Longshore sediment flux hindcast and potential impact of future wave climate change along the Gironde/Landes Coast, SW France
Résumé
The West-facing Gironde/Landes Coast, SW France, comprises about 230 km of high-energy, nonengineered, sandy beaches. It extends from the Gironde estuary to the North, and to the Adour estuary to the South. Overall, the coast erodes at a rate of about 1 to 2 m/yr, with localized areas with stronger erosion or with a stable or slow accretive trend. The gradients in longshore sediment transport and the sediment budgets are presumably the most important factor controlling decadal scale evolution of the shoreline along the Gironde/Landes Coast. It is established that the net longshore drift, which is only disturbed at the tidal inlet system of the Arcachon Lagoon and shoreward of the Capbreton Canyon, is oriented southward (Michel, 1997). However, its intensity and longshore variability, as well as how it will evolve in the framework of climate change, are still a matter of debate. This important issue is addressed in this contribution. First, longshore sediment fluxes are hindcasted over the period 1958-2002, using an ERA-40 wind forced wave modelling and the most commonly used empirical longshore drift formulae (Charles et al, 2012). Results are (1) discussed with respect to the existing estimations based on smaller periods and using different datasets; (2) compared the shoreline evolution measured during this period and (3) further analyzed to address the respective contributions of the different representative wave classes (e.g. swell, storm waves, wind waves) to the total longshore drift. Wave projections, based on wind fields from one climate model (ARPEGE-Climat) for several emission scenarios (A2, A1B and B1), are carried out to estimate possible wave climate change in 2061-2100 (compared to a so-called REF scenario covering the 1961-2000 period). It shows that, for instance for the A2 scenario and the summer season, the modal significant wave height is likely to decrease together with a 5° northward rotation of incoming wave incidence. Based on the wave model outputs, longshore sediment fluxes along the Aquitanian coast are computed for the four scenarios. The analysis of the relative sediment flux evolution (A2, A1B and B1 scenarios with respect to present day REF scenario) shows an overall decrease of the southerly longshore drift magnitude. A thorough analysis of the wave components evolution responsible for this decreasing trend is done, as well as an analysis of the longshore drift gradients along the Aquitanian coast. The potential impact of longshore drift evolution on the shoreline evolution at the 2061-20100 is discussed.