Rip Channels, Megacusps, and Shoreline Change: Measurements and Modeling

摘要

To investigate the relationship between alongshore rip channel migration rates and alongshore sediment transport rates, multi-year surf-zone video and wave datasets are examined at three sites along Monterey Bay, on the coast of California. Time-averaged, rectified video images are used to estimate daily rip migration rates, and the CERC formula is used to compute concurrent bulk alongshore sediment transport rates. Correlation coefficients between daily rates of transport and migration are low, but they improve with frequency- based filtering. While higher frequency migration events (on time scales shorter than eight days) are often obscured below the 'noise floor,' longer period oscillations (spring/neap tidal and seasonal cycles) show up more clearly. Cumulatively summed mean rip migration distance and net alongshore sediment transport correlate well (with correlation coefficient r = 0.76 - 0.94), indicating that an approximately linear relationship exists at longer timescales. To examine the nature of megacusp formation on rip channel bathymetries and identify dominant sediment transport components, five years of surf-zone video and ADCP wave data are analyzed and the XBeach 2DH nearshore model is applied in a series of simulations over realistic bathymetries. XBeach is shown to hindcast measured shoreline change with moderate skill for lower wave energies. A process-based analysis is used to identify significant forcing terms at mean, infragravity, and very-low-frequency timescales. Observations and model results both suggest that megacusps can form shoreward of either rip channels or shoals, depending on forcing conditions. In all model simulations, mean advective sediment transport plays the most important role in the creation of megacusps.