Infragravity wave energy changes on a dissipative barred beach: a numerical study

Abstract

The influence of bars on the infragravity wave energy changes in dissipative beaches is analysed through numerical simulations of random wave propagation. The analysis of the simulated data includes the cross-shore evolution of the significant short and infragravity wave heights, the phase-lag between the radiation stress and the infragravity wave oscillations and the balance between the infragravity wave energy flux gradient and the rate of work done by the radiation stress. These simulated data were subjected to a sensitivity analyses aiming to assess the influence of the water depths over the bar crest and of the JONSWAP peak-enhancement factor. The presence of a bar induces partial short wave dissipation through depth-induced breaking. Over the bar, the infragravity wave energy flux gradient is almost balanced by the work done by the radiation stress for the analysed range of water depths and JONSWAP peak-enhancement factors. Nonlinear energy transfers between short and infragravity wave frequencies are therefore the most important physical process to explain the infragravity wave energy changes that occur over a dissipative bar. For the shallowest test case, infragravity wave dissipation, probably induced by depth-induced breaking, can play a role on the downward slope of the bar. The water depth over the bar crest controls the infragravity wave energy changes that occur between the bar crest and the shoreline. A transition range associated to Ursell numbers between 0.3 and 1 separates two clusters associated to nonlinear energy transfers. For Ursell numbers below this range, nonlinear energy transfers occur from the short to the infragravity wave frequencies. In contrast, for Ursell numbers above this limit they occur from infragravity to the short wave frequencies. A parametrization widely used in operational wave models performs satisfactory for Ursell numbers between 0.1 and 1 over the simulated dissipative barred beach profiles.