Agustín Sánchez-Arcilla

Medium-term coastal response at the Ebro delta, Spain

Abstract Measurements of shoreline displacements along the Ebro delta coast have been used to estimate the medium-term evolution of the coastline. Aerial photographs and beach profile surveys covering the last 35 years have been used. Shoreline data obtained from beach surveys have been analysed using a linear regression technique in order to filter out short-term components. The medium-term scale has been derived from the asymptotic trend with this filtered analysis. Results have been compared with trends observed in the past and a similar qualitative behaviour has been found. Rates of change have decreased because the shoreline has slowly evolved towards a more stable configuration. A profile closure depth of 7 m has been estimated based on morphological changes in the submerged profiles and then used to convert shoreline displacements into yolume changes. From these, a medium-term sediment budget has been calculated. Based on the available evidence it has been assumed that these changes are mainly due to wave-induced longshore transport gradients. Assuming proper boundary conditions, the net longshore transport scheme has been estimated for each hemidelta. The results obtained using this method are in good agreement with the longshore transport rates predicted by well-known transport formulae.

Processes reshaping the Ebro delta

Abstract Due to the limited supply of sediment from the Ebro river, its delta is mainly subject to reshaping processes in the form of alongshore alternation of erosive and accretive coastal stretches, without a significant change in the overall subaerial surface of the delta. The main agents presently forcing these processes (waves and mean water-level oscillations) are analysed in detail as a function of their contribution to coastal processes. In this manner, waves are characterised in terms of short-term/seasonal changes, longshore sediment transport pattern, long-term cross-shore transport at the shoreface and storm impacts. Although the study area is a microtidal one, and waves are the main agent forcing coastal evolution at yearly scales, mean water-level oscillations (meteorological tides and long-term relative mean sea-level variations) play a non-negligible role. Additionally, a joint analysis of waves and storm surges has been done in order to characterise conditions inducing an ‘impulsive’ type of coastal response. At last, the classical ternary classification of deltas has been applied in a ‘dynamical way’ by considering the river vertex in terms of the variation in sediment supply. Moreover, the effects of considering mean water-level variations (other than tides) on the deltaic classification has also been tested. The inclusion of RSLR influence becomes relevant for ‘river-independent’ deltas since it indicates the inability of the delta to withstand it. The main problem to include this agent in the ternary classification is that RSLR is more of a ‘passive’ factor, whereas the other considered agents are ‘active’ in the sense that they induce more perceivable morphological processes.

Large-scale experiments on wave propagation over Posidonia oceanica

Posidonia oceanica, the most abundant seagrass species in the Mediterranean, supports a highly bio-diverse habitat and is crucial in protecting against coastal erosion. In this work, experiments in a large-scale facility have been performed, for the measurement of wave attenuation, transmission and energy dissipation over artificial Posidonia oceanica. The effects of submergence ratio corresponding to the seagrass height divided by water depth, and seagrass density as the number of stems per square metre on the above characteristics are investigated. Measurements of wave height at different locations along the vegetation meadow indicate the wave attenuation along the Posidonia oceanica for three different submergence ratios and two seagrass densities. Results are also analysed with regard to the wave-induced flow within the meadow, and the effects of the submergence ratio and the seagrass density on the mean flow characteristics, based on data of mean velocities taken at three locations within the seagrass.

Spatial distribution of nutrients in the Ebro estuary and plume

Abstract In this paper the results of two field campaigns (Ebro-I and Ebro-II) carried out during the spring and summer of 1997 are used to examine the differences in nutrient behaviour between the estuary and plume of the Ebro River in the Spanish Mediterranean coast. These campaigns are representative of typical conditions existing during a rainy season (spring) and a dry one (summer). In the estuary, the most determinant processes are nutrient transport—strengthened by agricultural irrigation—and phytoplankton death in the surface, followed by its sinking and breakdown below the saline interface. These processes lead to higher levels of ammonium and phosphorus (SRP) below the halocline. In the river plume region the most important process is the phytoplankton uptake which leads to very low levels of SRP in the points of higher salinity. Both ammonium and SRP are always lower below the saline interface than in areas adjacent to the river mouth.

Implications of Climatic Change on Spanish Mediterranean Low-Lying Coasts: The Ebro Delta Case

Abstract A preliminary estimate of the implications of climatic change on the Ebro delta coast (Spanish Mediterranean) is presented based on an understanding of how climate and other changes will influence the different driving factors that control the interacting formation and reduction processes acting on this low-lying coast. The formation processes are primarily of riverine origin and concern the supply of sediment and freshwater. The reduction processes considered are primarily of marine origin and include increases in inundation/flooding, decreases in storm return periods, coastal erosion, salinity intrusion, and changes in wave climate (wave height, direction, and storminess). For the most part, climatologically induced changes affecting deltaic behaviour, i.e., those of marine origin, are most important for the Ebro delta because those of riverine origin will be significantly damped by river regulation works. Hence, formation processes are suppressed whereas reduction processes will be unaffected by management policies, unless they are related to the coastal zone. Because of its morphology, relative sea-level rise (RSLR) will become the most important climate-induced potential hazard for the Ebro delta. When considering RSLR-induced inundation of deltaic areas below a given level (e.g., 0.5 m), although the deltaic surface below the projected level could be relatively large, impacts will be modulated by the “protection” offered by an active coastal zone that is able to react to the RSLR. Another direct result of sea-level rise will be a decrease in the return periods of maximum water levels, which due to the surge climate of the area will be very significant. Finally, the estimated shoreline retreat due to the RSLR was small when compared to present evolution rates. However, they must be also considered because they will act as an additional background erosion rate along the entire coast.

