Wave-driven vortex dynamics in the surf zone

A longstanding problem in coastal oceanography is the prediction of the longshore currents that are forced by breaking waves in the surf zone. Traditional models based on a ID momentum balance for the surf zone predict that current should be strongest in regions of the strongest wave breaking. However, current on a beach with a sandbar is sometimes observed to have a maximum in the "trough" of the bar, far from the region of maximum wave breaking.; In this thesis, we propose a mechanism for the development of this current based on vortex dynamics and show; based on idealized studies, that it can explain the broad features of the observed current. These studies are pursued using a new numerical model which exploits wave-mean interaction results in order to model the effect of breaking deep water waves, without resolving the waves themselves.; We focus on the role of wave group variation rather than bathymetric irregularities or shear waves, which we argue have been explored and found to be not enough to produce the expected behavior. Our first experiment examines beaches forced by isolated packets of obliquely incident waves. We find that current dislocation occurs on a barred beach (to the trough low) but not on a planar beach, and no dislocation occurs in the absence of wave group variations.; Our second experiment examines beaches forced by sinusoidally varying wave groups of the approximate spatial scale suggested by the observed wind speeds during the DELILAH experiment. We again find that current maxima develop in the trough region and that a relatively simple parameter, given the restricted set of forcings, will predict whether or not this occurs. This is a first step towards determining when realistically varying wave group forcing will produce current dislocation.; We also consider the existence of turbulence in shallow water with topography. We find that the physical scales governing the beach do not permit the development of vigorous two-dimensional turbulence, and specifically the generation of vortices larger than the spatial scale of circulation forcing.