Free-Wave Energy Dissipation in Experimental Breaking Waves

Several transient wave trains containing an isolated plunging or spilling breaker at a prescribed location were generated in a two-dimensional wave flume using an energy focusing technique. Surface elevation measurements of each transient wave train were made at locations before and after breaking. Applying a nonlinear deterministic decomposition approach to the measured elevation, the free-wave components of the transient wave train were derived by excluding the contribution from bound-wave components. The comparison of the amplitude or energy spectra of free-wave components before and after a breaker can accurately reveal the energy dissipation as a function of frequency. It is found that the energy loss is almost exclusively from wave components at frequencies higher than the spectral peak frequency. Although the energy density of the wave components of frequencies near the peak frequency is the largest, they do not significantly gain or lose energy after the breaking. It is also observed that wave components of frequencies significantly below or near the peak frequency gain a small portion (about 12%) of energy lost by the high-frequency waves. These findings are quite different from the empirical formulas presently used for determining wave dissipation due to wave breaking. Hence, they have important implications to the ocean wave energy budget, specially to the energy transfer at frequencies below and near the spectral peak frequency.