Consideration of the above examples leads to the generalization that 

 the LVTS effectively identifies prominent wave groups when they exist in a 

 record. However, the original time series should be checked to verify the 

 existence of the groups, especially if the LVTS shows an absence of peaks 

 which are large relative to the time-series variance. The parameters G and 

 Rtt are of some use in identifying the extent of high wave grouping in a 

 record. 



To further use the LVTS it is useful to subjectively identify each ana- 

 lyzed field data time series with some evidence of wave groups. A review of 

 Appendix B indicates that all the records suffice except for 1408.5 to 1417 

 at Columbia Light and 1500 to 1510 at South Pass. Thus attention is focused 

 on Appendix E, Figures E-1 to E-4 , E-6, E-7 (the last half), E-8 , and E-9. 

 The time between major wave groups is represented by the time between promi- 

 nent peaks in the LVTS. This time has a clear tendency to be longest in the 

 Columbia Light records and shortest in the South Haven records. The figures 

 indicate considerable variability in this time, but T h. computed by 

 equation (35) and shown in each figure, is a plausible estimate of a modula- 

 tion time scale. 



Quantitative estimates of modulation time scale for wave groups were 

 extracted from Appendix E by measuring the time between successive peaks in 

 the LVTS, to be referred to as TtuTS' Since small LVTS peaks may not be 

 relevant to dominant modulation processes in a record, they were ignored in 

 estimating Tj^y^s ^^ *-^^ difference in amplitude between a peak and either 

 adjacent trough is less than about 1.5 times the time series variance, a^. 



A plot of TTTTrpc versus T j from equation (35) shows considerable 

 scatter (Fig. 40). The South Haven and South Pass data points show evidence 

 of a small tendency for T^vtS ^° increase with TLnH' Further, some of the 

 data points closest to the line for which Tx„™,g and T h are equal repre- 

 sent time differences between the larger LVTS peaks. For example, of the six 

 TlvtS values obtained by considering only the two highest peaks of Figure E-8 

 and the six highest peaks of Figure E-9, four values are within 15 percent 



°f T^od- 



^LVTS values for Columbia Light are all less than Tj^od' although they 

 show a small tendency to be higher than TTxr-pg values for the other loca- 

 tions. One obvious difficulty in dealing with the Columbia Light data is the 

 magnitude of Tmod- If 500-second modulations are truly present, there are 

 only two repetitions in the 1,024-second record. Records continuous more than 

 at least 1 hour are needed to adequately examine the evidence supporting 500- 

 second modulations. The plots in Appendix E certainly do not preclude the 

 existence of 500-second modulations. Of the three T^vTS values obtained by 

 considering only the two highest peaks in Figure E-4 and the three highest 

 peaks in Figure E-6, tvro values are within 2.5 percent of T h. 



Wave modulation information was summarized by Lake and Yuen (1978) and 

 Longuet-Higgins (1980) as a plot of dimensionless modulation frequency versus 

 wave steepness. A comparable plot using dimensionless Ttwjc values deter- 

 mined by the wave period corresponding to peak spectral frequency is given in 



60 



