Nearshore subtidal bathymetry from time-exposure video images

[ 1] Time-averaged ( over many wave periods) nearshore video observations show the process of wave breaking as one or more white alongshore bands of high intensity. Across a known depth profile, similar bands of dissipation can be predicted with a model describing the time-averaged cross-shore evolution of organized wave and roller energy. This close correspondence between observed and modeled dissipation proxies is used to develop a new remote sensing technique, termed Subtidal Beach Mapper ( SBM), to estimate nearshore bathymetry. SBM operates on a time series of cross-shore intensity profiles to resolve the pattern in depth change on a morphological timescale ( including overall gain or loss of sediment) rather than to focus on the particular change induced by a single intensity profile. From each intensity profile, the breaking-induced component is isolated by removing the contribution of background illumination and persistent foam. The depth profile is updated based on a comparison between this video-derived dissipation proxy and a cross-shore profile of the dissipation of the roller energy. This updating is implemented through time-dependent mass balance equations for the seabed and a buffer layer above the bed. SBM was tested using 1 year of hourly video data collected at Egmond aan Zee, Netherlands. The dominant morphological changes observed from ground truth data were reproduced reasonably well, including the shoreward migration of the outer bar and the net sediment gain in the profile. Root-mean square differences between surveyed and SBM derived depth after 1 year of video-based depth updating with an average of about 70 intensity profiles per month were smallest (similar to 0.2 m) on the inner bar and largest (similar to 0.6 m) in the outer bar trough, with a profile average value of about 0.4 m. Despite the many processes included in SBM, the implementation of a heuristic scaling function in the mass balance equations to spatially adjust morphological growth rates was essential to these results, in particular near the shoreline, where otherwise the profile is prone to an unrealistic deepening.