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

Abstract

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.