Long to short term coastal changes and sediment transport in the Ebro delta; a multi-scale approach

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1 -In : Transformations and evolution of the Mediterranean coastline - Long to short term coastal changes and sediment transport in the Ebro delta; a multi-scale approach by Jose A JIMENEZ 1, Agustin SANCHEZ-ARCILLA1 and Andres MALDONADO 2 1 Laboratori d'enginyeria Marftima, Universitat Politecnica de Catalunya, c/. Gran Capita s/n, Barcelona 08034, Spain. 2 InstitutoAndaluz de Ciencias de la Tierra, CSIC/Universidad de Granada, Facultad de Ciencias, Granada, Spain. ABSTRACT Coastal changes, sediment budget as well as associated sediment transport patterns are analysed at the Ebro delta coast. The analysis is done at three temporal and spatial scales: long-term, medium-term and episodic, whereas the contribution of each component to the overall deltaic behaviour is identified and quantified. The results characterise the Ebro delta coast, under present conditions, as a closed system for the sand fraction, without a significant variation in total subaerial surface and sediment volume. The most important change is the reshaping of the coast due to the redistribution of sand by waves and currents. This behaviour is mainly induced by the existing net longshore sediment transport pattern although other processes as overwash and aeolian transport can be locally important. RESUME Nous analysons ici les changements cotiers dans le delta de l'ebre, aussi bien du point de vue du bilan que du transport sedimentaires. L'analyse est faite a trois echelles dans le temps et l'espace - long terme, moyen terme et evenementiel - identifiant et quantifiant la contribution de chaque compo- Bulletin de l'institut oceanographique, Monaco, n special 18 (1997) CIESM Science Series n 3 169

2 sante au fonctionnement global du delta. D'apres les resultats obtenus, la cote du delta de l'ebre se comporte aujourd'hui comme un systeme clos en ce qui concerne le sable, sans variation significative dans la surface totale et le volume de sediments. Le changement le plus important est le remodelage de la cote due a la redistribution du sable par les vagues et les courants. Ce phenomene est induit principalement par le transport c6tier des sediments bien que d'autres mecanismes tels le transport eolien et le lessivage puissent etre localement importants. INTRODUCTION When studying morphological changes, a broad spectrum of frequencies in time and space can be considered, with scales ranging from the high frequencies (e.g. shoreface bed-form dynamics) to the very low ones (e.g. offshore bar system dynamics). However, when the emphasis is on relatively large scale changes, this theoretical spectrum can be narrowed, so that only some of the principal components may be analysed to reproduce or to study the major part of the large scale morphological changes. This kind of analysis is usually based on a statistical approach or a qualitative/quantitative process-based approach. In this paper a mixed-type of analysis, combining both approaches is applied to the Ebro delta, as an example of active coastal change that could be of interest for the study of similar systems. The Ebro delta has a subaerial surface of 320 km 2 and a sandy coastline of about 50 km long developed from the sediment supplied by the Ebro river (Figure 1 ). After several centuries of growth, its evolutive trend changed a few decades ago mainly due to the nearly total reduction of sand supply following dams construction in the river course. In order to characterise coastal changes, the associated sediment budget as well as the corresponding sediment transport patterns, a multi-scale (both in time and space) approach has been followed. Three main scales are considered in this study: (i) long-term scale, (ii) medium-term scale and (iii) episodic term scale. To identify and to isolate the coastal processes corresponding to each frequency band, morphological, meteorological and oceanographic data, including maps, aerial photographs, beach profiles, wind data, wave and current data are analysed. SCALES AND PROCESSES The selection of scales to be used in the characterisation of coastal processes has a degree of subjectivity which results in a relatively large diversity of proposed classifications (see DEVRIEND, 1991; FENSTER et al., 1993, among others). In this study the definition of scales is based on the analysis of the physics governing the observed coastal changes in the Ebro delta (see JIMENEZ et al., 1993; SANCHEZ-ARCILLA and JIMENEZ, 1997). Although the corresponding time boundaries are difficult to establish, they have been selected and validated through the performed analysis. Long-term processes have been associated with changes at a temporal scale of decades and a spatial scale in which the complete deltaic coast is considered. The main changes at this scale are those in its overall shape and sediment budget, which are characterised by the corresponding net surface and volume changes. The main "driving" or "forcing" agents contributing at 170 Bulletin de l'lnstitut oceanographique, Monaco, n special 18 (1997) CJESM Science Series n 3

