On themechanisms of low-frequency wave attenuation by muddy seabeds

Low-frequency (LF) wave energy increases as the waves shoal into shallow waters. However, recent field observations reported an unexpected near-shore LF wave energy dissipation on muddy seabeds, which cannot be explained by the classic two-layer formulation. Therefore, this phenomenon has been ascribed to either direct dissipation or nonlinear energy transfer. We investigate, by means of a two-layer nonlinear model, the role of the wave nonlinearity and mud viscosity in controlling these two competing mechanisms of mud-induced LF wave attenuation. Bispectral analysis of the simulated cases reveals the existence of three distinct LF wave attenuation regimes, which determine if the LF wave energy losses are owing to either nonlinear energy transfer or direct dissipation. These regimes can be predicted based on the Ursell number, whereas the mud viscosity controls the amount of energy transfer. The present findings clarify apparent inconsistencies in the literature regarding the mechanisms of LF wave attenuation by mud.