Estimation and characterisation of the wave-induced turbulent kinetic energy and turbulent dissipation from ADCP data

Abstract

Turbulence in the water flow causes small-scale variations in the mechanical stress acting on submerged tidal turbines. As such it increases their fatigue loading and impacts greatly their lifetime. It is therefore essential for engineers to have an accurate knowledge and characterisation of turbulence at a given site as they design the structures to be deployed. The strength of the tidal currents is the main parameter influencing the intensity of turbulence through their friction with the sea bed. However, most potential tidal energy sites are located in a coastal environment with shallow enough water depths so that the direct impact of waves on turbulence can not be overlooked. Steepness-induced wave breaking is indeed observed to increase the turbulent mixing for such applications. In this context, we propose to estimate the contribution of surface processes to the total turbulence in Alderney Race, France, the most energetic tidal site in western Europe. The turbulent kinetic energy (TKE) specifically induced by waves and wind is characterised using measurements from a 5-beams ADCP deployed between 27/02/2018 and 06/07/2018.Analytical profiles are fitted to the data, the only fitting parameter of the model is an evaluation of the turbulence penetration depth, it determines how deep surface processes impact the water column. Its dependence towards mean wave and current parameters is studied. The results do not allow to conclude on the nature of turbulence observed in the mid water column.