the four faces of a cell must be considered. If more material enters a cell 

 than leaves, deposition of material occurs, and if more material leaves than 

 enters, erosion of material occurs within the cell. If S t is the total 

 transport rate out of a cell and S- is the total transport rate into a 

 cell, then 



(S. - S W 

 \in out/ 



** a A (1 - p) (97) 



where 



Ah = change in the bed elevation of a cell 



At = time-step 



A = area of cell 

 c 



p = porosity of sediment on bed 



71. When applied to each grid cell of the grid shown in Figure 2, 

 Equation 97 determines the erosion and deposition throughout the grid. It is 

 assumed that there is no net erosion or deposition within all the boundary 

 cells. S^ n and S t can be determined once the current and wave fields 

 have been determined by the numerical models described earlier. Within the 

 surf zone, both the sediment concentration C and the total water discharges 

 (tidal and wave-induced) are used to compute S, and S t 



Profile Response (Onshore-Offshore) Model 



Introduction 



72. In addition to littoral and tidal transport, beach profiles respond 

 to the wave climate through onshore-offshore transport of sediment. This 

 onshore-offshore transport occurs at a much (orders of magnitude) slower rate 

 than does littoral transport of sediment (Galvin 1983). However, although the 

 gross littoral transport of sediment is very large at any point in the surf 

 zone, the net deficit or surplus of sediment is usually fairly small. Con- 

 versely, the gross onshore-offshore transport of sediment is small, but the 

 net deficit or surplus can be equal to the gross over short periods of time. 

 For example, during a storm there may be offshore movement of sediment to an 

 offshore bar. Although the total amount of sediment that moves to the bar is 



54 



