Miguel Ortega-Sánchez

Impact of river regulation on a Mediterranean delta: Assessment of managed versus unmanaged scenarios

This work addresses the effects of the construction of a reservoir 19 km from the mouth on the dynamics of the Guadalfeo delta (southern Spain), a Mediterranean delta in a semiarid and high-mountain basin. The sediment volume transported as bed load and accumulated in the delta was estimated under two scenarios by means of a calibrated hydrological model: a managed scenario, considering the flows drained by the dam, and an unmanaged scenario, considering the absence of such infrastructure. Bathymetric and topographic measurements were analyzed and correlated with the fluvial and maritime forcing agents. Results indicate that the reservoir has significantly modified the dynamics downstream: the coast has lost almost 0.3 hm3 of sediments since the entry into operation of the dam, generating a 1.4 km coastline retreat around the mouth, with a maximum retreat of 87 m (92% of the initial). The beach profile decreased by up to 820 m2, whereas the average decrease around the mouth was equal to 214 m2. Under unmanaged conditions, more than 2 hm3 of bed load would have reached the coast. Based on the results, three new management scenarios of flows drained by the dam, in combination with bypassed sediment from the reservoir, were proposed to prevent more severe consequences in the delta and the silting of the reservoir. The proposed methodology for new management scenarios can be extended to other worldwide deltas, especially to those in semiarid and Mediterranean basins, and it represents an advanced tool for decision making.

Coastal Evolution, Sea Level, and Assessment of Intrinsic Uncertainty

Abstract The climate variability, its implication in coastal processses and the uncertainty that it therefore introduces in the morphological evolution of the coast are addressed. Historical evidences in the Iberian Peninsula allow relating the occurrence of significant variations in sea level position during the Holocene and its effect on the morphology, to natural climate changes. It is still unknown the way that long term climate variability will affect sea level position and the severity of other meteorological agents, which is a source of uncertainty that adds to the stochastic nature of coastal long term proccesses. In a decadal scale, under the assumption that sea level and other parameters that describe the climatic forcing remain stationary, the methodology by Baquerizo and Losada (2008) is used to predict the impact of the construction of a reservoir in the delta of the river Guadalfeo (Spain) and to illustrate how to deal with the uncertainties of the prediction for management purposes. Among other results, it is found that the probability that the shore retreats more than 120 m at any location is about 0.95, which allow to conclude that the construction of the dam will have a severe impact.

An integrated methodology to forecast the efficiency of nourishment strategies in eroding deltas

Many deltas across the globe are retreating, and nearby beaches are undergoing strong erosion as a result. Among soft and prompt solutions, nourishments are the most heavily used. This paper presents an integrated methodology to forecast the efficiency of nourishment strategies by means of wave climate simulations, wave propagations with downscaling techniques, computation of longshore sediment transport rates and application of the one-line model. It was applied to an eroding deltaic beach (Guadalfeo, southern Spain), where different scenarios as a function of the nourished coastline morphology, input volume and grain size were tested. For that, the evolution of six scenarios of coastline geometry over a two-year period (lifetime of nourishment projects at the study site) was modelled and the uncertainty of the predictions was also quantified through Monte Carlo techniques. For the most efficient coastline shape in terms of gained dry beach area, eight sub-scenarios with different nourished volumes were defined and modelled. The results indicate that an input volume around 460,000m3 is the best strategy since nourished morphologies with higher volumes are more exposed to the prevailing storm directions, inducing less efficient responses. After setting the optimum coastline morphology and input sediment volume, eleven different nourished grain sizes were modelled; the most efficient coastline responses were obtained for sediment sizes greater than 0.01m. The availability of these sizes in the sediment accumulated upstream of a dam in the Guadalfeo River basin allows for the conclusion that this alternative would not only mitigate coastal erosion problems but also sedimentation issues in the reservoir. The methodology proposed in this work is extensible to other coastal areas across the world and can be helpful to support the decision-making process of artificial nourishment projects and other environmental management strategies.

Estimating Final Scour Depth under Clear-Water Flood Waves

AbstractIn this paper we experimentally investigate the final scour depth around cylindrical piers under flood waves. Only clear-water conditions are considered. An approach to estimate the final scour depth under a flood wave is proposed, based on the local scour depth calculated with steady flow equations under peak flow conditions. The new experimental results are also used to investigate the performance of existing expressions or approaches to predict time evolution under steady and unsteady flow conditions.

A global model of a tidal jet including the effects of friction and bottom slope

This paper presents a global model describing the jet plane flow due to a tidal or river discharge. This model characterizes the geometry and the velocity field of the ebb-tidal jet in the whole domain including the effects of friction and bottom slope. The model does not have adjusting coefficients since they are obtained during the integration process. It can be concluded that higher values of the friction coefficient increase the jet width, whereas higher bottom slope reduces the jet width due to the incoming complementary water. Moreover, for small slopes, the area affected by the flow varies, essentially, with the friction coefficient, whereas for higher sloping bottoms this area can be assumed constant.

Integrating complex numerical approaches into a user-friendly application for the management of coastal environments

This paper presents a software platform to compute the total water level, one of the key variables for the environmental management of coastal zones. The platform integrates six modules: (1) simulation of deep-water wave variables, storm surge and river flow; (2) wave downscaling; (3) wave propagation; (4) contribution of the river discharge; (5) astronomical tide; and (6) total water level. It was applied to three case studies in southern Spain. The first case study consisted of designing the extension of a fluvial marina in a highly dynamic area (Guadalete estuary, Cádiz), and the maximum number of floating docks to avoid flooding events was obtained. The second case study involved calculating the operation conditions for navigation purposes in an inlet with sedimentation problems (Punta Umbría, Huelva), and a relationship between the percentage of operation hours and the dredged volume was obtained. The third case study consisted of estimating the number of overwash events as a function of the height of the berm on a deltaic beach with erosion issues (Guadalfeo, Granada), and a simple design curve to help managers during the decision-making process of artificial nourishment projects was provided. These results highlight the potential of the developed software, whose methodology is feasibly extensible to other coastal areas worldwide, to help managers handle a wide range of environmental problems related to the total water level. This is especially relevant due to the expected sea level rise in the coming years.

