Building a Model:Overland Flow Routing

The simplest GSSHA model consists of a grid with elevations from the DEM and roughness values assigned, allowing overland flow routing to be simulated. Overland flow routing should first be attempted with uniform values of rainfall and overland roughness and a small time step, on the order of 10 s. Spatially varied maps of depth should be output every few time steps. As described in Section 14.6, these maps are useful for locating problem areas in the watershed, and comparing areas that water ponds to independent topographic data. If the overland flow module will not run with a small time step and the very stable ADEPC overland flow routine, the depth maps should be consulted to identify potential problems in the watershed. The elevations in the watershed may be smoothed using algorithms in the WMS software, or the elevations may also be manually edited. If water is ponding along the edge of the watershed, these cells will either have to be removed from the grid or raised in elevation. Another potential solution to making the overland flow module run is to increase the grid size, which will reduce the Courant number, and smooth the elevations in the model.

Once the overland flow routine will run with uniform roughness parameters, initial roughness parameters and retention depths can be spatially distributed according to index maps of land use and vegetation.