To adjust for the proper value of a use Table 2 which indicates that (a/ 

 0.0081) 1/2 for f = 0.17 hertz and u = 68 miles per hour is 1.63. The above 

 wave heights must be multiplied by 1.63 yielding 8.3, 6.3, 5.2, and 2.9 feet 

 (2.6, 2.0, 1.6, and 0.9 meter). 



V. DISCUSSION 



The two example problems indicate that the depth-controlled wave height 

 (spectral) depends significantly on the values chosen for a and f . On the 

 lake, the waves have a higher a value but because of fetch limitations, f 

 cannot be very low. As a result, there is less spread in energy over the spec- 

 trum and the limiting form (eq. 2) is only integrated over frequencies where 

 the energy density is relatively small compared to cases where f p is lower 

 and E is much larger. The method presented here indicates that the upper 

 bound on wave energy and an estimate of the significant height, H , by 

 H varies with depth (nonlinearly) , the peak frequency associated with the 

 waves, and a parameter a associated with the wave generation process. The 

 method allows an estimation of the depth-limited conditions of H = 4(E) 17 2 to 

 be based on an analysis of the wave generation conditions. 



The difference between H (directly related to the energy) and H^ (based 

 on the largest single wave that can occur) is significant for most wave condi- 

 tions and especially when the site has short fetches. It is recommended that 

 H. only be used where it is necessary to determine the largest individual wave 

 or if the wave conditions are nearly monochromatic. In all other cases, and in 

 particular, where an estimate of the energy of the wave field is required, the 

 method in this report may be expected to provide a more accurate (though less 

 conservative) estimate. 



VI. SUMMARY 



A method has been presented for calculating a maximum value for H = 4(E) 1 ' 2 

 for wind sea situations where depth is the controlling factor. Estimation 

 requires input of depth, a lower bound frequency, and parameter a typical 

 of storm spectra at the site. Methods for estimating the latter two parameters 

 are also provided. The results indicate that, in the shallow-water limit, H 

 (which is also an approximation to the shallow-water value H g ) is propor- 

 tional to the square root of depth. The values obtained can be significantly 

 less than the monochromatic depth-limited wave height, H^, which is taken to 

 be the upper bound of individual single waves in the spectrum. Again it is 

 important to emphasize that the H defined here is directly related to the 

 total energy of the wind sea and approximates the average of the highest one- 

 third waves. The monochromatic value H d defined in SPM better approximates 

 the highest individual wave that might be expected in that depth. The selec- 

 tion of which approximation to use will depend on the design application. 



13 



