PREFACE
Muddy coasts are land-sea transitional environments commonly found along lowenergy shorelines which either receive large annual supplies of muddy sediments, or where unconsolidated muddy deposits are being eroded by wave action. Muddy coasts are found in all kinds of climates and under any tidal conditions. Accordingly, their geographic distribution ranges from low tropical to high sub-arctic latitudes and from microtidal to macrotidal coastal settings. The most conspicuous examples are the vast mangrove swamps of the tropics and the extensive salt marshes fringing the shores of estuaries and back-barrier lagoons of mid-latitudinal coasts. Muddy coastal environments harbour highly variable and fragile ecosystems which, for the most part, are still poorly understood. Today these ecosystems are not only threatened by the growing economic interests of man (e.g., tourism, fisheries, aquaculture, land reclamation) but also by the prospect of an accelerating sea-level rise in the wake of global warming. While the detrimental effects of the former are increasingly becoming evident, those of the latter are still largely unknown. In order to provide an up-to-date review of the state of the art in muddy coast research, and to identify gaps in our knowledge, both in a scientific and geographic sense, and to define priorities for future research, an international conference entitled "Muddy Coasts 97" was convened in Wilhelmshaven, Germany, in September 1997. The conference was co-sponsored by the Senckenberg Natural History Society (Frankfurt), the Terramare Research Centre (Wilhelmshaven), the Federal Ministry of Science and Technology (Berlin), the Deutsche Forschungsgemeinschaft (Bonn), and last but least the Scientific Committee on Oceanic Research (SCOR) under the able participation of Working Group 106. The book "Muddy Coast Dynamics and Resource Management" forms part of the proceedings and has been edited by the conference organisers. It presents 21 regional case-studies from different parts of the world, including the southern Baltic Sea of Germany (6), the German Wadden Sea (6), the Wash in the U.K. (1), Portugal (1), the U. S. A. (1), Cameroon (1), Tanzania (1), Korea (1), and China (3). The studies deal with hydrodynamics and suspended particulate matter in bays and back-barrier tidal basins, erosion, deposition, and sediment budgets on tidal flats, primary production, nutrient fluxes and mineralisation in shallow lagoons (Bodden), sediment geochemistry of salt marshes and Holocene marine deposits, impacts of sea-level rise and land reclamation, and resource management of muddy coasts. The book is designated as a companion volume to the proceedings of the SCOR Working Group 106 published under the the title "Muddy Coasts of the World: Processes, Deposits and Function" edited by Terry Healy (New Zealand) and Ying Wang (China). The editors wish to express their sincerest gratitude to the numerous unnamed referees who have contributed substantially to the high standard of the contributions.
Burg Flemming, Monique Delafontaine, and Gerd Liebezeit Wilhelmshaven, August 2000
CONTRIBUTORS (current addresses) M.O. Andreae Max Planck Institute for Chemistry P.O. Box 3060 55020 Mainz Germany C.A. Angwe
Research Centre for Fisheries and Oceanography PMB 77 Limbe South-West Province Cameroon I. Austen Mittelstr. 26 25709 Kronprinzenkoog Germany H.-D. Babenzien Institut fi~r Gew~isser6kologie und Binnenfischerei Alte Fischerh~tte 2 16775 Neuglobsow Germany H.W. Bange Max Planck Institute for Chemistry P.O. Box 3060 55020 Mainz Germany A. Bartholomii Senckenberg Institute Schleusenstr. 39a 26382 Wilhelmshaven Germany S. Berghoff Department of Biology Rostock University Freiligrathstr. 7/8 18051 Rostock Germany
H.J. Black Institut f6r Okologie Universit~it Greifswald Schwedenhagen 6 18565 Kloster Germany M.I. Ca~ador
Instituto de Oceanografia Departamento de Biologia Vegetal Universidade de Lisboa 1700 Lisboa Portugal K.-S. Choi
Department of Oceanography Seoul National University Seoul 151-742 Korea M. Collins Department of Oceanography Southampton Oceanography Centre University of Southampton SO14 3ZH Southampton U.K. S. Dahlke Institut fiir Okologie Universit~it Greifswald Schwedenhagen 6 18565 Kloster Germany M.T. Delafontaine
Senckenberg Institute Schleusenstr. 39a 26382 Wilhelmshaven Germany B.W. Hemming
Senckenberg Institute Schleusenstr. 39a 26382 Wilhelmshaven Germany
xii C.E. Gabche
Research Centre for Fisheries and Oceanography PMB 77 Limbe South-West Province Cameroon S. Gerbersdorf Institut fi.ir Okologie Universit~it Greifswald Schwedenhagen 6 18565 Kloster Germany M.-K. Han
Department of Geography Peking University Beijing 100871 P.R. China X. Ke
Department of Urban and Resources Science Nanjing University 22 Hankou Road Nanjing 210093 P.R. China B.-K. Khim Polar Research Center Ocean Research and Development Institute P.O. Box 29 Ansan 425600 Korea M. Kb'ster Institut ffir Okologie Universit~it Greifswald Schwedenhagen 6 18565 Kloster Germany
Y.-F. Liu Department of Geography Peking University Beijing 100871 P.R. China M.I. Madureira IPIMAR Av. Brasflia 1400 Lisboa Portugal S. Mai Eifelstr. 46 60529 Frankfurt-Schwarnheim Germany A.J. Mehta Coastal and Oceanographic Engineering Department University of Florida P.O. Box 116590 Gainesville, FL 32611 U.S.A. ]. Meyercordt Institut ftir Okologie Universit/it Greifswald Schwedenhagen 6 18565 Kloster Germany L.-A. Meyer-Reil Institut fiir Okologie Universit/it Greifswald Schwedenhagen 6 18565 Kloster Germany N. Mimura
Department of Urban System Engineering Ibaraki University Hitachi 316 Japan
xiii
O.U. Mwaipopo Institute of Marine Sciences University of Dar Es Salaam P.O. Box 668 Zanzibar Tanzania N. Nyandwi Institute of Marine Sciences University of Dar Es Salaam P.O. Box 668 Zanzibar Tanzania T.M. Parchure Coastal and Hydraulics Laboratory U.S. Army Engineer Waterways Experiment Station Vicksburg, MS 39180 U.S.A.
P. Santamarina Cuneo Senckenberg Institute Schleusenstr. 39a 26382 Wilhelmshaven Germany G. Schlungbaum Department of Biology Rostock University Freiligrathstr. 7/8 18051 Rostock Germany
U. Selig Department of Biology Rostock University Freiligrathstr. 7/8 18051 Rostock Germany
Y.-A. Park Department of Oceanography Seoul National University Seoul 151-742 Korea
I. Stodian Institut ftir Okologie Universit/it Greifswald Schwedenhagen 6 18565 Kloster Germany
R. Ramesh Max Planck Institute for Chemistry P.O. Box 3060 55020 Mainz Germany
C. Vale IPIMAR Av. Brasflia 1400 Lisboa Portugal
S. Rapsomanikis Max Planck Institute for Chemistry P.O. Box 3060 55020 Mainz Germany
A. Voigt Institut f~ir Gew~isser6kologie und Binnenfischerei Alte Fischerh~itte 2 16775 Neuglobsow Germany
T. Rieling Institut fihr Okologie Universit/it Greifswald Schwedenhagen 6 18565 Kloster Germany
I. Wang National Marine Data and Information Service State Oceanic Administration 93 Liuwei Road Tianjin 300171 P.R. China
xiv Y. Wang State Pilot Laboratory of Coast and Island Exploitation Nanjing University 22 Hankou Road Nanjing 210093 P.R. China
c. Wolff Institut ftir Okologie Universit/it Greifswald Schwedenhagen 6 18565 Kloster Germany L. Wu
Department of Geography Peking University Beijing 100871 P.R. China T.J. Youmbi Research Centre for Fisheries and Oceanography PMB 77 Limbe South-West Province Cameroon J. Zhang National Marine Data and Information Service State Oceanic Administration 93 Liuwei Road Tianjin 300171 P.R. China D. Zhu State Pilot Laboratory of Coast and Island Exploitation Nanjing University 22 Hankou Road Nanjing 210093 P.R. China
X. Zou State Pilot Laboratory of Coast and Island Exploitation Nanjing University 22 Hankou Road Nanjing 210093 P.R. China
Muddy Coast Dynamics and Resource Management, B. W. Flemming, M. T. Delafontaine and G. Liebezeit (eds.) 9 2000Elsevier ScienceB.V. All rights reserved.
Hydrodynamics of Chwaka Bay, a shallow mangrove-fringed tropical embayment, Tanzania N. Nyandwi* and O. U. Mwaipopo
University of Dar Es Salaam, Institute of Marine Sciences, P. O. Box 668, Zanzibar, Tanzania
ABSTRACT
Time-series data of currents, sea levels and temperatures from Chwaka Bay, Zanzibar were analysed with the view of understanding the water circulation of the bay. The analyses show that there is a tidal asymmetry in the bay, with peak ebb tidal currents in the deep channels (45 cm s being stronger than flood tidal currents (35 cm s-l), and ebb periods (7 hours) being longer than flood periods (5 hours). The velocity and time asymmetry as well as the asymmetry in the current direction are controlled by the morphological variations of the tidal basin. It was found that, as the water flows from the inner bay during the ebbing tide, it first drains towards the main tidal creek which leads to concentrated but delayed flows. The temperature variations in the inner part of the bay are predominantly diurnal, whereas at the mouth of the bay they are semi-diurnal. There is a general temperature gradient between the inner bay and the mouth, the highest temperatures being recorded in the inner bay (30.14~ This indicates high residence times of the bay waters, presumably resulting from entrapment.
1. INTRODUCTION The hydrology of many tropical, mangrove-lined bays are characterized by salinity gradients even in areas without visible river supply, and by the entrapment of water in the mangrove forests (e.g., Wolanski et al. 1980; Wolanski 1989). Similarly, spatial and temporal variations in tidal current velocities are commonly observed. Thus, in Coral Creek, Australia, peak current velocities are generally higher than 1 m s in the tidal creek, whereas they hardly exceed 0.07 m s-1 in the mangroves (Wolanski et al. 1980). Indeed, a tidal velocity asymmetry was actually reproduced in a numerical model using the Coral Creek data. Furthermore, it was observed that human activities such as land reclamation and the felling of mangrove trees tend to reduce the magnitude and asymmetry of the tidal currents (e.g., Wolanski 1992).
* Corresponding author: N. Nyandwi e-mail:
[email protected] 4
Nyandwi and Mwaipopo
Salinity variations are usually observed between the inner and outer parts of bays and creeks. Several factors which may produce salinity gradients have been identified, including groundwater infiltration, evapotranspiration, and surface freshwater influx (e.g., Wolanski et al. 1980; Mazda et al. 1990; Ridd et al. 1990). Dilution by freshwater influx into mangrove areas usually produces a pronounced salinity gradient between the bay and the mangroves. Groundwater infiltration, which commonly occurs along the landward reaches of tidal creeks, can have a similar effect. It is also thought to be an important flushing mechanism of salts left behind by evapotranspiration (Wolanski & Gardiner 1981). The only exception to the above rules are associated with the conditions in hot and dry environments where evapotranspiration may cause an increase in salinity landwards of mangrove creeks (Wolanski et al. 1980; Ridd et al. 1990; Wattayakorn et al. 1990). A landward increase in salinity under such circumstances can be attributed to the extraction of freshwater from seawater by mangroves (Wolanski & Gardiner 1981). The saline water resulting from evapotranspiration may thus induce an inverse estuarine circulation (Wolanski 1992). Another factor which may affect the circulation pattern is the trapping of water in mangrove ecosystems (Okubo 1973), the amount of trapped water appearing to determine general flushing rates (Wolanski 1992). In the case of Chwaka Bay, a mangrove-lined embayment along the east coast of Zanzibar Island, Tanzania (Fig. 1A), the existence of a velocity asymmetry was observed but not verified because data on current variations in the tidal creeks and mangrove areas were lacking at the time (Wolanski 1989). Similarly, water entrapment and groundwater infiltration have been suggested as possible factors contributing to the offshore decrease in water temperature and the increase in salinity in the bay (Wolanski 1989). The exchange of water between Chwaka Bay and the open sea is not well understood, and there is no information on the heat budget of the area. The collection of data on temperature distribution and temporal variation would therefore be an important first step towards establishing a local heat budget. At the same time, a better knowledge of current patterns in the bay would not only contribute towards a better understanding of nutrient dynamics, waste dispersal, and water quality in general, but could also help explain why muds accumulating in the mangrove forests are never flushed out to impair coral reef growth at the mouth of the bay.
2. S T U D Y AREA
Chwaka Bay is located on the east coast of Unguja Island (Zanzibar) which is situated off the East African coast centred around 6~ and 39 ~ 30'E (Fig. 1B). It is a shallow embayment with an area of approximately 50 km 2 at high water springs (HWS). Its mouth is barred by a living offshore coral reef. A dead reef lines the southern landward end wl~ch is fringed by a 1 to 3-km-wide mangrove forest. The bathymetry of the bay was first studied by Wolanski (1989), using a portable echo sounder from a small boat operating along east-west transects in the bay and the mangrove creek. Water depths relative to mean sea level (MSL) are mostly less than
Hydrodynamics of Chwaka Bay, Tanzania
5 m along the eastern side of the bay. There are several tidal creeks in the open water of the bay, some of which connect to the mangrove creeks in the south.
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Figure 1. A: Location of the study area off the African east coast. B: Position of measurement stations in Chwaka Bay. Current meters and tide gauges were deployed at stations I and 2, whereas a tide gauge only was deployed at station 3.