3 0 N l 0 beach profiles Eucaliptos MEDITERRANEAN SEA 5km 0 Figure 1 - The Ebro delta. this scale have been identified as: river sand supply, cross-shore sediment exchanges at the shoreface, relative sea-level rise (RSLR) induced changes, aeolian transport over the dune fields (or barrier) and overwash transport. Medium-term processes have been associated with changes at a temporal scale of several years and a spatial scale of several km. In this scale cyclic (e.g. seasonal) changes are filtered out in such a way that the net evolutive trend is only retained. Most of the observed changes at this scale have been related with the net longshore sediment transport processes and correspond to a coastal reshaping in which eroding stretches are feeding accreting ones. Although this scale is shorter than the previous one, it has a residual morphological effect visible or detectable at the long-term scale. Episodic events have been associated with hydrodynamic processes with a long return period, unknown periodicity (due to its nature) and a spatial scale defined by the length of the coastal response (as in the case of barrier beach breaching). The contribution at this scale, although not present in every climatic cycle, whenever existing is important enough to contribute significantly, in a matter of several days, to the medium-term and, even, long-term processes with an eroded volume equivalent to what would happen in a few years without episodic events. The main "driving" agent for Bulletin de l'lnstitut oceanographique, Monaco, n special 18 (1997) CIESM Science Series n 3 171

4 these events is the presence of very energetic sea states, generally characterised by the coexistence of storm surges and storm waves, which produced an associated coastal response of "extreme" erosion of vulnerable stretches. Analysing the partition of the coastal response into several scales, it is possible to estimate a sediment budget for the Ebro delta coast, the corresponding contributions of processes acting at different scales and their relative importance for the global behaviour. Moreover, this will allow to consider different climatic scenarios based, for instance, on the occurrence of episodic events or river discharges and to evaluate the corresponding budget implications. One of the first aspects to solve is the determination of the time frame for the morphological analysis. This must be selected in function of the objectives of the study, i.e. characterisation of present coastal processes, determination of evolutive stages, sediments budgets and processes, among others. Because the main objective of this paper is to determine coastal processes governing the Ebro delta present evolution (or better, under the existing boundary conditions and driving factors), the time frame will be that during which present conditions were achieved. Thus, since the main boundary condition for the Ebro delta coast development is the building of dams in the river course, the initial time for this analysis will be that corresponding to the Ebro delta configuration near and from the date of dam building. Otherwise, if previous configurations would be included in the analysis different results could be obtained. During the last two centuries, the evolution of the Ebro delta is characterised by a tendency towards a exponential decrease of the expansion rate of the delta plain (Figure 2). After a period of large expansion ( ), followed a period in which the expansion is attenuated reaching a "hypothe- Sa DATE {years) Figure 2 - Evolution of the subaerial surface of the Ebro delta during the last two centuries (JIMENEZ, 1996). 172 Bulletin de l'lnstitut oceanographique, Monaco, n special 18 (1997) CIESM Science Series n 3

