6 = primary wave direction 

 6-6 = wave direction difference, ranging from -nil to +ti/2 



The spreading parameter s controls 

 the magnitude of directional spread. 

 As the value of s increases, direc- 

 tional spread narrows. Wind waves 

 are typically represented by broad 

 spreads and swell by narrow spreads. 

 Recommended representative values 

 for each are given in Table 10 

 (Goda 1985). 



Spectral post-processing begins 

 with specification of the desired H s ,T p y, and s and the arrays of HARBD 

 amplification factors. A refined JONSWAP spectrum is computed with 

 1,000 points, where them's in Equation 1 1 are 



/, = 0.5% , f 2 = 0.502% , /, = 0.504% , ... , f I000 = 2.498% 



Table 10 



Guidance for Choosing s 



Wave Condition 



s 



Wind waves 



10 



Swell 



25-75 



The number of wave periods computed with HARBD is always much smaller 

 than 1,000, typically less than 20. These periods, converted to frequency 

 (reciprocal of period), can be used to define bands in the JONSWAP spectrum. 

 Bands are bounded by the midpoints between HARBD computational 

 frequencies. The highest and lowest frequency bands are assumed to be centered 

 on the highest and lowest HARBD computational frequencies, respectively. A 

 weighting factor for each HARBD-defined band is computed by summing values 

 from the refined JONSWAP spectrum which fall within the band and 

 normalizing by the total spectral energy. 



1,000 



(15) 



where 



w k = weighting factor for /fc'th HARBD computational frequency 



/*-i + /* 



N u = index of lowest JONSWAP frequency^ satisfying f { > 

 N a = index of highest JONSWAP frequency f, satisfying f { < 



2 



50 



Chapter 4 Numerical Model 