The water movement in Chwaka Bay is controlled mainly by tidal motions. According to tidal records from the harbours of Dar Es Salaam and Zanzibar, the tide in this part of the Indian Ocean is semi-diurnal, being dominated by the M 2 component (e.g., Lwiza & Bigendako 1988). Older measurements indicate that peak ebb currents are stronger than peak flood currents, suggesting a tidal asymmetry in the bay (Wolanski 1989). The mean spring tidal range in the bay is 3.2 m. The main ecosystems in the bay include mangrove swamps, coral reefs and seagrass meadows. There are large intertidal areas which have recently attracted seaweed farming. Although there is no obvious freshwater supply to the bay, salinity measurements in Mapopwe creek showed values of 29.5-35%o (Wolanski 1989), suggesting some freshwater input to the mangrove swamps. Since no surface runoff exists, freshwater can only be supplied by groundwater seepage. This type of freshwater input was, in fact, suggested by Mazda et al. (1990). Being part of the East African region, Chwaka Bay is subject to two alternating seasons, the south-eastern (SE) and the north-eastern (NE) monsoons. The former
6
Nyandwi and Mwaipopo
begins in April and ends in October, whereas the latter begins in November and ends in March. During the SE monsoon the winds blow predominantly from the south-east, being accompanied by heavy rains and thunderstorms. Heavy rains are particularly common between March and June. During the NE monsoon the winds blow mainly from the north-east. This season includes the 'short rains' between October and November. Normally the area is cool with occasional light rains between June and September. From December to March the weather is relatively hot and dry with only very occasional rain. Meteorological data from Tanzania indicate that the mean values for rain and evaporation along the coast are 120 mm month -~ and 4 mm day -~, respectively. The East African Coastal Current (EACC) flows northwards throughout the year but differs markedly between the two monsoon seasons (Newell 1959). Thus, during the SE monsoon surface current velocities reach 4 knots (2 m s-~),being amplified by the trade winds of the Indian Ocean. During the NE monsoon, by contrast, the EACC still flows to the north but its speed is reduced to about 0.5 knots (0.25 m s-~) due to the domination of north-easterly winds (Newell 1959).
3. MATERIALS A N D M E T H O D S
Water fluxes and circulation patterns in Chwaka Bay were measured with selfrecording current meters and tide gauges equipped with temperature sensors. The temperature data were used to study the heat flux in both Mapopwe creek and the bay. Sea-level data obtained from the tide gauges were used to show the temperature and flow variations in the course of a tidal cycle. Three tide gauges of the type Micro-Tide, and two Sensordata SD6000 recording current meters were deployed in Chwaka Bay for one month in August-September 1992. The Sensordata SD6000 is a compact vector averaging current meter with memory capacity for up to 6000 combined data sets of current speed, direction and water temperature. The tide gauges have a memory capacity of 200 MB, and can measure and record combined data sets of pressure (water level) and temperature. A tide gauge and a current meter were deployed at the entrance of Mapopwe creek, a mangrove creek in the south-western part of the bay (station 1). A similar set was deployed in the middle of the bay (station 2), whereas a third tide gauge was located at the mouth of the bay (station 3). Three reference points (Security House at Chwaka village, Ras Juja and Ras Michamwe; cf. Fig. 1B) were used to determine the exact positions of the instruments by means of triangulation. The tide gauges and recording current meters were programmed to measure and record at 10-minute intervals. The tide gauges essentially recorded without interruption over the whole sampling period of about one month. The current metres at stations 1 and 2 experienced short interruptions when their propellers were fouled by seaweed. A set of manually operated gelatine pendulum current meters (Haamer 1974; Cederl6f et al. 1995) were deployed from a boat at a number of different stations within the bay during peak tidal flow in order to compile a map of spatial current
Hydrodynamics of Chwaka Bay, Tanzania
speed and direction patterns. Measurements with the pendulum current meters were also undertaken on several occasions between 1992 and 1994, particularly at times of maximum ebb or flood currents (i.e. approximately 3 hours after high and low tide, respectively).
4. RESULTS 4.1. F l o w patterns
The temporal patterns of the tidal currents at stations 1 and 2 are illustrated in the time series of Fig. 2, whereas the spatial patterns within the bay are shown in Fig. 3. From Fig. 2 it is observed that the maximum ebb currents (positive values) at station 1 are stronger than the maximum flood currents, and the ebb phase is longer (about 7 hours) than the flood phase (about 5 hours), indicating both velocity and time asymmetry. At station 2, however, there is no time asymmetry, and the velocity asymmetry was found to be less, with the peak flood velocities being slightly higher than the ebb velocities. The flow directions in Fig. 3 suggest an asymmetry in the tidal current direction, especially on the west bank where the flood current flows southwards whereas the ebb current flows about north-north-eastwards.
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Ke and Collins
24
During a tidal cycle, tidal currents often showed a decrease from the beginning of the flood (Fig. 7), with a sharper decrease in the first phase of the flood which was maintained essentially throughout the whole flood tide. The currents became even weaker during the ebb, with a slight increase at the end of this phase (Fig. 7a-c). Under extreme circumstances, tidal currents decreased from the very beginning of the flood until the end of the ebb (Fig. 7d). Over the Arenicola sandflat, the maximum tidal currents varied in the range 0.19-0.56 m s -1, mainly in response to the varying tidal ranges. The maxima occurred mostly during the first phase of the flood, and were much stronger than those observed over the saltmarsh and the upper mudflat (Table 2). It is possible that higher current velocities occurred at the beginning of the flood phase of the tides, and these are likely to have been missed because most of the surveys were undertaken only when the water depths were in excess of 0.2-0.3 m (cf. Fig. 7). In three of the four surveys undertaken over the Arenicola sandflat, minor peaks of tidal currents (0.15-0.3 m s -1) occurred at around HW (Fig. 7a-c). This may be a consequence of the tidal pulse caused by the abrupt increase in the tidal volume when the tidal water begins to flow into the saltmarsh near HW. 4.6. Station 8 The tidal curves for this part of the intertidal flat were asymmetrical in character, the duration of the ebb tide being almost an hour longer than that of the flood. The mean tidal current velocities at mid depth fluctuated between 0.15 and 0.30 m s -1, and at near bed between 0.09 and 0.27 m s -1. The results of the second survey show that tidal current velocities at mid depth and near bed (7-11 cm above the bed) were similar (Fig. 8b). They were also similar to those of the Arenicola sandflat, but noticeably higher than those on the saltmarsh and upper mudflat (Table 2).
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25
Tidal characteristics of The Wash
At station 8, the higher tidal current velocities occurred during the early stage of the flood phase and the late stage of the ebb phase, with a distinct minor peak at about 0.5 hr after HW. As in the case of stations 6 and 7, the lowest velocities were 0.5-1.5 hr before HW and 0.5-1.5 hr after HW, respectively. Generally, the flood tidal current velocities were noticeably higher than those of the ebb (Fig. 8 and Table 2). The highest velocities (with a m a x i m u m value of 0.53 m s-~) usually occurred at the beginning of the flood at very shallow water depths (U7; Fig. 8b). 4.7. Station 9 Tidal curves at this location were asymmetrical, with a Tf/Te ratio of 0.69 (Table 2). Throughout the tidal cycle the currents displayed a smooth progression with few abrupt changes, although a minor tidal current peak immediately after HW was again detectable. Current velocities were similar to those on other parts of the intertidal flat, and were higher than those over the saltmarsh (an order of magnitude in terms of the near-bed velocities Ub, and twice the value of the maximum tidal current velocity Umax; Table 2). The maximum tidal current velocities at this station (0.43 and 0.29 m s-1 for flood and ebb, respectively) occurred at the very beginning of the flood and at the end of the ebb. Again, the flood currents were stronger than the ebb currents (Fig. 9). 0.5
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U21
Figure 9. Water depths and tidal current velocities at station 9 on the lower sandflat, Freiston Shore, surveyed in 1992 (for location, see Fig. 2). U21- tidal current velocity at 21 cm above the seabed.
5. DISCUSSION 5.1. Tidal wave types and velocity maxima The tides within The Wash embayment are characterised by a standing wave, with offshore m a x i m u m tidal current velocities around mid tide and slack water near HW and LW (Pond & Pickard 1978; Pugh 1987; Fig. 10). The present study has shown that
Ke and Collins
26
the propagation of such a wave onto the intertidal flats and saltmarshes causes maximum tidal current velocities to occur (i) immediately after the beginning of the flood and near to the end of the ebb at/between MLW and MTL; and (ii) at the very beginning of the flood and end of the ebb at/above MTL where inundation occurs only after mid tide (i.e. at the same time as they reach their maxima in the offshore waters). Hence, the tidal current curves change gradually from 'sinusoidal' in the offshore to 'co-sinusoidal' or 'trochoidal' (with a shorter tidal inundation, i.e. out of phase) over most of the area above MTL (Fig. 10). Such changes take place within very short distances, since the corresponding topographic profile between MTL and MLW is very short due to the steeper slope of the offshore channel here. The velocity pulse recorded at about 10 cm above the seabed during early/late stages over intertidal flats, and during the over-marsh flow stage across the saltmarshes (cf. Figs 7 and 8) have been modelled by a continuity-based exploratory model for the hydraulic regime of tidal saltmarshes. This predicts that the tidal current maxima ('velocity pulses') occur during the earlier and later flow stages when water depths are small, and at over-marsh flow stages (Allen 1994). Stations
1.2
S t a t i o n s
6--9
S t a t i o n =
21
,23
,""*", I
"
0'5"I I
/
.w "X
"
i..== t = I ~=
>
0.5
~ / IX
'\
'
UtOOcr
i/i ,i~l!i
Io -1 0 +I *2 Tame (hrs rel tire to HW}
:"
-2 -I 0 +1 +2 +3 Time (hrs relative to HW)
0
-,= ~
'
""~ 0,5 -~
I
U1OOcr
'
0 , -6
I
I
I I
i I
I ~3
i,
10 =
,i J v 9 III Ill , ,' , '. , !', 8 -4 -2 0 +2 +4 +6 Time (hrs relative to HW}
, i l!l
I
I I
=-= 1 :] Saltmarsh ; "~'t (Mz=0.008(Mz=0.mm) 008mm) 1 ~'-3 , 0 500
. . . . . .
"''''"''''""'"
"
"
I
O f f s h o r e channel Intertidal flat ..... : ~ [Mz=0.18 ram} (Mz=0,08 mm) i .....~::::"" ZMLW , 1000 1500 Distance to dyke (m)
2o'o0
Figure 10. Conceptual hydrodynamic regime across the accretional Freiston Shore profile, The Wash. Top: tidal current velocity (continuous line), Ucr (lower horizontal line), U100cr (upper horizontal line), and water depth (dashed line) over spring tide for the saltmarsh (stations 1-2), the intertidal flats (stations 6-9), and the offshore channel (stations 21 and 23). Bottom: the Freiston Shore tidal-flat profile. Hydrographical data for offshore channel extracted from survey undertaken at stations 21 and 23 in the Boston Deep (HRS 1974; Ke et al. 1996; for locations, see Fig. 1). Mean grain diameters (Mz) for saltmarsh and intertidal flats are average values for the three years of study (Ke 1995). Mz data for the offshore channel were extracted from BGS (1988). Ucr and U100cr are based upon Sundborg (1967), Miller et al. (1977), Kapdasli & Dyer (1986), and Ke et al. (1994). At stations 1 and 2, it was not practicable to plot U100cr because the water depths are generally less than 100 cm in this case. ODN: Ordnance Datum (Newlyn).
Tidal characteristics of The Wash
27
5.2. Tidal current structure
The maximum and mean tidal current velocities do not change gradually along the tidal-flat profile, but rather in a 'step-like' manner, i.e. current velocities in the offshore channels and over the intertidal flats and saltmarsh occur at different levels, with maximum tidal current velocities decreasing landwards. Rapid changes also occur over the narrow transitional zones between these sub-environments (Fig. 11). These locations coincide with sedimentary and/or geomorphological boundaries and distinct topographic or bathymetric changes, i.e. the presence of steeper slopes in the profile. Interestingly, clay contents of the surficial seabed sediments were found to be >10% landwards, and , o mO
r
/
. . . . . . . . . .
0.5 ~
,?" ~ ,./O (~ ----o . . . . . d o e "
o
..
~
o
- ~ . . . . . . . . . a-o
No data
.,_
0
I
I
i
!
500
1000
1500
2000
2500
4 3 Q
o tO
2
0
i
Upper "'
Saltmarsh ~udfldl,, t
-2
Intertidal sandflat
O f f s h o r e channel
-3
0
500
1000
1500
2000
2500
Figure 11. Tidal current velocities (Umax and Umean) as a function of distance from the dyke (top) and topographic elevation along the study profile (bottom).
The sharp decrease in current velocities from the open intertidal flats across the mudflats to the saltmarsh (see above) may be attributed to a variety of factors. For an initially smooth intertidal-flat profile, the uppermost section will only be inundated during HW slack. Sediment deposition is thus more likely to occur on the upper intertidal flats rather than on those located more seawards. The different vertical accretional rates between the two areas will enhance differences in surface elevations. However, once the surface of the upper part of the flats has reached a height which permits pioneering halophytes to grow, a saltmarsh will develop. In the presence of vegetation, additional sediment will be trapped and the surface of the saltmarsh will be
28
Ke and Collins
raised further. As a result, saltmarshes will always occupy higher ground than the open intertidal flats relative to the regional tidal frame. At the same time, a steeply sloping area has been created between the saltmarsh and the seaward intertidal sandflat, i.e. the so-called upper mudflat at Freiston Shore. The latter feature explains why the tidal current velocities within the saltmarsh are so low. Meanwhile, due to the steeper slope, the surface of the upper mudflat is strongly scoured, and gullies and oval-shaped depressions occur. These increase the seabed roughness considerably (see Ke et al. 1994) which, in turn, causes a decrease in the tidal current velocity. Abrupt changes in the current patterns have been predicted on the basis of analytical models for flow and sedimentation over tidal saltmarshes (Woolnough et al. 1995; Allen 1996). As a result, hydrodynamic energy (in terms of tidal current velocities a n d / o r (?) wave heights) in the offshore channel, and over the open intertidal flats and the saltmarsh reaches different levels. In a sense, these have a 'quantum' character, and the observations or predictions in the case of the macrotidal environment of The Wash are therefore inconsistent with the 'settling and scour lag' hypothesis (see Postma 1954; Figs 10 and 11 in van Straaten & Kuenen 1957; the "lag effect' of van Straaten & Kuenen 1958). Instead, the present study has shown that mechanisms for the deposition of sediments over tidally dominated intertidal environments include (i) flood tidal current dominance, (ii) a 'step-like' shoreward hydrodynamic energy gradient, and (iii) the occurrence of strong tidal currents especially at the beginning of the flood, and at the end of the ebb. 5.3. Tidal asymmetry The results of the present study show that the tidal curve is asymmetrical in the lower part of the intertidal zone, and that it becomes gradually more symmetrical across the intertidal flats towards the saltmarsh (Fig. 10 and Table 2). A similar trend was recorded during the course of an earlier investigation (Evans & Collins 1975). However, some other studies show an almost opposite trend (Collins et al. 1981; van Smirren 1982). Such variation may, at least in part, be due to the fact that the early surveys were undertaken during spring tides, as was the case in the present study. In contrast, all the stations surveyed on the lower intertidal flats in most subsequent studies were undertaken at neap tides. Moreover, the earlier measurements were carried out high on the matured saltmarsh prior to reclamation, and these may have been affected by the presence of creeks and local flow patterns. The implication of such observations is that tidal symmetries change in response to tides, seasons and local control mechanisms which require further study if they are to be clarified. Furthermore, tidal currents in the region are also asymmetrical in character. Flood currents are dominant, with both the mean and maximum velocities being generally higher than those of the ebb (Figs 10, 12, and Table 2). Perhaps as a result of prevailing onshore winds, the velocity pulse of the final phase of the ebb was not observed on some occasions and hence, tidal current velocities decrease from the beginning to the end of a tidal cycle.
