the diffracting tip will be set equal to the depth -limited wave height deter- 

 mined by the relation H b = 7^ . 



269. GENESIS Version 2.0 does not allow formation of a tombolo; i.e., 

 the model will fail if the shoreline reaches or comes close to the breakwater. 

 It should also be noted that common diffraction theories, including the one 

 used in GENESIS, are technically invalid if the structure is very short (a 

 fraction of a wavelength) or for distances from the breakwater less than about 

 one wavelength. Placement of detached breakwaters should be made carefully in 

 light of these limitations. 



270. A variety of configurations of detached breakwaters can be repre- 

 sented in GENESIS. Part VII gives examples of more intricate placements of 

 detached breakwaters and the associated instructions in the START file. 



271. Line G.l: IDB If there are detached breakwaters on the model 

 grid, the value 1 ("yes") of the flag IDB is entered here. If there are 

 no such structures on the grid, including the boundaries, answer with the 

 value ("no"), and skip Lines G.3-G.9. (If the value is placed at Line 

 G.l, Lines G.3-G.9 will not be read by GENESIS, and values remaining there may 

 be arbitrary.) 



272. Line G.3: NDB Enter the number of detached breakwaters NDB 

 that appear on the grid. 



273. Lines G.4 and G.5: IDB1 . IDBN . The flags IDB1 and IDBN 

 tell GENESIS if there are detached breakwaters crossing the boundaries 



(no = 0; yes =1). If a model boundary is placed across a detached break- 

 water, waves diffracted by the tip of the breakwater located outside the grid 

 will not be taken into account. Thus, such a structure will be regarded as 

 semi -infinite with only the tip of the breakwater lying within the grid to 

 produce diffraction. 



274. The capability of placing detached breakwaters across grid boun- 

 daries should be used with caution. If a groin is not simultaneously located 

 on the boundary, GENESIS will apply the default pinned-beach boundary condi- 

 tion, which may not be appropriate in the shadow zone of the detached break- 

 water. The true meaning of the pinned-beach boundary condition is "the beach 

 does not want to move"; if the pinned-beach boundary condition is improperly 

 used, it may incorrectly mean "the beach is not allowed to move." 



115 



