104. The basic assumptions define a flexible and economical shoreline 

 change simulation model that has been found applicable to a wide range of 

 coastal engineering situations. However, it should be kept in mind that the 

 assumptions are idealizations of complex processes and, therefore, have 

 limitations. In a strict sense, the assumption that the beach profile moves 

 parallel to itself along the entire modeled reach is violated in the vicinity 

 of structures. For example, the slope of the profile on the updrift or 

 accreting side of a jetty or long groin is usually more gentle than the slope 

 of the beach distant from the structure. GENESIS will show shoreline advance 

 in such a case, and a calibrated model may provide agreement with measured 

 shoreline change, but the change in beach slope and sand volume contained in 

 that change will not be reproduced. As a result, simulations in situations 

 where the beach slope is expected to change significantly should be inter- 

 preted carefully. 



105. Similarly, the depth of closure and the berm height along the 

 modeled stretch of beach may vary alongshore, whereas these quantities are 

 constant in the model. Values for berm elevation and depth of profile closure 

 representative of the entire beach must be carefully determined. The trans- 

 port rate formula contained in Version 2 of GENESIS describes longshore sand 

 transport produced solely by incident waves. It does not describe transport 

 produced by tidal currents, wind, or other forcing agents, indicating that the 

 model should not be used if breaking waves are not the dominant mechanism for 

 transport sand alongshore. As described below, GENESIS can account for the 

 vertical and cross -shore distributions of longshore sand transport at groins 

 and jetties in an empirical fashion. It does not account for the full 

 vertical and horizontal water and sand circulation, making it incapable, for 

 example, of describing transport by rip currents, undertow or return flow, or 

 other 3-D fluid and transport processes. 



106. The assumption that there must be a long-term trend in shoreline 

 evolution means that a boundary condition or some other systematic process, 

 for example, a river discharge, or a regular change in the wave pattern such 

 as produced by a detached breakwater, dominates the beach change. This will 

 normally be the case at engineering projects. 



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