Thus , one contour line can be used to describe change in the beach plan shape 

 and volume as the beach erodes and accretes. This contour line is conven- 

 iently taken as the readily observed shoreline, and the model is therefore 

 called the "shoreline change" or "shoreline response" model. Sometimes the 

 terminology "one-line" model, a shortening of the phrase "one-contour line" 

 model, is used with reference to the single contour line. 



100. A second geometrical- type assumption is that sand is transported 

 alongshore between two well-defined limiting elevations on the profile. The 

 shoreward limit is located at the top of the active berm, and the seaward 

 limit is located where no significant depth changes occur, the so-called depth 

 of profile closure. Restriction of profile movement between these two limits 

 provides the simplest way to specify the perimeter of a beach cross -sectional 

 area by which changes in volume, leading to shoreline change, can be computed. 



101. The model also requires predictive expressions for the total long- 

 shore sand transport rate. For open-coast beaches, the transport rate is a 

 function of the breaking wave height and direction alongshore. Since the 

 transport rate is parameterized in terms of breaking wave quantities, the 

 detailed structure of the nearshore current pattern does not directly enter. 



102. Finally, it is assumed that there is a clear long-term trend in 

 shoreline behavior. This must be the case in order to predict a steady signal 

 of shoreline change from among the "noise" in the beach system produced by 

 storms, seasonal changes in waves, tidal fluctuations, and other cyclical and 

 random events. In essence, the assumption of a clear trend implies that the 

 wave action producing longshore sand transport and boundary conditions are the 

 major factors controlling long-term beach change. This assumption is usually 

 well satisfied at engineering projects involving groins, jetties, and detached 

 breakwaters, which introduce biases in the transport rate. 



103. In summary, standard assumptions of shoreline change modeling are: 



a. The beach profile shape is constant. 



b. The shoreward and seaward limits of the profile are constant. 



c. Sand is transported alongshore by the action of breaking 

 waves . 



d. The detailed structure of the nearshore circulation is 

 ignored. 



e. There is a long-term trend in shoreline evolution. 



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