How the Wave-Current Interactions Are Modulated By the Horizontal Mixing inside a Rip System ?

Monday, 15 December 2014
Anne-claire Bennis, University of Lower Normandy, M2C laboratory, Caen, France, Franck Dumas, IFREMER, Dyneco/Physed, Plouzané, France, Fabrice Ardhuin, IFREMER, LOS, Plouzané, France and Bruno Blanke, Laboratoire de Physique des Océans, Brest, France
The mechanics of rip currents are complex, involving interactions between waves, currents, water levels and the bathymetry, that present particular challenges for numerical models. We study the effects of the horizontal mixing on the wave-current interactions with the 3D fully-coupled model MARS-WW3. First, very weak mixing are used at high resolution. Modifications of the vorticity and wave fields, by adding of feedback from the currents to the waves, are in agreement with previous studies. The flow is purely non-stationary and, after half an hour, several vortices appear inside both the rip neck and feeder. The decrease of the mixing accentuates the nonstationary effects and the generation of vortices. Second, we consider a grid-spacing dependent mixing. Comparisons with simulations that use a constant value for the viscosity coefficient are carried out for three different cases of spatial resolution. The mixing is stronger than previously, leading to stabilization of the flow and reduction of the rip intensity. Wave rays diverge from channels towards bar crests due to refraction by both the bathymetry and current field, showing that the wave motion depends on the rip intensity. When the mixing is based on grid spacing, a high resolution is necessary to observe significant effects of the feedback from the currents to the waves. A grid-independent solution is obtained for a constant viscosity. With a viscosity dependent on the grid spacing, the wave motion at low resolution is the same for both coupling modes because the change in wave direction due to the currents is weak. The fully-coupled simulations show similar depth-averaged rip velocities for all mixing cases and resolutions, showing the improvement in the numerical convergence of the velocity thanks to the feedback from the currents to the waves. Horizontal mixing and feedback have little impact on the vertical shear of the 3D velocity, but strongly affect the intensity of the rip velocity.