Environmental Geomorphology (Developments in Earth Surface Processes 4)

in Earth Surface Pro

4

ELSEVIER SCIENCE B. V. Sara Burgerhartstraat 25 P.O. Box 211, I 000 AE Amsterdam. The Netherlands

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Fig. 42. Evidence of flood plain splay with local scour (arrow: a) after a secular flood; circle shows the break in the main railbed whose layout crosses a meander bend cut off over one hundred years ago. River Tanaro during the flood of Nov. 1994 (cone. S.M.A. no. 429. 27 Oct. 1995).

109 The presence of abandoned riverbeds near the floodway is associated with the hazard of breaches caused by water infiltration under the embankment, since in these places the alluvial deposits are characterised by a coarser and more heterogeneous particle size than the surrounding sediments, thus making up a preferential site for through flow processes. The abandoned riverbeds, even at a considerable distance from the watercourse, control the overland flows. For example, in the mid Po Plain a historical flood of 1705 occupied the surrounding land over a 20 km wide area, distributing its waters along channels parallel to the valley axis, in accordance with the traces of abandoned riverbeds dating from the Bronze Age onwards (Castaldini & Piacente, 1995). During the more recent flood of November 1994, which affected the upper Po Plain basin, serious damage to man's infrastructures was recorded just in correspondence with ancient river courses which had been activated by greater inundation currents during the propagation of the highwater (Fig. 42). In the presence meandering river course, controlled by the uninundable terrace scarps, such as in meanders cut deeply or confined by embankments, the riverbed mobility is in marked contrast with the relative staticity of the imposed natural or man-made limits. The scarps and the embankments are always subjected to the erosion caused by the meanders' evolution within the floodway zone. In the case of embankments, the meanders traces convey the overbank currents straight onto the embankments and generate breaches (Fig. 43). As one of the river processes linked to the modelling of a watercourse which can induce a flood hazard situation due to the breaching of an embankment, bank erosion should be mentioned. It is produced by both the downstream migration of meander bends and the deposition of sediments |>;i.5|-;;:'| embanked floodway zone

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Fig. 43. Meandering trend of a watercourse and related flood plain zone bounded by embankments: situation of the River Po during the XIX century in the stretch near Piacenza 350 km from the outlet. The most recurrent breach situations are located in correspondence with abandoned channels, internal or external to the embankments.

110 within the riverbed, with the consequent formation of middle bars or islands along straight courses or downstream of confluences. The assessment of the inundation probability, based on the flood recurrence, should take into account, for a given investigation territory, the morphological changes of the riverbed and alluvial plain as well as the development of man-made infrastructures. Previous instability phenomena, caused by inundations, could no longer represent hazard situations, whilst new hazard conditions could be generated owing to the mobility of the fluvial system.

3.5 Marine hazard 3.5.1 Geomorphology 3.5.1.1 Coastal environment A coast is a stretch of varying extension between land and sea. It comprehends the shoreline which has a changeable morphology in space and time, according to short and long-term fluctuations caused by sea-level variations due to several causes: from the flux and reflux of waves and tides to the regional or worldwide changes of the sea level caused by tectonics, eustatism, etc. Among the processes that shape a coast, those involving waves are the most important. There are various ways in which waves break a shoreline. First of all, if a coast is not a sheer drop from a cliff to the sea, the waves approaching it lose some of their propagation velocity owing to friction on the shallow sea floor. In other words, if the wave approaches the coast in an oblique direction with respect to the isobaths, the wave portion that first reaches the shallow bottom undergoes a slowdown due to friction. Therefore, wave fronts tend to arrange themselves parallel to the isobaths, i.e., the direction of the wave energy tends to assume a direction normal to the coast. This phenomenon is named wave refraction (Fig. 44). As a consequence, wave energy is concentrated towards headlands and promontories whereas it is dispersed in bays and inlets. Besides undergoing a diversion of their fronts owing to wave refraction, when waves come in contact with a shallow sea bottom a deformation of their oscillatory motion also occurs with overturning of the crest into the hollow in front of them creating plunging breakers. The water is thrown forward and goes up the coast as far as its transport energy allows. Afterwards, the water returns down the shore to the sea under the force of gravity, with a more or less disorderly and forceful landward flow, called backwash. Although the waves hit the coast, although with reduced energy owing to friction and with a direction diverted by refraction, they are also subject to a process of reflection with an angle equal to the angle of incidence. As a result of the interference between the two wave systems, direct and reflected, a fairly complex composite wave is produced. The incident wave, to some extent diverted by refraction, the reflected wave and

Ill ivai/e segments of same energy in deep water

A

B

C

Fig. 44. Wave refraction, due to the concentration of wave energy on headlands and to the dispersion around bays, has an important influence on erosional and depositional processes affecting coastlines.

the backwash all have different angles and directions and determine a composite landward and seaward movement with a zig-zag trajectory, subparallel to the shoreline, called longshore current. Coastal regions, in particular those in direct contact with the sea, are characterised both by high humidity, owing to the constant presence of sea water and by scarceness of vegetation, owing to the saltiness of the air. These two characteristics create very particular conditions, quite different from those of other environments where abundant humidity is accompanied by luxuriant vegetation. In this way processes of concentrated and widespread runoff take place, as well as landslides and solifluction, owing to littoral rock and debris impregnation. Moreover, the presence of sea water is a cause of important and widespread phenomena of physical and chemical weathering of rocks: salt cracking, humidification, frost shattering, solution, hydrolysis, hydration, etc. Wind, besides being a wave generator, is an important agent which transports sea water towards territories not otherwise reached by the waves and also sand along the coast or to the inland in the form of littoral dunes. Tides are an important modelling agent. These periodical variations of the sea level along a stretch of coast (intertidal zone) are the cause of several processes: salt cracking, humidification-exsiccation, etc. They also broaden the zone subject to wave motion. Finally, with the rising and lowering of tides, conspicuous horizontal movements of masses of water, called tidal currents, take place. These are very important agents of sea-floor erosion, sediment transportation and deposition.

112 especially near harbours, bays and, in particular, lagoons and estuaries, where they add to the effects of fluvial currents. Rock characteristics, such as cohesion, fissuring, frost shattering susceptibility, permeability and cementation can favour littoral marine degradation to varying extents. Also particular tectonic arrangements may condition a coast's degree of stability; for example, a dip-downstream attitude can facilitate rock slides. Animal and vegetal organisms are a direct or indirect cause of several physical and chemical processes when alive: roots widening joints, animals such as date mussels digging through rocks either mechanically or by secreting acid substances, coastal plantations damping the wave motion, corals and calcareous algae building atolls and reefs, micro-organisms producing carbon dioxide and thus causing karst phenomena, etc. 3.5.1.2 Coastal processes Similarly to river erosion, diverse morphogenetic processes linked to the action of littoral sea waters are defined by the term coastal erosion. Like the categories previously defined regarding watercourses, four types of coastal erosion can be distinguished: Coastal erosion in the strict sense, means the removal and loading of detrital material from a sea shore. Cavitation means the mechanical action of the waves only against the coast. Abrasion means the mechanical hitting action of debris thrown by the waves against the coast. Coastal degradation is a whole complex of morphological phenomena linked to the presence of the sea. For example, the weathering of rocks owing to humidification and exsiccation or salt cracking processes determined by marine water; or mass movements following undermining activities at the toe of a sea cliff, etc. The load of marine waters is made up by organic and inorganic materials in solution and by detrital elements of the most varied dimensions. Only a small portion of these is derived from actions of direct coastal erosion; in most cases they are conveyed to the sea by watercourses, glaciers, winds, etc. Part of this detrital material undergoes wearing due to the movements of the sea, or is transported and finally abandoned off-shore or transferred from one point of the coast to another. As happens with fluvial currents, transportation trends and conditions depend on the one hand on marine water velocity, turbulence, density and temperature and, on the other hand, on the dimensions, nature, bulk density and shape of the materials. Solid matter can be transported in solution, in suspension, hy floating, saltation, rolling and dragging along. Marine currents transport solid matter exclusively in solution, suspension or by floating; tide or longshore currents may assume noticeable rates of velocity and turbulence and thus can transport also by rolling, dragging or saltation. The latter is typical of plunging breakers which have considerable kinetic energy. Backwash can transport even coarse materials, by means of rolling, dragging and saltation.

113 The deposition of the detrital load transported by sea waters may occur owing to a decrease of a current's energy, as happens with fluvial currents. In certain cases the deposition takes place owing to the interference of two currents having opposite directions, following the ''algebraic" annulment of their kinetic energy, as in the case of two opposite and contrary wave fronts meeting as an effect of wave motion refraction, or caused by the interference between coastal breakers and backwash current, or between tide ebb and fluvial current, etc. The movements of the sea, in particular wave motion, rearrange and distribute the grains of solid matter which are progressively deposited along the coast: in this way littoral sands seem to be "washed", that is deprived of finer silty and clayey elements. These finer particles are, however, present in fluvial deposit sands whose patterns of sedimentation and transportation make their removal more difficult. On the other hand, coarser elements can mix with littoral sands since, once they have been deposited by waves, they can hardly ever be removed by backwash. 3.5.1.3 Cliffs A marine cliff (Pig. 45) is a rocky bluff, generally without vegetation, in contact with the sea. It has a steep or even vertical slope angle and is created by the direct or indirect erosive action of the sea. Marine cliffs are found in competent rocks but also in sands and clays. On the other hand, not all steep or rocky coasts are cliffs: there are cases of false cliffs which have not been modelled by the sea but by other phenomena, such as scarps along littoral faults or subaerial erosion coasts submerged owing to transgression movements. A distinction should be made between live cliffs, at present in straight contact with the sea and subject to marine erosion, and dead cliffs, no longer active and separated from the sea by coastal deposits or an erosional shore platform. Erosion, in a broad sense, and the dismantling and retreat of a cliff all depend on both littoral abrasion and cavitation and the removal of debris, as well as the degradation phenomena due to the sea. Moreover, the processes depend on the type of wave motion, tide extension, rock resistance, the topographic features of the shoreHne, the steepness of the submerged scarp, the quantity and particle size

Fig. 45. Schematic example of a cliff profile.

114 distribution of the detrital materials transported, etc. An essential role in a cliffs morphological evolution is played by several characteristics: a difference in the degree of competence gives more or less steep profiles; mineralogical composition may favour to varying extents chemical weathering and therefore a scarp's degradation and retreat; the degree of homogeneity or the alternance of different lithological types conditions a cliffs shape; the strata's tectonic attitude or the presence of joints determines more or less steep and indented coastlines. The retrogradation mechanism of a cliff might be schematically simplified as follows. The starting point is given by a cliff of unspecified origin in direct contact with the sea: the processes of weathering increase the number of fissures and cause a certain fragmentation of the rock; at the same time the actions of cavitation and especially abrasion effected by coastal breakers dig a notch in correspondence with the mean sea level; after a certain lapse of time the fall of a portion of rock can occur in correspondence with the steepest and most fissured stretch of sea cliff The fallen debris which accumulates at the cliff's toe for a time protects the scarp from the attack of coastal breakers. The latter in the meanwhile gradually reduce the rock fragments by means of abrasive processes linked to the waves' landward and seaward motion; the smallest detrital fragments are progressively removed, owing to the erosive effect of the waves and in particular longshore currents. As the accumulation covering the cliff's toe is gradually dispersed, meteoric degradation may continue to produce and/or increase rock fissuring in the scarp. Moreover, the breakers' actions can dig a new notch and reestablish the morphological situation described at the beginning: thus, a retrogradation process may start all over again. Retrogradation of sea cliffs may occur with extremely varying rates: from imperceptible to 10 cm/year. Therefore, with other conditions being equal, the evolution of cliffs depends on the erosion processes which break up and demolish the rocky walls, those which determine a reduction and removal of the debris and finally the weathering process, and in particular the actions of marine waves and currents. Nevertheless, an important role seems to be played by the more general movements of the sea, i.e., fluctuations of its level in either a transgressive or regressive sense. Indeed, the retrogradation 6f a marine cliff leads to the formation of a wave-out erosional platform (Fig. 45) whose progressive extension obstructs the coastal breakers. The latter gradually lose their erosive power as they come to attack the cliffs toe since friction with the platform disperses a considerable portion of the waves' initial energy. If the general movements of the sea or, rather, the relative movements of sea and coast tend to a transgression, the evolution mechanism of sea cliffs does not stop, but the cliff is more and more subject to retrogradation, leaving at its toe an erosional shore platform which is gradually submerged by the transgressive sea. If on the contrary the movements are regressive, then the sea cliff cannot be lapped by the waves any longer and thus turns in time into a "dead cliff. In some cases cliffs have vast and irregular cavities of varying dimensions: notches, caves, arches, etc. Their origin can be due to physical or chemical degradation processes or to differential marine erosion of lithological types of varying resistance or rock portions having different degrees of fracturing. In limestones the