Tidal characteristics of The Wash
29
1.2 0
1E >" ===,
0.8 -
0 0 (D
0
> 0.6
-
0
o9
r-
(D t._
L040
+ .,
N "0 0 0
0.2
-I.-
o
o Maximum
-
+ Mean O
1
I
I
I
I
I
0
0.2
0.4
0.6
0.8
1
1.2
Ebb tidal current velocity (m/s)
Figure 12. Maximum and mean tidal current velocity asymmetries for stations from the saltmarsh to the offshore channel along the Freiston Shore profile, The Wash. Data for the saltmarsh and intertidal flats are based on the surveys undertaken in 1980 (van Smirren 1982) and in 1991-1993 (present study). Data for the offshore channel at stations 21 and 23: see HRS (1974) and Ke et al. (1996).
Similar tidal curves and tidal current patterns have been observed previously in the region (Collins et al. 1981), and can also be identified in data sets of other accretional intertidal flat/saltmarsh environments (Table 3). At least during spring tides under calm weather conditions, such tidal patterns may characterise hydrodynamic regimes. Above MTL during the early stage of the flood tide, current flow acts as a strong resuspension mechanism, whereby maximum velocities exceed the threshold velocity Ucr for sediments with a mean grain diameter Mz of 0.08 mm in the present case (Fig. 10). Similarly, the generally stronger flood flows would transport suspended particulate matter (SPM) originating offshore in a landward direction. The majority of the coarser-grained particles, together with some of the finer-grained material within the water column, may settle during the following slack-water period (cf. 'settling and scour lag' hypotheses). These may be resuspended at the end of the ebb when current velocities are generally above Ucr (Fig. 10).
30
Ke and Collins
Table 3. Various accretional intertidal flat/saltmarsh environments associated with hydrodynamic characteristics similar to those observed in the present study. Locations Intertidal flat (Ameland, Wadden Sea, The Netherlands) Saltmarsh (Norfolk, England)
Data sources van Straaten & Kuenen 1957
Bayliss-Smith et al. 1979; French & Clifford 1992 Saltmarsh (Burry Inlet, Wales) Carling 1982 Saltmarsh (west-central Florida, USA) Leonard et al. 1995 Intertidal flat (southern New Hampshire, USA) Anderson 1973 Mudflat (Avon River, Canada) Amos 1995 Intertidal flat(Jiangsu coast, China) .......... .......Zhu & Xu 1982; Zhang 1992
Since the ebb currents are generally weaker than the flood currents, there will be an enhanced difference between the flood and ebb values of (U-Ucr) 3 which is proportional to the bedload transport rate (Gadd et al. 1978). Hence, net flood (i.e. landward) bedload sediment transport and SPM flux will occur over the intertidal flats throughout each tidal cycle. Together with the 'step-like' shoreward hydrodynamic energy gradient, the present hypothesis emphasises the tidal asymmetry in explaining features such as the mudflat zone and sharp sedimentary boundaries. This is different from the 'settling and scour lag' concept which assumes a gradual decrease in tidal currents from LW to HW, and which emphasises the difference between critical scour velocity and critical settling velocity. Furthermore, the lag effect should exist not only on intertidal flats but also in many other sedimentary environments. Within the context of the present study, tidal asymmetry may be more important than the 'settling and scour lag' effect in explaining sediment movement on the intertidal flats of The Wash. 5.4. Tidal transport modes Using the near-bed current velocity and sediment grain size, the Sundborg (1967) model clearly illustrated the mode of sediment movement and transport. It has provided a useful tool to identify seabed sediment dynamics (modes), and has been applied extensively in sediment dynamics (Sternberg 1972; Reineck & Singh 1980; Sternberg et al. 1983). On the basis of established patterns of tidal currents over the saltmarshes, intertidal flats and offshore channels (Fig. 10), and in combination with the Sundborg model, the 'modes' of sediment transport during different stages of the tidal cycle may be identified (Fig. 10 and Table 4). The results show that (i) over the saltmarsh, Mode II (transport in suspension and net deposition of the suspended load) is the only mechanism to be anticipated; (ii) over the intertidal flat, Mode I (entrainment of grains from the bottom and transport in suspension with resulting net erosion) occurs during the first phase of the flood and at the end of the ebb, Mode IV (absence of prolonged transport with resulting deposition of bedload or suspended
31
Tidal characteristics of The Wash
load) around HW, and Mode II in between; (iii) in the offshore channel, symmetrical and periodic changes from Mode IV (see above) occur at the beginning and end of the tidal cycle, through a short period of Mode III (entrainment of grains from bottom and transport of bedload with resulting net erosion or net accumulation) to Mode I at mid flood and mid ebb (Fig. 10). In terms of duration, only Mode II occurs throughout the tidal cycle over the saltmarsh. Over the intertidal flats, Mode I is dominant during the flood (45% of the time), whereas Mode IV is dominant during the ebb (43%). Finally, in the offshore channel, Mode I is dominant (48% of the time) throughout the tidal cycle, followed by Mode IV (36%) and Mode III (16%; cf. Table 4).
Table 4. The time of occurrence (%) of different transport modes during a typical spring tidal cycle along the Freiston Shore profile, The Wash (see also Fig. 10). .......Environment Saltmarsh
Intertidal flats
Offshore channel .........
Tidal phase Flood Ebb Cycle Flood Ebb Cycle Flood Ebb Cycle
.
.
.
.
.
.
.
......... Transportmodepercentage I II III 0 100 0 0 100 0 0 100 0 45 27 0 27 30 0 34 29 0 48 0 15 47 0 17 48 0 16 .
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
IV 0 0 0 28 43 37 37 36 36
On the basis of this analysis, resuspension of seabed sediments into the overlying waters and net erosion of the seabed can occur only in offshore channels during mid flood and mid ebb, and over the intertidal flats during the early flood and late ebb phases. On the saltmarsh, by contrast, suspended sediment transport and net deposition take place throughout the tidal cycle. During the whole cycle, suspended sediment transport and deposition are the dominant processes along the profile, bedload transport being limited to the offshore channel, and being of short duration. This can also be verified by both field observations and laboratory experiments. Although sand ripples are commonly found on intertidal flats, field observations indicate that they can easily be erased by the tidal front at the very beginning of the flood tide. X-ray photographs of cores collected from the lower sandflat also show that horizontal bedding is the dominant sedimentary structure, and that cross bedding occurs only occasionally as a diagnostic sedimentary structure (Ke 1995). Although the asymmetry of both the tidal curve and current over the saltmarsh is not as marked as over the open intertidal flat at Freiston Shore, flood domination still persists, supporting a net landward SPM flux (Ke 1995). Elsewhere, tidal velocity
32
Ke and Collins
surges and asymmetry have been observed in the Warham tidal channel at Stiffkey Marshes, North Norfolk (Pethick 1980). At spring tide and under calm sea conditions, muddy sediments of the saltmarshes of The Wash are not resuspended because, throughout the various tidal cycles, current velocities were less than the threshold velocity Ucr for sediments with a Mz of 0.008 mm (Fig. 10). The dominant sedimentation process on the saltmarsh is therefore settling of SPM from the water column, in the present case. This leads to the accretion of the saltmarsh surface throughout each spring tidal cycle, particularly during slack water and on the ebb (see also Table 4). The above mechanism can be verified by the presence of well-developed muddy laminae which dominate the saltmarsh deposits in The Wash (Ke 1995). However, the low suspended sediment concentration (SSC is generally 0.9) than did the suspended mud fraction (r >0.7) (Santamarina Cuneo 2000), this being consistent with the theory (e.g., Thorne et al. 1991). The concentrations and fluxes under calm weather conditions (March 1997) are illustrated in Figs 4A and 4B, respectively. Concentrations of the mud fraction reached 50 mg 14 during the peak ebb and 60 mg 14 during the peak flood current, the contribution of sand being negligible in both cases. During slack water the concentrations dropped to about 10 mg 1-1,indicating that a substantial portion of the finegrained suspended matter had large enough settling velocities to settle out. However, total fluxes did not exceed 300 kg s-1 during either tide. This demonstrates that the mass transport induced by tidal flow alone (i.e. without wave action) is relatively small. Nevertheless, the transport of suspended matter was higher in the flood phase (2988 tonnes) than in the ebb phase (2003 tonnes). As a result, a net import of 985 tonnes was recorded in this case.
.-.
80
% "~
,
.A
,
,
,
,
,
,
,.
,
,
,
,
,
,
,
o--oSand
60 ~
40
9 -
8
:,
o ,, 8000
,
400
s
4000
200
o
o
0
~-4000
-200
~
I-_8000"
'
&
'
'
'
1'2
' Time
'
'
1'6'
'
'
2' 0
'
-400
(hr)
Figure 4. Concentration and flux of SPM under calm weather conditions over a complete tidal cycle. A: Concentrations of suspended sand and mud as a function of time. B: Flow volume and mass transport as a function of time. Negative values: export (ebb tide); positive values: import (flood tide).
Suspended particulate matter flux
47
In Figs 5A and 5B the calm weather situation of March 1997 is contrasted with the concentrations and fluxes observed under moderate wind conditions in May 1997. In this case, a northerly wind of 6 Bft was associated with 1 m high waves along the open coast. Wave-induced sediment resuspension in the nearshore zone explains the higher concentration of sand in May. Surprisingly, the concentration of mud remained at the same level as that recorded in the calm weather situation, indicating that waves 1 m in height are unable to resuspend mud known to occur in deeper waters further offshore (Figge 1981). Furthermore, the concentrations of both fractions were higher during the flood tide (positive velocities) than during the ebb tide (negative velocities). Whereas the sand fraction settled out during both slack tides, the higher wave-generated turbulence kept up to 20 mg 1-1 of mud in suspension.
80 v
E
I
I
I
_A
I
I
I
I
I
I
I
A
I
I
I
I
I
~-,41Sand
ir-,A Mud
60
cO
40 O
20
O (')
0
,,4,
,
,
,
,
,
,
,
,
,
~
,
8000
600
4000
400
200
~--
o
- - 9 Water flow
- - ~,
Ud -4000 - - t ,~--~ 0"0 San M ud -8000
'
' 8
'
_
%~ ~ '-" '
'
' ' 12
'
'
i -200-400 I--'-==~ ' ' 16
'
'
' 20
'
-600
Time (hr)
Figure 5. Concentration and flux of SPM over a complete tidal cycle under windy weather conditions (6 Bft wind speed and 1 m high waves along the open coast). A: Concentrations of suspended sand and mud as a function of time. B: Flow volume and mass transport as a function of time. Negative values: export (ebb tide); positive values: import (flood tide).
48
Santamarina Cuneo and Flemming
At 7557 tonnes, the mass transport was substantially higher during the flood phase as compared to the 4607 tonnes transported during the ebb phase. The much higher transport values in this case are evidently related to wave-induced resuspension along the open coast. The observed transport asymmetry thus resulted in a net import of 1640 tonnes of sand and 1310 tonnes of mud. In comparison to the calm weather situation of March 1997, the mass transport of SPM in the presence of 1 m waves along the open coast was about three times as high, thus emphasising the importance of wave action for the resuspension and transport of suspended matter in the region. Nevertheless, the data also demonstrate that, even under more windy conditions, overall concentrations of SPM in this part of the Wadden Sea remain low compared to values recorded in other environments, for example, the Severn Estuary, U.K. (Kirby & Parker 1983), the Changjiang Estuary, China (Shi et al. 1999), and the Amazon shelf (Kineke & Sternberg 1992).
