Alejandro López-Ruiz

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.

Importance of Nearshore Waves on Mixed Sand and Gravel Coasts

When waves propagate from deep water toward the coast, they modify their properties due to their interaction with the seabed. The Mediterranean basin is characterized by narrow and complex inner shelves that influence the properties of the nearshore waves. This chapter models the wave propagation patterns at the two study sites and highlights the importance of the role played by the inner shelves in the coastal hydrodynamics. A calibrated wave propagation numerical model is applied for that purpose. The results reveal the importance of the feedback process between the forcing (mainly nearshore waves) and the morphological response (changes in the bathymetry) in the evolution of these systems.

Morpho-Sedimentary Dynamics of Mixed Sand and Gravel Coasts

This chapter addresses the changes in the morphology and sedimentology of a micro-tidal mixed sand and gravel beach (Playa Granada, southern Spain) forced by wave and water-level variations, and human intervention through nourishment. Monthly and storm event-driven beach surveys, consisting of topographical measurements and sediment sampling in two selected areas, were carried out over a one-year period. Three prevailing sediment fractions (sand, fine gravel and coarse gravel) and two end-member morphological states of the upper beach profile (convex with multiple berms and concave with a single storm berm) were identified. Between them, several transitional profiles were formed, characterized by developing berms that progressively overlapped, generating sediment variability both across the beach profile and with depth. The results indicate that the total run-up (including water-level) reached during an event represents a more accurate threshold for differentiating between erosional and depositional conditions than wave height. They also suggest that mixed sand and gravel beaches recover faster from storm erosion than sandy beaches. The long-term benefit of the artificial nourishment that took place at the end of the survey period was very limited and this is attributed to the too fine sediment used for the nourishment and its placement too high on the beach. Clearly, nourishment interventions must take into account the natural sediment distribution and the profile shape to avoid rapid losses of the nourished sediment.

Littoral Drift and Coastline Evolution on Mixed Sand and Gravel Coasts

The evolution of the coastline is mainly driven by the gradients in the longshore sediment transport (LST). These gradients are mostly influenced by the incoming waves and the morphology of the coastline. Traditionally, the coastline is assumed to be quasi-rectilinear, and the formulations are not valid for curved shorelines. This chapter presents a new expression that not only accounts for the curvature of the shoreline but also includes the variability of the sediment size that is typically found along Mediterranean coasts. It is later applied to the study sites described in Chap. 2, and the relation between LST trends and coastline evolution is analyzed and discussed for both sites.