115 origin is often related to karst solution phenomena, simultaneous with or preceding coastal erosion processes. Sometimes the waves' action can isolate large rocks and stacks from the cliff. Usually a jagged and irregular coast with bays and headlands is subject to a regulation of its shoreline: in fact, owing to wave refraction beaches are formed on bayheads whereas promontories are eroded, with consequent attenuation of the embayed shoreline. Subsequently, baymouth bars, spits and erosion platforms develop further reducing wave energy towards the coast thus favouring littoral sedimentation. The latter takes place according to processes and landforms mainly governed by longshore currents. In some places, however, especially in correspondence with more or less resistant rocks, selective littoral erosion occurs which progressively forms and accentuates a more or less regular coastline. 3.5.1.4 Beaches The term beach is given to a littoral belt made up of present and recent cohesionless marine deposits. Seaward it is bordered by the low water zone called offshore and landward by sand dunes and the first bedrock outcrops (Fig. 46). A beach is subdivided into areas the first of which is called mshore. This is the portion ranging between the mean high and low-tide levels and the permanently emerged zone. Generally the inshore contains a low tide terrace and a surf zone. The latter corresponds to the steepest portion, where surging waves and backwash movements usually take place. The emerged beach is often longitudinally divided by a berm into two parts: the backshore and \ht foreshore. The berm is formed by an accumulation of materials abandoned by coastline breakers, beyond the backwash limits of influence, where water goes back to the sea mainly by underground flow. It can be increased also by the wind, depositing sandy material from the foreshore. Coasdine breakers of particular intensity may erode the berm portion facing the sea. The backshore is made up of present littoral deposits and only exceptionally can it be invaded by sea waters; in some places it contains shallow lagoons and on its surface houses and farmland can be found. In the offshore and inshore if the detrital material is not subject to violent actions by the waves, it is removed and is moved slightly forwards, in correspondence with backshore

foreshore

Fig. 46. Schematic example of a beach profile.

inshore

offshore

116 crests, and backwards, in correspondence with wave troughs: typical oscillation ripples can be observed. Along the surf zone, seaward current ripples are found if the backwash movement prevails; on the contrary, if the advancing movements are dominant, landward current ripples are formed. The study of these detailed seaforms can be applied to reconstruct the dynamics of coastline waters. On the other hand, when the plunging breakers are rather forceful, the grains of material are bluntly raised by turbulent movements: some are thrown on the emerged beach and others slowly fall down onto the sea floor. Backwash currents can remove the finest particles and carry them off-shore; they can also easily transfer those materials which, raised by the wave motion, stay for a while in suspension near the sea floor. The debris, once removed, transported, deposited and again removed, undergoes a continuous alternating landward and seaward movement, with trajectories variable in space and time, depending upon the waves' direction and energy, the force of backwash, the direction of longshore currents, etc. Afterwards, it will be deposited in lower turbulence, lower current velocity zones or in points where a compensation of two opposite currents occurs. The general dynamics of a beach is usually characterised by an alternance of two actions: an action of prevailing accumulation of debris and an action of erosion in a strict sense of the material. The alternance rhythms of these phases are extremely variable: daily, several days and so on up to completely seasonal. Indications of the degree of stability of this type of coast are derived from the equilibrium between supply and removal of detrital material. If the balance is equal over a certain time, then the beach may be considered stable. On the other hand, beaches can be affected either by progradation or retrogradation, whether the supply of material or its removal prevails. Particles are selected by waves according to their dimensions, weight and shape. The coarser ones, which are usually heavier, are abandoned on the beach, whilst the finest ones can be taken back, transported and deposited off-shore. The material can also be thrown by plunging breakers beyond the high tide limit or, in any case, beyond the sea level at that moment: if backwash does not take place in this stretch of beach, because the breakers' water goes back to the sea through underground passages made up of detrital permeable layers, even the finest debris can remain on the beach. As a consequence, the dryer and the more permeable a beach is, the more the process of sedimentation, and therefore of progradation is accentuated: this fact can easily be observed on Mediterranean beaches in summer. On the contrary, during rainy periods, especially with high and frequent waves, the beach sediments are saturated with water and therefore become practically impermeable; as a consequence, all the water goes back to the sea through the subsoil, with velocity directly proportional to the beach gradient. Moreover, rain water can add to sea water, thus increasing the erosion power in a strict sense and the transportation of backwash waves. To all this, also the removal and undermining action of plunging breakers should be added, with the increased likelihood of debris being taken on load by the waves, since the energy necessary to keep a detrital element in movement is lower than that required for its removal from the state of immobility.

117 For these reasons many beaches are subject to erosion activities in winter: on the one hand great plunging breakers and backwash currents attack and demolish the beach and shoreline bar, on the other hand they convey to the sea part of the debris that make up beach deposits. Generally in a prograding beach the prevailing sedimentation stage alternates with phases of partial erosion and marine and aeolian rearrangement. The debris accumulation on the seaward face of the shoreline bar causes progressive advancement of the crest, with consequent widening of the backshore. The beach slope gradient mainly depends on the size grade of its grains: fine sands make up very gentle seaward slopes whereas coarse sands or gravels build quite steep slopes: — 2° for 0.12 mm diameter sands: — 8° for 0.50 mm diameter sands; — 12° for 2 mm diameter sands; " 15° for 5 mm circa diameter gravels; — 20° and more for pebbles of 64 mm and over in diameter. It should be considered, though, that the slope angle depends also on the energy of the wave motion which, in turn, is partially linked to the longitudinal trend of the coastline, its irregularities and the extension of the sea front. Preferential places for debris accumulation and therefore for beaches are the sea stretches facing river mouths, because of the abundant supply of materials, or the bays and the areas between islands and coasts, owing to wave refraction. Sand dunes can be found landward, beyond the berm: they are accumulation landforms due to wind action and are similar to those of arid regions. With the latter they have in common a vegetation-lacking environment which favours wind deflation processes. The debris supply area is the beach whereas the accumulation area usually corresponds with the backshore or with inner stretches of coast, where the energy of the wind blowing from the sea is lower whereas the presence of vegetation progressively increases. Seaward, opposite the beach, longshore bars can be found on the sea floor: they are long and narrow submerged deposits, subparallel to the coastline. The patterns that lead to their formation are complex, various and different. In some cases the first stage of their formation could occur in neutral zones, where waves and currents are compelled to deposit part of their load because of the interference of oppositedirection energies. In other cases the deposit could be the result of progressive slowdown of the waves owing to friction with the sea floor. This would lead to a loss of energy with consequent sedimentation of the coarser elements taken on load after the dismantling of headlands. In other cases they could be remnants of ancient berms submerged following marine transgression. Bayhead beaches are formed due to the decrease in wave energy owing to the phenomenon of refraction at inlets. The same phenomenon can also give rise to a neutral zone between an island and the coast, following the collision of two opposite wave energies, with consequent deposition of debris: the progressive sedimentation can then build up a junction between an island and dry land with the formation of a

118 tombolo. Hooked spits are the result of littoral drift motion owing to a leeward wind loss of part of the load transported following a decrease in energy. Cuspate forelands are formed following the collision of two opposite directions of currents carrying debris which is thus accumulated. Littoral barriers mainly derive from the emersion of a shorebar following the progressive availability of material. Lagoons are small bodies of water separated from the sea by littoral barriers or tombolos and are in direct and active connection with the inland fluvial network, since they are linked to the open sea by means of inlets, which are the starting points of channels going through the lagoons. In the lagoon there are marshy areas which are nearly always emerged and covered by grass and shrubs, called schorre, and others emerging only at low tide called slikke. In lagoons the reflux waters of tidal currents are added to the fluvial ones, thus freeing the bottom of the channels from debris. A lagoon's life depends on many conditions: the amplitude of tides, the flow rate and sediment load of influent streams, the dimensions and shape of the lagoon itself, the width of the mouths, the direction of marine waves and currents, etc. Within the same lagoon a live part is distinguished from a dead inner part. In the latter the tidal currents arrive much reduced, the channels are transformed into closed-system channels where shores prevail over slikkes. When abandoned to natural processes, which tend to obstruct their mouths and to fill their basins with debris, lagoons slowly subject tend to fill up going through the stages of dead lagoon, coastal marsh and swamp. 3.5.1.5 The evolution of coastlines A coastline, whether marine cliff or beach, can show either a positive or negative balance depending on whether the supply, that is the sedimentation phase, is superior or inferior to the drawing, i.e., the erosion and removal phase of the materials making up a coast. Coastline supply can be derived from two different activities: — those resulting from the subaerial modelling processes of the mainland (especially fluvial, colluvial and aeolian); — those due to either the direct or indirect action of the sea (cliff dismantling and retrogradation, erosion in a strict sense, erosion of the continental shelf and biological contributions, such as shells of mussels). Also the drawing of solid matter can be the result of two different activities: — elements taken on load near the coast, transported by currents and abandoned on another coast or towards the open sea; — elements removed by aeolian deflation and dispersed over the open sea or far inland. The negative balances of coastlines, or the erosion of a coast in a broad sense, i.e., when removal actions prevail over supplying ones, have various causes: natural and man-induced. Among the former wave refraction, causing the erosion of promontories, has already been mentioned. Other factors can play an important role, such as: climatic

119 changes which modify the trends and amount of erosion and sedimentation in the mainland and can also change the winds' trend; changes in the path of watercourses near their mouths with consequent modifications, haltings or new supplies of material along coastlines. Finally, eustatic, isostatic and tectonic marine transgressions should be quoted since they lead to noticeable, although extremely slow, invasions on the mainland by the sea. In most cases the latter are relatively slow phenomena, especially compared with human life, and hardly ever can they be adequately contrasted. On the contrary, the case of phenomena caused by man's actions is different: in the last few centuries and with increasing intensity up to the present, many coastline areas of the Earth have suffered deep changes in their natural equilibria, as a consequence of man's various activities. This has led to various degrees of alteration which can be defined in terms of impact (see chapter 1.5). 3.5.1.6 Continental shelf and slope The connection between dry lands and sea floors takes place all over the Earth's surface along a stretch of territory defined as continental margin. Therefore, all continents are bordered by a continental margin which develops from the shoreline to the abyssal plain according to the following morphological units (Fig. 47): a) continental shelf; b) shelf break: c) continental slope: and d) toe of the slope. Whereas the origin and the structural attitude of continental margins reflect the geodynamic features of the continents, the forms that characterise them show constant elements derived from the homogeneity of the geomorphic processes (erosional and sedimentary) which modelled them. For this reason, continental shelves show the following characteristics: mildly sloping erosion or accumulation surfaces up to a depth of 120 to 200 m, shelf margins made up of a deeply convex surface formed by the prograding accumulations of fine sediments; the slope is a high-gradient surface linking the margin with the BEACH

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Low stana sea-level

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c

c

d

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a - submerged beach b - posidonia prainie c - beach rocks d - late quaternary deposits e - olocenic deposits f - slumpings area g - slope deposits area

Fig. 47. Schematic example of a continental shelf profile.

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120 bathyal plain and, finally, the toe of the slope where the materials mobiHsed by gravity forces along the slope surface accumulate. Usually the bathyal plain is found at a depth of 2,000 m and goes as far as 4,000 to 5,000 m.coastal hazard The surface of connection between the continental shelf and the bathyal plain, with a difference in height of a few thousand metres, is essentially affected by gravitational geomorphic dynamics. The terrigenous sediments, which accumulated on top of the slope also in the periods of maximum sea-level lowering in periglacial epochs, are normally found in unstable conditions, giving rise to slumping even over very wide areas. Moreover, landslide events repeated in time in areas characterised by high terrigenous intake from the continent produce turbidity currents along the slope, with high linear erosive energy. This phenomenon gives origin to deep cuts called canyons, and can also be favoured by particular lithological and structural conditions. Therefore, the canyon system of a slope reflects the morphodynamic and sedimentological structural characteristics of the whole continental margin. A landslide originating at a canyon's head on the edge of the shelf produces a turbidity current which can run for several kilometres along the whole cut, as far as the toe of the slope. Eventually the current reaches the bathyal plain where the materials in suspension progressively lose their energy and are deposited, forming large fans. Even without the presence of canyons, an accumulation of fine sediments is nevertheless produced at the base of the scarp making up the toe of the slope. 3.5.2 The hazard"^ *by Paolo ORRU' and Antonio ULZEGA'

3.5.2.1 Coastal hazard The problems related to coastal hazard are of considerable environmental importance because of their implications regarding risk: in fact, human activities now take place along most coastlines, sometimes with continuity. A coastal environment which for many significant reasons can be used as an example is that of the Mediterranean Sea (Fig. 48). This sea is a semiclosed body of water localised at midlatitudes and not subject to extreme meteorological and climatic events. Since the most remote times its coasts have been a location favoured for human settlements. Over the centuries this has led to the creation of harbours, to urban and industrial centres, to hydraulic land reclamations, to the excavation and discharge of materials and so on in places mainly chosen according to their physiographic characteristics. Usually these areas have also coincided with situations subject to rapid geomorphological evolution, such as fluvial estuaries, peninsulae, coastal plains, lagoons, etc. Settlements, linked initially only by sea, subsequently required the progressive construction of communications routes of growing importance along the coasts: first roads and then railways. Along coasts the highest levels of hazard are due mainly to the following conditions:

'A. Ulzega has also written paragraph 3.5.1.6.

