modeling effort. Coastal experience must be relied upon to identify unique 

 characteristics of the site or a normally minor factor that may, for some 

 reason, occupy a position of prominence in the coastal processes. These types 

 of problems may often be treated by creative exercise of GENESIS'S many 

 features, but sometimes special expertise is required to allow a description 

 of unique situations with GENESIS. 



Boundary Conditions 



80. As discussed further in Parts V, VI, and VII, boundary conditions 

 must be specified at the two lateral ends of the numerical grid. Boundary 

 conditions determine the rate at which sand may enter and leave the modeled 

 area and can have a profound effect on shoreline change. 



81. There are situations in which it may be possible to eliminate the 

 influence of boundary conditions by placing the boundaries far from the 

 project so as to have a negligible effect over the simulation interval. For 

 example, if a project is highly localized, such as a single detached break- 

 water on a straight sandy beach, the boundaries may be placed several project 

 lengths to either side and a condition of no shoreline change imposed, as the 

 breakwater system is expected to modify only the local area and not completely 

 block longshore sediment transport. In more regional applications, represen- 

 tation of the naturally occurring boundary conditions must be addressed as 

 part of the problem. 



82. In situations where the boundary conditions are ill -defined (which 

 is the typical situation in applications), it is of great help to monitor the 

 net and gross longshore sand transport rates calculated by GENESIS (Part V) in 

 addition to shoreline change. Boundary conditions control the magnitude of 

 the longshore sand transport rate. GENESIS provides information on the 

 calculated transport rate for comparison to empirically determined rates or to 

 rates that have to be specified by assumption (for example, at a rocky cliff). 

 In many cases, one or both boundaries are an integral part of the project, 

 such as shoreline change at a long jetty or shore -connected harbor breakwater, 

 blockage of longshore transport at an inlet or navigation channel, or termina- 

 tion of the beach at a headland. 



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