d. Computer execution time (which depends on the time step, 

 number of cells on the grid, and the simulation interval). 



325. The number of structures that can be included in the model depends 

 on the particular configuration of GENESIS which was loaded on the operating 

 system. The configuration was determined on the basis of hardware and 

 software limitations and the intended use. The maximum numbers of grid cells 

 and structures that can be expected in GENESIS Version 2.0 are: 



a. Grid cells: 600. 



b. Groins (total of nondif fracting and diffracting): 70. 



c. Detached breakwaters: 20. 



d. Beach fills: 50. 



It should be remembered that execution time increases substantially as the 

 number of diffracting structures increases. 



Representation of Structures 



326. This section describes capabilities and limitations in represent- 

 ing structures in GENESIS. Idealized examples of plan views of various 

 configurations and the appropriate section of the associated START file are 

 given for reference. The theory of "wave energy windows" and "transport 

 calculation domains," through which GENESIS operates in representing the 

 effects of most types of structures, is given in Part V. It is again noted 

 that structures are represented as inf initesimally thin objects in the model. 

 For example, a groin or jetty is located at the wall of a single cell and 

 cannot occupy the position of more than one wall. 



327. Four basic rules governing placement of structures are: 



a. The position of a structure is defined by the location of its 

 tip(s), and these positions are located at cell walls. 



b. If a lateral boundary (at either cell wall 1 or cell wall 

 N+l) is not explicitly specified to be a groin, GENESIS will 

 apply a pinned -beach boundary condition as a default. 



c. There must be at least two cells between groins. As an impor- 

 tant special case, a groin cannot be placed in the cell next 

 to a lateral boundary. 



d. The locations of the tips of diffracting structures can 

 coincide (be at the same longshore coordinate), but they 

 cannot overlap. 



139 



