GENESIS 



Assumptions. Shoreline change models generally have five basic 

 assumptions: (a) a constant beach profile shape, (b) constant shoreward and 

 seaward limits of the profile, (c) sediment transport is described as a function 

 of breaking waves, (d) the detailed structure of nearshore circulation is 

 neglected, and (e) a long-term trend in shoreline evolution (Hanson and Kraus 

 1989b). For wave transformation calculations, GENESIS assumes that the 

 beach profile conforms to an equilibrium profile shape, 



D=Ay m ( 16 > 



in which D is the water depth and A is an empirical scale parameter that 

 relates to the median beach grain size as follows: 



A = 0.4 lD^ 94 for D 50 <0Amm 



A = 0.23D^ n for 0.4mm £D 50 < 10. 0mm (ll) 



O 28 



A = 0.23£>5<; for 10.0wn^D 50 < 40.0mm 

 A = 0.46D 5 M for 40.0mm £D 50 



Consequently, only one point on the profile is required to determine its shape; 

 this point is typically taken as the mean high water shoreline. 



Sediment is assumed to be transported alongshore between two well- 

 defined elevations on the profile, the top of the active berm at the shoreward 

 limit, and the depth of closure offshore. Longshore sediment transport in 

 GENESIS is determined with an empirical formula, 



Q = H 2 b C gb ( ai smie bs - a^sB,^) 08) 



in which H b is the breaking wave height; C b is the wave group speed at 

 breaking, given by linear wave theory; 6 bs is the angle of breaking waves to 

 the local shoreline; and x is the longshore coordinate. The non-dimensional 

 coefficients a 2 and a 2 are given by 



K 



a, = 



(19a) 



02 = 



\6(S-Y)(\-P) 

 8(S-l)(l-/>)tan/S 



52 



(19b) 



Chapter 3 Tools for Prediction of Morphologic Response 



