Spectral adaptation of the HARBD model is done as a post-processing step 

 using the standard, regular wave output from the model. For a given set of 

 incident wave directions representing the range of possible approach directions, 

 HARBD is run for a number of wave periods spread between the shortest period 

 satisfying the grid resolution constraint of Equation 8 and the longest swell 

 period of interest. 



Spectral post-processing is based on the assumption that a consistent spectral 

 form can be applied at every node. This major assumption provides the basis for 

 a workable, reasonable spectral weighting which improves on the traditional 

 regular wave approach. The spectrum is represented as the product of two 

 functions: 



5(/,0) = S(f) D(/,8) (10) 



where 



S(f> 0) = directional spectral energy density function 



S(f) = spectral energy density function 

 D(f> 0) = angular spreading function 



The JONSWAP spectral form was chosen for S(f) (Hasselmann et al. 1973). 

 The JONSWAP spectrum is specified as (U.S. Army Corps of Engineers 1989) 



2 

 5(f) = _JU> e " yb 



(2tc)^ 5 (ID 



where SffJ = spectral energy density at frequency^. 



The parameters a and b are given by the following relationships: 



-1.25 



a = 



f-X 





4« 



b = e 2 * 2 



T„ - I) 2 



a = (107 



for ftf, 



= 0.09 



for f>-f p 



(12) 



48 



Chapter 4 Numerical Model 



