Wave-energy dissipation by bottom friction in the English Channel

Abstract

The energy dissipation by bottom friction of wind-generated surface-gravity waves is evaluated in relation to the seabed roughness magnitude in the English Channel (western Europe). The investigation is based on the phase-averaged wave model SWAN (Simulating WAves Nearshore) modified to account for a new parameterisation of the wind-drag coefficient at high wind speeds. Two formulations of the bottom-drag coefficient are evaluated: (1) the default constant empirical values derived from the JONSWAP experiment and (2) the eddy-viscosity model of Madsen et al. (1988) integrating the hydrodynamic conditions and the bottom roughness length scale considered successively constant and parametrised according to the grain size of bed sediments. Model performances are evaluated by comparing predictions with available measurements of the significant wave height and the peak period at (1) three offshore lightships and (2) two nearshore wave buoys off Le Havre and Cherbourg harbours. The heterogeneous bottom roughness length scale associated with the grain size of seabed sediments improves globally numerical estimates. Mappings of coastal regions influenced by bottom friction are produced exhibiting significant energy dissipation in areas of pebbles and gravels of the Normano-Breton Gulf and the surroundings of the Isle of Wight exposed to the incoming waves from the North-Atlantic ocean.