example, long headlands or long jetties at entrances and inlets. On the other 

 hand, engineering structures such as groins or seawalls may be present on the 

 internal domain of the grid. These barriers interrupt the movement of sand 

 alongshore and so constrain the transport rate and/or movement of the shore- 

 line. These constraints, which function similar to boundary conditions, must 

 be incorporated in the simulation. In the following, commonly used boundary 

 conditions are discussed. 



Pinned-beach boundary condition 



180. It is helpful to plot all available measured shoreline position 

 surveys together to determine locations along a beach that might be used as 

 model boundaries. In doing so it is sometimes possible to find a portion of 

 the beach distant from the project that does not move appreciably in time. By 

 locating the model boundary at such a section, the modeled lateral boundary 

 shoreline coordinate can be "pinned." Expressed in terms of the transport 

 rate, this means 



Qi - Q 2 (32) 



if implemented on the left boundary, and 



Q N+ i = Qn (33) 



if implemented on the right boundary. These relations can be readily under- 

 stood by reference to Equation 23; if AQ = at the boundary, then Ay = , 

 indicating that y does not change. The pinned-beach boundary should be 

 located far away from the project to assure that the conditions in the 

 vicinity of the boundary are unaffected by changes that take place in the 

 project. Details of the mathematical representation of this boundary condi- 

 tion in the double sweep algorithm are presented in Hanson (1987). 



Gated boundary condition 



181. Groins, jetties, shore -connected breakwaters, and headlands that 

 interrupt, partially or completely, the movement of sand alongshore may be 

 incorporated as a boundary condition if one is located on an end of the 

 calculation grid. If located on the internal domain of the grid, these 



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