Wave Breaking and Rip Current Circulation

This paper examines the effect of current-induced breaking on the modeling of wave breaking dissipation near rip currents. Simulations using a set of existing wave transformation models are compared to existing laboratory measurements made on a bar/channel topography in the presence of both strong and weak rip currents. The model/data comparisons demonstrate that existing models can effectively simulate the wave breaking near rips, but only by making large adjustments in the model coefficients used in the wave dissipation parameterizations. Since wave breaking formulations are typically calibrated without regard to the cross-shore currents, these results suggest that most model predictions will suffer large errors near rip currents. Furthermore, any errors in the modeled wave breaking dissipation will affect model predictions of the wave setup/setdown and, therefore, the wave driven circulation. A simple model exercise shows that predictions of the longshore pressure gradient, which is a driving force for the rip circulation cells, can be sensitive to the accurate simulation of wave breaking near rip currents. INTRODUCTION Rip currents are one of the more elusive features of the nearshore ocean; they are difficult to measure, difficult to model and predict. But despite their difficult aspects, it has been well established that, as with most nearshore motions, they are driven by wave breaking. In addition, since rip currents flow in the offshore direction, they can have a strong feedback on the incoming wave field. Typically, shoaling and wave breaking in the nearshore is induced by the steady decrease in depth as waves approach the shore. However, the presence of an opposing current will also cause waves to shoal, steepen, and eventually break. For waves propagating on seaward flowing rip currents, it is highly likely that some combination of depthand current-induced breaking occurs, and since rip currents tend to be very limited in their longshore extent, this implies that the dynamics of wave breaking can have strong variations in the longshore direction when rip circulations are present. WAVE DISSIPATION MODELING In order to model wave shoaling and breaking in the presence of opposing currents, it is convenient to use the wave action balance equation. In one horizontal dimension the wave action balance including dissipation is given by 1 Asst. Prof., Dept. of Civil, Constr., and Env. Eng., 202 Apperson Hall, Oregon State University, Corvallis, OR 97331-2302 USA. [email protected]. 2 Asst. Prof., College of Oceanic and Atmospheric Sciences, Oregon State University, Corvallis, OR, 97331-5503 USA. [email protected].