban construction is the modern, continuing cause. From the Kenny Hills and Damansara Heights, building work has spread outwards since 1960 (Figure 20.3). Urbanization in Kuala Lumpur has caused an increase of peak flows by an average factor of over 4. Run-off volumes increased by an average

of 250 per cent. The most serious effect has been the increased frequency of flash floods, with erosion by surface water leading to a major deterioration in river water quality. Frequent flooding also poses severe health hazards in low-lying areas (Ithnin 1988). The channel changes produced by the combination of increased peak storm discharges and high sediment loads are well illustrated by the Sungai Anak Ayer Batu, a small tributary draining the Damansara Heights and Pantai Valley areas to the west of the city centre. In the mid-1960s a large area at the head of this small stream was bulldozed clear of all vegetation in preparation for a major housing development. The deeply weathered, weakly metamorphosed sedimentary rocks were easily eroded, and every storm washed large volumes of sediment into the streams. Concentrations of suspended sediment during storms exceeded 81 000 mg l−1 and the 1969–7O annual sediment discharge was estimated to be 2000 t km−2 y −1 (Douglas 1975, 1978). The original narrow meandering stream could not cope with these greatly increased water and sediment flows and changed from a narrow, deep, meandering channel to a straighter, steeper, braided channel (Figure 20.4). Downstream of an old bridge, an immense amount of scour occurred, as the space under the bridge did not have adequate capacity for the increased discharges (Douglas 1985). Further downstream on the Anak Ayer Batu large natural channel migration occurred, through the process of ‘load substitution’ (Vogt 1962), in which rivers drop their load in one place and then acquire new load by bank erosion. The stream began to threaten to erode the foundations of important buildings. To prevent expensive damage, the authorities built a large sediment trap in the form of a small dam across the valley in 1978. By 1982 the storage capacity was entirely taken up with sediment. The nearly flat course of the stream across the filled area enables it to drop much of the sediment load, only for severe scour to take place again immediately downstream of the reservoir. As a result, three decades after the original land clearance, problems of channel aggradation and frequent overbank flooding remain. These cause severe difficulties in an urban area. Tributaries like the Anak Ayer Batu discharge pulses of sediment into the main river. Depending on the location of thunderstorm rains and the relative magnitudes and velocities of main river and tributary flows, a tributary pulse may suddenly raise the bed of the main river temporarily, increasing the risk of flooding. On the other hand, a high flow down the main river may prevent the water from the tributary from escaping,

Urban Geomorphology of Kuala Lumpur 353

Fig. 20.4. Sequence of changes to a small river in Kuala Lumpur as a result of urban construction

causing the side stream level to rise and flood adjacent land and property. Frequently the economics of the residential land and housing market mean that areas are left bare for many months or even years. One such situation arose in the Sungai Sering catchment in Ulu Klang. Here huge concentrations of suspended sediment were measured downstream of a gullied construction site. In two years 1579.8 t km−2 y −1 were supplied from the 6.7 ha badly eroded bare Mengkuang Heights area upstream (Mykura 1989). The bed of the Sungai Sering became aggraded and its banks were undercut and eroded. Large volumes of debris were washed from the Sering into the Sungai Klang. The aggradation of the bed, coupled with the rapid storm run-off from the eroded areas, produced frequent overbank flooding which affected nearby households, mostly squatters who had moved into the temporarily unused construction site. Another catchment area affected by land clearance for urban development on the edge of Kuala Lumpur, the 36 km2 Sungai Keroh catchment above the Segambut Bridge, suffered an estimated soil loss of 5927 t km−2 y−1, with a fluvial suspended sediment yield of 1759 t km−2 y−1. The only abandoned tin-mining area to exceed this sediment yield was the Jinjang catchment, from which 2283 t km−2 y −1 is released (Balamurugan 1991c). Minor stream channels are often culverted or diverted. Sometimes altered streams cause erosion in localities formerly considered safe. Frequent accounts of such bank erosion episodes occur in the Kuala Lumpur press. In June 1995, for example, a local newspaper reported the concern of a family at the way a diverted stream carrying extra run-off from a new highway had eroded

laterally for 9 m and was threatening to undermine their house. Following the great flood of 1926, the river underwent its first major channellization, with a double trapezoidal cross-section being created through the city centre. Despite this work, the 1938 Department of Irrigation and Drainage Report indicated that the annual recurrence of flooding in Kuala Lumpur at that time was attributed to the tin-mining activity prevailing around the city. After the second great flood in 1971, a new channellization occurred, with a rectangular section of much greater capacity being created in the core area of the city. The new works included ramps to allow excavation equipment to get into the concrete channel at low flows to remove accumulated silt and restore the capacity of the channel. These new river training works merely serve to transfer the water and much of the silt further downstream. While floods are alleviated in one reach, they may be aggravated elsewhere. Constant additions to such works are often required, such as plans developed since 1995 to widen a stretch of the Sungei Klang near the mosque in Jalan Tun Perak, in order to prevent flash floods in that part of the city. The deposition of sediment in the channellized urban reach is only part of the story. The finer fraction is carried in suspension over the weir at Puchong downstream into the tidal wetland area of the channel. Here not only does the new, often inert quartzitic material blanket the fertile organic matter of the mangrove swamps, but heavy metals from the urban area (Sham et al. 1983) have a deleterious impact on the wetland ecosystem.

354 Ian Douglas

Problems Related to Quaternary Sea-Level Change Influences on Foundation Conditions The Old Alluvium The plains around Kuala Lumpur are developed on an alluvial sequence that formed during the changing sea levels of the last 2.4 million years. Much evidence suggests that often in that time the climate became more seasonal, the vegetation perhaps more sparse, and that wide, multi-thread, braided, sand and fine-gravel bedded rivers carried large quantities of sand down to sea levels lower than the present (see Chapter 2). Between these relatively dry periods, which coincided with great expansions of the polar ice sheets, sea levels rose and marine clays were deposited over the fluviatile sands. The stratigraphy of this sequence of fluvial and marine deposits, the Old Alluvium, is complex. The former braided river channels give way laterally to overbank deposits, and vertically to marine sediments. Piles supporting foundations thus pass through a variety of materials. Fortunately for the first 100 years of Kuala Lumpur’s development, most of the Old Alluvium around the city contains a layer of fine resistant material, known to engineers as the ‘stiff clay’. This clay provided adequate support for the one- to three-storey buildings of the young, growing city (Figure 20.5).

When high-rise, multi-storey construction began, new foundation problems arose. The alluvial deposits provided inadequate support in many cases and deep piling had to be used. Sometimes firm bedrock was encountered, but on occasions the subsurface limestone was found to have great caverns and an irregular pinnacled surface. These caves and pinnacles were formed during the low Quaternary sea levels. However, techniques of multiple piles and concrete raft or mat supports for major buildings often helped to provide the firm foundations required. The old river channels contained the alluvial tin so important in the history of Kuala Lumpur. As the original near-surface deposits around the city became worked out, the deeper buried channels beneath the coastal plain and offshore became the main focus of tin-dredging, all the remaining activity now being well downstream of Kuala Lumpur. Knowledge of the Quaternary deposits is thus helpful in securing both good foundations and in accessing the mineral resources that are important for economic activity and urban growth in present-day Kuala Lumpur.

Karst Problems The limestones that form the prominent Batu Caves Hills north of Kuala Lumpur are part of a much larger buried limestone mass containing many caves. This buried karst plain was exposed as a set of limestone pinnacles at the bottom of many of the alluvial tin

Fig. 20.5. Types of foundation problem associated with subsurface conditions in Kuala Lumpur

Urban Geomorphology of Kuala Lumpur 355

mines north and east of the city in the 1960s. Not only is the surface of the buried plain thus very irregular, but the caves and voids beneath the pinnacled surface are difficult to detect and predict. The large cavities enable water to move beneath the plain rapidly. The tin miners used a lot of water in their hydraulic sluicing and had to pump water out of their pits constantly. This removed water from some of the adjacent limestone cavities and sometimes led to sudden subsidence of the overlying alluvium into the buried caves (Hasan 1984). Such rapid lowering of the ground surface should have been an indication of the problems likely to arise when the areas over the buried karst plain were developed for housing and industry. Many abandoned tin mines were used as landfill sites for the city’s domestic and industrial waste. Gradually a large part of the former sequence of tin mine ponds that had restricted expansion of urban development between Sentul and Ampang was filled in. The shortage of urban residential land in Kuala Lumpur has led to some of the filled areas being reclaimed for low-cost housing. In a few instances fill has later subsided into sinkholes, with, in one place, a row of low-cost houses collapsing into a reopened hole (Tan 1986). Such buried karst plains are composite, being the product of evolution in relation to changing sea levels (Chow 1988). The pinnacles exposed beneath the alluvium may be but the summit of a series of caverns which have been invaded by the sea and drained more than once. Boreholes often encounter a sequence of voids or loosely compacted clays, separated by hard limestone. Although Glazek (1966) suggested that caverns may continue to evolve below sea level, it is most likely that the present buried karst all developed when the limestones were exposed during the Quaternary drier, low-sea-level periods. The buried karst now poses serious problems for civil engineering works (Tan and Batchelor 1981; Bergado and Sebanayagan 1987). New high-rise buildings require deeper foundations than the low-rise buildings which sufficed until the 1970s. Many of the taller, multi-storey structures require piling into the underlying limestone. However, the irregularity of both the karst surface and the cavities within the buried karst means that foundation investigations have to be particularly circumspect (Yunus Abdul Razak 1988). Drillholes may strike limestone, unaware of whether it is buried rockfall material or a pinnacle, while a neighbouring hole might pass through several more metres of alluvium before hitting limestone. These foundation problems had to be taken into account during the preparations for building the Petronas Twin Towers, the world’s tallest building at the time (Pelli et al. 1997). Beneath the level surface of the site

the pinnacled surface of the karst plain sloped from 15 m depth under the northwestern corner to a depth of over 180 metres at the southeastern edge. In order to get a conventional level foundation for such a huge weight of concrete, the bedrock would have to be excavated at one edge, while at the other piles would have had to be sunk to a far greater depth than normal. The risks and costs were so great that the building was moved 60 m to the southeast to allow for more than 55 m of alluvium beneath each tower. To do this successfully, a foundation designed to withstand movements with the alluvium was required. The ingenious solution was to spread the load throughout the alluvium so that an even movement could occur. A concrete mat was designed to spread the weight of the building over a set of drilled 1.3 m diameter piles. These piles would transfer the weight of the towers to the soil more gradually than a simple mat would. Friction between the surface of a pile and the surrounding soil would prevent the foundation supports from sinking. Settlement would then occur between the pile tips and the bedrock. Pile lengths were varied so that their tips were all at the same height above bedrock, so as to avoid any tilting of the foundations. This technique introduced a new concern. The interlocking grains within the alluvium would cease to be locked together if excavation removed overlying material and allowed the alluvial mass to expand. The piles were therefore sunk from as near the ground surface as possible and the concrete mat was laid on top of them. The extremely careful investigations and detailed precautions taken during the planning of the foundations of the Petronas Twin Towers show just how important it is to have a full understanding of the ground and subsurface conditions of any major engineering structure. Virtually all the tall buildings in Kuala Lumpur have had to pay particular attention to foundation problems. The ‘stiff clay’ within the alluvium cannot support the multi-storey buildings unless special foundation works are undertaken. The cavernous pinnacled limestone poses serious problems owing to the many voids in the bedrock. Do the developers of residential estates have such detailed knowledge and take so many precautions over the subsurface conditions beneath the houses they sell to the general public?

Downstream Consequences of Urban Effects on Geomorphic Processes The delta of the Sungai Klang has been considerably transformed by urban activity upstream. Considerable areas

356 Ian Douglas

of the wetlands have been reclaimed for major projects, such as the new Kuala Lumpur International Airport. The reclamation has reduced the flood storage capacity of the coastal wetlands. At the same time the channellization of the river and the extension of the drainage network by storm drains collecting run-off from roofs and paved areas leads to much higher peak discharges following storm rainfalls. The erosion of the bare areas and roadsides and the entrainment of all kinds of urban debris means that these storm run-off events carry surges of sediment into the coastal wetlands, changing the sedimentation conditions in the mangroves and often obliterating them. The mangroves are part of a complex wetland on deep organic soils that have their own stability and geochemical problems. Drainage of the soils results in both peat shrinkage and the development of acid sulphate soils. In the freshwater areas of these wetlands, waters in streams are characteristically nutrient-poor, black, acid waters with a pH of less than 4.5. They are more than usually corrosive of concrete and brickwork. Peat shrinkage gives rise to local subsidence problems and necessitates special foundation works for important structures. Developments at Port Klang, now among the top twenty-five ports in the world, have covered hundreds of hectares of mangroves with concrete, greatly changing the dynamics of the coastline. Most of the formerly productive fishery, supplying fresh fish, prawns, and crabs to the Kuala Lumpur market, has been displaced to other river mouths, and the new container, liquid, and break-bulk port activities are meeting the demands of the industries and consumers of the Klang Valley. The whole Klang River Basin has thus undergone a major transformation, from the tourist and recreation facilities in the hills to the shipping in the delta. The environmental link between the two is still provided by the sediment, by the particles of eroded soil, carried downriver from the hills and deposited in the channels of the delta. Port operations now mean that those sediments have to be dredged and perhaps used as material for land reclamation for yet more construction.

of mud and silt deposition. Some have undergone the trauma of flooding or emergency evacuation because of a landslide. The urban authorities, both in the Federal Territory and in the State of Selangor Darul Ehsan, have a constant battle to cope with the consequences of the changed run-off and soil erosion conditions of an increasingly paved and channellized urban area. Many of the precautions about run-off and sediment discharge control on construction sites have been implemented. However, signs of poor site management, inappropriate construction on hillsides, and inadequate protection of cut-and-fill slopes abound. The applications of geomorphology have not taken root in the urban planning and administration of Kuala Lumpur. The problems could be reduced by paying more attention to ensuring that construction was appropriate to the type of terrain. Ideally removal of vegetation has to be kept to a minimum and increased run-off and soil loss from building sites has to be restrained. As soon as possible after construction is completed appropriate revegetation, slope drainage, and stabilization measures have to be implemented. The idea behind all such precautions is to try to keep the modified hydrologic system as close to that of the natural forest as possible. The water has to be retained on site and allowed to infiltrate, rather than having the opportunity to pick up sediments and pollutants as it runs over the surface of the ground towards streams and rivers. The idea of ‘design with nature’ in terms of urban geomorphology means building in an appropriate way for the conditions of the site. For many people, effective demonstration of the human ability to ‘conquer nature’ suggests that engineering ingenuity can overcome all site and foundation problems. However, the ‘precautionary approach’ would suggest that when there is a choice, as there is in Kuala Lumpur, the designs that are in harmony with nature and appropriate to the geomorphic and foundations conditions are better value. Such buildings are likely to be better for everyone, both the users of the building and people downslope and downstream. The need to ‘know the ground you build on’ is particularly relevant in Kuala Lumpur.

Conclusions Driven by the dynamics of high-intensity rainfalls, highly erodable weathering profiles, and the rapid land use changes associated with an expanding economy, the urban geomorphology of Kuala Lumpur dramatically illustrates how earth surface processes are a part of everyday urban life. Few Kuala Lumpur commuters escape the delays caused by highly localized flash flooding. Most have some experience of slope instability or

References Airriess, C. A. (2000), ‘Malaysia and Brunei’, in T. R. Leinbach and R. Ulack (eds.), Southeast Asia: Diversity and Development (Upper Saddle River, NJ: Prentice-Hall), 341–78. Balamurugan, G. (1990), ‘Soil Erosion and Sediment Balance in the Klang Basin’, MA thesis, University of Malaya. —— (1991a), ‘Some Characteristics of Sediment Transport in the Sungai Klang Basin, Malaysia’, Journal of the Institution of Engineers, Malaysia, 48: 31–52.

Urban Geomorphology of Kuala Lumpur 357 —— (1991b), ‘Tin Mining and Sediment Supply in Peninsular Malaysia with Special Reference to the Klang River Basin’, The Environmentalist, 11: 281–91. —— (1991c), ‘Sediment Balance and Delivery in a Humid Tropical Urban River Basin: The Klang River, Malaysia’, Catena, 18: 271–87. Bayfield, N. G., Barker, D. H., and Yah, K. C. (1992), ‘Erosion of Road Cuttings and the Use of Bioengineering to Improve Slope Stability in Peninsular Malaysia’, Singapore Journal of Tropical Geography, 13: 75–89. Bergado, D. T., and Sebanayagan, A. N. (1987), ‘Pile Foundation Problems in Kuala Lumpur Limestone, Malaysia’, Quarterly Journal of Engineering Geology, 20: 159–75. Chow Meng Sun (1988), ‘Geological Hazards in the Urban Centre of Kuala Lumpur’, in Atlas of Urban Geology (Bangkok: UN-ESCAP, i. 24– 60. Douglas, I. (1975), ‘The Impact of Urbanization on River Systems’, Proceedings IGU Regional Conference and Eighth New Zealand Geography Conference (Wellington: New Zealand Geographical Society), 307–17. —— (1978), ‘The Impact of Urbanisation on Fluvial Geomorphology in the Humid Tropics’, Geo-Eco-Trop, 2: 229–42. —— (1985), ‘Hydrogeomorphology Downstream of Bridges: One Mechanism of Channel Widening’, Applied Geography, 5: 167–70. Fookes, P. G., and Weltman, A. J. (1989), ‘Rock Slopes: Stabilization and Remedial Measures against Degradation in Weathered and Fresh Rock’, Proceedings of the Institution of Civil Engineers Part I, 86: 359–80. Gerrard, A. J. (1988), Rocks and Landforms (London: Unwin Hyman). Glazek, J. (1966), ‘On the Karst Phenomena in North Vietnam’, Bulletin of the Polish Academy of Sciences, Geographical and Geological Series, 14: 45–52. Gullick, J. M. (1983), The Story of Kuala Lumpur (1857–1939) (Singapore and Petaling Jaya: Eastern Universities Press). Gupta, A. (1984), ‘Urban Hydrology and Sedimentation in the Humid Tropics’, in J. E. Costa and P. J. Fleisher (eds.), Developments and Applications of Geomorphology (Heidelberg: Springer), 240–67. Hasan, M. A. (1984), ‘Mineral and Energy Resources: Its Exploitation Related to Mining Operation Hazards, Subsidence and Other Effects’, in Sahabat Alam Malaysia (ed.), Environment, Development and the Natural Resource Crisis in Asia and the Pacific (Penang: Sahabat Alam Malaysia), 131–55.

Ibrahim Komoo (1998), ‘Deep Weathering: Major Cause of Slope Failure in Wet Tropical Terrain’, Proceedings: Eighth Congress of the International Association of Engineering Geology and the Environment, Vancouver Canada (Rotterdam: Balkema), ii. 1773– 8. Ithnin, H. (1988), ‘Spatial Analysis of Changes in Surface Water and its Effects on the Environment Due to Urbanization: The Case of Kuala Lumpur, Malaysia’, Ph.D. thesis, Pennsylvania State University. Leigh, C. H. (1982), ‘Urban Development and Soil Erosion in Kuala Lumpur’, Journal of Environmental Management, 15: 35– 45. Lewis, K. V., Cassell, P. A., and Fricke, T. J. (1975), ‘Urban Drainage Design Standards and Procedures for Peninsular Malaysia’, Bahagian Parit dan Taliair Planning and Design Procedure, vol. i. (Kuala Lumpur). Mykura, H. F. (1989), ‘Erosion of Humid Tropical Construction Sites, Kuala Lumpur, Malaysia’, Ph.D. thesis, University of Manchester. Newbery, J. (1970), ‘Engineering Geology in the Investigation and Construction of the Batang Padang Hydroelectric Scheme, Malaysia’, Quarterly Journal of Engineering Geology, 3: 151– 81. Pelli C., Thornton, C., and Joseph, L. (1997), ‘The World’s Tallest Buildings’, Scientific American, 277/6: 64–73. Pushparajah, E. (1985), ‘Development Induced Soil Erosion and Flash Floods in Malaysia’, Ecologist 15/1–2: 19–20. Sham, S., Latiff, A., Badri, M. A., Muhamed, A., and Yazid, M. I. (1983), ‘Urbanization and Biophysical Environment of Kuala Lumpur, Malaysia’, Paper prepared for the Regional Seminar of Techniques for Analysis of Tropical Cities on an Ecosystem Basis, Nov. 1983 (Bangi: Universiti Kebangsaan Malaysia), 24–2. Tan, B. K. (1986), ‘Geological and Geotechnical Problems of Urban Centres in Malaysia’, in B. K. Tan and J. L. Rau (eds.), Landplan II Role of Geology in Planning and Development of Urban Centres in Southeast Asia, AGID Report Series, 12 (Bangkok), 10–14. —— and Batchelor, B. C. (1981), ‘Foundation Problems in Limestone Areas: A Case Study in Kuala Lumpur, Malaysia’, Proceedings of the International Symposium on Weak Rock, Tokyo (Rotterdam: Balkema), 1461–3. Vogt, H. (1962), ‘Les Facteurs de la dynamique de l’Adour moyen’, Revue de Geomorphologie dynamique, 13: 49–72. West, G., and Dumbleton, M. J. (1970), ‘The Mineralogy of Tropical Weathering Illustrated by Some West Malaysian Soils’, Quarterly Journal of Engineering Geology, 3: 25– 40. Yunus Abdul Razak (1988), ‘The State of the Art of the Application of Urban Geology to Planning in Malaysia’, in Atlas of Urban Geology (Bangkok: UN-ESCAP), ii. 145–50.

21

Subsidence and Flooding in Bangkok Noppadol Phienwej and Prinya Nutalaya

Introduction

The Physical Features

Bangkok, the capital of Thailand, is situated on flat, low land in the southern part of the Central Plain, one of the main physical units of the country (Figure 21.1). Through the heart of the city, the Chao Phraya flows from the north and discharges into the Gulf of Thailand, 25 km south of the city centre. The city was founded in 1782, and in its early years numerous klongs (canals) were dug for transportation and defence uses. These canals became corridors of early development, and banks were lined with houses, shop-houses, and temples, etc. With the beauty of its waterway landscape, Bangkok was once dubbed the Venice of the East. Unfortunately, such a resemblance no longer exists as most of the canals have been backfilled to make room for road construction in recent urbanization. The Bangkok metropolis, which at present has a population in excess of 10 million, has expanded rapidly on both banks of the river since 1950. It has encroached into surrounding provinces, covering an area of approximately 60 × 70 km. Owing to its flat topography and close proximity to the sea, flooding threatens the city annually. Modern urbanization has resulted in the drastic destruction or blockage of natural drainage paths, increasing the flood risk to the city. Severe land subsidence from excessive groundwater extraction since the 1960s has intensified the flood risk, as well as creating numerous foundation problems. At present the land surface in some areas is already below mean sea level. The city now has to rely on a flood protection system to prevent inundation. However, its effectiveness is only temporary because land subsidence has not yet ceased.

The Central Plain is formed by the Chao Phraya River, the largest in the country. The river basin stretches from the Northern Highland to the Central Plain and covers about one-third of the country (514 000 km2). The Central Plain can be divided into the Upper and Lower Central Plains. The former extends from Tak to Nakhon Sawan Provinces. Four main rivers, namely, the Ping, the Wang, the Yom, and the Nan, which originate in the Northern Highland, traverse the plain and join together at Nakhon Sawan, 240 km north of Bangkok, to form the Chao Phraya River. It then flows southwards, traversing the Lower Central Plain, through the cities of Chai Nat, Ayutthaya, and Bangkok, finally discharging into the Gulf of Thailand at Samut Prakarn. With a flat to very slightly undulating broad depositional surface dominating the topography, the Chao Phraya gives off many tributary branches. Important among them is the Tha Chin River, which leaves the west bank at Chai Nat, 170 km north of Bangkok, and flows southwards to discharge into the gulf at Samut Sakhon, 35 km southwest of Bangkok. Two other rivers branch out at Chai Nat but rejoin the Chao Phraya near Ayutthaya, 50 km and 60 km north of Bangkok. Besides the Chao Phraya, the Central Plain contains three other main rivers, namely the Mae Khong to the west, the Pa Sak to the east, and the Bang Pakong to the southeast. They play important roles in flooding the Lower Central Plain. However, only the Pa Sak contributes to flooding in the Bangkok area. The Pa Sak, which has a catchment of 15 806 km2, originates in the Central Highland. It joins the Chao Phraya at Ayutthaya.

Subsidence and Flooding in Bangkok 359

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Figure 21.2 shows a geomorphological map of the Lower Central Plain. The main rivers that traverse the plain are included in the figure. The Lower Central Plain is a large, flat plain consisting of young fluvial and marine deposits of the Chao Phraya Delta. It is bounded by a mountain range on the west, Nakhon Sawan province on the north, and Khorat Plateau on the east. Fans and terraces occupy the west and east marginal zones of the plain. Natural levées and back-marshes occupy areas along the river channel. The Chao Phraya Delta is very flat, with a gradient between 1 × 10−5 and

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Fig. 21.1. Physical units of Thailand

2 × 10−5. The delta is divided into an upper delta and lower delta. The upper delta of fluviatile sediments and brackish sediments is well developed between Chai Nat and Ayutthaya, and has higher natural levées on it. The lower delta can be subdivided into a tidal flat of brackish clays, a tidal flat of marine clays, and a tidal zone (Figure 21.2). It is a very flat plain between Ayutthaya and the Gulf of Thailand, with ground elevation ranging from 0 to 3 m above mean sea level. The area is also called Bangkok Plain. Low natural levées, 1 m higher than the adjacent floodplain, exist along the banks of

360 Noppadol Phienwej and Prinya Nutalaya

Fig. 21.2. Geomorphological map of the Lower Central Plain (Source: after Takaya and Thiramongkol 1982)

the main rivers. The deposit of the tidal flat of brackish clays is composed of soft clay containing shell fossils. A tidal flat of very soft to soft dark blue marine clay stretches along the coast. The topography and soils of the Lower Central Plain reflect the fact that it was covered by a shallow marine sea from about 7000 to 2000 years ago, at which time

a soft clay was deposited in shallow nearshore waters (Rau and Nutalaya 1983). Probably, during most of Middle to Late Holocene time the plain was a vast tidal flat that was gradually filled, resulting in the Bangkok Clay. Its thickness is about 4–5 m at Ayutthaya and increases to 30 m in the coastal area south of Bangkok. The sea withdrew about 2200 years ago, exposing

Subsidence and Flooding in Bangkok 361

the soft clay. There has been very little change since its deposition, and only the uppermost 2 m has been weathered. The Bangkok Plain could be perceived therefore in the past as a large bay which was fed by estuaries emerging both from the western mountain belt and from the neck of the plain near Ayutthaya. The environment at the mouth of the Chao Phraya today resembles the Bangkok Plain of the Middle and Late Holocene. The structure of the basement of the Bangkok Plain is still not well known. The best description was given by Nutalaya and Rau (1987). In the Bangkok metropolitan area, its depth varies from about 450 m to more

than 2500 m. The bedrock is of variable types and ages, consisting of folded metasedimentary rocks intruded by Mesozoic granites. The top of the basement is undulated. Block faulting in the Late Pliocene–Pleistocene broke the basement into a series of horsts and grabens trending in a north– south direction. Overlying the impervious basement is a complex sequence of semi-consolidated and unconsolidated deltaic and marine clastic sediments of Tertiary and Quaternary ages. Only the upper 550 m of the sediment is well defined, referred to as the Bangkok aquifer system. Figure 21.3 depicts the isometric view of geomorphology and stratigraphy of the Bangkok Plain.

Fig. 21.3. Geomorphology and stratigraphical section of the Lower Central Plain (Source: after JICA 1995)

362 Noppadol Phienwej and Prinya Nutalaya

A comprehensive overview of geology and geoenvironment aspects of Bangkok is given by the ESCAP Secretariat (1988). Among various geo-environment problems facing Bangkok, flooding and land subsidence are the most critical, and they are the topics of this chapter.

Flooding Flooding is a natural phenomenon in the Lower Central Plain, usually occurring from September to November. It is the result of one or more of the following causes: 1. severe rainstorms in the upstream catchments; 2. intense local rainfall; 3. flood waves from different tributaries meeting at a confluence; 4. high tides from the Gulf of Thailand. In this monsoon region, rainfall is highest from August to September, owing to the passage of depressions and tropical storms. Consequently, in Bangkok, which is located downstream in the basin, the flood season generally begins in September, but local torrential rainstorms can cause immediate flooding at almost any time between May and October. The most severe floods occur in October, when rivers draining northern Thailand bring their flood water into the Bangkok area. Moreover, the highest tides of the year occur at the same time and back water up the Chao Phraya, slowing the passage of the high flow. Flooding in the Lower Central Plain is caused by overland flow from upstream, and by overtopping and levée breaches along the Chao Phraya and Tha Chin Rivers. The inundation is widespread, and the water can be detained for a few months before it can be drained out to the rivers or the sea. The government has placed a high priority on addressing the problem of flooding and has had notable success in reducing the magnitude of floods over the last few decades through the construction of multi-purpose reservoirs and other flood control infrastructure. Although the extent of flooding has been reduced by those measures, the potential flood damage has increased owing to recent urbanization. This has resulted in a higher overall flood risk, especially for the Bangkok region. The city and surrounding areas have a dense network of canals which drain the area into the Chao Phraya and Tha Chin Rivers as well as directly into the Gulf of Thailand to the south. The ground surface elevation in Bangkok is very low, around + 0 to + 2 m msl. During flood seasons the water level in the Chao Phraya at Bangkok normally exceeds + 2 m msl. Thus, flooding is unavoidable without the flood protection infrastructure.

On the east bank of the river, the city area has a higher flood risk because the ground elevation is lower than that on the west bank. Moreover, most tropical storms are driven into Thailand from the South China Sea in the east. The most critical area is the depression on the east bank of the river, formed by the recent land subsidence in Bangkok. The ground elevation in the depressed zone is at present below mean sea level. In the year 2000 the lowest point was around − 1.00 m msl. Figure 21.4 shows a map of the ground surface elevation in Bangkok. Major east–west-oriented canals drain water from the eastern suburban areas and the ricefields beyond into the Chao Phraya River. Hundreds of smaller canals, feed water into the main canals, passing through the heart of the city and crossing the subsidence bowl. The canals are essentially at zero gradient or are concave in some places. Tidal surge in the river sends water back into the canals during high-tide periods. A flood protection system of flood walls along the riverbanks and water gates at canal outlets have therefore been installed to prevent river water from intruding into the canal system. But when the city is flooded and the river is at its highest, the water in the canals must be pumped over the gates and discharged into the river. Even so, during a local heavy rainstorm the volume of water that must be pumped out may exceed the capacity of the system, resulting in flood waters being retained in the city area for an extended period. On the west bank, there are numerous canals that drain the area into the Chao Phraya. In the distant suburban areas, a system of major dug and natural canals drain the area to the Gulf of Thailand to the south as well as to the Tha Chin River to the west. With this dense canal system and the higher ground elevation, the western side of the city is less susceptible to flooding than the eastern side. Urbanization in Bangkok and the nearby provincial cities since 1960 has increased the potential for flood damage. Urbanization has progressed rapidly, and the Bangkok metropolitan area of 51 km2 in the 1950s has grown to more than 550 km2 at present. Consequently, not only has flood damage potential increased owing to the rising population and development, but floods have also become more serious owing to a decrease in the run-off detention capacity. The floodplains on the west and east banks of the Chao Phraya in the vicinity of Bangkok metropolis (which extends over 2000 km2 between the Chao Phraya and Tha Chin Rivers and 2000 km2 between the Chao Phraya and Bang Pakong Rivers) serve as water conservation areas that have long played a vital role in protecting Bangkok during extreme floods (Takaya and Thiramongkol 1982). However,

Subsidence and Flooding in Bangkok 363

Fig. 21.4. Ground surface elevation map of Bangkok (Source: Duc 1999)

the recent rapid changes in land use and lifestyle of the inhabitants have made it difficult to expect such a role in some areas. Moreover, the forest area in the upper region of the basin has drastically diminished from 166 000 km2 before 1950 to 92 000 km2 in the 1990s. It has resulted in an increase in flood discharge of the rivers. Flood embankments in the form of flood protection dykes and highway and railway embankments prevent flooding to the protected zones on the floodplains, but they also confine the flow within the river. The increases in discharge and water level in the river result in more rapid flood waves propagating downstream. The embankments also obstruct natural paths of overland flood flows, seriously detaining or slowing down the flow, resulting in a higher and longer flood duration outside the protected areas.

To understand better the flood risk in Bangkok, its great floods should be reviewed. The biggest recorded flood in Bangkok was in 1942. In this flood the highest water level marked at the Memorial Bridge (the reference gauge of the city) was + 2.27 m msl, and the entire Bangkok city area suffered severe damage from prolonged inundation. Flood protection measures in the basin as well as in areas around Bangkok were almost non-existent at that time. However, the urban area was much smaller than at present. In the last two decades, 1980, 1983, 1995, and 1996 are rated as flood years, among which the 1983 and 1995 floods were the most serious. The flood in 1983 inflicted severe damage on the Bangkok city area that led to the installation of the present comprehensive flood protection infrastructure. In Bangkok and adjacent

364 Noppadol Phienwej and Prinya Nutalaya

areas, floods started in August because of the effects of several tropical storms which brought heavy rainfall, 575 mm in August and 454 mm in September. Exceptionally heavy rainfalls were recorded in the upper Chao Phraya Basin from September to November. In October the rivers from the north added their flows to the widespread flooding in the south, resulting in serious flooding of inhabited areas along the Chao Phraya, Tha Chin, and Bang Pakong Rivers. The river level in Bangkok was high because of unusually high tide levels. The peak water level in the Chao Phraya registered at the Memorial Bridge gauge was 2.04 m. It was impossible to drain flood water entering the city from the unprotected eastern floodplain to the Chao Phraya via major canals passing through the city area. The installed pump capacity at the canal outlets to the river was strikingly insufficient. Consequently, extensive inundation occurred around Bangkok, especially in the eastern suburbs. The hardest-hit area was the subsidence bowl, which suffered inundation for more than three months by water depths exceeding 1 m. The damage was estimated at $US250 million. After the event the need for a comprehensive flood protection system for Bangkok was realized, and the installation of major facilities began. A number of studies and measures were taken to solve the flood problem. The most important work following the 1983 flood was the construction of a flood control dyke along the eastern boundary of the city. The dyke is known as King Dyke because it was suggested by His Majesty King Bhumipol, who has long played an important role in engineering the current flood protection system of Bangkok. Another measure taken was to increase the pumping capacity to accelerate the transfer of water from canals by lifting it over the floodgates to the river and the sea. The flood in 1995 caused severe inundation over an area of nearly 4700 km2 in the Central Plain. The damage to infrastructure inflicted by the flood was estimated to be around $260 million. A series of tropical storms from the end of July to early September brought very heavy rainfall to the upper catchment area. Water in the reservoirs of two major dams in northern Thailand, the Sirikit and Bhumipol Dams, was at or near their storage capacity. In September spillage occurred at the Sirikit Reservoir for the second time since its completion in 1972, resulting in flooding downstream and high discharge into the Chao Phraya. This, combined with considerable discharge from other rivers draining the upper and central areas of the Chao Phraya Basin, brought large flood discharges to the Lower Central Plain. In September the flow of the Chao Phraya River at Nakhon Sawan reached 4800 m3 s−1, whereas the

discharge capacity of the river was only 3000 m3 s−1 downstream at Bangkok. Consequently, the area was severely flooded. Some sections of the dyke along the Chao Phraya channel in the lower reaches of the river were breached, resulting in widespread flooding of the lower plain. Overflows from the Chao Phraya and Tha Chin Rivers caused severe floods in the areas surrounding Bangkok. The situation was worsened by the high flow from the Pa Sak River, which brought additional water into the Chao Phraya at Ayutthaya. However, the metropolitan area of Bangkok was mostly safeguarded from the flood waters by the just completed dykes and the regulating gates installed in canals entering the city along its eastern, northern, and western limits. The only city area affected was confined to a narrow strip along the riverbank outside the flood dyke, where flood walls had not yet been installed. Figure 21.5 shows the location of the dykes that have been completed along the periphery of the Bangkok metropolitan area, some sections of which are incorporated into embankments of main roads and railways. This infrastructure prevented the flood water from draining freely through major drainage canals passing through the city. Consequently, the flood water was retained outside the protected zone, and severe flooding and heavy damage occurred in the suburban areas and surrounding provinces. The inundation was widespread and lasted for one to two months in many areas. Flood height in the northern and western areas outside the Bangkok dyke was as high as 2 m, threatening housing and industrial and government complexes, including the campus of the Asian Institute of Technology, which relied on their own polder dykes to protect them against flooding. In several residential complexes, overtopping or breaching of the polder dyke occurred, resulting in severe damage to property. Vongvisessomjai, MacDonald, and Cowley (1997) and JICA (1999) give excellent accounts of the flooding and flood management plan implemented for the Chao Phraya Basin and Bangkok. The principal means of managing floods is through the physical control of water movements by large-scale civil engineering works. This involves storage and delay of flood flows by reservoirs and detention basins, diversion of flood flows away from affected areas, enhanced drainage, and excluding flood water by means of dykes and other flood defence structures. Storage of flood flow in reservoirs has been practised on a large scale in the Chao Phraya River Basin since the construction of the Bhumipol Dam on the Ping River in 1964. Currently, the Bhumipol and Sirikit Reservoirs, with a combined storage volume of some 16 320 million m3, have had

Subsidence and Flooding in Bangkok 365

LEGEND RID Pump Station Pump Station (> 20 M/S) Pump Station Regulator Dyke Small Dyke Monkey Check (Bangkok City) Monkey Check (Irrigation Department)

Pathum Thani

Nonthaburi

Samut Prakan

Fig. 21.5. Flood protection infrastructures of Bangkok metropolitan area

considerable success in reducing flood peak flows in the lower basin. The Pa Sak Dam, completed in 1988, which is situated within the Lower Central Plain, despite its small storage capacity of 780 million m3 has helped to alleviate flood intensity in areas downstream from its confluence with the Chao Phraya at Ayutthaya which include Bangkok. Prior to the flood of 1995 the flood protection and drainage facilities for the urban areas of Bangkok metropolis and provincial cities in the Chao Phraya Basin were not fully installed. After that flood further improvements were carried out. The governing body of Bangkok city, the Bangkok Metropolitan Administration, has continuously attempted to protect the metropolis from floods and to improve the drainage condition. Such efforts have concentrated on the construction of (1) polder dykes along the periphery of the urban areas on both right

and left banks of the Chao Phraya, (2) flood walls along both banks, (3) drainage pumping stations with a flood forecasting centre, and (4) improvement of drainage channels. At present, the authorities in Bangkok and its adjoining provincial suburbs are constructing complete flood walls along both banks of the Chao Phraya River to protect the urban area of about 890 km2 (650 km2 on the left bank and 240 on the right). By 2001 out of a total of 83 km of flood wall planned for the city, 35 km have been completed. The flood wall is designed with a crest elevation of + 2.75 to + 3 m msl to protect against the probable 100-year flood level (+ 1.9 to + 2.5 m msl). A freeboard of 0.5 m has been provided to account for future land subsidence. The total capacity of the drainage pumps installed for the city is 692 m3 s−1 (452 for the left bank and 240 for the right), most of which is discharged directly into the Chao Phraya.

