Parameterization of nearshore wave breaker index

The performances of phase-averaged parametric nearshore wave transformation models depend significantly on a reliable estimate of the wave breaker index gamma (the breaker height-to-depth ratio), a free model parameter that essentially controls the amount of breaking energy dissipation. While the previous studies have suggested separate relationships between gamma and the offshore wave steepness (s0) or the normalized local water depth (kh), the parameterization of gamma still requires further investigation considering a wider variety of conditions and a sounder physical basis. In this study, we use the field datasets of wave height and the inverse modelling approach to reveal a composite dependence of gamma on both s0 and kh. Specifically, the results show a positive dependence of gamma on kh for larger s0, and a negative dependence of gamma on kh for smaller s0. Based on such composite relationships, a new gamma formula is proposed, and its performance is verified against the available datasets of wave height in three coasts and 14 laboratory tests. Implementation of this new formula in a parametric wave model leads to the error reduction of wave height prediction by 10-24% (mean = 19%) relative to seven widely used models in literatures. In particular, a remarkably higher model accuracy is obtained under wave conditions with small offshore wave steepness, which is important for studying onshore sediment transport and beach recovery. Two counteractive physical mechanisms for wave nonlinearity effects, namely the breaking intensification mechanism and the breaking resistance mechanism, are suggested to explain the opposite gamma-kh relationships within different ranges of s0.

Automated summary

The study reveals a composite dependence of the wave breaker index gamma on both offshore wave steepness and normalized local water depth, proposing a new gamma formula that significantly improves wave height prediction accuracy. There are two counteractive physical mechanisms at play, the breaking intensification mechanism and the breaking resistance mechanism, explaining the opposite relationships between gamma and water depth under different offshore wave steepness conditions.