5. DISCUSSION AND CONCLUSIONS This study has demonstrated that, if adequately calibrated, the intensity of the backscattered signals of ADCPs can be used effectively to infer suspended matter concentrations and fluxes. The method is fast and efficient, at the same time being sufficiently accurate to be as good as or superior to much more elaborate procedures such as round-the-clock profiling and pumping at fixed anchor stations (e.g., Jones et al. 1989). It was shown that the efficiency and accuracy of the approach was dependent on the calibration procedure, the method described in this paper having proved the most effective to date. Whereas stationary ADCPs will provide continuous data through the water column at a study site, mobile instruments will integrate over entire survey transects, for example, channel cross-sections. In addition, velocity measurements acquired simultaneously over the same spatial and temporal scales allow the calculation of SPM fluxes, and hence provide good estimates of sediment budgets. In this respect, ADCPs are superior to any other instrumentation currently available. It was further demonstrated that separate calibrations for individual size fractions increase the overall accuracy of the SPM estimates, the results complying with theoretical expectations and suggesting that there is room for further improvement. In particular, it would be of interest to investigate a larger number of size fractions, for example, by distinguishing between the flocculated and/or aggregated mud and the non-flocculated silt fractions as well as between fractions of different petrographic and geochemical compositions. The good results achieved in this study should also encourage ADCP manufacturers to upgrade the resolution of the analogto-digital conversion of the backscattered signal intensity. At the moment, this signal is still being regarded as a methodological by-product rather than a feature of high scientific value in its own right. The application of an accurately calibrated high-resolution 1.2 MHz ADCP to study SPM concentrations and fluxes into and out of a mesotidal back-barrier basin
Suspended particulatematterflux
49
has shown that the method is suitable for obtaining quantitative time-integrated estimates of material fluxes (excluding bedload transport) under different weather conditions. Wave-induced nearshore resuspension processes at wind speeds around 6 Bft. resulted in a substantial material import over a single tide, with an unexpectedly high proportion of suspended sand. This reflects the relatively low concentrations of SPM in southern North Sea waters (e.g., Eisma 1993), and implies that I m waves are ineffective in resuspending offshore muds. It also emphasises the necessity of calibrating the ADCP backscattering signals for different size fractions, including sands. Significantly, no net export was observed under the weather conditions covered in this study which ranged from calm weather to wind speeds up to about 6 Bft. A fragmentary data set collected under storm conditions (>9 Bft) indicated high SPM concentrations during the ebb phase. However, since the measurement programme had to be terminated prematurely because of technical problems, it was not possible to calculate net fluxes in this case. By implication, the evidence that a net long-term export of fine-grained material in the course of continued sea-level rise is linked to strong wind events and/or episodic storm action has remained inconclusive and awaits verification in future studies. A critical test of the net export hypothesis would obviously be a mass balancing of SPM fluxes recorded under weather conditions rougher than the range covered in this study (wind speeds >7 Bft). In particular, it would be necessary to develop ADCP-based survey techniques capable of handling severe storms. Other studies have shown that under such conditions flood currents can be reduced to almost zero by the backflow of dammed-up water masses. As a result there is no import of suspended material during the flood phase, although strong wave action in the backbarrier basin will keep remobilized muds in suspension. This material is then flushed out during the subsequent ebb surge which has been shown to reach velocities up to 65% higher than those of ebb currents under more benign conditions (Koch & Niemeyer 1978). As recently shown by Bartholdy & Anthony (1998), such episodic flushing events are evidently capable of exporting most of the material accumulated in calmer interim periods, much like the dramatically elevated sediment discharges associated with severe river floods.
ACKNOWLEDGEMENTS
We wish to extend our thanks to the captain and crew of the research vessel Senckenberg for their assistance during the station work and the surveys with the motorboat. The help of Astrid Raschke in carrying out much of the laboratory work was highly valued. The first author was supported by a bursary from the Deutscher Akademischer Austauschdienst (DAAD), whereas the Senckenbergische Naturforschende Gesellschaft provided running money and ship time. The generous support of both organisations is gratefully acknowledged.
50
Santamarina Cuneo and Flemming
REFERENCES
Austen, G., Fanger, H.-U., Kappenberg, J., Mfiller, A., Pejrup, M., Ricklefs, K., Ross, J. & Witte, G. (1998) Schwebstofftransport im Sylt-Romo Tidebecken: Messungen und Modellierung. In: G~itje, C. & Reise, K. (eds), Okosystem Wattenmeer: Austausch-, Transport und Stoffumwandlungsprozesse. Springer, Berlin, pp. 185214. Bartholdy, J. & Anthony, D. (1998) Tidal dynamics and seasonal dependent import and export of fine-grained sediment through a backbarrier tidal channel of the Danish Wadden Sea. In: Alexander, C.R., Davis, R.A. & Henry, V.J. (eds), Tidalites: Processes and Products. SEPM Spec. Publ. 61: 43-52. Bartholom~i, A., Flemming, B.W. & Delafontaine, M.T. (2000) Mass balancing the seasonal turnover of mud and sand in the vicinity of an intertidal mussel bank in the Wadden Sea (southern North Sea). In: Flemming, B.W., Delafontaine, M.T. & Liebezeit, G. (eds), Muddy Coast Dynamics and Resource Management. Elsevier, Amsterdam (this volume). Bunt, J.A.C., Larcombe, P. & Jago, C.F. (1999) Quantifying the response of optical backscatter devices and transmissometers to variations in suspended particulate matter. Cont. Shelf Res. 19: 1199-1220. Davis, R.A. Jr. & Flemming, B.W. (1991) Time-series study of mesotidal bedforms, Martens Plate, Wadden Sea, Germany. In: Smith, D.G., Reinson, G.E., Zaitlin, B.A. & Rahmani, R.A. (eds), Clastic Tidal Sedimentology. Can. Soc. Petrol. Geol. Mem. 16: 275-282. Dijkema, K.S. (ed.) (1989) Habitats of the Netherlands, German and Danish Wadden Sea. Research Institute for Nature Management, Texel, and Veth Foundation, Leiden, 30 p. Eisma, D. (1993) Suspended Matter in the Aquatic Environment. Springer-Verlag, Heidelberg, 315 p. Figge, K. (1981) Sedimentverteilung in der Deutschen Bucht, Blatt Nr. 2900. Deutsches Hydrographisches Institut, Hamburg. Flemming, B.W. & Bartholom~i, A. (1997) Response of the Wadden Sea to a rising sea level: a predictive empirical model. Ger. J. Hydrogr. 49: 343-353. Flemming, B.W. & Davis, R.A. Jr. (1994) Holocene evolution, morphodynamics and sedimentology of the Spiekeroog barrier island system (southern North Sea). Senckenbergiana marit. 24: 117-155. Flemming, B.W. & Nyandwi, N. (1994) Land reclamation as a cause of fine-grained sediment depletion in backbarrier tidal flats (southern North Sea). Neth. J. Aquat. Ecol. 28: 299-307. Flemming, B.W., Schubert, H., Hertweck, G. & Mfiller, K. (1992) Bioclastic tidalchannel lag deposits: a genetic model. Senckenbergiana marit. 22: 109-129. Groen, P. (1967) On the residual transport of suspended matter by an alternating tidal current. Neth. J. Sea Res. 3: 564-574. Hales, L. (1995) Accomplishments of the Corps of Engineers dredging research program. J. Coast. Res. 11: 68-88.
Suspended particulate matter flux
51
Hanes, D.M., Vincent, C.E., Huntley, D.A. & Clarke, T.L. (1988) Acoustic measurements of suspended sand concentration in the C2S2 experiment at Stanhope Lane, Prince Edward Island. Mar. Geol. 81: 185-196. Holdaway, G.P., Thorne, P.D., Flatt, D., Jones, S.E. & Prandle, D. (1999) Comparison between ADCP and transmissometer measurements of suspended sediment concentration. Cont. Shelf Res. 19: 421-441. Jay, D.A., Uncles, R.J., Largier, J., Geyer, W.R., Vallino, J. & Boynton, W.R. (1997) A review of recent developments in estuarine scalar flux estimation. Estuaries 20: 262-280. Jones, P.D., Head, P.C. & Whitelaw, K. (1989) A data recording station to measure water and solids fluxes through the Mersey Narrows. In: McManus, J. & Elliott, M. (eds), Developments in Estuarine and Coastal Study Techniques. Olsen & Olsen, Fredensborg, pp. 91-100. Kineke, G.C. & Sternberg, R.W. (1992) Measurements of high concentration suspended sediments using the optical backscatterance sensor. Mar. Geol. 108: 253-258. Kirby, R. & Parker, W.R. (1983) Distribution and behaviour of fine sediment in the Severn Estuary and Inner Bristol Channel, U.K. Can. J. Fish. Aquat. Sci. 40 (Suppl. 1): 83-95. Koch, M. & Niemeyer, H.D. (1978) Sturmtiden-Strommessungen im Bereich des Norderneyer Seegats. Forschungsstelle Norderney Jber. 29: 91-108. Kr6gel, F. (1997) Einflut~ von Viskosit~it und Dichte des Seewassers auf Transport und Ablagerung von Wattsedimenten (Langeooger R/ickseitenwatt, s/idliche Nordsee). Ber. Fachber. Geowissens. Universit~it Bremen 102, 168 p. Kr6gel, F. & Flemming, B.W. (1998) Evidence for temperature-adjusted sediment distributions in the back-barrier tidal flats of the East Frisian Wadden Sea (southern North Sea). In: Alexander, C.R., Davis, R.A. & Henry, V.J. (eds), Tidalites: Processes and Products. SEPM Spec. Publ. 61: 31-41. Kr6gel, F., Flemming, B.W. & Delafontaine, M.T. (2000) High-resolution sediment distribution patterns and dynamics in the Accumer Ee tidal basin: subtle effects of Europipe. In: Delafontaine, M.T., Flemming, B.W. & Vollmer, M. (eds), Environmental Impacts of Europipe. J. Coast. Res. Spec. Issue 27 (in press). Osborne, P.D., Vincent, C.E. & Greenwood, B. (1994) Measurement of suspended sand concentrations in the nearshore: field intercomparison of optical and acoustic backscatter sensors. Cont. Shelf Res. 14: 159-174. Postma, H. (1961) Transport and accumulation of suspended matter in the Dutch Wadden Sea. Neth. J. Sea Res. 1: 148-190. Santamarina Cuneo, P. (2000) Fluxes of suspended particulate matter through a tidal inlet of the East Frisian Wadden Sea (southern North Sea). Berichte, Fachbereich Geowissenschaften, Universit~it Bremen (in press). Shi, Z., Ren, L.F. & Hamilton, L.J. (1999) Acoustic profiling of fine suspension concentration in the Changjiang Estuary. Estuaries 22: 648-656.
52
Santamarina Cuneo and Flemming
Streif, H.J. (1989) Barrier islands, tidal flats, and coastal marshes resulting from a relative rise of sea level in East Frisia and the German North Sea coast. Proc. KNGMG Symp. Coastal Lowlands: Geology and Geotechnology, pp. 213-223. Suk, N.S., Guo, Q. & Psuty, N.P. (1998) Feasibility of using a turbidimeter to quantify suspended solids concentration in a tidal saltmarsh. Estuar. Coast. Shelf Sci. 46: 383-391. Thorne, P.D., Vincent, C.E., Hardcastle, P.J., Rehman, S. & Pearson, N. (1991) Measuring suspended sediment concentrations using acoustic backscatter devices. Mar. Geol. 98: 7-16. van Straaten, L.M.J.U. & Kuenen, P.H. (1957) Accumulation of fine grained sediment in the Dutch Wadden Sea. Geol. Mijnb. 19: 329-354.
Muddy Coast Dynamics and Resource Management, B. W. Flemming, M. T. Delafontaine and G. Liebezeit (eds.) 9 2000 Elsevier Science B.V. All rights reserved.
55
Surface erosion of fine-grained sediment revisited A. J. Mehta a* and T. M. Parchure b
aCoastaland OceanographicEngineering Department, University of Florida, Gainesville, FL 32611, U.S.A. bCoastaland Hydraulics Laboratory, U.S. Army Engineer Waterways Experiment Station, Vicksburg, MS 39180, U.S.A. ABSTRACT
For applications in waters with low to moderate concentrations of suspended finegrained sediments, the formula of Kandiah (1974) for the rate of bed surface erosion remains a convenient model for simulating scour due to steady or quasi-steady flows. Arulanandan et al. (1980) show that the two parameters characterizing this formula, namely the erosion rate constant and the bed shear strength with respect to erosion, seem to be related in such a way that the rate constant decreases with increasing shear strength. Other studies have shown that the shear strength correlates with bed density. We have used these findings to develop a formula for estimating the rate of erosion from bed density for sediments which are largely inorganic. While this formula cannot replace the need for laboratory or prototype testing of sediment beds for an accurate determination of erosion rate, it may be used to obtain "first cut" values of the rate characterizing parameters in situations where they are unavailable from measurements. Recent experimental results suggest that the same formula may also be useful for estimating the rate of erosion of organic-rich sediments.
1. INTRODUCTION Modeling the erosion of fine-grained sediment beds continues to pose problems largely due to a lack of clear understanding of the way in which the bed-water interface responds to a flow-induced stress. For steady or quasi-steady (e.g., tidal) flows, numerous formulae relating the rate of surface erosion to the bed shear stress have been proposed. In this mode of erosion, particles or particulate aggregates at the bed surface are detached and entrained in the flow, thus causing bed scour. Some of the earlier formulae have been summarized by Mehta et al. (1982). These stress* Corresponding author: A. Mehta e-mail:
[email protected] 56
Mehta and Parchure
based formulae are generally applicable to cases of low to moderate suspended sediment concentrations. At high concentrations exceeding 4-20 g 1-1, settling of sediment is hindered, being controlled by the rate of upward seepage of interstitial water. Under these conditions, a layer of fluid mud may form over the bed due to the deposition of suspended sediment. The mechanism by which this layer erodes is not modeled well by stress-based formulations. In any event, to various degrees all such formulae are empirical-phenomenological approximations of very complex flowparticle interactions which ultimately cause bed particles and aggregates to dislodge, rupture and entrain. The formula proposed by Kandiah (1974) is
~;--gM
'L'b- "~s / Ts
(1)
in which ~ is the erosion rate or mass flux (mass eroded per unit bed area per unit time), % is the bed shear stress, "rs is the bed shear strength with respect to erosion, and the erosion rate constant ~Mis the value of r when zb = 2zs. Equation 1 is characteristically applicable to homogenous, uniform density, uniform shear strength beds, and indicates that ~ varies with the excess shear stress %-~s. Thus, a plot of r versus ZD-~s ideally appears as a straight line, as shown by, among others, Kandiah (1974) through careful laboratory experimentation on the erosion of clay and clay/silt mixtures of uniform density. This is exemplified in Fig. 1, in which the erosion rate and the shear strength (as determined by the intercept of a given line with the horizontal axis) is seen to depend on the percentage (by weight) of montmorillonite in the Yolo loam + montmorillonite mixture. Also observe that the effect of the highly cohesive montmorillonite was to decrease ~M(line slope) due to an increase in the shear strength of the mixture. 9
Yolo I.oam
+ Montmonllonlte (
3.5 10% =
3
~2.5 =' =r =
2 1.5
..,
,-.
1
0.5 0
i
0
2
4 6 Shear Stre~s, "~b(Pa)
8
Figure 1. Erosion rate versus bed shear stress for mixtures of Yolo loam and montmorillonite (percentage values indicate montmorillonite by weight; adapted from Kandiah 1974).
