EXPERIMENTAL-STUDY OF WAVE GROUPS IN DEEP-WATER RANDOM WAVES

Results from a laboratory study of wave groups in random seas are presented for Bretschneider and JONSWAP sea states. A distinction is made between general crest-envelope crossings of a threshold level and those involving groups of two or more wave crests. Measurements show that the average duration of a run of high waves in groups and the average interval between such groups are significantly greater than the values found for general envelope crossings, since many large waves occur by themselves and not in groups. Experimental results for these average values, as well as for probability density functions of time durations and intervals, are found to be characterized broadly by linear similarity relationships, with non-linear effects of enhanced crest amplitudes altering this similarity at high threshold levels. Average envelope durations and intervals are then shown to be consistent with standard envelope theory of random waves when used with parameters derived from truncated amplitude spectra and non-linear crest statistics. Probability density distributions for envelope durations and intervals are shown to be described approximately by Gamma functions rather than the commonly assumed exponential function. Average durations and intervals for groups of two or more wave crests, as well as their associated probability density functions, are also shown to be theoretically related to the corresponding envelope statistics.