121

Fig. 48. Location of quoted examples: 1) Capo Altano, cliff in rotational slide; 2) Baia del Poetto in Cagliari, active cliff and beach erosion processes: 3) Baia di Carbonara, littoral erosion processes; 4) Muravera coastal plain, inundation processes in coastal plain and lagoon; 5) Venetian lagoon and R. Po delta, lagoon earth filling processes; 6) Ria di Olbia. fluvial progradation processes in a harbour area; 7) Gulf of Asinara, submerged paleocliff with rotational slide; 8) Capo Caccia. submerged paleocliff with rock falls; 9) Strait of Messina, unstable submerged paleocliff owing to neotectonics; 10) Gulf of Orosei, active canyon; 11) Gulf of Gioia, canyon's headward retrogradation processes; 12) Gulf of Taranto, scarp deltaic processes; 13) Strait of Corynthus. scarp landslide processes owing to neotectonics; and 14) Channel of Sicily, sediment dynamics due to dragging currents,

1) 2) 3) 4)

active geodynamics owing to neotectonics, volcanism, seismicity and subsidence; geological-structural attitude favourable to instability; high fluvio-deltaic and littoral sedimentation; high meteo-marine energy.

3.5.2.2 Hazard affecting cliffs With respect to marine cliffs, the hazard is linked to retrogradation, whose evolution velocity is function of: — energy of incident wave motion; — geological-structural conditions; — seismo-tectonic activity. In terms of geomorphological risk, the situations of highest hazard are found where mass movements associated with retrogradation affect constructions on the cliff top's

122 surface; as for people's safety, the risk consists in the possibihty of sudden falls from unstable scarps. For example, the case of the Sella del Diavolo promontory, east of the township of Cagliari in Sardinia, is illustrated. It corresponds to a small horst made up of a Miocene sequence of sandstones which give way towards the top to marly-arenaceous Umestones and eventually to Tortonian calcareous reef banks. The coast shows a tectonically-derived cliff which is morphologically active owing to the continuous undermining effect at its toe caused by sea waves. In different epochs, gravitational phenomena of varying intensity have taken place, such as slumps, rock detachments and partial falls of the slopes. In proximity of the most unstable part of this promontory there is a busy tourist harbour and a beach which is very crowded during the summer. This has led to situations of serious risk which lately have caused the loss of human lives. In order to mitigate the undermining process at the base of the sea cHff, the Ministry of Civil Defence of the Italian Government has constructed a vast cliff which has locally reduced the direct risk but, at the same time, has triggered erosional processes in the nearby beach, with noticeable damage to seaside-resort activities. In some places, also nonactive marine cliffs, that are no longer undermined at their toe by sea waves can show conditions of potential instability, given by: — dormant deep-seated gravitational slope deformations; — fault surfaces. The geomorphological risk is often associated with works which owing to load changes or modification of the internal runoff, alter the scarp's conditions of equilibrium. Deep-seated gravitational deformations consequent to Pleistocene variations of the sea level have affected the Capo Altano sea cliff, in south-western Sardinia, as illustrated in the block diagram of Fig. 49. At present, the direct sea action does not affect the sea cliff; nevertheless the recent construction of a major coast road on the slip surface of an ancient landslide has determined its reactivation, thus causing the complete destruction of the work itself. Transport infrastructures and residential and industrial settlements along coasts affected by uplift neotectonic movements, are particularly subject to risk, as shown by the accelerated retrogradation processes along the Italian coasts of Liguria, Tuscany and Calabria. 3.5.2.3 Hazard affecting beaches Along most of the Mediterranean coasts, a general increase of the erosional processes derived from the present rising of the sea level can be recorded. This phenomenon is particularly evident on beaches, where even minimal variations of the sea level can induce large-scale effects. Inevitably, these phenomena are related also to vulnerability as the natural process of erosion affecting a beach can determine risk conditions for the works constructed in the high-beach active zone. Moreover, the concomitance of a natural event with unsuitable use of the beaches can exacerbate and accelerate erosional processes.

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Fig. 49. Capo Altano, south-western Sardinia — block-diagram showing the fossiHsed coastal rotational slide from the "Eolianiti di Funtana Morimenta" (Middle Pleistocene). The recent construction of a road embankment (a) in correspondence with the slip surface reactivated the slide which caused a partial destruction of the works. Lithology: 1) marlstones. sandstones and conglomerates (Oligocene); 2) rhyolitic tuffs (Oligo-Miocene); 3) ignimbritic lavas (Oligo-Miocene); 4) aeolianites (Middle Pleistocene); 5) aeolianites and colluvial deposits (Upper Pleistocene); 6) conglomeratic beach-rocks (Holocene); 7) sandy colluvial deposits (Holocene-Present); 8) sands and gravels from present beaches (modified after Orru & Ulzega, 1984).

Finally, the construction of engineering works on the shore line could itself determine immediate or subsequent impact effects, such as the long-shore dispersion of a beach's sandy materials, and therefore trigger accelerated erosional processes. In any case, even if erosional processes do not coincide directly with the works, the loss or degradation of a beach always means serious economic consequences. As an example, the case of the Poetto beach, in southern Sardinia, is now reported. This beach originated during the Versilian emersion of a 10 km long littoral barrier closing a complex lagoon system of Tyrrhenian-related backshore. The full area is no longer provided with the feeding of solid materials owing to the complete channelisation of watercourses, the presence of quarries in the riverbeds and the construction of reservoirs upstream. Moreover, since the Second World War the beach has been subjected to the removal of several million cubic metres of sand used for the building industry in both its submerged and dry portions. Over the past fifteen years a general acceleration of erosional processes has been recorded; they have produced a 70 m retrogradation of the shoreline and a general lowering of the beach profile. This new arrangement allows the coast roads and seaside resort areas to be reached by the most intense sea storms. With marine meteorological conditions of low energy, sandy residual deposits witnessing the presence of paleobeaches can be preserved, although in a precarious

124 equilibrium. This is the case of Spiaggia del Riso near Villasimius, placed at the south-eastern extremity of Sardinia, where the particle size and chromatic characteristics of its exclusively quartz grains made it famous as a seaside resort. The recent construction of a large tourist harbour nearby has altered the beach's dynamic equilibrium, causing total removal of the sand. The extremely rapid erosional process occurred after sea storms during two winter seasons, with the formation of an erosion shore about 4 m high over the Pleistocene clastic substratum, whose retrogradation is still taking place and is threatening the structures of a local resort (Fig. 50). 3.5.2.4 Hazard affecting lagoons Another problem regarding coastlines of great environmental and economic importance is found in lagoons, marshes, deltas or estuary wetlands. In these areas the development of aqua-culture activities is in contrast with the need to preserve biotopes of international importance. Wetlands in general are subject to filling processes which are taking place more or less rapidly according to the management trends of surrounding territories. In the lagoons of Veneto, in the northern part of the Adriatic Sea, a variation in the hydraulic regime, with prevalent transportation of fine sediments by the affluent rivers, is favouring the development of eutrophic conditions leading to the overproduction of biogenic material. Moreover, a further alteration of the ecological balance of these environments as been caused by the intake of waste substances from high-density urban settlements and farming activities. This new tendency has reduced

Fig. 50. Villasimius coast, "Spiaggia del Riso" after a year from the construction of the new tourist harbour. Legend: 1) granites with basic dykes: 2) quartz sands in a highly oxidised silty matrix (Versilian); 3) present sands; a) emerged abrasion shelf: b) transgressive pebble deposit; c) erosion shoreline subject to accelerated retrogression shows some pedogenetic levels; d) original shoreline.

125 and in some cases ruined the productivity of these lagoons as food resources. The same problems, with varying levels of gravity, are involving the Adriatic lagoon system of the River Po delta which makes up one of the most significant natural environment of the Mediterranean. In the Mediterranean morphoclimatic domain the filling up processes affecting coastal wetlands are often caused by occasional high-water events. In the coastal plain of the Flumendosa River, in south-eastern Sardinia, the inadequacy of the slopes and the man-induced degradation of the vegetal cover owing to fires, sheep grazing, etc., produces great quantities of detrital materials. Dismembered slope materials are therefore loaded up and transported towards the sea in concomitance with catastrophic floods which periodically occur at the end of every dry season. The detrital mass carried by the river passes through the coastal plain, damaging farming activities and transport infrastructures and is finally dispersed into the lagoons. In this way a devastating effect on the biological balance in fish-breeding plants is produced, accompanied sometimes by the total destruction of high value food resources. The lagoon systems are goverened by extremely complex equilibria between physical and biological processes. Sometimes interventions of hydraulic engineering aiming at a rational water exchange with the sea, produce deep changes in the natural equilibria instead with consequent diffusion of salt water into the environment. This is the case of Cabras lagoons, in centre-western Sardinia, whose fish productivity used to be among the highest in Europe but has now fallen to extremely low levels. 3.5.2.5 Hazard affecting continental shelves On a continental 5/z^//hazard conditions are defined as follows: 1) rapid progradation of deltaic apparatuses; 2) high sea-floor dynamics of sediments; 3) active geodynamics. Moreover, on a continental shelf the impact of human settlements is felt to a greater extent, such as the dumping of industrial waste and the input of urban sewage. An example of the former can be found in north-eastern Sardinia, where rias coast morphologies are frequent, i.e., derived from fluvial valleys submerged by the sea. In this area the Olbia ria is located which is occupied in its innermost part, 4 km from its outlet, by an important tourist harbour and industrial port as well as by the city of Olbia itself. Access is made through a narrow, 100 m wide and 10 m deep, navigable channel. The ria itself was utilised as an anchoring and trade landing place since the Punic period (sixth century B.C.), whereas in Roman times it was the most important port in Sardinia. At present the ria is affected by accumulation processes of fluvial sediments resulting from the increase of solid load in the Rio Padrongianu, whose delta is placed at the southernmost tip of the inlet. While a small portion of sediments is taken in load by littoral drift and transported towards the outer mouth, the high content of organic particles from the sewage waters of the city of Olbia and concentrated in the marine environment amalgamates with most of the fluvial sediments, thus hindering their dispersion. Therefore the filling processes of the ria

126 act directly on the access channel to the harbour, causing a serious risk for maritime traffic and compelling port authorities to undertake continuous, extremely expensive dredging and reclamation operations (Fig. 51). In the Mediterranean marine prairies made up of Phanerophytes {Posidonia oceanica) are the main element controlling the trophic state of the sea environment and a fundamental regulation element of the geomorphic processes taking place in the upper continental shelf and submerged beaches. The extension of marine prairies depends on the general, climatic-morphological conditions of the environments in which they are found and reacts prompdy to global changes. It should be emphasized that at present the prairies investigated show critical stability conditions and even small actions from the outside can produce rapid retrogradation processes in its lower and upper limits or widespread degradation. The external man-induced actions are brought about by mechanical demolition in the case to create anchoring points and by the practice of trawl-net fishing (Fig. 52). Moreover, urban and port settlements along the coasts give rise to covering processes produced by the sedimentation of suspension particles, whereas chemical degradation is caused by industrial and agricultural products spilled over the territory. The maintenance of optimal quality for the marine environment is the best way of preserving very important resources for

Fig. 51. Aerial photograph of the southern shore of Ria di Olbia (north-eastern Sardinia). 1) Progradating delta of Rio Padrongianu; 2) dredged canal for access to Olbia's commercial and industrial harbours.

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Fig. 52. Side scan sonar image of the continental shelf of the Gulf of Orosei (centre-eastern Sardinia) showing the damage caused by trawl-net fishing on the Posidonia oceanka marine prairies. Legend: 1) canal filled by sand ripples; 2) furrows due to mechanical erosion; 3) sea-floor profile (modified after Orrii & Ulzega, 1987).