366 Noppadol Phienwej and Prinya Nutalaya

The flood protection dykes that surround the metropolitan area help to alleviate the flood risk to the city, but they intensify flooding in the unprotected suburban areas outside the dykes. After the construction of dykes in the eastern suburbs in 1984, flood flow from the ricefields and the suburban zones to the east of the city was prevented from entering the canal drainage system through the city area. Subsequently, the water level outside the flood control dyke used to rise, and after a period of heavy rainfall could be as much as 1.2 m higher than on the protected city side. Gradually, steps have been taken to alleviate flood problems outside the dyke by accelerating drainage from these areas. The drainage capacity of the areas outside the dyke, especially in the eastern suburbs, has increased along existing canals and drains. The capacity of canals running through the populated suburban areas was increased to prevent overflow. Major canals running in a north–south direction were deepened so that the excess water can be conveyed quickly to the sea. Where they cross railways or highways, the waterway channels have been enlarged to prevent necking of flood flows. However, owing to the very flat and low-lying topography of the area, the canals have almost no gradient. Thus, pumping was needed to accelerate the drainage of flood water through these canals before it could be discharged to the sea. Major regulating gates and pumping facilities have been installed at the outlets to the sea of all major canals draining the areas on both eastern and western suburbs (Figure 21.5). Even so, the concave topography of the area in the eastern suburb outside the King Dyke made the pumping facilities installed in the major canals ineffective during high flood periods. The solution to this problem is the construction of a storm drain tunnel, which is now being planned, to bypass the concave area. The Monkey Cheek project is another flood protection scheme for the suburban areas surrounding Bangkok suggested by His Majesty King Bhumipol, focusing on flood control and drainage improvement by means of regulating existing and newly created retention basins located close to the seashore, synchronously with the fluctuation of tidal levels. The scheme is to help to accelerate the discharge of flood water during high-tide months. The project, which is now under implementation, is primarily intended to minimize flood damage to the suburban areas on the west bank of the Chao Phraya River.

Land Subsidence in Bangkok The evidence of land subsidence is visible everywhere in Bangkok as ground and footpaths adjoining buildings

subside and pavements buckle around and over bridge foundations. Land subsidence in Bangkok was first reported by Cox (1968). Because Bangkok is underlain by a thick, soft marine clay, the subsidence may have various causes: consolidation of the soft clay due to loading from landfill or buildings, lowering of the shallow perched water-table in the upper zone of the soft clay layer, erosion of sandfill below pavements or around drainpipes, and groundwater pumping. However, the comprehensive study made by the Asian Institute of Technology (AIT 1981) clearly confirmed that excessive pumping of groundwater from a large number of deep wells sunk into the aquifers underneath the city was primarily involved.

Bangkok Aquifer System The Bangkok aquifer system consists of eight main aquifers that are constituted of sandy and gravelly sediment intercalated by aquitards of clay layers, the stratigraphy of which is shown in Figure 21.6. As the lower Chao Phraya Basin is capped by a thick layer of soft and stiff clay aquitard, groundwater recharge to the aquifers could only be expected from the peripheral areas of the basin. The uppermost aquifer is called the Bangkok Aquifer (50 m depth zone) and is commonly found between 16 and 55 m depth (Figure 21.6). However, it is no longer potable owing to both high salinity and being partly contaminated by exposure from borrow pit excavation. The productive aquifers are the second aquifer (Phra Pradaeng Aquifer, 100 m zone), the third aquifer (Nakhon Luang Aquifer, 150 m zone), and the fourth aquifer (Nonthaburi, 200 m zone). Most wells in Bangkok extract water from these aquifers. Extraction from deeper aquifers has recently become necessary in some zones of the provincial suburbs owing to excessive drawdown and the high salinity of groundwater in the three productive aquifers following earlier heavy exploitation. Extraction from the aquifers as deep as 600 m has been made in some areas in southern Bangkok. The groundwater extraction caused large drops in the piezometric head in the pumped aquifers as well as in the adjacent aquitards. Consequently, the effective stresses in both of them increased, causing instantaneous compression of the sand aquifers and delayed consolidation compression of the clay aquitards, which in turn resulted in the land subsidence. The seriousness of land subsidence of Bangkok was first noticed in the early 1970s. However, owing to the inability of the government to reduce groundwater pumping during the last twenty years, the subsidence continues, and the affected zone has expanded into the newly developed

Subsidence and Flooding in Bangkok 367

Fig. 21.6. Hydrological profile of the Bangkok aquifer system in a north–south direction (Source: after Ramnarong 1983)

industrial and residential areas in the provincial suburbs surrounding Bangkok.

Groundwater Extraction in the Bangkok Aquifer System Deep groundwater extraction in Bangkok started in the early 1900s, but it was not until the early 1950s that large-scale extraction from the aquifer system of Bangkok began. Wells of large capacity were drilled by the Public Works Department for the supply of tapwater to the city (Ramnarong 1983). The daily pumping rate was recorded at 0.65 million m3 in 1975, which rapidly jumped to 1.2 million in 1980. The chronological record of groundwater extraction in Bangkok and the surrounding provincial suburbs is shown in Figure 21.7. The record shows that groundwater extraction has continued to increase over the entire period from 1950 to 2001, the only pause being between 1983 and 1993. The rate of groundwater pumping increased rapidly after

1993, at the start of the latest economic boom of the country. During that period Bangkok rapidly expanded in all directions, with a large number of residential and industrial development projects starting in the outskirts. Almost all of them had to rely on groundwater supply. The overall pumping rate increased to 2 million m3 in 1997 and was estimated to be 2.4 million in 1999 (Chula-Unisearch 2000). This has resulted in land subsidence in the Bangkok metropolitan area continuing beyond ad 2000.

Piezometric Drawdown and Land Subsidence The chronological development of Bangkok land subsidence and piezometric drawdown in the Bangkok aquifer system were well reported and summarized by Nutalaya et al. (1989), Ramnarong et al. (1997), Duc (1999), and Phienwej (1999). The extensive pumping in the 1970s–1980s led to significant drawdown in the piezometric head of up to 40–50 m in the exploited aquifers. The drawdown in the Nakhon Luang Aquifer

368 Noppadol Phienwej and Prinya Nutalaya

Groundwater Usage (1955–1997) 2.2

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Fig. 21.7. Chronological record of groundwater extraction rate in the Bangkok Plain (Source: after Ramnarong et al. 1997)

recorded in 1981 is shown in Figure 21.8. This drawdown occurred as a depression cone covering the entire city area, especially the east bank of the river, indicating the continuity of the aquifer below the basin. The centre of the cone was located in east Bangkok, which was then the nucleus of residential and industrial growth. The piezometric decline led to the compression of the Bangkok aquifer system, which manifested in severe widespread land subsidence. According to the record, the largest magnitude of subsidence over a fifty-four-year period (1933– 87) was 1.6 m in the worst-affected area, which was then the eastern suburbs of Bangkok (Nutalaya et al. 1989). It subsequently increased to 1.9 m by 1997. This area coincided with the centre of the cone of piezometric drawdown in the

pumped aquifer. It also coincided with the area that suffered the largest rate of subsidence during the early 1980s. The largest rate of subsidence was recorded in 1981 at 120 mm yr−1 (Figure 21.9). In 1978 the Groundwater Act was first enforced to control use of groundwater in Bangkok. Following the conclusion of the comprehensive study of groundwater exploitation and land subsidence (AIT 1981), a resolution entitled The Mitigation of Groundwater Crisis and Land Subsidence in Bangkok Metropolis was issued by the government. Several initiatives were implemented with an attempt to regulate and reduce groundwater extraction. One of the measures was to cease gradually the dependence of the Metropolitan Water Works Authority of Bangkok on groundwater for raw water

Subsidence and Flooding in Bangkok 369

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supply. The authority consumed about 40 per cent of the daily pumpage during that period. It required the authority to expand the water treatment facility to surface water sources (mainly the Chao Phraya River) and to extend distributing piping network to cover the entire inner city area as well as the expanding outskirts. Other measures included the establishment of the Groundwater Board to control closely the issuance of groundwater permits. The measures led to a reduction in groundwater pumping in the central area of Bangkok, which resulted in a brief recovery of piezometric pressure

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in some areas observed in the period 1983–7. But after 1988 the recovery in groundwater level stopped as the volume of groundwater pumping increased while the city underwent a new wave of economic growth and infrastructure development. A large number of wells were sunk after 1990, mostly in the outer areas of the Bangkok metropolis. The number of groundwater wells was estimated at more than 14 000 in 1992. Around 60 per cent of the groundwater extraction was for the industrial sector. The continued increase in the rate of groundwater extraction during the 1990s resulted in

370 Noppadol Phienwej and Prinya Nutalaya

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further drawdown of the piezometric pressure in the Bangkok aquifer system. The piezometric level in the Nakhon Luang Aquifer in 1998 is shown in Figure 21.10 (Duc 1999). The maximum drawdown at the centre of the cone further increased by an additional 20 m after 1981. The deepest groundwater level was at 71 m below the ground surface in the Nakhon Luang Aquifer, which occurred in the eastern suburb. In addition, the size of the cone expanded in a north– south direction following the recent pattern in development of the industrial and residential zones of the city. Moreover, a separate cone developed in the Samut Sakhon industrial area, near the coastline on the west bank of the Chao

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Fig. 21.9. Land subsidence of Bangkok, 1981 (Source: After AIT 1981)

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Phraya River. These subsidence cones coincided with the distribution of the groundwater wells sunk in the Bangkok Plain. Monitoring of land subsidence in Bangkok first started in 1978 with the comprehensive study by AIT (1981) mentioned earlier. It has continued since then, and at present there are more than 200 monitoring points installed across the Bangkok Plain. The distribution map of Bangkok land subsidence during the period 1978–97 is shown in Figure 21.11, and the latest updated map of the subsidence rate in Figure 21.12 (Duc 1999). In comparison with the situation in 1981, the area affected by land subsidence has much enlarged

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by spreading into the outer areas of the city and provincial suburbs following the rapid urban and industrial growth of the city. In 1997 the critical areas of subsidence were located in the east, southeast, and southwest industrial and residential zones. The subsidence was occurring at the rate of 30– 45 mm yr −1 in those outlying areas. For central Bangkok, where groundwater pumping did not happen after the late 1980s, the land subsidence still continued, but at a reduced rate of 5–10 mm yr −1. This slow continuation was due to the lingering effect of the past extraction

within the area on the delayed compression behaviour of the clay aquitards, particularly the soft Bangkok Clay at the surface as well as the rapidly increased extraction in the outlying areas. Figure 21.13 shows the rate of land subsidence in the inner Bangkok area as well as in the suburban industrial areas. Subsidence in Bangkok has occurred at a relatively uniform rate over the 1990s. A summary of the subsidence in various years along a north–south profile through the centre of Bangkok is shown in Figure 21.14. The higher subsidence zone shifted towards the coastal area with time.

372 Noppadol Phienwej and Prinya Nutalaya

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Environmental Impact of Land Subsidence Land subsidence and the drawdown of the Bangkok aquifer system has impacted adversely on the environment and ecological system in Bangkok. In comparison to other subsidence-affected cities around the world, the problems are particularly critical here because of the city’s very low-lying topography, its close proximity to the sea, and the presence of a thick, soft clay layer at the subsurface. Numerous environmental and engineering problems result from excessive pumping and drawdown of groundwater from the aquifers. In Bangkok, the most

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serious ones are the lowering of the ground elevation and the intensification of the flood threat to the city. The land subsidence that has affected the entire area of the Bangkok Plain has increased vulnerability to flooding in Bangkok. The flood protection infrastructure in Bangkok has become less efficient with time as the crests of flood protection dykes and walls continue to subside along with the ground. The cost of pumping storm and flood water over the floodgates to drain the water from the city to the river and sea will increase progressively over time. The subsidence has also resulted in a loss of seashore south of Bangkok at the head of the Gulf of Thailand.

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Over the last fifty years a strip of seashore about 500–1000 m wide has subsided and been inundated by the sea for about 80 km along the coastline from the Tha Chin River to the Bang Pakong. This coastal subsidence and loss of land has resulted in destruction of the ecological system. Mangroves have been drastically reduced, leading to further coastal erosion by wave attack. Because the ground surface of Bangkok is already almost at mean sea level, further lowering of the ground surface can be regarded as a critical anthropogenic hazard to the city environment in the future. Subsidence and groundwater pumping data for the entire Bangkok

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Fig. 21.12. Land subsidence rate in Bangkok, 1997

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metropolitan area during 1994–8 indicate that for each cubic metre of water being extracted from the aquifers, on average about 0.12 m3 of ground loss occurred at the surface (Phienwej 1999). Common practice in land development in Bangkok involves raising the area by landfill to protect it against local flooding, which indicates that continued land subsidence will worsen the need for landfill, leading to increased cost and exploitation of natural resources. At present the cost of landfill per cubic metre of subsidence is about 100 times higher than the revenue the government collects from the user per cubic metre of groundwater extracted. This is a

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1996 1997

800 900 1000

Fig. 21.14. Chronological change in accumulative land subsidence along a north–south section of Bangkok

terrible situation from the point of view of sustainable natural resources utilization and management. The intrusion of salt water into the aquifers along the coastal front to the southeast and southwest of Bangkok is another significant impact of excessive groundwater

pumping from the Bangkok aquifer system. According to the groundwater monitoring data for 1992– 8, the salinity front continued its encroachment into the productive aquifers in the industrial zones southeast of Bangkok at an approximate rate of 0.5 km per year

Subsidence and Flooding in Bangkok 375

Pore water pressure (kPa) 0

50

100

150

200

250

300

350

0

Weathered clay

5

Depth (m below ground surface)

Soft clay

10

15

20

Stiff clay

25 Sand

30

35 Hydrostatic 31 yrs 71 yrs

1 yr 41 yrs 81 yrs

11 yrs 51 yrs 91 yrs

(Chula-Unisearch 2000). A similar situation has also developed in the coastal area to the southwest, but to a lesser degree. The saltwater intrusion resulted in a great loss to the aquifer system, which is hard to remedy. At present it is necessary to use deeper aquifers for groundwater supply in the affected areas (Ramnarong et al. 1997). Apart from the direct environmental impacts mentioned above, land subsidence from deep-well pumping also causes numerous problems in foundation engineering practice. This is quite critical because of the existence of the highly compressible soft marine clay of 15–30 m in thickness below the ground surface in the Bangkok city area. Although the wells extract groundwater from deep aquifers, the piezometric drawdown is felt up to the bottom part of the soft clay layer (Figure 21.15). As a consequence, compression of the soft clay occurs, which is responsible for a considerable portion of the overall subsidence at the surface. According to numerical analyses

21 yrs 61 yrs measured 1980

Fig. 21.15. Piezometric drawdown in the shallow soft and stiff clays of Bangkok

and latest monitoring data (Premchitt 1978; Phienwej 1999), approximately 30–50 per cent of the total land subsidence that occurs at the surface in the inner city area is caused by the compression of the soft and stiff clay layers above the first sand layer in the shallow zone of the Bangkok aquifer system. This pattern of land subsidence leads to the problem of differential settlement between adjoining structures resting on foundations of different depths. Normally, buildings and other structures in Bangkok are founded on piles having tips extending down to the stiff clay or the first sand layer below the soft clay to attain sufficient bearing capacity and avoid problems of settlement from structural load. However, such structures experience less subsidence settlement than adjoining structures founded on shallower foundations such as pavements, footpaths, and lightweight buildings. Hence differential settlement occurs with surface cracking, which may jeopardize the intended function of the structures. As the land subsidence has

376 Noppadol Phienwej and Prinya Nutalaya

(a)

Fig. 21.16. Differential settlements between structures caused by land subsidence (b)

continued to occur in the Bangkok area since the 1970s, and so far has shown no sign of terminating, differential settlement and building cracks have been common, and are visible everywhere in the city (Figure 21.16). The problem cannot be effectively solved as long as the

land subsidence is not under control. In fact, it is one of the major constraints in the design of the recent underground infrastructure development projects in Bangkok, such as the mass transit line, the sewerage system, and underground power lines. Figure 21.17

Subsidence and Flooding in Bangkok 377

Fig. 21.17. Schema of differential settlements between underground structures caused by land subsidence (Source: after Phienwej 1999)

illustrates various levels of differential settlement that may occur between different types of structure joined together. Prediction of the magnitude of the differential settlement between the structures is rather uncertain owing to difficulties in attaining reliable records on the historical development of piezometric drawdown in the underlying soil layers and land use at the site under consideration. One such attempt, utilizing coupling hydraulic and consolidation finite difference analysis, was discussed by Thepparak (2001), and appears to be the most promising one to date. At the end of the twentieth century land subsidence in Bangkok continued, with no sign of termination in the near future because the city was still in need of groundwater in the surrounding suburbs. The rate of groundwater pumping over the entire basin was about twice the permissible yield of the aquifer system. The permissible yield, below which groundwater can be extracted without causing environmental impact in terms of land subsidence and sea-water intrusion, was estimated at 1.2 millions m3 per day (Chula-Unisearch 2000). The land subsidence caused various environmental impacts on

the city, the total economic loss being $US400 million per year (at the year 2000). The government resorted to supplementary measures to mitigate this crisis by planning a scheme for artificial recharge to the aquifers through wells, and for gradually raising the groundwater tariff to a level more comparable to the cost of municipal water from surface sources. In 2000 the tariff for groundwater use per cubic metre was about one-fifth to a quarter of the tapwater supply. The first rise (about 60 per cent higher than the original rate) had been approved and implemented by the government. The plan for the artificial recharge has been studied by the Ministry of Industry and Chula-Unisearch (2000), and presented to the government. According to the plan, the first stage of the recharge would involve preparation of an infrastructure for water treatment and recharging of 628 000 m3 per day at four sites of high priority in Bangkok. However, the project was subsequently put on hold in 2001 owing to the very high cost of recharge and the continued economic recession in the country. The ultimate solution is to install and expand treatment facilities for surface water sources and distributing

378 Noppadol Phienwej and Prinya Nutalaya

networks to cover the entire Bangkok metropolis, and the adoption of a pricing scheme for a groundwater tariff to persuade users to revert to the piped-water supply. However, it is unlikely that these measures will be effectively implemented in the near future. As of 2003 the land subsidence remains an unsolved anthropogenic geo-environmental hazard facing Bangkok.

References AIT (Asian Institute of Technology) (1981), Investigation of Land Subsidence Caused by Deep Well Pumping in the Bangkok Area, Research Report no. 91, Division of Geotechnical and Transportation Engineering (Bangkok: Asian Institute of Technology). Chula-Unisearch, Chulalongkorn University (2000), A Study for Concept Design of Artificial Recharge of Groundwater for Conservation of Environment, Final Technical Report submitted to the Department of Mineral Resources (Bangkok). Cox, J. B. (1968), A Review of the Engineering Characteristics of the Recent Marine Clays in Southeast Asia, Research Report 6 (Bangkok: Asian Institute of Technology). Duc, N. A. (1999), ‘Updating and Analysis of Bangkok Land Subsidence Caused by Deep Well Pumping with Emphasis on Shallow Soil Settlement’, M.Sc. thesis, Asian Institute of Technology, Bangkok. ESCAP (Economic and Social Commission for Asia and the Pacific) Secretariat (1988), Geological Information for Planning in Bangkok, Thailand. Proceedings: Seminar on Geology and Urban Development, vol. i: Atlas of Urban Geology (Bangkok: United Nations, ESCAP), 24–60. JICA (Japan International Cooperation Agency) (1995), The Study on Management of Groundwater and Land Subsidence in the Bangkok Metropolitan Area and its Vicinity, Report submitted to the Department of Mineral Resources and Public Works Department, Kingdom of Thailand (Bangkok: JICA).

—— (1999), The Study on Integrated Plan for Flood Mitigation in Chao Phraya River Basin, Final Report submitted to the Royal Irrigation Department, Kingdom of Thailand (Bangkok: JICA). Nutalaya, P., and Rau, J. L. (1987), ‘Structural Framework of the Chao Phraya Basin, Thailand’, in Proceedings: Symposium on Cenozoic Basins Thailand: Geology and Resources (Bangkok), 106–29. —— Yong, R. N., Chumnankit, T., and Buapeng, S. (1989), ‘Land Subsidence in Bangkok during 1978–1988’, in Nutalaya, P., Phienwej, N., and Sophonsakulrat (eds.), Proceedings of the Workshop on Bangkok Land Subsidence: What is Next?, Bangkok, June (Bangkok), 1– 48. Phienwej, N. (1999), ‘Bangkok Land Subsidence and its Problems in Foundation Engineering’, in Phienwej (ed.), Proceedings of the Seminar of the Engineering Institute of Thailand, Bangkok, July (Bangkok). Premchitt, J. (1978), Analysis and Simulation of Land Subsidence with Special Reference to Bangkok, AIT doctoral diss. no. D37 (Bangkok: Asian Institute of Technology). Ramnarong, V. (1983), ‘Groundwater Depletion and Land Subsidence in Bangkok’, in Proceedings of the Conference on Geology and Mineral Resources of Thailand (Bangkok: Department of Mineral Resources). —— Buapeng, S., Chootnatut, S., and Loupensri, A. (1997), Groundwater and Land Subsidence Crisis in Bangkok Metropolitan and Vicinity, Technical Report no. 3/1998 (Bangkok: Department of Mineral Resources). Rau, J. L., and Nutalaya, P. (1983), ‘Geology of Bangkok Clay’, Bulletin of the Geological Society of Malaysia 16: 99–116. Takaya, Y., and Thiramongkol, N. (1982), Chao Phraya Delta of Thailand, Asian Rice-Land Inventory Descriptive Atlas no. 1, Southeast Asian Studies (Kyoto: Kyoto University). Thepparak, S. (2001), Analysis of Settlement and Compression of Shallow Soil Strata due to Drawdown of Groundwater in an Underlying Aquifer from Well Pumping in Bangkok Area, M. Eng. thesis no. GE-00-01 (Bangkok: Asian Institute of Technology). Vongvisessomjai, S., MacDonald, A., and Cowley, J. E. (1997), ‘Chao Phraya Flood Management Review’, Water Resources Journal of Economic and Social Commission for Asia and Pacific, 82–90.

22

Urban Pollution in Southeast Asia Sham Sani

Introduction This chapter looks specifically at the pressures imposed by urbanization on the physical environment in Southeast Asia, leading to its degradation and a decline in the quality of life. This is followed by a discussion on the management responses highlighting some common concerns that need to be addressed in order to plan and manage urban systems better. Like many of their counterparts in the developing world, levels of urbanization in Southeast Asia are low by world standards. However, the growth rates of the urban population are high: 3–5 per cent per annum (Jones 1993). The relatively low levels of urbanization, nevertheless, are by no means a reflection of the failure of cities in the region to reach substantial sizes. Indeed, three of the very large cities of Southeast Asia, Jakarta, Bangkok, and Manila, carry 10 million people. The current trends and direction of urban growth are expected to continue, although the rates are likely to be somewhat retarded within these few years owing to the economic downturn recently experienced by the Southeast Asian countries. Such continued growth and rapid urbanization can only result in greater burdens to the already very strained urban systems, in terms of both the provision of an urban infrastructure and social services and the biophysical environment.

Impact on Infrastructure and Services One notable consequence of urban growth and population concentration in Southeast Asian cities is the pressure they generate on the provision of an infrastructure and essential services that eventually affects the

environment, health, and quality of life. Here, the problems of providing an adequate infrastructure are immense, especially given the budgetary constraints. Policy response is often highly inadequate compared to the scale of the problems. Singapore’s special position as a city-state has enabled it to overcome problems that other Southeast Asian cities have not been able to cope with, particularly as it is not affected by the perennial problem of rural–urban migration. One major problem which is shared by many Southeast Asian cities is overcrowding and lack of proper shelter. Virtually all major cities have squatters. Squatters are basically illegal occupants of urban land that belongs to the government or private individuals. Usually they come from economically depressed areas of the city, although ‘squatter landlords’ and ‘affluent squatter groups’ as described by Khairuddin Yusof and Low Wah Yun (1995) are not uncommon. In Metro Manila, 3 million people, more than a third of the total population of the metropolis, live in slums and squatter settlements. Many of them encroach on the waterways or live within existing open dump sites where conditions are extremely hazardous to health (Rosario 1995). Similar situations also exist in Bangkok and Jakarta. In Kuala Lumpur, the number of squatter settlements has risen to 216 over the last ten years. A recent unofficial City Hall count suggested that approximately 350 000 people, about a third of the city population, lived in such settlements, although Mohammed Razali Argus (1997) gave a lower figure of around 200 000. This is despite the relocation of 45 000 squatter families by the City Hall between 1978 and 1988. Many endemic diseases such as diarrhoea, typhoid, intestinal parasites, and food poisoning are common among the squatters. Besides, the cramped conditions in which they live lead

380 Sham Sani

to easy transmission of communicable diseases such as influenza and tuberculosis. Apart from shelter, another basic need, water, is becoming a problem. For many cities access to a supply of drinking water not only is limited but also varies from one section of the community to another. In certain instances, possibly 80 per cent of the supply goes to only 20 per cent of the population (UNDP 1992). To compound the problems, a substantial amount of the available water supply is lost through theft or leakage. Nearly half of Jakarta’s water supply is lost in this fashion. In Bangkok and the Klang Valley–Kuala Lumpur, the figures for water loss through leakage were 40 and 30–35 per cent respectively. With increasing demand for drinking and potable water in the cities of Southeast Asia, the water supply problem now becomes critical during the dry season. Kuala Lumpur and the Klang Valley area declared water rationing for more than five months from March 1998. The reservoirs behind three major dams (the Klang Gate Dam, the Semenyih Dam, and the Langat Dam) that supply water to the Klang Valley area were unable to do so adequately to the public owing to low water levels during the dry season. Although El Niño was blamed for the exceptionally dry conditions in 1998, the way water was being managed in the Klang Valley was not entirely blameless. The rationing exercise could have been avoided had the water management plan been followed carefully. Indeed, in most Asian cities intermittent supplies of piped water are a common feature (ESCAP 1993). In several Southeast Asian cities, where groundwater is used to complement water supply, other problems occur. In many cities, groundwater is becoming exhausted and partly contaminated. Besides, taking water indiscriminately from aquifers leads to other consequences. In metropolitan Bangkok, where a third of the population has no access to piped water, the unlicensed pumping of groundwater is possibly half as much again as licensed withdrawal. This scale of consumption has lowered the water-table, inducing an inflow of saline water to the aquifers and irregular land subsidence, reported to be up to 1.6 m between 1960 and 1980. In Jakarta, two-thirds of the city’s population receive their water supply exclusively from groundwater (ESCAP 1993). Since almost all wells are owned and operated by firms or households, groundwater utilization in cities is commonly unplanned and uncontrolled. The number of wells, amount of extraction, and depth of extraction in Jakarta remain largely unknown. As a direct consequence of groundwater overdraft and sea-water intrusion, many wells have been abandoned. The quality

Table 22.1 Degree of traffic congestion in selected Southeast Asian cities City

Estimated speed of traffic flow during peak periods (km per hour)

Bangkok Jakarta Kuala Lumpur Manila Singapore

20.9 26.2 20.9 11.6 60.0

Sources: World Bank (1987); Camp (1990).

of the city’s aquifers has been adversely affected up to 8 km from the coast in northern Jakarta. In parts of central Jakarta, the land surface has subsided up to 0.8 m. Parts of the core of Indonesia’s capital city may be subsiding at a rate of 1–3 cm per year, and up to 6 cm per year in the northern part (Sharma 1986; Hadiwinoto and Clarke 1990). Besides water supply and lack of proper shelter, another major problem shared by many cities is traffic congestion. The rapid growth in the number of automobiles operating in city areas (well over 10 per cent per year in higher-income areas) will compound the congestion problem if no meaningful alternative can be found soon (ESCAP 1993). With the exception of Singapore, the cities of Southeast Asia are facing serious congestion problems (Table 22.1). First, the wasted time causes productivity losses amounting to 1–10 per cent of the local economic turnover (ESCAP 1993). Secondly, traffic congestion wastes fuel. It is estimated that for every one-third drop in average vehicle speed, fuel efficiency drops by 30 per cent. In Thailand, traffic jams are reported to cause $US1.5 billion worth of fuel to be wasted annually. Thirdly, traffic congestion generates excessive pollution of many kinds, including impact of lead emissions on humans. In Bangkok, average lead levels in the blood ranges between 40 and 45 µg per decilitre, four times the United States standard. The possible solutions suggested to alleviate the congestion problem include modifying road networks to create a smoother flow of traffic. In Tokyo, this has been done using multi-level road systems. Computerized traffic signals also improve the traffic flow, and in Singapore this is reported to have saved millions of litres of fuel. Another option is to adopt a diversity of transport modes, e.g. buses and railways. A number of cities in the ESCAP region are considering various new mass transit systems to complement the existing ones. In Kuala Lumpur, electric commuter trains and light rail transit (LRT) systems have been introduced, and have received

Urban Pollution in Southeast Asia 381

One early symptom of rapid urban growth is the transformation and, in many cases, the deterioration of the natural environment. In many areas, forests and other types of natural vegetation have been replaced by infrastructure, residential buildings, offices and places of business, industries, and commercial buildings. In more recent years luxury apartments appear to have become popular, and their number has increased substantially in virtually all cities of Southeast Asia. Too often there is inadequate provision of open space and green areas.

the heat generated by many forms of combustion process, traffic, and industry, and the amount of pollution released into the city’s atmosphere all combine to create a climate quite distinct from that of the surrounding rural areas. Almost all of the major metropolises of the world have been shown to have their own characteristic climate affecting dispersion of air pollutants, human comfort, energy consumption, architecture, health, and other social costs. Although there are few empirical studies for Southeast Asian cities, all the major urban areas may be expected to show such effects. Indeed, studies on the impact of urbanization on local climate in Kuala Lumpur and the Klang Valley region (Sham 1980) show that, in agreement with earlier works in the mid-latitude regions, all factors of climate were affected (Chandler 1965; Oke 1979). Probably the single most important aspect which has been extensively examined in the study of urban climate in Malaysia is the changing form and intensity of the heat island. Observations carried out over the last several years in the Klang Valley indicate that commercial centres are usually several degrees warmer than the surrounding countryside (Sham 1973a,b, 1979, 1980). On average, the difference in mean annual temperature between the city and the old Subang Airport was approximately 1–2°C, but on calm and relatively clear nights the urban– rural temperature differential could rise to 6–7°C (Sham 1984, 1989, 1991). Apart from the city’s effect on horizontal temperature and hence on human comfort, health, and energy requirements for cooling, the urban thermal influence also extends upwards to 200–300 m and even 400 m and above (Oke 1976). This development has been observed to affect urban ventilation and air pollution concentration, dispersion, and transport, and is particularly evident if the cities are located in a valley or close to the sea. In such a situation, the combined emissions from these urban centres may form a giant plume affecting the surrounding area. If the spacing between cities is insufficient, their alignment with the wind may cause a cumulative pollution build-up. In addition, the plume from one city can become fumigated into the atmosphere of a second one downwind in the manner as described by Oke (1976, 1986).

Climate

Air Pollution and Haze

Local climate is one area in which the impact of urban growth on the natural environment has been clearly demonstrated. In the city, the effects of complex urban surfaces, the shape and orientation of buildings, the overconcentration of structures, the characteristic thermal and hydrological properties of the urban morphology,

Available information on the quality of air in the cities of Southeast Asia suggests that the ambient concentration of some selected pollutants is high. This is particularly true in the case of suspended particulate matter. Singapore is perhaps the only regional city where the air quality has remained within the long-term World

a positive response from the public. The second phase of the LRT programme was temporarily shelved owing to the economic slowdown but has now been resumed. No study has been made, however, to ascertain the reduction in the number of car journeys due to LRT and the electric commuter trains. A third option to control congestion is to increase the cost of owning and/or driving a motor vehicle. Fuel taxes, for example, are being applied in varying degrees in many countries with varying results. Singapore has the most comprehensive programme for making private motor transport expensive. To add to the already high customs duties, vehicle registration fees, and road taxes, all private vehicles must pay a fee for driving in the central business district during peak hours. A quota system has also been emplaced to restrict the number of new vehicles that can be purchased, requiring payment for a Certificate of Entitlement. Finally, it is imperative that long-term urban planning and design be improved and made more compact and integrated. Unlike its Southeast Asian counterparts, Singapore has excellent public transport services. In addition to public bus services, the mass rapid transit (MRT) network was commissioned in 1987 and received overwhelming public support. MRT services have reportedly won over car commuters to public transport, with one in every ten passengers being a former car commuter (Singapore, Inter-Ministry Committee for UNCED Preparatory Committee 1992: 22).

Impacts on the Biophysical Environment

382 Sham Sani

Health Organization guidelines during normal periods, especially from 1984 onwards. For certain parts of the year (usually during the dry months of February–April and June–September) the problem of air pollution may be compounded by the intrusion of transboundary atmospheric pollution, locally known as haze. In Malaysia, particularly in Kuala Lumpur and the Klang Valley area, haze and haze episodes are not exactly new phenomena. Haze occurrences were already recognized as early as the 1960s. Haze, however, became a problem only relatively recently, when it began to affect visibility significantly, disrupt air travel schedules and shipping, and aggravate health. The September 1982 haze and those of 1990 and 1996 attracted a great deal of public attention, disrupted air and sea traffic, and posed health hazards. At its peak, the August 1990 haze recorded suspended particulate levels exceeding 780 µgm−3 (Sham et al. 1991). But perhaps the longest and the worst episode, and the one that received the most attention, occurred during September–November 1997. It was caused not only by external air pollution sources, which exacerbated the contribution from internal sources, but also by its coinciding with El Niño, which prolonged the dry season until the middle of the following year, thus providing an environment conducive to haze formation. In the September–November 1997 haze episode, Indonesia, Singapore, Brunei Darussalam, and Malaysia were severely affected. While internal sources such as open burning and pollutant discharge from motor vehicles and industries were contributory factors, the main cause of the haze was intentional land-clearing variously attributed to commercial oil palm and timber plantations, government-sponsored transmigration projects, and local smallholders. Azman (1997) argued that in the 1997 haze about 20 per cent of the haze composition in Kuala Lumpur was attributable to local pollution, largely from vehicles and industries; the rest came from outside the national boundary. An estimated 70 million people in Singapore, Malaysia, Brunei Darussalam, and parts of Indonesia (especially Kalimantan and Sumatra) were affected by the haze. Conservative estimates by the World Wildlife Fund put the total amount of burnt forests at 2.5 million ha. Samples of air pollution levels and air pollution indices taken during the haze period of September and October 1997 for selected urban centres in the Klang Valley, Malaysia, are shown in Table 22.2. The deterioration in air quality, together with increases in potentially polluting activities, is a cause for concern in Southeast Asia. This is particularly disturbing as the climate in this region, with its variable winds, relatively high percentage of calms, and stable

Table 22.2 Frequency of occurrence of days with specific air pollution index categories for four urban centres in the Klang Valley during the haze period September–October 1997 (%) API category

Kuala Lumpur

Petaling Jaya

Klang

Kajang

Good (0–50) Fair (51–100) Unhealthy (101–200) Very unhealthy (201–300) Hazardous (301–500)

0.0 30.6 49.0 18.4 2.0

8.2 34.6 49.0 8.2 0.0

4.2 32.0 59.6 4.2 0.0

20.0 26.0 50.0 4.0 0.0

Note: The air pollution index (API) is derived from five air pollutants: SO2, O3, CO, NO2, and PM10. For PM10 and SO2, the mean concentration is averaged for an hour after twenty-four hours of exposure. For CO, the one-hour reading is taken after eight hours of exposure, and for O3 and NO2, the readings are taken after one hour of exposure each. Indices for each of the pollutants are then computed. The highest index recorded is taken as the API reading. Source: Calculated; based on data from Malaysia, DoE (1997).

atmospheric conditions, has a high potential for pollution (Sham 1979, 1980). The atmospheric ability to disperse and flush out pollutants in the cities of Southeast Asia is restricted because of its climatic attributes. This, together with the near-total dependence on motor vehicles for transport and the abundance of sunshine, provides the necessary set of conditions required for photochemical smog formation.