5 tical" equilibrium surface. The peak of the curve at 1989 is hypothetical because a steady situation has not necessarily been reached, but it may represent a point of reversal in the trend. The main dam complexes in Ebro river course were built in the beginning of the 1960's and on the basis of the delta plain surface evolution, the time frame for this analysis was selected from 1957 to present. This period, during which the Ebro delta coastline is precisely known, is characterised by an evolutive stage of minor delta plain growth, reflecting the reduced sediment supply by the river in comparison to previous stages of the delta evolution. The selection of this time frame is in accordance with the methodology developed by FENSTER et al. (1993). Analysing methods to predict shoreline positions, FENSTER et al. (1993) identified the presence of a critical point (change in the observed trend) as a timing indicator of a change in the long-term trend, and they recommend to split the observed evolution in periods during which shoreline changes can be assumed linear. The analysis of ancient deltaic coastal changes would be necessary when the historical evolution is the main object of interest (e.g. MALDONADO, 1972, 1986; WRIGHT and COLEMAN, 1973). LONG-TERM/LARGE-SCALE COMPONENT The study of the long-term component of deltaic changes is based on different deltaic shorelines obtained from maps and aerial photographs. The time span between "available" coastlines with an adequate accuracy for the analysis is about 16 years, covering the period between 1957 and The use of these data is one of the best way to assess the effects of the long-term component of the coastal behaviour since, as the time span between data is large, most of shorter-term responses/processes are filtered (see e.g. DOLAN et al., 1991; CROWELL et al., 1993). Discussions about the accuracy of such kind of data can be seen in ANDERS and BYRNES (1991) and CROWELL et al. (1991) among others. The Ebro delta coastline evolution between 1957 and 1989 is shown in Figure 3, where a significant coastal reshaping may be observed. The most "apparent" behaviour is the alternance of erosive and accretive zones, with a maximum shoreline recession of about 1,700 m at Cap Tortosa, and maximum shoreline progradations of about 1,000 m and 900 m at the apex of the northern and southern spits respectively. When these shoreline changes are converted to subaerial deltaic area changes, a slight increase is detected (Figures 2 and 4). This area increase is larger during the first period ( ) at a rate of about 25,000 m 2 /yr, decreasing down to about 12,000 m 2 /yr for the period Main zones experiencing growth correspond to the river mouth area as well as both spits, while erosion concentrates in the abandoned river mouth at Cap Tortosa (MALDONADO, 1986; JIMENEZ et al., 1993) This difference in the surface rates of change between both periods can be explained by the implementation of the new conditions governing the Ebro delta evolution. Thus, the first period ( ) corresponds to the period of dam building and therefore to the initiation of the sediment input supply decrease. Under this situation waves and induced currents began to act on the coastal deposits and the deltaic coast was rapidly reshaped. Bulletin de l'institut oceanographique, Monaco, n special 18 (1997) C!ESM Science Series n 3 173

6 Figure 3 - Ebro delta coastline evolution from 1957 to 1989 (JIMENEZ et al., 1993). During the second period ( ) although the wave transport capacity was the same, as the coastline had been already reshaped towards a more stable situation, the changes were of lesser magnitude. The river sediment supply during the second period, in addition, has totally adjusted to the drastic reduction of bed load imposed by the building of the dams (VARELA et al., 1986). Moreover, some of the differences in the net increment of subaerial surface can be associated with the fact that during the first period ( ) the river water discharge was larger and therefore, the potential river transport was also higher (JIMENEZ, 1996). The wave transport capacity can be assumed as steady since existing wave data do not show any significant temporal variation during this time span. Due to the low values obtained and considering the estimated accuracy of shoreline positions used in the analysis, this result must be understood as indicative that the present long-term/large scale budget of the Ebro delta 174 Bulletin de l'institut oceanographique, Monaco, n special 18 (1997) C!ESM Science Series n 3

7 '>::;' $ 0.3 s 'C> t:j, 0.15 <!) bjj 0.1 = ro.a 0.05 u 4-< 0 0 ~ ro ('<$ -0.1!':: ro OJ ro :;.1 "' GAIN LOSS NET 1973/89 Figure 4 - Subaerial surface rates of change in the Ebro delta from 1957 to 1989 (JIMENEZ et al., 1993). coast is nearly nil. In fact, by converting deltaic area changes to volume changes - assuming realistic values for the depth of closure and taking into account alongshore variations due to the existing morphology of bays, spits and open coast - a slight volumetric increase of about 22,000 m 3 /yr was obtained (SANCHEZ-ARCILLA et al., 1995; JIMENEZ, 1996). This means that the net result of all the processes acting as sources and sinks are in equilibrium in such a way that, under present conditions, their resulting net contribution to the long-term/large scale sediment budget, considering only the sand fraction, is zero or close to zero. One of the key points to consider in order to etablish the closure depth of this budget is that there exists a mud belt surrounding the coastal sandy sediment in the Ebro prodelta (GUILLEN, 1992). This belt is located at around 15 m depth. In former times the sandy sediment supplied by the river was able of bypassing and to nourish coastal stretches located southwards of the delta, but at present no sand migration across this belt has been detected (Figure 5). This has been established based upon geophysical campaigns and sediment sampling surveys (MOPU, 1979, GUILLEN, 1992). The presence of this belt can be used to establish a general boundary condition for the Ebro delta : the coastal system can be considered as closed for the sand fraction. In order to understand the obtained results, the potential contribution of processes acting at this scale has been calculated. Positive contributions These contributions to the sediment budget, considering only sandy sediments (medium and fine sands) are due to river sand supplies and crossshore transport at the shoreface level. Although the Ebro river water discharge seems to have suffered an important decrease during the last decades Bulletin de l'lnstitut oceanographique, Monaco, n special 18 (1997) CIESM Science Series n 3 175