Mediterranean countries, regarding both tourism and fishing activities. Among the risk situations connected to the degradation of sea prairies there is the reduction of fishing stocks owing to the lack of adequate nurseries, as well as the increase in the extension of submerged beaches, with consequent degradation of the dry beaches used for seaside resort activities. At varying depth on Mediterranean continental shelves, rock scarps, which evolved during the Quaternary glacio-eustatic regressive and transgressive phases into sea cliffs, are often preserved. At the moment of their morphogenesis, these cliffs were subject to coast dynamics which produced rock falls, rotational slides, deepseated gravitational slope deformations with basal accumulations of both landslide masses and debris fans. Their submersion with respect to the present sea level stopped the undermining processes at the foot of the cliffs but did not nullify their instability. For example, in the gulf of Asinara in Sardinia at a depth of about 70 m there is a 20 m high paleocliff, which developed in Miocene calcarenites with monoclinal attitude. Its front is affected by a succession of rotational slides alternating with debris fans (Fig. 53). Another example is found on the shelf in north-western Sardinia, off the coast of Alghero, where at a depth of 80 to 120 m a paleoshore of structural origin, with alternating high cliffs and inlets, is present. This landform is made up of Mesozoic carbonatic rocks and on the walls of the cliffs shear surfaces, open fractures and heaps of unstable blocks are preserved. These structures are all related to large rock falls (Fig. 54).

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Fig. 53. Block-diagram of the submerged cliffs of Gulf of Asinara (northern Sardinia). 1) Miocene calcarenites; 2) coarse clastic sediments and blocks: 3) sandy and silty sediments; F) presumed fault; a) sharp rock edge and detachment crown; b) rotational slide rock body; c) accumulations at the toe of the rock edges; d) rock edges buried by fme sediments; e) layer's heads bounded by sharp edges; f) cuts buried by fine sediments (after Ulzega et al., 1986).

In these situations of potential risk in conditions of tectonic stability, engineering projects, such as the construction of platforms for the exploitation of hydrocarbons or the laying of submarine pipelines, should start only after carrying out a careful and precise check of the geomorphological conditions of the sea floor. A different problem is given by paleocliffs in tectonically active areas, as emphasized by the example of the straits of Messina. The sandstones and conglomerates of the Messina Formation (Middle-Upper Pleistocene) are intersected by marine abrasion shelf at present dismembered into several portions found at varying depths, up to 90 m. The neotectonic movements and the seismogenetic activity keep the rotational sliding surfaces of the continental shelf edge active. Moreover, the cover of the superficial sediments is affected by gravitational sliding producing detachment niches at the top and landslide accumulations at the toe. The channel between Sicily and the Italian peninsula is affected by the presence of various works such as gas and oil pipelines, electric and telephone cables (Fig. 55). These engineering works are subject to continuous monitoring because of the high geomorphological and sedimentological dynamics of the straits of Messina, whereas the planning and constructing of major communication infrastructures (road and railway bridges or tunnels) is made extremely complex by the same problems. 3.5.2.6 Hazard affecting continental slopes The passage from continental shelves to deep sea floors usually takes place along an

129

Fig. 54. Block-diagram of the buried cliffs of the Alghero coast (north-western Sardinia). 1) Mesozoic limestones and dolostones; 2) bioclastic sands and gravels: 3) silty sands; a) 75—80 m deep abrasion surface; b) step-like edges and outmost boundary of buried abrasion shelves; c) closed depression filled by fme sediments; d) fractures; e) mega-ripples fields; f) presumed faults; g) 20-30 m high subvertical edge; h) canyon; i) riverbeds; 1) fan-deltas; m) buried abrasion shelf (after Ulzega & Orrij, 1990).

inclined connection surface on which important geomorphological processes occur. On the continental shelf break and slopes the main cause of hazard is given by gravitational instability, related to progradation of sediments on the break, seismic and volcanic activity and the dynamic evolution of canyons. Sediment mobilisation due to gravitational energy, takes the form essentially of slumpings, turbidity currents and debris flow. The transportation of sediments from the slope to the bathyal plains assumes extremely high hazard values especially in correspondence with tectonically active margins. An example of canyon dynamics in a situation of tectonic stability is offered by the continental shelf of western Sardinia, deeply cut by the heads of several canyons. Among them, the Gonone canyon shows retrogradation due to regressive erosion which leads the head itself to move upstream up to a depth of 50 m, at a distance of 200 m from the high sea cliffs of the coast (Fig. 56). The shallow depth of the

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Fig. 55. Block-diagram of the Strait of Messina, Sicilian side near Mortelle. 1) Poorly cemented sandstones and conglomerates of the Messina Formation (Middle-Upper Pleistocene); 2) cross-bedding littoral sands (Holocene); 3) shelf's silty sands and sandy silts (Holocene-Present); a) marine abrasion surfaces disjointed by neotectonic activity: b) gravitational slide surfaces; c) detachment crowns of superficial sediments in boundary zone; d) basal landslide heaps; e) methane pipelines (after Orru et al., 1994).

canyon head has allowed its dynamics to be studied and the evolutional process in course to be defined by means of scuba diving investigations; among these phenomena the linear movement of the shelf sediments, the block slides of present biogenic formations and Pleistocene beach-rocks and the formation of wide slumping surfaces are studied. The construction of a submarine pipe for the dumping of urban sewage is now being planned; its outlet will be placed in correspondence with the canyon head. If on the one hand the active processes of sedimentary dynamics will favour the disposal of sewage, on the other hand active gravitational sliding is a risk for the work itself and special foundations and continuous maintenance interventions are necessary. Another example referring on the other hand to a tectonically active area is offered by the Tyrrhenian coast of Calabria, where the continental shelf is of such limited extent that in some cases the canyon heads reach the proximity of the shoreline. A series of step-like normal faults affect the whole continental margin favouring the formation of sliding surfaces. In these morpho-structural conditions a great industrial harbour is being constructed whose entrance is placed exactly between the two heads of the Gioia Tauro Canyon. At the moment of its construction, the materials resulting from the digging of the port basin, about 5.5 million cubic metres, were thrown into the sea directly on the canyon's margin. On June 12, 1977 at

131

Fig. 56. Block-diagram of the Gonone canyon (centre-eastern Sardinia). 1) Mesozoic limestones and dolostones; 2) stratified sediments (Miocene-Pliocene:'); 3) basalts (Quaternary); 4) deltaic and lagoon silts with peat (Upper Pleistocene); 5) paleoriver deposits; 6) sandstones and beach conglomerates (Upper Pleistocene); 7) organogenous silty sands (Holocene); 8) littoral sands (Holocene); a) canyon linear cuts; b) heads subject to erosion with cropping out of bedrock; c) edge progradation; d) slumpings; e) basaltic plateau with emission centre; f) paleodelta; g) beach-rocks; h) paleolagoons; i) paleocliffs with structural control; 1) lithothamnos plain; m) Posidonia oceanica sea-prairie; n) active sea cliff with Pleistocene and Holocene caves and breaker furrows; o) delta; p) lagoon; u) submarine pipeline. Detail of the canyon's head; 1) basement; 2) red algae bioconstructions; 3) ripple-mark sands; 4) silts; a) sea rocks; b) biogenic reef subject to landsliding; c) gravitational sliding furrows of organogenous coarse sediments; d) gravitational sliding of fine sediments, slumpings (after Orrij & Ulzega. 1987).

7:20 h, a submarine landslide was triggered in the dumping area whose current of turbidity, travelling at a speed of 15 17 km/h. caused at 8:12 h the rupture of a cable at a depth of about 600 m. At the same time, on the surface a lowering of the sea level by about 2 m first occurred, soon followed by a 5 m high wave which hit the harbour structures and the coast thus causing the demolition of one of the quays and other widespread serious damage (Fig. 57). The subsequent study of the Gioia Tauro canyon, carried out by scuba divers and submarine explorations along its slopes and axis up to a depth of 1,000 m, clearly showed that the event could be foreseen, due to a situation of great geomorphological risk. A similar event took place on October 16, 1979 in Nice where the waste materials derived from the extension works of the

132

Fig. 57. Gioia Tauro canyon (southern Calabria), a) Schematic topographic map of the canyon showing its two heads near the piers off the Gioia Tauro industrial harbour; b) longitudinal profile of the canyon showing the flow time of the turbidity current generated by the landslide (after Colantoni et al., 1992).

airport were dumped into the sea at the head of the Var-canyon. Among marginal areas, in the continental slope geomorphological dynamics is noticeably reduced owing to the limited availability of sediments and the decrease of slope gradients and, therefore, of gravitational energy. In any case, at the depths pertaining to these environments, 200—2,000 m, works and activities of economic interest are rare. Fig. 58. Block-diagram of the western margin of the Gulf of Taranto (north-eastern Calabria). 1) Submarine fan of the R. Trionto with linear flow channels along the whole scarp: 2) submarine fan of the R. Crati with top flow channels and basal mass transportation lobes; 3) scarp furrows showing retrogressive heads cutting through the submerged beach.

133

134 Nevertheless, situations of hazard can be determined in particular conditions, for example in tectonically active areas with margins in compression, where a very narrow margin with nearly nonexistent shelf corresponds to the rapid uplift of the dry lands. A significant example of geomorphological evolution of this type is observed along the southern margin of the Gulf of Taranto, in the Ionian Sea: where neotectonics has produced a great increase of energy in the slopes of the emerged reliefs, these give way without interruptions to the continental slope, with an anomalous accumulation of materials in correspondence with the coast, which give origin to complex sedimentary bodies in conditions of instability along the slope. Migration of the sediments towards the deep sea floor occur by means of linear movements in cone-shaped structures, the formation of lobes in prodeltaic environment, fan-delta or directly on the upper slope, where the narrowness of the shelf hinders littoral drift. In these conditions, along the whole margin in correspondence with the heads of the inflow channels, retrogradation niches are formed which directly affect the submerged beach: this creates situations of risk, first of all for maritime structures but it can also cause sudden withdrawal of the shoreline, with hazard for the roads running along the coast and for seaside resorts (Fig. 58). Another example of continental margin characterised by an important geodynamic activity is offered by the Hellenian arc. The activity in this case consists of high

Fig. 59. Channel of Sicily, about 20 nautic miles north of Scherki shoal. Side scan sonar image showing the sea floor at a depth of-400 m; (a) sand ripples; pipeline subjected to both burying and undermining processes: the darkest echoes (b) represent rock block heaps (c) lain down in order to stabilise the pipeline.

135 seismicity, with shelf uplift and slope instability. Moreover, active faults affect poorly cohesive evaporitic rocks, situated in the bathyal plain and at the base of the continental slopes. Owing to these contributory causes successive landslides triggered along the Corinth slope caused the repeated rupture of underwater electric and telephone cables. In morphological high places channelised in correspondence with straits, strong currents are usually created, due to the compression of the flux lines of large water masses; this phenomenon takes place with a preferential direction and triggers tractive currents flowing with a velocity of some metres per second on the sea floor. The displacement of sediments gives rise to parabolic dunes with high translation velocity. In the western Mediterranean, this phenomenon was observed in the Straits of Messina and in the Channel of Sicily. The latter is a body of water stretching from Cape Bon in Tunisia to Cape Feto in southern Sicily. The morphology of the channel includes Adventure Shoal to the north and Scherki Shoal to the south, in turn separated by an axial zone structured by a horst-graben system forming ridges and parallel channels with a NW-SE direction. On the bottom of the channels intense dynamic processes affect the coarser sediments which migrate towards the Pantelleria Basin to the south in the shape of parabolic dunes up to 20 m long and 3 m high. Along this path, three parallel methane pipelines which are the main energy link between North Africa and Europe were constructed. The movement of parabolic dunes can cause the undermining at the base of the pipelines which, in order to protect them, are weighed down with a cover of rock blocks and by regular periodical inspections (Fig. 59).

3.6 Glacial and periglacial hazard* *by Alberto CARTON

3.6.1 Introduction The situations of environmental risk in high mountain areas are mainly linked to the presence of ice and snow masses. Moreover, there are other phenomena that progressively lead towards gravitational movements in a strict sense; they are mainly due to the action of ice, snow and melt water. Since the areas where these phenomena take place are often remote and with a very scarce human presence, the resulting risk is often underestimated. In fact, the consequences often affect territories located even at noticeable distances from the place where a certain phenomenon has occurred. In the particular case of an alpine environment, the effects can be felt as far as the valley floor. As a consequence, in a period when mountain exploitation for sport and tourism purposes is very intense both in summer and winter, the hazard no longer concerns the single user but rather permanent anthropic structures or small and big sites alike, which are often very busy with people, although only seasonally. The hazard rate of certain phenomena is in certain cases very high owing to their

136 suddenness which is accompanied by a poorly defined or totally lacking series of premonitory signs linked to several variables and not always completely known or adequately taken into account. The prevention of these types of processes is moreover under-evaluated since within the field of natural hazard phenomena those regarding the presence of snow, and even more of ice masses, certainly do not represent a high percentage. Finally, in the present phase of retreat or immobility of the glacial fronts, also ice dynamics seems to be underestimated or neglected by most people, apart from specialists and some other frequent visitors of the mountains. On the contrary, one should consider that climatic changes occurring even in a short time (e.g., little Ice Age) can produce deep variations in the ice mass which in turn could affect recently developed areas or predispose future hazard situations. Hereafter, the hazard phenomena (Fig. 60) that can be found in a mountain environment are listed: — ice rock avalanches; — supraglacial debris fall/slide outside the lateral moraine; — ice fall from snout of glaciers (ice avalanches); — rapid advance of snout of glaciers (surges); — emptying of internal water-pocket; — emptying of proglacial lake; — emptying of ice-dammed lake; — debris flows caused by bursts of dammed ice;

Fig. 60. Typical example of periglacial hazard: in the foreground the destructive and accumulation effects of an avalanche; in the background an avalanche cone (photo M. Panizza, Caucasus, June 1974).

