Modeling rip current circulations and vorticity in a high-energy mesotidal-macrotidal environment

Abstract : In June 2007 an intense 5 day field experiment was carried out at the mesotidal-macrotidal wave-dominated Biscarrosse Beach on a well-developed bar and rip morphology. Previous analysis of the field data elucidated the main characteristics of a tide-modulated and strongly evolving rip current driven by low- to high-energy shore-normal waves. Here we present a modeling strategy based on the vertically integrated and time-averaged momentum equations accounting for roller contribution that is applied to the Biscarrosse experiment. Wave and flow predictions in the surf zone improve significantly when using a spatially constant time-varying breaking parameter by Smith and Kraus (1990). The model correctly reproduces the main evolving behaviors of the rip current. An advection-diffusion equation governing the mean wave-driven current vertical vorticity is further derived from the momentum equations. Vertical vorticity is driven by a forcing term that depends on the breaking wave energy dissipation and on the wave propagation direction. Spatial gradients in depth-induced broken-wave energy dissipation therefore determine both the strength and the sign of the wave-driven circulation rotational nature. When applied to the Biscarrosse experiment, the vorticity efficiently predicts the main characteristics of the evolving rip current such as its width, cross-shore extension, and intensity. In addition, good correlations are found between the maximum rip current intensity and the deviation of the forcing term. Thus, we determine precisely the rotational component associated with the wave forcing which is less direct through the traditional radiation stress approach.