Modeling of very low frequency motions during RIPEX

Abstract

Numerical computations are used to explain the presence of Very Low Frequency motions (VLF's), with frequencies less than 0:004 Hz, in the rip current velocity signals observed during the RIPEX ¯eld experiment. Observations show that the VLF-motions are most intense within the surfzone and then quickly taper o® in the o®shore direction. By comparing computed VLF-intensity (Urms;vlf ) distributions in both the cross-shore and alongshore direction with observations in a qualitative sense, the most important contributions to the VLF-dynamics are established. VLF-motions at neighboring rip-channels are seen to interact in the computations, with stronger surfzone intensity for increasing bathymetric variation. The intermittent forcing by spatially varying wave-groups is essential in obtaining the correct Urms;vlf distribution in the cross-shore direction suggesting this is the predominant mechanism responsible for the generation of the VLF-motions observed during RIPEX. Computations also suggest that VLF-motions can occasionally propagate o®shore but are mostly con¯ned to the surfzone corresponding to surfzone eddies.