Protection of gravel-dominated coasts through wave farms: Layout and shoreline evolution

The impacts of wave farms (arrays of wave energy converters, or WECs) on the nearshore must be fully understood for wave technology to develop and thus contribute to a sustainable, carbon-free energy mix in the near future. The objective of this work is to investigate the role played by the farm layout on the wave propagation patterns leewards and the implications for longshore sediment transport (LST) and shoreline evolution on a gravel-dominated deltaic coast. Changes in wave propagation in four scenarios, corresponding to as many wave farm layouts, are computed by means of a spectral numerical model (Delft3D-WAVE) under (i) low-energy and storm conditions, and (ii) westerly and easterly waves - the two prevailing wave directions. On this basis, sediment transport rates are computed and changes in the shoreline position assessed using a one-line model. To quantify the impact of the wave farm on the nearshore wave conditions, sediment transport and shoreline, we define three ad hoc indicators: the non-dimensional wave height reduction, the non-dimensional LST rate reduction and the non-dimensional shoreline advance. Significant wave heights decrease in the lee of the wave farm, with the consequent reduction in LST rates. As a result, the dry beach area increases in every scenario under both westerly and easterly waves. We find that case studies with the WECs arranged on fewer rows but covering a greater stretch of coastline provide better coastal protection. These results confirm that wave farms can be used not only to generate carbon-free energy but also to protect gravel-dominated coasts.

Wave farm effects on the coast: The alongshore position

For wave energy to become a fully-fledged renewable and thus contribute to the much-needed decarbonisation of the energy mix, the effects of wave farms (arrays of wave energy converters) on coastal systems must be addressed. The objective of this work is to investigate the effects of wave farms on the longshore sediment transport and shoreline evolution of a gravel-dominated beach and, in particular, its sensitivity to the longshore position of the farm based on eight scenarios. Nearshore wave propagation patterns are computed by means of a spectral wave propagation model (SWAN), variations in sediment transport rates induced by the farm are calculated, and a one-line model is applied to determine the shoreline position and dry beach area. The significant wave height at breaking is reduced in the lee of the wave farm, dampening sediment transport. We find that changes in the dry beach area induced by the wave farm are highly sensitive to its alongshore position, and may result in: (i) erosion relative to the baseline scenario (without wave farm) in three of the eight scenarios, (ii) accretion in three other scenarios, and (iii) negligible effects in the remaining two. These results prove that the alongshore position of the wave farm controls the response of the beach to the extent that it may shift from accretionary to erosionary, and provide evidence of its effectiveness in countering erosion if appropriately positioned. This effectiveness opens up the possibility of using wave farms not only to generate carbon-free energy but also to manage coastal erosion, thus strengthening the case for the development of wave energy.

Prodeltaic Undulations and Hyperpycnal Flows (II): Evolutionary Trends

Undulation fields found on Mediterranean prodeltaic slopes usually display sub-surface precursory features recognized through seismic profiling and sediment core data. In this contribution we examine the stratigraphic patterns of sub-surface undulations occurring in wedge-shaped Holocene deposits that are in agreement with the episodic activity of hyperpycnal flows. Shallow sub-surface undulation facies show two fundamental features: lateral continuity of reflections and upward migration trends. Upward evolutionary trends may also be observed at the scale of the entire prodeltaic wedges. The most simple cases document repeated wavy stratified patterns or upward transition from sub-parallel to undulate. In some cases, however, the upward evolution can be very complex, with several stages ranging from non-development of undulations to pronounced wave migration, in addition to intermediate aggradational stages. Sedimentologically, sets of undulations exhibit variations between coarse and fine fractions. The sub-surface stratigraphic patterns of prodeltaic undulations are suggestive of enhanced development during evolving environmental conditions, mediated by the alternation of wet and dry climates characteristic of Mediterranean regions. Those conditions would have eventually favoured the activity of episodic hyperpycnal flows. Lateral changes of the internal undulation patterns also reveal the modification of fluvial flows and their imprint in the submarine environment.

Prodeltaic Undulations and Hyperpycnal Flows (I): Morphological Observations

Fields of submarine undulations occurring over prodeltaic slopes may be caused in some circumstances by the imprint of high-density sediment flows (i.e. hyperpycnal flows). This is the case of numerous Mediterranean settings, where marked seasonal climates and abrupt inshore physiography are conducive to the generation of these flows. In this contribution we examine the morphological patterns of undulations that are compatible with such an origin. Prodeltaic undulations occur off the offlap breaks on relatively low slopes (1°–2°) and show dominance of landward-directed forms. Morphological parameters of undulations such as heights, widths and lengths exhibit distinctive dimensions and tend to follow consistent patterns around nearby river mouths. In particular, height distributions tend to follow symmetric patterns, and vertical form indexes are lower than those of other sediment waves with a different genesis. These observations are compatible with geological processes under the influence of high-density sediment flows, such as different deposition rates due to enhanced bedload transport. Lateral changes of river mouths involving modifications of sediment flows are also imprinted in the geomorphological parameters of this kind of undulation.