Jets, Circulation Cells, and Meanders: What Controls Flow Patterns in Bar-Channel Systems?

Nearshore circulation patterns driven by breaking waves are controlled by the geometry of nearshore bathymetric features including crescentic or channel-incised sandbars. In these systems, the surfzone circulation pattern can range from rip-current jets with symmetric feeder currents, rip-current jets pushed to one side of a channel, pairs or single circulation cells, and meandering or uniform alongshore currents. These complex circulation patterns impact sediment transport, morphological evolution, transport of biota and pollutants, and swimmer hazards. Although the forcing that drives alongshore currents and bathymetric rip currents is well known, further work is needed to understand what controls the spatial patterns–including cross-shore and alongshore positions of flow maxima and reversals–of surfzone jets, circulation cells, and meanders. Here, idealized numerical modeling of circulation and waves (COAWST: coupling ROMS and SWAN) in channel-incised sandbar systems is used to investigate the circulation-pattern response to the size and asymmetry of channels during a range of incident wave heights and directions. The model results and an analysis of wave-averaged momentum balances are used to develop a framework that classifies circulation patterns as a function of wave forcing and bathymetry.