Video observations and model predictions of depth-induced wave dissipation

Time-averaged video observations of the nearshore zone show the process of wave breaking as one or more white alongshore bands of high intensity, corresponding to the preferential location of breaking-wave dissipation on one or more sandbars. Across a known depth profile, similar bands of dissipation can be predicted with existing wave transformation models based on the wave energy balance. This opens up possibilities for estimating bathymetry from video observations by processing observed intensities into a model-predicted proxy of wave dissipation and the inverse modeling of this dissipation proxy into depth. The objectives of this paper are: 1) to present a technique for processing time-averaged cross-shore image intensity profiles into profiles that solely contain the contribution due to wave breaking and 2) to empirically determine which modeled wave dissipation proxy relates best to the cross-shore shape of the obtained breaking-induced intensity. The processing technique involves the removal of background illumination and noise, and, most importantly, a correction for white foam that after being generated in the breaking process remains floating at the water surface. This foam thus contributes to high image intensities but is not predicted by wave transformation models. Based on video and bathymetric data collected at the double-barred beach at Egmond aan Zee, The Netherlands and a standard wave transformation model containing the wave and roller energy balances, we find that the modeled cross-shore distribution of the dissipation of the energy of the surface roller (the white aerated mass of water at the breaking-wave front) matches the observed cross-shore shape of breaking-induced intensity well. Other hypothesized dissipation proxies result in a larger (> 10 m) seaward bias in the locations of maximum breaking-induced intensity and model-predicted dissipation or do not predict the cross-shore change in the area below the breaking-induced intensity bands correctly.