VI. INTERPRETATION 



Several analyses related to wave grouping characteristics and detailed 

 spectral structure of field wave records were presented in the previous sec- 

 tion. The objective of this section is to interpret the analyses, using the 

 six hypotheses in Section I. 



The wave data analyzed were taken at sites in three different water bod- 

 ies: Lake Michigan (South Haven), Pacific Ocean (Columbia Light), and Gulf of 

 Mexico (South Pass). The data represent moderate to high wave conditions with 

 unimodal spectra and reasonably constant significant height and peak spectral 

 period. The data represent actively growing waves at two sites (South Haven 

 and South Pass) and old swell at the third site (Columbia Light). These simi- 

 larities and differences affect interpretation of the analyses. 



Many of the time series selected for analysis show evidence that high 

 waves tend to occur in groups, e.g., the time series for 1720 to 1737 at South 

 Haven, 1300 to 1308.5 at Columbia Light, and 1510 to 1520 at South Pass (App. 

 B) . However, there are several notable exceptions in which the high waves do 

 not appear in clear groups, e.g., the time series for 1408.5 to 1417 at Colum- 

 bia Light and 1500 to 1510 at South Pass. Thus, both grouped and ungrouped 

 time series are considered in the analysis. 



The field data time-series plots from South Haven and Columbia Light (App. 

 B) appear to have a few unusually low wave troughs. The tendency for very low 

 troughs may be exaggerated relative to the South Pass time series. There is 

 a possibility that low, sharp troughs result from imperfect response of the 

 Waverider buoy gage to high waves . Since low troughs seem to coincide with 

 groups of high waves, the troughs are not a deterrent to a study of wave 

 groups. However, they may exaggerate the appearance of existing groups. 



One procedure presented earlier to aid in identifying wave groups in a 

 time series was the Local Variance Time Series (LVTS) analysis. A comparison 

 of the LVTS in Appendix E with the field data time series in Appendix B for 

 1300 to 1317 at Columbia light (a well-grouped record) shows that the peaks of 

 the LVTS effectively indicate the presence of six high wave groups and their 

 location in the time series. Note that each time series plot covers 512 

 seconds while each LVTS plot covers 1,024 seconds. Similarly, the LVTS peaks 

 for 1540 to 1600 at South Pass (another well-grouped record) indicate the 

 presence and location of high wave groups in the corresponding time series. 

 Both the Columbia Light and South Pass records also gave high values of G 

 and Rtj, the two parameters considered indicative of grouping (see Tables 4 

 and 5). 



By contrast, the LVTS for 1400 to 1417 at Columbia Light (a poorly grouped 

 record) indicates few peaks which are high relative to the variance of the 

 original time series. There is also a lack of low valleys in the last half of 

 the LVTS, which indicates that groups are not well separated by low variance 

 sections of record. Values of G and Ru are relatively low for the 

 record. The LVTS for 1500 to 1510 at South Pass (another poorly grouped 

 record) shows an absence of peaks which are high relative to the time-series 

 variance. Values of G and Ru for this record were computed for a 20- 

 minute record which includes some well-grouped waves , so they are not partic- 

 ularly helpful in this case. 



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