Water Pollution Apart from its influence on the atmosphere, urbanization also considerably changes water quality, especially in rivers passing through cities. Ill-managed construction sites can cause severe problems downstream as sediments generated tend to choke channels, raising their beds and reducing their ability to carry flood flows. Silting from construction and other land-clearing activities within the drainage basin have long been a problem for the Sungai Klang and its tributaries in Kuala Lumpur. This has also affected the harbour area in Port Klang downstream, costing millions of ringgit for dredging works in the navigation channels. In many of the large cities of the region, the urban waters are heavily polluted with domestic sewage, industrial effluent, and solid wastes. Typical examples include the Chao Phraya and the klongs (canals) in Bangkok, the Pasig River in Metro Manila, and the Klang River in Kuala Lumpur. With the exception of Singapore, where about 96 per cent of the population enjoy modern sanitation, the rest of the cities have no such comprehensive central sewage system. In Metro Manila, only about 12 per cent of the population, about 7.15 million in 1986 (Chrifa 1988), are served by a sewage collection

Urban Pollution in Southeast Asia 383

system. For the majority of the residents not served by a piped system, sanitary facilities range from septic tanks to nothing at all, especially in slums and squatter areas (Nierras 1988). The major rivers of Metro Manila, the Pasig and the Tenejeros–Tullahan, are said to be biologically dead, and Manila Bay’s nearshore waters have become unfit for swimming or for growing shellfish. Water pollution in these rivers has been attributed to sewage, indiscriminate dumping of garbage, and industrial discharges (Rosario 1995). In Bangkok, where there is no central sewerage system, a similar situation exists (Sivaramakrishnan and Green 1986). Human waste is disposed of mainly through septic tanks and cesspools, and the effluents are discharged into stormwater drains or klongs. Inefficient drainage, together with periodic flooding and a high water-table, makes water pollution a health hazard. It has been estimated that about 50 per cent of the city’s daily production of 2500 t of garbage finds its way into klongs (Sivaramakrishnan and Green 1986). The situation is not much different in Jakarta. In several moderate and low-income kampungs (villages or neighbourhoods) in Bandung, a single canal serves as both drainage and sewerage system, with rivers and streams clogged by uncollected garbage and inadequate drainage resulting in periodic flooding. About half the population meet their water needs from unprotected shallow wells and rivers (ESCAP 1993). In Kuala Lumpur, the Klang River, which flows through the middle of the city, has been classified by the Department of Environment as one of the most polluted rivers in the country (Malaysia, DOE 1997). Heavy metals such as lead, zinc, copper, and cadmium have been detected. Although the level for many metals is still below the World Health Organization guidelines, the concentration of some, especially lead and zinc, could well be in excess of these standards (Badri and Sham 1986). The Klang River is also badly affected by sewage discharges from the city population, of whom only a small percentage is served by a central sewerage system. For several of these urban rivers, however, efforts are continually being made to clean and manage them. Nine refuse traps are positioned strategically in the Klang River to catch some 20 000–40 000 t of trash discharged into it daily. In 1997 the Kuala Lumpur City Hall spent 20 million ringgit (approximately $US7.1 million) just to manage rivers in the Kuala Lumpur area. Perhaps one of the most ambitious plans of action in cleaning up urban rivers in Southeast Asia was that of the Singapore River and the Kallang Basin, which covers about a fifth of Singapore’s total land area. In 1977 a ten-year clean-up programme was launched. It

involved many government agencies with the Ministry of Environment as the coordinator. The entire programme cost the Singapore government some $S200 million ($US150 million). The action plan involved four phases: (1) cleaning and dredging the waterway, (2) phasing out polluting activities in the basin, (3) removing and/or relocating farms, hawkers, and polluting workshops, (4) developing a suitable infrastructure, factories, housing, and food centres for those affected by the relocation. The initiative was hailed as one of the most successful of its kind in Southeast Asia. Principally, this was due to the integrated and well-coordinated approach taken involving several government agencies. Furthermore, the people displaced were well taken care of by relocation of required infrastructure such as housing, food centres, and workplaces. The clean-up programme was given a specific deadline, and targets were set throughout the programme for frequent monitoring and evaluation. Finally, there was a strong commitment on the part of the government and an effective education programme to support the whole exercise. The phased progress and achievements of the ten-year programme are summarized in Table 22.3.

Municipal and Hazardous Wastes Closely related to water supply and water quality are waste and its disposal. Waste is at the core of urban environmental problems and usually involves four interrelated aspects: basic drainage of the urban area, sewerage, the disposal of solid wastes, and the disposal of hazardous and toxic wastes. In all of these, the situation in the cities of Southeast Asia is, to say the least, substandard, and, in some cases, getting worse. The basic drainage system, where it exists, is frequently blocked with abandoned solid waste. The capacity of the sewerage systems has in most cases long been exceeded. Solid wastes remain a long-standing problem in urban areas. Many landfills are not properly managed and are sources of open burning. Metro Manila, for example, used to produce about 5525 t of refuse per day, which in 1995 Rosario projected to double within the next twenty years (Rosario 1995). Only about 70 per cent of this amount was collected. The rest was dumped into creeks and along roadsides, exacerbating drainage and flooding problems, and constituting a health hazard. This is further compounded by the fact that Metro Manila is vulnerable to natural hazards in the form of typhoons, floods, tsunamis, and storm surges. In the Bangkok metropolitan area in the early 1990s, the total amount of solid waste generated per day was about 4030 t. Only about 60–70 per cent of this waste was collected (Tabucanon 1991). Of the

384 Sham Sani Table 22.3 Progress of the ten-year clean-up programme for urban rivers in Singapore Removal of pollution sources 1982 All pig- and duck-farming activities within the Singapore River and Kallang Basin catchments phased out or relocated. Majority of families (> 26 000) resited 1983

800 barges moored at the Singapore River moved to Pasir Panjang Wharves. Relocation of the Kallang Basin boatyards nearly completed by 1985

1986

Food centres built by Housing Development Board. Urban Redevelopment Authority and Ministry of Environment enabled resiting of all 5000 street hawkers

> 2800 backyard and cottage industries relocated. 21 000 unsewered premises phased out

Cleaning up Once sources of pollution removed or controlled, river beds dredged and accumulated rubbish, dumped over a century, removed. Upgrading and improvement works then undertaken by the Public Works and Parks and Recreation Departments. Improvements • Tremendous improvement to the waterways, cleaner and free from stench. • By 1983 fish and prawn could be caught in the rivers. • In 1986–8 about 80 000 sea bass fingerlings (Lates calcarifer), 8500 cherry snapper fingerlings (Oreochromis niloticus hybrid), and 630 000 banana shrimp fry (Penaeus merguiensis) stocked in the Boat Quay area. • Improvement in water quality within the first three years of the programme. Between 1978 and 1981 biological oxygen demand in the Singapore River dropped from 21 to 5 ppm; in the Kallang River, from 335 to 79. Suspended solids, ammoniacal nitrogen, and dissolved oxygen also improved significantly. Long-term management • After the phasing out of pollution sources, follow-up action maintained. • Public education stepped up to overcome anti-social habits. • Mass media mobilized in the appeal for public cooperation. • Long-term education programme for schoolchildren initiated. Source: Chou (1998).

amount collected, 60 per cent was disposed of by composting and the rest dumped at open dump sites, into canals and/or rivers, or directly into the drainage systems (Tasneeyanond 1984). In the Federal Territory of Kuala Lumpur, some 1930 t of refuse are generated daily, and for the greater Klang Valley region the figure is in the order of 3400 t. A large proportion of the refuse is disposed of through sanitary landfills. Kuala Lumpur, however, is no longer able to provide land for such landfill operations and will need to resort to incineration in the future. Solid-waste disposal in Singapore is more systematically organized and better managed. The Engineering Services Department operates two refuse incineration plants with a combined capacity of 3600 t per day. A third incineration plant began operation in 1992 with a capacity of 2400 t. In addition, the department operates a refuse transfer station which can handle about 1500 t of refuse per day (Singapore, Ministry of Environment 1989). With increased industrial growth and the use of chemical and metal inputs to production in and around urban centres, toxic and hazardous wastes and their disposal have now become a growing problem. In Malaysia, it is estimated that industries generate some 380 000 m3 of toxic wastes yearly. These are mainly acids,

possibly heavy metals comprising 22 per cent by volume, heavy metal sludge (15.4 per cent), mineral sludge (12.6 per cent), and asbestos (9.2 per cent). The rest are components such as paint or pigment in water, dust, ashes, alkali, oil, and hydrocarbons (New Straits Times, 18 January 1990). The major sources of these toxic and hazardous wastes are metal-finishing industries, textile industries, gas processing, foundry and metal works, and asbestos factories. The dangers of locating plants that might produce toxic substances close to populated areas are considerable. But even where the danger is recognized, housing often grows up spontaneously around the plant. Legislation and codes of regulations to monitor the production, movement, storage, and disposal of toxic substances exist in most countries, but the institutional capacity and expertise to check the operation of new plants and assess the risk of wastes in existing plants is frequently lacking. This lack of technical capacity to enforce legislation very often provides an opportunity for companies to ignore the regulations and dump their wastes accordingly.

Flash Floods In addition to pressures exerted on infrastructure, provision of facilities, and services, urbanization, through

Urban Pollution in Southeast Asia 385

changes in the physical surface, also affects the magnitude of surface run-off and hence flash floods. In many cities, about 50 per cent of the surface is impervious. Roofs, streets, and parking lots increase the run-off. Leopold (1968) has identified two main physical effects of urban land use on water balance: (1) changes in the total volume of run-off and (2) changes in peak flow characteristics. Both these are dependent on the percentage of impervious surfaces and the rate of water flow across the land. Although not all urban fabrics are impervious (bricks, for example, are absorbent), by and large run-off from urban areas is greater than from similar rural areas (Reagen, Livermore, and Sterans 1971; Thorpe 1973). Such effects are clearly illustrated by the run-off coefficients and percentage imperviousness of the surface. Run-off coefficients of fully builtup areas range between 0.85 and 0.9 (indicating that nearly all storm rainfall will run off directly), while those of parks and forested areas range between 0.3 and 0.35 (Fricke and Lewis 1976; Douglas 1984). The degree of imperviousness for different types of land use is also very significant in determining run-off. The area used for terraced houses, for example, has a higher percentage of imperviousness (Table 22.4) as compared to that for semi-detached houses and detached bungalows (Douglas 1984). The implication of these altered relationships in the water balance equation is that run-off in a built-up area becomes peaked and flashy. Generally, an area which is 50 per cent impervious and has no storm-water drains will have a discharge 1.6–1.8 times greater than an otherwise identical rural area. With 50 per cent of the area sewered, the discharge will be 2–3 times greater, and with all the area sewered the discharge will be 2.8–4.7 times greater (Leopold 1968). Changes in the time lag between peak precipitation and peak discharge are crucial aspects of the urban water balance, affecting the timing of flood peaks and the occurrence of overbank flows.

Table 22.4 Percentage of impervious surface as a function of land use Land use

House density (houses/ha)

Impervious surface (%)

Terraced house

5.4 5.9 2.3 3.2 1.2

80 85 52 62 41

Semi-detached house Bungalow Source: Douglas (1984).

Green Areas The benefits of vegetation as a moderator of air temperature, as a buffer against intense raindrops hitting the ground surface, excessive noise, and air pollution in an urban setting have long been recognized (Oke 1976; Douglas 1983). In the United States, an Urban Forestry Act has been in place for many years to ensure that city areas are not completely filled with concrete blocks. In cities of Southeast Asia, green areas have not received as much attention as their counterparts in the West. Only in the last few years have efforts been made to improve the extent of green areas in cities. Singapore’s greening programme and her efforts to set aside 5 per cent of her total land area as nature reserves are commendable. The Kuala Lumpur Structure Plan (1984) allocated close to 10 per cent of its total land area by the year 2000 as green areas. All other cities also have their respective plans and programmes of action on city greening. But with increasing population pressures and demands for parks and recreation areas, the current limited percentage allocated for green is far from adequate. More commitments are needed to ensure that reasonable proportions of green areas in the cities are preserved. While the competition from other more economically viable land uses has, in the past, marginalized the importance of green areas in cities, this has to change. Other considerations such as quality of life, health, recreational facilities, and aesthetics should replace purely economic payback.

Management Responses and Outstanding Issues This brief overview of the impact of urbanization on infrastructure, services, and the environment, all of which are interrelated, illustrates the kinds of issue and extent of the problems faced by major cities in Southeast Asia. While many of the problems are city-specific, a number are common to cities all over the region. Although some aspects of the urban landscape have been enhanced and improved, the general state of the environment and quality of life for many parts of cities is deteriorating despite preventive and mitigating measures taken by governments through land planning, the provision of basic amenities and social services, legislation, and environmental impact assessment (EIA) procedures. In Malaysia, for example, several measures are being taken with varying degrees of success. The Environmental Quality Act, which was passed by Parliament in 1974, forms the basis of much of the legislation. Currently, over twenty pieces of legislation have been promulgated

386 Sham Sani

under the Act and are being enforced by the Department of Environment. In addition, several other pieces of legislation have been enacted but are being enforced separately by local governments and other government agencies. These include the Housing Developers (Control and Licensing) Act 1966, the Streets, Drainage, and Building Act 1974, the Local Government Act 1976, and the Town and Country Planning Act 1976. The Environmental Quality Act 1974 itself has been amended three times—in 1985, 1996, and 1998—with additional provisions that are more stringent in terms of control, heavier in terms of penalties, and wider in terms of jurisdiction. Under Section 34A of the Environmental Quality (Amendment) Act 1985, an environmental impact assessment is mandatory for certain selected activities. This section empowers the minister in charge of the environment to label any activity that is likely to have a significant impact on the environment as a ‘prescribed activity’ requiring an environmental impact assessment study prior to project approval. In general terms, the Malaysian example reflects the availability and spread of environment-related legislation and other instruments in countries of Southeast Asia. The issue, however, is not the lack of legislation, but rather the effectiveness of enforcement of such laws. Many governments in Southeast Asia have long been aware of what economic and population changes are doing to the environment. They lack neither environmental legislation and regulations nor ingenuity in technical and other applications to improve the environment. Yet, despite environmental policies and declarations, the political will to act appears to be lacking. And, even if it is in place, the administrative capacity to implement the policy is weak. A review of the state of the urban environment in major Southeast Asian cities indicates that environmental problems are complex and closely related to larger national agenda. Likewise, their management is also strongly influenced by policies at the national level. Indeed, almost all aspects of national policy directly or indirectly affect environmental conditions in cities; these are not necessarily confined to environmental policy per se. Sometimes economic instruments and other public policies are much more powerful than an explicit environmental policy. The efforts by Singapore to clean her major rivers are good examples of how comprehensive planning with support from national government and the public succeeds. As much as the commitment and support from national government are important, the role of local government is also crucial. This is precisely the area of government that is weakest in many Southeast Asian

countries. The institutional and legal structures of local governments are generally not equipped for effective environmental management despite their direct involvement with the environment. The lack of access to an adequate financial base is a major weakness. Most local governments have difficulty getting sufficient revenue to cover their operating expenses, let alone make new investment to extend services and facilities. To become effective agents of development, local governments and municipalities need enhanced political, institutional, and financial capacities, notably access to financial facilities and support. Standards, too, must be appropriate to local conditions if environmental policy is to work, particularly the standards on ambient air and water quality and on allowable discharge limits. Too often, standards appropriate to a developed country are far too costly relative to the benefits, and cheaper alternatives exist. The high costs mean that the standard is not enforced, and there are far too many cases of non-compliance that need to be taken care of. While the role of legislation and institutions in the administration of policies and programmes regarding the urban environment is recognized, public support is equally essential in ensuring the success of such programmes. Public support, however, can only be expected from well-informed citizens who are aware of the problems, committed, and willing to do something about them. At both the federal and local levels, public education and dissemination of environmental information need to be expanded. Since environmental education is basically aiming for community action, efforts to reach the different target groups must be varied, involving both government institutions and a wide range of nongovernmental organizations (NGOs), including private and commercial enterprises and the mass media. The contribution from the NGOs can be particularly helpful, as many conservation works and restoration projects involve their efforts. Some NGOs initiate campaigns to help keep the city clean through education and the media. Others participate in official government committees at a high level and influence very important decisions.

Conclusions One important advantage for Southeast Asian city managers is the opportunity to share experience in city management. A review of the state of the environment of any major urban area in the region indicates that, while many of the problems are city-specific, a number are common to cities all over the region, and experience

Urban Pollution in Southeast Asia 387 Table 22.5 Activities to help determine priorities for action for cities in Southeast Asia Action by

Policy activity

National governments

Formulating appropriate urban environmental health policies. Funding basic health care in critical areas (water, sanitation, solid wastes, drainage infrastructure). Preventive health measures and primary health care. Creating and enabling environment for local governments and civil society to control pollution Formulating appropriate economic policies: (a) cost recovery; at present, cost recovery for most municipal utilities is inadequate; (b) a mix of regulatory and economic instruments including pollution charges, tax rebates, tax write-offs, subsidies Strengthening local government capacity by ensuring that they have access to adequate trained personnel and financial resources to carry out their responsibilities

Local authorities

Integrating environmental considerations in urban development plans Improving infrastructure investment programmes Conducting environmental impact assessment at the project level Conducting environmental risk analysis Strengthening capacity for implementation and enforcement Support NGOs and community-based initiatives

Non-governmental organizations and communities

Developing innovative joint government–community programmes Enhancing greater citizen awareness and involvement

International agencies

Institution-building support Development assistance and technology transfer support Monitoring, data collection, research, and policy analysis support

Source: ESCAP (1993).

can be shared among city managers to their advantage. The review also shows that environmental problems are complex and closely related to socio-economic and population factors. Any attempt to plan and manage the urban environment will inevitably have to be concerned not only with the city system but also with its interdependence with the rural hinterland, other city systems, and the outer world. A national urbanization strategy could provide a set of goals and priorities for the development of an acceptable urban system in which large, intermediate, and small centres can be incorporated. Within such a framework, the traditional tools of urban policies, including land use planning and pollution control, could stand a better chance of being effective. Once this is achieved, the next step will be to strengthen the capacity of local governments so that effective solutions to local urban problems can be identified and implemented. Establishing priorities for action on urban environmental management is a complex task because of the multiplicity of concerns expressed by a host of actors who often have conflicting perspectives and objectives. This task is further complicated by the lack of comprehensive environmental data, difficulties in measurement

and analysis, and the need to address efficiency, equity, and issues of scale simultaneously. The task is also sitespecific, because each city has a unique set of problems with differing physical, political, social, and cultural contexts. However, there are a number of commonly shared activities that can be considered to help determine priority for action for a given city. Obviously, a number of these policy activities are already in place for many cities in Southeast Asia. The details may vary from one city to another, but Table 22.5 could be useful as a general checklist and a guide. In the scheme shown in Table 22.5, four major actors have been identified—the national government, the local authorities, the NGOs and the community, and the international agencies—each of which has its own role to play. Naturally, the lead will have to come from the national governments in formulating basic policies relating to environmental health, appropriate economic policies to recover costs, and local government as an effective institution. The local authorities, on the other hand, will need to look into the detailed day-to-day running of the city and putting plans into action on the ground. The role of the NGOs and the international agencies is to support specific programmes and enhance citizen awareness.

388 Sham Sani

References Azman Zainal Abidin (1997), The Star (25 Sept. 1997). Badri, M. A., and Sham, S. (1986), ‘Heavy Metal Pollution Problems in a Developing Tropical City: Case of Kuala Lumpur, Malaysia’, in L. Pawlowski, C. Alberts, and W. L. Lacy (eds.), Chemistry for Protection of the Environment (Amsterdam: Elsevier Science Publications), 71–87. Camp, S. L. (ed.) (1990), Cities: Life in the World’s 100 Largest Metropolitan Areas (Washington, D.C.: Population Crisis Committee). Chandler, T. J. (1965), The Climate of London (London: Hutchinson). Chou, L. M. (1998), ‘The Cleaning of Singapore River and the Kallang Basin: Approaches, Methods, Investments and Benefits’, Ocean and Coastal Management, 38: 133–45. Chrifa, R. D. (1988), ‘Tropical Urban Ecosystems: Case of Metro Manila, Philippines’, Tropical Urban Ecosystems Studies, 4: 109–27. Douglas, I. (1983), The Urban Environment (London: Edward Arnold). —— (1984), ‘Water and Sediment Issues in the Kuala Lumpur Ecosystem’, in Yip Yat Hoong and Low Kwai Sim (eds.), Urbanization and Ecodevelopment with Special Reference to Kuala Lumpur (Kuala Lumpur: Institute of Advanced Studies, University of Malaya), 101–21. ESCAP (Economic and Social Commission for Asia and the Pacific) (1993), State of Urbanization in Asia and the Pacific (New York: United Nations). Fricke, T. J., and Lewis, K. V. (1976), Flood Estimation for Urban Areas in Peninsular Malaysia, Hydro Procedure 16 (Kuala Lumpur: Ministry of Agriculture Malaysia, Drainage and Irrigation Department). Hadiwinoto, Suhardi, and Clarke, G. (1990), ‘The Environmental Profile of Jakarta’, Paper prepared for the Metropolitan Environmental Improvement Program Workshop, Honolulu, 18–20 Dec. Jones, G. W. (1993), ‘Industrialization and Urbanization in SouthEast Asia’, in H. Brookfield and Y. Byron (eds.), South-East Asia’s Environmental Future: The Search for Sustainability (Kuala Lumpur: UN University Press/Oxford University Press), 47–71. Khairuddin Yusof and Low Wah Yun (1995), ‘Healthy SelfSustaining Communities for Squatters’, in Azman Awang, Mahbob Salim, and J. F. Halldane (eds.), Toward a Sustainable Urban Environment in Southeast Asia (Skudai, Johor: Institute Sultan Iskandar of Urban Habitat and Highrise, Universiti Teknologi Malaysia), 71–92. Leopold, L. B. (1968), Hydrology for Urban Land Planning: A Guidebook on the Hydrologic Effects of Urban Land Use, US Geological Survey Circular 554. Malaysia, DoE (Department of Environment) (1997), Environmental Quality Report 1997 (Kuala Lumpur). Mohammed Razali Argus (1997), ‘Urban Growth, Poverty and the Squatter Phenomenon’, in Jamilah Ariffin (ed.), Kuala Lumpur in Poverty amidst Plenty: Research Findings and the Gender Dimension in Malaysia (Kuala Lumpur: Pelanduk Publications), 127–51. Nierras, J. U. (1988), ‘Tropical Urban Ecosystem Study: Case Study of Metropolitan Manila’, Tropical Urban Ecosystem Studies, 4: 92–108. Oke, T. R. (1976), ‘Inadvertent Modification of the City Atmosphere and the Prospects for Planned Urban Climates’, Proceedings, WMO Symposium on Meteorology as Related to Urban Regional Land Use Planning, World Meteorological Organization, Publication no. 444 (Geneva), 150–75. —— (1979), Review of Urban Climatology 1973–76, WMO Technical Note 169 (Geneva).

—— (1986), ‘Urban Climatology and Tropical City’, in Oke, T. R. (ed.), Urban Climatology and its Applications with Regard to Tropical Areas, World Meteorological Organisation, Publication no. 652 (Geneva), 1–25. Reagen, R., Livermore, N. B., and Sterans, J. C. (1971), Environmental Impact of Urbanization on the Foothills and Mountainous Lands of California (State of California Resources Agency and Department of Conservation). Rosario, Elenida del (1995), ‘Recent Directions in Philippines Urban Environment Policy’, in Azman Awang, Mahbob Salim, and J. F. Halldane (eds.), Toward a Sustainable Urban Environment in Southeast Asia (Skudai, Johor: Institute Sultan Iskandar of Urban Habitat and Highrise, Universiti Teknologi Malaysia), 269–76. Sham, S. (1973a), ‘Observations on the Effects of a City’s Form and Function on Temperature Patterns: A Case of Kuala Lumpur’, Journal of Tropical Geography, 36: 60–5. —— (1973b), ‘The Urban Heat Island: Its Concept and Application to Kuala Lumpur’, Sains Malaysiana, 2: 53– 64. —— (1979), Air Pollution Climatology of a Tropical City: A Case of Kuala Lumpur–Petaling Jaya, Malaysia (Bangi: UKM Press). —— (1980), The Climate of Kuala Lumpur–Petaling Jaya, Malaysia (Bangi: UKM Press). —— (1984), ‘Urban Development and Changing Patterns of Night-Time Temperatures in Kuala Lumpur–Petaling Jaya Area, Malaysia’, Teknologi, 5: 27–36. —— (1989), Pembandaran dan Iklim, Syarahan Perdana Jawatan Profesor, Universiti Kebangsaan Malaysia (UKM) (Bangi: UKM Press). —— (1991), ‘Urban Climatology in Malaysia: An Overview’, Energy and Buildings, 15/1: 105–17. —— Cheong, B. K., Leong, C. P., and Lim, S. E. (1991), The August 1990 Haze in Malaysia with Special Reference to the Klang Valley Region, Malaysian Meteorological Service Technical Note no. 49 (Petaling Jaya). Sharma, M. L. (1986), Role of Groundwater in Urban Water Supplies of Bangkok, Thailand and Jakarta, Indonesia, Working Paper (Honolulu, Hawaii: Environment and Policy Institute, East–West Center). Singapore, Inter-Ministry Committee for UNCED Preparatory Committee (1992), Singapore’s National Report for the 1992 UN Conference on Environment and Development Preparatory Committee (Singapore). Singapore, Ministry of Environment (1989), Annual Report 1989 (Singapore: Singapore Government Printer). Sivaramakrishnan, K. C., and Green, L. (1986), Metropolitan Management: The Asian Experience, EDI Series in Economic Development (Singapore: Oxford University Press). Tabucanon, M. S. (1991), ‘Toward a Sustainable Environmental Future for the Southeast Asian Region’, Paper presented to the Conference toward a Sustainable Environmental Future for the Southeast Asian Region, Yogyakarta, Indonesia, 6–10 May. Tasneeyanond, P. (1984), Country Monograph on Institutional and Legislative Framework on Environment (Thailand) (Bangkok: United Nations Economic and Social Commission on Asia and the Pacific). Thorpe, G. R. (1973), ‘Forecasting Runoff from Storms Moving over Partially Urbanized Catchments’, Paper 8, presented to the CIRIA Symposium on Rainfall, Runoff, and Surface Water Drainage of Urban Catchments, Bristol, Apr. UNDP (United Nations Development Programme) (1992), The Urban Environment in Developing Countries (New York: United Nations). World Bank (1987), Urban Transport Databook, 1987 (Washington, D.C.: World Bank).

23

Coastal Zone Development in Southeast Asia P. P. Wong

Introduction Coastal environments of Southeast Asia have been discussed in Chapter 11. This chapter focuses on the utilization of the region’s coastal resources, reflecting not only its varied physical characteristics but also the traditional practices and more modern economic influences that have developed along the coastal regions. Historically, the region serves as an important link between trading routes to Western and Eastern Asia. Many sea battles were fought here between local potentates and foreign powers to win control of the spice trade. A number of the coastal villages developed into important coastal cities, e.g. Cebu, Malacca, Singapore, or in recent years, into coastal tourist resorts, e.g. Pattaya, Kuta. Within the region, there are still strong cultural traditions in the use of coast, although these are being eroded or replaced by more modern or economic practices. For example, the beach forms the traditional recreational area for farmers after the harvest season in Lombok and the east coast of Peninsular Malaysia. Traditionally, the Balinese attach a low economic value to the coast, but this has been replaced in modern times by new and high economic values for tourism, residence, and other uses. The demands for the coastal areas for different uses have various impacts, many of which are detrimental to the coastal environment and may lead to conflicts between users. This chapter relates people with the coastal environment in terms of living and non-living marine resources. Specifically, it discusses several major coastal uses, and their impacts and attempted solutions, to development-related problems. A holistic approach in coastal zone management to solve the problems is

advocated, and the implementation and success of this approach assessed. This is also considered within the future and wide-ranging context of climate change and attendant sea-level rise. The definition of a ‘coastal zone’ in Southeast Asia is variable and difficult, as not all states have coastal zone management acts or legislation to define the coastal zone. For the purposes of this chapter, the coastal zone is taken as a variable area defined by not only biogeomorphological characteristics but also the major types of use. This corresponds to the area from the onshore zone to the subtidal– offshore zone in Viles and Spencer (1995) but excludes the continental shelf. The coastal zone can be viewed as an important zone for population and various economic activities of different magnitude.

Coastal Population and Settlements The broad physical structure of Southeast Asia has influenced markedly the distribution of the coastal population concentration. In mainland Southeast Asia, many of the coastal population centres are in the deltaic areas formed by the large rivers (Irrawaddy, Chao Phraya, Mekong, Sông Hóng). In contrast, the areas of fertile volcanic materials in insular Southeast Asia, particularly in Java, are the main centres of population. Given its extensive coastline and thousands of islands, Southeast Asia has been a region of long-established maritime traditions. Today people from remote coastal villages still travel along the coast. The sea nomads are probably the oldest surviving example of this group of people, but their activities have very little impact on the coastal environment. They inhabit the Mergui

390 P. P. Wong

Archipelago, Riau–Lingga Archipelago, eastern Sumatra, Sulu Archipelago, eastern Borneo, Sulawesi, and the islands of eastern Indonesia, and make use of the richness of the coral reef and mangrove ecosystems (Sather 1997). Some groups are able to develop a broad base of adaptation to both reef and mangrove environments; others only to either mangroves or reefs. Outside the coastal cities, the typical Southeast Asian coast is characterized by villages, sometimes built on stilts, especially along mangrove coasts and estuaries. These villages are traditionally dependent on agriculture and fishing, while some are exclusively fishing villages depending on fish resources in the inshore areas. Thus, they are closely associated with the mangrove and reef coasts. In the Philippines, more than half of 1500 municipalities and 42 000 villages are coastal by location (Primavera 2000). More than 7100 villages in the coastal area of Indonesia are involved in traditional agriculture and fishing. The heavy pressure of coastal communities has led to the destruction of coral reefs and mangroves. Although some of the traditional knowledge has protected the resources, these have been abandoned because of economic pressure and overexploitation. For example, corals are mined extensively around Pulau Seribu, near Balikpapan, and around Lombok and Bali. Substantial coral-mining has also taken place in the Riau Archipelago, the Sunda Strait, the Karimunjawa Islands, on the northern coast of Java, and in southern Sulawesi (Haeruman 1988). In contrast to the coastal villages are the coastal cities. Southeast Asia has at least a dozen coastal cities, each with a population over 1 million (Table 23.1). Many are located in similar physical settings, sharing Table 23.1 Population of coastal cities in Southeast Asia with more than 1 million inhabitants Coastal city

1996 population (million)

Projected 2015 population (million)

Metro Manila Jakarta Bangkok Yangon (Rangoon) Ho Chi Minh City Singapore Surabaya Medan Palembang Kuala Lumpur Hanoi Davao

9.6 8.8 6.7 4.0 3.6 3.4 2.3 1.7 1.3 1.3 1.3 1.0

14.7 13.9 9.8 6.8 4.8 4.0 3.6 2.7 2.2 1.9 1.8 1.7

Source: UN Population Division (1997).

problems of extensive and poorly drained backswamps, soft clays, thick peat deposits, salt-water intrusions, periodic typhoons, etc. (Rau 1994). They are affected by extreme events, such as earthquakes and tsunamis, and by long-term hazards, such as relative sea-level changes and coastal erosion (Arthurton 1998). The urban coastal zones are utilized for a broad range of socio-economic activities which place a great strain on the coastal environment. The population problem is at the base of nearly all threats to the coast. Within the region, 143 million live in the coastal zone, and the number is increasing. Humaninduced stresses on the coastal zone arise from mining, drilling, and production of oil, diversion of fresh water (including pumping from coastal aquifers), forest exploitation, massive deforestation, conversion to agriculture and aquaculture, coastal development (housing, commercial establishments), and pollution (sewage discharge to the sea). An overview of the region’s environmental problems and pollution is given by Gomez (1988) and Hungspreugs (1988). Figure 23.1 highlights some of the major humaninduced stresses in the coastal zone of Southeast Asia, which include (Bryant et al. 1998; ESCAP 2001; TalaueMcManus 2000): 1. extensive destruction of mangroves in Indonesia, the Philippines, Thailand, and Cambodia; 2. blast fishing in the Philippines, eastern Indonesia, and eastern Sumatra; 3. shrimp farming and environmental impacts on land and water along the gulf coast of Thailand and the Mahakam Delta; 4. marine pollution in the Strait of Malacca and the north coast of Java; 5. coastal pollution hot spots off the cities of Bangkok, Manila, Jakarta, Surabaya, Palembang, Hanoi, and Ho Chi Minh City; 6. coastal tourism impacts in Phuket, Pattaya, and Koh Samui in Thailand. Figure 23.1 indicates that the main causes of environmental problems in the region are associated with spatial use of the coasts (different coastal types have different physical and ecological characteristics), coastal resources use (overuse and improper use give rise to environmental deterioration), and poor-quality environmental management (lack of financial and personnel resources in administration and management) (Sato and Mimura 1997). This chapter reviews four major environmental impacts: coral reef degradation, shrimp-related mangrove degradation, the effect of industrialization on the coast, and tourism-related degradation.

Coastal Zone Development 391

Fig. 23.1. Coastal development in Southeast Asia: selected major uses and impacts

Degradation of Coral Reefs Coral reefs support a rich assemblage of marine life. They support the fish, molluscs, and crustaceans on which many coastal communities depend and, with other coastal habitats, provide nutrients and breeding grounds for many commercial species. UNEP/lUCN (1988) noted that coral reef fisheries have been estimated to comprise 8–10 per cent of the overall fishery production in the Philippines, 5 per cent in Indonesia, and in excess of 20 per cent in Sabah (Malaysia). In the Philippines, the coral reefs ecosystem supports an estimated 1 million small-scale fishermen and provides more than 50 per cent of the animal protein for consumption (White, Vogt, and Arin 2000). Direct and indirect human pressures pose the greatest threat to the Southeast Asian coral reefs. The majority of the people live along the coast, and development pressures have caused the loss of many reefs, particularly those close to large population centres. The extent of damage is noted in various reports.

According to the Reefs at Risk report from the World Resources Institute (Bryant et al. 1998), over 80 per cent of the reefs in Southeast Asia are at medium and higher risk (56 per cent are at high risk). Risk is based on potential risk associated with human activity and not actual reef conditions resulting from natural causes. Most of the coral reefs of the Philippines, Sabah, eastern Sumatra, Java, and Sulawesi face a high potential threat from various disturbances. Overfishing, destructive fishing practices, sedimentation, and pollution associated with coastal development are the biggest threats. This report has been updated to October 2000 (WRI 2001). Preliminary results suggest that 86 per cent of all reefs in the region are at medium or higher threat. The most pervasive threats, overfishing and destructive fishing, affect 60 per cent and 50 per cent respectively. Coastal development and sedimentation from inland sources each threatens approximately 20 per cent of reefs. Only 4 per cent of the reefs are in partially effective and good management and the remainder with inadequate management. Degradation of coral reefs is increasing with time.

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Another update is by Chou (2000). Degradation continues despite increased awareness of the value of reefs. Chou (2000) identified overfishing by Cambodia, Malaysia, Philippines, Thailand, and Viet Nam as the single major cause of reef degradation in an assessment of transboundary problems; next were destructive fishing methods and sedimentation; followed by pollution associated with coastal development. Overfishing and destructive fishing prevail throughout East Malaysia, the Philippines, and Indonesia. The situation is most serious in Indonesia. With the economic downturn from 1997 and the breakdown of administration, damage to reefs has increased. Widespread destructive fishing practices were reported in the national reports from the Ninth International Coral Reef Symposium held in Bali in 2000. Some startling facts are as follows. As many as four bomb blasts per hour were reported in many offshore reef areas in East Malaysia; blasting fishing reduced the coral cover by 50–80 per cent of many Indonesian reefs, although those in eastern Indonesia are in comparatively better condition (Chou 2000). The Philippines are the most adversely affected, with less than 5 per cent of the reefs considered to be in excellent condition. The reasons for degradation include the use of sodium cyanide and other destructive methods in fishing, and increasing damage from anchors of diving boats and impacts of divers, especially in tourist areas (White, Vogt, and Arin 2000). In Indonesia, about 6–7 per cent of reefs are in good condition. Destruction occurs mainly through the illegal use of fish bombing, cyanide poisoning, coral-mining, and pollution (Down to Earth 2000). Of natural factors affecting corals, the strongest influence is that of the annual monsoon, which reverses current flows and introduces fresh water into the coastal areas, thereby lowering salinity and increasing sedimentation. Typhoons affect the Philippines, Viet Nam, and Thailand, while volcanic and tectonic activity occurs in Indonesia and the Philippines. Isolated instances of Acanthaster plagues have occurred. Widespread coral bleaching occurred over many of the region’s reefs in the early half of 1998. This was triggered by elevated sea-surface temperatures connected to the El Niño phenomenon (Chou 1998). Although coral reefs have the capacity to regenerate rapidly after catastrophic tropical storms, recovery often takes fifteen to twenty years. Also, the 1998–9 bleaching had a greater impact than previous bleaching events (Chou 2000). Over the past fifty years major increases in stresses to coral reefs in the region have come from direct and indirect human activities. Reef degradation can only be reduced when there is better understanding of reef functions and the need

for marine protected areas (MPAs). A widely held view is that conservation through MPAs can be an effective measure (White 1988). While various countries in the region have legislation for MPAs, owing to lack of adequate financial resources, personnel, etc., there is little enforcement. The size of area to be conserved seems to be important in the success and type of management system to be used. Experiences in the Philippines, Indonesia, and Thailand indicate that small areas can be managed successfully by local communities. Community-based management systems with various models adapted to the local conditions therefore have been more successful. Integrated coastal management can also be used, as it involves the local community (Pomeroy 1995). Many ‘house reefs’ are well-protected by tourist resorts. For large areas, co-management by government agencies, local communities, and non-governmental organizations together is more effective. In the end, conservation requires full support from all sectors of government, users, and the public (Chou 2000). Other measures, such as artificial reefs and coral transplantation, have been tried for reef rehabilitation. In general, artificial reefs contribute little to enhance the marine environment (Chou 1997). Despite widespread interest in using coral transplantation for reef rehabilitation, this measure is still largely experimental, unlike mangrove rehabilitation. It is best to leave degraded sites to recover unaided (Edwards and Clark 1998). Policy-makers are increasingly aware of the economic value of coral reefs and the need to invest in coral reef conservation. For example, tourism associated with coral reefs provides major economic benefits to the region. The annual value of 1 km2 of healthy reef in the Philippines with tourism potential is estimated at $US31 900–113 000 (White, Vogt, and Arin 2000). Although managed reefs provide sustained benefits, they require effective management beyond the declaration of MPAs. Various regional efforts have been carried out since 1995. The main strategies include integrated management, capacity building, research and monitoring and mechanisms for coordination, and implementation of activities by the International Coral Reef Initiative (Kirkman and Cao 1998).

