Sandy Stocks on a Rocky Coast: States and Monthly to Pluriannual Evolutions
Résumé
In context of rocky coast, sandy stocks accumulated on the beaches can undergo very fast fluctuations. Erosion or deposition processes can be significant even sometimes at the temporal scale of a tidal cycle. Thus, knowledge of the available sediment stocks at the scale of the entire coastal system (from cliffs to subtidal zones) and transit dynamics within the system are fundamental to understand beach evolutions. The study site is located in the district of Bidart on the French Basque coast. The site is characterized by a succession of cliffs, rocky platform, perched and embayed beaches leading to a complex sedimentary dynamics, strongly constrained by the geological context. The longshore and cross-shore migrations of the sediments result of a fluctuating dynamics in time, depending of the tide, the seasonal waves energy, and the waves direction. This study proposes, from the compilation of several dataset and different observation tools (topo-bathymetric surveys, seismic surveys, sediment coring, airborne LiDAR, oblic-photography), (i) several quantification of the available sedimentary stocks at the scale of the coastal system, (ii) an analysis of the morphological evolutions of the subtidal and intertidal zones at interannual, seasonal and at event time scales. These results are discussed with high resolution bottom currents modelling from a phase-resolving non-hydrostatic wave model to identify the hydrodynamic conditions at the origin of the sedimentary movements and potential circulations between the subtidal and intertidal zone. Based on these works a quantified conceptual model is proposed in order to synthesize the complex exchanges between the various compartments of the shore and give new insight to guide a future sediment management plan.