Surf Zone Currents

A large fraction of the water-wave energy incident on beaches is dissipated as the waves break and travel towards the shore through the surf zone. However, the momentum associated with the incident waves is not destroyed and drives other motions within the surf zone. An analysis is given of the unsteady and irregular currents that may occur in the surf zone. The forcing due to a discrete group of waves, and the vorticity of the surf-zone currents are the major topics. In particular, the almost two-dimensional nature of the flow implies that significant eddies are likely to be generated. There is evidence that eddies arise from long-shore currents, and the combination of eddies of opposite sign gives rip currents. It is noted that generation of vorticity by non-uniformities in bores may be a useful way of considering the forcing of surf zone motion. Expressions for the rate of increase in the circulation about material circuits and the vorticity generated at bores are derived. Received 30 January 1997 and accepted 16 May 1997