OBSERVATIONS OF TRIAD COUPLING OF FINITE DEPTH WIND-WAVES

Fetch limited finite depth wind waves are subjected to a bispectral an alysis 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 sig nificant 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 spect ral peak frequency. Previous studies of triad coupling have generally been confined to relatively shallow water. Values of the relative dept h, k(p)d (k(p) is the wavenumber of the spectral peak, d is water dept h) for these previous studies have ranged between 0.14 and 1.13. The p resent data set extends available data to values of k(p)d between 1.39 and 2.35, The existence of triad coupling at these water depths indic ates that models which are to be used to predict waves in the transiti onal water depths found on many continental shelves may need to includ e the effects of such interactions. Previously, it has been assumed th at triad interactions were generally only significant in the shoaling region. (C) 1998 Elsevier Science B.V.