137 — — — — — — — — — —

debris flows caused by bursts of ice-marginal lakes; debris flows resulting from bursts of moraine dammed lakes; debris flows resulting from bursts of subglacial and englacial water pockets; high water events connected with rapid glacier ice melting; lahars; hazards caused by mountain climbing activities; mass falls due to glaciopressure; avalanches; advance of rock glaciers; debris flows caused by short and intense precipitations on scattered glacial deposits or by talus deposits. Before considering the series of phenomena that take place in the mountain environment, the formation of debris flows should be emphasized in particular, since they are a recurrent phenomenon among those connected with glacial dynamics. Their formation is mainly determined by sudden releases of large amounts of melt water as well as by the abundance of unconsoHdated sediments which are usually found near the unstable glacier margins in the form of thick moraine ridges or as scattered glacial deposits or, more generally, as ice-contact debris. Moreover, the constant retreat of the glacial fronts, starting from the middle of last century (which coincided with the maximum Holocenic expansion), has uncovered very vast surfaces, at present covered by large amounts of detritus not yet fixed by the vegetation and often lying on polished rocks. On the same surfaces a hydrographical network deriving from a glacier is found, characterised by extremely variable and unforeseeable flow-rates and capable of loading remarkable quantities of loose glacigenic sediments. As regards the waters more specifically, it should be remembered that the particular land surface arrangement (high relief energy, deep valleys, etc.) emphasizes, from the hydraulic standpoint, the effects of surface running waters, conferring them with high energy: their flow is often confined in narrow, steep and uneven spaces (gorges, troughs, canyons). In cases when glaciers release sudden amounts of water, some torrents are compelled to run in valleys which are inadequate with their new occasional flow; as a consequence, an anomalous raising of their level takes place with the depositing of various types of debris located on slope portions usually untouched by the flow of watercourses. 3.6.2 Ice-rock

avalanches

In some morphological and structural situations, ice and rock avalanches involving large volumes of material can occur. This phenomenon affects at first only portions of rocks that by sliding drag along also ice masses in their movement (Fig. 61). The so-formed melange is extremely mobile and can run very long stretches. The presence of ice, both in the form of blocks and/or melt water, modifies the mechanical behaviour of the landslide body by reducing the shear strength angle, making it very fluid and providing a "lubricated" sliding surface. More seldom the phenomenon can

138

Fig. 61. Schematic example of supraglacial rock fall outside the lateral moraine.

take place when a suspended glacier lies on fractured, easily degraded rock as a result of the infiltration of large amounts of water repeatedly turning into the solid state. Even seismic events can trigger ice-rock avalanches. The consequences of such a phenomenon can be both of direct and indirect type. An ice-rock avalanche can directly affect works connected with man's activities or create barriers to the hydrographical network with the consequent formation of temporary impoundment areas. In the latter case the heap's morphological features and dimensions can change with time, following the melting of the entrapped ice: variations will thus be a function of the amount of ice involved in the fall. The development times of the phenomenon are extremely short and prediction is almost impossible. Only in some cases can the starting of isolated rock falls foreshadow the event. Usually the rock portions affected by this kind of disarrangement are located in inaccessible places or at least in areas which are not normally frequented and are therefore not normally kept under direct observation. When a slope's proneness to movement is detected, it will be possible to set up a traditional monitoring system on the unstable rock mass if the consequences are assessed as negative for human activities. Ice-rock avalanches of large proportions are reported in the specific bibliography

139 regarding Peru: among catastrophes of this kind perhaps they were the most disastrous. From the top of Mount Huascaran in 1962 a portion of the ice cap detached (3.5 million cubic metres). A second avalanche of about 300,000 m \ detached from just below, followed the previous one. The great volume of ice dragged along a large quantity of rock and subsequently hit and removed a portion of a glacier. Eight villages were submerged and destroyed. Following an earthquake in 1970 an ice-rock avalanche was formed from the same Mount Huascaran; the towns of Ranrahirca and Jungay were devastated. Also in the Caucasus several cases of ice-rock avalanches were recorded. At the toe of Kazbek, from the Defdoraki glacier, rocks, mud and ice fell into the underlying valley in 1766, 1785, 1808, 1817, 1832. In the central Caucasus, the valley of Guenal Dom was devastated in July 1902 by two ice-rock avalanches coming from a glacial cirque bordered by the Giumaraykhkn, Kazbek, Marly Knokh and Fombal Fziti peaks. In the Alps, on the Simplon Pass, in 1901 on the flank of the Fletschorn a large volume of stones was detached and fell on the Rossbode glacier thus forming a gigantic avalanche rated at 2^3 million cubic metres. In literature other cases of ice-rock avalanches are described on the Italian side of Mont Blanc in Val d'Aoste (Val Ferret and Val Veny) in 1717, 1728 and 1920. In Switzerland in the Valais in 1714 and 1749 a vast amount of rock below the Glacier de Diablerets slipped perhaps owing to the glacier's melt water percolating within the rock fissures. 3.6.3 Supraglacial debris fall/slide outside the lateral moraine This phenomenon can take place during a glacier's accretional stage with a prevalently vertical development. At times the ice thickness increase can reach or overcome the height of the lateral/frontal moraine. The event takes place preferentially in the glaciers whose top is covered with abundant supraglacial debris (blackglacier) or when large boulders which have fallen on the glaciers are conveyed towards zones more favourable topographically to this type of phenomenon. The thickness increase of a glacier tongue is not necessarily determined by an expansion phase, especially when glaciers with abundant floating moraine are considered. In the latter, in fact, ablation is strongly inhibited by the debris cover and in difficult situations of frontal and superficial ''disposal" the mass can react by swelling up. Once it has reached the top of the moraine, the coarser detritus can slide outwardly as far as the scarp's toe (Fig. 62). The effects produced by this kind of event are normally of small entity and fall in the category of risk case histories only if they directly involve anthropic works (usually roads and paths). The phenomenon occurs suddenly when it actually takes place but its preparation develops slowly and is easily controlled. Usually only modest amounts of debris or more often isolated boulders are involved. Premonitory signs are given by a series of small slides along the morainic bank's outer scarp or by the proximity of large boulders to the glacier margins. When the debris' coarser fraction shows instability,

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Fig. 76. Legend for morpho-neotectonic features (after Panizza, Castaldini et al., 1985). 1) Linear ridge; 2) planar discontinuity of ridge; 3) altimetric discontinuity of ridge; 4) planar-altimetric discontinuity of ridge; 5) alignment of peaks; 6) non-coincidence between alignment of peaks and watershed; 7) mountain pass; 8) scarp; 9) reverse slope; 10) landslide; 11) area with particular recurrent and/or aligned forms of erosion; 12) karst and pseudo-doline forms; 13) valley asymmetry; 14) rectilinear valley; 15) gully; 16) (a) river bend and (b) double river bend; 17) barbed confluence; 18) step or anomaly in the longitudinal profile of a watercourse; 19) (a) blind valley and (b) dry valley; 20) drainage pattern asymmetry; 21) centripetal drainage pattern; 22) radial drainage pattern; 23) river capture; 24) rectilinear drainage; 25) rectilinear paleodrainage; 26) terrace edge; 27) converging and diverging terraces; 28) aggradation; 29)straight coast; 30) (a) triangular facet and (b) trapezoidal facet; 31) isolated relief; 32) line of undefined nature; 33) distinct tectonic line; 34) circular structure; 35) paleosurface; 36) anomaly on paleosurface; 37) fissure; 38) sand and/or mud volcano.

Stated on the data sheet. All analyses performed in the field or on laboratory samples should also be specified. It is extremely important to carry out accurate investigations before associating tectonic causes to displacements or foldings, even if they are evident and affect Quaternary deposits. For example, neotectonic studies near Lake Garda in northern Italy (Castaldini & Panizza, 1988) have permitted to distinguish landforms linked to tectonic causes from gravitational processes or collapses derived from melting of dead-ice blocks or "glacio-tectonic" phenomena, etc.

171 On the basis of the above specified morpho-neotectonic studies, a subdivision of the lineaments according to the following categories is established (Panizza, Castaldini et al., 1987). 1) Active tectonic element: recorded displacement and/or deformation of rocks and/or significant forms. 2) Tectonic element supposed to be active: so defined on the basis of numerous qualified and congruent geomorphological indications, or of other sources. There are no visible rock displacements and/or deformations nor significant landforms. The presence of an inactive fault or tectonic deformation is in any case ascertained from geological evidence. 3) Tectonic element supposed to be inactive: so defined on the basis of scanty, non qualified and noncongruent geomorphological indications, or of other sources. There are no visible rock displacements and/or deformations nor significant landforms. The presence of an inactive fault or tectonic deformation is in any case ascertained from geological evidence. 4) Inactive tectonic element: displacements and/or rock deformations are not ascertained. The presence of an inactive fault or tectonic deformation is in any case confirmed. 5) Qualified lineament: with numerous, qualified and congruent geomorphological indications or of other kind, but showing no outcrops capable of confirming displacement and/or deformation. 6) Unqualified lineament: with unquahfied or noncongruent geomorphological indications and showing no outcrops capable of confirming displacement and/or deformation. 7) Lineament not corresponding to tectonic elements: so defined when showing numerous and/or qualified and/or congruent geomorphological indications, but without any ascertained displacement and/or rock deformation. It should be noted that the term "significant" is used for rocks and forms to indicate that their age lies within the neotectonic interval considered. The elements derived from morpho-neotectonic studies are listed in a specific thematic document called morpho-neotectonic map (Fig. 77). These morpho-neotectonic studies contribute to the definition of a neotectonic synthesis, together with investigations in other fields of Earth Sciences: for example, the results of seismo-tectonic investigations, or those concerning the main fracture directions of rocks. The synthesis document is a neotectonic map, like the one shown in Fig. 78. Also in this sector of Geomorphology, as for the treatment of all geomorphological phenomena, the temptation to indulge in schematizations must be resisted as these could lead to inappropriate deductions and interpretation errors due to the geomorphological convergence of landforms which are outwardly similar but in fact are genetically different. Remaining in the field of morphoneotectonics, the significant example could be quoted of asymmetrical valleys which show tilting phenomena in one direction or the other, according to the erodibility of the rocks, the rate of tectonic movements and the intensity of fluvial erosion processes. Another example

172

2) 0

a, b, c

Fig. 77. Examples of morpho-neotectonic map. Legend: 1) classified lineaments; 2) field check-points; see Fig. 76 for morpho-neotectonic symbols.

is offered by a series of fluvial terraces distributed on various levels: the latter can show several orders corresponding to different stages of evolution or to the same order involved in various tectonic displacements. 3.7.3 Geomorphology and seismic susceptibility As mentioned above, there are particular geomorphological situations that can condition seismic susceptibility. In other words, these situations can offer local responses to seismic acceleration, causing mitigation or amplification of earthquake intensity. In order to calculate seismic acceleration, the following variables should be known: — magnitude, frequency of occurrence and distance of seismic events; — material sequences for each point of the terrain, as well as geotechnical data such as density and seismic wave velocity;

173

ACTIVE

1) 0 0 0 3)-

HELD TO BE ACTIVE 4)

•

-

5)

—

6)

+

7)

> ^

Fig. 78. Examples of neotectonic map. Legend: 1) anticline; 2) syncline; 3) fault; 4) qualified lineament; 5) area undergoing subsidence; 6) area undergoing uplift; 7) tilting. Compare with Fig. 77.

