Free Long Wave Generation: Breakpoint Forcing Versus Bound Wave Release

When wind waves break in the nearshore, free long (infragravity) waves are generated through two mechanisms: breakpoint forcing and bound wave release. Previous studies have highlighted that lower frequency groups breaking on steep slopes favor long wave generation by breakpoint forcing, whereas higher frequency groups breaking on mild slopes favor bound wave release. However, no study has established and demonstrated quantitative thresholds for when each mechanism will dominate. In this paper, we use a one-dimensional linear numerical model to quantify each mechanism and assess their relative dominance in free long wave generation, as a function of wave and bathymetry parameters. The results show that the individual contributions of breakpoint forcing and bound wave release do not add up linearly to the total long wave energy generated from both mechanisms due to the phase differences between free long waves generated from each mechanism. The combination of mechanisms always results in a net smaller long wave amplitude. The normalized bed slope parameter is shown to be effective in differentiating between the dominance of the breakpoint-forced and bound wave release mechanisms. The efficiency of breakpoint forcing is found to be independent of bed slope, wave group frequency and short-wave period. Therefore, the relative importance of each mechanism to long wave generation depends mainly on the efficiency of the bound wave release, which varies with these parameters. Overall, the bound wave release mechanism tends to dominate under most conditions, except for cases with low infragravity frequencies that occur on steep slopes (e.g., typical of reef environments).