Model Construction:Verifying the basic model
For every new GSSHA model a basic simulation with uniform roughness should be run to determine the overall quality of overland flow. The minimum parameters that must be defined to run a basic simulation are surface roughness and rainfall. As described above, the time-step, total time, and outlet information must also be defined. The required steps are described below.
- In the GSSHA Job Control Parameters dialog, make sure that all optional processes (routing, sediment, infiltration, etc.) are turned off. Enter a value for total simulation time in minutes (a few hours) and a small time-step in seconds (5 to 10 sec). In the GSSHA Job Control Parameters dialog, select Output Control, toggle on Surface depth under the Data Set Map Options. Toggle on the ASCII map Type. Input a Write Frequency (minutes) such that maps of surface depth are written out every 15 to 30 min.
- For the uniform rainfall event, enter an intensity of 10 to 50 mm/hr and a duration of 60 to 120 minutes.
- Assign a uniform overland roughness coefficient, as described in Chapter 4, with a value of 0.05.
- Save the project and run GSSHA.
The model should run to completion and produce a hydrograph at the outlet. If the model runs but does not produce flow at the outlet, then either increase the total time of your simulation, your rainfall duration, or your rainfall intensity and rerun the model. Do this until there is output. The model may or may not run to completion as flow is produced.
If the model does run to completion, use the methods described in Chapter 14, Post-processing, to view the outlet hydrograph and the overland flow depth maps. These maps are useful for locating problem areas in the watershed and comparing areas of ponded water to independent topographic data. If water is ponded on the watershed at the end of the simulation (ponded water shows up as blue areas on the overland flow depth maps), compare these locations to topological maps and ensure that the ponded areas correspond to real depressions. If these areas should drain, you may have to go back and do more smoothing on the DEM or manually edit the values of elevation in the affected grid cells, as discussed in Chapter 2. Even if the ponding areas correspond to natural depressions, you may still wish to smooth the DEM or edit the grid elevations to drain these areas, as computation of overland flow with significant backwater effects requires a small time-step. Experience has shown that DEM smoothing has minimal effect on streamflow predictions.
If the overland flow routine crashes, information on problem areas will be printed to the screen and also to the run summary file. If the overland flow module will not run you can try to change the overland flow routing method to ADE-PC, reduce the time-step, or decrease the uniform rainfall intensity or duration. If the model will not run with a small time-step and the very stable ADE-PC overland flow routine, the depth maps should be consulted to identify potential problems in the watershed. The DEM may be smoothed using algorithms in the WMS software, or the elevations in the grid may also be manually edited. The information provided by GSSHA will tell you where to target editing of grid cell elevations. Zoom in on these identified problem areas; turn on the color fill contours, and display the grid cell elevations. You may have to remove flat spots, dams, or depressions that are causing the overland flow model to crash. 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.
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