Modelling infragravity motions on a rip-channel beach

A non-linear shallow water wave model operating on the time-scale of wave groups is compared with measurements of infragravity motions on a rip-channel beach to verify the model concepts and assess the model performance. The measurements were obtained during the RIP-current EXperiment (RIPEX) in concert with the Steep Beach Experiment (SBE) performed at Sand City, Monterey Bay, CA, during the spring of 2001. The nearshore bathymetry was made up of shore-connected shoals incised by relatively narrow rip-channels spaced approximately 125 m apart. The comparison considers a 20-day period during which significant changes in both the offshore wave climate and nearshore bathymetry occurred. The temporal variation in infragravity conditions during the experiment is strong, with computational results typically explaining 70% to 80% of the observed infragravity motions within the nearshore. In contrast to the temporal variation, the alongshore spatial variation in infragravity intensity during the experiment is generally weak, even though the underlying bathymetry shows strong depth variations. Model computations suggest preferential coupling between the computed edge wave motions and the quasi-periodic bathymetry is present, a prerequisite for strong spatial variability. However, the infragravity field is dominated by cross-shore infragravity motions, which are only weakly coupled to the quasi-periodic bathymetry, resulting in a weak alongshore variability of the total infragravity motions.