Spectral Energy Balance of Breaking Waves within the Surf Zone*
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The spectral energy balance of ocean surface waves breaking on a natural beach is examined with field observations from a cross-shore array of pressure sensors deployed between the shoreline and the outer edge of the surf zone near Duck, North Carolina. Cross-shore gradients in wave energy flux were estimated from spectral changes observed between closely spaced sensors. Direct, empirical estimates of. nonlinear energy exchanges between different frequency components of the wave spectrum were obtained from observed bispectra using Boussinesq theory for near-resonant triad wave-wave interactions. The large decrease in energy flux observed across the surf zone in the energetic part of the wave spectrum is balanced closely by the estimated nonlinear energy transfers from the spectral peak to higher frequencies. In the high-frequency tail of the spectrum, observed energy flux gradients are small and do not balance the nonlinear energy transfers. This analysis indicates that the observed decay of wave spectra in the surf zone is primarily the result of nonlinear energy transfers to higher frequencies, and that dissipation occurs in the high-frequency tail of the spectrum where energy levels are relatively low.