Fine-grained sediment erosion
57
For beds which are stratified with respect to density and shear strength, formulae which account for the variation in ~s with depth have been developed, e.g., by Parchure & Mehta (1985). Although these formulae differ from Eq. 1, in all of them the erosion rate varies with the excess shear stress. This similarity, as well as experience from modeling applications, suggest that Eq. 1 can also be used for stratified beds with a reasonable degree of accuracy by allowing ~s to vary with depth, i.e. by replacing zs by ~(z) where z denotes the vertical coordinate (Hayter & Mehta 1986). Vinzon (1997) used measured time-series of near-bed velocities and suspended sediment concentrations at sites on the Amazon Shelf off Brazil to develop the linear plot shown in Fig. 2. The observed relationship is akin to the lines in Fig. 1, and therefore conforms to Eq. 1 but with a considerably greater scatter of data points, as would be expected in field data. The shear strength ~s was obtained from a formula discussed below. Finally, with reference to Eq. 1 it is also interesting to note that a compilation of erosion rate formulae for wind- as well as mechanically-generated waves in laboratory flumes indicates the validity of the functional form of Eq. 1 (Mehta 1996). This information is summarized in Table 1, in which characteristic parameters are given for the expression (2)
'rb - ~ s ~s
E =E M
For 6 = 1, Eq. 2 reduces to Eq. 1. As seen from Table 1, experimental data at times have yielded values of 6 close to unity. In Eq. 2 % is the peak value of the bed shear stress during the wave cycle, and ~s can differ from that associated with currentinduced erosion due to the effect of cyclic loading on the soil matrix (Maa & Mehta 1987; Mimura 1993).
3 ..............
i. . . . . . . . . . . . . .
io
~1.5 "=.
!~ ...........
o
o o~ !
11 ........
os[ "
~
...................
.....~---;o ............ i ..............
..A'o..o ~ o
0
o
:
.
.
.
.
............................. i
0.05 0.1 0,15 Excess Shear Stress, ;b-~, (Pa)
0,2
Figure 2. Erosion rate versus excess shear stress based on the field data analysis of Vinzon (1997).
58
Mehta and Parchure
Table 1. Parameters for Equation 2 for w a v e - i n d u c e d erosion. Source
Mode of wave generation
Sediment
Parameter a ranges a (cm); co(rad s-1 ); k (cm1)
Parameter values in Eq. 2 EM
Ts
(Pa)
8
(g m -2 s -1)
Alishahi & Krone (1964)
Wind
Bay mud
0.9 < a _ i o W ,i
20
10
"o
I
0
'
I
2
'
I
4
'
I
'
6
I
8
'
I
10
'
I
12
salinity [PSU]
Figure 6. Relationship between nitrous oxide concentration and salinity for the cruises in March and April 1997. Spearman rank order correlation coefficient r = -0.0969, n = 171, p = 0.207 (not significant).
146
Dahlke et al.
Table 2. Concentrations of N20 during stratification at Kleines Haft, March 13 1997. Depth
I m below surface I m above bottom
Salinity [PSU]
Water temperature [~
N20 concentration [nmol 1-']
2.3 5.3
5.6 4.3
19.3 25.8
Incubation of sediment cores showed both consumption (0.2 ~mol N m "2 h -1) and release (up to 0.7 ~mol N m -2 h -1) of N20. Surprisingly, no correlation was found between nitrification or denitrification in the sediment cores and N20 formation. This might be caused by the high spatial variability of the data, associated with the fact that these measurements were carried out in different sediment cores. Highest N~O formation occurred in sediments of the outer Bodden waters, namely in the Greifswalder Bodden and not in the inner Oder estuary. A significant correlation was found (Spearman rank order correlation coefficient r = 0.83, n = 94, p.,
DO
DO
EO
EO (a) 0
6
Ee
EQ
(b)
I
I
I
I
8
10
12
14
16
8
I
i
9
10
TOC [mg cm3] Q
July 1996
11
CIN O
October1996
Figure 3. Relationships between a) microbial decomposition activities (FDA hydrolysis) and total organic carbon concentrations (TOC), and b) microbial decomposition activities (FDA hydrolysis) and C / N ratios for selected depth horizons in sediments of the Rassower Strom in July and October in 1996 (A = 0.0 to 0.2, B = 0.2 to 0.5, C = 0.5 to 1.0, D = 4.0 to 5.0, and E = 9.0 to 10.0-cm depth horizon).
In summer, the decomposition of organic matter resulted in steep gradients of ammonium and phosphate concentrations in the pore water of the sediments in the Rassower Strom, leading to high nutrient fluxes into the overlying water (Figs 4, 5). The amounts of nutrients liberated agreed well with the findings of earlier studies in the Baltic Sea and adjacent areas (cf. Koop et al. 1990). Ammonium fluxes reached a mean value of 68 lJmol m 2 h -1 in the dark, and 23 lamol m -2 h -1 in the light, corresponding to a reduction of ammonium fluxes of 65% during the light period. No liberation of nitrate or nitrite from the sediment was detected in either July or October. Phosphate fluxes accounted for 17 lamol m -2 h -1 in the dark. Compared to these values, only 10% of the phosphate flux occurred during the light period. To explain the reduction of nutrient fluxes observed during the light period, we used the Redfield ratio (Redfield 1934) to estimate the nitrogen and phosphorus demand of the microphytobenthic community at the study site. Benthic gross primary production was calculated by concurrent measurements of oxygen fluxes in incubation experiments (for more details, see Gerbersdorf et al. 2000). The results show that the decrease in ammonium flux during the light period was of the same order of magnitude as the estimated demand, implying nutrient assimilation by the microphytobenthos (cf. Bruns & Meyer-Reil 1998). In contrast, the phosphorus demand was not sufficient to explain the decrease in phosphate flux under light
Organic matter decomposition and nutrient fluxes
181
conditions, indicating other processes to be also involved in the reduction of this flux. One such process could be the deeper penetration of oxygen into the sediment under light conditions, and the chemical immobilization of phosphate (Carlton & Wetzel 1988; Rizzo et al. 1992; James et al. 1996). NH 4 [pmol ml ~ p~] 0
200 I
i
0
400 ~
9
.=
600 ..... I
800 ,
_
)
)
-2
N
-4
c
.E_
-a
if)
-8
-10
). ---0-
.
.
.
July 1996
..-. -0--
October 1996
Figure 4. Downcore NH 4 concentrations in the pore water of the Rassower Strom.
Figure 5. Fluxes of dissolved inorganic nitrogen (DIN) and dissolved inorganic phosphorus (DIP) as well as biological oxygen demand (BOD) in the Rassower Strom (values include both dark and light incubations; error bars indicate standard deviations).
182
Rieling et al.
No significant seasonal trend was found for fluctuations in the magnitude of DIN fluxes out of the sediment but the stochiometric relationship between the oxygen and nitrogen fluxes suggests that different processes are involved in the liberation of DIN in summer and autumn. During aerobic decomposition of organic matter, approximately 6.6 Mol 02 are consumed for every atom of nitrogen released as ammonium (Redfield 1934; Koop et al. 1990). During autumn, the m e a n O 2 / N H 4 flux ratio was close to the Redfield ratio (12.8:1), explaining the DIN fluxes as the result of aerobic decomposition. During summer, the O 2 / N H a flux ratio was much lower (1.5:1), indicating that ammonium release was mainly the result of anaerobic decomposition. This interpretation is supported by the high concentrations of ammonium recorded in the pore water of the upper anoxic sediment layers in summer. During autumn, no gradients of ammonium or phosphate concentrations were found in the pore water, implying a liberation of ammonium by the heterotrophic aerobic community at the sediment-water interface. To estimate the influence of the DIN and DIP fluxes on phytoplankton primary production, we calculated the daily nitrogen and phosphorus demand of the phytoplankton, again using the Redfield ratio as well as the concurrent measurements of primary production carried out by Gerbersdorf et al. (2000) at the same study site. Sediment DIN and DIP contributions to nutrient demand in the water column were higher in summer than in autumn. In summer, nutrient fluxes from the sediment could support 8% of the phytoplankton N demand, and 20% of the phytoplankton P demand. During autumn, sediment nutrient release covered only 6% of the N demand, and 9% of the P demand in the water column. Earlier studies have found a significant coupling between benthic decomposition processes and pelagic production processes in several shallow-water systems worldwide (e.g., Koop et al. 1990; Reay et al. 1995), showing that sediments are temporarily important sources of inorganic nutrients. The present work demonstrates that sediment-water exchange processes play a major role in the nutrient and carbon budgets of the Bodden ecosytem, too.
ACKNOWLEDGEMENTS
This study forms part of the interdisciplinary project OKOBOD (Okosystem Boddengew~isser - Organismen und Stoffhaushalt) of the BMBF (Bundesministerium fiir Bildung und Forschung). The authors are especially grateful to T. Br~iggmann, M. Gau, S. Kl~iber and I. Kreuzer for their excellent technical assistance.
REFERENCES
Barnett, P.R.O., Watson, J. & Conelly, D. (1984) A multiple corer for taking virtually undisturbed samples from shelf, bathyal and abyssal sediments. Oceanol. Acta 7: 339-409.
Organic matter decomposition and nutrient fluxes
183
Boetius, A. & Lochte, K. (1996) Effect of organic enrichments on hydrolytic potentials and growth of bacteria in deep-sea sediments. Mar. Ecol. Prog. Ser. 140: 239-250. Boschker, H.T.S. & Cappenberg, T.E. (1998) Patterns of extracellular enzyme activities in littoral sediments of Lake Gooimeer, The Netherlands. FEMS Microbiol. Ecol. 25: 79-86. Boucher, G., Clavier, J. & Garrigue, C. (1994) Oxygen and carbon dioxide fluxes at the sediment-water interface of a tropical lagoon. Mar. Ecol. Prog. Ser. 107: 185-193. Bruns, R. & Meyer-Reil, L.-R. (1998) Benthic nitrogen turnover and implications for the budget of dissolved inorganic nitrogen compounds in the Sylt-Romo Wadden Sea. In: G~itje, C. & Reise, K. (eds) Okosystem Wattenmeer - Austausch-, Transportund Stoffumwandlungsprozesse. Springer Verlag, Berlin, pp. 219-232. Carlton, R.G. & Wetzel, R.G. (1988) Phosphorus flux from lake sediments: effect of epipelic algal oxygen production. Limnol. Oceanogr. 33: 562-570. Gerbersdorf, S., Black, H.J., Meyercordt, J., Meyer-Reil, L.-A., Rieling, T. & Stodian, I. (2000) Significance of microphytobenthic primary production in the Bodden (southern Baltic Sea). In: Flemming, B.W., Delafontaine, M.T. & Liebezeit, G. (eds) Muddy Coast Dynamics and Resource Management. Elsevier, Amsterdam, (this volume). Grasshoff, K., Ehrhardt, M. & Kremling, K. (1983) Methods of Seawater Analysis. Verlag Chemie, Weinheim, 419 p. HELCOM (1988) Guidelines for the Baltic Sea Monitoring Programme for the Third Stage, Part 27D. Baltic Marine Environmental Protection Commission, Helsinki. Herndl, G.J. (1992) Marine snow in the northern Adriatic Sea: possible causes and consequences for a shallow ecosystem. Mar. Microb. Food Webs 6: 149-172. James, W.F., Barko, J.W. & Field, S.J. (1996) Phosphorus mobilization from littoral sediments of an inlet region in Lake Delavan, Wisconsin. Arch. Hydrobiol. 138: 245-257. Koop, K., Boynton, W.R., Wulff, F. & Carman, R. (1990) Sediment-water oxygen and nutrient exchanges along a depth gradient in the Baltic Sea. Mar. Ecol. Prog. Ser. 63: 65-77. K6ster, M. & Meyer-Reil, L.A. (1998) Enzymatischer Abbau von organischem Material in Sedimenten - Standardisierung und Anwendungsbeispiele. VAAMMethodenhandbuch-Mikrobiologische Charakterisierung aquatischer Sedimente. Oldenbourg Verlag, M~inchen, pp. 74-86. K6ster, M., Dahlke, S. & Meyer-Reil, L.-A. (1997) Microbial studies along a gradient of eutrophication in a shallow coastal inlet in the Southern Baltic Sea (Nordrtigensche Bodden). Mar. Ecol. Prog. Ser. 152: 27-39. K6ster, M., Babenzien, H.-D., Black, H.J., Dahlke, S., Gerbersdorf, S., Meyercordt, J., Meyer-Reil, L.-A., Rieling, T., Stodian, I. and A. Voigt (2000) Significance of aerobic and anaerobic mineralization processes of organic carbon in sediments of a shallow coastal inlet in the southern Baltic Sea. In: Flemming, B.W., Delafontaine, M.T. & Liebezeit, G. (eds) Muddy Coast Dynamics and Resource Management. Elsevier, Amsterdam, (this volume).
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Reay, W.G., Gallagher, D.L. & Simmons, G.M. Jr. (1995) Sediment-water column oxygen and nutrient fluxes in nearshore environments of the lower Delmarva Peninsula, USA. Mar. Ecol. Prog. Ser. 118: 215-227. Redfield, A.C. (1934) On the proportions of organic derivatives in seawater and their relation to the composition of plankton. James Johnstone Mem. Vol., University Press, Liverpool, pp. 176-192. Rizzo, W.M., Lackey, G.J. & Christian, R.R. (1992) Significance of euphotic, subtidal sediments to oxygen and nutrient cycling in a temperate estuary. Mar. Ecol. Prog. Ser. 86: 51-61. Verado, D.J., Froelich, P.N. & McIntyre, A. (1990) Determination of organic carbon and nitrogen in marine sediments using the Carlo-Erba NA-1500 analyzer. DeepSea Res. 37: 157-165.
Muddy Coast Dynamics and Resource Management, B. W. Flemming, M. T. Delafontaine and G. Liebezeit (eds.) 9 2000 Elsevier Science B.V. All rights reserved.
