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Chapter 8 Performing an Unsteady Flow Analysis Drops in the Bed Profile. Significant drops in the bed profile can also be a source of model stability problems, especially at low flows. If the drop is very small, then usually an increase in baseflow will drowned 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. 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 example of a profile drop that causes a model stability problem is shown in Figure 842. Unsteady with smaller event Geom: Base Geometry Data Flow: Brunner River Wolfe Reach 494 Legend WS 10FEB1999 0100 492 Crit 10FEB1999 0100 Ground 490 Elevation (ft) 488 486 484 482 480 478 20000 25000 30000 35000 40000 45000 50000 Main Channel Distance (ft) Figure 8-42. Stability Problem caused by drop in bed profile. When an Inline Structure (weir) is added to the above data set, the model is able to obtain a stable and accurate solution of the profile (Figure 8-43). 8-71
