Tidal influence on the littoral drift of a beach with complex bathymetry: example of the Vougot beach, Guissény (France)
Résumé
Rocky coastal zones, including beaches constrained by rocky headlands, represent approximately 80% of the world's coastline. In such coastal environments, with complex topo-bathymetry, the offshore wave conditions can be very different compared to the conditions at the wave breaking point due to refraction and diffraction processes that play an important role in nearshore wave transformation. The obliquity of waves at the breaking point controls the direction and intensity of the longshore drift and the induced sediment transport. In addition, on macrotidal beaches, tides induce water level variations that strongly impact nearshore current circulation. The present study analyses longshore currents in the surf zone of a sandy beach in a complex coastal macrotidal environment: Vougot beach (France). Wave refraction is shown to invert the predicted longshore current (wave direction changed by almost 90°). In the surf zone, the current is first driven by the tidal current. The wave climate influences the magnitude: the westward longshore current reaches 0.6 m/s during an energetic event and is proportional to the significant wave height. Commonly used formula to predict wave-generated longshore currents failed to reproduce observations. Numerical modelling is in progress to test the influence of tides and waves on surf zone currents.