Etude de l'hydrodynamique d'une zone côtière anthropisée: L'embouchure de l'Adour et les plages adjacentes d'Anglet

Over the last decades, coastal areas have become a favourite site for touristic, recreational activities, and housing estates. Thus, they draw considerable attention and pressure to development. Traditionally this attention has not been matched with knowledge, resulting in a proliferation of coastal problems. In many places, the natural erosion of sandy coasts has often been accelerated by anthropic activities. The Anglet coastal area is an example of this. The beaches have been decreasing for decades and the Adour River mouth, located northward, is subject to important silting, which results in dredging activities twice a year. Several groins have been built since 1970 in order to control these two phenomena but they are only partially effective. In this context, the conurbational council of Bayonne-Anglet-Biarritz had financed a research program. The thesis involved is aimed at improving the hydro-sedimentary behaviour of the coastal area by experimental and numerical means. The continuous tendency and constant rate of erosion were underscored by an analysis of the evolution of the beaches based on 24-year period surveys. The hydrodynamic forcings, and especially the offshore wave climates, were studied (Chap. II). A field campaign was carried out in March 2003 in order to increase the knowledge of the physical processes and to obtain field data for the evaluation of numerical models. Under low energetic conditions, an energy concentration due to refraction of waves above the dredging spoil was observed in the south of the River mouth. In this local point, the currents were directed to the mouth at ebb and flood tides (Chap. III). The selected numerical models (phase-resolving and phase-average wave propagation models & 2DH hydrodynamic model) were assessed by comparing field measurements of March 2003 with numerical results (Chap. IV). A coupled Tomawac/Telemac 2D approach was adopted to describe the circulation induced by representative forcings (wave, tide, wind, river flow). Hydrodynamic regimes were obtained under realistic conditions based on statistical analysis. The coupled contribution of waves and tide was also studied (Chap. V). The action of tides is underscored at the boundaries of the system where the sediment transport rate is not significant even under high energetic conditions. Moreover, the combined effect of waves and tides accounts for the sediment transit from the South to the North which results in the erosion of Anglet beaches and the silting of the River mouth. Finally, the major effect of waves is brought under attention near the Adour River mouth where the sediment transport rate is significant; all the more as the energetic conditions are high (storm events and swells).