Hybrid Functions for Nonlinear Energy Transfers in Third-Generation Wave Models: Application to Observed Wave Spectra

Hybrid functions (hereafter, HBL) has been newly presented as an efficient method to the full Boltzmann integral (FBI) for quadruplet wave–wave interactions at finite depths and also been tested for idealized wave spectra. Herein, the method is implemented in TOMAWAC, the third-generation wave model, and is validated between the existing approximate methods such as discrete interaction approximation (DIA), multiple discrete interaction approximate method (MDIA) and quasi-exact method, namely Gauss quadrature method (GQM), for observed wave spectra. The measured wave spectra obtained from the off-Kochi, along the southwest coast of India at a water depth of 21 m, are analyzed for southwest monsoon period in the year 2016, with an objective to understand the effect of nonlinear coupling due to quadruplet resonant interactions on composite spectra. The swell and sea components of the spectra are separated using peakedness algorithm, and the analysis shows that most of the spectra are swell dominated. The nonlinear coupling between swell and sea spectra is studied thoroughly, and it is ascertained that coupling is generally much larger in the vicinity of closer intermodal distance and mean directions.