Bottom Friction for Wind Sea and Swell in Extreme Depth-Limited Situations

Abstract

A severe depth-limited storm and a case of extreme swell, which both occurred in the southern North Sea, are hindcasted with a regional version of the third-generation WAM model. This is done using different expressions for the bottom-dissipation source term: an empirical expression, the Hasselmann and Collins drag-law expression and an expression based on the eddy-viscosity concept. An efficient approximation to this eddy-viscosity model is presented, which is easy to compute and which yields accurate results. It is proposed to take a constant roughness value kN = 4 cm in this model, which corresponds to small sand ripples. The validity of this assumption is investigated. The hindcast results are compared with measurements from different stations and an estimate is made of the influence of the tide on the wave dissipation in the southern North Sea. The eddy-viscosity model is found to be a promising alternative to the empirical expression, which is used now in the WAM model.