Most investigators of wave groups seem to prefer dealing with the serial 

 variation in individual wave heights. Rye (1974), Houmb and Overvik. (1977), 

 and Arhan and Ezraty (1978) reported autocorrelations between individual wave 

 heights from field records. Autocorrelations averaged between about 0.2 and 

 0.3, indicating a weak correlation between heights of successive waves. Arhan 

 and Ezraty found essentially no correlation between a wave height and the 

 height of the second or third following wave. 



Arhan and Ezraty (1978) also reported evidence from 169 North Sea storm 

 records that the correlation between a wave height and the succeeding wave 

 height depends on the individual wave height. They showed an increasing cor- 

 relation when the wave height is greater than about 0.75 times the significant 

 height. When the wave height is lower than 0.75 times the significant height, 

 it is uncorrelated with the height of the following wave. Siefert (1976) also 

 observed that the tendency for high waves to occur in succession increases 

 with wave height. These interesting results raise questions about the value 

 of a single autocorrelation between wave heights as a parameter indicative of 

 wave grouping. 



Statistics of runs of consecutive waves with heights above some specified 

 level have also indicated a weak tendency for high waves to occur in groups. 

 Literature on the subject includes Goda (1970, 1976), Wilson and Baird (1972), 

 Ewing (1973), Chakrabarti, Snider, and Feldhausen (1974), Rye (1974), and 

 Burcharth (1980). 



Nolte and Hsu (1972) studied wave groups by examining statistics of the 

 wave envelope. They found good agreement between their theory and field 

 data. Chou (1978) developed an analytical method for constructing the wave 

 envelope, based on the assumption of a stationary random Gaussian sea state. 



Both the serial variation in wave heights and the statistics of the wave 

 envelope are in theory useful approaches to studying wave grouping. However, 

 real ocean wave records typically include numerous small bumps and erratic 

 variations which introduce subjectivity into the definition of individual wave 

 heights and wave envelopes. An appealing alternative has been devised and 

 applied by Sedivy (1978). A wave group is considered to be a short section 

 of wave record which has high variance relative to the variance of the whole 

 record. The local variance is computed throughout the record so as to iden- 

 tify all areas of high local variance (i.e., wave groups). Statistics derived 

 from numerous field records by this approach are given by Sedivy (1978), 

 Nelson (1980), and Thompson and Sedivy (1980). 



Sedivy (1978) experimented with various lengths of record for computing 

 the local variance to most clearly identify prominent wave groups. His final 

 choice was two times the peak spectral period for the record. Nelson (1980) 

 confirmed that choice. 



Another approach to studying wave grouping involves computing the spectrum 

 of the squared data points in a wave record. The spectrum shows energy from 

 wave groups at very low frequencies while energy at the dominant individual 

 wave frequency appears at about twice the individual wave frequency. Thus, 

 energy and frequencies associated with wave groups can be effectively iso- 

 lated. This approach is well described and illustrated in Funke and Mansard 

 (1979), and is also discussed in Funke and Mansard (1980). 



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