213. Gated boundary . A gated boundary condition (Figure 20) is 

 implemented at a terminal grid cell (grid cell walls 1 and N+l) if the 

 modeler specifies a groin (or jetty or shore -connected breakwater) in the 

 respective cell. The amount of sand entering or leaving the grid at a gated 

 boundary is determined by the distances from the shorelines on either side of 

 the groin to the seaward end of the groin, the beach slope at the groin, and 

 the permeability of the groin. In Figure 20, the distance from the shoreline 

 to the end of jetty outside the grid on the right boundary y GN was made very 

 long (as specified in the model input) and the permeability set to 0. (Such 

 a condition might occur if an inlet is located to the right of the grid.) The 

 jetty therefore appears infinitely long and high from outside the grid on the 

 right, and no sand will be transported onto the grid. However, transport off 

 the grid at the right boundary may occur and will depend on the distance from 

 the shoreline to the end of the jetty inside the grid and the wave conditions 

 (which determine the depth and location of the longshore sand transport) . 



SHORT EFFECTIVE JETTY LENGTH; LONG EFFECTIVE JETTY LENGTH 



SAND CAN PASS FROM ErTHER SIDE ON OUTSIDE; SAND CAN LEAVE 



GRID BUT NOT ENTER 



Figure 20. Gated boundary condition 

 100 



