PART VII: REPRESENTATION OF STRUCTURES AND BEACH FILL 

 Types of Structures and Their Effects 



321. GENESIS simulates the effects of common coastal structures and 

 engineering activities on the shoreline position. Generic types of structures 

 that can be represented are groins, jetties, harbor breakwaters (with respect 

 to their functioning as a jetty or groin); detached breakwaters; seawalls; and 

 the "soft structure" of beach fill. Considerable flexibility is allowed in 

 combining these basic structures to produce more complex configurations, e.g., 

 T-shaped groins, Y-shaped and half-Y groins, and jetties with spurs. Combina- 

 tions of these types of structures are also possible. 



322. In shoreline change modeling, structures exert two direct effects: 



a. Structures that extend into the surf zone block a portion or 

 all of the sand moving alongshore on their updrift sides and 

 reduce the sand supply on their downdrift sides. Blocking can 

 be direct, as by a groin or jetty, or indirect, as by the 

 calmer region of water in the formed lee of a detached 

 breakwater. 



b. Detached breakwaters and structures with seaward ends extend- 

 ing well beyond the surf zone produce wave diffraction. The 

 diffraction pattern causes the local wave height and direction 

 to change, altering the longshore sand transport rate. 



Grid Cells and Numbers of Structures 



323. For design mode modeling, it is recommended that at least nine 

 grid points (eight cells) be placed behind detached breakwaters and between 

 adjacent groins. In a scoping mode application or if a wide coastal extent is 

 being covered for which detail at any one structure is not vital, it is recom- 

 mended that at least four cells be used. 



324. Grid spacing in the modeling system should be selected through a 

 balance of the following four conditions: 



a. Resolution desired. 



b- Accuracy of measured shoreline positions and other data. 



c. Expected reliability of the prediction (which mainly depends 

 on the verification and quality of input wave data) . 



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