8 medium and fine sands(> 125 microns) fine and very fine sands (63 to 125 microns) N Figure 5 - Sediment characteristics surrounding the Ebro delta coast (modified from GUILLEN, 1992). (see e.g. MALDONADO, 1986; CALLIS et al. 1988; GUILLEN and PALANQUES, 1996) and now most of the drainage basin is regulated, it is assumed that in the non-regulated area, the river is still able to transport sediment under high discharge conditions. The threshold river water discharge for sandy sediments, with a sediment grain size similar to that present in the coastal zone, has been estimated at about 400 m 3 /s (JIMENEZ et al., 1990). These authors have made an estimation of the "potential transport capacity" of the river, obtaining a transport rate of about 30,000 m 3 /yr, which is similar to that estimated by other authors (e.g. GUILLEN, 1992). Even though the river sand supply is a long-term component of the sediment budget (it contributes to the global deltaic budget), it does not act as a "continuous" agent but as a pulsing one, i.e. during the course of river floods. When river flow conditions permit to supply sand to the coast, the sediment is mainly deposited in the northern part of the delta due to the present river mouth orientation towards the North (Figure 1). GUILLEN and JIMENEZ (1995) explained the differences in the coastal sediment grain sizes between the northern and southern parts of the delta (finer in the northern one) by the effect of river supplies. The cross-shore shoreface transport has been identified as one of the main sources of sediment for many coasts around the world (see e.g. NIEDORODA et al., 1985; DEAN, 1987). In fact, some authors assume that this contribution plays an important role for zones in which rivers have declined 176 Bulletin de l'institut oceanographique, Monaco, n special 18 (1997) CIESM Science Series n 3

9 their function as sediment sources (see e.g. LEE and OSBORNE, 1995). Generally speaking, main driving agents contributing to this transport are wave action (wind-waves as well as infragravity waves), currents and gravity induced contributions (see WRIGHT, 1987, for a detailed description). The best option, and maybe the only realistic one, is to analytically estimate this contribution by shoreface measurements, but such information is not available for the Ebro delta coast. A modelling approach has been used therefore as an alternative similar to that of WRIGTH et al. (1991 ). Using the energetic models of BOWEN (1980) and BAILARD (1981) this contribution has been estimated at the Ebro delta shoreface at 10 m depth, excluding the long wave contribution, by JIMENEZ (1996). This depth was selected as a more or less "subjective" boundary between the upper and the lower shoreface based on the existing wave climate and the cross-shore sediment distribution. Results indicate that the yearly averaged potential net transport capacity at the shoreface level would be about 4.5 m 3 /m/yr directed landwards, and mainly induced by the most energetic waves (JIMENEZ, 1996) which is in the order of magnitude of more than 6 times of the expected sandy supply by the river. Alongshore variations in the calculated transport rates can be found along the Ebro delta coast due to changes in the shoreface profile (bottom slope). This onshore transport implies that if it was the only active process, it would induce a steeping of the shoreface profile because sand actually laying in the lower shoreface (between 10 m and 15 m depths) would be transported towards the upper shoreface where it would be deposited. In any case, this positive contribution does not imply a sediment feeding of the Ebro delta with allochthonous material but the already existing material redistribution from deeper parts of the shoreface to the shallower ones. Negative contributions These contributions to the sediment budget, considering only sandy sediments (medium and fine sands), are due to aeolian transport over the dune fields, overwash transport and RSLR induced changes. Aeolian and overwash transports have been estimated using morphological data such as shoreline changes in the backbarrier of the Trabucador Bar and dune fields evolution along the deltaic coast. Both processes will induce a net cross-shore transport in the subaerial part of the beach in a "landwards" direction. In the first case, the net transport will be the result of the storage of sediment in the dune fields due to the wind action on the emerged part of the beach, whereas in the last case it will be the result of the landwards transport of sediment across the subaerial part of the beach produced under high water levels by bores propagating over the beach under storms action. The importance of such processes can be seen in the evolution of the Trabucador Bar, where not only the outer coast is modelled by coastal processes but also the backbarrier evolves in time (Figure 6). This behaviour is typical of most barrier beaches and it is associated to overwash transport and inlets formation (e.g. LEATHERMAN, 1979). Since, there was no inlet formation of significance during the period of study, this behaviour has been mainly associated with overwash processes (SANCHEZ-ARCILLA and JIMENEZ, 1994). The yearly averaged estimated joint contribution of both factors has been estimated at about 3 m 3 /m/yr (SANCHEZ-ARCILLA et al., 1993; JIMENEZ, 1996). Bulletin de l'institut oceanographique, Monaco, n special 18 (1997) CIESM Science Series n 3 177

