Laboratory study of non-linear wave-wave interactions of extreme focused waves in the nearshore zone

Abstract

Extreme waves play a crucial role in marine inundation hazards and coastal erosion. Prediction of non-linear wave-wave interactions is crucial in assessing the propagation of shallow water extreme waves in coastal regions. In this article, we experimentally study non-linear wave-wave interactions of large-amplitude focused wave groups propagating in a two-dimensional wave flume over a mild slope (β = 1 V 25). The influence of the frequency spectrum and the steepness on the non-linear interactions of focused waves are examined. The generated wave trains correspond to Pierson- Moskowitz and JONSWAP ( = 3:3 or = 7) spectra. Subsequently, we experimentally approach this problem by the use of a bispectral analysis applied on short time series, via the wavelet-based bicoherence parameter, which identifies and quantifies the phase coupling resulting from nonresonant or bound triad interactions with the peak frequency. The bispectral analysis shows that the phase coupling increases gradually and approaches 1 just prior to breaking, accordingly with the spectrum broadening and the energy increase in high-frequency components. Downstream breaking, the values of phase coupling between the peak frequency and its higher harmonics decrease drastically, and the bicoherence spectrum becomes less structured.