Shrimp Aquaculture and Mangroves Globally, Southeast Asia is home to one of the largest areas of mangroves in tropical and subtropical regions. The mangroves are distributed in large stretches in

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Indonesia (e.g. east Sumatra, most of Kalimantan, Papua), Malaysia (west coast of the Malay Peninsula), Myanmar and Thailand (e.g. the eastern side of the Gulf of Thailand). They flourish in relatively sheltered lowlands away from strong wave action, and on aggrading coasts characterized by deltas (e.g. the Irrawaddy, Mekong) (Eisma 1982). The plants provide a wide variety of products for the coastal population of Southeast Asia (Furukawa 1994). They are also an important nursery to young fish and are positively correlated with the fishery resources of Indonesia, the Philippines, and Malaysia (Baran and Hambrey 1998; Primavera 2000). Use of mangroves for aquaculture goes back several centuries in Southeast Asia, with the culture of fish in brackish water ponds dating back to 1400 in Java (Primavera 1995). In the traditional method, small fish or prawns are trapped and held in enclosed brackish ponds (tambaks) until they reach a marketable size. Problems of stocking and some incidences of pollution are present in traditional mangrove aquaculture. These are, however, minor compared to the more serious problem of mangrove destruction and degradation resulting from extensive conversion of mangroves to ponds largely for milkfish (Chanos chanos) culture in the 1960s in the Philippines and to shrimp ponds in Thailand. Typically, the patterns of shrimp pond development in mangrove areas involve healthy mangroves, and their profitable conversion spread until all or nearly all mangrove areas were converted to ponds (Nickerson 1999). Mangrove destruction from shrimp aquaculture is extremely serious in Thailand. Historically practised for more than fifty years, Thai shrimp aquaculture has evolved from coastal flooding of the lowlands in the 1950s based on natural processes to open intensive farming with 5–40 per cent water exchange, complete diet feeding, and deeper but smaller ponds. The yield has increased twentyfold, and brought social and economic benefits to numerous individuals. From 1987 to 1993 Thailand was the world’s largest exporter of shrimp, with the 1992 shrimp product valued at $US124 million (Dierberg and Kiattisimkul 1996). From the mid-1980s to mid-1990s, shrimp aquaculture reached a level to cause major environmental concern (Dierberg and Kiattisimkul 1996). Between 1979 and 1993 the loss of mangroves to shrimp aquaculture is estimated at 16–32 per cent of total mangrove area destroyed in the country (Dierberg and Kiattisitmkul 1996). A higher estimate is given by Piyakarnchana (1999), with mangroves reduced from 367 000 ha in 1961 to 173 600 ha in 1991, of which 64 per cent was used for mariculture, mostly shrimp ponds and other

uses including salt farms, tin-mining, and residential and industrial development. Shrimp aquaculture has several major environmental impacts on mangroves. Apart from their destruction, the major problem is the acid sulphate condition associated with most mangroves converted for shrimp aquaculture (Ong 1995). The development of shrimp aquaculture in Thailand is associated with destruction of mangroves, deterioration of coastal water quality, salt-water intrusion, salt-water contamination in groundwater and ricefields, sediment disposal problems, abandoned shrimp ponds, and displaced traditional livelihoods (Flaherty and Karnjanakesorn 1995; Dierberg and Kiattisimkul 1996). A recent wave of ‘low-salt’ shrimp aquaculture —the result of high market prices for shrimp and the feasibility of trucking salt water inland within a two-hour drive of the sea— is envisaged to cause even more serious environmental problems (Flaherty and Vandergeest 1998). On the gulf coast of Thailand, the conversion of ricefields to shrimp ponds and migration of shrimp ponds inland has accelerated coastal erosion at various locations. As ponds nearest to the sea are breached by waves, erosion leaps landward to the bund of the next pond, accelerating the process of coastal erosion. In Indonesia, 1.5 million ha of mangroves have been wiped out in almost two decades. The major causes of mangrove destruction are conversion to shrimp and fish ponds, pollution (especially from the oil industry), overlogging for timber and charcoal, and conversion for residences and industries. With a few exceptions, little attention is given to the need to protect mangroves. In the Mahakam Delta, east Kalimantan, only 10 per cent of 150 000 ha of mangroves are left, with most of the rest converted to shrimp farms (Down to Earth 2000). Of an estimated 110 000 ha of mangrove forests before 1965 on the coast of south Sulawesi, about 80 000 ha were cleared for timber, fuel wood, and conversion to tambak by the early 1990s (Nurkin 1994). Tambak is one of the commonest land uses in coastal south Sulawesi and has created serious land use conflict. Various revegetation projects have been adopted, including the establishment of a 200 m wide ‘green belt’ along the coast (Nurkin 1994). Largely owing to aquaculture, the area under mangroves in the Philippines estimated at 450 000 ha in 1920 decreased to 288 000 ha in 1970. By 1993 only 123 000 ha were left, i.e. a loss of 70 per cent in roughly seventy years (Nickerson 1999). Half of the loss is due to brackish water pond construction and the rest to exploitation for fuel wood, conversion to agriculture, salt beds, industry, and settlements (Primavera 2000).

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Pond development peaked in the 1950s and 1960s with 5000 ha per year. Milkfish constituted 95 per cent of brackish water pond culture for a long time, while the shrimp industry took off in the 1980s. Milkfish is now a popular food fish for domestic consumption, while shrimp is a cash crop for the urban and export markets. Intensive shrimp-farming leads to other associated problems, such as pollution of coastal waters and decline in domestic food crops. The loss of mangroves to brackish water ponds is most extensive in western Visayas and central Luzon (Primavera 1995). Since the 1970s numerous legislative measures have attempted to preserve the remaining mangroves in the Philippines. Despite their protected status, the mangroves suffered an annual loss of 11 000 ha between 1981 and 1990. A mangrove buffer zone as required by law is absent in the majority of culture ponds. Even up to 1985 the mangroves were treated as ‘swamplands available for development’ by the Bureau of Fisheries and Aquatic Resources. Other factors include the lack of effective law enforcement and the entry of powerful political and business interests in the pond industry (Primavera 1995). Mangrove management and conservation gained impetus in the 1980s, with revised guidelines for zoning of forestlands into fishponds and areas for conservation. Mangrove rehabilitation and conservation were hindered by factors such as the promotion of aquaculture, low economic rent for mangroves, conflicting policies, and ineffective government management. About 600 kg of fish and shrimp can be obtained from each hectare of protected or restored mangroves (Primavera 2000). In Viet Nam, 40 per cent of the mangroves were destroyed by military spraying of herbicides (MacIntosh 1982). Also, extensive shrimp culture and unplanned coastal development have contributed to the loss of mangroves. To reduce the problem of mangrove loss, integrated shrimp–mangrove farming systems were first introduced at the southern tip of Viet Nam in the Ngoc Hien district. Economic analysis shows that shrimp culture with mangrove coverage of 30–50 per cent of the pond area gives the highest annual economic returns, thus demonstrating the importance of maintaining mangroves within the farming systems (Binh, Phillips, and Demaine 1997). In total, the estimated percentage loss of mangroves from its original cover in Indonesia, the Philippines, Thailand, Cambodia, Viet Nam, and Malaysia is 83, 80, 70, 50, 37, and 12 per cent respectively (TalaueMcManus 2000). From a summary of mangrove rehabilitation projects of Southeast Asian countries, Indonesia, the Philippines, and Viet Nam put in most

effort into the rehabilitation of degraded areas. More mangrove planting for timber and charcoal production is carried out in Malaysia and Thailand (Field 1998). The importance of coastal fisheries is often the main reason for rehabilitation. Other reasons include the redevelopment of disused shrimp ponds, e.g. in Thailand, and for sea-dyke protection in Viet Nam. However, the region is far from achieving sustainable management of mangroves. The Matang mangroves in Malaysia provide a rare case of successful sustainable management (Ong 1995). Its success is due to monospecific stands and to the unique cycle of thinnings involving two thinnings (formerly three) at fifteen and twenty years and clear-felling at thirty years to complete one rotation. A similar system is used in Thailand and Indonesia, with different techniques in clear-felling and the period of rotation. A different method is used in Sabah and Sarawak. Despite the seemingly conflicting situation, mangroves and aquaculture can be compatible. Mangrove-friendly aquaculture is best for small-scale, family-based operations and suitable in areas for mangrove conservation and restoration. It can be considered in the wider context of integrated coastal zone management or integrated coastal area management, coordinating various sector demands such as fisheries, aquaculture, forestry or industry (Primavera 2000). Ong (1995) also made an important point on mangrove conservation. As mangroves represent interface ecosystems that straddle land and sea, from fresh water to sea water, the boundary of mangrove national parks should extend into the sea as well as into the freshwater catchments.

Coastal Industries and Infrastructure In Southeast Asia, the coastal zone continues to attract industries and infrastructural development, especially where the population is concentrated. Many of the land use problems result from a lack of understanding the geological conditions of the coastal lowlands, leading to subsidence, salt-water intrusion, coastal flooding, or coastal erosion (Rau 1994). These also include development normally accompanying the draining of peatlands and consequent compression of as much as 3 m, the large sediment loads of river systems which actually close part of the river channels and river and port development. There is thus a need for a better understanding of the following ecosystems: river estuary, river mouth, beach, coastal lagoon, offshore bar, and coral reef.

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Many population centres in the coastal zone have inadequate sewerage treatment, and the disposal of waste water remains a problem (Phillips and Tanabe 1989). The major problem areas are Jakarta Bay (Indonesia), the Strait of Malacca, Manila Bay, and the Upper Gulf of Thailand (Hungspreugs 1988). A large proportion of the litter on the beaches of the Thousand Islands in Jakarta Bay is from Jakarta, lying to the south of the bay. The region’s pollution hot spots are associated with the coastal cities (Figure 23.1) and associated mainly with untreated domestic waste runoff from cities and agriculture or industry (TalaueMcManus 2000). About 75 per cent of the large cities in Indonesia are located on the coast and discharge untreated wastes into the coastal waters. Untreated sewage discharged by Metro Manila through a pipeline 2 km offshore returns to the bay and renders it unfit for recreation and shellfish culture. The quality of the Upper Gulf of Thailand depends much on the Chao Phraya. Its low-flow season in April affects water quality, although the water is still suitable for shellfish culture. Piyakarnchana (1999) reports that untreated wastes from the hinterlands and cities into the Gulf of Thailand has led to frequent occurrence of red tides in the estuarine regions of large rivers in the Inner Gulf. Although the harmful effects of red tides was known in the early 1970s, the situation is expected to worsen with increased input of untreated wastes. Within Southeast Asia, the Philippines has been most affected by red tides, with Manila Bay being the worst case (Azanza and Taylor 2001). Eutrophication is a serious problem in the region. Ports and shipping also affect the coastal areas negatively. Sulphur dioxide from shipping heavily affects the coastal areas bordering the straits (Streets, Carmichael, and Arndt 1997). Heavy-metal contamination of the coastal waters and sediments from industrial and domestic sewage discharges or direct waste-dumping are a problem, although the picture is not clear owing to lack of reliable data. High levels of heavy-metal pollution have been reported from Jakarta Bay, Surabaya, and the Strait of Malacca (Hungspreugs 1988). Associated with the shipping activity in Singapore is the release of anti-fouling paints from boats (Goh and Chou 1997). Analysis of samples of mussels from the Philippines and Thailand indicate that the butyltins and organochlorines in the coastal waters are due mainly to aquaculture and boating activities, causing increasing concern for the aquatic environment of the region (Tanabe et al. 2000). Organotin anti-fouling was used by ocean-going vessels until banned by the International Maritime Organization in 1998.

Although surveys in the 1980s show a low level of oil pollution in the coastal waters of the region (Chansang 1988b), accidental oil spills occur along the oil transport routes or points of loading and discharging for oil tankers. This is particularly true in the Strait of Malacca, an important international navigation channel, where several major oil spills have occurred. A number of minerals are mined in the coastal zone of Southeast Asia. The most important mineral mined in the coastal and nearshore areas is cassiterite, but the list also includes mineral sands of economic importance such as zircon, monazite, rutile, ilmenite, and gold (Rau 1994). Until recently, tin-mining in the coastal areas was an important activity along the Andaman coast (Thailand), and Bangka and Belitung Islands (Indonesia). It created a serious environmental problem on land and in the nearshore area, as illustrated on the west coast of Phuket Island. The mineral is found within the alluvial deposits on land and in coastal waters. On land, gravel-pumping is used, in which unconsolidated deposits are liquefied in the open with high-pressure jets and the mixture is pumped to a sloping sluice box to remove the tin and heavy minerals. Lighter materials are washed away as mine tailings. The tailings are often fed into old mine pits or bunded for the solids to settle out, and water is recycled or discharged into natural waterways. Dredging is carried out to recover tin deposits from mangrove areas and shallow waters to about 30 m deep. It is done by a bucket ladder or a suction pipe, and the tin is removed by using a similar gravitational method and tailings discharged directly from the rear of the dredge. Sediment plumes are generated during almost the entire operation of the dredgers (Chansang 1988a). Various development activities, such as dredging, construction, and reclamation, contribute to the destruction or damage to coastal habitats and water pollution. Coral-mining for building stone and lime-making is a traditional activity that continues to threaten the reefs in the Philippines and Indonesia (Figure 23.2). Photographic transects off the northwest coast of Phuket showed the destruction of corals due to tin-dredging activities. Corals were smothered by sedimentation and also affected by a reduction in light intensity, which is required for coral growth at certain depths (Chansang 1988a). About 60 per cent of the coral reefs in Bang Tao Bay, on the west coast of Phuket, are polluted by tinmining activities (Ruyabhorn and Phantumvanit 1988). In the mangrove area of Phang-Nga Bay, northeast of Phuket, years of tin-mining had reduced the organic matter and nutrients, making mangrove recolonization

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Fig. 23.2. Corals removed from reefs for construction or lime-making, Lombok, Indonesia

impossible for a period of six to twenty years (Chansang 1988a). In the Philippines, mine tailings affect Calancan Bay (Marinduque) and Tanon Strait (between Cebu and Negros). Sand extraction affects some sectors of the Malaysian coasts. Granite quarrying affects the coasts of the Karimun Islands. An estimated 20–25 per cent of the seagrass areas in Indonesia, Malaysia, the Philippines, and Thailand are damaged by a combination of coastal development and activities (ESCAP 2001). Landfill for the creation of space for economic activities takes place along the sea for major coastal cities. For example, more than 10 per cent of the total area of Singapore is derived from landfill to provide space for industries, airport, highways, and urban development resulting in substantial destruction of the mangroves and coral reefs (Hilton and Manning 1995). Coastal activities also interfere with the geomorphological processes associated with accretion and deposition of coastal sediments. Some infrastructures have a negative impact on the coast. The extension of the runway of Ngurah Rai Airport in Bali in the 1970s and 1980s interrupted the longshore drift from south to north, leading to coastal erosion along a 300 m stretch. An erosion rate of up to 7.5 m per annum was reported less than 1 km north of the runway extension (Lubis, Kridoharto, and Sikumbang 1989). Coastal erosion is a serious problem in Malaysia, affecting nearly every state. By 1998 coastal erosion had affected 29 per cent of Malaysia’s coastline mainly as a result of human activities (ESCAP 2001).

Coastal and Marine Tourism Tourism, particularly coastal tourism, is important in many Southeast Asian countries. Blessed with ample sunshine, beaches, and coral reefs, many coastal areas and islands have been developed for tourism. Although providing economic benefits, coastal tourism comes into conflict with traditional types of land use and also impacts negatively on the coastal environment, especially when unplanned development takes place. The growth of Pattaya, Kuta, Batu Ferringhi, and Boracay as coastal resorts illustrates the extent and degree of unplanned coastal development in Southeast Asia (Wong 1998). The major negative impacts to the coastal and marine environment include environmental degradation through removal of natural tree cover, discharge of poorly treated waste water, beach erosion from poor design or location of coastal structures, and damage to coral reefs from anchoring and tourist activities. Although integrated coastal resorts, such as Nusa Dua, Phuket Laguna, and Bintan Beach International Resort are considered as ideals for planned coastal tourism, they are limited to certain areas and are not easily implemented owing to size and high investment cost limitations. Ignorance or lack of knowledge have also contributed to the negative impacts of tourism on the coastal environment. For example, unnecessary or ill-conceived sea walls (Figure 23.3). to protect resorts lead to coastal erosion (Wong 1998). Other negative impacts are related to tourist-related activities, e.g. anchor damage to coral

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Fig. 23.3. A sea wall within reach of waves in front of a resort encourages beach erosion, Phuket, Thailand

reefs arising from marine tourism in Sombrero Island, Batangas, Philippines (Palaganas 1991). The mass tourism market is still driven by costs and comfort levels, thus causing continuous development of new areas. Various small coastal areas are specially at risk. For example, Puerto Galera, a small (4 km2) semi-enclosed bay on the northeast coast of Mindoro, Philippines, has a variety of coastal habitats including corals, seagrasses, mangroves, and beaches. It is a tidally dominated environment, and its quality of water is determined by tidal flows and seasonal changes. As a popular tourist resort, increased tourism development has created problems and conflicts affecting the quality of local marine resources. A high nutrient pool has built up in the inner part of the bay as a result of the low flushing rate and high nutrient input from freshwater run-off, and sewerage and waste water discharge (Diego-McGione, Villanoy, and Aliño 1995). Recent demands for coastal ecotourism and marine tourism have put increasing pressure on the coral reefs (Wong 2001). Ecotourism remains a small niche market, but is often cited as justification for the creation of MPAs that promote sustainable management practices.

Coastal Management and Sustainable Development Coastal degradation is often aggravated by conflicting multiple uses of the coastal environment. For example, conflicts on the north coast of Java arise from use of

interacting resources, coral-collecting, clam shell collecting, and mangrove utilization (Taufik 1987). In Puerto Galera, Philippines, the problems include erosion due to gold- and marble-mining, coral reef destruction from unregulated beach-mining, and tourism-related activities (Kintanar and Fama 1987). In the past two decades the region has undergone various developments in coastal management. From the early 1980s, with the implementation of regional study programmes on the protection of the marine environment, the development of coastal zone management moved to a more enlightened approach (Chou 1994). Useful lessons were learned on various management mechanisms for Cilacap, Indonesia, and Lingayen Gulf, Philippines (Paw and Chua 1991b). Lingayen Gulf is a pioneering effort in the management of a coastal area larger than small islands (Talaue-McManus and Chua 1997). The integrated management approach for marine living resources in the ASEAN region was a step in the right direction (Chua and Garces 1994). Equally important is the participation of local people in the assessment and management process. Of various approaches to coastal zone management in Southeast Asia, the most successful has been the community-based coastal resources management, with emphasis on integrating sociological, economic, and environmental information at the community level and generating recommendations for local management of marine resources in a bottom-up approach. This is most widespread in the Philippines (Christie and White 1997).

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The Philippines is the most advanced country in Southeast Asia in the practice of coastal management. Since the mid-1980s at least forty-five coastal management programmes and projects involving 150 sites have been implemented, but not under a single agency or national integrated coastal management plan ( Jacinto et al. 2000). With 832 coastal municipalities along its 18 000 km coastline, coastal resources are under threat from a variety of activities and impacts and plagued by issues of declining fisheries, mangrove forest and coral reef destruction, and poverty among coastal communities. Up to the mid-1990s such threats have been tackled by coastal resources management focusing on damage to habitats and decline of fishery production based mainly on community-based management. But these have not solved the critical problems. In the current programme started in 1996, the new approach is integrated coastal management replacing habitat-specific management and fisheries development. Also, local government units assume more responsibility given the decentralization of authority from the early 1990s (Courtney and White 2000). Based on various studies and reviews, communitybased coastal resource management in the Philippines is clearly a response to failures of more centralized approaches (Pomeroy and Carlos 1997; Rivera and Newkirk 1997; Alcala 1998). This is particularly successful in the Philippines on small island communities surrounded by coral reefs. The success has been due to the close-knit nature of island communities, isolation from outside destabilizing forces, and the ecological nature of coral reefs to respond rapidly and positively to sustainable measures (Christie and White 1997). Faced with a variety and complexity of problems in the coastal areas, coastal zone management programmes in Thailand are characterized by sectoral rather than holistic or integrated approaches. Six areas of conflict have been identified: sustainable shrimp aquaculture; rehabilitation of abandoned shrimp ponds; sustainable small-scale fisheries; protection, management, and rehabilitation of mangrove forests; management of coastal marine protected areas; and marine biodiversity conservation. Mismanagement is attributed mainly to poor legal framework and policy, and lack of understanding and enforcement. From experience and examples, co-management is seen as a promising approach to bring the stakeholders into discussion and management (Sudara 1999). Indonesia has more than 7100 coastal villages which rely on inshore fishing and are characterized by poverty, underdevelopment, and low educational level. Its national strategy on coastal community development emphasizes infrastructural development, socio-economic

improvements, and optimal sustainable use of coastal resources. Various agencies with overlapping interests at the coast create an excessively complex governance, and coastal management is highly sectoral (Sloan and Sugandy 1994). Innovative management practices were achieved in Proyek Pesisir, the Indonesia Coastal Resources Management Plan (1996–2001) implemented in north Sulawesi, Lampung, and East Kalimantan provinces. In 1999 management of Indonesia’s coasts and seas came under a single ministry. With the 1999 law on regional autonomy, there has been devolution of central government control, with substantial powers given to the regency and municipality governments. There have been some successes with community-based management under various projects and initiatives and increasing participation of non-governmental organizations and international agencies (Wong 2001). Malaysian law favours a centralized approach in coastal management. The island reef management programmes are not as effective as the state governments control land use. This power is used to promote shoreline development, which either displaces coastal communities or impacts on marine resources. ‘To be effective, coastal management must be tailored to the cultural, social, educational, and legal context in which it works’ (Christie and White 1997: 164). Southeast Asian countries are increasingly aware of sustainable coastal development as in Agenda 21. For example, in the implementation of the Second LongTerm Development Plan (1994/5–2019/20), Indonesia is still highly dependent on natural resources, including coastal resources. Marine and littoral habitats are under increasing pressure from development projects and dependence of an expanding population on marine and coastal resources for food, utilities, and market products. Thus, sustainable development is crucial. For Malaysia, sustainable use of the coastal zone relates in particular to the use of mangroves in coastal protection. Mangroves are used as a natural system of coastal protection with a mangrove strip separating the sea and bunds behind, which usually are agricultural land. This system has been implemented by various methods (Othman 1994). ‘Linear’ parks are being developed along the bunds (Figure 23.4). In Thailand, laws and regulations in the implementation of sustainable use have been promulgated (Piyakarnchana 1999). Several national laws incorporate sustainable use and development in an attempt to protect the environment. Since 1992 the Fisheries Act, the Wildlife Conservation Law, and the Promotion of National Environmental Quality Act have been amended. But problems of enforcement still remain.

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Fig. 23.4. Pedestrian walk equipped with lighting being developed on a bund on the landward side of a protective belt of mangroves, south Johor, Malaysia

Sea-level rise is of greatest concern to islands and coastal areas in the region (Amadore et al. 1996). The implications of a future sea-level rise as a result of climate change on coastal area utilization and management in Southeast Asia have been discussed (Paw and Chua 1991a). A variety of country studies have also been carried out and some specific measures evaluated. In Viet Nam, the potential impact of a 1 m sea-level rise has been examined in relation to its shoreline vulnerability to climate change and response strategies to integrated coastal management (Zeidler 1997). More specifically, mangrove rehabilitation is being examined to enhance sea defence systems in three coastal districts of northern Viet Nam (Tri, Adger, and Kelly 1998). The potential impact of a sea-level rise on the coastal resources of Manila Bay has been studied and adaptive responses suggested (Perez et al. 1996).

Conclusion Southeast Asia is characterized by a high population in the coastal zone. This places a tremendous strain on the coastal resources. Overuse and improper use of such resources resulting in environmental degradation and destruction of coastal habitats are common. More importance should be given to coastal management within the national planning framework. Archipelagic nations such as Indonesia and the Philippines continue to depend much on coastal management. Whatever approach is adopted, there is a need to raise

coastal communities from poverty and towards the sustainable use of coastal resources. The task is not easy as it requires effort from the villages, municipalities, provinces, the central government, and development agencies involved. It takes too long for a central agency to deal with the issues. These efforts have been supplemented by non-governmental organizations and external aid. There is also a need to avoid prevarication and to tackle issues directly. More direct involvement is required instead of elaborate plans whereby none of the villagers benefit from money spent on gathering information and refining research tasks.

References Alcala, A. C. (1998), ‘Community-Based Coastal Resource Management in the Philippines: A Case Study’, Ocean and Coastal Management, 38: 179– 86. Amadore, L., Bolhofer, W. C., Cruz, R. V., Feir, R. B., Freysinger, C. A., Guill, S., Jalal, K. F., Iglesias, A., Jose, A., Leatherman, S., Lenhart, S. Mukherjee, S., Smith, J. B., and Wisniewski, J. (1996), ‘Climate Change Vulnerability and Adaptation in Asia and the Pacific: Workshop Summary’, Water, Air, and Soil Pollution, 92: 1–12. Arthurton, R. S. (1998), ‘Marine-Related Physical Natural Hazards Affecting Coastal Megacities of the Asia–Pacific Region: Awareness and Mitigation’, Ocean and Coastal Management, 40: 65– 85. Azanza, R. V., and Taylor, F. J. R. (2001), ‘Are Pyrodinium Blooms in the Southeast Asian Region Recurring and Spreading? A View at the End of the Millennium’, Ambio, 30: 356– 64. Baran, E., and Hambrey, J. (1998), ‘Mangrove Conservation and Coastal Management in Southeast Asia: What Impact on Fishery Resources’, Marine Pollution Bulletin, 37: 431– 40.

400 P. P. Wong Binh, C. T., Phillips, M. J., and Demaine, H. (1997), ‘Integrated Shrimp–Mangrove Farming Systems in the Mekong Delta of Viet Nam’, Aquaculture Research, 28: 599–610. Bryant, D., Burke, L., McManus, J., and Spalding, M. (1998), Reefs at Risk (Washington: World Resources Institute). Chansang, H. (1988a), ‘Coastal Tin Mining and Marine Pollution in Thailand’, Ambio, 17: 223–8. —— (1988b), ‘Effects of Oil Pollution in Marine and Coastal Living Resources of the East Asian Seas Region’, in Oil Pollution and its Control in the East African Seas Regions, UNEP Regional Seas Reports and Studies, no. 96 (Geneva), 41–55. Chou, L. M. (1994), ‘Marine Environmental Issues of Southeast Asia: State and Development’, Hydrobiologia, 285: 139–50. —— (1997), ‘Artificial Reefs of Southeast Asia: Do they Enhance or Degrade the Marine Environment?’, Environmental Monitoring and Assessment, 44: 45–52. —— (1998), ‘Status of Southeast Asian Coral Reefs’, in C. Wilkinson (ed.), Status of Coral Reefs of the World: 1998 (Townsville: Australian Institute of Marine Science); www.aims.gov.au/pages/ research/coral-bleaching/scr1998/scr-06.html (25 Oct. 2001). —— (2000), ‘Southeast Asian Reefs—Status Update: Cambodia, Indonesia, Malaysia, Philippines, Singapore, Thailand and Vietnam’, in C. Wilkinson (ed.), Status of Coral Reefs of the World: 2000 (Townsville: Australian Institute of Marine Science), 117–29. Christie, P., and White, A. T. (1997), ‘Trends in Development of Coastal Area Management in Tropical Countries: From Central to Community Orientation’, Coastal Management, 25: 155–81. Chua, T. E., and Garces, L. R. (1994), ‘Marine Living Resources Management in the ASEAN Region: Lessons Learnt and the Integrated Management Approach’, Hydrobiologia, 285: 257–70. Courtney, C. A., and White, A. T. (2000), ‘Integrated Coastal Management in the Philippines: Testing New Paradigms’, Coastal Management, 28: 39–53. Diego-McGlone, M. L., Villanoy, C. L., and Aliño, P. M. (1995), ‘Nutrient Mediated Stress on the Marine Communities of a Coastal Lagoon (Puerto Galera, Philippines)’, Marine Pollution Bulletin, 31: 4–12. Dierberg, F. E., and Kiattisimkul, W. (1996), ‘Issues, Impacts, and Implications of Shrimp Aquaculture in Thailand’, Environmental Management, 20: 649–66. Down to Earth (2000), no. 45 (May); www.gn.apc.org/dte/45CRC.htm (25 Oct. 2001). Edwards, A. J., and Clark, S. (1998), ‘Coral Transplantation: A Useful Management Tool or Misguided Meddling?’, Marine Pollution Bulletin, 37: 474–87. Eisma, D. (1982), ‘Asia, Eastern Coastal Geomorphology’, in M. L. Schwartz (ed.), The Encyclopedia of Beaches and Coastal Environments (Stroudsburg, Pa.: Hutchinson Ross), 76–82. ESCAP (Economic and Social Commission for Asia and the Pacific) (2001), State of the Environment in Asia and the Pacific 2000 (New York: United Nations). Field, C. D. (1998), ‘Rehabilitation of Mangrove Ecosystems: An Overview’, Marine Pollution Bulletin, 37: 383–92. Flaherty, M., and Karnjanakesorn, C. (1995), ‘Marine Shrimp Aquaculture and Natural Resource Degradation in Thailand’, Environmental Management, 19: 27–37. —— and Vandergeest, P. (1998), ‘ “Low-Salt” Shrimp Aquaculture in Thailand: Goodbye Coastline, Hello Khon Kaen’, Environmental Management, 22: 817–30. Furukawa, H. (1994), Coastal Wetlands of Indonesia: Environment, Subsistence and Exploitation, trans. P. Hawkes (Kyoto: Kyoto University Press).

Goh, B. P. L., and Chou, L. M. (1997), ‘Heavy Metal Levels in Marine Sediments of Singapore’, Environmental Monitoring and Assessment, 44: 67–80. Gomez, E. D. (1988), ‘Overview of Environmental Problems in the East Asian Seas Region’, Ambio, 17: 166–9. Haeruman, H. (1988), ‘Conservation in Indonesia’, Ambio, 17: 218–22. Hilton, M. J., and Manning, S. S. (1995), ‘Conversion of Coastal Habitats in Singapore: Indications of Unsustainable Development’, Environmental Conservation, 22: 307– 22. Hungspreungs, M. (1988), ‘Heavy Metals and Other Non-Oil Pollutants in Southeast Asia’, Ambio, 17: 178– 82. Jacinto, G. S., Aliño, P. M., Villanoy, C. L., Talaue-McManus, L., and Gomez, E. D. (2000), ‘Philippines’, in C. R. C. Sheppard (ed.), Seas at the Millennium: An Environmental Evaluation, vol. ii (Amsterdam: Pergamon), 405–32. Kintanar, R. L., and Fama, R. P. (1987), ‘Coastal Zone Management of Puerto Galera’, in Coastal Zone ’87: Proceedings of the Fifth Symposium on Coastal and Ocean Management (New York: ASCE), iii. 2925–38. Kirkman, H., and Cao, S. (1998), East Asia: A Review of Actions for Coral Reefs in the East Asian Seas, www.reefbase.org/Summaries/pdf/ ITEMS98EASIA.pdf (21 July 2001). Lubis, S., Kridoharto, P., and Sikumbang, N. (1989), ‘Preliminary Study of Coastal Erosion in Kuta Beach, Bali, Indonesia’, in Proceedings of the Twenty-Fourth Session: Technical Reports, pt. ii, Bangkok, Thailand, 28 Oct.–7 Nov. 1987 (Bangkok), 190–7. MacIntosh, D. J. (1982), ‘Asian, Eastern, Coastal Ecology’, in M. L. Schwartz (ed.), The Encyclopedia of Beaches and Coastal Environments (Stroudsburg, Pa.: Hutchison Ross), 67–76. Nickerson, D. J. (1999), ‘Trade-Offs of Mangrove Area Development in the Philippines’, Ecological Economics, 28: 279– 98. Nurkin, B. (1994), ‘Degradation of Mangrove Forests in South Sulawesi, Indonesia’, Hydrobiologia, 287: 281– 6. Ong, J. E. (1995), ‘The Ecology of Mangrove Conservation and Management’, Hydrobiologia, 295: 343–51. Othman, M. A. (1994), ‘Value of Mangroves in Coastal Protection’, Hydrobiologia, 285: 277– 82. Palaganas, V. P. (1991), ‘Anchor Damage on the Coral Reef of Sombrero Island, Batangas, Philippines’, in Coastal Zone ’91: Proceedings of the Seventh Symposium on Coastal and Ocean Management (New York: ASCE), 3318– 29. Paw, J. N., and Chua, T. E. (1991a), ‘Managing Coastal Resources in Cilacap, Indonesia, and Lingayen Gulf, Philippines: An ASEAN Initiative’, Marine Pollution Bulletin, 23: 779– 83. —— —— (1991b), ‘Climate Changes and Sea Level Rise: Implications on Coastal Area Utilization and Management in South-East Asia’, Ocean and Shoreline Management, 15: 205– 32. Perez, R. T., Feir, R. B., Carandang, E., and Gonzalez, E. B. (1996), ‘Potential Impacts of Sea Level Rise on the Coastal Resources of Manila Bay: A Preliminary Vulnerability Assessment’, Water, Air and Soil Pollution, 92: 137– 47. Phillips, D. J. H., and Tanabe, S. (1989), ‘Aquatic Pollution in the Far East’, Marine Pollution Bulletin, 20: 297– 303. Piyakarnchana, T. (1999), ‘Changing State and Health of the Gulf of Thailand Large Marine Ecosystem’, in K. Sherman and Q. Tang (eds.), Large Marine Ecosystems of the Pacific Rim: Assessment, Sustainability, and Management (Malden, Mass.: Blackwell Science), 240–50. Pomeroy, R. S. (1995), ‘Community-Based and Co-Management Institutions for Sustainable Coastal Fisheries Management in Southeast Asia’, Ocean and Coastal Management, 27: 143– 62.

Coastal Zone Development 401 —— and Carlos, M. B. (1997), ‘Community-Based Coastal Resource Management in the Philippines: A Review and Evaluation of Programs and Projects, 1984–1994’, Marine Policy, 21: 445–64. Primavera, J. H. (1995), ‘Mangroves and Brackishwater Pond Culture in the Philippines’, Hydrobiologia, 295: 303–9. —— (2000), ‘Development and Conservation of Philippines Mangroves: Institutional Issues’, Ecological Economics, 35: 91–106. Rau, J. L. (1994), ‘Urban and Environmental Issues in East and Southeast Asian Coastal Lowlands’, Engineering Geology, 37: 25–9. Rivera, R., and Newkirk, G. F. (1997), ‘Power from the People: A Documentation of Nongovernmental Organizations’ Experience in Community-Based Coastal Resource Management in the Philippines’, Ocean and Coastal Management, 36: 73–95. Ruyabhorn, R., and Phanthumvanit, D. (1988), ‘Coastal and Marine Resources of Thailand: Emerging Issues Facing an Industrialising Country’, Ambio, 17: 229–32. Sather, C. (1997), The Bajau Laut (Oxford: Oxford University Press). Sato, A., and Mimura, N. (1997), ‘Environmental Problems and Current Management Issues in the Coastal Zones of South and Southeast Asian Developing Countries’, Journal of Global Environmental Engineering, 3: 163–81. Sloan, N. A., and Sugandy, A. (1994), ‘An Overview of Indonesian Coastal Environmental Management’, Coastal Management, 22: 215–33. Streets, D. G., Carmichael, G. R., and Arndt, R. L. (1997), ‘Sulfur Dioxide Emissions and Sulfur Deposition from International Shipping in Asian Waters’, Atmospheric Environment, 31: 1573–82. Sudara, S. (1999), ‘Who and What is to be Involved in Successful Coastal Zone Management: A Thailand Example’, Ocean and Coastal Management, 42: 39–47. Talaue-McManus, L. (2000), Transboundary Diagnostic Analysis for the South China Sea, EAS/RCU (East Asian Seas Action Plan/Regional Coordinating Unit) Technical Report Series, no. 14 (Bangkok: UNEP). —— and Chua, T. E. (1997), ‘Lingayen Gulf (Philippines) Experience: If we Have to Do it Again’, Ocean and Coastal Management, 37: 217–32.

Tanabe, S., Prudente, M. S., Kan-Atireklap, S., and Subramanian, A. (2000), ‘Mussel Watch: Marine Pollution Monitoring of Butyltins and Organochlorines in Coastal Waters of Thailand, Philippines and India’, Ocean and Coastal Management, 43: 819– 39. Taufik, A. W. (1987), ‘Marine Resource Management in the Java Sea’, in Coastal Zone ’87: Proceedings of the Fifth Symposium on Coastal and Ocean Management (New York: ASCE), 4488–502. Tri, N. H., Adger, W. N., and Kelly, P. M. (1998), ‘Natural Resource Management in Mitigating Climate Impacts: The Example of Mangrove Restoration in Vietnam’, Global Environmental Change: Human and Policy Dimensions, 8: 49– 61. UN Population Division (1997), World Urbanization Prospects Database, the 1996 Revision (New York: United Nations). UNEP/IUCN (United Nations Environmental Programme/International Union for Conservation of Nature and Natural Resources) (1988), Coral Reefs of the World, vol. ii: Indian Ocean, Red Sea and Gulf (Nairobi: UNEP; Gland: IUCN). Viles, H., and Spencer, T. (1995), Coastal Problems: Geomorphology, Ecology and Society at the Coast (London: Edward Arnold). White, A. T. (1988), Marine Parks and Reserves: Management for Coastal Environments in Southeast Asia, ICLARM Education Series, no. 2 (Manila: ICLARM). —— Vogt, H. P., and Arin, T. (2000), ‘Philippine Coral Reefs under Threat: The Economic Losses Caused by Reef Destruction’, Marine Pollution Bulletin, 40: 598– 605. Wong, P. P. (1998), ‘Coastal Tourism Development in Southeast Asia: Relevance and Lessons for Coastal Zone Management’, Ocean and Coastal Management, 38: 89– 109. —— (2001), ‘Trends in Coastal Ecotourism in Southeast Asia’, UNEP Industry and Environment, 24/3– 4: 20– 4. WRI (World Resources Institute) (2001), Reefs at Risk Southeast Asia October 2000, www.wri.org/marine/pdf/rr_seasia_10_1_00.pdf (11 July 2001). Zeidler, R. B. (1997), ‘Continental Shorelines: Climate Change and Integrated Coastal Management’, Ocean and Coastal Management, 37: 47–62.

