PART III: STORM SURGE PLUS TIDE SIMULATION 



23. The WES Implicit Flooding Model (WIFM) was used as the hydrodynamic 

 storm surge model. A detailed description will not be given in this report. 

 The numerical and hydrodynamic features of WIFM are discussed in Butler (1978) 

 and the application of WIFM to coastal studies is demonstrated in numerous 

 reports (including Butler 1983). WIFM solves the vertically integrated, time- 

 dependent, shallow-water wave equations of fluid motion using an alternating 

 direction, implicit, finite-difference algorithm. The model allows subgrid 

 barriers which can be non-overtoppable, overtoppable, or submerged. An impor- 

 tant feature of WIFM is the capability for using an exponentially stretched 

 numerical grid which permits a concentration of grid resolution in areas of 

 interest. Also included in the code is the capability to flood or .dry indivi- 

 dual cells during a simulation. 



Grid Development 



24. In order to model storm surge, it is usually necessary to extend 

 the computational grid past the edge of the continental shelf and into deep 

 water. Since it also is desirable to have small cell sizes in areas of 

 interest, a very large number of grid cells may be necessary to model a study 

 area using one grid. Consequently, in locations with a wide continental 

 shelf, as in the present study, a two-grid system is usually developed. A 

 global grid with coarse resolution extends throughout the study area and out 

 past the edge of the continental shelf. A nearshore grid which extends only 

 over the immediate study area but with much finer resolution is also devel- 

 oped. A surge plus tide event is first simulated on the global grid. Then, 

 using boundary conditions saved during the global run, the event is simulated 

 on the nearshore grid. 



25. The present study does not use this two-grid system. Because of 

 the project's proximity to the NOS tidal gage in Boston Harbor, a method was 

 devised to use the Boston tide gage in place of a global grid. Use of the 

 single grid resulted in considerable savings avoiding both simulation on an 

 outer grid and stage-frequency curve generation at a connection point between 

 two grids. This process involved setting up a single grid (Figure 6) and then 

 calibrating the model to produce correct water levels throughout the study 



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