185
Significance of aerobic and anaerobic mineralization processes of organic carbon in sediments of a shallow coastal inlet in the southern Baltic Sea M. K6ster, a* H.-D. Babenzien, b H. J. Black, ~S. Dahlke, ~S. Gerbersdorf, ~J. Meyercordt, ~ L.-A. Meyer-Reil, a T. Rieling, ~I. Stodian, a and A. Voigt b
alnstitut fidr Okologie der Ernst-Moritz-Arndt-Universit~t Greifswald, Schwedenhagen 6, 18565 Kloster/Hiddensee, Germany blnstitut fidr Gew~sser&ologie und Binnenfischerei, Alte Fischerhidtte 2, 16775 Neuglobsow, Germany ABSTRACT
The significance of aerobic and anaerobic organic carbon mineralization was investigated in sandy m u d surface sediments of the Nordri,igensche Bodden (southern Baltic Sea) in different seasons in 1996 and 1997. By measuring oxygen penetration depths, rates of oxygen uptake, the release of manganese and iron as well as denitrification and sulfate reduction, it could be shown that sulfate reduction was the dominant respiration process in sediments sampled in summer, autumn, and winter. The ratio of the relative percentage of anaerobic to aerobic organic carbon oxidation varied between 3:1 and 15:1. In spring 1997, however, oxygen respiration was dominant. Carbon oxidation by manganese, nitrate and iron was insignificant at all times. A total benthic respiration of 0.8 to 2.3 mmol C m -2 h -1 was estimated. Between 20 and 89% of the total gross pelagic and benthic primary production was respired by heterotrophic organisms.
1. INTRODUCTION In shallow coastal waters sediments play an important role in the carbon cycle for the modification and remineralization of organic matter. Up to 50% of the organic matter production may be degraded in the bottom sediments of such areas (Jorgensen 1982, 1983). The driving forces of organic matter degradation are microbially mediated electron transfer processes which are controlled by the input of organic carbon (electron donors), and the availability of electron acceptors for organic carbon oxidation (Froelich et al. 1979). For the microbial degradation processes, a sequence of electron acceptors is used which follows increasing sediment * Corresponding author: M. K6ster e-mail:
[email protected] 186
KOster et al.
depth, decreasing redox potential, and availability of energy. When oxygen is depleted by aerobic respiration, manganese, nitrate, iron, sulfate, and carbon dioxide may serve subsequently as secondary electron acceptors for anaerobic degradation processes (Canfield et al. 1993). In coastal marine sediments, the decomposition of organic material via oxygen and sulfate respiration has been studied intensively whereas the role of other mineralization processes, in particular manganese, nitrate and iron respiration, has been given less attention (see Stodian et al. 2000). In organic-rich coastal sediments, aerobic microbial respiration processes are restricted to a narrow zone in the uppermost surface sediments. Below this oxic zone only a few millimeters thick (Mackin & Swider 1989), anaerobic degradation processes occur. In the case that oxygen consumption by microbial respiration exceeds the diffusion of oxygen from the bottom water, anaerobic decomposition of organic material may even occur directly at the sediment/water interface. In fine-grained coastal sediments, much of the sediment oxygen uptake is used to reoxidize the reduced products of anaerobic respiration (Canfield et al. 1993). Aerobic and anaerobic degradation processes lead to the release of mineralization products which diffuse into the overlying water where they become available for primary producers. Within the framework of the interdisciplinary research project OKOBOD (Bodden Ecosystem), major aerobic and anaerobic degradation processes were studied in sandy mud sediments of a shallow coastal inlet in the Nordrtigensche Bodden (southern Baltic Sea, Germany) in 1996 and 1997. The Nordrtigensche Bodden are located on the coast of Mecklenburg-Vorpommern, and form a transition zone between the open sea and the mainland. They act as filters for natural and anthropogenic terrestrial inputs. Because of the shallow water depths, the sediments play a significant role in the turnover of organic matter in these coastal lagoons.
2. MATERIALS AND METHODS 2.1. Sampling sites and sampling period Sampling campaigns took place in summer (July) and autumn (September) in 1996, and in winter (January) and spring (April) in 1997. The main sampling site, the Rassower Strom (water depth of 4 m), is located in the outer part of the Nordrfigensche Bodden and is strongly influenced by exchange processes between the inner part of the Bodden and the open sea. In January 1997 the Rassower Strom could not be accessed because of extensive ice coverage. A more readily accessible neighbouring site at Klosterloch (water depth of 3 m) was sampled instead by drilling a hole into the ice. With respect to inorganic nutrient loads, both locations are regarded as mesotrophic to eutrophic. Salinity ranged between 8.2 and 9.1 PSU at the sampling sites. Water temperatures were highest in July and September in 1996 (16 and 12~ respectively), and lowest in January and April in 1997 (0 and 6~ respectively).
Mineralization of organic carbon
187
2.2. Biological and chemical measurements Undisturbed sediments were collected by means of a modified multiple corer (Barnett et al. 1984) or a hand corer in 10-cm diameter plexiglas tubes. Sediment cores were sectioned into intervals of 0.2-1.0 cm (0-0.2, 0.2-0.5, 0.5-1.0, 1-2, 2-3, 4-5, 6-7, and 9-10 cm) in a glove box flushed with argon. Sediments were classified by determining the percentage by weight of the sediment fractions 30 year indicate a SLR of about 1.2 m m per year (Pirazzoli 1986). This value was confirmed in the 1990s, a global * Corresponding author: C.E. Gabche Fax: ++237332376351357
222
Gabche et al.
relative sea-level change of 1 m s-~are not uncommon (Keita et al. 1991). The tidal wave penetrates far up into the estuaries (Morin & Kuete 1989), travelling upstream by as much as 40 km in the case of the Mungo river. As a result, large intertidal flats and sand banks are exposed at low tide in the estuaries. This not only
226
Gabche et al.
obstructs boat traffic but, together with the high freshwater discharge, also inflicts a high level of disturbance onto the aquatic ecosystems, intertidal organisms being particularly strongly affected. In addition, the intermittently active volcano near Buea constantly threatens the stability of Cameroon's coastal zone (UNEP 1984), thereby making it even more vulnerable to SLR. Cameroon's coastal zone is exposed to monsoonal winds of the Guinea type. They blow predominantly from the south-west in the wet season, producing a humidity close to saturation. During the dry season the winds blow from the north-east. These so-called Hamattan winds reach average speeds of 0.5-2 m s-1 over most of the year, but can reach 18 m s1 in April. The coast is dominated by two types of climate, the so-called Guinean and Cameroon types. These can show local modifications due to the topography of the coast. The Guinean type extends from the Kribi region in the south up to the mouth of the Sanaga river to the north (Fig. 2). It is characterised by 4 distinct seasons (a long and a short rainy season, and a long and a short dry season). The Cameroon climate type is mainly found in the south-western coastal region, but it also occurs in the neighbourhood of Mount Cameroon. It is characterised by two seasons, a wet one lasting about 8 months, and a dry one of about 4 months duration. Maritime climate conditions extend from the west coast to the mouth of the Nyong river. They are characterised by constant high temperatures and humidity. For example, the seaward facing slopes of Mount Cameroon receive the full force of the south-west monsoon, as a result of which the town of Debundscha receives a rainfall of roughly 10,000 mm per year, making it the second wettest place on earth. 2.4. Anthropogenic influences The population of Cameroon's coastal zone (amounting to 1,399,870 for an area of only 27,064 km 2 in 1987) is concentrated in the urban centres where 60% of the country's manufacturing industries are found. The highly urbanised population around the Cameroon river estuary (890 km 2) in the Wouri Division was estimated at 83,4471 in 1987, whereas the census for the Rio-del-Rey estuary (6275 km 2) in the Ndian Division was 87,435, being mostly made up of fishing communities. These population groups are highly vulnerable to SLR. Anthropogenic impacts on the coastal zone include agricultural land use which represents 30% of the gross domestic product (GDP), 70% of which is income from export revenue, with 75% of the labour force being employed in this sector. This is followed by the fishing industry (about 25,000 fishermen in the artisanal sector), the manufacturing sector (17% of the GDP, 16% of the trade), transport, communication, a promising petroleum production (industrial activities represent 60% of the national production), and tourism which concentrates on ethnic and ecological diversity (e.g., sandy beaches, mud flats, and waterfalls in Kribi, Edea, Tiko and Limbe).
Sea-level rise and coastal resources in Cameroon
227
3. T R E N D S , P R E D I C T I O N S A N D A S S U M P T I O N S
In this study, trends have been estimated for Cameroon's main marine fisheries resources for the period 1970-1990, based on the production of fin and shellfish which make use of the m u d d y coastal regions as breeding grounds, nurseries and transition zones. The data used in this study were partly provided by the Ministry of Livestock, Fisheries and Animal Industries of Cameroon, and partly extracted from Seki and Bonzon (1993). The actual trends of the last few decades were then used to estimate projected fish demand for 1990-2010, and artisanal fisheries resources at risk in the wake of SLR. The diversity and distribution of faunal and floral components in muddy-coast aquatic ecosystems were, amongst others, chosen as criteria for gauging possible losses resulting from SLR. The proposed scenarios were based on the following assumptions: 9 SLR is the only factor causing major coastal changes. 9 SLR estimates of IPCC (1990), and Abe and Kaba (1996, for Takoradi in Ghana) are applicable to Cameroon's m u d d y coast as well. 9 The Cameroon and Rio-del-Rey estuaries experience subsidence as a result of natural compaction and anthropogenic influences such as crude oil extraction from offshore wells. 9 Because of the low topography, the m u d d y coastal regions of Cameroon will be flooded and eroded in the course of SLR. 3.1. Fish production trends and values at risk
Trends in the annual catches of Cameroon's marine fish industry (total catch, and fish and shrimp catches) show that the total production fluctuated between ca. 30 and 45 * 103 metric tons (t) in the time interval from 1970-1978. This was followed by a sudden increase to more than 70 * 103 t in the years from 1979-1981 (Fig. 3). 80-
70-
60o
50
~4o "~ r c~
3020-
10
i
0 --
./ I
19"/0
A ' ~ " A ~ A ~ = ' A " =
'
I
1915
'
s.r,mO I
1980
'
'
'
'
I
1985
'
'
~'"
"
I
1990
year
Figure 3. Marine fisheries production trends in Cameroon for the period 1970-1990.
228
Gabche et al.
A few years later, a less-marked but short-lived increase to ca. 75 9 10~ t occurred, followed by a general decrease which lasted up to the year 1990 at least. The general decrease observed in the total annual catches also applies to fish production. The annual shrimp catches, however, remained steady throughout this period. These decreasing trends can be attributed to increasing fishing effort, and to ecological degradation resulting from SLR. Table 3. Projected fish demand for Cameroon (1990-2010). Population
Year
(1000s)
Fish supply Per capita Total (kg year -I) (1000s t) 2.6 149.5
Projected demand (1000s t)
1989-1990
11,833
2000
14,787
.
.
.
.
187
2010
19,286
.
.
.
.
244
m
~
Table 4 gives the quantities and monetary values for the total export of marine fisheries, on the one hand, and that of crustaceans, on the other hand, for the period 1980-1990. Total exports had a mean annual monetary value of ca. 4.5 million US $. Although crustaceans made up only ca. 7% of the total mean annual export mass, their mean annual monetary export value amounted to 3.9 million US $, i.e. ca. 86% of the value of the total fisheries export. This emphasises the point that crustaceans, which thrive particularly well in the muddy coastal areas, are clearly Cameroon's most valuable export commodity (Fig. 4).
Table 4. Quantities and monetary values for the export of fish and crustaceans (shrimps and others). Total export Year 1980
US $ ( x 106) 4.0
mass (Mt) 4018
Export of crustaceans US $ mass ( x 106) (Mt) 3.4 609
Contribution by crustaceans (%) US $ mass ( x 106) (Mt) 85.0 15.2
1982
1.6
4981
1.1
291
68.8
5.8
1984
2.7
10,952
2.4
461
88.9
4.2
1986
3.2
18,609
2.1
306
65.6
1.6
1988
7.1
8661
6.0
752
84.5
8.7
1900
8.2
2484
8.1
1106
98.8
44.5
Total
26.8
49,705
23.1
3525
86.2
7.1
Mean
4.5
8284
3.9
588
86.1
7.1
229
Sea-level rise and coastal resources in Cameroon
The projected fish demand stands at 187 and 244 * 1 0 3 t for the years 2000 and 2010, respectively (Table 3). These give close estimates of expected losses if we consider that a SLR of 3.4 mm per year would result in a total sea-level rise of 1.02 and4.42 cm, respectively.
...
a
m
70 &
n
E
6-
v
rj~ 5 &
..m.
.C::::: 4" ~0 r
-~
/.
total shrimp
r
3-
_er___
e~ 2 -
fish XqX
0
-|--1970
I 1975
1980
1985
1990
year
Figure 4. Monetary values (US $) for Cameroon's marine fisheries production.
Artisanal fishery is practised in the estuaries, mangroves, coastal rivers and up to 2 nautical miles offshore. It is dominant in the Rio-del-Rey, Cameroon and Sanaga regions. Of all the coastal regions, the Wouri coastal division has by far the highest population density (ca. 940 per km 2, 1987 census), harbours about 27% of the artisanal fishermen, and maintains roughly 22% of the landing camps (Table 5). This region, in which 80% of Cameroon's industrial fisheries are established, will clearly incur the greatest losses in the wake of SLR. Although the Rio-del-Rey region has a low population density (ca. 14 per kin2), it has the largest number fishermen and the greatest potential for fisheries development (notably shrimps), besides its potential for offshore petroleum drilling and mining.
230
Gabche et al.
Table 5. Population densities (1987 census), sizes of mangrove areas as well as the numbers of fishermen and their landing camps in Cameroon's coastal divisions. Landing camps (no.)
(no. km -2)
Mangrove area (ha)
Ndian
13.9
150,000
9387
33
Fako and Ocean
120.0
20,000
4908
40
Wouri
937.6
180,000
6484
46
Sanaga Maritime
14.7
180,000
2415
34
Whole coast
51.7
350,000
24,136
206
Coastal division
Population density
Fishermen
(no .)
Investigating the values at risk in the wake of SLR for 3 fishing camps in the Douala-Edea Reserve (Sanaga Maritime division), Gabche (1997) reported a total value of 1293.4 billion FCFA (approx. 2.4 billion US $), with a mean value of 430.8 billion FCFA (approx. 0.86 billion US $; Table 6). The total value for the 34 camps of the Sanaga Maritime division (Table 5) is thus estimated at ca. 29.2 billion US $.
