Wave and Roller Transformation Over Barred Bathymetry

The cross-shore transformation of breaking-wave roller momentum and energy on observed barred surfzone bathymetry is investigated with a two-phase Reynolds Averaged Navier Stokes model driven with measured incident waves. Modeled wave spectra, wave heights, and wave-driven increases in the mean water level (setup) agree well with field observations along transects extending from 5-m water depth to the shoreline. Consistent with prior results the roller forcing contributes 50%-60% to the setup, whereas the advective terms contribute similar to 20%, with the contribution of bottom stress largest (up to 20%) for shallow sandbar crest depths. The model simulations suggest that an energy-flux balance between wave dissipation, roller energy, and roller dissipation is accurate. However, as little as 70% of the modeled wave energy ultimately dissipated by breaking was first transferred from the wave to the roller. Furthermore, of the energy transferred to the roller, 15%-25% is dissipated by turbulence in the water column below the roller, with the majority of energy dissipated in the aerated region or near the roller-surface interface. The contributions of turbulence to the momentum balance are sensitive to the parameterized turbulent anisotropy, which observations suggest increases with increasing turbulence intensity. Here, modeled turbulent kinetic energy dissipation decreases with increasing depth of the sandbar crest, possibly reflecting a change from plunging (on the steeper offshore slope of the bar) to spilling breakers (over the flatter bar crest and trough). Thus, using a variable roller front slope in the roller-wave energy flux balance may account for these variations in breaker type. As ocean waves break in shallow water near the shore, they generate a "roller," the foamy white bubble-laden air-water mixture at the top of the breaking wave. The water in the roller carries energy and momentum onshore, which affects the magnitude of flows and water level changes caused by breaking waves. Here, the roller evolution is investigated with a numerical model that includes the breaking-wave air and water mixture. The model simulates breaking waves and water levels observed near a sandbar, and shows that the roller transports momentum shoreward and damps wave energy. In addition, the results suggest that the depth of water over the sandbar may affect the shape of the breaking waves, with the roller relatively more important when water is deeper. Other mechanisms, such as turbulence in the water column, friction from the seabed, and the inertia of currents may affect how momentum is distributed in the system. A two-phase RANS model accurately simulates random wave transformation and wave-driven setup observed on barred ocean beaches 15-25% of the wave energy transferred to the roller is dissipated in the water column below and the rest is dissipated within the roller The simulations suggest that using a variable roller front slope in roller parameterizations may account for differences in breaker type