24

Coral Reefs of Southeast Asia: Controls, Patterns, and Human Impacts T. Spencer and M. D. Spalding

Introduction The intricate coastline of Southeast Asia, and its many islands and island groups— Indonesia alone has over 17 500 islands—contains 32 per cent (91 700 km2) of the world’s shallow coral reefs (Spalding, Ravilious, and Green 2001). While sedimentary regimes appear to restrict reef development in the East China Sea, the Gulf of Thailand, the South China Sea, and around the island of Borneo, reefs are well developed elsewhere. Fringing reefs characterize island coastlines, and there are also barrier reefs and, in the deeper waters of the South China Sea and to the east, atoll-like reef structures. Although the region has a distinguished history of reef studies—in which the pioneering work of R. B. Seymour Sewell, J. H. F. Umbgrove, and Ph. H. Kuenen on the Snellius expedition (1929–30) come particularly to mind —the lack of detailed information about many areas remains considerable. The coral reefs, and their associated shallow-water ecosystems, within this region are the product of both historical and contemporary processes. A wide range of hypotheses to explain coral distributions have been proposed. These include the importance of the widespread availability of suitable shallow substrates for coral growth with submergence histories determined by regional tectonic and sea-level dynamics (e.g. Hall and Holloway 1998), the variety of habitats present (e.g. Wallace and Wolstenholme 1998), and the more contemporary roles of high sea-surface temperatures and ocean current circulation patterns, including the dynamics of western Pacific Ocean–eastern Indian Ocean connectivity (Tomascik et al. 1997a). Both sets of controls show wide variation across the region. Thus, for example,

geological settings range from tectonically stable platforms to rapidly uplifting plate collision zones of considerable seismic and volcanic activity. Present-day environments vary from equable, tranquil interior seas to cycloneand swell wave-dominated environments on the region’s margins. Added to these controls are the perturbations introduced by, for example, periodic coral bleaching and biological catastrophes (e.g. Crown of Thorns starfish infestations; Lane 1996). Taken as a whole, therefore, the coral reefs of Southeast Asia demonstrate enormous complexity and considerable dynamism. These reef resources are, however, under considerable pressure from large, and growing, populations and economic development. Southeast Asian dietary requirements are strongly marine-based, with shallow seas supplying 60 per cent of animal protein ( Yong 1989). Rising demand has led to both increased pressures on traditional reef sources of food and new impacts on more remote reef sites. As well as these increased and new patterns of exploitation, Southeast Asian reefs are at risk from organic and inorganic pollution from sewage, agricultural, and industrial discharges. Sedimentation is associated both with shoreline development and with changing land use patterns and practices in coastal hinterlands. Bryant et al. (1998) conclude that these reefs are among the most threatened in the world, and it has been estimated that some 88 per cent are at risk from anthropogenic impacts (Burke, Selig, and Spalding 2002). This chapter is divided into three sections. In the first, the nature of constraints and controls on reef development, and how these factors nest within one another over a range of temporal and spatial scales, are considered. In the second section, new mapping of the distribution

Coral Reefs of Southeast Asia 403

of coral reefs and mangrove forests in Southeast Asia is presented, accompanied by descriptions of the regional variety of reef environments. Finally, in a third section, the chapter considers the threats that these reefs face at the present time.

Geological Controls on the Structure and Distribution of Southeast Asian Coral Reefs Geological History and the Distribution of Shallow Marine Environments The structure and distribution of coral reefs and other shallow marine ecosystems in Southeast Asia have been, and continue to be, influenced by a tectonic setting of enormous complexity and dynamism. The dominant process of convergence, from the interaction of the Eurasian, Indian, Australian, Philippine Sea, and Pacific Plates that developed after the break-up of the southern supercontinent of Gondwanaland, has led to the subduction, deformation, fragmentation, and reorganization of numerous micro-plates within an ever-decreasing area, albeit accompanied by the creation of new marine basins associated with island arc development. Hall (1998) identifies three periods of significant interaction; all have implications for the history of shallow marine environments in the region. The first period, at c.50 Ma bp, was particularly important on the western margins of this region; here the collision of India with Asia not only led to the reorientation of plate boundaries but also, through the uplift of the Himalaya and other mountain chains, determined changes in climate, drainage systems, and sediment supply to coastlines. The flooding of shallow shelf areas with fluvial sediments and the growth of large delta systems (Woodroffe 2000), both directly inhibited coral growth and more generally introduced barriers to biogeographical exchange at this time. Twenty to thirty species of coral are typically recorded from the Upper Triassic (e.g. Timor; Martini et al. 1977) to the Upper Jurassic (e.g. Sarawak; Beauvais 1986) in Southeast Asia. Thereafter, however, palaeontological analysis has long demonstrated (e.g. Umbgrove 1946) that Early Tertiary (Eocene – Oligocene) carbonate deposits in the region are dominated by benthic foraminifera and coralline algae with only sparse coral faunas. The second major revision of plate boundaries and motions, at c.25 Ma bp, was the result of the reorganization of western Pacific Ocean Plate margins and, more importantly, the collision of the north Australian margin with island arcs to the north. This created not only new, longitudinal linkages between Australia and

Southeast Asia via Sulawesi but also new barriers from mountain-building and associated delta growth (e.g. the development of new drainage divides on the island of Borneo), the widening of existing shallow seas (e.g. South China Sea; Taylor and Hayes 1983), and the formation of new, often deep, ocean basins (e.g. the opening of the Sulu and Banda Seas). The development of the deep trough (1500–2500 m water depth) of the Makassar Strait, variously attributed to extensional tectonic rifting (Hamilton 1979) or crustal subsidence (Bergman et al. 1996), may also date from this phase. The collision both greatly reduced Southeast Asia’s isolation (the water gap to Australia was 3000 km at the beginning of the Tertiary and the northern Great Barrier Reef did not reach tropical latitudes until 25–16 Ma bp; Davies et al. 1987) and initiated a major phase of carbonate platform and reefal development, slightly more extensive than that of the present day (Wilson and Rosen 1998). The third set of reorganizations began at c.5 Ma bp, possibly as a result of arc– continent collision in the vicinity of Taiwan, and resulted in renewed tectonic activity and increases in land and highlands throughout the margins of Southeast Asia. At the present time, the north-northwest-moving Philippine Sea Plate and the north-northeast-moving Indo-Australian Plates are converging with Eurasia at 8–10 cm a−1 and 7–8 cm a−1 respectively; the Eurasian Plate itself is static or slow-moving (0.4–1 mm a−1) (McCaffrey 1996). There is now a greater area of land in Southeast Asia than at any time during the last 30 million years, explaining the shrinkage in shallow-water carbonate platform area with enhanced terrestrial sediment influxes (Hall 1998). These plate tectonic processes have resulted in four structural and physiographic units (Figure 24.1) which have strongly influenced coral reef locations and growth in the past and continue to constrain reef development at the present time. These are: 1. The southeastern promontory of the Eurasian Plate, the Sundaland Craton, an area of 1.85 million km2 extending from the coasts of the Malay Peninsula and Indo-China to the Java Sea, including eastern Sumatra, northern Java, and the island of Borneo. The Sunda Shelf is covered by silts and clays (Tjia 1980) and water depths rarely exceed 50 m, except near its margin with the South China Sea. Extensive areas are less than 20 m deep. Following Molengraaff and Umbgrove, palaeoriver drainage systems have been mapped in general by Tjia (1980) and in more detail by Emmel and Curray (1982), who describe a submerged shelf edge delta complex (delta front, backswamps, river floodplain) on the northwest approaches to the Malacca Strait.

404 T. Spencer and M. D. Spalding 110°

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2. The Australian Continental Craton, extending into southern Irian Jaya and Papua New Guinea and incorporating the Arafura Shelf and the Sahul Shelf. Although deeper than the Sunda Shelf seas, water depths over these shelves rarely exceed 100 m. As a result of high rainfall, steep terrain, and high sediment yields, much of western Papua is devoid of reefs. However, fringing reefs are characteristic of the tip of the Bird’s Head peninsula and the large neighbouring islands, and extensive fringing reefs, barrier reefs, and platform reefs are all found in Cenderawasih Bay on the north coast of Papua (UNEP/IUCN 1988). Extensive fringing reef development characterizes the eastern coasts of the Aru Islands in the Arafura Sea (Tomascik et al. 1997b). 3. A tectonic transition zone between regions 1 and 2, with active seismicity and volcanism. This zone runs both west–east and south–north. The longitudinal element extends from the Sunda Arc to the Banda Arc. Convergence is expressed by the subduction of oceanic lithosphere west of Sumatra and south of Java, with a narrow, fault-bounded, and tectonically active (Malod and Kemal 1996) continental shelf.

4. Marginal seas, described by Hutchison (1989) as ‘the most characteristic feature of south-east Asia, distinguishing it from all other regions of the world’. They are associated with the opening of back-arc basins and can be grouped into (a) the South China Sea, the Sulu Sea, and the Celebes (Sulawesi) Sea on the eastern margins of the Sundaland Craton and (b) the Banda Sea (and its extension into the Flores Sea) marginal to the Australian Craton. In strong contrast to the continental margins, water depths typically vary between 3.5 and 5 km, reaching maximum depths (e.g. Weber Deep, Banda Sea) of over 7 km. Some of the basin floors are flat, with considerable thicknesses of sediment; elsewhere bottom topography is rugged, with numerous deep basins. Of particular importance are the sills that connect the deep basins as these control water circulation and throughflow patterns. This chapter considers regions 1, 3, and 4 in more detail below; much of region 2 lies outside the geographical scope of this volume.

Coral Reefs of Southeast Asia 405

Quaternary Vertical Movements of Land and Sea, the Distribution of Reef Types, and Implications for Coral Biogeography As well as these large-scale lateral changes, there have been considerable vertical displacements, which are continuing to this day. Outside the stable platforms, plate collision processes are reflected in the presence of raised coral reef sequences and in the restriction of modern reef development to island fringing reefs. Thus, for example in the east of the region, the Banda Arc is divided into an inner (northern) and an outer (southern) arc, the former an extension of the Sunda Arc and the latter a non-volcanic, continental– subduction zone collision (various models have been proposed for the nature of this interaction; Richardson and Blundell 1996). Extensive sea-level fringing reefs (e.g. the Tanimbar Archipelago, the Kai Islands) and raised coral reefs characterize both arcs. On the inner Banda Arc, raised coral reef terraces have been described from Wetar (Nishimura and Suparka 1986) and Alor, where a sequence of six reefs reach 700 m above present sea level. Dating of terraces between Last Interglacial and Holocene age suggest an uplift rate of 1–1.2 mm a−1 (Hantoro et al. 1994). On the outer Banda Arc, uplifted reef terraces have been described from Atuaro Island (consequent upon uplift rates of 0.43– 0.5 mm a−1) and Timor (Chappell and Veeh 1978), from West Timor, Rote, and Sabu (0.2–0.45 mm a−1; Hantoro, in Tomascik et al. 1997a), and the Kai Islands (Charlton et al. 1991). In particular, the probable Sundaland fragment of Sumba supports spectacular sequences of raised reef terraces, thought to represent a 1 million year record of uplift (0.49 mm a−1; Pirazzoli et al. 1993). Over geological time these uplift processes have interacted with fluctuating eustatic sea levels. This interaction is best known from the Huon Peninsula, Papua New Guinea, where continued tectonic uplift (rate = 0.5–4 mm a−1) has resulted from the collision of the Australasian Plate with the Pacific Plate. Superimposed sea-level change has produced a staircase of preserved reef complexes representing high sea-level stands from near present sea level to 600 m (Bloom et al. 1974). The detail within this record is considerable. Thus, for example, a record of Last Interglacial sea levels is preserved in reef terrace vii. Deglaciation began at c.140 ka bp with sea levels rising to a peak at 135 ka bp, 14 m below present levels (Esat et al. 1999). Thereafter, at 130 ± 2 ka bp, sea level appears to have fallen by 60–80 m, in association with a 6– 8°C fall in western Pacific Ocean sea-surface temperatures (McCulloch et al. 1999). After this time sea level rose

again in response to the major solar radiation insolation maximum at 128–126 ka bp, to above present mean sea level. Younger terraces show a sequence of progressively lower high stillstands, indicating the long-term trend of declining water volumes with the growth of the continental ice sheets towards the last glacial maximum at c.18 ka bp. At this time sea level reached 125 m below present sea level. From the different structural and physiographic units identified above, it is clear that the biogeography of the region was differentially affected by such Pleistocene sea-level changes. While the western part of the region was repeatedly subaerially exposed during each glacial lowstand, the eastern part retained deep-water habitats during these periods. For this reason, it seems likely that the western, northern, and southern margins of the region would have been repopulated by corals and associated organisms on sea-level rise from eastern Indonesian refugia, explaining the high levels of diversity but low degree of endemism (generally < 5 per cent) in the region (Wallace 1997; Best 1999). The combination of phases of rapid sea-level transgression, tectonic uplift, and active volcanism must have subjected Pleistocene reefs to considerable environmental stress. Boekschoten et al. (1989) illustrate this point with reference to the reefs of Ambon, where the absence of significant reef framework-building acroporids suggests an extinction event followed by a subsequent failure of recolonization on rapid sea-level rise.

Holocene Sea-Level Change and Reef Growth An Early Holocene sea-level curve from the stable Sunda Shelf has recently confirmed the sea-level rise record established at Tahiti and Barbados, including the presence of a major meltwater pulse between 14.6 and 14.3 ka bp which raised sea level by up to 16 m in 300 years (Figure 24.2a; Hanebuth, Stattegger, and Grootes 2000). For the Late Holocene, a considerable body of morphological evidence — variously in the form of raised abrasion platforms (or ‘Daly shorelines’), raised intertidal notches, raised beachrock and beachridge crests, and fossil oyster, clam, and coral evidence —was assembled from the 1950s onwards from both Peninsular Malaysia (+ 0.5– + 9 m above present sea level; Tjia, Fujii, and Kigoshi 1977) and the Indonesian archipelago (+ 0.5– + 5 m above present sea level; Tjia 1970; Thommeret and Thommeret 1978). Dated sealevel curves were produced for Peninsular Malaysia and the Malacca Strait (Tjia, Fujii, and Kigoshi 1977; Geyh, Kudrass, and Streif 1979; Tjia and Fujii 1992), the South China Sea (Tjia, Fujii, and Kigoshi 1983), and the Sunda Strait ( Jouannic, Hantoro, and Indarto 1985), and all suggested Late Holocene emergence to varying

406 T. Spencer and M. D. Spalding

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(b) Holocene sea-level curves for Singapore (Hesp et al. 1998), Straits of Malacca (Geyh, Kudrass, and Streif 1979), and Peninsular Malaysia (Tjia and Fujii 1992). Present tidal data for Singapore. MHWS = Mean High Water Springs; MSL = Mean Sea Level; MLWS = Mean Low Water Springs. (c) 14c corrected age (correction = 410 years) for fossil reef flat corals against height above highest living open-water corals at Ko Taphao Yai, Phuket, south Thailand

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degrees. However, as Pirazzoli (1991: 115) notes, the precision of these often oscillatory curves is ‘illusory . . . [and] the character of sea-level change (smooth, marked by steps, or by erratic high and low stands) remains uncertain’. A more recent, better-constrained sea-level curve from Singapore (Hesp et al. 1998) indicates that for this apparently tectonically stable region, present sea level was reached at 7– 6.5 ka bp and then rose to + 3 m before beginning to fall to present mean sea level after 3 ka bp (Figure 24.2b). The Singaporean record is echoed by the dating of massive corals protruding from the intertidal reef flat at Phuket, southern Thailand which show that reef growth commenced at this location at 6.0 ka bp when the low spring tide level

was at least 1 m above its present level (Figure 24.2c; Scoffin and Le Tissier 1998). Elsewhere, as might be expected in such a geologically complex region, mid to late Holocene coral reefs record ongoing tectonic instability. Thus, for example, on the Huon Peninsula, Papua New Guinea, six terraces represent episodic uplift events, typically of c.3 m magnitude at intervals of c.1000 years (Ota and Chappell 1996). More detail is provided in this region at Madang, where sea level falls of c.4.5 m (at 3 ka bp) and c.3 m (within 1 ka bp) bracket sea-level rises of 1.5 m (2.4 ka bp) and 0.5 m (1.2 ka bp), indicative of sudden vertical displacements associated with coseismic events (Tudhope et al. 2000).

Coral Reefs of Southeast Asia 407

Contemporary Controls on the Structure and Distribution of Southeast Asian Coral Reefs Many of the geological processes described above continue to influence the structure and functioning of shallow-water ecosystems in Southeast Asia and coral reef development. Again, a fundamental distinction can be drawn between the stable platforms and the tectonically active transition zones that lie around and between them. On the former, sedimentation processes provide important controls, whereas the latter are affected by a range of tectonically driven processes. These controls are in turn overlain by the contemporary dynamics of a highly distinctive regional climatology and oceanography.

Fluvial Inputs, Sedimentation, and Reef Growth Total river discharge on the Indonesian portion of the Sunda Shelf is 9.1 × 1011 m3 a−1; of this, 61 per cent drains Kalimantan, 28 per cent Sumatra, and 11 per cent Java (Edinger and Browne 2000). Many rivers show strong seasonal variations in discharge: thus the Solo River, northeast Java, experiences daily flow rates of 80–250 m3 s−1 during the southeast monsoon but 300–1800 m3 s−1 during the northwest monsoon. Similarly, the Brantas River discharges an additional 600–1200 m3 s−1 per day in the wet season (Hoekstra, Nolting, and van der Sloot 1989). Active tectonism and rapid uplift, heavy rainfall, rapid weathering, both physical and chemical, and catchment degradation, most notably through poorly regulated deforestation, result in very high sediment loads (Ludwig and Probst 1998). Milliman, Farnsworth, and Albertin (1999) estimate that six islands (Sumatra, Java, Borneo, Sulawesi, Timor, and New Guinea) discharge 4.2 × 109 t of sediment annually (or c.3000 t km−2 a−1; Milliman and Syvitsky 1992). Thus although these islands account for only 2 per cent of the land area draining into the global oceans they are responsible for 20–25 per cent of the sediment export (Milliman, Farnsworth, and Albertin 1999). Not surprisingly, rates of coastal progradation are high (Woodroffe 2000). Seaward shoreline advances of c.30 m a−1, and up to 100 m a−1, have been reported from Java (Tomascik et al. 1997a) and Sumatra (Burbridge, Koesoebiono, and Dahuri 1988) respectively. There is evidence to suggest that coral reefs close to large rivers support only low-diversity reef flat communities dominated by a few coral species known to be stress-resistant. Localized impacts may structure shallow

marine communities at the shore; thus van Woesik, De Vantier, and Glazebrook (1995) documented 85 per cent coral mortality at shallow depths (c.1.3 m), and subsequent reef replacement by algal turf communities, following a major storm that reduced inshore salinities to just 8 per mille. However, it has been further suggested (Tomascik et al. 1997a) that these fluctuations result in lowered wet-season salinities (to 16 per mille in Jakarta Bay) sufficient to prevent coral reef growth up to 130 km from river outlets. Furthermore, on the inner Great Barrier Reef, seasonal variations in growth rate have been correlated to patterns of fluorescence in coral skeletons, as this records the incorporation of humic and fulvic acids derived from terrestrial run-off (Boto and Isdale 1985). However, while strong fluorescence has been recorded in coral colonies sampled in the Seribu Islands, Java Sea, no seasonal pattern has been detected (Scoffin, Tudhope, and Brown 1989), suggesting that the seasonal signal seen in suspended sediment concentrations within Jakarta Bay is not preserved offshore. Rather, freshwater plumes typically only affect the top 2 m of the water column, and rapid mixing with sea water dissipates such effects within 2 km of the coastline. A further widely held view is that high turbidity results in low rates of coral growth, partly as a result of the direct smothering of coral surfaces (whose removal has an energetic cost to corals that might otherwise be directed towards growth) and partly as a result of reduced light levels for light-enhanced calcification (e.g. Dodge, Aller, and Thomson 1974). However, in a study of coral growth along a hydraulic energy gradient at Phuket, south Thailand, the highest linear extension rates were observed in the most sheltered but also most turbid sites (Scoffin et al. 1992). However, overall calcification in corals is the product of linear extension and skeletal bulk density. As skeletal bulk density increases with exposure (and see also Brown, Sya’rani, and Le Tissier 1985 for similar findings in the Seribu Islands, Java Sea), overall there appears to be no great variation in coral growth rate between sites of varying turbidity and exposure in south Thailand.

Volcanic, Earthquake, and Tsunami Impacts in Southeast Asia The plate tectonic setting of Southeast Asia exposes its coral reef environments to a range of tectonic hazards. There are, for example, eighty active volcanoes in the Indonesian archipelago, which also experiences 10 per cent of the world’s earthquakes, many of magnitude M = 8 or greater (Tomascik et al. 1997a). Volcanic and seismic events also make the region highly prone to tsunami impacts.

408 T. Spencer and M. D. Spalding

In the short term, volcanic eruptions destroy reefs. The mechanisms include the covering of reefs by new lava flows, the smothering of reefs by volcanic ash, the killing of corals through hot-water stress, and the termination of coral growth by the rapid collapse and subsidence of reef-bearing island flanks. For example, the 1988 eruption of Gunung Api, Banda Sea, blanketed 40 000 m3 of fringing reef with lava and smothered other adjacent reefs with tuffs and volcanic ash. The eruption of Mount Pinatubo, Philippines, in June 1990 led to a reduction in live coral cover from 60–70 per cent prior to the eruption to 10 to 20 per cent a week after the eruption (Atrigenio et al. 1992). However, over more geological timescales volcanic activity can provide valuable new substrates for coral colonization and reef growth. Thus, for example, the rapid collapse of Krakatau during the 1883 eruption eliminated the extant reef communities. A 1981 resurvey of Krakatau revealed a relatively diverse coral assemblage of ninetyseven species representing sixty-two genera. However, these figures suggest relative impoverishment compared to recolonization rates associated with active volcanoes further north and east in the Indonesian archipelago. At Gunung Api, five years after the eruption, a lava flow at a sheltered site supported 124 coral species (some 40 per cent of all species recorded for eastern Indonesia), a high mean coral cover (over 60 per cent), and high growth rates (15 cm a−1 radial extension) in tabular Acropora colonies. Tomascik, van Woesik, and Mah (1996) suggest that these characteristics are the result of the presence of a stable, topographically complex, and predator-free substrate, perhaps also characterized by high nutrient concentrations from adjacent hydrothermal vents. By comparison, neighbouring pyroclastic debris and lavas exposed to wave attack have been subjected to repeated slumping and collapse and only maintained simple coral communities (Tomascik, van Woesik, and Mah 1996). Such localized differences, controlled by substrate type, may explain varying rates of reef recovery after volcanic eruptions elsewhere (e.g. the variable reef development on the north coast of Sumbawa after the 1815 Tambora eruption). Stoddart (1972), at Medang, New Guinea, and Tomascik et al. (1997a), for the Sunda Strait islands, Gunung Api (Banda Sea), and Manumere Bay (Flores Sea), have described earthquake impacts on coral reefs. These include the dislocating, splitting, and overturning of corals on reef tops and the destructive cascading of dislodged corals down reef front slopes. At Medang, this destruction was shortly followed by the influx of terrestrial sediments from land surfaces exposed by earthquake-triggered landslides; at Manumere Bay,

earthquake activity produced debris that was transported a year later under cyclone-generated wave attack. Earthquake activity can also be accompanied by uplift and the widespread mortality of corals and reef-associated organisms, as documented by Pandolfi, Best, and Murray (1994) on the Huon Peninsula, New Guinea, for an event in May 1992. It is estimated that thirty-five tsunamis have impacted the Indonesian archipelago since the catastrophic Krakatau tsunami of 1883, which reached heights of 40–50 m along the coastline of the Sunda Strait (Carey et al. 2001). Recent tsunamis have included the December 1992 Flores event, which reached + 26 m in northeast Flores (Yeh 1993); the Banyuwangi event, on the south coast of east Java, of + 4 m in June 1994 (Tsuji et al. 1995); the Biak event, Papua, of February 1996 (Matsutomi et al. 2001); and the northern Papua New Guinea event of + 15 m in July 1998 (Geist 1998). It is clear that tsunamis transport coral boulders to considerable heights above sea level, but it has proved difficult to isolate the effect of a tsunami on a reef from the effects of the earthquake that generates it.

Oceanography of Southeast Asian Seas The oceanography of the core region of the Southeast Asian seas is strongly influenced by the seasonally reversing wind systems of the Asian monsoon and its regional variation (Figure 24.3a; Salm and Halim 1984). By contrast, the margins of the region are affected by the climatology and oceanography of the bounding Pacific and Indian Oceans, and large wind wave and ocean swell waves characterize the outer boundaries to the region. Tidal range varies from the micro-tidal (e.g. Seribu Archipelago ) to the macro-tidal (e.g. 8 m at Selat Muli, Papua). Diurnal tides are characteristic of the Java Sea, with semi-diurnal tides to the west and mixed tides to the east and north. Strong tidal currents characterize narrow straits, such as the Lombok Strait. Tidal regimes are varied because of tidal propagation from both the Pacific and Indian Oceans and their interaction with a complex bathymetry and coastal configuration. This dynamic points to the significance of the island archipelagos of Southeast Asia in acting as a complex barrier to the linkage of the Pacific and Indian Oceans. The movement of water from the ‘warm pool’ of the western Pacific Ocean to the eastern Indian Ocean, described by the term ‘Indonesian Throughflow’, has important consequences for global heat, mass and salinity transfers, and, through the coupled ocean–atmosphere system, to global climate and its variability. During the boreal winter within the Indonesian archipelago, rainfall is high over Sumatra, Java, and Borneo, leading to surface waters of lower salinity (30 per mille)

Coral Reefs of Southeast Asia 409 SOUTH CHINA SEA

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(b) Schematic pathways of the Indonesian Throughflow NECC = North Equatorial Counter Current; SEC = South Equatorial Current. (Source: After Gordon and Fine 1996)

but high suspended sediment load being driven eastwards, at current speeds of 25–38 cm s−1, by winds from the northwest through the Java Sea. During the northern summer months both wind directions, surface currents (typically 25 cm s−1), and suspended sediment pathways are reversed (Phipps and Roberts 1988). Mapping in the Seribu Islands, Java Sea, in the 1920s by Umbgrove showed that the reefs are typically characterized by shingle ramparts, sand cays, and, in some cases, lagoons, all resting asymmetrically on an underlying coral platform. Umbgrove (1929) ascribed the dominant north, northeast, and east locations of ramparts, and the typical southwest position of the sand cays, to a wind-wave climate driven by winds from the south east. Annual reversing winds were also responsible for seasonal sediment redistributions on sand cays (Umbgrove 1930a). Subsequently, Verstappen (1954) was able to demonstrate inter-decadal changes in sand cay position from longer-term variations in the relative strengths of the monsoon wind directions. The continued dynamism of these islands is evident from Stoddart’s (1986) more recent remapping (and see also Verstappen 1988). Similar patterns characterize the Sulu Sea, which shows a cyclonic flow during the northeast monsoon and anticyclonic circulation during the southwest monsoon. The circulation system of the South China Sea also shows strong seasonal variability as a result of its semi-enclosed configuration and seasonally reversed wind forcing. During the northeast monsoon, inflow is greatest from the Pacific through the Luzon Strait at the northern extremity of the Philippines, and coastal currents flow southwards along the coast of Taiwan, Viet Nam, and the eastern coast of Peninsular Malaysia, advecting cooler and more saline waters from the north. The narrow and shallow opening to the Java Sea constricts these flows and promotes northward currents along the west coasts of Borneo, Palawan, and Luzon. These currents form the eastern limb of a cyclonic gyre in the northwestern South China Sea. During the southwest monsoon, current flows along the Asian continental margin and through the South China Sea are predominantly to the north, with a strong western boundary current, bringing warmer and less saline waters from the south, particularly to the eastern South China Sea (Chu, Edmons, and Fan 1999). These reversals are also reflected in changes in the position of sea-level highs and lows, as deduced from satellite altimetry (Shaw, Chao, and Fu 1999). As might be expected from this oceanographic forcing, reef development on carbonate platforms in the South China Sea is concentrated on northeast and southwest margins,

410 T. Spencer and M. D. Spalding

with detrital material being transported to infill the areas between these two growth foci. As the intensity of the northeast monsoon increases to the north, so reef growth and morphology becomes increasingly asymmetrical, with, for example, northeast reef flats becoming wider than those to the southeast. The interior seas are all fetch-limited and lie between the two tropical storm belts. They are, therefore, largely unaffected by tropical cyclones and suffer only localized squalls. However, when storms do occasionally enter these waters, they can cause considerable damage. Thus cyclone Lena destroyed fringing reefs in Manumere Bay, Flores Sea, in January 1993, not only from wave attack but also by burial under terrestrial sediments brought into the bay as a result of high rainfall associated with the cyclone (Tomascik et al. 1997a). The north coast of Papua and Halmahera are affected by swell waves associated with cylones impacting the Philippines to the north. Furthermore, the eastern part of the region is affected by a ‘storm corridor’, running northeast– southwest over the island of Timor (Tomascik et al. 1997a). Typhoon tracks typically pass to the north and east of the Philippines in early season (June–August) but then track progressively further south as the season develops, reaching the central Philippines in November– December. An average of nineteen tropical cyclones per year pass through the archipelago (Jacinto et al. 2000). Tropical cyclones also affect the Andaman Sea reefs and coasts of north Thailand.

The Indonesian Throughflow The prime control on the Indonesian Throughflow are the southeast trades, which normally result in higher ocean levels in the western Pacific Ocean (c.+ 15 cm) mirrored by comparably lower ocean levels in the eastern Indian Ocean (i.e. − 15 cm). This pressure gradient, greatest in the eastern part of the region, sets up a transport, largely restricted to the upper few hundred metres of the water column and variously estimated at 5–15 Sverdrups (Sv; 1 Sv = 1 × 106 m3 s−1) from observations (Gordon and Susanto 1999), with some higher estimates from physical modelling. Salinity, temperature, and chemical tracer data show that the Indonesian Throughflow is dominated by two components, reflecting the fact that it draws water from near the division of the North Pacific and South Pacific water mass fields. The eastern coastline of the Philippines archipelago at 5–15°N forms a barrier to the westward-flowing, low-salinity North Equatorial Current of the Pacific Ocean. Here the current splits into a northward-flowing branch (which forms the root of

the Kuroshio Current) and a southward-flowing branch known as the Mindanao Current (Bingham and Lukas 1994). This in turn splits into two flows, one feeding the return flow of the Equatorial Countercurrent and a western limb which enters the Celebes (Sulawesi) Sea over the Sarangani Sill (1629 m water depth; the sill thus blocks deeper flows) and then flows south through the Makassar Strait. About 20 per cent (i.e. c.2 Sv) of the flow through the strait exits directly through the Lombok Strait (Arief and Murray 1996). The remainder accumulates in the Flores Sea and Banda Sea, where it is modified by high precipitation inputs and strong, tidally driven vertical mixing (Ffield and Gordon 1996), before entering the Indian Ocean through the Timor and Ombai Straits. Some shallow flow from the North Pacific also enters the Molucca Sea, passing over the Lifamatola Sill (1950 m) into the Banda Sea, but the major contribution here is from the second component of more saline South Pacific waters at depths below 300 m (Figure 24.3b). There is considerable variability in the Indonesian Throughflow over a range of timescales. On an intraseasonal basis, DwiSusanto et al. (2000) detected 67– 100-day periodicities in wind-forced Kelvin waves from the Indian Ocean through the Lombok Strait into the Makassar Strait and 48– 62-day period Rossby waves from the Celebes Sea into the Strait. Between the seasons, the throughflow is greatest during the southeast monsoon ( June –August) and lowest during the northwest monsoon (December–February). Detailed patterns of flow are, however, complex, as a result of the disposition of islands and inter-island sills and basins, and because the flows themselves are affected by precipitation, run-off, and a wide range of intense internal mixing processes promoted by the rugged bathymetry. Thus, for example, in the southeastern Sulu Sea, large internal solitons are generated by the interaction of strong tidal flows with the Pearl Bank Sill. The solitons have wavelengths of 516 km, periods of 35–55 minutes, phase speeds of 1.8–2.6 m s−1, and a travel time to the Palawan coast of 2.5 days, where they drive coastal sieches (Giese et al. 1998). These mixing processes are also associated with regional upwelling. This occurs off northwest Luzon (Shaw et al. 1996) and southwest Sumatra during the northeast monsoon (December– February), in the Makassar Strait, east Banda Sea, and Arafura Sea during the southeast monsoon, and along the south coast of Java, Bali, and Sumba during the peak annual development ( June –August) of the southeast trades. Upwelling in the Banda Sea reduces sea-surface temperatures from 29–30°C to 26°C and, by bringing nutrients into surface waters from depths of

Coral Reefs of Southeast Asia 411

100–200 m, produces average gross production rates here and in the Arafura Sea of 500 g C m−2 a−1, with recorded maxima of 2555 g C m−2 a−1 (Gieskes et al. 1990). Field measurements have shown how relatively cool upwelling waters rise from depths of 60–90 m to spill over onto the fringing reefs of the Banda Islands and may be an important nutrient source for these reefs (Wetsteyn et al. 1990). Furthermore, the central and northern Mahakam Shelf (Roberts and Sydow 1997), and the carbonate platform of Kalukalukuang Bank, rising from the deep (600 m) sea floor of the southern Makassar Strait (Phipps and Roberts 1988), are characterized in places by 20–50 m thick bioherms of the calcareous alga Halimeda; it has been argued (H. H. Roberts, Aharon, and Phipps 1988) that these algal growths outcompete reef growth under the high nutrient conditions introduced by the periodic upwelling of nutrient-rich Pacific throughflow over the bank margins.

El Niño Southern Oscillation Events and Coral Bleaching in Southeast Asian Waters The surface waters of the tropical Pacific Ocean are usually characterized by a relatively deep (100–200 m) western ‘warm pool’ (29–30°C) and a shallow (< 75 m) cool tongue (22–4°C) in the east, the latter resulting from the upwelling of cold, nutrient-rich ocean deep water driven by the trade winds converging on the warm pool. This state can be described by the sea-level pressure differences between Darwin, north Australia (as indicative of ascending warm and moist air over the western warm pool), and Tahiti (measuring the dry descending air of the South Pacific High). This difference, in standardized form, is known as the Southern Oscillation Index and major perturbations to the oceanatmosphere conditions it represents are known as El Niño Southern Oscillation (ENSO) events (Philander 1998). Warm phase perturbations, when pulses of warmpool water move rapidly eastwards and ocean warming occurs across the entire equatorial Pacific Ocean, are known as El Niño events. These events last from eighteen to twenty-four months and occur every two to ten years (x¯ = 3.2 years). Their intensity varies: thus the last two decades have seen very strong events in 1982–3 and 1997–8, a moderate event in 1986–7, and a weak but unusually prolonged episode in 1992– 4 (Slingo 1998). An El Niño event is often followed by unusually cool conditions, characterized by vigorous upwelling along the coast of South America and along the Equator, a phenomenon known as a La Niña event (Palmer and Webster 1997). These reorganizations of El Niño and La Niña events are also accompanied by sea-

level changes which have implications for the Indonesian Throughflow. Windstress across the equatorial Pacific Ocean usually sustains relatively high sea levels in the western Pacific, on average 45 cm higher than the eastern Pacific. When the southeast trades weaken or even reverse to westerlies with the onset of El Niño, warm water is able to move rapidly eastwards and western Pacific Ocean sea levels fall. Thus, for example, levels of − 8 to − 36 cm and − 20 to − 60 cm below mean sea level were recorded during the 1976–7 moderate and the 1972–3 strong events respectively (Wyrtki 1979). In the 1982–3 event, sea levels 15 cm below non-El Niño years were recorded in the Java Sea (Tomascik et al. 1997a). Under such conditions, Indonesian Throughflow transport is significantly reduced. Conversely, during La Niña episodes, as a result of strengthened trades, sea levels are higher than usual in the western Pacific, the warm pool well developed, and the thermocline deep. These characteristics drive a strong throughflow. Thus while Makassar Strait transports averaged 12 Sv in the La Niña period of December 1996–February 1997, they reached only 5 Sv during the El Niño months of December 1997–February 1998 (Gordon and Susanto 1999). At the same time as El Niño events take place in the Pacific Ocean, it is now becoming clear that a similar sequence of events characterizes the Indian Ocean. These episodes are, however, a mirror image of the Pacific fluctuations, with low sea levels in the east and high sea levels in the west (Chambers, Tapley, and Stewart 1999). Anomalously low sea levels have been described from western Sumatra (Potemra and Lukas 1999) and Thailand (Brown and Dunne 2001), in the latter case, reinforcing high solar irradiance receipts by reef flat corals (see below). El Niño warm events have also been linked, through migrations of warm-season ocean ‘hot spots’ and localized heating, to coral bleaching. Corals appear to have narrow thermal tolerances and to live near the upper limit of that tolerance, being adapted to local mean maximum summer temperatures (Jokiel and Coles 1990). When this threshold is exceeded (perhaps accompanied by associated changes in solar irradiance and/or other environmental factors), corals respond by whitening or ‘bleaching’ (Hoegh-Guldberg 1999). Bleaching is the visible manifestation of the loss of symbiotic unicellular algae, zooxanthellae, from coral tissues, and/or the loss of photosynthetic pigmentation from the zooxanthellae (Brown 1997). The zooxanthellae play key roles in photosynthesis and carbon fixation and their loss or reduced function is accompanied by reduced skeletal growth (Goreau and Macfarlane 1990). Furthermore, bleached corals show reduced reproductive ability which

412 T. Spencer and M. D. Spalding

Fig. 24.4. Coral bleaching events recorded from Southeast Asia in 1998 Points show intensity of bleaching from various records, from darkest to lightest, representing: dark grey = high (> 50 per cent corals bleached); mid-grey = medium (10–50 per cent); light grey (< 10 per cent). Open circles represent areas where there was no bleaching. (Source: Spalding, Ravilious, and Green 2001, with permission)

may slow coral recruitment after disturbance (Glynn 1996). As different coral genera show differing susceptibilities to extreme sea-surface temperatures, these events may restructure reef communities. Thus, for example, Hoeksma (1991) reported that shallow-water fungiids showed less bleaching than other corals during the 1983 bleaching event in the Java Sea. Southeast Asian reefs might be thought to be particularly susceptible to bleaching because of usually high (27–28°C) equatorial sea-surface temperatures, the presence of clear skies during El Niño episodes (as occurred in the Java Sea in 1983; Brown and Suharsono 1990), and high UV penetration in the still waters of the internal seas. Coral bleaching and bleaching-related coral mortality across the major coral reef regions in 1997– 8 was the most extensive since 1982–3 (Wilkinson 2000), but while bleaching was reported throughout Southeast Asia there was no clear pattern to the bleaching incidences (Figure 24.4). More informative are longer-term records of bleaching, and related recovery and coral mortality, from individual sites. Such detailed records are available for two sites in the Southeast Asian region and offer considerable insights into the complexity of the bleaching process and coral ecological responses to it. Reefs in the Pari Group, Seribu Islands, in the Java Sea experienced severe ENSOrelated bleaching in April–May 1983 with decreases in live coral cover of 84–92 per cent on shallow reef flats (Brown and Suharsono 1990). Interestingly, recovery was quite different at sites separated by only 2.5 km, showing the importance of the sequencing of environmental

impacts. At the more exposed South Pari site, recovery was initially rapid with pre-bleaching coral cover levels being reached by 1990. In 1991, however, a storm carried old debris from the 1983 event onto the reef flat, where it abraded and blanketed the live coral, setting in train a reduction in coral cover to less than 5 per cent by 1994. By comparison, at the sheltered South Tikus site, no storm damage resulted and thus coral cover at this site in 1994 was similar to pre-bleaching levels (Brown 1997). At Phuket, south Thailand, bleaching was extensive in 1991 and 1995, when sea-surface temperatures exceeded seasonal (May) maximum by 0.65– 0.66°C, with a threshold to bleaching established at threshold of 30.1°C (Figure 24.5a; Brown, Dunne, and Changsang 1996). In 1997 and 1998, when maximum temperatures reached 30.5°C, bleaching was considerably reduced compared to earlier events (Figure 24.5b). Brown and Dunne (2001) argue that this is because corals were subjected to periods of reef flat exposure and high solar irradiance prior to the onset of potentially damaging sea-surface temperatures; this pre-conditioned corals by building up photoprotective mechanisms ahead of the potentially most damaging conditions. The relationship between solar irradiance, water depth, and turbidity is clearly an important control on bleaching, as has been demonstrated at other sites in the Andaman Sea. Thus after the 1995 bleaching event, Acropora spp. on inshore reefs in turbid waters showed very little bleaching-related mortality, whereas there was extensive bleaching and subsequent mortality (c.80 per cent)

Coral Reefs of Southeast Asia 413

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a progressive decline over the period 1990–5, which appears to be due to the interaction of bleaching episodes with continued sediment-loading from shipping movements and repeated channel-dredging adjacent to the reef (Brown 1997). Finally, it seems likely that ENSO events have structured Southeast Asian reefs over long timescales; fossil reefs from the Last Interglacial, at 124 ka bp, in northern Sulawesi (Hughen et al. 1999) record an El Niño signal. Studies by Glynn and Colgan (1992) have estimated that 12–30 El Niño events have interrupted reef-building in the eastern Pacific over the last 6000 years, and in the period 1728–1983, Quinn, Neal, and Atunez de Mayolo (1987) identified forty-seven ‘strong’ to ‘very strong’ El Niño warm events.

