Statistics on the occurrence of multiple spectral peaks identified 

 by the above techniques (presented in the next section) are based only 

 on the part of the spectrum between 0.03 and 0.5 hertz for the staff and 

 accelerometer-buoy gages. The high-frequency cutoff for the pressure 

 gages was 0.33 hertz in the Great Lakes and 0.31 hertz at Pt. Mugu as 

 discussed earlier. 



(b) Spectral Peakedness . It is highly desirable to param- 

 eterize a spectrum. Various parameters have been considered in the lit- 

 erature, but most are derived from moments of the spectrum as defined in 

 equation (1). 



In practical application equation (1) becomes 



f 

 m„ = J S(f) f"df (2) 



where f^p is the low- frequency spectral cutoff, and ffjE the high- 

 frequency spectral cutoff used for computation. When applied to field 

 records of finite length, the integral in equation (2) becomes a 

 summation. 



Equation (2) shows that the higher the value of ffip> the greater 

 the magnitude of m^ provided that S(f) is greater than zero at high 

 frequencies. In practice, the choice of ff^p has a significant influ- 

 ence on the values confuted for m^, especially for the higher moments. 

 The choice of fj^p is less ambiguous in practice because contributions 

 to vHf^ from the low-energy and low-frequency part of the spectrum are 

 generally negligible. 



The sensitivity of m^ and parameters based on m^ to the choice 

 of fjip was investigated by Rye (1977). He considered two theoretical 

 spectral shapes: a sharply peaked JONSWAP spectrum and a flat JONSWAP 

 spectriim equivalent to a Pierson-Moskowitz spectrum. Rye recommended 

 three spectral parameters for general use: 



(a) Significant wave height, Hg where 



Hg = 4v^ (3) 



(b) period of the spectral peak, Tp 



(c) spectral-peakedness parameter, Qp where 



K ff[S(f)]2df (4) 



Bin J 



The spectral-peakedness parameter was originally proposed and shown to 

 be directly related to the average number of high waves oceurring in 



27 



