Observations of triad coupling of finite depth wind waves

Fetch limited finite depth wind waves are subjected to a bispectral analysis to determine the extent of triad nonlinear coupling. Very long yet stationary time series are utilized, enabling the determination of bicoherence values with significantly smaller confidence limits than have been achieved previously. The bispectral analysis indicates a significant degree of phase coherence between the spectral peak frequency and higher frequencies. It is concluded that this phase coherence is as a result of non-resonant or bound triad interactions with the spectral peak frequency. Previous studies of triad coupling have generally been confined to relatively shallow water. Values of the relative depth, k_pd (k_p is the wavenumber of the spectral peak, d is water depth) for these previous studies have ranged between 0.14 and 1.13. The present data set extends available data to values of k_pd between 1.39 and 2.35. The existence of triad coupling at these water depths indicates that models which are to be used to predict waves in the transitional waterdepths found on many continental shelves may need to include the effects of such interactions. Previously, it has been assumed that triad interactions were generally only significant in the shoaling region.