Observations of waves' impact on currents in a mixed-energy tidal inlet: Arcachon on the southern French Atlantic coast

Coastal morphodynamic processes around tidal inlets in mixed-energy environments are particularly complex due to severe tide and wave conditions. Collecting data in this area is generally very challenging. Here we present the first hydrodynamic data collected in the outer inlet of the Arcachon lagoon, a mixed-energy inlet situated on the southern part of the French Atlantic coast. Data consist in vertical current profiles and sea surface elevations collected on the offshore edge of the ebb delta under various tidal and wave conditions. In particular data were collected during the severe Joachim storm associated to wave heights up to 8 m. Preliminary results indicate that current profiles, intensities and direction are very sensitive to wave conditions. Under energetic conditions, the vertical profile of currents become uniform and the longshore drift is enhanced (up to 1.5 m/s). Nevertheless data show that under severe wave conditions (Hm(0) > 6.0m) and despite high incidence of waves, the longshore drift is weaker (I<1 m/s) than under 'usual' storm conditions associated to Hm(0) = 4 5m. More surprisingly, directions of the longshore drift are not always consistent with wave incidence under those severe storm conditions and the cross-shore components are very weak while under less energetic conditions we observe intense offshore currents (up to 0.8 m/s).