Surf zone currents over irregular bathymetry: Drifter observations and numerical simulations

Surf zone currents over irregular bathymetry were observed with GPS-tracked drifters and modeled with the depth-integrated nonlinear shallow water equations. Trajectories of drifters released in 1-2 m depth sometimes defined rip currents and surf zone eddies, features that have been difficult to resolve with fixed instruments. The drifter-delineated surf zone circulation evolved as the tidal level changed during each 4-6 hour deployment. In one case, as the tide dropped, a shore normal rip current present for the first 2 hours evolved to a more shore parallel flow. In a second case, on a rising tide a well-developed bifurcated rip current was replaced by a weak, amorphous circulation. Rip current velocities were strongest near the surf zone edge and decayed by an order of magnitude within 2 surf zone widths from the shoreline. An eddy was observed to persist over a bathymetric depression within the surf zone for at least 1 hour. Observed and numerically simulated drifter trajectories define similar flow features, but the observed and modeled velocities differed by roughly a factor of 2, and flow evolution observed during roughly 0.5 m changes in tidal level were reproduced only if the simulated water levels varied by about 1 m. The alongshore momentum balance was spatially variable, with wave forcing balanced by pressure gradient, friction, and advective terms. The cross-shore momentum balance was everywhere dominated by wave forcing and pressure gradient terms of about equal magnitude. In the vicinity of a simulated rip current the residual of the cross-shore wave forcing and pressure gradient increased and was balanced by advection terms.