cell spacing should be determined such that major shoreline features are 

 resolved. Distances are read on the grid with cell wall 1 as the origin; 

 that is, the y-axis intersects the x-axis at grid wall 1 , not at "zero." 



209. GENESIS Version 2 uses a uniform alongshore grid, and the spacing 

 between all shoreline positions is Ax . Positions on the grid defining the 

 ends of structures, of which terminal groins or jetties are a typical case, 

 are located at a distance Ax/2 from adjacent shoreline position cells, since 

 sediment transport rates are calculated at grid cell walls. In the example of 

 Figure 19, a tip of a detached breakwater (groin, seawall) is assigned to 

 position i ; GENESIS will place the tip of the structure at cell wall i and 

 not at shoreline position i , which is in the middle of the cell. As another 

 example, the jetty located on the left boundary of the grid is a distance 

 Ax/2 to the left of shoreline position coordinate y x ; the shoreline starts 

 at the location of y x , not at the jetty. Concerning beach fills, since a 

 fill moves the position of the shoreline, the grid locations of the two 

 lateral ends of a fill are at shoreline positions, not cell walls. 



210. All historic shoreline position data must be translated to the 

 coordinate system and placed on the grid. Structures are usually assigned the 

 cell number at which they would naturally reside, but the modeler is free to 

 use judgment. For example, if an already short detached breakwater would be 

 further shortened by following standard procedure in placing it on the grid 

 (due to roundoff to the nearest cell position) , one tip could be "moved" to 

 the next cell to increase the effective length of the structure. 



211. It is also possible to simulate shoreline change along a subsec- 

 tion of the grid, in which case consideration must be given to boundary condi- 

 tions at the two ends of the subsection. It is recommended to check the 

 results of preliminary model runs for longshore and offshore locations of 

 topographic information to confirm that it was entered correctly on the grid. 



Lateral boundary conditions 



212. As described in Part V, GENESIS allows two types of lateral 

 boundary conditions to be implemented, a "gated" boundary and a "pinned-beach" 

 boundary. The default condition is the pinned beach; if a groin is not placed 

 on cell wall 1 or N+l , the boundary will be treated as a pinned beach, 

 allowing sand to freely cross it from both sides. 



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