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(b) Fig. 24.5. (a) Monthly mean sea-surface temperatures (1990–2000) for the sea area around Phuket, south Thailand 30.1°C is regarded as the local temperature threshold to coral bleaching

(b) Percentage of total coral cover by visual bleaching categories on line transects at Phuket, May 1991, 1995, 1998 (Source: after Brown and Dunne 2001)

to depths of 6 m and 10 m in the Surin and Similian Archipelagos respectively, suggesting increased tolerance in inshore corals and/or greater light penetration at well-flushed offshore sites (Phongsuwan 1998). Furthermore, long-term monitoring of reef flat coral cover at Phuket since 1979 highlights the importance of the interaction of these natural processes with anthropogenic impacts. The major impact on mid- and outerreef flats occurred in 1987 as a result of dredging associated with deep-water port construction (Brown et al. 1990). Coral cover at these locations, and on the outer reef slope (Tudhope et al. 1993), has registered

All modern biogeographic syntheses (e.g. Stehli and Wells 1971; Veron 1995; Wallace 1997) identify a centre of high coral diversity in the Indo-Pacific province delimited by the Philippines–Borneo–Papua New Guinea triangle. In this core 70 coral genera and over 400 species are present (Figure 24.6). This core region has been further refined by Best et al. (1989) who identify a 75-genera region which itself includes an inner core of 79– 80 genera (Hoeksma, in Tomascik et al. 1997a). Tomini Bay, northeastern Sulawesi, supports 77 species of Acropora alone (Wallace 2000). Such patterns are mirrored in similar centres of biodiversity for, among other taxonomic groups, mangroves (Spalding 2001), seagrasses, marine molluscs, and reef fish (Thresher 1991; C. M. Roberts et al. 2002). Space precludes a detailed review of the various hypotheses which have been proposed to explain these patterns. A simple distinction separates out those hypotheses that see the area as a centre of origin for corals and subsequent outward dispersion (e.g. Rosen’s (1984) ‘eustatic diversity pump’ model) and those that see the region as a sink or accumulation area for species which have evolved elsewhere (e.g. Jokiel and Martinelli’s (1992) ‘vortex’ model). A third, ‘survivalist’ model sees these distributions as a vestige of an older, rich, and cosmopolitan distribution (e.g. McCoy and Heck 1976). Further elaborations of these ideas look at how both sets of hypotheses might be combined and incorporate the plate tectonic, climatological, and oceanographic controls outlined in this chapter (e.g. Paulay 1997; Wallace 1997; Wilson and Rosen 1998). In the context of this review, regional reef, and other, distributions

Fig. 24.6. Patterns of diversity in hermatypic Scleractinian corals in Southeast Asia and adjacent waters Isolines separate areas of different diversity, with the darkest shading representing > 400 species and subsequent paler shades representing 300–400 species, 200–300, 100–200, 50–100, and < 50 respectively. (Source: Spalding, Ravilious, and Green 2001, with permission; data kindly provided by J. E. N. Veron)

Coral Reefs of Southeast Asia 415

emerge (Plate 3) when these biogeographic patterns are superimposed under the geological, sedimentological, and oceanographic controls already described.

5000 a BP sea level

Sundaland In much of this region, substrate type and terrestrial sediment inputs control the distribution of shallow marine communities (Woodroffe 2000). The Gulf of Thailand is a semi-enclosed basin reaching maximum depths of c.80 m. It is affected by freshwater discharge and sediment inputs from Thai rivers (and probably the Mekong River to the east) and characterized by high turbidity under the reversing monsoon wind systems. Reefs are restricted to shallow depths (2–8 m) at monsoon-sheltered sites on the gulf margins and to rocky outcrops further offshore (Phongsuwan 2000). Similarly, on the west coast, facing the Andaman Sea, fringing reefs are associated with sites sheltered from the southwest monsoon. At more exposed sites, with greater water clarity, true fringing reefs are not present; instead clumps of massive and branching corals sit in plains of skeletal sands and gravels. The fringing reefs of southeast Phuket, southern Thailand, are typical of reef development under conditions of high sedimentation and poor underwater visibility. Intertidal reef flats are up to 300 m wide, with a morphology and species composition that varies with sedimentation rates and wave exposure (Ditlev 1978). Interestingly, certain species of massive coral on these reef flats are affected by almost annual ‘solar bleaching’, where between February and April coral bleaching is restricted to discrete patches on the western sides of colonies (Brown et al. 2000). The Phuket reefs terminate in a narrow reef front of large living coral colonies above a muddy forereef only a few metres deep. Tudhope and Scoffin (1994) have shown how these reefs prograde by the splitting, through bioerosion from boring shrimps and bivalves, of reef-front corals (often of Porites lutea) and the subsequent toppling of large blocks into the reef-front muds. The blocks are able to protrude above the mud and thus regenerate and take the reef front forward, at a rate of 40 mm a−1 at sheltered sites (Tudhope and Scoffin 1994) and 17 mm a−1 at more exposed sites (Scoffin and Le Tissier 1998) over the last 6 ka years; a model of reef development is shown in Figure 24.7. Further north, the Surin and Similian Archipelagos are located offshore and near the shelf margin respectively and are characterized by reefs reaching water depths of 25–30 m. This compares to coral growth to only 3–10 m at inshore, turbid sites in this area. These reefs form the southern extension of the c.800 islands that comprise the Mergui Archipelago, characterized by diurnal tides

rock foundation

3000 a BP sea level

1000 a BP sea level

intertidal sand waves

Present intertidal reef flat sea level fore-reef muddy sediment

Fig. 24.7. Model of Late Holocene reef growth in a muddy environment with assumed slight (1 m) sea-level fall over last 5000 years (Source: after Tudhope and Scoffin 1994)

and a macro-tidal range, which is finally terminated at c.13°N by the muddy shallows of the Gulf of Martaban and the Irrawaddy Delta. The Malacca Strait is characterized by extensive subtidal and intertidal mudflats, large areas of mangrove forest, and some seagrass beds. Reefs are largely restricted to the Singaporean offshore islands and to the Riau and Lingga Archipelagos (Mohamed and Badaruddin 1991; Chua et al. 2000). On the eastern side of the Malay Peninsula, mangrove-fringed estuaries and lagoons and tidal flats are also characteristic, with fringing reefs on rocky offshore islands (Mohamed and Badaruddin 1988; Ibrahim et al. 2000). Borneo is largely a mudflatand mangrove-fringed island, a product of large river

416 T. Spencer and M. D. Spalding

systems, high sediment loads, and, in many places, a wide continental shelf on which terrestrial sediments can accumulate. It has been argued, for example, that the Mahakam Delta, on the western margin of the Makassar Strait, prevents reef development. Deltaic deposits 50 m thick have accumulated over the last 7000 years, during which time the delta has prograded 50 km onto the continental shelf (Roberts and Sydow 1997). With a discharge of 1500 m3 s−1 and a typical sediment yield of between 3.78 (Dutrieux 1991) and 10 × 106 tonnes a−1 (Eisma et al. 1989), the sediment plume from the delta may extend up to 50 km offshore and 400 km to the southeast and be responsible for the paucity of reefs in south Kalimantan (Hopley and Suharsono 2000). In addition, southwestern Borneo reefs are restricted to small patch reefs between the major rivers and to offshore islands. However, fringing reefs are well developed in northwest Borneo, offshore from Kota Kinabalu, and particularly along the Bornean east coast, north of the Mahakam Delta (Oakley, Pilcher, and Wood 2000). Here barrier reef complexes, with numerous patch and platform reefs, characterize the delta-front environments of the Mangkalihat Peninsula and the Berau River (Oakley, Pilcher, and Wood 2000). Further offshore are three large lagoonal shelf reefs—Muaras, Maratua, and Kakaban—the latter two having raised limestone rims up to 200 m above present sea level (Kuenen 1933; Umbgrove 1947; Tomascik et al. 1997b; Hopley and Suharsono 2000). Perhaps the most remarkable reef system is the little-known Great Sunda Barrier Reef, with a length of over 600 km (well over twice as long as any other barrier reef in the Indonesian archipelago), which lies 60 km offshore, equidistant between Borneo and Sulawesi. The reefs of the Tambelan, Anambas, and Natuna Archipelagos are found on the shelf between the Malay Peninsula and southeast Borneo. In the Java Sea, platform reefs are well developed around Bangka and Belitung Islands and in the Karimata Archipelago to the west and in the Karimunjawa, Bawean, Sapudi, and Kangean Archipelagos. These reefs are far enough from coastal margins for water clarity and nutrient conditions to be favourable for reef growth to depths of 25 m (Edinger and Browne 2000). In the Java Sea, the Kepulauan Seribu (or Thousand Islands) form a chain of over 300 reef platforms (supporting over 700 individual reefs) stretching 80 km in a northwesterly direction from Jakarta Bay. The islands have a long history of study, from J. H. F. Umbgrove in the 1920s and J. Verwey in the 1930s, through Verstappen in the 1950s, to reassessments in the 1980s (Ongkosongo and Sukarno 1986). Individual

reef groups sit on a submerged ridge but separated by east-west–trending deep channels. The islands of the Pari Group at the southern end of the Seribu island chain are typical: the islands are surrounded by extensive fringing reefs characterized by a sand flat, reef flat, reef edge, and reef slope which drops to − 20 m. The fringing reefs are dominated at their outer margins by branching Acropora spp. and foliose Montipora spp. (Brown et al. 1983).

Tectonic Transition Zones Many fringing reefs associated with the central Sunda Arc are only 30–100 m wide, with reef flats that dry at low tide and steep seaward margins (Hopley and Suharsono 2000), although more extensive reefs are known. In southern Bali, for example, fringing reefs are typically 400–500 m wide, with a 0.5–1 m deep lagoon behind an algal ridge margin. At sites exposed to Indian Ocean swell, the algal ridge reaches + 1.7 m above datum and dries at low spring tides, the forereef slope is characterized by good coral cover, and a spur-and-groove topography and coral growth extends to − 15 m. At more sheltered and turbid water sites, the algal ridge is at + 0.5 m, there is no spur-and-groove, and coral growth extends to only − 8 m (van Woesik 1997). Further east, at Komodo, similarly swell-exposed reefs are dominated by encrusting or low-branching coral growth forms among considerable skeletal debris (Borel-Best, Moll, and Boekschoten 1985). To the north and west, the Sumatran islands associated with the accretionary ridge of the subduction complex, and near the shelf edge, all support fringing reefs and/or barrier reefs; inner shelf reefs and mainland fringing reefs are more restricted in extent (Kunzmann 1997). Coral micro-atolls record historical changes in sea level in these locations and show that while the Mentawai Islands have been submerging at 4–10 mm a−1 over the last forty to fifty years, the mainland coast has remained relatively stable (Zachariasen et al. 2000). The subduction zone complex continues north of Sumatra into the volcanic island chain of the Nicobar and Andaman Islands, an archipelago of some 340 reef-fringed islands (Sewell 1922, 1935; Scheer and Pillai 1974). To the east, beyond the Banda Arc, the tectonic transition zone reaches northern Papua, where it interacts with the Sorong Fault system. This system has stripped continental terranes from the northern margins of the Australian block and fed them westwards to accrete onto Eurasia (e.g. Ambon–Seram, Buton and southeast Sulawesi, east Sulawesi and the Sulu spur). The coral communities of Ambon Bay have been described by Borel-Best, Moll, and Boekschoten (1985); colonies tend

Coral Reefs of Southeast Asia 417

to be small, composed of sediment tolerant species, and separated by patches and channels of silty sand. The meridional element of this transition zone extends through Sulawesi and the Moluccas to Mindanao, and further north to Luzon, in the Philippine archipelago. This transition is similarly structurally complex. The Molucca Sea, for example, is bounded to the west by the highly active (Whitten, Muslimin, and Henderson 1987) Sangihe volcanic arc, supporting the Karkaralong and Sangir reef archipelagos and to the east by the Halmahera Arc with its extensive fringing reefs (Tomascik et al. 1997b); between the two is the Talaud–Mayu ridge, with the reefs of the Nanusa and Talaud Archipelagos, representing the uplifted product of the collided accretionary wedges from both arcs (Moore et al. 1981). The island of Sulawesi and its surrounding waters support extensive fringing reefs, barrier reefs, atolls, and platform reefs. The island’s perimeter, at almost 5000 km in length, is almost continuously fringed with reefs up to 200 m wide (Hopley and Suharsono 2000). Tomascik et al. (1997b) list thirty-four individual barrier reefs with a total length of over 2000 km; eight of these systems are over 100 km in length. Two of these systems have been well studied. In northeastern Sulawesi, in Tomini Bay, a barrier reef system, with fringing reefs, patch reefs, and atoll-like structures, extends around, and eastwards from, the Togian Islands (Umbgrove 1939a, 1940; Moll 1986; Wijsman-Best, Moll, and De Klerk 1981). Under the prevalent low-energy conditions, the barrier lacks well-developed algal ridges, reef-front spur-and-groove topography, or shingle ramparts. In southwest Sulawesi, the Spermonde Archipelago lies 60 km offshore and consists of four lineations of coral islands, submarine reefs, patch reefs, and fringing reefs (Umbgrove 1930b; Wijsman-Best, Moll, and De Klerk 1981). Coral growth on these structures is asymmetric, with vigorous coral growth and reef flat development to the west but poor coral growth on eastern flanks, probably as a result of high rates of sedimentation. What have been variously termed atolls, platform reefs, and open-water reefs occur in clusters to the south of Sulawesi in the Flores Sea. Three types of island groups have been identified. The first type are large structures over 30 km in length and 10 km in width, consisting of an island rim of large table reefs separated by channels and enclosing a lagoon filled with large patch reefs. Take Bone Rate (UNEP/IUCN 1988) is typical of this type. The second type are medium-sized atolls with a continuous rim enclosing a deep lagoon, generally breached by a single channel. Finally, there are isolated oceanic platforms of small size and without welldeveloped lagoons. Some island groups clearly record

complex histories. Thus the Tukang Besi Archipelago, southeast Sulawesi, comprises a linear array of sea-level atolls and raised reefs (Umbgrove 1947), indicative of localized faulting and uplift on a subsiding platform of complex topography (Kuenen 1933).

Marginal Seas The South China Sea is characterized by a deep central basin, reaching a maximum depth of 4.6 km; continental shelves, wide to the south and north and typically less than 250 m below present sea level; and a continental slope indented with deep submarine troughs and canyons between the shelf edge and the deep basin. True oceanic coral atolls are found at c.15°N; Scarborough Reef (Huangyan Dao), for example, is a large, submerged atoll based on a volcanic basement which rises from a water depth of over 4 km (Wang 1998). Further west are the shelf atoll and platform reefs of the Paracel Islands, the fringing reefs around the island of Hainan (Wang, Lu, and Quan 1990), and the fringing and barrier reefs in the Gulf of Tonkin. Dominant numerically, however, are the shelf edge reefs. Over 200 shoals, reefs, and islands characterize the Spratly Islands, which cover an area of over 1000 km2, 200 km west of the Philippines. One hundred and forty species of corals from 58 genera have been recorded at Layang Layang (Swallow Reef) (Pilcher and Canaban 2000). Atolls on the continent slope are aligned northeast– southwest or east–west, reflecting underlying structural tectonics. Contemporary reef morphologies show considerable variability in terms of overall dimensions (range of atoll diameters: 2– 62 km), degree of lagoon closure by encircling reefs (from totally open to completely closed), reef flat width (2.5–5.5 km), and lagoon depth (3–100 m). They appear strongly controlled by the topography of underlying Pleistocene carbonate basements as Holocene reef growth is typically less than 20 m in thickness (Wang 1998). The Sulu Sea is divided into two basins by the Cagayan Ridge, which supports the atoll-like systems of the Cagayan Islands (Alcala, in UNEP/IUCN 1988) and the Tubbataha Reefs. The northern basin is a relatively shallow, sediment-covered shelf, whereas the southern basin is deeper and floored by oceanic crust. The northern margin of the basin extends from northernmost Borneo, through Palawan, to Mindoro in the Philippines, and is characterized by a submerged, double barrier reef type structure on the western margin of Palawan as well as extensive fringing reefs and the atoll-like Apo Reef in the Mindoro Strait. The southern boundary, from northeast Borneo to Zamboanga, Philippines, supports the extensive (400 +)

418 T. Spencer and M. D. Spalding

reef systems of the Sulu Archipelago; to the south is another marginal sea, the flat-floored, 4–5 km deep Sulawesi Sea. Unlike the shallower seas of the region, the interior of the Banda Sea is not characterized by extensive platform reef systems. Reefs are restricted to fringing reefs around active volcanoes, such as Gunung Api, 120 km south of the island of Seram.

Threats to the Reefs of Southeast Asia The wider region of Southeast Asia is home to 500 million people, 90 per cent of whom live within 100 km of a coast (Burke et al. 2000). On the island of Java alone, the urban–industrial complexes of Semarang, Surabaya, and Jakarta have populations of 1.5, 3, and 10 million respectively, with population densities reaching over 1000 persons per km−2. It seems inevitable that these pressures will increase. Thus, for example, with a population of 68 million, the Philippines is the ninth most populous country in Asia. At the current growth rate of 2.3 per cent, the population will double to 128 million by 2025. Major environmental threats to the reef seas of Southeast Asia come from rural, industrial, and domestic land-based pollution and from a range of marine resource extraction activities, including oil production, shipping and refining, capture fisheries, aquaculture, and coral-mining (Brown 1986; Edinger and Browne 2000). Coral reef surveys in the 1990s showed 49 per cent of western, 37 per cent of central, and 29 per cent of eastern Indonesian reefs to be in poor condition (Edinger et al. 1998). In East Malaysia (Sabah and Sarawak), only 10 per cent of reefs have less than 10 per cent dead coral (Pilcher and Canaban 2000), and in the Philippines, only 4.3 per cent of coral reefs have been assessed as being in excellent condition (at least 75 per cent live coral cover) (Licuanan and Gomez 2000).

Terrestrial Impacts on the Coastal Zone Rural Land surface modification and clearance associated with agriculture, commercial forestry, and mining magnify the naturally high delivery rates of sediments and nutrients to coastal waters in this region (Wolanski and Spagnol 2000). These impacts are further exacerbated by the removal of coastal forests and mangroves for forest products, land clearance, and conversion to aquaculture (Ong 1995). In the 1980s Ong (1982) estimated that 1 per cent of Malaysian mangrove was being lost each

year; further recent figures suggest a 75 per cent loss of total original cover by the early 1990s (Burke et al. 2000). Mangrove areal loss rates have been estimated at 25 per cent (for the period 1975–99) for Sarawak, 42–81 per cent (1982–90) for Kalimantan (Oakley, Pilcher, and Wood 2000), c.50 per cent (1961–91) for the Gulf of Thailand (Hungspreugs, Utoomprurkporn, and Nitithamyong 2000), and 67 per cent (by late 1980s) for the Philippines (Burke et al. 2000). TalaueMcManus (2001) suggests that all Southeast Asian mangrove forests will be gone by 2030. Agricultural run-off is a growing problem as it contains very large quantities of fertilizers and smaller quantities of pesticides. Total fertilizer use has been estimated at 0.11, 0.18, and in excess of 5.6 × 106 tonnes for Viet Nam, the Philippines, and Indonesia respectively. Pesticides contain significant toxic elements, including persistent organic pollutants; for Indonesia, loading of 29 × 103 tonnes a−1 between 1992 and 1996 has been estimated (Chia and Kirkman 2000). Mining activities generate high inshore turbidity from mining processes and introduce metals into the shallow marine environment, both of the product mined e.g. tin (Phuket reefs; Brown and Holley 1982; Changsang 1988) and aluminium (Bintan, Riau Archipelago; Burbridge, Koesoebiono, and Dahuri 1988) and from by-products of the mining process (e.g. mercury pollution of estuaries in Kalimantan from within-river mining of placer gold deposits; MacKinnon et al. 1996). In addition, untreated sewage and poorly regulated industrial effluents are discharged into the nearshore environment. Complete loss or removal of reefs is far less common than reef degradation, but in Singapore some 60 per cent of the shallow reef area has been buried beneath reclamations (Hilton and Manning 1995; Chou and Goh 1998).

Urban and Industrial Waste-water treatment has not kept pace with domestic demand in the rapidly growing mega-cities of Southeast Asia; not one major coastal city in Indonesia, for example, had a sewage system in place in 1998 (Edinger et al. 1998). Particularly during low flow conditions, urban–industrial discharges are low in dissolved oxygen and high in organic content, nitrogen, and phosphorous, thus encouraging the eutrophication of coastal waters. Thus, for example, sewage discharges along the Sumatran coast deliver loadings of biological and chemical oxygen demand, total nitrogen, and total phosphorous estimated at 167 000, 381 000, 74 000, and 7000 tonnes a−1 respectively to the Strait of Malacca (Wong 2000). In addition, it is estimated (Edinger and Browne

Coral Reefs of Southeast Asia 419

2000) that 30–40 per cent of solid waste is either inappropriately disposed of or simply not collected in the major urban centres; thus reefs and shallow marine environments can become choked with rubbish, much of it plastic (Willoughby, Sangkoyop, and Lakaseru 1997). Significant industrial pollution is generated by agro-based and manufacturing industries, including palm oil, rubber, and tapioca processing, sugar-refining, tanneries, and pulp and paper manufacturing. High concentrations of lead, zinc, mercury, nickel, cadmium, and copper have variously been reported from coastal waters in Malaysia, Sumatra, and Singapore (Chua et al. 2000), western Indonesia (Edinger and Browne 2000), and the Upper Gulf of Thailand (Hungspreugs and Yuangthong 1983). The extension of agriculture to lands cleared during commercial timber operations and the intensification of production on traditional agricultural lands has had major implications for the quantity and quality of agricultural run-off to coastal margins. The development of a Southeast Asian agro-industry has been accompanied by the appearance of organochloride pesticide residues in river waters and in shellfish tissues, which may in turn be linked to dinoflagellate blooms and incidences of paralytic shellfish poisoning.

Marine Resource Extraction Exceedingly high exploitation of regional fishery resources takes place in Southeast Asian seas in an environment of ‘limited regulatory enforcement, minimal scientific understanding of the fisheries, and incomplete and inaccurate catch and effort data’ (Edinger and Browne 2000: 391). The significant exploitation of the offshore fishery in the 1960s can be correlated to the introduction of new trawling and purse seine net technologies. By the late 1970s the dermersal and prawn fisheries were considered to be fully exploited and trawl fishing was banned in western Indonesia in 1981. This had the effect of shifting fishing effort to purse seining for pelagic species, which in turn was seen to be above maximum sustainable yield by the end of the 1980s (Edinger and Browne 2000). Fisheries decline has been further exacerbated by mangrove forest clearance (Fortes 1988) and by destructive fishing practices. Reef fisheries represent a small proportion of the total reported catch, but their contribution to artisanal communities, local economies, and health and food security is far greater than can be ascertained from national-level reporting. Being a local resource, many, perhaps most, reef fisheries are artisanal and unreported. One of the better-studied examples is Bolinao in the Philippines, where some 17 000 people are ‘employed’ in the utilization of some 68 km2 of reef (Spalding,

Ravilious, and Green 2001). Reef fishes are also of considerable external value and have increased rapidly; thus ornamental fish exports from the Indonesian seas increased one hundred-fold between 1968 and 1991 (Tomascik et al. 1997b). Fishing practices which destroy the fished habitat and/or the physical structure of that habitat are prevalent throughout Southeast Asia; they include blast fishing, cyanide fishing, muro-ami (drive-in net fishing), bubu trap fishing, and inshore trawling. ‘Bombing’ reef fish, with home-made fertilizer-based devices of c.1 kg, is thought to be practised by c.15 per cent of Indonesian reef fishers, with 10– 40 per cent of reef catches coming from such operations (Erdmann 2000). Edinger et al. (1998) have recorded reductions in coral diversity of c.50 per cent on bombed reefs: individual corals are toppled, fractured, and shattered by explosions and reef substrates marked by 1– 4 m diameter blast craters. Repeated blast fishing can reduce a reef to a field of shifting rubble. Coral reefs at Bunakan, northeast Sulawesi, which have been protected from blast fishing since the 1980s have shown no signs of recovery of hard coral cover, being characterized instead by zooanthids and soft corals (Erdmann 2000). This is not surprising as not only is it difficult for coral planulae to settle on a moving substrate but also the destruction of the three-dimensional reef topography makes blasted sites less receptive to migratory adult and settling larval fishes. Economic models have demonstrated that while initially blast fishing is much more rewarding than conventional methods, these advantages are lost within a few years at most. After twenty years blast fishing yields an income only one-fifth of that which can be realized by more sustainable fishing practices (Pet-Soede, Cesar, and Pet 1999). The use of potassium or sodium cyanide to stun fish and invertebrates desired for live collection is also prevalent throughout Southeast Asia. Although originally used selectively to target key ornamental species for the aquarium trade, in recent years its use has widened to gather lobsters and large reef fish for live export to the gourmet restaurants of the major Asian cities. Cyanide fishing is highly efficient, not only in removing the ‘target species’ populations but also in proving lethal to other reef organisms, including hard corals. As reefs near centres of population—such as the Seribu Islands and the Spermonde Archipelago— have degraded and now have considerably reduced yields, so certain commercial fishing operations have moved to more remote locations. High-value ‘target species’ are now being taken from even remote reefs in the region. In many cases these species are large individuals which play a critical role in the replenishment of fish stocks in general.

420 T. Spencer and M. D. Spalding

With rapid expansion since 1980, shrimp farms in South and Southeast Asia now account for 75 per cent of the total world production of farmed shrimp. The development of this industry has, however, been at great environmental cost, exacerbated by the limited lifespan of most operations and the lack of ecosystem restoration following abandonment (Chua, Paw, and Guarin 1989). Shrimp pond construction has become one of the principal reasons for mangrove forest destruction in Sumatra, Kalimantan, south Sulawesi, and Java; in the latter, 82 per cent of the north-coast mangroves had been cleared for shrimp farming by 1991 (Tomascik et al. 1997b). The removal of the natural buffering function of these forests has led to widespread reports of high rates of coastal recession. Furthermore, some of these clearances—notably in south Sumatra (Burbridge, Koesoebiono, and Dahuri 1988) and Kalimantan (MacKinnon et al. 1996)—have subsequently been abandoned as aeration of pyritic peat forest soils has liberated sulphuric acid and then mobilized iron, and particularly aluminium, to toxic levels. Functioning ponds shed high-nutrient, organic carbon and bacterial loads into nearshore waters, often in concentrated flushes when the ponds are drained for harvesting. Typical releases of inorganic nitrogen and phosphorus per tonne of shrimp are equivalent to the sewage produced by 80 and 130 individuals respectively; for Indonesia as a whole, this equivalence comes to the sewage of 5.2 (nitrogen) to 8.4 million people (Briggs and Funge-Smith 1994). High stocking densities (10–30 shrimp m−2) mean that the use of antibiotics, fungicides, and organo-metal compounds as molluscicides is widespread, raising fears about their impact on natural coastal communities (Philips et al. 1993). The direct mining and quarrying of coral reefs is a widespread practice throughout Southeast Asia, at both subsistence and commercial levels (Hopley and Suharsono 2000). Coral is mined for house-building materials, for road foundations, to be fired to produce lime for mortar, and for simply decorative purposes, both locally and through export. Immediate physical impacts create further problems, including reduced coral growth and fisheries decline through increased water turbidity on extraction. Coral removal may result in a loss of wave energy dissipation and lead to beachlowering, sediment loss, and shoreline retreat. Remedial shoreline stabilization is often undertaken at great cost. Cesar (1997) has estimated that hotels on Bali and Lombok spend over $US100 000 per year on beach protection and nourishment, often using the placement of coral blocks removed from the reef itself.

Southeast Asia produces considerable volumes of both oil and liquefied natural gas, and its shallow marine habitats—mangroves, seagrasses, and coral reefs —are all highly sensitive to oil pollution impacts, albeit localized. Beach tar (Uneputty and Evans 1997) and heavy metals in coral skeletons (Scott and Davies 1997) in the Seribu Islands have been linked to the operation of offshore oil production platforms in the Java Sea. At the subsequent refinement stage, oil leakage from refineries has been implicated in the pollution of estuaries in eastern Sumatra (Whitten, Muslimin, and Henderson 1987). Problems additionally arise from the production of liquefied natural gas, of which Indonesia is the world’s leading producer. Hot-water discharges from the processing plant at Bontang Bay, south Kalimantan, have killed corals on adjacent fringing reefs and natural gas-derived mercury discharges are a pernicious pollutant in the Malacca Strait and northern Java Sea (Burbridge, Koesoebiono, and Dahuri 1988). Major petroleum shipping lanes run through the Malacca Strait, South China Sea, and the Makassar and Lombok Straits. Between 1974 and 1994 thirty-four major oil spills were reported, spilling the equivalent of over 1 million barrels, half as crude oil (Edinger and Browne 2000). A large proportion of all oil pollution incidents are, however, the result of regular deballasting and tank-cleaning operations. High concentrations of tri-butyl tin have been reported from the shipping lanes (Hashimoto et al. 1998) and linked to imposex in muricid snails and neo-gastropods at several locations in the Malacca Strait (Chua et al. 2000). In parts of Southeast Asia it is clear that these impacts, often acting synergistically, are driving many reefs to the point of terminal decline. One location where this process can be demonstrated is Jakarta Bay, where water transparency and reef descriptions from the 1920s (Verwey 1931a,b; Umbgrove 1939b) can be compared with those of the 1990s. Water transparency has always been poor close to the mainland as a result of terrestrial run-off. However, the decline in offshore transparency has been marked (Figure 24.8a), owing to the increased offshore spread of algal blooms in increasingly eutrophic waters. Not surprisingly, the maximum depth of coral growth has decreased dramatically (Figure 24.8b). These statistics are a surrogate for other measures of reef health. Thus of the ninety-six species of coral described from Nyamuk Besar in 1929 (Umbgrove 1939b), only sixteen remain and at only low levels of abundance, leading Tomascik et al. (1997b: 1236) to conclude that ‘the once “thriving” reefs of Batavia ( Jakarta) Bay are functionally dead at present’.

Coral Reefs of Southeast Asia 421 10

Distance from mainland

Depth (metres)

8

8.5

4.9

6 4

2.8

3.5

Onrus

Kelor

2 0

Ubi Besar

Air Besar

(a) 1929

14

1985

1993

Depth (metres)

12 10 8 6 4 2 0 Onrus

Kelor

Ubi Besar

Air Besar

(b) Fig. 24.8. (a) Temporal and spatial comparison of water transparency (Secchi disc depths) between four patch reefs in Jakarta Bay Numbers above the columns indicate distance from mainland.

(b) Temporal and spatial comparison of maximum depth of living coral at same stations. (Source: after Tomascik et al. 1997b)

Addressing the Threats Efforts to redress many of these anthropogenic threats to coral reefs are being addressed on a number of fronts. Three broad approaches cover fiscal incentives, legal regimes, and public education. Typically all of these approaches may be combined in integrated coastal zone planning but such integrated measures have not yet been widely applied in Southeast Asia. Probably the most widespread regimes for coastal management involve legal measures. These include controls on fishing

practices (blast fishing is illegal across the region; Burke et al. 2002), but more especially the designations of particular areas as marine protected areas (MPAs). Traditionally MPAs have fallen under the domain of conservation authorities. However, more recently the role, particularly from small ‘no-take’ zones, of fisheries management has increasingly been recognized. A number of no-take areas have been established across the region. Where these have been effectively enforced, such as the well-studied site of Apo Island in the Philippines, they have had a dramatic effect on fish stocks, increasing yields from adjacent reefs and yielding considerable social and economic benefits to adjacent villages (Russ and Alcala 1996). The total area of marine resources which need to be set aside in MPAs is dependent on a number of factors, including the intensity of human use of an area, the strength of the protection regime within MPAs, and the ultimate objectives of protection. Strict maintenance of natural systems, or recovery of depleted or damaged systems, clearly requires greater levels of protection than maintenance of fishery yields. Irrespective of these requirements, however, the protection of 20 per cent of marine areas from fishing has been seen as a minimum requirement, with 20– 40 per cent being seen as likely to increase greatly the long-term yields (C. M. Roberts and Hawkins 2000). Burke, Selig, and Spalding (2002) recently completed a regional assessment of the extent and effectiveness of the MPA network in Southeast Asia. Across the region covered in this chapter some 430 MPAs have been designated (not including the Andaman and Nicobar Islands). However, the majority of these are not closed to all fishing, and there are considerable problems of management effectiveness. Typically the total area of coral reef protected in each country falls below 10 per cent, with the exception of Viet Nam (11 per cent) and Thailand (38 per cent). Furthermore, an expert assessment on the management effectiveness in 308 of these sites has shown that only 15 per cent of these sites has good management (Burke, Selig, and Spalding 2002). Critical problems with MPAs include inadequate funding, poor integration of local communities, and poor design and management. Models from the region and elsewhere have shown the considerable social and economic benefits which can be derived from MPAs. However, these need to be applied with the support and involvement of local communities and designed to be sustainably financed and managed over long time-frames. In addition, it is necessary to consider the external anthropogenic influences and to seat protection measures within a wider framework of integrated

422 T. Spencer and M. D. Spalding

management (C. M. Roberts and Hawkins 2000; Cesar 2000), taking account of the wide range of constraints and controls outlined in this chapter.