10 km l!l!ll deposition [JJ erosion N 1 l Figure 6 - Trahucador Bar evolution from 1957 lo 1989 (SANCHEZ-ARCILLA and JIMENEZ, 1994). The remaining contribution is due to RSLR induced changes. It is world-wide assumed that an increase in the RSLR, considering eustatic changes and local induced effects (subsidence and compaction) will produce an erosion of the shoreface and a deposition in the offshore, in such a way that the profile will maintain a constant shape suffering a net landwards and upwards translation (e.g. BRUUN, 1962). In any case, these changes and the associated transport rates are not due to RSLR itself, but they would be the result of a change in the governing conditions acting on the coast (a different mean sea level) and it would be like the result of the acting processes to reach a Jong-term equilibrium situation under wave action. This means that the effect of RSLR will be an indirect one although the corresponding coastal response will be associated with this effect. Unfortunately, there are no tidal records long and accurate enough to estimate local sea level changes in the Ebro delta area. Moreover, there also exists a lack of direct estimations for subsidence rates and so only indirect estimations have been possible (see GUILLEN, 1992; SANCHEZ-ARCILLA et al., 1993, 1995; IBANEZ et al., 1995). These estimations have been obtained through different methods such as by comparing the depth of ancient lobes, by sedimentary budget considerations and by comparing the elevation of the ancient levees. SANCHEZ-ARCILLA et al. (1995) made a first indirect estimation of RSLR in the Ebro delta area based on sedimentary budget considerations obtaining a RSLR of 2-3 mm/yr. More recently JIMENEZ (1996), following the same approach but with more detailed information and using the shoreface profile model of STIVE and DEVRlEND (1995), obtained a range of RSLR between l.6 mm/yr to 5.3 mm/yr, which leads to a volumetric loss of m 3 /m/yr. Although the obtained calculated rates show a significant variation, they serve to bound RSLR in the area. 178 Bulletin de l'jnstitut oceanographique, Monaco, n special 18 (1997) CIESM Science Series n 3

11 MEDIUM-TERM COMPONENT The study of the deltaic coastal changes at the medium-term scale supposes to work in a level of "aggregation" lower than the previous one (temporal scale of several years). Data used to estimate this component are beach profile surveys and shoreline positions taken from 1988 till 1992 with a time span of about 3 months. Due to the frequency of these data (they will retain cyclic/seasonal effects) a simple linear-regression technique was applied to filter out shorter-term components (see DOLAN et al., 1991 ). The estimation of the medium-term evolutive trend for one location experiencing erosion can be seen in Figure 7. (a) nuulium-term trend observed changes -80+-~~~~~~~~~~~~~~~~~~~~~~~~~-< DATE Figure 7 - Estimation of the medium-term shoreline rate of change (JIMENEZ and SANCJIEZ ARCILLA, 1993). Figure 8 shows the so-calculated shoreline evolution trends along the Ebro delta coast for the analysed period. Results show an alternance of erosion and accretion zones along the coast. These zones are similar to those obtained in the analysis of the long-term component although the shoreline migration rates of change are different. This agreement between the location of erosion and accretion zones can be used as an indication that processes acting at this scale have a residual effect visible at a longer-term. The differences in the magnitude of the rates of change can be explained if one assumes that the coastline is evolving towards an equilibrium state under wave action. Thus, the rates calculated for the first period ( ) are the largest ones because the delta coast began to experience the controls of the new factors governed by the drastic reduction of sediment supply due to dam construction. The shoreline changes of the subsequent periods are, in consequence, lower because the shoreline morphology has slowly evolved towards a more stable configuration under present conditions. This estimated medium-term budget shows an alternance of sediment losses and gains which, when integrated, gives a zero net sediment balance (Figure 9). Bulletin de l'institut oceanographique, Monaco, n special 18 (1997) CIESM Science Series n 3 179