Table 6. Estimated values at risk (xl03 FCFA) for 3 fishing camps along Cameroon's muddy coast (extracted from Gabche 1997). Mangrove Total values wood stocks 401,706 4365 14,744 224,630 577,923 2588 10,282 309,050 313,774 1500 7954 222,320 1,293,403 8453 32,980 756,000 430,811 2818 10,660 165,333 252,000 Mean a Fishing economic units for fishing gear, canoes, engines and accessories b Fish-processing (smoke-drying) units Fishing camp Mbiako Yoyo I Yoyo II Total
FEUs a
Kitchens and living houses 158,000 256,000 82,000 496,000
Bandas b
3.2. Muddy-coast aquatic ecosystems Most coastal rivers and estuaries in the region are dominated by a freshwater fauna including osteoglossiforms, perciforms and characiforms (Tengels et al. 1992). The estuarine zone, characterised by an accumulation of sediments and swamp organic matter, contributes to a high zooplankton production. This provides sustenance for stocks which spawn and use this zone as a nursery. Common organisms in this area are oysters, periwinkles, mullets and catfish (Table 7).
231
Sea-levelriseand coastalresourcesin Cameroon Table 7. Estuarine and mangrove fish species and their habitats. Species
Common name
Habitat Spat on aerial roots of mangroves and at intertidal levels
Crassostrea gasar
Oyster, bivalve
Tympanotonus fuscatus
Periwinkle
Mud in swamps
Callinectes marginatus
Crab
Brackish estuarine mangrove swamps
Periopthalmus hoelferi
Mud skipper
Mud in swamps
Mugil spp.
Mullet
Penaeus notialis
Pink shrimp
Nematopalaemon hastatus White shrimp
waters
and
Flats in swamps of Cameroon and Rio-del-Rey estuaries Juveniles in brackish waters and muddy deposits Juveniles in brackish waters and muddy deposits
Macrobrachium spp.
Giant river prawn
Ethmalosa fimbriata
Bonga
Sardinella maderensis
Strong kanda
Arius heudeloti
Catfish
Demersal, estuaries
Cynoglossus spp.
Sole
Demersal, muddy sediments at 15-100 m
Lutjanus spp.
Snapper
Demersal, estuaries
Polydactylus quadrifilis
Shrine nose
Demersal, estuaries
Sphyraena piscatorium
Barracuda
Demersal, estuaries
Pseudotolithus typus and Croaker P. elongatus
Riverine and brackish waters Pelagic, estuarine and mangrove transit/nursery zones Light sandy-muddy habitats at 6-30 m
Demersal, estuaries
Mud deposits in the estuarine zone are also beneficial for the shrimp fishing industry. The abundance and distribution of the fish and shrimp stocks in these muddy regions are largely determined by the physicochemical conditions resulting from freshwater input and tidal fluctuations within the mangrove forests. The continental shelf fisheries comprise artisanal fishing inshore and industrial fishing beyond the 2-nautical mile zone up to a water depth of 200 m. Species composition and catch estimates for both fisheries show that clupeids dominate in the artisanal, whereas sciaenids are more important for the industrial fisheries.
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Gabche et al.
Table 8. Zonation of floral species in mangroves. Intertidal Species Habitat .Rhizopora Range is 20-45 m, racemosa zones are (epiphytic, 4-8 min parasitic) back zones .Nypa fruticans Zones flooded at high tide .Hibiscus Occurs in mixed filaiceus com.Inu.Acrostichium nity aurium .Drepanocarpus lunatus .Dalbergia ecastaphyllum 9Carapa procera Forina.Chrysobalamus tion of rarely orbicularis flooded .Manilkara banks obovata .Crudia klainei .Cinometra manii .Oxystignia manii
Intermediate Species i Habitat ,Rhizophora Flooded, horrisonii muddy upper ,Pandamus stratum candelabrum .Dalbergia 1Flooded, ecastaphyllum 'muddy ,Drepanocarpus discontinuous lunatus .Oramacarpum 'middle stratum verrucosum .Conocarpus erectus .Fimbristhylis Flooded, muddy ferruginea lower .Eleocaris stratum geniculata '(grassy) .Paspalum vaginatum .Becope decumebs ,Acrostichium aureum .Utridulation spp. 9Xyris anceps
Mangroves Species Habitat .Antocleista Always flooded, vogetli common .Atlophyllum freshwater inophyllum 9Phoenix swamp rechinata species .Rhizophora horrisonii ,Drepanocarpus lunatus .Arcrostichum aureum .Ormacapum verracosum
The zonation of floral species within the mangrove ecosystem (Table 8) shows a high species diversity. Of these, 12 species occur in the intertidal, 13 in the intermediate, and 7 in the mangrove swamp areas. The fauna is dominated by 19 fish species, followed by 4 mollusc, 2 oyster, 1 cephalopod, and 1 mussel species. In addition, about 14 bird species and various monkey families are found. The human population is dominated by foreign nationals. Man-induced influences include the exploitation of mangroves for the construction of houses and canoes, as well as for medicinal and fish processing purposes such as providing wood for smoke-drying of fish. Further uncontrolled exploitation of the mangroves will result in a serious decline in biodiversity, changes in coastal morphology, and accelerated accretion. These negative impacts would be compounded by more frequent flooding and salt-water intrusions in the wake of SLR.
Sea-level rise and coastal resources in Cameroon
233
3.3. Impacts of sea-level rise Complex, multifaceted links exist between fisheries resources, aquatic ecosystem stability, agro-industrial activities, industries, sustenance of the population, and anthropogenic influences. The immediate impacts of SLR would come from flooding, causing the loss of land and livelihood for traditional fishing communities. The intrusion of saline waters into flood-plain soils will also have the effect of "pushing back" the habitats of salinityintolerant flora, e.g., the water hyacinth Eichornia crassipes, further upstream. On the positive side, this will result in a reduced disruption of coastal fishing. On the negative side, increasing saltwater intrusions will induce changes in the dynamics and chemical composition of groundwater and the water regimes at river mouths. As a result, water-logged soils will be converted into mud, necessitating extensive road construction and also the desalination of freshwater aquifers. Despite the fact that the use of contaminated waters for domestic purposes is prohibited, such bans are being widely ingnored. Since the improvement of water-processing techniques is very costly, an increase in the loss of lives through infectious diseases must be expected. The promotion of a greater utilisation of rainwater from roof catchment systems will result in exorbitant prices for roof sheeting. Coupled with reduced production from muddy-coast agriculture, serious economic losses must be anticipated for many sites. Salinity stress will induce a redistribution of species assemblages and result in the destabilisation of the ecological balance between estuarine and mangrove domains. The losses of ecological niches and nursery grounds for juvenile fishes will reduce production and export earnings, and lead to an impoverishment of the fauna and a decreased fish protein supply. Long-lasting floods will result in root decay and mass tree kills with recruitment failure due to anoxic conditions. Other possible effects include changes in sediment deposition patterns, increased storminess resulting in beach erosion, changes in ocean circulation and nutrient availability, higher risks at sea for fishermen, and a general decline in coastal property values. Pollution will increase in the fishing grounds and coastal ecosystems because of greater influxes of anthropogenic pollutants such as heavy metals and petroleum products. Land shortage will result in the use of the swamps as dumps and toilets, thereby promoting epidemics. Water dispersal will increase the virulence of water-borne diseases such as typhoid and cholera which would also spread into the hinterland. In addition, there will be a higher prevalence of malaria since the larvae will have more extensive breeding grounds. It is evident that SLR will cause the widespread destruction of coastal ecosystems, notably the mangroves. Since these serve as important breeding grounds, and also contribute effectively to coastal defence (e.g., Mazda et al. 1997), their decimation will seriously undermine the social and economic survival of the coastal population.
234
Gabche et at.
4. A D A P T I V E RESPONSES A N D POLICIES TO C O M B A T SEA-LEVEL RISE
In combination with intensified educational programs, adaptive measures to combat the negative effects of sea-level rise should include: 9 the installation of tide gauges by trained manpower; such data will promote the timely implementation of effective coastal management strategies; 9 the upgrading of agro-forestry practices, thereby limiting harmful CO2 emissions; 9 the promotion of local skills in coastal defence; for example, the use of sand bags to reinforce barrier weirs, and of higher gabions with additional diaphragms; 9 the relocation of fishing grounds, and the establishment of conservation zones, including nurseries; this will improve biodiversity and safeguard endemic species; 9 the relocation of roads and residential areas in less vulnerable regions; 9 the establishment of nurseries for mangrove trees and salt-tolerant coconut and oilpalm strains; 9 the promotion of public awareness about the negative effects of deforestation; 9 the promotion of more efficient fishing techniques, especially the use of fuel-saving fish-smoking kilns which conserve the mangrove fuel; 9 the introduction of mariculture and other farming techniques based on local materials; 9 the improvement of the existing mesh size of gears, and the strict definition of fishing-ground limits for artisanal/industrial fisheries; this will reduce competition and loss of gear in the wake of SLR; 9 the installation of domestic solar-driven desalination units for the production of drinking water; 9 the construction of permeable, non-concrete breakwaters and groynes using local materials for the purpose of increasing fisheries yield; 9 the mapping of fisheries resources, aquatic ecosystems, and related infrastructure, using advanced mapping techniques in combination with geographical information systems; 9 the support of short and long-term research programmes focused on fish conservation, mariculture, muddy coast nutrient cycling, and physical dynamic processes; 9 the organisation of public-awareness campaigns focused on sustainable ecosystem use, deforestation, coastal erosion, etc.; this should maintain the quality and improve the biomass of ecosystems as well as discourage the illegal exploitation of resources. In conclusion, a holistic approach to the impacts of SLR is proposed for Cameroon's fisheries resources and aquatic ecosystems, accounting for multifaceted aspects such as increased temperature, flooding, destabilization of fish-processing practices, and impaired human health. To assess and alleviate these impacts, an integrated coastal management plan would thus be required. In anticipation of continued SLR along Cameroon's coast, strategic policy trusts to promote technology as well as research programmes are needed, also focusing on the complex interactions between anthropogenic influences and natural factors. Because Cameroon's coastal zone characteristics are typical for other countries in the Gulf of
Sea-level rise and coastal resources in Cameroon
235
Guinea, it is suggested that such an approach would be meaningful for the region as a whole and, for that matter, also for similar regions worldwide (cf. IPCC 1996). ACKNOWLEDGEMENTS
We express our gratitude to the Ministry of Livestocks, Fisheries and Animal Industries for making their data sets available for this study, and to the organising committee of the international conference Muddy Coasts 97 for financial support to the main author during his stay in Wilhemshaven, Germany. REFERENCES
Abe, S. & Kaba, N. (1996) Problems and management strategies of the Ivorian coastal zone. In: Ibe, C., Kothias, J.B.A. & Ajayi, T.D. (eds), Perspectives in Coastal Areas Management in the Gulf of Guinea Region. Tech. Publs Series GOG-LME, 15 p. Barth, M.G. & Titus, J.G. (1984) Greenhouse Effect and Sea Level Rise. Van Bostrand Reinhold, New York, 325 p. Binet, D. (1997) Climate and pelagic fisheries in the canary and Guinea currents 1964-1993: the role of trade winds and the southern oscillation. Oceanol. Acta 20: 177-190. Gabche, C.E. (1997) An appraisal of fisheries activities and evaluation of economic potentials of the fish trade in the Douala-Edea reserve- Cameroon. Fish. Consult. Rep. Cameroon Wildlife and Conservation Society, Yaounde, 39 p. Gabche, C.E. & Angwe, C.A. (1996) Coastal erosion and sedimentation in Cameroon. Int. Sem. Coastal Zone of West Africa: Problems and Management. 25-29 March 1996, Accra, 18 p. IPCC (1990) Strategies for adaptation to sea-level rise. Rep. Coast. Zone Subgroup, IPCC Working Group III. Rijkswaterstaat, The Netherlands, 122 p. IPCC (1996) Second Assessment Report of Climate Change. Chapter 9: Impacts of climate change on coastal zones and small islands. Cambridge University Press, Cambridge, pp. 289-324. Karen, C.D., Niang-Diop, I. & Nicholls, R.J. (1995) Sea level rise and Senegal: potential impacts and consequences. J. Coast. Res. 14: 243-261. Keita, M.L., Johnson, R., Diallo, E.H.A. & Nzegge, E.J. (1991) La courantologie dans l'estuaire de la Bimbia (Cameroon). Atel. Rech. Conj. Product. Estuaires Mangroves Afrique de l'Ouest. UNESCO/COMARAF Rapp. Tech. Projet, pp. 5-9. Mazda, Y., Magi, M., Kogo, M. & Hong, P.N. (1997) Mangroves as a coastal protection from waves in the Tong King delta, Vietnam. Man. Salt Mar. 1: 127-135. Morin, S. & Kuete, M. (1989) Le littoral Cameroonais. Probl6mes morphologiques. Trav. Lab. G6ogr. Phys. Appl. Inst. G6ogr. Univ. Bordeaux III, II: 5-53. Nwilo, P.C., Onuoha, A.E. & Mike, P.T. (1995) Monitoring global sea-level rise/relative sea-level rise in a developing country. The Nigerian experience. Proc. Int. Conf. Coastal Change 95. Bordomer-IOC, Bordeaux, 1995, pp. 24-31.
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Pirazzoli, P.A. (1986) Secular trends of relative sea level changes indicated by tide gauge records. J. Coast. Res. Spec. Publ. 6: 11-5. Pirazzoli, P.A. (1996) Sea-Level Changes - the Last 20,000 Years. John Wiley & Sons, Chichester, 211 p. Schneider, W. (1992) Fiches FAO d'identification de la p~che. Guide de terrain des resources marines commerciales du Golfe de Guin6e. Collaboration Bur. R6gion. FAO Afrique. Rene FAO, 268 p. Seki, E. & Bonzon, A. (1993) Selected aspects of African fisheries: a continental overview. FAO Fish. Circ. 810, 158 p. Tengels, G.E., Reid, G. & King, R.P. (1992) Fishes of the Cross River Basin (Cameroon -Nigeria): Taxonomy, Zoogeography, Ecology and Conservation. Mus6e Royal de l'Afrique Cental, Tervuren. Ann. Sci. Zool. 226, 132 p. UNEP (1984) The marine and coastal environment of the West and Central African region and its state of pollution. UNEP Region. Seas Rep. Stud. 46. Volonte, C.R. & Nicholls, R.J. (1995) Uruguay and sea-level rise: potential impacts and responses. J. Coast. Res. 14: 262-284. Wauthy, B. (1983) Introduction a la climatologie du Golfe de Guin6e. Oc6aonogr. Trop. 18(2): 103-138.
