Modelling the Risk by Submerged Structures in Sandbar Systems

Submerged structures have been used to protect beaches from erosion while, at the same time, maintaining visual appealing for tourism. Particularly, multi-purpose structures, inspired in coral reefs, have been of interest due to their improvement of surfing conditions (Black and Mead, 2001). Unlike emerged structures, the resulting currents are not straightforwardly deduced. For example, Ranasingue et al. (2006) identified two current modes (2-cell system and 4-cell system) mainly dependent on the distance of the multi-purpose structure from the shoreline. Later, Ranasingue et al. (2010) studied the current response to a shore-parallel submerged breakwater, reaching similar conclusions. In both cases, a non-barred profile was assumed. It is interesting that despite the ubiquity of sandbars in the coastal area, their influence has been overlooked so far. To the authors’ knowledge, only Bouvier et al. (2017) looked at the effect of a submerged structure on the morphology of a sandbar system in France. Although Bouvier et al. did not study the impact on the sandbar-related currents, the morphological response of the system is an indicative of the strong perturbation of nearshore currents. The present work aims to study the strengthening/weakening of rip currents by submerged structures in the presence of sandbars, and the associated risk to users. We first reproduce numerically the currents predicted by Ranasingue et al. (2006 & 2010) and then study a bathymetry characterized by a profile with a sandbar located in three different sites with respect to the structure (offshore, coupled and onshore) to take into account the natural cross-shore migration cycle of bars.

Black, K., Mead, S., 2001. Design of the gold coast reef for surfing, beach amenity and coastal protection: surfing aspects. Journal of Coastal Research, Special Issue 29, 115-130.

Ranasingue, R., Turner, I.L., Symonds, G., 2006. Shoreline response to multi-functional artificial surfing reefs: A numerical and physical modelling study. Coastal Engineering, 53, 589-611.

Ransingue, R., Larson, M., Savioli, J., 2010. Shoreline response to a single shore-parallel submerged breakwater. Coastal Engineering, 57, 1006-1017.

Bouvier, C., Balouin, Y., Castelle, B., 2017. Video monitoring of sandbar-shoreline response to an offshore submerged structure at a microtidal beach. Geomorphology, 295, 297-305.