objects will act to constrain the transport rate and shoreline change, 

 automatically calculated by GENESIS. The representation is the same for both 

 cases, although it occurs in different places in the numerical solution 

 scheme . 



182. The effect of a groin, headland, or similar object located on the 

 boundary is formulated in terms of the amount of sand that can pass the struc- 

 ture. Consideration must be given to sand both entering and leaving the grid. 

 For example, at a jetty located next to an inlet with a deeply dredged 

 navigation channel, sand might leave the grid by bypassing the jetty during 

 times of high waves; in contrast, no sand is expected to cross the navigation 

 channel and jetty to come onto the grid. The jetty/ channel thus acts as a 

 selective "gate," allowing sand to move off but not onto the grid. This 

 "gated boundary condition" was termed the "groin boundary condition" in 

 previous descriptions of GENESIS. 



183. The most appropriate mathematical representation of the gated 

 boundary condition is a subject of active research (Gravens and Kraus 1989), 

 and GENESIS is expected to undergo revision in this capability. At present 

 two approaches are under study, one in which the amount passing the boundary 

 is proportional to the transport rate at the immediately updrift grid cell 

 (Perlin and Dean 1978) and the other in which the amount is proportional to 

 the potential longshore transport rate at the location of the boundary (Hanson 

 and Kraus 1980). In any case, the gating action on a boundary is controlled 

 by the combined actions of sand bypassing and sand transmission. 



184. Sand bypassing . In GENESIS, two types of sand movement past a 

 structure are simulated. One type of movement is around the seaward end of 

 the structure, called bypassing, and the other is through and over the 

 structure, called sand transmission. Bypassing is assumed to take place if 

 the water depth at the tip of the structure D G is less than the depth of 

 active longshore transport D LT . Since the shape of the bottom profile is 

 known (Equation 6), D G is determined from knowledge of the distance between 

 the tip of the structure and the location of the shoreline. However, since 

 structures are located at grid cell walls between two calculated shoreline 

 positions, this depth is not unique. In GENESIS the updrift depth is used. 



88 