12 <:- g 10 w 0 C> z <( -10 ::c (.) -20 LL w -40 ~ south accretion erosion 1957 / / ALONGSHORE DISTANCE (km) Figure 8 - Shoreline rates of change along the Ebro delta coast for the periods 1957 /73, 1973/89, 1988/92. In order to explain the obtained results, main driving terms were analysed and related to the observed changes. As it was previously stated, it is assumed that the main driving factor acting at this scale is the longshore sediment wave-induced transport pattern along the coast. To see if the observed pattern of volume changes is mainly conducted by longshore transport, volume changes were converted to net longshore transport rates (Figure 9). This was done using the continuity equation applied to the conservation of sand, and imposing proper boundary conditions. Boundary conditions, derived from observed coastal behaviour and taking into account that no sand migrates out of the deltaic coastal system, are zero transport southwards of the southern spit and the "potential" river sand supply for the northern part of the delta. At this scale, the river sand supply does not act as an intrinsic condition (i.e. its main effect will not be at this scale) but as an extrinsic condition (i.e. it will act like a boundary condition at this time scale). The main result is that volume changes at this scale can be fully explained using the net longshore sediment transport pattern. Moreover, the derivation of this transport pattern from coastal behaviour reproduces reversals in the transport direction as that verified at the ancient river mouth (Figure 9). This transport pattern can be reproduced using sediment transport formulae fed by the existing wave climate although calibration coefficients for the employed formulae have to be fitted (e.g. JIMENEZ and SANCHEZ-ARCILLA, 1993; JIMENEZ, 1996). As a result, the initial hypothesis that medium-term sediment budget is mainly or fully driven by the longshore transport pattern is assumed to be correct. The most significant conclusion is that the present medium-term processes produce a redistribution of the sediment along the Ebro delta coast, but without a net contribution to the coastal sediment bud- 180 Bulletin de l'institut oceanographique, Monaco, n special 18 (1997) CJESM Science Series n 3

13 accretion erosion mouth area N ~ 70 5km amounts in thousands of m 3 /yr.~ Figure 9 - Medium-term sediment budget and associated net longshore sediment transport rates along the Ebro delta coast (adapted from JIMENEZ and SANCHEZ-ARCILLA, 1993). get. Due to this, the medium-term component will have a "residual" morphological effect on the longer term component. This effect will reflect in the coastal reshaping observed at the long-term/large-scale behaviour, although without a contribution to the global sedimentary budget for the sand fraction (Figure 3). EPISODIC EVENTS The contribution of this component, by its nature, is deterministically unpredictable. It will only act under the action of driving terms of long return period, and its action will be restricted to a very short time period, which is the duration of the event. The morphological effects, however, will have a longer time scale. Moreover, although the driving terms conducting episodic changes will act on the entire deltaic coast, only vulnerable stretches such as barriers and beaches with a narrow subaerial surface will Bulletin de l'institut oceanographique, Monaco, n special 18 (1997) CIESM Science Series n 3 181

