CoD^arison of mean spectra in Appendix B for the same height-period 

 intervals at different locations provides additional perspective. Mean 

 spectra for the Atlantic coast locations are generally quite similar to 

 each other (Fig. 16, a and b). Mean spectra for the gulf coast location 

 differ from the Atlantic coast spectra in that spectral energy at fre- 

 quencies lower than 0.1 hertz is absent in virtually all cases. Mean 

 spectra for the Great Lakes locations show an even more pronounced 

 absence of low-frequency energy, especially for low wave conditions. 

 This effect is illustrated in Figure 16a. The absence of low-frequency 

 energy gives the gulf and Great Lakes mean spectra a tendency to have 

 higher peaks than the mean spectra for corresponding height -period 

 groups at Atlantic coast locations. 



Mean spectra for the Pacific coast locations show that peak periods 

 longer than 10 seconds are more common than for Atlantic coast locations. 

 Reduced amounts of energy are also shown at frequencies higher than about 

 0.15 hertz in conqsarison to Atlantic coast spectra with long peak periods 

 (Fig. 16b). Hence, the Pacific coast long-period mean spectra generally 

 have higher peaks than their counterparts for the Atlantic coast. The 

 few Pacific coast mean spectra shown with peak periods shorter than 9 

 seconds also show a distinct swell energy concentration at frequencies 

 less than 0.1 hertz. 



b. Grouped by Height, Period, and Water Level . Numerous field 

 studies have indicated that shallow-water spectral shape is dependent on 

 local water depth (e.g., Bamett 1969; Hasselmann, et al,, 1973; Wang and 

 Yang, 1976). Therefore, many of the results in this report are expected 

 to be strongly influenced by the water depth at the gage site. The higher 

 wave conditions at many of the sites can represent situations where waves 

 occasionally break seaward of the gage, or, in the extreme, cases where 

 the gage is actually in the surf zone as discussed in Section II. 



To give more perspective on the systematic influence of water depth 

 on spectral shape, the spectra within each significant height-peak period 

 interval for the gage at Nags Head were further grouped according to mean 

 water level in the record. Three water level groups were arbitrarily 

 defined. Plots of average spectra by water level for height -period in- 

 tervals in which there were a reasonably large number of cases are shown 

 in Appendix C. Each plot is labeled with the height and period intervals 

 represented and with a mean water level designator. A mean equal to 1 

 indicates the mean water level was relatively low; a mean of 2 indicates 

 midtide water levels and 3 high water levels. Water level 2 encon5)assed 

 a range of 49 centimeters. The differences between mean spectra as a 

 function of mean water level are small. 



3. Spectral and Sea-Surface Elevation Distribution Function Parameters . 



a. Secondary Spectral Peaks . The computerized procedure described in 

 Section III and Appendix D was used to identify major peaks in all spec- 

 tra considered for this report. For five of the six ocean locations only 

 about one-third of the shallow-water spectra are single peaked (Fig. 17) . 



38 



