Spectral modeling of wave dissipation on negative current gradients

Hindcast studies for the Dutch Wadden Sea using the spectral wind wave model SWAN have shown the significant influence of currents on wave predictions in the tidal inlets. In a number of cases with strong gradients in opposing, partially blocking current, wave heights are significantly overestimated. Ris and Holthuijsen (1996) propose that such overestimations are due to insufficient steepness dissipation of waves on an opposing current gradient. The present paper presents a new formulation for the enhanced breaking dissipation of waves on negative current gradients (accelerating opposing current; decelerating following current). Nonlinear effects are not included in detail for these partial blocking conditions, but handled parametrically. Unlike the expression by Ris and Holthuijsen (1996), the proposed expression isolates the steepening effect of the current gradient on the waves, so that inherently steep young wind sea is not overly dissipated. This expression contains one additional unknown parameter, which was calibrated using laboratory observations. Validation of this enhanced dissipation term for field cases of the Amelander Zeegat tidal inlet (Dutch Wadden Sea) shows an improvement in the tidal channel for both opposing and following current situations with negative gradients. In particular, the results for the young wind sea on the tidal flats are not significantly affected, as desired, unlike with the expression of Ris and Holthuijsen (1996). However, since the remaining dissipation terms in SWAN have been calibrated without this enhanced dissipation term, the addition of the proposed formulation results in some deterioration of the overall statistics. (c) 2012 Elsevier B.V. All rights reserved.