alternating cells. An artificial limit on the absolute value of the wave an- 

 gle, 86 deg, is written into the model. Problems of this nature can sometimes 

 be eliminated by using finer grid resolution. 



98. Cell size is the second factor defining the grid system. Accuracy 

 of model solutions computed using finite difference approximations is depen- 

 dent upon cell size; smaller cells result in less error. Diffractive effects 

 can become important on spatial scales with orders of magnitude smaller than 

 the wave lengths being considered. In modeled regions of interest where dif- 

 fraction is important (very complicated bathymetry) cell sizes may need to be 

 some fraction of the wavelength in order to simulate wave propagation accu- 

 rately. Fine resolution is not a requirement for model applications where the 

 bathymetry is very regular. The physical processes of importance, whether it 

 be refraction or a combination of refraction and diffraction, dictate cell 

 sizes which should be used. 



99. The examples in the next two sections represent two different kinds 

 of problems. Example I deals with wave propagation over a rather small region 

 with complex bathymetry. Diffraction was presumed to be an important physical 

 process; therefore, finer grid resolution was used in an attempt to represent 

 this effect more accurately. Example II illustrates the applicability of 

 RCPWAVE for propagating waves over a very large area dominated by very regular 

 bathymetry. Assuming that refractive effects would dominate wave transforma- 

 tion throughout the region, cell sizes were increased to a value on the same 

 order of magnitude as the wave length. These two examples show the diversifi- 

 cation of problems which can be addressed using the model. 



Example I: FRF Pier, Duck, North Carolina 



100. Bathymetry in the vicinity of CERC's research pier at Duck, North 

 Carolina, is quite complex as seen in Figures 11 and B2. Figure 11 was 

 plotted using similar bathymetric data digitized onto the rectangular grid 

 system comprised of 75 cells in the offshore (x) direction and 50 cells in 

 the longshore (y) direction. Again, cell sizes are 12 and 24 m in the x- and 

 y-directions, respectively. All depths on the bathymetric file are given in 

 metres below MSL. A constant tidal elevation of 0.90 m is added to the 

 bathymetric data. Two wave conditions are considered: (a) 2-m-deep water 

 wave height, 12-sec wave period, and +20 deg deepwater incident angle (refer 



54 



