Geographical limits for a grid of depth values take into account the 

 probable origin points for waves of all deep-water (d/Lo^O.5) approach 

 angles of interest. By convention, the grid origin is located in the 

 southwestern corner of the area to be covered, with the X axis extending 

 eastward along the southern border and the Y axis extending northward 

 along the western border. The grid interval is selected such that, in a 

 given cell, bottom contours can be represented by straight and parallel 

 lines. (This representation will be discussed in detail later.) 



Actual or interpolated depths at grid intersections are recorded to 

 the nearest foot, and all real depths are made positive. Contours are 

 then drawn of depth values extending several grid units out from shore. 

 At this point, contours that are symmetrical reflections of nearshore 

 bottom contours are drawn on the land, over a perpendicular distance from 

 the shore of two grid units. "Depths" at the grid points on land are then 

 derived in the region of the symmetry contours on land; these "depth" 

 values are made negative. All other land "depths" (that is, those farther 

 than two grid units from the shore) are made zero. (The procedure of 

 assigning negative values for nearshore "depths" on land is required for 

 the fitting of wave-velocity surfaces.) The depth values so obtained are 

 prepared as input to the DISTV program (Appendix B) . 



Example of Input. An area of the mid-Atlantic Bight (Figure 1) is 

 chosen to illustrate the method of data input and the computation of wave 

 refraction. A specific wave-ray target area 1*6 the region covered by the 

 depth grid is the shoreline of the Borough of Virginia Beach in the City 

 of Virginia Beach, Virginia (Figure 1). 



The wave input data used here are not representative of any given 

 wave spectrum observed or hindcast. They approximate the 15 most commonly 

 observed combinations of wave height, period, and direction observed at 

 the Chesapeake Lightship (Figure 1). These combinations were determined 

 and condensed for input to the computer at the request of the Coastal 

 Engineering Research Center. The method used in determination of the 

 combinations is described in Appendix G. The authors have used the six 

 wave period — direction combinations (Tables 1 and 2), extracted from 

 the 15 height-period-direction combinations of Appendix G, merely to 

 illustrate the method of ray construction. 



The depth grid (Figure 1) of the example was chosen so that the 

 origin points of six second waves approaching Virginia Beach from 60° and 

 90° T would be positioned in water depths of greater than 92 feet 

 (d/Lo>0.5 for T = 6 sec). 



Outlines of the 99 X 81 unit depth grid used in the example are 

 shown on Figure 1; the grid origin is located at 7601.9'W, 36039.,5'N, and 

 the grid interval is 3,040 feet. U. S. Coast and Geodetic Survey (boat- 

 sheet) charts 5988, 5990, 5991, 5992, 5993, and 6595 were used in obtaining 



