Immediately after completion of a project, the beach is far from equilibrium, 

 and changes resulting from longshore sand transport usually dominate over 

 storm and seasonal changes, with the possible exception of a beach fill. 

 Shoreline change calculated over a short interval will probably be reliable in 

 such a case. As the beach approaches equilibrium with the project, the 

 simulation interval must extend to a number of years. Stated differently, the 

 shoreline change model best calculates shoreline movement in transition from 

 one equilibrium state to another. 



26. The spatial extent of a target region ranges from the single 

 project scale of hundreds of meters to the regional scale of tens of kilo- 

 meters. The modeled longshore extent will depend on the physical dimensions 

 of the project and boundary conditions controlling the sand transport. 

 Dimensions of the project are typically at a local scale, whereas placement of 

 appropriate model boundary conditions may require extension to a more regional 

 scale. Evaluation of possible effects of the project on neighboring beaches 

 may also dictate extension of the spatial range of the simulation. Shoreline 

 change numerical models require modest computer resources and are well suited 

 for regional scale engineering studies. 



27. Shoreline change models are designed to describe long-term trends 

 of the beach plan shape in the course of its approach to an equilibrium form. 

 This change is usually caused by a notable perturbation, for example, by 

 jetties constructed at a harbor or inlet. Shoreline change models are not 

 applicable to simulating a randomly fluctuating beach system in which no trend 

 in shoreline position is evident. In particular, GENESIS is not applicable to 

 calculating shoreline change in the following situations which involve beach 

 change unrelated to coastal structures, boundary conditions, or spatial 

 differences in wave -induced longshore sand transport: beach change inside 

 inlets or in areas dominated by tidal flow; beach change produced by wind- 

 generated currents; storm-induced beach erosion in which cross-shore sediment 

 transport processes are dominant; and scour at structures. Table 1 gives a 

 summary of major capabilities and limitations of Version 2 of GENESIS, which 

 will be discussed in succeeding chapters. 



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