External Wave Transformation Model : RCPWAVE 



155. In many applications offshore contours cannot be considered as 

 plane and parallel. In these cases accurate modeling of shoreline change 

 requires calculation of the nearshore waves using the actual bathymetry. For 

 the open-coast situation, the linear wave transformation model RCPWAVE 

 (Ebersole 1985; Ebersole, Cialone, and Prater 1985) has advantages for use 

 with GENESIS: 



a. It solves for wave height and angle values directly on a grid. 



b. It is efficient, allowing wide-area coverage. 



c. It includes diffractive effects produced by an irregular 

 bottom, thus reducing caustic generation as well as providing 

 better accuracy than a pure refraction model. 



d. It has proven to be very stable. 



156. RCPWAVE places values of wave height and direction at grid points 

 on a nearshore reference line, shown schematically in Figure 9b. From this 

 line the internal wave transformation model in GENESIS brings waves to 

 breaking. Figure 15 shows GENESIS and RCPWAVE in the overall calculation 

 flow. 



157. Shoreline change simulation intervals are typically on the order 

 of several years, and the extent of the modeled reach several kilometers, 

 requiring hundreds of grid cells. Since the time step for the simulation is 

 typically 6, 12, or 24 hr , thousands of wave calculations must be performed. 

 It is impractical to run a wave transformation model such as RCPWAVE for each 

 time step because of the enormous execution time involved. A general wave 

 model runs on a two-dimensional grid, and its execution time is proportional 

 to N 2 , where N is on the order of the number of grid cells in the x- and 

 y-directions . In contrast, GENESIS is a one -dimensional model, and its 

 execution time is proportional to N . Therefore, it is unbalanced in 

 computational effort to perform a general wave calculation at every shoreline 

 simulation time step. As a related physical consideration, time series of 

 offshore waves are usually not available or, if available, contain uncertain- 

 ties, implying that an expensive, accurate numerical wave transformation 

 calculation would not be in balance with approximate input data. 



Ik 



