WAVE GROUPS - A CLOSER LOOK AT SPECTRAL METHODS

Two well known spectral approaches to wave group analysis are critically examined: the envelope theory combined with the discrete counting correction scheme (Longuet-Higgins, 1984), and the Kimura theory as modified by Battjes and Van Vledder (1984). In both cases, the mean length of discrete wave groups, j ($) over bar, is related through the narrow spectral bandwidth approximation, to some characteristics of the wave spectrum: the spectral width, v, and the spectral correlation coefficient, gamma(s), respectively. comparisons between the predictions of the models and the groupiness characteristics of both numerically simulated and field data are used to identify some deficiencies of the two methods over a wide range of ocean wave conditions. The limitations of the two methods due to the various assumptions employed are closely examined, and their effects on the models' predictions quantified. The discrete counting correction scheme of the first method is shown to use an incorrect probability distribution for the wave groups and also to neglect an important splitting effect. In the second approach, the spectral correlation coefficient is found to be consistently smaller than the putatively equivalent discrete wave correlation parameter, gamma(h), resulting in a systematic underestimation of the mean group length by up to 12%.