— groundwater levels; — topographic conditions. There is specific literature on the various geomorphological, geological, geotechnical, hydrogeological situations, among others, that can influence seismic effects in a negative or positive manner. Only specifically geomorphological parameters will be discussed in the following. Those of a geological type (lithology, contacts between different rocks or deposits, presence of fractures, etc.), geotechnical type (particle size distribution of the deposits, degree of consolidation, mechanical characteristics, etc.), hydrogeological type (presence and depth of the water table) or others will not be discussed here and specific works should be consulted on these topics. It should be mentioned that research in this sector of applied sciences is still at an initial stage, in which the results have not yet permitted the indication of precise methods of evaluation, model procedures or seismic effect calculations, unless the circumstances are relatively uncomplicated. In most cases, qualitative assessments may be formulated, based mainly on the consequences of recent earthquakes and the interaction of their effects.

174 The most important geomorphological situations that can condition seismic susceptibihty, are the following: ~ Slope angle. ~ Debris. ~ Morphology. ^ Degradational slopes. — Paleolandslides. — Underground cavities. 3.7.3.1 Slope angle It is well known that, in a static situation, with other variables being equal (lithology, moisture, etc.), slope instability increases with the slope angle. In other words, slope stability is inversely proportional to slope angle. In the dynamic situation of an earthquake, the instability of a slope and consequently that of any buildings on it will be greater than in a static situation. For the towns located in the Campania and Basilicata regions affected by the earthquake of the 23 November 1980, the Italian National Research Council (CNR) Finalised Project "Geodynamics" (1983) adopted a correction coefficient (Sp) for the buildings and structures located in seismic areas, which depends on the slope angle, as follows: 5/7=1 + 1.5/ where / is the mean slope angle expressed as a percentage. The ^/^-coefficient, which expresses the increase in seismic activity, is multiplied by the seismic coefficient C, which defines the force employed for the seismic testing of constructions (C = 0.1 for the earthquake regions in the first category; C - 0.07 for those in the second category). In any case, the CNR Finalised Project '^Geodynamics" (1983) advises against construction under the following conditions in the case of slopes with an mean slope gradient /° greater than the established limit. This limit depends upon the type of rock or deposit, as shown in Table 13. 3.7.3.2 Debris The slope deposits made up of cohesionless materials, as in the case of talus, morain, TABLE 13 Slope angle limits for several types of terrains, according to stability during seismic shocks /° limit Incoherent debris (including cataclastic debris), with water table depth >10 m Incoherent debris (including cataclastic debris), with water table depth 10 m Clays of compact texture, but very "tectonized" and/or fissured

20° 10° 30° 25° 10°

175 TABLE 14 Increases in seismic intensity for some types of debris, compared to granite Rock types

Seismic intensity

Granite Pebbles and gravel Sands Clays

0 1—1.6 1.2-1.8 1.2-2.1

alluvial deposits, etc., generally give rise to geomorphological situations that can amplify seismic susceptibility. More specifically, increases in seismic intensity can be linked to three factors: a) surface slope angle of the debris; b) its particle size characteristics; c) thickness of the deposit. The debris slope angle has already been considered in the more general case of the last section and the reader is referred to it for details. Particle size distribution can significantly enhance seismic intensity. It is outlined in Table 14, modified from Medvedev (1965) with some simplifications. Some cohesionless rock types are compared with compact granite, under conditions where there is no water infiltration. This infiltration can further increase seismic intensity up to values of 3—4, approximately. The thickness of the deposit also plays a part in amplifying seismic intensity. In the case of deposits thicker than 5~10 m, Siro (1985) indicates critical thicknesses on the basis of the most probable frequencies of the arriving vibrations and the average propagation velocities of the transverse waves, which determine the amplifications of seismic intensity. For velocities of transverse waves from 200 to 600 m/s, the critical thicknesses resulting are shown in Table 15, along with the proposed corresponding coefficients of amplification ranging from 1.5 to 2.5. 3.7.3.3 Morphology An amplification of seismic intensity resulting in an increase in damage to buildings has often been observed in correspondence with particular morphological situations

TABLE 15 Critical thickness of debris, compared with the velocity of transverse seismic waves, with ampHfication coefficients ranging from 1.5 to 2.5 Average velocities (m/s)

Critical thickness (depth in m)

200 300 400 500 600

8-40 11-60 15-80 19-100 23-120

176 such as crest lines, terrace borders, scarp edges, or abrupt variations in slope gradient. These amplifications appear to be due to phenomena involving the concentration of seismic rays that are reflected as a result of their different inclination angles, with respect to the vertical, corresponding to sudden topographical changes. Studies aimed at investigating this topic (Castellani et al., 1982) indicate amplifications as great as 3.5. Similar amplification phenomena have been observed in correspondence with buried morphological structures, such as paleoriverbeds or fossil terraces or forms of erosion masked by superficial deposits. There are also cases involving seismic ray concentration phenomena, due, however, to refraction phenomena where the angles of incidence are shown to differ from the vertical. In the intervention projects planned for the towns of the Campania and Basilicata regions (southern Italy) affected by the earthquake of November 1980 (CNR Finalised Project "Geodynamics", 1983), 10-20m reserve zones were advised in correspondence with rocky crests, isolated summits, terrace edges and border areas of steep scarps. 3.7.3.4 Degradational slopes The amount of weathering on slopes and therefore the degree and type of rock degradation taking place can affect seismic susceptibility. For example, in the Friuli earthquake of 1976 (northern Italy), there were numerous debris falls from the slopes of the Tagliamento river valley and those of its tributaries. The debris ranged from small or isolated rock fragments to large boulders and rock falls of vast proportions (Fig. 79). The survey and mapping of these mass movements by Govi and Sorzana (1977) revealed that most of the rock

Fig. 79. Rock fall caused by earthquake in 1976, Braulins (Friuli, Italy) (Photo M. Panizza, May 1976).

177 falls occurred prevalently in places where there had previously been other gravitational events. Geomorphological surveys have revealed the different sources of the debris accumulations and their different ages of formation. Observations on the area led to the indication that the phenomena, occurred prevalently in calcareous rocks showing intense tectonic fracturing, which are subject to strong frost weathering. This type of disturbance was much less frequent in the case of the earthquake of southern Italy in 1980, even though there are slopes in the area made up of calcareous rocks, some of which have been intensely fractured by tectonic activity. Frosty weathering is lacking in this area or, in any case, it is much more limited compared with the Alps. Frost action exerts a slow and progressive degradation of the rocks. In the case of seismic shock, the fragments detached are not only unbalanced debris, which would have fallen in any case on account of frost action and gravitational processes, but also include material that is still partially attached to the rock or on slopes lower than those with detached fragments. It is as if the seismic shocks complete a sort of "cleaning operation" on the slopes, eliminating the weathered material. Studies aimed at the assessment and identification of areas subject to this type of seismic susceptibility must consider not only the sector beneath slopes or scarps undergoing degradation processes, which can be affected by rock falls or other landslides, on the basis of gravitational mechanisms alone. They must also consider those areas that can be reached by boulders thrust by strong seismic shocks beyond the gravitational fall limit or set into a rolling motion and movement because of the so-called "vibrating table" effect. An investigation on the limits of maximum advance of fallen boulders following the 1976 Friuli earthquake, was carried out by Onofri and Candian (1979). By means

Fig. 80. Slope parameters considered by Onofri and Candian (1979) for an investigation on the limits of maximum advance of fallen boulders following the 1976 earthquake in Friuli (Italy).

178 of a statistical analysis on 98 cases of rock fall, the authors have set up two diagrams for the definition of the boulders' maximum advance limits for each single slope angle (Fig. 80). In the first diagram (Fig. 81) the parameters taken into account are: H (altitude of fall or vertical height between the detachment point and the arrest one) and D (projection on the horizontal plane of the distance between A and B). These parameters totally prescind from the morphological pattern of the slope on yi = D

m 1400

1300

1200

1100

1000 H

900 H

800

700 H

600

500^

400 limit of predictability 95%

300

limit of predictability 90% limit of predictability 80%

200 H

100

#^ Xi = H 100

200

300

400

500

600

700

800

900 m

Fig. 81. Diagram of the limits of maximum advance of fallen boulders following the 1976 earthquake in Friuli (Italy) prescinding from the morphological pattern of the slope (after Onofri and Candian, 1979).

179 which the boulders fall. In the second diagram (Fig. 82), also the influence of the slope morphology on the path followed by the boulders is considered. The parameters taken into account are: the ratio between the area Ap subtended by the slope profile, with respect to the horizontal plane, and the square number of the fall height (H'); and the 0^ angle (inclination of the energy line). For each of the two diagrams the limits of confidence of 80, 90 and 95%, respectively, have been calculated. 3.7.3.5 Paleolandslides Earthquakes sometimes set into motion older landslides (see 3.7.4). The villages of Calitri, Bisaccia, Senerchia and other localities in southern Italy affected by the November 1980 earthquake, are emblematic examples of this. The landslides occurred in mainly clayey rocks, in flysch facies, sometimes chaotically arranged or highly tectonised. They affect stretches of land several kilometres long with a transport involving hundreds of millions of cubic metres of material. A slide affecting the village of Calitri in the province of Avellino started to move a few days after the seismic event, involving about 23 million cubic metres of material. Its sliding surface was calculated at a depth of over 100 m. The movement corresponds to a partial reactivation of a pre-existing rotational slide (Hutchinson & Del Prete, 1985) evolving downslope into earth flows that reached the Ofanto river, modifying its course. Cotecchia (1986) maintained that pore-water pressure, which increases noticeably in concomitance with seismic vibrations in clayey and flyschoid rocks in tectonic contact with water-saturated calcareous strata, must have played a very significant role in triggering into motion the paleolandslides of this region. It has been demonstrated (Agnesi et al., 1983) that earthquakes lead to an increase in the overall size of landslide bodies, yet without significant modifications of their shape and typology. 3.7.3.6 Underground cavities Earthquakes in calcareous areas with underground karst processes can cause the fall or collapse of cave roofs or the removal of debris from rock cavities. The consequences on the surface are depressions or subsidence of the ground with the resulting danger of collapse for the buildings located above. limrt of predictabiWy

95<Mi •

limft of pnedtctabiNty

90% •

limit of predictability

80% •

Fig. 82. Diagram of the influence played by the slope morphology on the maximum advance of fallen boulders following the 1976 earthquake in Friuli (Italy) (after Onofri and Candian, 1979).

180 3.7.4 Earthquake-triggered mass movements'^ *by Doriano CASTALDINI

3.7.4.1 Worldwide examples Earthquakes are considered to be a major triggering cause of mass movements in many geological materials. One of the first earthquake-triggered landsHde was documented as early as 372—373 B.C. when Helice, a Greek city on the northern coast of the Peloponnese, sHd into the sea after having been razed to the ground (Marinatos, 1960; Seed, 1968). Many other examples occurred in historical times such as the mass movements produced by the earthquake at New Madrid (Missouri) in 1811, at Kansu (China) in 1920, at Chait (USSR) in 1949, in California in 1957, in Alaska in 1964 and in Japan in 1978 (see Hansen and Franks, 1991). Data from 40 historical worldwide earthquakes were studied by Keefer (1984) in order to determine the characteristics, the geological environment and hazards of landslides caused by seismic shocks. One of the main results obtained by Keefer (1984) was to define relative levels of shaking that triggered landslides in susceptible materials. Four types of internally disrupted landslides — rock falls, rock slides, soil falls and disrupted soil slides — are started by the weakest shaking. More coherent materials with deep-seated slide surfaces, require stronger shaking while lateral spreads and flows are triggered by even stronger shakings, but the strongest shaking is probably required for highly disrupted rock avalanches and soil avalanches. Such topic was further investigated by Keefer and Wilson (1985, 1989). At a more general level, many detailed investigations have been carried out all over the world in order to assess the relationships between earthquakes and mass movements (see Plafker et al., 1971; Nilsen & Brabb, 1975; Solonenko, 1977; Youd, 1978; Harp et al., 1981; Perissoratis et al., 1981; Sorriso-Valvo, 1986; Wue Cai & An Ning, 1986; Geh et al., 1988; Rymer & White, 1989). In more recent times a conference on slope stability in seismic areas was organised (Faccioli and Pecker, 1992); a special session on "Seismicity and Landslides" was held during the 16th International Symposium on Landslides (see Bell, 1992) and landslides certainly triggered by earthquakes have been described in Japan (Yamagishi et al., 1993) and in California (Jibson et al., 1994). More specifically, as reported by Bell (1992), a huge landslide in Iran, occurring during the 21 June 1991 Manjil-Roodbar earthquake, with M 7.3 magnitude, was analysed (Anvar et al., 1992). This slide started the day after the earthquake and continued for several weeks threatening the blockage of a road used for emergency operations and the filling of a large reservoir of a dam along a downstream river. In Crozier (1992) a methodology for determining paleoseismicity is discussed and demonstrated for three earthquake-triggered landslides in New Zealand, while according to Naumann & Savigny (1992) the five major rock avalanches that have occurred in SW British Columbia since the end of the last glaciation are at least likely to have been triggered by high pore-water pressure increase caused by seismic shock.