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Index Bold numbers denote reference to illustrations Abacan channel 262 Abies 111 Acacia 108, 109 Acacia mangium 110 Acanthaster 392 accelerated erosion and sedimentation 239– 48 acid sulphate soils 210, 224, 233, 393 accretionary prism 9, 11, 15, 39 accretionary wedge, see accretionary prism acrisols 95, 99, 100, 102, 103 Acropora 188, 189, 408, 412, 413, 416 Acrostichum 232 Acrostichum aureum 225 aerosol 89 Africa 15, 89, 102, 110, 185, 302, 318 Agathis 26 Agathis alba 97, 98 Agathis dammara 98 Agenda 21 398 agriculture 95, 107, 108, 111, 113, 114, 117, 150, 155, 169, 170, 174, 199, 207, 209, 212, 214, 219, 240, 241, 307, 311, 318, 340, 390, 402, 418, 419 agroforestry 106, 110 Agung 151, 152, 268, 269 Agusan Lowlands 62 alang alang, see Imperata cylindrica alert levels 273 Alor 11, 405 allophane 97 alluvial fan 52, 55, 62, 96, 239, 338 Amazon 31, 193 Ambon 310, 416 Ambon Bay 188, 310, 416 Ampang 349, 355 Anak Krakatau 179, 266 Anambas Archipelago 182, 416 Andaman: coast 395 Fault 15 Islands 9, 416, 420 Sea 7, 8, 15, 20, 21, 52, 180, 186, 222, 410, 412, 415 Andaman-Nicobar Ridge 15 andosols 95, 97, 102, 103, 148 Angat River 340 Angeles 262 An Giang 214 Angkor 307 Angkor Wat 66 Anjer 257 Annamite Chain 42, 53–55, 54, 56, 76, 193, 203 Annamite Mountains, see Annamite Chain Annamite Ranges, see Annamite Chain Antarctica 7, 24 Antipolo 329 anthropogenic activities, see anthropogenic modification anthropogenic alteration, see anthropogenic modification

anthropogenic changes, see anthropogenic modification anthropogenic modification 38, 39, 40, 78, 240, 244, 245, 246, 421 anthrosols 95, 101, 103, 104 Aparri 184 Apo 136, 183, 421 Apo Duat 56 Apo Reef 417 Apu River 276 Aquifoliaceae 26 Arafura Sea 3, 26, 33, 180, 404, 410 Arafura Shelf 31, 404 Arakan Coast 179, 180, 182, 184 Arakan Yoma 14, 40, 42, 44, 74, 81, 222, 301 arcuate islands of Indonesia 42, 56–60, 58 Ardisia elliptica 184 Argo Abyssal Plain 3, 7, 14 Aru Islands 32, 33, 403 arenosols 95, 96, 102, 103 Asahan River 144 ASEAN 91, 311, 397 ash 32, 59, 88, 89, 95, 96, 97, 255, 269, 408 ash fall 255, 291, 292 Asia 9, 24, 26, 31, 33, 38, 80, 82, 85, 89, 177, 179, 185, 389, 403, 418 Asian Development Bank 202, 211, 212 Asian Institute of Technology 364, 366 Assam 9, 38 Assam Himalaya 42 Assam-Yunnan Syntaxis 9, 15 Aswan Dam 233 Atauro 230 Atlantic Ocean 13, 24, 89, 185 Atlas 20 atmospheric effects 266–7 Atuaro Island 405 Australia 3, 7, 8, 11, 17, 20, 25, 26, 27, 31, 33, 34, 90, 109, 111, 132, 135, 136, 197, 209, 411 Australian Craton 40, 404 Australian Plate 33, 56, 60, 157, 403, 405 AVHRR 245 Avicennia 187, 222, 225 Avicennia marina 225 Awu 268 Ayutthaya 27, 75, 230, 358, 359, 361, 364 Baao 327 Babi Island 179 Babuyan Island 143 back-arc seas 11–13 backswamp 96, 390, 403 Bacolor 329 Bagamanoc 329 Baguio 20, 83, 86, 314 Bako 182 Balai Sungai dan Sabo 297 Bali 26, 59, 88, 96, 97, 114, 142, 150, 151, 152, 154, 179, 180, 182, 183, 188, 189, 241, 268, 307, 389, 390, 392, 396, 410, 416, 420

Balikpapan 390 bamboo 108 bamboo forests 106, 109 Banda 179, 411 Bandaapi 268 Banda Arc 11, 14, 404, 405, 416 Banda Ridges 14 Banda Sea 8, 14, 31, 142, 179, 180, 182, 403, 404, 408, 410, 418 Banda Volcanic Arc 157 Bandung 59, 266, 317, 322, 329, 330, 337, 342, 383 Bandung Basin 29, 154 Bandung-Chimahi alluvial fan 329 Bandung Plateau 330 Banghiang River 203 Bangka 51, 98, 100, 129, 180, 255, 395, 416 Bangkok 27, 39, 66, 75, 89, 175, 202, 230, 233, 316, 318, 319, 321, 323, 326, 328, 336, 337, 338, 339–40, 342, 358–78, 370, 372, 373, 379, 380, 382, 383, 390 aquifer system 366–78, 367 Bay 245 canals 323, 338, 340, 362, 382 Clay 230, 233, 360, 371 marine clay 359, 366, 375 Metropolitan Administration 365 Metropolitan Water Works Authority 320, 339, 368 piezometric drawdown 367–71 Plain 361, 372 Public Works Department 367 Bangladesh 231, 234 Bang Pakong River 51, 228, 358, 362, 364, 373 Bang Tao Bay 395 Ban Houei Sai 202 Banjak Island 188 Banjaran Titiwangsa, see Main Range (Malaysia) Ban Kangvit 213 Banten 149, 154 Banten Bay 154, 155 Banthat Range 52 Banyuwangi tsunami 408 Baram Delta 15, 17, 221, 226– 8, 227, 233 Baram River 226–8 Barisan Mountains 57, 59, 81, 85, 144, 300 Barbados 405 Barito Basin 17, 21 Barito River 56 Barito peat swamp 56 Barren Island 9 Barringtonia 183, 184, 185, 207 Barringtonia Asiatica 184 base surge 255 Bashi Channel 26 Bassac 55, 77, 207, 223, 232 Bassein River 222 Batam 317, 342 Batanes 86 Batang River 283

430 Index Batangas 397 Batavia, see Jakarta Batu Putih 130 bats 168, 174 Battembang 207, 326 Batu Arang 17 Batu Caves 174 Batu Cave Hills 346, 354 Batu Ferringhi 396 Batu Gumbar Orang 131 Batu Island 188 Batukau 151 Batu Pahat 53 Batur 151, 152, 268 Baturaden 85 Bau 21 Bawdwin 21 Bawdwin Volcanic Series 21 Bawean Archipelago 416 Bay of Bengal 7, 46, 82, 83 beach 27, 62, 179, 180, 182, 183, 184, 240, 244, 246, 389, 394, 397, 420 beach conglomerate 180 beach ridge 27, 225, 233, 330 beach vegetation 106, 109–10, 184 Bebeng 282, 287, 294 Bedok 282, 341 Bekasi 338 Belawai 228 Belitung 20, 21, 51, 98, 100, 129, 182, 395, 416 Bellwood, P. 33, 34 Benain River 72 Bengal Basin 7 Bengal Fan 7, 231 Bengkalis Graben 15 Bengkulu Province 241, 245 Benham Plateau 14 Benioff zone 8, 9, 11, 39 Benkoelen, see Benkulu Benkulu 257 Benom Complex 125 Bentong-Raub Line 17 Berau River 416 Bezymianny 289 Bhamo 73 Bhumipol Dam 74, 364, 366 Biak 408 Bicol 62, 327 Bien Hoa 318 Bilin River 46 Billiton, see Belitung Bintan 182, 183, 185, 189, 190 Bintan Beach International Resort 396 Bintulu 332 biodiversity 25, 174, 177, 187, 190, 191, 193, 199, 203, 210, 211, 212, 413 Bird’s Head 142, 404 black leaf monkey, see Trachypithecus francoisi Black River, see Da River Blauktaung 50 Blongkeng 282 Boea 167 Bogor 338 Bohol Island 62, 169 Bok Bak Fault 51 Bolinao 419 Boloven Plateau 55 Bombay 257 Bontang Bay 420 Boracay 183, 396

Borassus flabellifer 109 Borneo 3, 7, 13, 15, 17, 26, 31, 32, 42, 56, 57, 66, 67, 69, 72, 81, 82, 87, 89, 90, 98, 99, 100, 101, 102, 104, 107, 108, 112, 113, 116, 125, 129, 142, 157, 158, 168, 172, 177, 186, 226, 230, 232, 245, 300, 306, 314, 328, 402, 403, 407, 408, 409, 413, 415, 416, 417 Bos sauvli 210 bottled water 342 Boyong River 244, 276, 278, 280, 282, 287, 294 brackish water swamp forest 106, 111 Brahmaputra 7, 231 Brantas 65, 219, 244, 407 Brassey Range 56 Brazil 89, 117 Brinchang 129 Bruguiera 187, 222, 225 Bruijnzeel, L. A. 302, 305 Brunei 17, 39, 88, 89, 90, 114, 117, 157, 158, 182, 186, 300, 306, 338, 382 bryophytes 110 Bubu 53 Bucao Valley 262 Buchanan, F. 94 Bujong Melaka 130 Bukit: Bald 131 Berembun 301, 304 Chemargong 131 Lamar 125 Lanchar 125 Nanas 346 Panbunga 131 Tahun 347 Bukit Besi 21 Bukit Peninsula 182 Bukit Timah Canal 327 Bukittinggi 59 Bukit Tinggi Fault 51 Bulacan Province 340 Bulusan 267 Bunaken Island 186 bun bang fai 307 Bun Lung 211 Bureau of Fisheries and Aquatic Resources, Philippines 394 Burmah Plate 9, 20 Burretiodendron 172 Buton 416 Butterworth 87 Cabagnaan 267 Cagayan Islands 417 Cagayan Ridge 14, 417 Cagayan River 62 Calamian micro-continent 21 Calancan Bay 396 Calaya solfatara field 145 Calcutta 7, 231 Caldera 143– 4, 151, 256, 266 Calophyllum 183 Calophyllum inophyllum 184 Cam 87 Camah 130 Camarines reef bank 188 Ca Mau 207, 214, 215 cambisols 95, 97– 8, 100, 102, 103 Cambodia 20, 51, 55, 72, 77, 78, 80, 82, 98, 99, 100, 106, 114, 157, 182, 186, 193,

197, 199, 203, 204, 207, 210, 211, 212, 214, 216, 223, 234, 245, 305, 306, 307, 308, 326, 390, 392, 394 Cameron Highlands 83, 129, 131 Camiguin Island 62, 183 Campnosperma 228 Ca Na 132 Canavalia 184 Candidasa 189, 190 Canlaon City 330 Canning Basin 17, 20 canopy 106 Cap Saint Jacques, see Vung Tau Cap Varelle, see Mui La Caramoran 329 carbon 90, 116, 117 carbon dioxide 32, 90, 91, 116, 117, 161, 212, 319 carbon emission 116, 117 Cardamom Hills 52, 55, 204 Caroline Plate 32, 142 Carstensz, see Mount Jaya cassava 100, 110 cassiterite 20, 21 Castanopsis 32 Casuarina 25, 96, 109, 183, 215, 222, 232 equisetifolia 109, 184 junghuhniana 111 sumatrana 98 Catanduanes 329 catfish, see Pengasius Cathaysia 3, 15, 20 Catlacarpio siamensis 207 cave 164, 165, 167, 168, 169, 170, 171, 172, 354, 355 Cebu 389 Cebu Island 20, 396 Celebes Sea 14, 143, 404, 410 Celebes Sea Plate 143 Cemoro River 33 Cenderawasih Bay 404 Central Cordillera 62, 86 Central Myanmar Lowland 42, 44–6, 45, 50, 51, 65, 81 Central Highland of Malay Peninsula 42, 49, 50–1 Central Plain of Thailand 42, 47, 50, 51, 52, 65, 95, 99, 339 Central Range of Hills 42, 46–51, 50, 52 Central Range, Papua 40 Central Sumatra Basin 15, 17 Ceriops 187, 222 Certificate of Entitlement 381 Chai Nat 75, 358, 359 Champion 106 Chang 52 Changi Airport 332 Chang Jiang, see Yangtze Chanos chanos 393, 394 Chao Phraya 26, 50, 51, 52, 55, 65, 67, 71, 74–5, 99, 112, 169, 182, 228, 308, 320, 339, 358, 361, 362, 364, 365, 366, 369, 370, 382, 389, 395 Chao Phraya Delta 221, 230, 233, 245, 359 charcoal 111, 113, 188, 207, 393 Cheduba Strait 179 Cherry Hill Housing Estate 329 Chhibber, H. L. 130 Chiang Khong 199, 202 Chiang Mai 169 Chiang Rai 67

Index 431 Cidurian 309 China 3, 8, 11, 14, 15, 20, 26, 27, 32, 34, 42, 67, 75, 78, 81, 82, 83, 112, 170, 173, 193, 197, 199, 202, 212, 318 Chindwin 44, 46, 65, 74 Chin Hills 40, 42, 44, 81, 301 Chi River 203 Chocolate Hills 62, 169 Chromolaena odorata 109 Chuping Formation 20 Chu Tong Sin 129 Chuxiong 199 Cilacap 397 Cilegon 189 Ciliwung 326, 338 Cilutung 242, 245 Cimanuk 231, 242, 245 cinchona 97 cinder cone 146 Cirebon 245 Ciremai 144, 145, 269 Citarum 231 Citunduy 245 Ciwidey 302 Clearwater cave 162, 166, 167 climate 80–93, 101, 105, 188, 225, 314, 403 climate change 29–32, 35, 90, 116–17, 164, 177, 189, 219, 232 climax vegetation 106 cloud forest 110 coastal and marine tourism 396 environment 177–91, 389 erosion 190, 394 management 397–9 Coastal Plain of Kra Isthmus 42, 49, 52–3 Coastal Plain of Kra Isthmus and Malay Peninsula 42, 49, 52–5 Coastal Plain of Southeastern Thailand 42, 47, 52 coastal processes 179 reclamation 53 zone development 389–99 Cobra Cave 165 cocoa 100, 110, 302 coconut 88, 96, 110, 112, 184 cocos nucifera, see coconut Code River 289, 294, 297 coffee 97, 100, 102, 110, 144 cogon, see Imperata cylindrica cold surge 81, 82 Combretocarpus 228 CCOP (Committee for the Co-ordination of Joint Prospecting of Mineral Resources in Asian Offshore Areas) 39 Compayrés, see corestones cone karst 157, 161, 170 conservation 117–18 construction impacts 348–9 continental shelf 13 continental shelf seas 13 continental shields 33 convergent plate margins 8–11 copper 20, 162 coral 11, 25, 27, 29, 32, 60, 62, 154, 155, 177, 179, 180, 182, 183, 186, 188, 189, 190, 191, 230, 232, 240, 246, 397, 411, 412 bleaching 188–9, 392, 402, 411–13, 412 reef 26, 188–90, 338, 390, 394, 395, 396, 402–22; degradation 391–2, 397, 398; earthquake impacts 407– 8; tsunami

impacts 407– 8; volcanic impacts 407–8; see also coral threats 403, 418–22 core-boulders, see corestones corestones 68, 125, 127–9, 128, 135, 137, 347 Corypha utan 109 Cotobato Lowland 62 cotton 96, 97 crater lake 255, 256 Crocker Range 15, 17, 56 Crown of Thorns Starfish 402 Culiatan 168 Curray, J. R. 403 Cyclone Lena 410 Cyclops Mountains 31 Cyperacea 96 Cyperus pedunculatus 184 stoloniferous 184 Dacade Volcanoes 273 Dachaoshan 199 Dacrycarpus 26 Dacrydium 31 Dacrydium elatim 98 Dactylocladus 228 Da Lat Plateau 203 Daluo 199, 202 Dam, M. A. C. 266, 309 Damansara Heights 352 Da Nang 55, 56, 89, 316, 318 Danau Matano 33 Danau Poso 33 Danum Valley 302, 304 Da River 42, 170 Darwin 411 Darwin Crater 33 Davao Gulf 183 Dayang Jiang 73 debris avalanche 276, 282, 284, 289 flow 282, 287 deciduous dipterocarp forest 108 deciduous forest 32 Deer Cave 165, 166, 168 deforestation 34, 35, 90, 95, 113, 114, 116, 117, 169, 211, 216, 240, 241, 242, 245 delta 15, 17, 27, 39, 40, 46, 53, 55, 56, 59, 66, 67, 69, 75, 98, 177, 186, 190, 193, 197, 204, 207, 208, 212, 219–34, 239, 240, 242, 245, 246, 308, 330, 333, 337, 347, 403, 416 deltaic-estuarine plains 219 delta morphology and process 220 Denchin 193 Denpasar 190 Dent Peninsula 14 Department of Irrigation and Drainage (Malaysia) 353 depocentre 220 desalination 342 diamond 21 Dieng 268 Dieng Plateau 83, 149, 150, 264, 269, 284 Dipterocarpacae 26, 107, 108, 167, 172 dipterocarp forest 99, 301 diversity, see biodiversity Djuli River 230 doi moi 215, 318 doline 157, 164, 169, 170, 173 dome collapse 257, 266, 284, 287, 292 Dong Giao 170

Dong Nai 318 Dong Rek Range 51, 55, 204 Douc langurs, see Pygthrix nemaeus Douglas, I. 68, 126, 241, 304 drought 86, 88, 108, 170, 315 dry deciduous forest 108 dry evergreen forest, see tropical seasonal forest dryland forest 90 Dry Zone (Myanmar) 81, 108, 306 Du Lang 171 Dulit Range 56 dune 182, 184, 185, 222, 244 East Asia 117 East China Sea 402 Easter island 34 Eastern Coastal Plain of the Malay Peninsula 42, 49, 53–5 Eastern Range (Malaysia) 51 Ebulobo 268 edifice collapse, see dome collapse Eleocharis spp. 225 Elephant Hills 52, 55 Elephant and Cardamom Hills 42, 47, 52 El Nido 182, 244 El Niño Southern Oscillation 29, 32, 87–8, 91, 113–14, 315, 336, 340, 343, 380, 382, 392, 411–13 emergents 106, 109 Emmel, F. J. 403 Endau River 53 endemic diseases 332–3, 379 environmental impact assessment (EIA) 385 environmental Management Bureau, Philippines 324 Environmental Quality Act (Malaysia) 385 Equator 31, 72, 80, 83, 85, 88, 303, 305, 411 Equatorial Countercurrent 410 erosion plot 240 ESCAP (United Nations Economic and Social Commission for Asia and the Pacific) 39, 362, 380 escape tectonics, see extrusion tectonics eucalyptus 31, 97, 109, 111 Eucalyptus urophylla 110 Eurasia 15, 20, 403 Eurasian flora 25 Eurasian Plate 3, 32, 40, 60, 67, 125, 142, 143, 157, 158, 177, 403 Europe 88 evaporation 83, 300, 309 evapotranspiration 302 Excoecaris agallocha 184 extrusion tectonics 15, 67 Eya 222 Fang Basin 17 Fan Si Pan Ridge 42 Fan Si Peak 42 FAO 114 FAO-UNESCO soil classification system 94 Federal Territory (Malaysia) 356 FELDA (Federal Land Development Authority) 241 Feldspar phenocryst 127 fengcong 159, 170 fenglin 159, 170 ferralsols 95, 100, 102, 103 ferricrete 51, 52 Ficus 183

432 Index figs, see Ficus fir, see Abies fire 31, 107, 108, 109, 111, 112, 113, 114, 318 fire and mud 269 firewood 108, 113, 207, 214, 215 fish 188, 193, 207, 214, 338, 340, 356, 413, 419, 420, 421 fishery, see fish fish ponds 111 Fiske, R. S. 256 flake industry 34 flood 51, 72, 78, 79, 197, 209, 211, 219, 240, 241, 245, 269, 280, 289, 294, 305, 307, 309, 314, 315, 327– 8, 339, 344, 352, 353, 358–78, 383, 384–5, 393, 394 floodplain 68, 204, 239, 241, 244, 337, 344, 362, 403 Flores 33, 34, 60, 88, 179, 408 Flores Sea 179, 182, 188, 404, 408, 410, 417 flutings 134–5, 134 fluvisols 95– 6, 102, 103 forams 189 forest degradation 113–14, 116, 220, 343, 390 distribution 114, 115 fire 90, 305 loss 113 forest on extreme soil types 106, 108 forest on ultramafic rocks 106, 108 Forest Research Institute, Malaysia 304 4000 Islands 77 Fraser’s Hill 127, 128, 131 freshwater mangrove, see freshwater swamp forest freshwater swamp 27, 53 freshwater swamp forest 106, 111–12, 230 Frog Island 131 fuelwood, see firewood fumarole 254, 264, 276 fumarolic stage 143 gallery forest 107 Galunggung 90, 148, 267, 268 Gamalama 268 Ganga 7, 231 Ganga-Brahmaputra-Meghna Delta 231 Ganga-Brahmaputra-Meghna system 219, 220 Gangow Range 44, 73 Gap 129 Garden of Eden 164 gas deposits 11, 39, 212, 220, 420 Gedeh 147, 154, 155, 269 Gendol 276, 278, 280, 282, 289 General Circulation Model 91 General Santos 85 Genting Highlands 349 Genting Sempah Tunnel 349 Georgetown 129 geothermal station 149 Gergasi Cavern 163 giant muntjac 172 giant tortoise 34 Gigantopteris 20 gilgai 97 Gili Islands 184 giong, see beach ridge glaciation 13, 20, 25, 29, 31, 32, 111, 137, 164, 168, 177, 232

glacier, see glaciation gleysols 95, 98, 102, 103 global climate models 117 global warming 91, 116, 164, 343 Gmelina arborea 110 gold 20, 418 Golden Triangle 202 Gombak River 344 Gondwanaland 3, 17, 20, 26, 403 Gonystylus 228 Gonystylus bancanus 112 Good Luck Cave 165, 166 Gopeng 126 Gopeng Beds 161 Gorontalo Bay 143 graben 17, 67, 75, 144 granite 7, 13, 40, 57, 59, 67, 68, 123–38, 124, 182, 244 topography 129–31 weathering 125–9 grasses 108, 111, 224 grassland 32, 107, 117, 202 Great Barrier Reef 189, 403, 407 Great Sunda Barrier Reef 416 Great Tenasserim River 46 Greater Sunda Islands 26 green areas 385 Green Cave 167 Green Revolution 309 greenhouse gases 117 Greenland ice cap 33 groundnut 96, 97, 100 groundwater 100, 149, 155, 300, 307, 310, 320–3, 324, 328, 337, 338, 339, 340, 342, 367, 368, 373, 380, 393 grus 125 Gua Nasib Bagus, see Good Luck Cave Guanxi 159, 170 Gua Tempurung 160, 161, 162 Gubug 309 gufeng 159, 170 gugup, see corestones Guilin 159, 171 Guizhou Plateau 170 Gulf of Martaban 11, 33, 46, 180, 222, 415 Thailand 17, 21, 27, 33, 50, 51, 52, 67, 75, 180, 199, 215, 224, 228, 339, 358, 359, 362, 372, 393, 395, 402, 415, 418, 419 Tonkin 67, 180, 417 Gumuk ash 280, 289 Gunung: Api 164, 165, 168, 172, 173, 408, 418 Benarat 164, 165 Bintang 131 Buda 164 Belumut 131 Gagau 131 Layang Layang 135 Ledang 129–30, 138 Lesong 131 Melaka 131 Mulu 158, 167 Mulu National Park 164–8, 165, 174 Panti 131 Pulai 131 Semangko 131 Sewu 170 Stong 130 Tahan 51, 130, 136 Tempurung 160, 161, 162 Tera 130

Gyaing 46, 50 gymnosperms 31, 32 Hadley cell 86 haematite 127 Hainan 26, 417 Haiphong 316, 317, 318 Halimeda 411 Halmahera 11, 60, 112, 143, 153, 417 Ha Long 317 Ha Long Bay 171, 172, 173, 174–5, 182 Hanoi 174, 308, 316, 317, 320, 322, 323, 326, 328, 342, 390 Hanoi Rift System 67 Hawaii 34 hazard zonation 267–8, 273, 275, 276, 284–97, 285, 286 hazard zone mapping, see hazard zonation hazards, Merapi 275–99 haze 381–2 heath forests 106, 108, 167 Henzada 74 herbaceous swamps 106, 112 herbicides 394 Heritier fomes 222, 231 Heritiera 272 Heritiera littoralis 184, 225 Hermyingi Mine 21 Hevea brasiliensis, see rubber Hevea gardens 110 Hibiscus tillaceus 184 Hibok-Hibok 62, 183, 267 Hidden Valley 164, 166 Hills of Northern Thailand and Lao PDR 48, 50 Himalaya 7, 9, 38, 40, 82, 193, 403 Himalayan flora 26 Hinatuan 85 hippopotamus 33 histosols 95, 101–2, 103, 104 Hkakbo Razi 42, 110 Hmong 211 Hoabinhian 34 Hoang Lien Son 203 Ho Chi Minh City 66, 316, 318, 320, 323, 338, 390 Hoekstra 230 Holocene 11, 24, 26, 27, 31, 32, 34, 35, 38, 44, 56, 65, 66, 67, 154, 155, 170, 179, 219, 220, 224, 225, 226, 228, 230, 231, 233, 360, 361, 405–6, 417 hominids 25 Homo erectus 33 Homo sapiens 34 Hong Kong 82, 129, 132, 213, 214 hongs 169 Hon Trung Lon 131 Hon Trung Nho 131 Hooghly 231 horned cattle, enormous 33 Hose Range 56 hot springs 170 Hua Hin 131 Huangyuan Dao, see Scarborough Reef Hué 56, 245 Hukawang Valley 44 hunter-gatherers 116 Huon Peninsula 232, 405, 406, 408 Hydnocarpus 207 hydroelectricity 175, 212 hydrology 300–11

Index 433 Ia 268 Igan 228 ignimbrite 59, 70, 144, 154 Ijen 268 Ili Boleng 268 illite 97, 99 Illitic clay, see illite Imperata cylindrica 35, 99, 102, 109, 202 India 3, 9, 13, 15, 20, 26, 33, 38, 40, 42, 44, 59, 83, 94, 231, 318, 403 Indian Ocean 3, 7, 14, 15, 31, 34, 56, 70, 80, 177, 179, 180, 183, 408, 410, 411, 416 Indian Ocean Ridge 7 Indian Plate 67, 403 Indo-Australian Plate 3, 9, 25, 32, 125, 142, 177, 403 Indo-Burman Ranges 7, 9, 14 Indochina 3, 13, 15, 18, 83, 86, 89, 129, 157, 403 Indochina Block 20 Indonesia 20, 21, 26, 27, 31, 32, 33, 34, 38, 40, 51, 56, 60, 70, 79, 80, 82, 83, 85, 88, 90, 96, 97, 98, 102, 108, 125, 142, 143, 145, 149, 150, 157, 173, 175, 177, 179, 182, 183, 186, 188, 189, 190, 219, 230, 240, 242, 245, 250, 264, 266, 267, 268, 269, 273, 297, 305, 306, 307, 309, 317, 318, 328, 329, 330, 338, 339, 342, 382, 390, 392, 393, 394, 395, 396, 397, 398, 399, 402, 405, 407, 408, 416, 419, 420 Indonesia, eastern 39, 107, 108, 109, 110, 111, 180, 186, 390, 408, see also Indonesia Indonesia, western 58, 113, 114, 116, 129, 419, see also Indonesia Indonesian Throughflow 408, 410–11 Indosinian Massif 125, 202 orogeny 3, 15, 20, 21 Indo-West Pacific biogeographical region 180, 220 insect life and vector-borne diseases 332–3 interception 301–2 interglacials 31, 177 IPCC (Intergovernmental Panel on Climate Change) 190 International Decade of Natural Disaster Reduction 273 International Date Line 88 IUCN 175 ITCZ (Intertropical Convergence Zone) 72, 80, 85 Investigator Ridge 7 Ipo River 340 Ipoh 21, 31, 131, 160 Ipomoea pes-caprae 184 Iran Range 56 Irian Fault 33 Irian Jaya, see Papua Ironstone, see laterite and beach conglomerate Irrawaddy Delta 27, 182, 190, 221, 222, 223, 232, 245, 415 Irrawaddy River 11, 15, 38, 40, 42, 44, 46, 50, 55, 65, 67, 71, 73– 4, 74, 96, 99, 112, 222, 308, 389, 393 irrigation 91, 95, 96, 97, 98, 99, 100, 101, 149, 169, 197, 209, 211, 215, 300, 307–10, 311 island arcs 20, 25, 33, 40, 56, 143, 403

Jabotek 338 Jakarta 175, 245, 257, 266, 316, 317, 318, 319, 321, 322, 326, 328, 332, 336, 337, 338–9, 340, 342, 379, 380, 390, 395, 418 Jakarta Bay 183, 332, 338, 339, 395, 407, 416, 420 Jalan Tun Perak 353 Japan 82, 97, 212, 213 Japan International Corporation Agency 297 Java 8, 9, 13, 15, 29, 32, 33, 34, 40, 59, 67, 70, 83, 85, 86, 96, 97, 98, 99, 101, 102, 110, 113, 114, 129, 136, 142, 144, 147, 148, 149, 151, 153, 154, 170, 173, 177, 179, 180, 182, 183, 184, 186, 189, 230, 231, 232, 240, 241, 242, 244, 245, 254, 255, 256, 257, 264, 266, 267, 268, 269, 276, 280, 289, 301, 302, 307, 309, 311, 338, 390, 397, 403, 407, 408, 410, 418, 420 Java Sea 9, 17, 65, 67, 179, 180, 403, 407, 409, 411, 412, 416, 420 Java Trench, see Sunda Trench Javelosa, R. S. 259 Jehosophat Mine 162 Jembangan 148 jet stream 82 Jinghong 199 Johor 51, 53, 131, 245, 320 Johor Baru 342 Johor-Riau-Singapore growth triangle 317 Johor River 53 Strait 131, 134 Jolo 143 Jugra 131 Juncus 209 Jurong industrial estate 332 Kabuh Formation 33 Kachin Hills 44 Kai Islands 405 Kakaban Reef 416 Kaladan 44 Kalaukalukuang Bank 411 Kalimantan 20, 21, 25, 26, 27, 31, 33, 35, 56, 85, 90, 96, 111, 113, 157, 174, 177, 182, 183, 188, 225, 233, 241, 250, 382, 393, 398, 407, 416, 418, 420 Kali Mas 326 Kalisat 83 Kaliurang 280, 297 Kallang Basin 383 Kaloatoa Island 182 Kambaranga 137 Kampung Improvement Project, Indonesia 339 Kampung Tambatuhan 127 Kanchanaburi 50 kangaroo 34 Kangean Archipelago 416 kaolinite 95, 99, 126, 347 kaolinitic clay, see kaolinite kapok 97, 100 Kapuas Hulu Range 56 Kapuas River 56, 186 Karang Volcano 149 Karangetang 268 Karimata Archipelago 416 Karimun Island 129, 396 Karimunjawa Islands 390, 416 Karkaralong Reefs 417

karst 39, 40, 50, 53, 55, 59, 62, 99, 153, 157–75, 193, 203, 328, 354–5 karst, Indonesia 170 karst, Malaysia 158–68 karst, Thailand, 169–70 karst, Viet Nam 170–2 Kawah Mati 276 Kayan River 56 Kedah 88, 131, 187, 307, 308 Kelantan 130 Kelantan River 50, 51, 53 Kelian 20 Kelimutu 268 Kelud 244, 256, 268, 269 Kelvin waves 410 kenaf 96, 98, 99 Kendeng Ridge 154 Kengtung 202 Kenh Ga 170 Keningau 314, 315 Kenny Hill Formation 348 Kenny Hills 346, 352 kentongan 297 Kepulauan Seribu, see Pulau Seribu kerangas forest 98, 164, 167 kerangas soils 98 Kerguelen Plateau 7 kernels, see corestones Ketungau Basin 15 Khammuan Plateau 55 Khao Luang 52, 129 Khemmarat Rapids 203 Khmer Empire 66 Khone Falls 55, 77 Khorat 193 Khorat Basin 20, 52 Khorat Plateau, see Khorat Upland Khorat Upland 40, 47, 51–2, 55, 69, 72, 75, 76, 203, 204, 308, 359 Kinabalu, see Mount Kinabalu King Dyke 364 King Edward Peak 137 King George Peak 137 Kinta Limestone 160 Kinta River 326 Kinta Valley 127, 131, 158, 159, 163, 173 Kinta Valley tower karst 158–64 Klang 344 Klang Delta 230, 233 Klang Gate Dam 380 Klang Gates Ridge 123, 346 Klang River, see Sungai Klang Klang Valley 318, 380, 381, 382, 384 klongs 323, 340, 358, 382, 383 Knema malayana 302 Kobe 136 Koh Samui 390 Kolbano unit 11 Komodo 416 Komodo dragon 34 Ko Nang Yuan 184 Kong River 55, 77, 203 Kontum Massif, see Kontum Plateau Kontum Plateau 20, 55 Korea 212, 214 Kota Kinabalu 327, 332, 416 Kota Marudi 315 Kota Tampan 88 Kra Isthmus 42, 49, 50, 52 Krabi 169, 170

434 Index Krakatau 9, 59, 88, 179, 256, 266, 268, 269, 408 Kramat Pulai 21 Kranji 341 Krasak 282, 294 Kratié 193, 211, 212 Kuala Lumpur 123, 174, 242, 318, 328, 336, 337, 338, 343, 344–56, 345, 346, 379, 380, 381, 382, 383, 384 International Airport 356 slope failure 351 stability 348 Kuala Lumpur Structure Plan 385 Kuala Pergau 130 Pilah 301 Selangor 131 Kuantan 21 Kuching 324, 330, 331, 332 Kuching River 324, 330 Kukusan Fault 276, 280 Kulim 53 Kumphawapi 34 Kunming 199, 202 Kunning 276, 278, 282, 289 Kuroshio Current 410 Kut 52 Kuta 190, 389, 396 Kutei Basin 15, 17 Kwai Noh 50 Kwai Yih 50 Labuan Amuk 151 Labuk 245 Laguna de Bay 306, 326, 327, 340 Lahad Datu 14, 56 Lahar 62, 70, 145, 244, 254, 255, 257– 63, 266, 267, 269, 275, 276, 280, 281, 282, 283, 287, 291, 292, 294, 297, 298, 329 Lai Chau 170 Lake: Kutubu 33 Maninjau 59 Ranau 59 Sentarum 31 Singkarak 59 Tawar 59 Toba 33, 59 lakes 59, 262, 300, 306–7 lalang, see Imperata cylindrica Lalidijiwo 83 Lâm Hà district 211 Lamongan 268 Lampang 169 Lampung 144, 398 Lampung Province 241 La Niña 87– 8, 91, 411 Lancang Jiang 193, 199 land reclamation 332 landfills 339, 383 Langat Dam 380 Langkawi 18, 52, 131 Lan Tsan Chiang, see Lancang Jiang Lao PDR 31, 34, 42, 50, 52, 55, 75, 76, 77, 78, 80, 82, 100, 108, 113, 114, 157, 172, 174, 193, 197, 199, 202, 203, 204, 210, 212, 213, 216, 242, 305, 306, 307, 310 Lassa 228 last interglacial 405, 413 laterite 20, 99, 100 lateritic soils, see laterite

lava dome 275, 276 flows 254–5, 267, 284, 285, 407 Layang Layang 417 Lebong 20 Le Havre 257 Lehmann, H. 170 Lembang 266 Lembang Fault 330 Lembeh Island 182 leptosols 95, 100–1, 103 Leptospernum 25 Lesser Sunda Islands 33, 102, 182 Leucaena leucocephala 110 Leyte 143 Lhasa Block 20 lianas 106 Lich River 326 LRT (light rail transit) 380, 381 Limbang River 164 Lim Chu Kang 324 limestone 59, 108, 151, 153, 157–75, 180, 182, 188 limestone forests 106, 108 limestone topography, see karst Lingayan Gulf 33, 190, 397 lithosols 104 liverworts 110 logged forests 106, 109 logging 35, 108, 109, 111, 113, 116, 199, 207, 211, 241, 243, 300, 305, 393 Lokon 268 Lombok 29, 32, 60, 88, 142, 143, 150, 151, 152, 154, 179, 183, 184, 241, 389, 390, 420 Lombok Strait 408, 410, 420 Long Xuyen Quadrangle 214, 224 Lop Buri River 75 Lorong Halus 324 Lower Seletar Reservoir 340 lowland vegetation 106–10 Low’s Gully 137 Low’s Peak 136, 137 Lubok Mandi 21 Luang Prabang 75 luvisols 95, 98–9, 102, 103 Luzon 13, 14, 20, 32, 33, 60, 62, 70, 86, 87, 97, 143, 144, 168, 244, 256, 257, 306, 314, 327, 394, 409, 410, 417 Luzon Strait 26, 409 MacRitchie Reservoir 340 Mactan Island 182 Madagascar 34 Madaw Island 9 Madura 96, 99, 184, 241, 245 Mae Chaem 310 Mae Hong Son 169 Mae Klong 50, 51, 228, 320, 326, 358 Mae Sai 199, 202 Magaung River 73 magma plug 255 Mahakam Delta 15, 27, 221, 225, 226, 232, 245, 390, 393, 416 Mahakam River 56, 65, 67, 186 Mahakam Shelf 411 Mahasahakhan Formation 204 Mahawu 268 mahogany, see Swietenia macrophylla Mai Chau 171 Main Range (Malaysia) 20, 21, 42, 50, 131, 244, 344, 346, 347

maize 96, 97, 98, 100, 110, 169, 204 Makassar Sea 225 Makassar Strait 15, 40, 177, 403, 410, 411, 416, 420 Malacca, see Melaka Malacca Strait, see Strait of Malacca Malaya 340 Malay Basin 17 Malay Peninsula 25, 31, 32, 33, 34, 39, 42, 50, 51, 52, 67, 69, 72, 81, 90, 107, 108, 112, 123, 125, 131, 137, 142, 160, 162, 183, 186, 230, 233, 301, 330, 340, 393, 403, 415 Malaysia 9, 13, 17, 18, 20, 21, 26, 33, 40, 42, 46, 52, 56, 80, 82, 83, 85, 86, 87, 88, 90, 91, 98, 99, 100, 101, 102, 104, 113, 114, 116, 117, 118, 123, 125, 129, 130, 134, 157, 158, 161, 164, 167, 169, 173, 179, 182, 184, 186, 188, 190, 240, 242, 244, 245, 302, 303, 304, 305, 306, 307, 308, 309, 310, 311, 315, 318, 320, 326, 327, 328, 332, 336, 338, 340, 341, 344, 351, 381, 382, 384, 385, 386, 389, 391, 392, 393, 394, 396, 398, 405, 416, 418, 419 Malaysia, East 131, 136, 182, 305, 342, 392, 418, see also Malaysia Malaysia, West 135, 136, 138, 241, see also Malaysia Malaysian International Biological Programme 302 Malaysian Rural Water Supply Scheme 310 Malesian flora 25 Mali Hka 44, 73 Maluku 34, 99, 107, 112, 113, 417 Maluku Islands 268 Mambucal 267 Mamut 20 Manado 319 Mandai Basin 15 Mandalay 44, 46, 74 Mangin Range 44 Mangahan Floodway Project 327 Mangkalihat Peninsula 416 Mangman 199 mangrove 27, 31, 44, 53, 57, 66, 69, 70, 96, 114, 177, 180, 182, 185–8, 190, 191, 193, 207, 208, 209, 215, 222, 224, 225, 226, 228, 230, 232, 240, 246, 333, 347, 356, 373, 390, 392–6, 397, 398, 403, 415, 418, 420 mangrove forests 106, 111, 112, 220 Manila 266, 316, 317, 318, 323, 324, 325, 326, 327, 329, 332, 336, 337, 340, 342, 379, 382, 383, 390, 395 Manila Bay 190, 324, 326, 338, 383, 395, 399 Manila Trench 11, 14, 60, 143 Manumere Bay 408, 410 Manwan 199 Mapanuepe Lake 262 Maratua Reef 416 Marella Valley 262 marginal basins 3 marginal seas 13–15, 404 Marikangen Volcano 155 Marikina 327 Marinduque 326, 396 marine karst 173 marine protected area (MPA) 392, 421 marine resource extraction 419–20

