General Considerations:Elevation Map
Elevation data are input for every active grid cell in a GRASS ASCII map file specified with the ELEVATION project file card. The elevation data are perhaps the most important inputs for GSSHA modeling. The quality of the elevation data plays a major role in success of GSSHA simulations. Elevations in the grid are derived from digital elevation model data (DEM).
DEMs always contain errors. Large flat areas in the DEM may be due to the limited vertical resolution of elevation data from which the DEM was derived. Extensive flat areas usually cause problems for the 2-D explicit diffusive-wave overland flow routing used in GSSHA. Digital dams, pits, and depressions in the DEM may be artifacts of the interpolation scheme used to rasterize digitized contours, or due to coarse resolution in regions of concave topography.
In addition, the grid size used in a GSSHA model is normally coarser than the available DEM data. Elevation data in the grid must be somehow interpolated from the DEM data and lumped into larger areas. This process can introduce additional error in the elevation of grid cells. As a rule, the user must cross check the elevation values with in-field observations or topographic maps of the area. Digital topographic maps are often available and can be displayed as a background image in WMS.
One way to discover potential errors in the elevation data is to perform a simulation with the most basic GSSHA model: a single event with uniform rainfall, overland flow, a relatively short time step, and no other options. Surface depth output maps should be written frequently (see DEPTH and MAP_FREQ project file cards). If the simulation finishes without an error, the surface depth maps should be examined to determine where most water accumulates and whether such accumulations are justified by the topographic map of the watershed. Alternatively, the model may crash. The location of grid cells where problems occurred will be printed on the screen and also at the bottom of the run summary file.
Editing of the elevations in the grid is often necessary to impose the actual drainage trend observed in the topographic map. Digital dams, pits and depressions must be removed since they trap surface runoff that would otherwise contribute to the outlet discharge. Using grids with raw elevations requires shorter computational time steps, while properly prepared grids, particularly those with coarser resolution, allow use of longer time steps. If channel routing is performed, care should also be taken to ensure that overland flow runoff reaches grid cells that contain channel links. If the stream network is delineated independently from the DEM, e.g. from a digital line graph (DLG), then the elevations of the grid cells containing channel nodes should be checked to insure they are not higher than those of the surrounding cells. Otherwise the overland flow will not be correctly passed to the channels.
The cleandam.exe routine can be used to do much of the above. Cleandam.exe is described in a another section of the manual.
WMS offers the option of importing and displaying vector stream location files (DLG) to aid in stream channel delineation. WMS users can also automatically delineate streams from the DEM using the tools in the WMS software. GRASS users may use the r.watershed command to automatically delineate the streams. As either automatic delineation relies on the DEM to determine stream locations, there may be substantial differences from the DLGs. If an automated method is used to locate the stream locations from the DEM data and the grid resolution used in the GSSHA model is coarser than the available DEM data the stream may not fall in the lowest elevation grid cells. However the stream is located, the elevations of the cells containing the stream may need to be manually adjusted to ensure proper overland-channel interaction.
- 4 General