the number of grid cells to approximately the 1.5 power, a regular grid with 

 100-ft cells would require almost 250 times as much computational time as the 

 variable grid shown in Figure 2. 



9. The grid shown in Figure 2 covers a portion of Pamlico Sound, approx- 

 imately 12 miles of ocean coastlines on Bodie and Pea Islands north and south 

 of Oregon Inlet, and a section of the Atlantic Ocean extending out to the 

 60-ft mean low water (MLW) contour. The ocean boundary of the grid is far 

 enough from the inlet to be in a region where the contours are straight and 

 parallel. Thus all the effects of bathymetric complexities on waves and cur- 

 rents are included in the grid computations. 



Wave Propagation Model 



10. The wave climate at Oregon Inlet is available through the Waterways 

 Experiment Station Wave Information Study (WESWIS). Twenty-year hindcast data 

 were obtained at a water depth of 60 ft off the coast of Oregon Inlet. The 

 development of a wave propagation numerical model was necessary to propagate 

 waves over the complex bathymetry around the inlet and to determine wave- 

 current interactions in the vicinity of the inlet. 



11. The traditional method of propagating waves over the nearshore re- 

 gion is wave ray tracing, in which the paths of individual wave rays are pro- 

 pagated from deeper water to shore. One disadvantage of ray tracing is the 

 difficulty of converting a large number of wave rays into arrays of wave 

 heights, wave numbers, and directions of propagation for specific grid points. 

 Such arrays are needed by the numerical models that calculate wave-induced 

 currents and sediment transport. It is difficult to convert a wave ray field 

 into arrays providing information at specific grid points because it is not 

 possible to know a priori where individual rays will propagate and thus where 

 rays must be started in order to go through given points. Another disadvan- 

 tage of wave ray tracing is that wave-current interaction calculations cannot 

 be made, since ray theory assumes that energy cannot cross wave orthogonals. 

 This eliminates the possibility of currents sweeping wave energy across 

 orthogonals. 



12. In order to eliminate the problems of wave ray tracing, a numerical 

 model was developed to calculate wave propagation on a gridded system. The 

 model considers the same system of differential equations considered by Noda 



11 



