Multiple major spectral peaks are evident in many of the spectra in 

 Appendix A. However, some of the multipeaked spectra and many of the 

 single-peaked spectra appear to approximate the form of the JONSWAP spec- 

 trum for waves undergoing active generation (Hasselmann, et al., 1973). 

 Only seven of the spectra selected for inclusion in Appendix A were col- 

 lected during hurricanes. This small sample does not illustrate any 

 clear differences between spectra generated by hurricanes and spectra 

 generated by extratropical storms. 



2. Mean Spectra . 



a. Grouped hy Height and Period . Spectra from each gage site were 

 groi5)ed into sets by significant height and peak spectral period. Inter- 

 vals of 30.5 centimeters were used for significant height; intervals of 

 about 1 second were used for peak period (Table 2). 



Mean spectra were plotted for all height -period intervals containing 

 more than one case with heights greater than a cutoff height, which 

 varied with location. In addition, mean spectral plots were generated 

 for height-period intervals with heights below the cutoff if there were 

 a reasonably large number of cases in the interval. This approach pro- 

 vided mean spectral plots for the most statistically meaningful low wave 

 cases and for all high wave cases. Energy computed for periods longer 

 than 30 seconds was omitted from plots for Great Lakes and Pacific coast 

 locations as discussed earlier. 



The mean spectral plots grouped by location are given in Appendix B. 

 The approximate relative water depth and ratio of significant wave height 

 to water depth for each location are listed in Table B-1; the dates 

 covered are in Table 1. 



The range of significant wave height and peak spectral period repre- 

 sented and the number of cases are listed on each plot. Each plot repre- 

 senting more than one case contains several dashlines in addition to the 

 solid line showing the mean spectrum. The dashline nearest the mean and 

 above it represents the mean plus one standard deviation. Similarly, 

 the dashline nearest the mean and below it represents the mean minus one 

 standard deviation. When the mean minus one standard deviation is nega- 

 tive, it is plotted as zero. For plots representing more than two cases, 

 another dashline above the mean shows the highest single-energy density 

 value occurring in each band. This line forms an envelope inside which 

 all of the individual spectra would fall. The dashlines should not be 

 treated as a spectrum, and are included only as indicators of the observed 

 variability of energy density in each spectral band. Some of the plots 

 show a relatively large concentration of energy at the lowest frequency 

 (0.005 hertz). This concentration is at a frequency which is irrelevant 

 to wind-generated gravity waves and should be ignored. 



The mean spectral plots for each location in Appendix B are arranged 

 in order of increasing significant height and peak period in most cases; 

 i.e., all mean spectra in a column represent the same peak -period interval 



32 



