Free-wave energy dissipation in experimental breaking waves

Several transient wave trains containing an isolated plunging or spilling b reaker at a prescribed location were generated in a two-dimensional wave fl ume using an energy focusing technique. Surface elevation measurements of e ach transient wave train were made at locations before and after breaking. Applying a nonlinear deterministic decomposition approach to the measured e levation, the free-wave components of the transient wave train were derived by excluding the contribution from bound-wave components. The comparison o f the amplitude or energy spectra of free-wave components before and after a breaker can accurately reveal the energy dissipation as a function of fre quency. It is found that the energy loss is almost exclusively from wave co mponents at frequencies higher than the spectral peak frequency. Although t he energy density of the wave components of frequencies near the peak frequ ency is the largest, they do not significantly gain or lose energy after th e breaking. It is also observed that wave components of frequencies signifi cantly below or near the peak frequency gain a small portion (about 12%) of energy lost by the high frequency waves. These findings are quite differen t from the empirical formulas presently used for determining wave dissipati on due to wave breaking. Hence, they have important implications to the oce an wave energy budget, specially to the energy transfer at frequencies belo w and near the spectral peak frequency.