40. Lateral boundary conditions for a row are specified at the conclu- 

 sion of calculations for that row. The value of all variables at cells j=N 

 and j=l are set equal to their values at cells j=N-l and j=2 , respec- 

 tively. This boundary condition implies that the change in the variable in 

 the y-direction is zero. The condition is most valid when the bathjonetric 

 contours are nearly straight and parallel to the y-axis. For this reason the 

 grid is oriented so that the y-axis is nearly parallel to bottom contours 

 along the lateral boundaries. 



41. Boundary conditions along the offshore boundary of the grid are 

 used to initiate the shoreward marching algorithm. They are computed from 

 deepwater wave input supplied by the user along with the following assumption. 

 Bottom contours extending from the offshore grid row (i=M) out to deep water 

 are assumed to be straight and parallel to a line making an angle of with 

 the y-axis. In other words, Snell's law is assumed to be valid from deep 

 water to the outer boundary of the grid system. No inshore boundary condi- 

 tions (along row i=l) are required because of the forward marching solution 

 scheme. 



Wave transformation 

 inside the surf zone 



42. Waves approaching the very nearshore zone tend to steepen and 

 eventually break because of decreasing water depths. Shoreward of this break- 

 ing point dissipative energy losses due to turbulence strongly influence the 

 wave height. Linear theory does not allow for prediction of the breaker loca- 

 tion nor for wave transformation across the surf zone. Instead, empirical and 

 approximate methods must be used to describe the breaking process. 



43. The first aspect to consider in surf zone transformation of waves 

 is incipient wave breaking. RCPWAVE uses the following criterion of Weggel 

 (1972): 



bh^ 



"b = ^ ^^^^ 



1+^ 



where 



H = breaking wave height 



28 