Muddy Coast Dynamics and Resource Management, B. W. Flemming, M. T. Delafontaine and G. Liebezeit (eds.) 9 2000 Elsevier Science B.V. All rights reserved.
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Impacts of sea-level rise and human activities on the evolution of the Pearl River delta, South China M.-K. Han, a* L. W U l a Y.-F. Liu a and N. Mimura b
aDepartment of Geography, Peking University, Beijing 100871, P.R. China bDepartment of Urban System Engineering, Ibaraki University, Hitachi 316, Japan ABSTRACT
Research based on Landsat image interpretation, GIS-topographic mapping, historical records, and ground truthing indicates that the evolution of the muddy coast and the expansion of the Pearl River delta have been strongly affected by human activities in historical times. In recent decades the region has experienced severe man-induced siltation coupled with rapid but premature reclamation of muddy tidal flats in the wake of economic development and population expansion. At present, there is practically no natural coastal landscape left, the shoreline being characterized by man-made dikes throughout. In addition, most of the delta plain is poorly protected, being situated below local high-tide and storm-surge levels. The delta region is thus exposed to natural disasters such as typhoon-driven storm surges and ground subsidence caused by local sediment compaction and regional tectonics. These effects are compounded by the threat of accelerated relative sea-level rise which has been estimated to reach 0.5 m within the next 50 years. Without massive protection works this would lead to the inundation of 96.5% of the delta region, and would include the destruction of even entire cities such as Guangzhou. We contend that the effects of human interventions in the Pearl River delta region have reached the same significance as those associated with geological processes. This important aspect has to be taken into account when studying the recent evolution of the delta, especially when seeking sustainable solutions for the economic development of the region.
1. INTRODUCTION The Pearl River (Zhujiang) delta (Fig. 1) is the third largest in China, being surpassed in size only by those of the Yangtze River (Changjiang) and Yellow River (Huanghe). The delta receives its sediment mainly from the west, the north and the * Corresponding author: M.-K. Han e-mail:
[email protected] 238
H a n et al.
east in association with the three large tributaries comprising the Xijiang (Western River), the Beijiang (Northern River), and the Dongjiang (Eastern River), respectively. Since the Holocene marine transgression peaked at about 6000 years B.P., the former shallow estuarine embayment, dotted with numerous rocky islands, has been completely infilled by deltaic deposits, in the process leading to the development of the Pearl River. Today it has an average runoff of 302,10 ~ m 3 year -1, and a sediment load of ca. 84,106 t year -1, with an average sediment discharge of 0.306 kg m -3. The delta has been utilized for agriculture as early as the Han Dynasty (206 B.C.-220 A.D.), and for aquaculture since the Song Dynasty (618-1279 A.D.; cf. Han et al. 1988; Li et al. 1991; Fig. 2).
Figure 1. Locality map of the study area.
2. HUMAN ACTIVITIES 2.1. Types of activities The Pearl River delta evolved by the southward expansion of its frontal margin. Since historical times the muddy coastal fringe has been increasingly affected by growing human interventions. These activities have in recent decades led to increased siltation in the wake of rapid land reclamation to combat fast urbanization and the resulting expansion of agriculture, aquaculture, and industrial development exerted by the policy of opening China to the outside world (Yan 1984, Han et al. 1995; Li et al. 1995).
Sea-level rise and human activities in the Pearl River delta
239
;igure 2. Historical evolution (after Li et al. 1991), and distribution of elevations in he Pearl River delta region based on GIS-topographic mapping. Shore line since I the Qing Dynasty, II) the Ming Dynasty, III) the Song Dynasty, IV) the Tang )ynasty, V) the Han Dynasty, VI) 4000 B.P., and VII) 6000 B.P. (elevations relative to ?cal datum at Pearl River mouth, i.e. 0.6 m above national datum or mean sea level ,f the Yellow Sea).
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Most natural river deltas are characterized by extensive m u d d y fringes along the coastal margin, the estuaries, and the banks of distributary channels. Such lands are commonly reclaimed once they have aggraded above mean sea level (MSL). In the Pearl River delta, however, the local people developed innovative techniques by which the creation of new land along the m u d d y coast was speeded up. This new method of land reclamation was implemented in two stages. To reclaim an intertidal area, rubble dikes were at first constructed at low tide with their crestlines at the elevation of mean sea level. During the rising tide these dikes were overtopped, and the basins in their rear were filled with m u d d y tidal waters. During the following ebb tide the suspended matter in the trapped water body settled out to form a thin mud deposit. This process repeated itself with each tidal cycle until the level of the deposited mud reached the elevation of the mean sea level. The dikes were then progressively raised until the mud deposits reached the hightide level. The land reclaimed in this way was initially cultivated with salt-tolerant plants. Later, when the remaining salt in the soil had washed out, the cultivation of rice could begin. In a second stage, the separately diked areas were merged into larger reclaimed regions by upgrading the main dikes, and by building sluices and structures for the control of irrigation waters and river floods. The remaining diked but low-lying and frequently water-logged grounds were then converted into so-called mulberry (or fruit-tree) dike fish-pond systems. These were artificial ecosystems which promoted more efficient land use (Han et al. 1988). To accelerate the land-reclamation process, the former manual procedure was mechanized over the past three to four decades. Thus, the junks formerly used for the transport of rubble were replaced by motor vessels, and the construction of dikes is now conducted by elevator-ships. Also, the diked but water-logged low-lying grounds are being drained by pump ships, and then filled with silts dredged from nearby distributary channels. This procedure has led to a situation where the elevation of the tidal flat to be reclaimed is now as low as 0.3 m, and in some cases even 0.5 m below mean sea level. We consider this to be a premature land-reclamation process which may threaten the lives and livelihood of future generations in the wake of climate change and accelerated sea-level rise. Our evaluation of Landsat TM data for the years 1986, 1988, 1992, 1994, and 1996 as well as a comparison with the topography of 1966 show that the land reclaimed along the margin of the Pearl River delta over the past +30 years (1966-1996) has attained an area of 344 km 2, corresponding to a mean reclamation rate of 11 km 2 year -~(Figs 3, 4).
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~igure 3. Growth of the Pearl River delta resulting mainly from land reclamation in he last 100 years (based on Landsat imagery; see Fig. 4 for framed area).
.2. Consequences of human activities The accelerated land reclamation and the increased man-induced siltation has ;enerated a number of features which today characterize the muddy coasts of the 'earl River delta.
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A. Natural intertidal mudflats, which commonly line the coastal fringe of large deltas, have become a rare sight in the Pearl River delta. Instead, the coast displays a rather narrow stretch of embryonic tidal flat which either imbricates against the foot of the dikes along the frontal margin of the delta or may be expressed as a small patch at the apex of some rocky embayments. Any expansion of the tidal flat immediately triggers reclamation. As a result, most of the coastline along the Pearl River delta margin has an artificial, man-made appearance. B. The land-reclamation process in the Pearl River delta region lacks an integrated and comprehensive coastal management plan. All reclamation projects primarily pursue the purpose of creating new land either for agriculture and aquaculture or for urban and industrial development, without any effort to design and implement an ecologically sound environmental protection and preservation scheme. As a result, ecologically important habitats such as wetlands, especially the mangrove zones, have been completely eliminated. Similarly, in the development of aquaculture, interest is focussed only on building shrimp and fish-breeding pond systems without considering the maintenance of breeding grounds for shellfish, oysters, etc., on the tidal flats. The latter are not only much more profitable, but would also increase the biodiversity in the region. C. Because of the premature land-reclamation procedure, most of the delta region comprises very low-lying muddy ground (Fig. 2 and Table 1) which is very vulnerable to many kinds of natural disasters. The constant acceleration of land reclamation has extended the lengths of river outlets, thereby creating complicated distributary networks which have difficulty in discharging river flood waters into the sea. This greatly enhances the risk of flooding. In some cases, peak flood levels, being constrained within high levees on both river banks, have generated such large hydrostatic pressures that the groundwater has spurted out upwards from the low-lying farmland behind the levees. Clearly, such silt-laden gushing waters can easily develop into destructive floods, incurring huge economic losses. This has recently been experienced behind the eastern bank of the Beijiang River northwest of the city of Guangzhou. Furthermore, in many places the reclaimed delta plain is not sufficiently protected against the frequent typhoon-generated storm surges. This is because the dikes are neither high enough nor in good enough condition for the purpose. Consequently, serious economic losses have been suffered in such inadequately-protected regions. Areas on the delta underlain by 5 to 10-m-thick mud deposits experience continuous subsidence due to compaction of the sediment. This may bring about safety problems during construction if it is not considered already at the early planning stages. For example, the Huangpu Economic Technological Development Zone, located east of Guangzhou near the outlet of the Dongjiang River and built only in 1988, has already experienced such problems. In combination with crustal subsidence and sea-level rise, the lowering of the ground level caused by sediment compaction has resulted in the flooding of the container wharf (originally designed
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close to high-tide level) once in every two or three years. Some buildings have been deformed and even cracked due to the uneven subsidence. Here again, costly cleaning-up, repair, and maintenance operations are incurred by the authorities.
Figure 4. Growth of the Pearl River delta resulting from the reclamation of the muddy coast in the vicinity of the Moudaomen Outlet in the recent 100 years, especially in the period 1966-1996 (based on Landsat imagery).
3. SEA-LEVEL RISE
Against the background of the above discussion, it is clear that any sustained sealevel rise would create serious problems in the Pearl River delta environment, thereby exacerbating the already existing disaster risks. The eustatic rise in sea level due to global warming is estimated at 20-30 cm over the next 50 years (until 2050). However, the relative sea-level rise in the Pearl River region is expected to triple. This is mainly due to the additional impacts of crustal subsidence, man-made subsidence triggered by the extraction of groundwater, oil and gas, and sediment compaction in the reclaimed lands. Thus, for the Pearl River delta area the relative sea-level rise over the next 50 years has been estimated at
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0.42 m by the State Oceanic Information Center (reported by the China Environmental News, Chinese edition, 10 August 1996, page 1). From Table 1 it can be seen that, if the Pearl River delta were not protected by dikes, the area which would be flooded at times of maximum high-tide levels would today amount to ca. 65.3% of the whole deltaic plain. This would increase to ca. 89.0% when adding a 1-m elevation in water level during typhoon storm surges, and to ca. 96.5% when a relative sea-level rise of 0.5 m is considered in addition. In the latter case, all the cities on the delta, including Guangzhou, would be under water.
Table 1. Surfaces areas between individual elevation intervals of the Pearl River deltaic plain based on GIS-topographic mapping and calculations (the surface areas of rocky hills and islands are not included). Elevations are relative to MSL of the Yellow Sea (national datum of China) or MSL of the South China Sea (local datum) which lies 0.6 m above national datum. Mean tidal range: 0.86-1.64 m. Maximum tidal range: 2.53 m. Elevation local datum (m) Mangroves
D Research
D
coral reefs
~ Planning
~> Erosion
D Wetlands D Shore protection
D Utilization
I Siltation
D Pollution ~> Sea-level rise
~> Stormsurge effects
Figure 5. Proposed scheme for systematic integrated coastal area management (modified after Wang et al. 1997).
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It clearly shows the links between the natural environment and human impacts in the form of coastal law (regulation), education and ICAM to protect and conserve the natural environment and its resources (cf. Wang et al. 1997; Healy & Wang 2000).
ACKNOWLEDGEMENTS
This study was supported by the State Laboratory of Coast and Island Exploitation, Nanjing University (Contribution No. SCIEL21198118).
REFERENCES
Anonymous (1993a) The Noordwijk guidelines for integrated coastal zone management. The World Bank Environment Department, Land, Water and Natural Habitats Division. World Coast Conference 1993, November 1993, Noordwijk, pp. 1-6. Anonymous (1993b) China Population Statistics Yearbook 1993. State Statistical Bureau P.R. China, China Statistical Publishing House. Anonymous (1995) Statistical Yearbook of China 1995. State Statistical Bureau P.R. China, China Statistical Publishing House. Fu, W.X. & Li, G.T. (1996) Ocean pollution and ocean environment protection. In: Marine Geography of China (Chap. 23). Ocean Science Press, Beijing, pp. 474-517 (in Chinese). Healy, T. & Wang, Y. (eds) (2000) Muddy Coasts of the World: Processes, Deposits and Function. Elsevier, Amsterdam (in press). Wang, Y. (1983) The mudflat system of China. Can. J. Fish. Aquat. Sci. 40 (Suppl. 1): 160-171. Wang, Y. & Aubrey, D.G. (1987) The characteristics of the China coastline. Cont. Shelf Res. 7(4): 329-349. Wang, Y. & Zhu, D.K. (1994) Tidal flats in China. In: Oceanology of China Seas (Vol. 2). Kluwer, Dordrecht, pp. 445-456. Wang, Y., Luo, Z. & Zhu, D. (1997) Economic development and integrated management issues in coastal China. In: Haq, B.U., Haq, S.M., Kullenberg, G. & Stel, J.H. (eds) Coastal Zone Management Imperative for Maritime Developing Nations. Kluwer, Dordrecht, pp. 371-384. Wang, Y., Ren, M.-E. & Syvitski, J. (1998) Sediment transport and terrigenous fluxes. In: The Sea (Vol. 10). John Wiley & Sons, New York, pp. 253-292. Zhu, D.K., Martini, I.P. & Brookfield, M.E. (1998) Morphology and land-use of the coastal zone of the North Jiangsu Plain Jiangsu Province, Eastern China. J. Coast. Res. 14: 591-599.
