Numerical study of rip currents interlaced with multichannel sandbars

Abstract

Rip currents have become one of the primary coastal hazards globally. In the present work, the dynamics of a multichannel rip system are revealed by a group of numerical simulations over truncated sandbar bathymetry under varying wave conditions. It is found that increasing wave height may disperse rip currents due to strong wave-current shear in opposite directions and the resulting microbreaking, which is contrary to the widely accepted proportional theory. The pump-feed phenomenon between adjacent channels shows that rip currents might be absent in small channels when water flows through neighboring broader pathways. Rip currents are indicated highly sensitive to wave direction and deflect into undulant alongshore currents around sandbars as waves deviate from normal incidence. The decay of rip currents with wave angle did not show a clear dependence on channel width, which was found in previous studies. Vortices usually appear near bar-channel edges and by sides of rip currents. The magnitude of the vortex is not explicitly related to the wave height but increases with the wave angle. The water setup shoreward by sandbars substantially increases with wave angle, while the surface depression over rip channels is less observed. This indicates a lower pressure differential to drive feeder flows toward the channels and explains why rip currents cannot persist when incident waves become slightly oblique.