Dynamics of wave-induced currents over an alongshore non-uniform multiple-barred sandy beach on the Aquitanian Coast, France

Abstract

This paper presents field investigation and numerical modelling of waves and wave-induced currents on a wave-dominated and non-alongshore uniform multiple barred beach. This study aims at establishing the first analysis of the dynamics of horizontal flows on the French Aquitanian coast. The spectral wave program SWAN is coupled with the time- and depth-averaged (2DH) coastal area model MORPHODYN. This coupled-model is applied to Truc Vert Beach, and results are compared with field data. From the 14th to the 19th of October 2001, a field experiment was carried out in order to characterize hydrodynamics and sediment transport over a complex bathymetry in the presence of oceanic wave conditions. From this data we calibrated three parameters: the bottom friction for wave propagation from the Aquitanian continental shelf to the nearshore zone, the spatially constant bottom friction coefficient due to waves and currents, and lateral mixing. Despite model approximations and the fact that the offshore wave boundary condition was located 15 km off the coast, the model is in good agreement with measurements. During weak wind conditions, computed waves and longshore currents fit well with field data on the ridge and runnel system. The strong tidal modulation of surf zone processes over this system is revealed. Hydrodynamics are strongly controlled by the beach morphology. For near-normally incident swells, the ridge and runnel system is responsible for a strong rip current located at the runnel outlet, associated with a circulation cell. Prediction of the tidal modulation and the sensitivity of the rip current to offshore wave conditions are in agreement with observations. Maximum rip current flow velocities occur approximately at mid-tide, which differs from what most researchers have found in other environments.