Significant drops in the bed profile can also be a source of model stability problems, especially at low flows. Significant drops in the elevation of the channel bed can cause flow to pass through critical depth and results in an unstable model solution. An example of this type of problem is shown in the figure below.
If the drop is very small, then usually an increase in base flow will drown out the drop, thus preventing the model from passing through critical depth. If the drop is significant, then it should be modeled with an inline structure using a weir profile at the top of the drop. This will allow the model to use a weir equation for calculating the upstream water surface for a given flow, rather than using the unsteady flow equations. This produces a much more stable model, as the program does not have to model the flow passing through critical depth with the unsteady flow equations. HEC-RAS automatically handles submergence on the weir, so this is not a problem. An additional solution to this problem is to use the cross section rating curve option at the top of the drop, which causes the program to interpolate the water surface from the rating curve, rather than solving the unsteady flow equations through the drop in the bed profile.