181 Other authors, from a general point of view, show how the effects of an earthquake on a slope depend on several natural factors (such as the characteristics of the shock, the slope angle, the properties of the rocks and soils involved, the porewater pressure, etc.) as well as irrational human activities (such as deforestation, inadequate land use, etc.) contribute to further deterioration. After analysing the relationships between earthquakes and mass movements on a world-wide scale, Cotecchia (1987) remarks that the behaviour of a slope affected by an earthquake depends on the nature of the ground motion, the slope geometry and its composition. Indications of the likelihood of mass movements under nonseismic conditions do not always hold good for landsHdes triggered off by earthquakes. Ground shaking can produce accelerations within the soil mass, accompanied by a system of stresses that can completely alter the strength of the materials. Thus when considering the response of a slope to accelerations that vary in amplitude and direction, the dynamic properties of the ground cannot be ignored (Cotecchia, 1992). Moreover, it is opportune to remember that Cendrero & Dramis (1994), in a general survey on landslides in Europe, noticed that in areas subject to seismic influence, both the size of individual landslides and the total areas affected tend to be somewhat greater, but no significant differences exist in typology and morphometry between recent earthquake-triggered movements and older, presumably climate-triggered ones. This suggests that seismicity does not change the style of landscape evolution, but it probably increases its rate. This is confirmed by the fact that only 5% of the area affected by landslides after an earthquake corresponds to first-motion movements. The remaining ones are reactivations of older movements. 3.7.4.2 Examples in Italy One of the first reports on earthquake-triggered landslides in Italy is by Vivenzio (1788) who described the geomorphological effects produced by a catastrophic earthquake which struck Calabria (southern Italy) in 1783 (Cotecchia et al., 1969). Several other historical records and oral traditions exist about gravitational movements triggered by earthquakes in Italy (see, for example, Oddone, 1915 and 1931; Govi, 1977; Govi & Sorzana, 1977; Genevois & Prestininzi, 1981; Serva, 1981; Dramis et al., 1982; Pellegrini & Tosatti, 1982; Crescenti et al., 1984; Zecchi, 1987; Murphy, 1993; Mazzini, 1994). The characteristics of some mass movements connected with the two major earthquakes occurring in Italy in the past 20 years (Friuli, north-eastern Italy, and Campania and Basilicata, southern Italy) will now be examined. The earthquake that occurred in Friuli on May 6, 1976, with a main shock of magnitude 6.4 and intensity IX-X (Mercalli scale), produced several surface effects over an area of more than 1,600 km' (Weber & Courtout, 1978). The most striking surface effects were given by the numerous landslides activated by the earthquake (Govi & Sorzana, 1977). They mainly consisted of rock falls and, to a lesser extent, block-slides involving rock volumes of up to 100,000 m \ especially on calcareous slopes with high relief energy. Fractures, visibly opened as a result of the earthquake, appear to outhne many of the gravitational forms (Govi, 1977).

182 The results of the above mentioned investigations on the relationships between the landslides and the Friuh earthquake are here given in detail: — the photo-interpretative method for the survey of landslides connected with an earthquake was found to be adequate for rapidly acquiring an overall picture of the effects triggered on the slopes by seismic shocks; — most of the landslides triggered by the Friuli earthquake were rock falls which took place mainly where these phenomena had already occurred in the past; — important factors influencing the landslides at an equal distance from the epicentre, were the weakening of the rocks by intense tectonic fracturing and the steepness of the slopes controlled by lithological and structural conditions. Further studies (Girardi et al., 1981) carried out in the area struck by the 1976 earthquake, have emphasized that the whole area along the front of a pede-Alpine overthrust shows evidence of the close connection between seismic events and largescale landslides ascribed to Late Pleistocene. The latter appear to be rock slides along bedding planes or faults. The latest high-intensity earthquake in Italy (with a main shock of 6.8 magnitude and intensity X (Mercalli scale) happened on November 23, 1980 and struck a vast area in southern Italy, especially in Campania and Basilicata regions (Deschamps & King, 1983; Westway & Jackson, 1987). The most evident surface effects of the earthquake were mainly due to mass movements of various types, in relation with the lithological and structural characteristics of the bedrock (Cantalamessa et al., 1981; Cherubini et al., 1981; Genevois & Prestininzi, 1981; Cotecchia, 1982; Maugeri et al., 1982; Agnesi et al., 1983; Crescenti et al., 1984: Cotecchia & Del Prete, 1984; D'Elia et al., 1985 and 1986; Hutchinson & Del Prete, 1985: Cotecchia, 1986; Carton et al., 1987; Fenelli et al., 1992; Bisci & Dramis, 1993). These events mainly occurred immediately after the main shock, their movement becoming completely or almost exhausted in a short period. Block-slide phenomena, bordered by earthquake-induced fractures were fairly frequent. The huge landslide 3 km in length that occurred in a pelitic-arenaceous bedrock at San Giorgio in Molara was of this type. From eye-witness accounts and historical data, it seems that this has only moved in association with earthquakes (Genevois & Prestininzi, 1981; Dramis et al., 1982). At Trevico, Dramis & SorrisoValvo (1983) observed lateral spreading in an area of conglomerates resting on clays. The event consisted of the deepening, amounting to some decimetres, of a small graben-like depression (50 m long, 15 m wide and 2 m deep) located on a hill top. For this form too, testimonies from local inhabitants refer to activation corresponding to previous earthquakes and no disturbances within aseismic periods. Another huge mass movement showing recurrent activity and accompanying seismic events involved the whole town of Bisaccia in Campania, even if the displacements were limited. This landslide has been described also recently by several researchers: Crescenti et al. (1984), D'Elia et al. (1985), Esu et al. (1985 and 1987), FeneUi (1988), Fenelh & Picarelh (1990), D'Elia (1991), Fenelh et al. (1992), Bisci & Dramis (1993).

183 3.7.4.3 Study methodologies in Italy At present various studies on the evaluation of landslide hazard in seismic areas are being carried out in Italy. For example, the investigations in north-western Tuscany require detailed geological and geomorphological surveys (1:5,000 scale) in order to draw up a stability map. This will show active and dormant landslides, as well as areas potentially exposed to mass movements due to their morphological and lithological features (see D'Amato Avanzi et al., 1993). An interesting study project on earthquake-triggered landslides is being carried out by Genevois (1994). It is schematically represented in Fig. 83 and is here summarised. The existing data bank phase is based on bibliographical research on earthquakes (consulting seismic catalogues for historical, instrumental and macroseismic data and a consequent selection of the study areas on the basis of isoseismal maps, e.g., minimum VII of intensity (MCS scale), and on the distribution of epicentres and landslides (references of catalogues and historical archives, requests to public authorities such as Council Boards, Districts, Regions, research agencies and Universities; detailed reviews of scientific articles and magazines). The phase of identifying earthquake-triggered landslides implies the correlation and assessment of the data acquired in the previous phase. The identification of earthquake-triggered landslides is carried out by means of correlations that can be of direct or indirect type. With relation to the direct correlation, the method is based on the identification of landslides occurring simultaneously with the seismic event {direct landslides) or certainly set into motion some hours or days after the seismic shock {indirect landslides). Indirect correlation is carried out on the basis of the morpho-structural and/or topographic characteristics (very low gradient angle) of prehistoric landslides or on the basis of documented simultaneity between seismic event and mass movement. In the latter, with temporal concomitance a 4—5-day time-interval is intended, between earthquake and landslide, although obviously some problems remain when the date of the earthquake or of the subsequent landslide is not known. The evaluation of the correspondence between earthquakes and landslides can be good (if direct or indirect correlation is io\\nd)\ fair (if the interval is a little longer than the time span or if a careful location of the landslide is not possible, e.g., more than one in the same area, or an approximate date of the earthquake) or no correlation (time interval too long, uncertain location of the landslide, very ancient landslide, too vague a date for the earthquake). The next step consists on the compilation of landslide cards. This phase of the research is based on a collection of data (geological, geomorphological, hydrogeological, tectonic and neotectonic, geotechnical, geomechanical, climatic, seismological) and other possible integrations (geological and geomorphological surveys, photogeological analysis, in situ investigations, laboratory tests, landslide classification, statistical data analysis). The final content is given by a complete geological model, that is a geotechnical-geomechanical model, an isoseismic-line map

184 (^HISTORICALAND BIBLIOGRAPHICAL RESEARCHES^ _[ LANDSLIpis")

(EARTHQUAl

(D Subregional

Local

/ Projects

Programmes

Plans

Policies

Category of action Fig. 97. Categories of action and levels of government within a comprehensive EIA system (after Lee & Wood, 1978, modified).

Policy Act") established the principles of environmental policy in that country. It introduced a series of norms for the preventive evaluation of impacts: the ''Environmental Impact Statement" (Council of Environmental Quality, 1973). Afterwards, other countries, in particular France, the United Kingdom, Canada and Australia, adopted similar procedures. On 27 June 1985 also the European Community acknowledged this need and adopted a Directive making environmental assessments mandatory for certain categories of projects. This Directive is applied to projects that owing to their nature, size and location can determine a considerable impact on the environment. Two kinds of project categories should be taken into account in this evaluation: those that in any case produce serious effects on the environment and therefore necessarily imply an Environmental Impact Assessment (EIA); those that may have serious environmental consequences according to circumstances: for the latter category the compulsoriness of the EIA will have to be established for each individual case. As regards the impact studies, it is necessary to identify those environmental components which are not only particularly significant to the environment itself but also sensitive to the work that is going to be carried out in a certain territory; these are named environmental indicators. They are employed both in the description and interpretation of the environment, in the planning of the location of a certain project

225

and within the radius of influence of the project itself, as well as in the impact identification and evaluation phase, concerning the influence produced on the indicators. For identifying and assessing impacts, surveying and monitoring systems are used which refer to environmental indicators and transfer the data into adequate control lists or cause-effect matrices (see Leopold et al., 1971). A subsequent phase will consist in attributing a weight, that is an environmental value to each indicator, in order to assess impact and identify a priority order of importance. This allows corrective operations to be carried out in order to reduce or compensate the impact effects and assess the compatibility of the planned work with the environmental characteristics of the territory. Here it is not the case to go deeply into the complex general themes of EIA, regarding which the reader is referred to the above mentioned authors and their relative references, but in the following chapters the problems concerning Geomorphology more specifically will be dealt with. Also the conceptual and methodological bases used for some research of an European Union contract' (see Marchetti et al., 1995; and more in particular: Bollettinari, 1995; Panizza, 1995; Rivas et al., 1995; see also the final publication: Panizza et al., 1996) will be illustrated.

6.2 Concepts First of all, research must be carried out to identify the three main groups of geomorphological components, which may be treated differently (see Panizza, 1995): processes, landforms and raw materials (Fig. 98).

HAZARD (if hazardous, e.g. for landslides)

PROCESS — (e.g. cliff evolution)

> O -i

O X

LANDFORM (e.g. sea beach)

-^

ASSET (if valuable, e.g. for social value)

RESOURCE (if used, e.g. as sea-side resort)

RAW MATERIAL — (e.g. littoral deposits)

->

ASSET (if valuable, e.g. for economic value)

RESOURCE (if used, e.g. as sand quarry)

a. cc

O

o UJ

o

Fig. 98. Conceptual basis of the relationships between two of the geomorphological components (landforms and processes) and a project.

'Human Capital and Mobility Contract ERBCHRXCT 930311 (Coordinator: M. Panizza).

226 Figures 99 and 100 show in synthesis the conceptual bases of the simpHfied relationships between the above mentioned geomorphological components and the project. In particular, the processes which when hazardous are geomorphological hazards, may interfere with a project, which is always characterised by specific vulnerability and cost (Fig. 99). The activity of these geomorphological processes may produce damage for the project, that is to say a risk for the project. This is the case of a landslide (geomorphological hazard) which may damage a motorway (project). In this situation the natural component of the environment shows an active role and the project a passive role. Furthermore, particular landforms which if valuable are geomorphological assets, characterised by specific fragility and value, may be affected by a project (Fig. 99). The effects on geomorphological assets deriving from the implementation of a project make up a direct impact, which causes environmental damage for the same assets. This happens, for example, when the construction of a road ruins a glacial cirque. In this case the natural component of the environment has a passive role with respect to the project which plays an active role itself. Nevertheless, it should always be remembered that there are some processes that can be considered as beneficial (e.g., a particular kind of erosion which may be considered as an educational example). However, these processes produce a landform and then can be included in it. The same considerations can be made for particular raw materials which if valuable constitute geomorphological assets: the interferences with a project may produce a risk or a direct impact (Fig. 100). landforms

environmental damage

GEOMORPHOLOGY

| processes

damage to the project a = in active position p = in passive position

Fig. 99. Conceptual basis of the relationships between two of the geomorphological components (raw materials and processes) and a project.

