Effects of Triad Interactions on Wave Attenuation by Vegetation

Abstract

Wave attenuation in vegetated shallow water is mainly attributed to actual drag-induced dissipation and near-resonant triad interactions. The latter is neglected in existing theoretical models for vegetation-induced dissipation. In this study, a set of evolution equations describing the spatial evolution of three near-resonant wave components was extended to include vegetation effects. The contributions of triad interactions to the attenuation of individual harmonics were investigated. The authors found that neglecting the energy cycling caused by triad interactions gives an underestimation of damping rates. The triad interactions have greater effects on the attenuation of higher harmonics. A fully nonlinear, fully dispersive wave model was used to explore the effects of triad interactions on spectral dissipation of random waves. After isolating the energy transfers caused by triad interactions and the actual drag-induced dissipation, the authors found that triad interactions transfer energy to higher harmonics, which experience greater damping. Due to the energy transfers from spectral peak (fp) to high frequencies (2fp), the energy losses in fp and 2fp are overestimated and underestimated, respectively, by existing models based on linear wave theory.