Best practice for the estimation of extremes: A review

Task 2: Estimation of Extremes of the European Union research project FLOODsite was dedicated to analysing single and joint probability extremes in river, coastal and estuarine environments. It considers the sources of risk, such as river flow and level, wave height, period and direction, and sea level. Herein the work done within Task 2 is reviewed. Several statistical models and various fitting techniques are described. Planning an appropriate extremes analysis involves an understanding of the problem to be addressed, selection and preparation of source data, selection of methods for analysis and parameter fitting, and use of the derived extremes to address the problem. The applications described illustrate some of the pitfalls and difficulties associated with extreme predictions, particularly for the case of more than one variable. Understanding the assumptions and interpreting the obtained results are important for extreme analysis.

Wind waves off the Ebro Delta, NW Mediterranean

Abstract A descriptive investigation on the wind wave climate off the Ebro delta has been conducted on the basis of time series of data collected after 1978. Sea states have been classified into phenomenological cathegories according to available directional data. For selected records, illustrating the different wave conditions, statistical and spectral analyses have been performed so as to assess the short-term characteristics of the wave climate. Regarding the longer-term evolution, correlations among the main wave parameters have been established. Theoretical probability distributions have been fitted to the observed frequency histograms in order to complete the description of the average wave climate. Furthermore, some statistics of extreme wave heights have also been considered.

Impacts of sea-level rise on the Ebro Delta: a first approach

Global climatic change is taking place and it will likely affect Mediterranean deltas and other low-lying coastal regions in terms of sea-level rise, salinity increase and changes in temperature and weather patterns. This will have serious implications because these deltas are very valuable in terms of natural resources and related economic activities. This study focuses on one of the northwestern Mediterranean deltas, namely that of the Ebro river. There is an enormous lack of information about and understanding of the integral functioning of this type of system. The objective of this ongoing study is to determine the vulnerability and response of this deltaic system to climate change, such that informed decision-making can be made. In order to do this it is proposed to make combined use of existing and new field measurements (sedimentation, soil formation and coastal fringe response) and an integrated (physical/ecological) conceptual model of deltaic behaviour. Using these tools, organized in relation to the interaction with socio-economic components, a number of intervention scenarios, aiming to cope with the effects of climate change, will be examined in a later stage of the research project. It is expected that these results will provide valuable information for integrated, comprehensive approaches to determine whether management plans are sustainable.

Coastal dynamics and wetlands stability. The Ebro delta case

Coastal wetland stability and structure can be significantly affected by littoral processes when they are close to the shoreline. Thus, under certain conditions, the combination of shoreline dynamics and direct wave action during storms can influence the stability of marshes and vegetation community composition. This interaction between littoral dynamics and coastal wetlands is illustrated by analysing processes taking place in the Buda Island (Ebro delta, NW Spain), where a coastal lagoon very close to a retreating shoreline exists. Two main time scales have been found to be relevant for interaction of coastal processes with ecosystem dynamics, the decadal and episodic scales. The decadal scale determines the average trend in beach width and directly controls the potential loss of wetland surface. The episodic scale is linked to the occurrence of wave and storm surge events and it determines a pulsing stress in the ecosystem through flooding, being key parameters to determine their direct influence the intensity and repetition of these events.

The narrow shelf concept: couplings and fluxes

Abstract The continental shelf domain has the nearshore zone as land-ward boundary while the shelf slope plays the role of sea-ward boundary. Shelf dynamics are, therefore, influenced by terrestrial and marine processes and are an important component for any integrated assessment of the coastal zone. The quantitative influence of nearshore/slope processes is, however, seldom considered and many continental shelves are considered “wide” (e.g. parts of the North Sea coast) or “narrow” (e.g. parts of the Mediterranean coast) based on intuition and implicit knowledge. No objective and quantitative assessment of the relevance of such processes in controlling the resulting shelf dynamics has been presented so far. The main aim of this paper is to examine the mechanisms “isolating” and “linking” the shelf to its neighbouring domains (viz. nearshore and slope) and, from this, to obtain a “natural” definition of the shelf category. The resulting critical shelf width, dividing shelves controlled by “border” processes from those controlled by their internal dynamics, turns out to depend on the considered process and its associated time/space scales. The paper will, thus, start by presenting the main mechanisms isolating shelf and slope, viz. the geostrophic and potential vorticity “constraint”. This will be followed by a discussion of the main processes linking shelf and slope, viz. non-linearities, boundary layer and unsteadiness effects and stratification. The same approach will be applied to the analysis of shelf and nearshore links, highlighting the distinction between isolating and coupling mechanisms. From this a “natural” evaluation of the critical shelf width will emerge, allowing a hierarchical classification of the various shelf width assessments and illustrating the relative importance of the different processes involved for the Ebro delta coastal sea (Spanish Mediterranean coast). The performed theoretical analysis, although based on field observations from the Ebro delta site, is also expected to be of use for other shelf environments, where there is a need to distinguish between the competing influences of nearshore and slope “boundaries”.