Wave climate and longshore drift on the South Aquitaine coast

The Aquitaine coast, the southern part of the French Atlantic coast, is exposed to very energetic waves, inducing large sediment transport rates and shoreline changes. Local wave characteristics are, however, not accurately defined. Neither is the longshore drift along the shore. In this paper, statistical parameters of directional waves are determined by using outputs over 8 years from numerical model WAVEWATCH III. Wave statistics obtained point out a very weak variability of peak directions, almost all events coming from the same 300 degrees direction, highly energetic extreme events and a predominance of swells over wind sea states. Representative sea states are calculated using a classification process. They are set as boundary conditions for the numerical model SWAN to compute the wave parameters along the breaking line and the corresponding longshore drift with the Kamphuis formula. The net annual longshore drift is assessed by taking into account the occurrence of the wave classes. Results show a reversal of the longshore current at the town of Capbreton. To the north, the current is directed southwards while it is northwards but very weak to the south. Locally, the pattern is reversed due to the influence of the submarine canyon of Capbreton. This new result is supported by in situ current measurements carried out along the shore. The annual average amount of sediment transported longshore is about 40,000 m(3)/year to the north of Capbreton and only 1000 m(3)/year to the south. However, along this latter area, the drift shows larger variations than in the north. Energetic swells account for more than two-thirds of the net annual longshore drift while they only occur 25% of the year. Moderately energetic swells and sea states with higher steepness, though more frequent, contribute less to the total drift. Around the town of Capbreton, results are dependent on the frequency spectrum set at the boundary of the numerical model, highlighting the complex role played by the submarine canyon and suggesting the need for further investigations in that area. (c) 2006 Elsevier Ltd. All rights reserved.