Impact of extreme wave-current interactions on the tidal stream energy estimate
Résumé
Tidal energy is a promising way to produce electricity from tidal current. The most energetic tidal site in western Europe is the Alderney race, located between La Hague Cape and the Alderney island. High winds sweeping across the English Channel make wave-current interactions could be crucial to the tidal energy estimate. Pioneer works of Lewis et al. (2014) and Guillou et al. (2016) investigate how wave-current interactions modulate this estimate for several UK tidal sites and Fromveur (France). Bennis et al. (2018) continue these former studies by performing realistic numerical simulations specially for Alderney Race, that are validated with in-situ data. They show that ocean waves reduce or increase the tidal estimate by 10% at 10 meters depth, which is in agreement with former conclusions. Moreover, the inclusion of waves significantly improves the simulation of the vertical structure of the flow, with an acceleration or deceleration of the current velocity near the surface according to the direction of current and waves. These results are for low/medium hydrodynamic conditions (waves and tides). This work is pursued with an analysis of the consequences of the wave-current interactions in extreme conditions on the tidal stream energy estimate: wave height is about 8 meters and tidal range is about 10 meters. On the whole, in-situ measurements and model results are in agreement. The wave model underestimates a few the wave height, while wave period and wave direction are correctly simulated. Refraction effects of waves by current are strong leading to large changes in wave direction. Alderney Race having shallow to intermediate waters, we observe a strong enhancement of the bottom friction by waves for this extreme sea state, that changes the flow near the bottom. Stokes drift effects are dominant when the tidal flow becomes weak between two tides. Near the surface, wave breaking and Stokes drift significantly affect the vertical shape of the flow, producing an homogenization due to mixing and a shearing caused by Stokes drift. In the future, we will work on wave breaking and its impact on the water column in terms of turbulence.