An Analysis of Wave Data for Wave Grouping

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Abstract
This paper has examined the change of wave grouping of storm waves during wave growth and decay by analyzing the field wave data observed off Gobo Coast in Wakayama Prefecture and at Lake Biwa in Shiga Prefecture (Japan) with some parameters to describe the magnitude of wave groups. For the mean lengths of runs and total runs of wave heights, the comparison of the observed results with the theoretical ones by Kimura (1980) has been done. Besides, the effect of shallow water on wave grouping has been discussed. The main results are as follows: (1) To describe the magnitude of wave groups, at least two parameters are needed; one is the magnitude of the sequence of high waves in a time series, and the other is the magnitude of the variation of wave energy or wave height around the mean value. (2) Concerning the wave data observed at Gobo, wave grouping was pronounced in the main stage of storms where three parameters of the correlation coefficient of successive wave heights, the mean length of runs and the variation coefficient of wave heights were large. (3) Concerning the wave data observed at Lake Biwa, wave grouping was not so noticeable in the main stage of storms, due to the effect of shallow water. (4) The mean lengths of runs and total runs become large in shoaling water and take maximum values around h/Ho=1.5, in which h is the water depth, and Ho the significant wave height in deep water. The groupiness factor proposed by Funke and Mansard (1978) becomes small in shoaling water and takes a minimum value around h/Ho=1.0. (5) The theory of run by Kimura can explain the observed mean lengths of runs and total runs generally. However, when the correlation coefficients of successive wave heights are larger than about 0.4, the theoretical values are a little smaller than the observed ones.

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