Experimental study of bound triad interactions across a dissipative surf zone under different wave breaking conditions

This study experimentally investigates the development and decay of bound energy fractions due to positive and negative interactions (b(p)(2) and b(n)(2), respectively) applying bispectral analysis. It was revealed that the Ursell number is the most successful parameter to estimate the variation patterns of b(p)(2) and b(n)(2). The experimental waves included 27 waves with three offshore wave heights, three periods, and three offshore water depths propagating over a mild slope of 1:30. A wide variety of wave nonlinearity and bound coupling intensities were observed across the experimental dissipative surf zone. For a wave propagating toward the shore, the corresponding Ursell numbers for the relative maximum values of b(p)(2) and b(n)(2) are 0.27 and 0.22, respectively. The ratio of significant wave height to water depth corresponding to these extreme values for positive and negative triads are -0.4 and -0.32, respectively. The highest values of b(p)(2) and b(n)(2) for a progressive wave can be predicted as a function of the Ursell number using two new proposed experimental formulations. The sub-harmonics are more persistent during wave-breaking conditions than super-harmonics due to lower wave steepness. The biphase values of self-coupling of the spectral peak are attained negative for waves with Ursell numbers greater than 0.35.