Index 435 Martaban submarine canyon 222 Martapura 21 Martinique 255 MRT (mass rapid transit) 381 Matang mangroves 394 Mataram 280 Matunggong 315 Maubisse-Aileu unit 11 Mawchi Mine 21 Maxwell Hill 301 Mayon 62, 267, 269 Medang 408 Medalam Gorge 164 Medalam River 165 Median Graben 59 Megamuntiacus vuquangensis 172 Meiktila 46 Mekong 26, 39, 40, 42, 44, 50, 52, 55, 65, 66, 67, 71, 72, 75– 8, 76, 77, 87, 99, 112, 193–216, 222, 232, 243, 245, 246, 306, 308, 311, 389, 393, 415 Mekong-Chi-Mun transfer 308, 309 Mekong Committee 197 Mekong Delta 27, 56, 67, 89, 95, 182, 190, 207–10, 208, 221, 222–5, 224, 232, 233, 245 Mekong Lowland 42, 54, 55 Mekong River Basin 193–216, 194, 311 Mekong River Commission 78, 199, 211, 306 Mekong, water resources 212–14 Melaka 53, 138, 245, 338, 389 Melaleuca 109, 112, 214–15, 225, 232 cajuputi 214 leucadendron 96, 102 mélange 14 Melinau Gorge 164, 167 Melinau Limestone Formation 164 Melinau River 165, 167 Melinau-Paku River 165 Valley 164, 165 Memorial Bridge, Bangkok 364 Mengla 202 Mengkuang 351, 353 Mentawai 7, 15, 416 Merak 257 Meramang dyke 155 Merapi 70, 244, 255, 257, 266, 268, 269, 273, 275–99, 277, 278, 279, 329 Merapi Volcano Observatory 297 Meratus Mountains 17, 56 Merbok Estuary 230 Mergui 46, 389 Mergui Archipelago 46, 415 Mergui Ridge 40 Mergui Terrace 15 meteor strike 33 Meteorological and Geophysical Centre, Yogyakarta Airport 287 MWSS (Metropolitan Waterworks and Sewage System), Manila 340 methane 117 Metroxylon sagu 112 migration 116, 316–17 Mikir Hills 9 milkfish, see Chanos chanos Minabalac 327 Minahasa 143 Minas oilfield 17 Mindanao 26, 60, 62, 85, 100, 112, 143, 145, 186, 306, 417

Mindanao Current 179, 410 Mindanao Trench 177 Mindoro 397, 417 Mindoro Strait 417 mineral deposits 20–1 minor surface solution forms 173 Miri 17, 342 MISEDOR core 225 Moc Chau 170 mogote 169 Mohr, E. C. J. 94, 148 moist deciduous forest 106, 108 Mokol Volcano 155 Molengraff, G. A. F. 31, 67, 403 Molengraff River 26 Molucca Sea 8, 11, 14, 60, 177, 417 Moluccas 34, see also Maluku Mondulkiri 211 Monkey Cheek project 366 monoculture 110 monsoon 38, 62, 67, 71, 78, 80, 81, 81–3, 85, 90, 170, 177, 179, 180, 184, 197, 215, 256, 300, 305, 315, 392, 408, 415 northeastern 53, 65, 71, 79, 81–2, 86, 88, 179, 180, 183, 407, 409, 410 southwestern 65, 71, 72, 75, 79, 82–3, 85, 179, 197, 243, 319, 407, 409, 415 monsoon trough 83 montane vegetation 106, 110–11, 114, 167, 168 Montipora spp. 416 montmorillonite 96 mosses 98, 110 mossy forest 110 motor vehicle emissions 319 Moulmein 46 Mount: Baru 152, 153 Canlaon 267, 329 Gadjahmunkur 276 Jaya 29, 32, 127 Karang 155 Kerinci 143 Kinabalu 15, 17, 20, 29, 32, 40, 56, 110, 136–7 Lamongan 144 Maranat 145 Matutun 145 Merbuk 151 Ophir, see Gunong Ledang Patas 151 Pelée 255 Popa 8, 9, 46 Punikan 151 Pusonglondon 276 Semeru 254, 257 Wilhelm 32 Muang Khannouan 203 Muang Sing 199 Muar 53 Muaras Reef 416 Muda 53, 307, 309 Muda Irrigation Scheme 308, 309 mud volcano 179 Mui La 129 Mukdahan 193, 203 Mulu 162 Mulu Formation 164 Mun River 52, 69, 72, 76, 204 municipal and hazardous wastes 383–4

Muong Khen 170 Muong Tai-Pi Toong 170 Muong Trai Formation 170 Musi Banyuasin Estuary 190 Myanmar 3, 7, 9, 15, 17, 18, 20, 21, 38, 40, 42, 44, 46, 50, 73, 74, 78, 80, 81, 96, 97, 100, 102, 105, 106, 107, 108, 110, 113, 114, 117, 118, 129, 130, 157, 169, 179, 182, 184, 188, 190, 193, 199, 202, 211, 212, 301, 306, 308, 310, 393 Myitkyina 46, 73 Mykura 351 My Phuoc 210 Naga City 327 Naga Hills 9, 42, 44 Nahai Nam Theun National Biodiversity Conservation Area 213 Nakai Plateau 203, 213 Nakhon Luang Aquifer 367, 369, 370, 371 Nakhon Sawan 51, 74, 358, 359, 364 Nakhon Si Thammarat 52 Nakhon Si Thammarat Range 52 Nam: Ca Dinh 203 Kading 203 Khong 169 Lang 169 Mae Kok 202 Mun, see Mun River Ngum 197, 202 Ou 42, 239 Theun 193, 203, 213 Theun 1 213 Theun 2 202, 213 Theun Hinboun 202, 213 Nan 50, 51, 74, 358 Nanusa Archipelago 417 Nan-Uttaradit suture 17 Narcondam Islands 9 National Environment Quality Act 398 Natuna Archipelago 182, 416 natural hazards 250, 254 Negeri Sembilan 301, 344 Negritos 116 Negros 14, 62, 143, 330, 396 Negros Trench 143 Nepenthes muluensis 167 Netherlands 197 Neumann van Padang, M. 268, 287 New Guinea 25, 26, 29, 31, 32, 33, 34, 86, 105, 107, 111, 112, 116, 129, 142, 172, 173, 177, 230, 407, 408 New Zealand 34, 131 Ngoc Linh 129 Ngupasan 297 Ngurah Rai Airport 396 Nias 7, 15 Niah Cave 34 nickel 20, 108 Nicobar Fan 7 Nicobar Islands 416, 421 Ninety-East Ridge 7 Ninh Binh 170 Ninth International Coral Reef Symposium 392 Nipa palm, see Nypa fruticans nitisols 95, 99–100, 102, 103 nitrogen oxides 117, 212, 318, 319 nitrous oxide, see nitrogen oxides Nmai Hka 44, 73

436 Index Non-governmental organizations (NGOs) 385, 386, 392, 398, 399 Nong Pa Kho 32 North Equatorial Current 179, 410 Northern Mountainous Region 42, 43, 50, 53, 73, 76 North Moluccas Plate 143 North Sumatra Basin 17 North Sunda River, see Molengraff River North Viet Nam Plain 42, 54, 55– 6 Nothofagus 26, 111 Novaliches River 340 Nuées ardentes 254, 255 Nui Con Voi Ridge 42 numerical modelling/simulation 275, 276 Nunbei 72 Nusa: Dua 188, 396 Penida 151 Tenggara 32, 60, 107, 142, 268 Nyamuk Besar 420 Nypa fruticans 27, 111, 188, 209, 225, 228, 231 oaks, see Quercus ocean trenches 7, 8, 9, 11, 13, 14, 39 oil, see petroleum oil palm 35, 91, 97, 99, 100, 102, 110, 209, 241, 301, 382, 419 oil spills 420 Okinawa Trench 14 Old Alluvium 40, 161, 162, 163, 341, 354 Ombai Strait 410 Oncosperma 232 Oncosperma tigillarum 225 ophiolite 9, 14, 17, 20, 21 orchids 98, 167, 168 overfishing 391, 392 ozone 89, 90 Pabelan River 283 Pacific 13, 15, 20, 25, 31, 34, 80, 82, 83, 85, 86, 87, 91, 179, 180, 220, 405, 408, 410, 411 Pacific Plate 3, 60, 142, 143, 157, 177, 403, 405 Pacitanian stone tool 33 Padang 182 padang soil, see podzols Padas 245 paddy, see rice paddy soils, see anthrosols Pahang 125, 128, 130, 135, 306 Pahang River 50, 51, 53, 65, 67, 69, 72, 230 Paka Cave 136, 137 Paksé 203 Palau-Kyushu Ridge 14 Palaeotethys ocean 17, 20 Palawan 17, 21, 168, 182, 186, 188, 409, 410, 417 Palembang 187, 230, 390 palms 26, 167 Palu Valley 85 PAM Jaya (Perusahaan Daerah Air Minum) 338 Pa Mong Dam 197, 199, 202 Panay 62 pandans 168 Pandanus 168, 183, 184 Pandeglang 189

Pangalengan 83 Pangasius larnaudii 214 micronemus 214 Pangrango 83 Panjang Island 155 Pantai Valley 352 paperbark, see Melaleuca Papua 3, 32, 34, 40, 83, 98, 99, 100, 101, 102, 104, 105, 110, 111, 112, 114, 116, 142, 182, 186, 241, 250, 301, 393, 404, 408, 410, 416 Papua New Guinea 32, 33, 157, 173, 232, 404, 406, 408, 413 Paracel Islands 417 Parakasak Volcano 155 Paranchangan 127 Paraserianthes falcataria 110 Pari Group Reefs 412 Pa Sak 51, 75, 358, 364 Pa Sak Dam 365 Pasarbubar atrio 276, 280 Pasig catchment 262 Pasig-Potrero 259 Pasig River 326, 327, 340, 382, 383 Pasoh Forest Reserve 301, 302, 304 Patkoi Hills 42 Pattani River 230 Pattaya 389, 390, 396 Paungchon Taung 46 Pau Tsi Luing 129 Payo 329 Payung Volcano 155 Pea Bullock 29 Pearl Bank Sill 410 peat 31, 35, 70, 79, 90, 98, 102, 112, 116, 167, 226, 228, 230, 241, 330, 333, 390, 420 fire 35 swamp, see peat swamp forests 106, 112, 226, 228, 230, 232, 233 peaty soils, see histosols pebble tools 34 Pedu 308 Pegu Yoma 44, 74 Peirce Reservoir 340 Pekanbaru 319 Peléean 255, 287 Pelepah Kanan 21 Pematang Formation 15 Penan 174 Penang 53, 87, 129, 131, 245, 301, 315, 342 Penggaron Timur 309 Pengkalan 53 pepper 104 white 100 Perak 128, 158, 164, 304 River 46, 51, 53, 230 Perangtritis 182, 184 Perhentian 131, 183 Perlis 88, 127, 307, 308 permatang 53 Perth 135 pes-caprae formation 184 pesticide 418, 419 Petra-Jaya housing estate 331 petroleum 3, 39, 220, 228, 390, 418 Petronas Twin Towers 355 Peusangan Delta 230 Phang Hoei Range 51

Phang-Nga 15, 169 Bay 169, 182, 395 Phapheng Falls 77, 78 Phattalung 169 Phetchabun Mountains 51 Philippine Fault 11, 60 Islands 42, 60–2, 61, 177, see also Philippines Mobile Belt 143 Sea Plate 3, 11, 32, 60, 142, 143, 403 Trench 11, 14, 60, 143 Philippines 3, 8, 11, 13, 20, 21, 26, 32, 33, 38, 39, 60, 70, 79, 80, 81, 82, 83, 85, 86, 87, 88, 89, 100, 102, 113, 114, 116, 117, 118, 136, 143, 144, 145, 154, 157, 158, 169, 177, 179, 180, 182, 183, 186, 188, 189, 190, 219, 239, 244, 245, 250, 255, 256, 257, 267, 269, 273, 297, 305, 306, 314, 317, 320, 323, 324, 326, 329, 330, 390, 391, 392, 393, 394, 395, 396, 397, 398, 399, 408, 410, 413, 417, 418, 419, 421 Philippines, karst 168–9 Phitsanulok Basin 17 Phivolcs 267, 269, 273 Phnom Kravantes, see Cardamom Hills Phnom Penh 55, 77, 78, 207, 223, 224, 318, 337, 338, 342 Phong Nha Cave 172, 173 Phong Nha Karst Region 171 phreatic explosions 263–4 Phuket 15, 18, 20, 21, 52, 245, 390, 395, 396, 406, 407, 412, 413, 415 Laguna 396 Range 52 Phu Phan Hills 52, 203 Phyllocladus 26, 32 physiographic provinces 38, 40–62, 41 phytokarst 167, 168 Pinang Volcano 155 Pinatubo 60, 62, 70, 89, 90, 143, 145, 244, 255, 256, 257, 259, 261, 262, 266, 267, 269, 282, 297, 329, 408 pine, see Pinus pineapple 96, 112 Ping 50, 51, 74, 358 pinnacle karst 160, 165, 168, 171, 172–3, 328, 354, 355 pinnacles, see pinnacle karst Pinus 25, 31, 111 kesiya 110 mercusii 97, 98, 110 pitcher plant, see Nepenthes muluensis pitting 135 placer deposits 21 Plain of Jars 42, 216 Plain of Reeds 209, 214, 224 plantation 102, 104, 110, 114, 117, 169, 241, 304, 352 Platanus 172 plate tectonics 3–21, 32–3, 38, 39, 142, 154, 231, 250, 402, 405, 407, 413 Plawangan 276, 280 Pleikrong 203 Pleiku 216 Pleistocene 9, 13, 20, 24, 25, 29, 31, 32, 33, 34, 38, 39, 40, 46, 52, 56, 65, 67, 68, 75, 88, 110, 125, 146, 151, 153, 154, 161, 171, 177, 179, 190, 208, 220, 224, 230, 254, 328, 329, 405, 417 Plinian eruption 255, 280, 287, 289, 292

Index 437 plinthite 100 plinthosols 95, 100, 102, 103 plywood 113 Podocarpus 26, 31 podzols 95, 98, 102, 103, 104 poisonous gas 264, 269 polished blades 34 polje 168, 169, 170, 171, 173 pollen 29, 31, 34, 224 pollutants 248, see also pollution pollution 306, 311, 320, 338, 339, 340, 341, 390, 391, 392, 393, 395, 402, 418, 419, 420 air 314, 317, 318, 319, 320, 332, 381–2 urban 379– 87 water 306, 307, 311, 324, 382–3 polygonal karst 168 Pompengan 309 Population 95, 96, 102, 112, 113, 116, 117, 118, 169, 170, 174, 210, 212, 216, 219, 254, 275, 289, 291, 292, 294, 300, 314, 316–17, 336, 379, 383, 391, 394, 395, 402, 418 Porites lutea 415 Porong Delta 231 Port Elizabeth 257 Port Klang 332, 356, 382 post-construction impacts 349 Prawirodirjan 294, 295 prawn 209, 215, 230, 233, 356, 390, 392– 6, 420 precipitation 300–1 Prinya Nutalaya 367 Progo River 70, 244, 266, 289, 291 Prokasih (Clean River Programme) 326 Proto South China Sea 15 Province Wellesley 308 Proyek Pesisir 398 Pucangan Formation 33 Puchong 353 Puerto Galera 397 Puger 184 Pulasari Volcano 155 Pulau: Indah 332 Ketam 310 Pinang 327, 332 Semakau 324 Seribu 183, 390, 395, 407, 409, 412, 419, 420 pumice 143, 152, 155, 276 Puncak Jaya 110 Putih Valley 152, 282 Pygathrix nemaeus 172 pyroclasts: blast 280 density current 275 eruption, see flow below flow 62, 70, 239, 244, 254, 255, 256, 257, 262, 266, 267, 269, 275, 276, 280, 281, 282, 284, 287, 289, 291, 292, 297 surge 255, 267, 275, 280, 284, 287, 289 Quang Nam 318 Ninh 318 Ninh Province 171 Quaternary 24–35, 39, 46, 51, 52, 67, 78, 142, 144, 151, 157, 158, 170, 173, 177, 179, 219, 220, 225, 228, 232, 330, 333, 339, 341, 354–5, 361, 405

Queen Alexandra Park 137 Quercus 26, 32 Ragang 145 rain-day 85 rainfall 65, 80, 83–5, 84, 91, 209, 215, 225, 239, 241, 257, 297, 300, 301, 305, 306, 311, 316, 320, 336, 342, 344, 348, 404, 407 rainforest 26, 31, 38, 39, 65, 97, 100, 116, 172, 240, 241, 303, 316, 327, 348 Rajang: Delta 221, 226– 8, 227, 233 Group 15 Gulf 13 River 56, 72, 226– 8 Rajmahal Traps 7 Rakhine Hills, see Arakan Yoma Rakhine Yoma, see Arakan Yoma ramin, see Gonystylus bancanus Ramu River 219 Ranau Lake 266 Rangoon River 222 Ratburi 15 Ratburi Limestone 20 Ratanakiri 211 rattans 106, 116 Raub 21 Raung 147, 268 Rawang 344 recycled water 342 Red River, see Sông Hóng Red River Fault 40 red soils 94 red tides 395 regolith 123, 125, 126, 127, 135, 138 Rembang Ridge 154 Resettlement scheme, see transmigration Rhizophora 187, 225 Stylosa 187 Rhododendron 26, 111, 167, 168 Riau-Lingga Archipelago 182, 185, 390, 415 rice 35, 88, 91, 96, 97, 98, 99, 100, 101, 102, 110, 112, 113, 149, 151, 169, 204, 209, 215, 219, 225, 230, 232, 233, 307, 308, 309, 310 ricefields 106, 112, 149, 199, 203, 216, 242, 308, 393 rillenkarren 170 Rinjani 152, 153, 154 Rio de Janeiro 126 risk 250, 254, 292, 293, 297 risk, Merapi 275–99 rivers 65–79, 125, 183, 190, 199, 239, 240, 241, 245, 300, 306, 337, 407, 415 Rizal Province 317, 324 rock basins 135 rock coasts 180–3 rockfalls 257 Romang 11 roof water collection 320 root systems, mangrove 187 Rossby waves 410 Rote 405 Royal Irrigation Department, Thailand 309 rubber 91, 99, 100, 102, 110, 199, 241, 301, 302, 352, 419 rural water supply 300–11 Rutaceae 26 Ryukyu 39

Sabah 14, 17, 20, 29, 40, 56, 87, 88, 89, 113, 125, 127, 131, 135, 136, 157, 158, 179, 182, 188, 190, 241, 242, 302, 304, 305, 315, 327, 332, 391 Sabah Orogeny 15, 17 Sabo dam structure 297 Technical Centre, see Balai Sungai dan Sabo Sabu 405 Saccharum sp. 228 Sagaing 74 Sagaing Fault 9, 15, 40, 46, 74 sago palm, see Metroxylon sagu Sahul Shelf 25, 26, 39, 404 Saigon, see Ho Chi Minh City St John’s Peak 137 St Paul Subterranean River National Park 168–9 St Paul Underground River 168–9 Sakon Nakhon Basin 52 Salacca rupicola 167 saline intrusion 209–10, 342 salinity 180, 204, 392, 407, 408, 410 salinization 309, 340 salt ponds 111, 230 salt-water intrusion 300, 374, 375, 380, 390, 393, 394 Salween 38, 42, 46, 50, 65, 68, 193, 199, 245 Delta 245 Sambisari ash 280, 289 Temple 280 Sambor 212 Samet 52 Samut Prakarn 358 Sakhon 78, 358 San Carlos City 330 Sandakan 14 sandy coast 183–5 Sangean Island 154 Sangeang Api 268 Sangihe 11, 143, 417 Sangiran 33 Dome 33 Sangir Reef Archipelago 417 Sankamphaeng Range 51 San Mateo landfill 324 San Miguel 329 San River 55, 77, 203 Santo Thomas Valley 259, 260 Sanur 183, 188, 189, 190 Sapudi 416 Saraburi Limestone 20 Sarangani Sill 410 Sarawak 13, 17, 21, 34, 56, 87, 88, 89, 91, 98, 102, 113, 135, 157, 158, 162, 164, 172, 173, 174, 182, 226, 245, 310, 318, 324, 330, 332, 342, 418 Chamber 166 Orogeny 15 Sat 282 satellite images 240, 244, 245 Sattahip 52 Satuan Koordinasi Pelaksanaan Penanggulangan Bencana Alam 298 Satun 169 savanna 100, 107, 111, 112 forest, see deciduous dipterocarp forest Savannakhet 76, 203 savannas and grasslands 106, 109 sawnwood 113 sea grass 189–90, 397, 413, 415, 420

438 Index sea-level change 13, 25, 26–9, 35, 44, 56, 66, 67, 78, 91, 158, 162, 169, 170, 177, 179, 190, 191, 219, 220, 222, 232, 248, 328, 330, 343, 354–5, 399, 405– 6 sea-level rise, see sea-level change sea surface temperature 180, 412 Se Bang Hiang, see Banghiang River Sebuku 186 Scaevola taccada 184 Scarborough Reef 417 Seamounts 13 Schomburgk’s deer 112 Schwaner Range 56 scoria 276, 284 Scrivenor, J. B. 127, 130, 131 secondary forest 106, 108–9 sediment 68, 72, 219, 221, 222, 304, 391, 392, 394, 402, 403, 404, 407, 415, 416, 417, 418, 420 yield 239– 48 Segama 305 Se Kong, see Kong River Selangor 128, 131, 301, 304, 310, 356 Selangor River 320, 326 Semangko fault zone 144, 154 Semarang 418 Semenyih Dam 380 Semeru 136, 268, 269 Sengdiku 189 Senggigi 183 Senowo 282, 289 Sentani 34 Sentul 355 Sepik River 219 Seraja 151 Seram 112, 416, 418 Serang 189 Se San, see San River Se San 3 203 4 203 Sesquioxides 99 Setap Shale 17, 164, 167 Shan Highland 9, 18, 40, 44, 46, 48, 50, 51, 73, 222 Plateau, see Shan Highland States 21, 169, 310 Shifting agriculture 97, 99, 100, 102, 109, 110, 113, 169, 174, 197, 202, 203, 210, 211, 214, 241, 243, 244, 300, 305 Shifting cultivation, see shifting agriculture Shilin 173 Shillong Plateau 9 Shorea 232 albida 228 shrimp, see prawn shrimp farming, see prawn shrubland 32 shrublands and thickets 106, 109 Siberia 82 Sibumasu, see Sinoburmalaya Si Chang 52, 188 Sierra Madre Range 62 Silanglaya 151 silica solution 126 siliceous speleothems 126 silicon dioxide 148 Similian Archipelago 413, 415 Simkin, T. 256 Sinbo 73 Singa Formation 18

Singapore 31, 39, 40, 46, 51, 53, 80, 82, 83, 85, 89, 90, 114, 117, 126, 127, 135, 175, 180, 182, 188, 190, 213, 241, 242, 243, 305, 306, 317, 318, 320, 324, 327, 328, 332, 336, 337, 338, 340–2, 341, 380, 381, 382, 383, 384, 385, 386, 389, 395, 396, 406, 415, 419 Singapore River 324, 383 Sing Buri 75 Sinkep 129 sinkholes 135, 328 Sinoburmalaya 3, 9, 15, 18, 20 Sipitang 315 Sip Song Chau Thai Ridge 42 Sipsongpanna 199 Sirikit Dam 74, 364 sisal 100 Sittang 46, 96 Slamet 149 slash and burn 90 slums 379 smectite 129 Snellius expedition 402 snow line 29 Soc Trang Province 210 soil erosion 123, 204, 241, 349–52 formation 95 soils 94–104, 202, 219 solar bleaching 415 solfatara fields 143 solid waste 323– 4, 340, 383, 419 soliton 410 Solo Delta 221, 230, 231 River 219, 230, 231, 232, 407 Sombrero Island 188, 397 Sông Ca 67 Sông Ch¯ay 55, 67 Sông Hóng 38, 40, 42, 55, 65, 67, 112, 182, 231, 305, 389 Sông Hóng Delta 95, 113, 190 Son La 170 Sonneratia . . . 187, 222, 225 Caseolaris 225 Soputan 268 Sorikmerapi 149, 150 Sorong Fault 8, 416 South America 102, 110, 411 South Asia 117 South China Sea 17, 26, 31, 32, 40, 50, 51, 53, 55, 65, 67, 81, 82, 83, 86, 87, 91, 177, 179, 180, 193, 199, 207, 215, 219, 223, 362, 402, 403, 404, 409, 417, 420 South China Sea Basin 13 South Korea 82 South Pacific High 411 South Sumatra Basin 17 Southern Ocean 24 soybean 96, 97 speleothem 161, 169, 170, 173 Spermonde Archipelago 417, 419 Spinifex littoreus 184 spit 27, 179, 183 spitzkarrren, see pinnacle karst SPOT 244, 276, 294 Spratly Islands-Dangerous Grounds 17, 417 springs 169, 171 squatter settlements 353, 379 Sra Kaeo 17 Srepok River 55, 77, 203 Sri Lanka 7 staghorn coral 189

stalactites 126, 167 stalagmites 126 Stegodon 33 stemflow and throughfall 302 steppe 32 stone forest, see Shilin Strait of Madura 231 Strait of Malacca 3, 15, 46, 51, 53, 58, 65, 67, 69, 180, 186, 233, 338, 344, 390, 395, 403, 405, 415, 418, 420 Strait of Surabaya 230 stratovolcano 59, 143, 145, 147, 149, 151, 152, 155 Stung Sangkor 326 Stung Treng 197, 211 subak 307 Subang Airport 381 subduction 3, 9, 14, 15, 56, 142, 151, 250 subsidence 233, 314, 328, 333, 339, 342, 358–78, 394 subtropical forest 32 sugar 35 sugarcane 88, 96, 97, 98, 102 Sulawesi 3, 13, 15, 20, 26, 27, 29, 31, 33, 34, 60, 85, 88, 99, 106, 108, 112, 113, 125, 143, 157, 173, 174, 180, 182, 183, 186, 188, 268, 309, 319, 390, 391, 393, 398, 403, 407, 413, 416, 417, 418, 419, 420 Sulawesi Sea, see Celebes Sea sulphur 89, 149, 162 dioxide 89, 90, 212, 318, 319 Sulu Archipelago 14, 142, 390, 418 Sulu Sea 14, 143, 403, 404, 409, 410, 417 Sulu Spur 125, 416 Sumatra 3, 7, 8, 9, 13, 15, 17, 20, 26, 29, 31, 33, 34, 35, 39, 40, 56, 57, 58, 59, 67, 69, 70, 81, 85, 90, 96, 97, 98, 99, 100, 101, 102, 104, 107, 108, 110, 111, 112, 113, 116, 125, 129, 142, 143, 144, 149, 153, 154, 177, 179, 180, 182, 183, 186, 187, 188, 190, 230, 233, 241, 245, 255, 257, 266, 267, 301, 306, 319, 382, 390, 391, 393, 403, 407, 408, 410, 411, 418, 419, 420 accretionary prism 7 Fault 15 Trench 7, 9 Sumba 34, 144, 179, 230, 405, 410 Sumbawa 9, 60, 88, 179, 266, 408 Sunda arc 154, 404, 405, 416 Sundaland 3, 13, 15, 20, 26, 40, 57, 67, 403, 404, 405, 415–16 Sunda Plate 125 Sundarbans 231 Sunda Shelf 13, 25, 26, 31, 39, 59, 60, 107, 116, 151, 168, 171, 177, 180, 190, 219, 221, 225, 230, 232, 233, 403, 404, 407 Sunda Strait 9, 31, 59, 142, 144, 154, 257, 390, 408 Sunda Trench 3, 7, 9, 14, 56, 177 Sunda volcanic arc 157 Sungai Anak Ayer Batu 352 Gombak 126, 304 Jinjang 241, 353 Kampar 162 Keroh 353 Klang 126, 338, 344, 352, 353, 355, 382, 383 Lembang 21

Index 439 Lui 301 Marong Kanan 304 Mentawai 167 Merbok 187 Sering 351, 353 Setoh 159 Tekala 304 Tekam 302, 303, 305 Tutoh 164 Suoh pull-apart basin 141, 154 Surabaya 337, 342, 390, 395, 418 Surat Thani 245, 305 surface water supplies 320 Surin Archipelago 413, 415 suspended particulate matter 318, 319, 381 sustainable development 199, 211, 215 suture 9, 15, 17, 18, 20 Swallow Reef, see Layang Layang swamp forest 90, 116 Swatch-of-No-Ground 7, 231 Sweden 197 sweet potato 96, 97, 100, 110 swidden, see shifting agriculture Swietenia macrophylla 110 Switzerland 197, 212 Taal 62, 144, 266, 267, 273, 297 Ta Chin 99 Tadarida plicata 168 Tahiti 405, 411 Taiping 301 Taiwan 3, 8, 11, 14, 26, 34, 143, 213, 403, 409 Tak Province 358 Taka Bone Rata 188, 417 Takhek 213 Talang 268 Talaud Ridge 11, 417 Tama Abo Range 56 tambak 186, 393 Tambanen 151 Tambelan Archipelago 416 Tambora 9, 60, 88, 144, 255, 266, 269 Tampin 134, 135 Tangkuban Perahu 265, 266, 268, 329 Tangkuban Perahu-Sunda Complex 264, 266, 269 Tangkulan 145 Tanimbar Archipelago 405 Taninthayi 301 Tanjing Bugel 245 Tari 33 taro 169 Tasik Bera 306 Chini 306 Tasmania 33 Tavoy 21, 46, 130 tea 99, 102 teak, see Tectona grandis Tectona grandis 97, 99, 108, 110 tectonic depressions 144 telaga 170 Telisa Formation 17 Telok Betong 257 Teluk Bahang 332 Tembelan 182 Tembeling River 69 Temburong 301 Tempe Depression 27 temperature 83, 315 water 180

Templer Park 347 Tenasserim Coast 42, 46, 47, 50, 69, 182, 184, 245 Tenasserim Hills 46, 47, 50, 51, 52 Tenasserim Yoma, see Tenasserim Hills Tenejeros-Tullahan River 383 Tengerang 338 Tengger-Jambangang 269 tephra 255, 257, 264, 266, 275, 276, 280, 281, 284, 287, 288 tephra falls, see tephra Tepus 170 Terminalia 207 cattapa 184 Tertiary sedimentary basins 15–17 Tha Chin 75, 358, 362, 364, 373 Thailand 9, 15, 17, 18, 20, 21, 25, 26, 27, 31, 32, 33, 34, 40, 46, 50, 51, 52, 67, 75, 78, 80, 82, 83, 86, 87, 97, 98, 99, 100, 102, 106, 108, 112, 113, 114, 117, 126, 129, 131, 157, 173, 174, 179, 182, 183, 184, 186, 188, 190, 193, 197, 199, 202, 203, 211, 212, 213, 214, 230, 241, 242, 301, 304, 305, 306, 307, 308, 309, 310, 311, 317, 319, 320, 323, 325, 326, 328, 358, 362, 380, 390, 392, 393, 394, 395, 396, 398, 406, 407, 410, 411, 412, 415, 421 Thaton 9 Thespesia populnea 184 thorn forest 108 Thousand Islands, see Pulau Seribu Three Pagodas Pass Fault 17, 46, 50 Thuarea involuta 184 Thungyai-Huai Kha Khaeng Wildlife Sanctuaries 170 Tibet 20, 32, 38, 40, 65, 71, 73, 75, 193, 199 Tibetan Plateau, see Tibet tidal creek 220 tide 180, 188, 408 Tiga Puloh Mountains 15, 18 timber 108, 233, 382, 419 Timor 11, 31, 32, 60, 72, 142, 230, 239, 405, 407, 410 Sea 180 Strait 410 Trench 11 tin 3, 7, 20, 21, 31, 39, 123, 160, 161, 162, 163, 306, 326, 353, 354, 355, 393, 395 Tioman 131, 135 Toba 33, 59, 88, 144, 154, 266, 306 tobacco 96, 97, 98, 99 Togian Islands 417 Tokyo 380 Tomini Bay 413, 417 Tompo Volcano 155 Tonlé Sap 52, 55, 67, 78, 204–7, 205, 206, 212, 223, 306, 307 fishery 207 River 55, 78, 193, 223 total suspended particulates, see suspended particulate matter tourism 150, 174, 175 tower karst 39, 157, 160, 161, 168, 169, 173, 182, 328, 346 Trachypithecus francoisi 172, 174 T.f. hatinhensis 172 T.f. laotum 172 traffic congestion 380 Trannih 42

transboundary atmospheric pollution, see haze Transkei 135 transmigrasi, see transmigration transmigration 102, 232, 233, 241, 382 tree crops 110 tree plantations 110 Trengganu 131 Trising River 289 troglobitic species 168 tropical cyclone 85, 86–7, 91, 179, 190, 239, 244, 256, 306, 314, 329, 390, 392, 410 tropical deciduous forests 106, 107–8 tropical logs 113 lowland evergreen rainforest, see tropical rainforest rainforests 106–7 seasonal forests 106, 107 storms, see tropical cyclone tsunami 59, 179, 191, 256–7, 390, 408 Tubbataha Reefs 417 tuff 59, 144, 154, 155, 266, 276, 408 Tukang Besi Archipelago 417 Tukung Volcano 154, 155 Turgo 276, 280, 284 typhoon, see tropical cyclone Typhoon: Babs 326, 327 Bart 87 Ryan 87 Ubin 131, 134, 135 Ubol 193 Ukay Heights 349 Ulu Gombak 349 Ulu Klang 349, 351, 353 Ulu Langat 303, 304 Ulu Pandan 324 Umbgrove, J. H. F. 403, 409 Umiray River 340 Umtata 135 Una-Una Volcano 143 UNEP/IUCN 390 United Nations 197, 311 UNDRO (United Nations Disaster Relief Co-ordinator) 250, 273 UNESCO 273 United States 212, 383 United States Geological Survey 267 Unzen 287 urban atmosphere 318–20 climate 381 development, see urbanization environment 39, 314–33 rivers 324 slope failure 332, 348, 349 urbanization 35, 53, 111, 116, 240, 327, 343, 347, 349, 352, 362, 379, 382, 384, 385 USDA Soil Taxonomy 95 Vaccinium 26 Van Bau 129 van Baren, F. A. 148 van Bemmelen, R. W. 144, 268 van Steenis, C. G. G. J. 106 vegetation 105–18, 173, 182, 186, 187, 203, 209, 219, 222, 232, 239, 240, 242, 245, 246, 301, 302, 315, 328, 351, 354, 381, 385

440 Index vermiculite 126 Verstappen, H. Th. 25, 31, 59, 409 vertisols 95, 96–7, 102, 103 Vidal, J. E. 106 Vientiane 52, 75, 76, 193, 202, 204, 337 Viet Nam 13, 20, 27, 31, 34, 38, 42, 55, 65, 67, 72, 78, 80, 82, 86, 87, 89, 96, 98, 99, 100, 102, 114, 118, 131, 132, 157, 173, 175, 179, 180, 182, 183, 184, 186, 190, 193, 197, 202, 203, 209, 210, 211, 213, 214, 215, 216, 231, 234, 239, 245, 255, 301, 305, 306, 308, 314, 316, 317, 318, 320, 322, 323, 326, 328, 392, 394, 399, 409, 418, 421 Viga 329 Vinh Hoa 215 Visayas 86, 394 volcanic hazards 250–73 islands 142–55 landforms 143– 4 mudflow, see lahar zones 250 Volcanological Survey of Indonesia (VSI) 256, 268, 273, 280 vulcanian 255 vulnerability 250, 275, 276, 289, 290–1, 291, 294, 296, 297 Vung Tau 129, 318 Vung Tau Graben 67 Wahgi River 33 Wallaby Plateau 7

Wallace, Alfred Russell 130, 138 Wallace’s Line 26, 105 wang 50, 74, 173, 358 waste water 323, 326, 338, 339, 341, 395, 396, 418 water balance 65, 385 water, cities 336– 43, 380 Waterfall Forest Reserve 301 weathering 40, 95, 96, 97, 99, 100, 101, 123, 125–9, 304, 316, 346, 347, 407 Weber 31 Weber Deep 14, 404 West Burmah Plate, see Burmah Plate West Crocker Formation 15, 17 West Natuna Basin 17 West Philippine Sea Basin 14 Western Coastal Plain of Malay Peninsula 42, 49, 53 Western Myanmar Hills 42–4, 45 Wetar 11, 142, 405 wetland 106, 111 Wharton Basin 15 Wharton Ridge 7 Wheeler, Raymond 197 Whiteodendron moultanian 98 Whitmore, T. C. 106 wildlife 114–16 wild ox, see Bos Sauveli Witty Range 56 Wonosari 170 World Conservation Monitoring Centre (WCMC) 115, 117–18

World Health Organization (WHO) 319, 320, 338, 382, 383 World Reference Base for Soil Resources (WRB) 94 World Resources Institute 391 World Wildlife Fund 382 Woro 276, 278, 283, 289 wrench fault 9, 15, 17, 20, 33, 125 Xe Bang Fai 213 Xiaowan 199 Xieng Khuang 42, 193 Xizang, see Tibet Yali Falls Dam 203 Yaly Reservoir 211 Yang Plateau 83 Yangon 33 Yangtze 112, 193, 199 Yen Chau 171 Yogyakarta 255, 266, 276, 280, 287, 288, 289, 294, 297, 298, 310 Yom 50, 51, 74, 358 Yuas Marine Transfer Station 324 Yunnan 29, 32, 38, 42, 46, 173, 193, 197, 199, 202, 213, 215 Zambales 20, 62 Zamboanga 14, 417 Zingyaik Range 130 Zooxanthellae 411