For simplicity, only longshore transport of sand is considered. It is 

 straightforward to generalize Equation 1 to formally include contributions 

 for cross-shore transport, as well as sediment sources and sinks. An equation 

 given by Hallermeier (1979, 1983) for a limiting depth of sand motion in terms 

 of the incident wave conditions has been recommended by Kraus and Harikai 

 (1983) for use as the depth of closure (see also Kraus 1984). 



Model Input Requirements and Boundary Conditions 



36. In order to solve Equation 1, three kinds of information are re- 

 quired: (a) the initial location of the shoreline with respect to some coor- 

 dinate system (Figure 4) in which the x-axis is oriented along the trend of 

 the coast and the y-axis points offshore, (b) an expression for the longshore 

 sand transport rate, Q , and (c) boundary conditions for either y or Q at 

 the two lateral ends of the beach. Of these, the initial position of the 

 shoreline is readily obtained or assumed. 



y n 



LATERAL BOUNDARY 

 CONDITION: JETTY 



LATERAL BOUNDARY 

 CONDITION: NATURAL 

 (FIXED) BEACH- 



Figure 4. Definition sketch for coordinate system, shoreline, 

 seawall, and lateral boundary conditions 



37. The longshore transport rate, Q , is usually calculated from the 

 "CERC" formula (SPM 1984, Chapter 4): 



Q = K' (H 2 C g ) b sin 2 e bs 



(2a) 



16 