227 raw materials

GEOMORPHOLOGY ^ processes Geomorphological b hazards j

damage to the project a = in active position p = in passive position

Fig. 100. Types of thematic maps for Geomorphology and EI A studies.

Also in this case there are some materials that can be considered as hazards (e.g., salty soils meta-stable sands). However, these materials are the result of particular processes and therefore can be included in the latter.

6.3 Methodology Research should be carried out following the scheme below (Panizza, 1995). 1) Types of Projects. 2) Investigation phases. 3) Mapping. 4) Indicators. 5) Evaluation of Hazards and Assets. 6) Evaluation of Impacts l.s. 7) GIS Methods. 6.3.1 Types of Projects The norms of the European Union for EIA's studies cover essentially the following types of projects of the 1st category, here subdivided into groups with analogous characteristics for geomorphological research: a) — refineries; — chemical plants, plants for coal aeration, asbestos mining processes, etc.; — thermic and nuclear power plants; b) radioactive and toxic waste plants; c) transport infrastructures (motorways, railways) and airports;

228 d) commercial harbours and navigation lines. Owing to the connections with Geomorphology, also the following projects from the second category can be taken into account: e) tourism infrastructures; f) land use change; g) mining activities. 6.3.2 Investigation phases The investigation phases can be defined as follows, with progressive increases of detail and scale. I = Reconnaissance — general evaluation — site selection: general studies on a small scale in a large area. II = Site and specific investigations: verification of the technical suitability of the selected sites. III = Detailed planning of engineering works: research and detailed measures. IV = Monitoring. V = Mitigation. There are moreover three steps normally followed for the development of a project: 1) design; 2) operation: — construction; — functioning; and 3) decommissioning. EIA investigations should be carried out in Phases I and II so that the results can be included in the detailed planning and design of the project (Phase III). This planning phase should include proposals of monitoring during construction and recommendations for mitigation measurements to be included. As construction proceeds, more information will become available, particularly about the materials and the subsurface conditions. Also a failure of the design may occur. In carrying out diagnosis and remedial measures it may be necessary to modify the predicted EIA. The procedures to be followed will, however, be the same. During the functioning operation it is recommended that the degree of success of the predictions and impacts is evaluated as a guide to maintenance routines and future design practice. In the decommissioning operation it will be necessary to design a new EIA, in order to predict the effects of the abandonment or dismantling of the plant (e.g., pollution, uncontrolled mine drainage, etc.). 6.3.3 Mapping The types of thematic maps used for these EIA studies are summarised in Fig. 101. They are both base maps and derived maps, to be used at different scales according to the investigation phases: for example a small or medium scale map for phase I, a detailed one for phases II or III.

229 3.3.a = geomorphological map 3.3.b= geotechnical map 3.3.C = hydrogeological map

}

}•

base

3.3.x = hazard map (from: 3.3.a - 3.3.b - 3.3.c) 3.3.y = landforms - assets map (from: 3.3.a) }^ derived 3.3.Z = raw materials - assets map (from: 3.3.a - 3.3. b) J Fig. 101. Conceptual and methodological scheme of the role of Geomorphology (landforms and processes) for the EIA and a project.

As for thematic base mapping, it is possible to make reference to various geomorphological, geotechnical and hydrogeological maps, well-known in literature. As for thematic derived mapping it is possible to make reference to some examples of stability and hazard maps made by some groups of researchers; however, examples of thematic maps concerning geomorphological assets are rarer. The derived thematic maps can be integrated with data from literature, archives, databases, etc. During investigation phase I more general maps like morphometric and morphographic maps can be used. For example, with respect to the hazard maps (concerning geomorphological processes) references may be made to the contents of chapter 3.9. On the other hand, as regards mapping of geomorphological assets, reference should be made to the contents of chapter 2.3. 6.3.4 Indicators In the specific literature various definitions of the term indicator have been given since this word may be applied to all the variety of environmental aspects, ranging from geological indicators to urban ones, to quote just two. Although varying considerably from one author to another, involved in Environmental Impact Assessment, all the definitions of this term show the need to emphasize in a significant and summarised way an environmental phenomenon. In the specific case, a geomorphological indicator should describe a situation, a geomorphological process and, according to the situations, also its evolutive trend and its susceptibility to an external intervention. Therefore appropriate indicators should be selected for each assessment investigation in order to acquire an exhaustive but concrete picture of the specific situation; at the same time, all those not strictly necessary should be omitted, in order to avoid accumulating an excessive burden of data which would make the work unmanageable from an economic point of view. Impact is expressed by the changes affecting an environmental unit following the implementation of a project. These changes will be emphasised just by the indicators which depend both on the environment and on the project. Example. Let us consider an arenaceous slope subject to natural erosion producing

230 sandy debris. The detritus accumulates in a riverbed at the bottom of a slope and is subsequently carried as far as the sea and deposited along the coast. It is obvious then that any human intervention on the slope will alter the pre-existing equilibrium. In fact, if the slope is totally or partially covered with concrete (project), erosion will either stop or decrease and therefore the production of detritus will tend to zero. As a consequence, the river will no longer receive the input of material that determined its pristine balance and the energy previously dissipated in the load and transport of the sandy sediments will be mainly directed to fluvial erosion activities. Supposing the riverbed is made up of clayey rocks, the material removed will no longer contribute to the replenishment of the pre-existing beaches, contrary to what happened when the transported material was made up of sand. Consequently the beaches, thus deprived of adequate solid supply, will retreat. This example is shown just to quote some of the processes that take place when the slope conditions undergo changes. Also other processes could occur, such as interference with the groundwater or undercutting at the slope toe. In such a case the process indicators to be found are: one in the fluvial system and the other in the coastal one. In order to evaluate indicators for hazards and assets, it is important to consider the scale and the density' of particular hazard processes and/or geomorphological assets. The lifetime of the project must be considered in order to evaluate the hazardous processes which can be at risk for the project or can be induced by the project. It is therefore necessary to prepare a list of indicators for hazards and assets. From this point of view, the frequency of the processes must be evaluated and compared with the lifetime of the project. According to the main groups of geomorphological components, the investigation phases and the scale of investigation, the indicators will be subdivided into three groups: A. Morphometric and morphographic. B. Hazards. C. Resources. 6.3.5 Evaluation of Hazards and Assets The evaluation of hazards should be developed using different methods: 1) Direct measures (e.g., on scarp retreat). 2) Mechanical models and calculations (e.g., geotechnical measurements). 3) Crossing of ''causes" (e.g., overlapping thematic maps). 4) Statistical approach of ''effects'' (e.g., recurrence of landslides). The first method consists of direct measures of some indicators. In the case of an active cliff the measure of the risk for a project on the top of the cliff itself, may be represented by the direct measure of the retreat of the slope or the variation in the slope angle, etc. The second method is developed using geotechnical measures and engineering models. For example in the cliff case geotechnical parameters and engineering models may be used in order to evaluate the retrogradation of the cliff.

231 The third method takes advantage from the crossing of different thematic maps in which the indicators are considered. For example, in the case of the above mentioned chff the evaluation should be estimate by the overlapping of different maps such as lithological, morphometric, vegetational, etc. The fourth method requires a good knowledge of many parameters in a large area. On the basis of statistical behaviour a forecasting of hazardous events can be evaluated. For example, in the cliff case, the retreat of the slope can be predicted on the basis of the general retreat along all the coast. The evaluation of assets is described in 2.3. The hazards and assets will be related to the following groups of morphogenetic units: 1) Weathering. 2) Slope: a) soil erosion/sedimentation; and b) mass movements. 3) Periglacial (including special mass movements). 4) Glacial. 5) Fluvial. 6) Coastal. 7) Aeolian. 8) Karst. 9) Subsidence. 10) Groundwater. 6.3.6 Evaluation of Impacts Starting from Figs. 98 and 99 it is possible to indicate a conceptual and methodological scheme of the role of Geomorphology for the EIA of a project, with the specification on how the active and passive elements combine in giving different types of impact is. (broadly speaking) (Cavallin et al., 1994). Figure 102 takes into account the geomorphological components landforms and processes. Beside the consequences in terms of risk and direct impact shown in section 6.2, a project during its implementation, functioning and decommissioning, may produce induced hazards, i.e., hazards which did not exist in the area before the introduction of the project. These induced hazards may give rise to three kinds of induced direct and/or indirect effects: direct risk, indirect risk and indirect impact (Fig. 102). Direct risk can be delineated as the effect on the project of a hazard induced by the project to itself; in this case a reflexive action takes place. For example, the construction of a road may cause the instability of the slope where this road is built, thus endangering the project. Indirect risk consists in a hazard induced by a project which damages the surrounding settlements. This is the case of a landslide induced by a road cut, which endangers a village located in the vicinity of the project.

232

INDIRECT RISK

environmental damage

K

""T^ on the surrounding settlements

RISK T

damage to the project a = in active position p = in passive position r = in reflexive position

Fig. 102. Conceptual and methodological scheme of the role of Geomorphology (landforms and processes) for the EIA and a project.

Finally, indirect impact refers to the effects of hazards induced by a project on geomorphological assets existing in areas surrounding the same project. For example, the filling of a lake, which is considered a geomorphological asset, due to a landslide triggered by the construction of a road. The same considerations can be made by taking into account the geomorphological components processes and raw materials. For the evaluation of the different types of impact l.s. mentioned above, for each type of project (6.3.1) and for each investigation phase (6.3.2), the main research instruments are the maps (6.3.3) and the indicators (6.3.4). 6.3.7 GIS techniques Geomorphologists have long recognised the importance of morphometric studies. The availability of altitude data in digital format, and the possibility of preparing and analysing Digital Terrain Models (DTM) may be important tools for quantitatively analysing topographic elements (see Pike, 1993). Software packages specifically designed to produce high fidelity DTM are now available (e.g., Carla et al., 1987; Carrara, 1988) and to produce derivative maps such as slope, aspect and so on. The principal aim is to design and apply a structured method for EIA studies in the field of geomorphology, including data collection, updating, modelling and analysis, using GIS techniques to automate and optimise the decision-making processes. Part of the required parameters for an evaluation of the impact on assets consists of subjective analytical expertise, weight setting, application of nonspatial indices, etc., that cannot be applied directly in a GIS. Nevertheless, some of the criteria need a spatial definition a priori, such as all phenomena occurring in the study area or in

233 the area of influence where GIS apphcations can be partially or completely included. In most cases, though, it becomes evident that a GIS represents a convenient tool for interpreting the data necessary for the assessment in terms of feasibility analysis, robustness of the impact measurements, sensitivity and comparability analysis with spatially referenced information. Much work is still required, however, to bring spatial data analysis, geomorphology and EIA into the realm of routine procedures using widely accepted and significant standards (Patrono, Fabbri & Veldkamp, 1996).

6.4 Quantification of impact* *in collaboration with Mauro MARCHETTI

6.4.1 Raw materials The methodology proposed by Rivas et al. (1995) is presented below. It has been used in the already cited Human Capital and Mobility Contract. The types of direct impact on geomorphological raw materials which can take place as a result of different activities are: — consumption as a consequence of direct extraction; — sterilisation as a result of activities which make the resource unusable; — permanent sterilisation; — temporal sterilisation; and — degradation due to pollution which can alter the properties of the material. In order to quantify the impact on existing raw materials in a given area, the following parameters have to be considered: V: total volume of the deposit (m^); v: % total volume affected; u: % of useful material; P: price ($m'^). The price of a resource is usually directly related to its quality, rarity and exploitability. R: reversibility of action (dimensionless: 1 to (-1)). Obviously the degree of reversibility depends on the type of project; we can consider the minimum reversibility (-1) for those projects which imply the irreversible covering (construction on top) of the deposit for an unlimited time (nuclear power station, urban areas, roads, dams, etc.); reversibility near to 0 is applied to activities such as forestry, agricultural uses or holiday villages, where the technical measures for decommissioning are more feasible (-0.01 times years of implantation of the activity); -0.5 for legal limitations, such as National Parks, etc. When the project does not limit the use of the resource, the value of reversibility is 1; a: relative abundance or rarity is the affected resource volume or area divided by the total resource volume or area (dimensionless); the geographical area used as reference could be the region, province, or the area within a certain distance of the project; A: accessibility is a measurement obtained by the division of the number of potential

234 users (inhabitants) within a certain radius of the resource and the distance (D, in metres or km) of the resource from the nearest road (H/D); the quotient A^JA^^^, will always be