Alongshore rip channel migration and sediment transport

Abstract

The relationship between alongshore rip channel migration and sediment transport is investigated using time-averaged video images to identify the positions of rip channels at three sites along southern Monterey Bay, California, over 1–3 year periods. Daily migration rates are calculated for each site. The basic Coastal Engineering Research Center (CERC) formula for bulk alongshore sediment transport is used to estimate alongshore transport rates at each site, based on hourly input of directional wave spectra measured at an offshore buoy and shoaled and refracted to the nearshore. The wave transformation model is validated by comparison with measured nearshore directional wave spectra in 13 m water depth. An expanded CERC formulation that accounts for the additional forcing of alongshore wave height gradients is also tested but rejected. Correlation coefficients between daily rates of calculated alongshore transport and rip channel migration are low: 0.01, 0.44, and 0.38 at the Sand City, Stilwell, and Marina sites, respectively. The problem of digitization noise, associated with video-based rip channel identification, is discussed and several filtering methods are used to increase the signal to noise ratio and obtain improved correlations. Results indicate that higher frequency migration events (on time scales shorter than 8 days) will likely be obscured below the digitization “noise floor”, but that longer period oscillations (such as tidal and seasonal cycles) show up clearly in both rip migration and sediment transport datasets. Daily rates are cumulatively summed to generate time series of representative overall rip migration distance and net alongshore transport, and the cumulative summing procedure is shown to be similar to applying a low-pass filter. Mean migration is demonstrated to lag behind net transport in the summertime, suggesting that a minimum level of alongshore forcing is required to generate detectable rip migration. Rip channel migration and net alongshore transport have correlation coefficients that range from 0.76 to 0.94 at the three sites.