The impact of vegetation on the intensity and beat length of wave-wave interaction

This study investigates the impact of vegetation on the nonlinear wave-wave interaction. Both rigid cylinders and model salt marshes with varying meadow densities were tested under regular waves. The spatial evolution of wave amplitudes, skewness, asymmetry, energy spectrum, and bispectrum was analyzed to quantify the impact of vegetation on nonlinear wave transformation. Energy exchange between the primary and higher harmonics occurs periodically and reversibly, with a periodic length known as beat length. The presence of following (opposing) currents increases (decreases) the intensity and beat length of nonlinear wave-wave interaction. Under pure waves, the presence of vegetation slightly reduces the intensity without altering the beat length. However, submerged vegetation significantly enhances the influence of current on nonlinear wave-wave interaction, a new finding not reported previously. Specifically, the beat length increases (decreases) with the meadow density under waves with following (opposing) currents. We extended the classic theoretical prediction of beat length for pure waves to current-wave coexisting conditions, incorporating the vegetation effects through the frequency dispersion relationship for combined current and waves, with the near-surface time-mean current velocity adjusted by vegetation canopy.