249. Line C.3: DCLOS . The closure depth DCLOS (called "D c " in 

 the main text) defines the seaward limiting depth of profile movement. It is 

 entered here, referenced to the same datum as the average berm height. 



D. Nondiffracting groins 



250. The lengths of groins and short jetties are normally on the order 

 of the average width of the surf zone; wave diffraction produced by such 

 structures can be considered to be negligible, since in shallow water the 

 waves will arrive almost normal to the tip of the structure or will have 

 already broken. Thus, typical groins used for shore protection and short 

 jetties should be treated as nondiffracting structures. 



251. GENESIS distinguishes between groins (and jetties) that produce or 

 do not produce wave diffraction. Model computation time associated with a 

 diffracting structure is much greater than for a nondiffracting structure; 

 therefore, the number of diffracting groins should be minimized. The diffrac- 

 tion option, starting at Line E.l, is mainly used to describe long jetties 

 (jetties with lengths on the order of several surf zone widths) and harbor 

 breakwaters that act as a long jetty by almost completely blocking longshore 

 sand transport; these types of structures extend well beyond the surf zone 

 where waves may arrive at a large oblique angle, resulting in a wide diffrac- 

 tion zone. They also block sand transport alongshore and, therefore, are 

 functionally equivalent to groins with regard to shoreline change. 



252. GENESIS can accommodate a large number of simple groins and more 

 complex structural configurations composed in part of simple groins. Part VII 

 gives examples of START file instructions for complex configurations of 

 structures including groins. 



253. Line D.l: INDG Line D.l asks if there are groins and short 

 jetties on the calculation grid used in the particular simulation, setting the 

 flag INDG . The great majority of groins as well as jetties at small 

 channels do not extend beyond the average width of the surf zone; therefore, 

 they should be treated as nondiffracting structures that interrupt the 

 movement of sand alongshore. Bypassing of sand seaward around such structures 

 is automatically calculated by GENESIS. If the value 1 ("yes") is placed at 

 Line D.l, then responses are required at Lines D.3-D.5. If there are no short 

 (nondiffracting) groins or jetties on the grid, a value of ("no") should 



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