Energy Dissipation of Unsteady Wave Breaking on Currents

Abstract

Energy dissipation for unsteady deep-water breaking in wave groups on following and opposing currents, including partial wave-blocking conditions, was investigated by detailed laboratory measurements. A range of focusing wave conditions, including current strengths, wave spectrum slopes, and breaking intensities, were examined. Observations show that weak following and opposing currents do not alter the limiting wave steepness. The kinematics of unsteady breaking can be characterized as the one without currents simply by the Doppler shift. In contrast, strong opposing currents can cause partial wave blockings that narrow the spectral frequency bandwidth and increase the mean spectral slope. Dependence of the significant spectral peak steepness on the spectral bandwidth parameter was identified, confirming threshold behavior of breaking inception of nonlinear wave group dynamics. Loss of excessive energy fluxes due to breaking was found to depend strongly on the mean spectral slope. Wave groups of a steeper spectral slope yield fewer energy losses. In addition, the spectral distribution of energy dissipation due to breaking has the following two main characteristics: (a) significant energy dissipation occurred at frequency components that were higher than the spectral peak frequency, and little energy change at the peak frequency was found; (b) below the spectral peak frequency a small energy gain was observed. The energy-gain-to-loss ratio varies with the spectral bandwidth parameter. Higher gain? loss ratios (up to 40%) were observed for breakers on strong opposing currents under the partial wave-blocking condition. Comparison and assessment of proposed and existing parameterizations for breaking-wave energy dissipation were made using the measured data. The new proposed form provides the features for addressing these two main spectral energy distribution characteristics due to breaking with and without currents.