Routing:Troubleshooting channel routing problems
The explicit diffusive wave scheme employed in GSSHA naturally diffuses, or smoothes, the water surface profile between sharp transitions. The scheme also contains a number of internal stability checks and will typically run on a properly prepared channel network. As discussed above, essential to a properly prepared channel network are smooth transitions between channel cross sections and thalweg elevations.
Problems typically arise when cross sections change abruptly, the thalweg elevation changes abruptly, there are regions of adverse slope in the channel section, or the channel thalweg is above the overland cell elevation. These situations should be eliminated from the beginning. If regions of adverse slope (slope is in the general upstream direction) really exist in the channel network, then it may be possible to include them in the channel network.
Once a problem occurs during channel routing, the task of the user then is to identify the location of the problem. When channel instability occurs, usually because of negative depths caused by numerical oscillations, the model ceases execution and prints warning statements identifying the problem nodes. The user should look at the node/link pairs involved and try to identify the problem. The problem can typically be corrected by smoothing the channel thalweg and/or increasing the number of channel cross sections, or reducing the depth increment in look-up tables. If a surveyed longitudinal profile is causing the trouble, it may be necessary to reduce the channel time-step or do some minimal smoothing of the profile.