version of the table is in Appendix B. ) The parameter a is related to 

 dimensionless fetch and can be either obtained from a measured deepwater 

 spectrum or can be inferred as a function of a peak, frequency of a spectrum 

 and windspeed. The adjustment of H(f c , h, 0.0081) for variation in a is 

 discussed in the next section. 



If the parameter, w, , defined in equation (3) is one or less for the 

 main energy containing frequencies in a sea state, the H(f , h, 0.0081) can be 

 approximated by 



0.161 h 1 / 2 



H(f , h, 0.0081) = for h in feet 



f„ 



0.089 h 



1/2 



(5) 

 for h in meters 



This equation predicts that in areas where the wave energy is controlled by 

 depth, H varies with the square root of depth, not approximately linearly 

 with depth as does the monochromatic H,. 



III. SIMPLIFIED METHOD 



To use Table 1 or equation (5), estimates of f • and a are necessary to 

 estimate H in depth, h. 



1. Selection of f. 



f is defined as the lowest frequency in which there is appreciable wave 

 energy. Typical storm spectra in shallow water (60 feet or less) have very 

 sharp peaks. If there is a given spectrum offshore where the values of H 

 are desired in a depth, h, it is recommended that f be set to 90 percent 

 of f , the peak frequency of the sea spectrum. 



f c =0.9f p (6) 



If more conservatism is required, f can be set lower. If no spectral data 

 are available at a site, then f can be estimated either by estimating f 

 via hindcast curves or by selecting a reasonable but conservative value of 

 f based on similar conditions elsewhere. 



2. Selection of a. 



The parameter of a can be directly estimated from measured spectrum by 

 fitting equation (2) to the spectrum. If measurements are unavailable, a 

 can be estimated by fitting equation (2) to good quality hindcast spectra. If 

 neither hindcasts nor measured data are available, then a can be estimated 

 from Hasselmann, et al. (1973) 3 



.- 0.076 \- } 



22 



3 HASSELMANN, K. , et al. , "Measurements of Wind-Wave Growth and Swell Decay 

 During the Joint North Sea Wave Project JONSWAP," Deutsches Hydrographisches 

 Institut, Hamburg Germany, 1973. 



10 



