Wave overtopping of smooth impermeable seawalls under unidirectional bimodal sea conditions

Abstract

Reduction of risks due to overtopping of coastal defences is an essential requirement for the design, management and adaptation of coastal structures. Extensive knowledge is available on the prediction of overtopping of smooth, sloping and vertical impermeable seawalls. The existing prediction methods provide less certain guidance for combined wind and swell conditions. This experimental study focuses on overtopping characteristics of coastal seawalls under random bimodal waves. Both unimodal and corresponding bimodal conditions were simulated under different slopes and crest freeboards. Proportions of swell in the bimodal sea were varied in each test. Mean wave overtopping discharges were calculated for non-breaking random waves over at least 1000 waves. EurOtop formulae agree closely with our measurements from the unimodal experiments whereas they underpredict the observed wave overtopping rates for bimodal conditions even when using the recommended Tm−1,0 period for such cases. A modification to the EurOtop formula is proposed on the basis of the experimental results, to better capture the overtopping discharge under bimodal conditions.