214. On the left boundary of the grid in Figure 20, the jetty of the 

 same length as that on the right boundary may allow sand to enter as well as 

 leave the grid since its effective length on the outside y G1 was made 

 comparatively short. The gated boundary condition thus allows considerable 

 flexibility to control the rate of sand transport across the boundaries. 



215. Pinned -beach boundary . The pinned-beach boundary condition repre- 

 sents a beach that has exhibited a long-term trend of stability. This 

 condition is implemented as a default boundary condition. A pinned-beach 

 boundary can be used in situations where a long sandy beach is located far 

 from the project and has not or is not expected to change greatly in position. 



216. The four possible combinations of the lateral boundary conditions 

 are illustrated in Figure 21. The boundary conditions are independent and 

 represent the modeler's interpretation of the physical situation. For small 

 projects, pinned-beach boundaries are sometimes used and placed far from the 

 project (for example, five project lengths to each side). The independence of 

 the result on this distance should be checked by varying the distance. Care 

 must be taken if the simulation interval is long or the transport intense. 



GROIN AT 





» i-1 



- 



- 







PINNED 



BEACH AT 



i-N+1 



. ■ ■ ■ ■ ■ • . »• : .■ 







GROIN AT 





i=N+1 



PINNED 







BEACH AT 

 1=1 







■ 







PINNED BEACH AT 

 1=1. i=N+1 



Figure 21. Combinations of lateral boundary conditions 



101 