14 mainly suffer this kind of changes. These can be characterised by a nearly instantaneous and "catastrophic" erosion. In order to illustrate the possible effects and corresponding contributions of this component to the deltaic budget and behaviour, an erosive event occurred in the Ebro delta coast will be used. This event can be classified as episodic because no similar case has been reported during the second half of this century. The event consisted in the action of a storm during October 1990 during which the Trabucador Bar, the barrier beach linking the main body of the delta with the southern spit, was breached as well as part of the beach at Cap Tortosa (Figure 1 ). The main morphological effect of the storm action was the opening of a breach about 800 m long in the middle of the Trabucador. The involved sediment volume change, considering only that removed from the breached stretch, was estimated at about 70,000 m 3 in few days and it was transported towards the inner bay and deposited in a shallow platform behind the barrier (SANCHEZ-ARCILLA and JIMENEZ, 1994). The driving factors producing such volume changes were longshore and cross-shore (offshore) and overwash transports induced by the littoral dynamics generated under the simultaneous action of high waves and a storm surge. To estimate the relative importance of each acting agent, the different contributions to the observed volume changes were estimated. Thus, it was detected that most of the morphological changes were due to overwash processes in such a way that most of the eroded sediment (60,000 m 3 ) of the breached stretch was transported towards the inner bay, producing a net sediment loss from the outer coast dynamics. The amount of sediment transported towards the inner bay under this event (3-4 days) significantly exceeds the yearly averaged overwash transport rates obtained in a longterm perspective (in the order of 16,000 m 3 /yr for the entire Trabucador Bar). Under "normal" years, the overwash transport rate is about 5 m 3 /m/yr (averaged over the last 30 years), while during the event the integrated transport rate was about 60 m 3 /m in 4 days. The contribution of such events to the global sedimentary budget of the outer coast will be a sink of sediment. However, this sediment will not be lost from the deltaic area, it will remain in the inner bay, where it will contribute to the landward displacement of the spits and coastal bars. In summary, the main morphological effect of such episodic events will be reflected at the longest time scale. Breaching will be counteracted and closed by the natural outer coast littoral dynamics through the deposition of sediment transported alongshore and across-shore after the storm. However, the sediment deposited in the inner bay will be removed from the outer coastal system due to the absence of driving factors energetic enough to mobilise this sediment in the bay and to transport it back offshore. This process represents a significant contribution to the landwards displacement of the backbarrier (e.g. Figures 3 and 6). CONCLUSIONS The Ebro delta coast can be considered at present as a closed system for sandy sediments. The net sediment budget for the whole deltaic coast is determined by the long-term component. At present, different contributions, 182 Bulletin de /'Institut oceanographique, Monaco, n special 18 (1997) C!ESM Science Series n 3

15 positive and negative, seem to be balanced in such a way that the deltaic coast does not present a significant variation in the volume of coastal sandy sediments. The observed large scale coastal reshaping is due to the "residual" effect of driving terms acting at the medium-term scale, a lower level of "aggregation". The observed morphological changes at this shorter scale do not involve a net change in the global sediment budget, but a redistribution of the sediment along the coast. This process is induced by the net longshore wave-induced sediment transport. Episodic events, when present, produce over of few days a significant morphological response involving a mobilisation of large sediment volume, such as breaching of barrier beaches. These processes are restricted to specific deltaic coastal stretches, specially sensitive to such events (e.g. Trabucador Bar). The contribution of this component to the global sediment budget and coastal response will be more significant at the long-term scale, since it will contribute to the landward displacement of the backbarrier of the spits and bars, and it will remove significant proportions of sandy sediments from the outer coastal system. The analysis of the sediment budget of coastal systems, and of the Ebro delta coast in particular, at different scales allows to assess the different factors controlling coastal evolution. The contribution of each factor to the net increment of volume of the coastal system can be analysed to estimate its real contribution to the sediment balance and redistribution. Moreover, since each component has a specific contribution to the coastal response, this can be assessed under different climate scenarios by combining the different contributions of each factor. ACKNOWLEDGEMENTS This work was carried out as part of the MEDDELT and PACE projects. It was partly supported by the EU Environment Research and MAST programmes, under contracts N EV5V-CT , MAS3-CT and by the Spanish Ministry of Education and Science through DGICYT (UE ). This work has been done in association with LOICZ/IGBP. REFERENCES ANDERS F.J. and BYRNES M.R., Accuracy of shoreline change rates as determined from maps and aerial photographs. - Shore and Beach, 59, 1: BAILARD J.A., An energetics total load sediment transport model for plane sloping beach. - Journal of Geophysical Research, 86, Cll : BOWEN A.J., Simple models of nearshore sedimentation; beach profiles and longshore bars. - In: The Coastline of Canada, Geological Survey of Canada., McCann, S.B.: BRUUN P., Sea-level rise as a cause of shore erosion. -Journal of Waterway and Harbours Division, 88 WWI: Bulletin de l'jnstitut oceanographique, Monaco, n special 18 (1997) CIESM Science Series n 3 183

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