where M is the total number of gridlines in the x direction, N the total 

 number of gridlines in the y direction, and T the total time simulated in 

 seconds. The number 3,000 is the approximate number of computational points 

 per second the model processes on an IBM 3033; on a CRAY computer, it would 

 be approximately 20,000. 



It is evident that the grid region, grid spacing, time step, wave charac- 

 teristics, and depth are interrelated and that certain limits are imposed on 

 the grid spacing and the time step for proper model performance. A number of 

 parameter variations may be required before an acceptable combination is 

 determined. If waves of different heights and periods are used in the model, 

 each case should be checked for conformance to limitations. 



2. System 21 Mark 8 Model Topography . 



Once the model grid has been established, certain information must be 

 supplied at each gridline. First the elevation of each gridline must be 

 specified. This process is automated since the large number of gridlines 

 precludes it being done manually. 



If elevations on the base map are randomly spaced, they can be located in 

 the grid by the x and y gridline that is closest. Values of x, y, and 

 z (elevation) can then be processed by the interpolation program described in 

 Herchenroder (1981)3 to obtain elevations along each gridline. The resolution 

 of topographic features in the region is directly related to how much infor- 

 mation is supplied to the interpolation program. If a topographic feature is 

 not defined by spot values, it will not be present in the interpolated values. 



An alternative method to specifying the spoj: value of an elevation is to 

 specify the location of contours of an elevation. By choosing the contour 

 interval, control is kept over which features are to be resolved in the region. 

 The contours can be digitized to obtain a set of x, y, and z values for 

 input into the interpolation program. This program, in turn, supplies the 

 depth values along each gridline. 



III. SYSTEM 21 MARK 8 MODEL COMPONENTS 



1. Initialization Unit (lU) . 



The lU creates the input to the computational unit (CU) and also checks for 

 errors in the data. Further, it allows the user to make changes or additions 

 to a set of basic model parameters that define the type run to be made. Ex- 

 amples of such parameters would be the number of time steps, the size of a time 

 step, and any options used to print and store the results. In addition, it 

 allows for the adjustment of parameter fields, such as the initial surface 

 level, the frictional bed resistance eddy coefficients (which are related to 

 the diffusion of momentum), and the porosity coefficients (which are related 

 to energy loss due to flow through porous media) . Boundary data are also 

 specified through the lU. These data designate which sections of the grid 



^HERCHENRODER, B.E., "Products From Two Computer Programs Which Process 

 Digital Bathymetric Data," CETA 81-13, U.S. Army, Corps of Engineers, Coastal 

 Engineering Research Center, Fort Belvoir, Va., Oct. 1981. 



