Strong Waves-Current-Turbulence Interactions in a complex environment : application to Alderney Race

Alderney Race, located in the English Channel, has strong tidal current (up to 5 m/s), a large tidal range (up to 11 m), wind-waves and swells, highly energetic marine turbulent cells (10-30 m width) and a very rough and uneven bottom with marine cliffs and dunes. In such conditions, it is necessary to investigate how these processes interact together to understand the hydrodynamics of the site and to assist the implementation of tidal converters. For this purpose, different techniques were employed during the HYD2M project (2016-2019). Real-time measurements by High-Frequency radar (1.5 years of data) showed a circulation pattern dominated by a main current vein flowing along the isobaths in a north-east, or south-west direction depending on the tidal stage. In addition, eddy-like structures were observed near the coast, especially during current reversal. In situ measurements (3 ADCPs, 1.5 years of data) have highlighted three-dimendimensional (3D) turbulent structures, characterized by a strong velocity around 3 m/s. Moreover, ADCPs have recorded energetic sea states, including a storm event with waves of 8 m height. Numerical modelling with a 3D fully-coupled wave-current model, that has been successfully validated with ADCP and HF data for this site, showed that the tidal current strongly affected sea states throughout refraction processes that changed wave direction and modulated the significant wave height, and current-induced wave breaking increasing the turbulent mixing inside the water column. Wave effects influenced the vertical profile of the current, even for small waves (wave height less than 1.5 m) and for strong surface current (up to 2.3 m/s). When the current became too high (in spring tide), wave effects on current were diluted. Vertical profile was mainly impacted by Stokes drift effects and by the enhancement of the bottom friction and also by the wave-induced turbulence near the surface and bottom. In contrast, the highly energetic turbulent structures of 10-30 m width were not very well reproduced by the model. However, thanks to recent developments dealing with the implementation of the Leray-alpha and LANS-alpha turbulent closures in our model, we were able to renergized the flow, that is a great sign for the future.