Interaction of Short Wind Waves and Swell: An Experimental and Numerical study

The accurate modelling and prediction of bimodal sea states, combining swell and wind waves, is of upmost importance for many applications such as wave overtopping of coastal protections. Yet, the discrepancy in the field observations and the wave model limitations make the modeling of this common sea state condition rather complex. The question guiding this paper is: are wind waves generated the same way with and without pre-existing swell? The approach we chose starts with laboratory measurements in a wind-wave facility showing that wind-sea growth is modified in the presence of long waves (representing swell). To upscale this observation to open oceans, a numerical spectral wave model is firstly validated by comparison with laboratory results, and then at coastal scale using in situ bimodal sea-state observations collected during the SHOWEX campaign. By a separation of the physical processes involved in wind-wave generation, numerical simulations allow to assess the role each physical process plays in the wind-wave growth when a swell system is present.