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If you’re having trouble with your unsteady flow model, please carefully read through these references.
#Hec ras hydraulic reference manual how to
Here are some very helpful references that deal directly with how to troubleshoot HEC-RAS unsteady flow models that are unstable and/or crashing. That understanding will allow you to make informed and intelligent decisions on what techniques to use to stabilize your model. This is why it is important to understand the theory behind the computations in HEC-RAS. Keep in mind, stabilizing your model may require more than one “change”. Also understand what accuracy (if any) you are sacrificing to achieve the increased stability. By methodical, I mean understand what you are doing, and why it can improve numerical stability. Making methodical, logical, and beneficial changes to your model is a much better approach.
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Haphazardly making changes and adjustments to your model without any forethought or strategy, in an attempt to make it stable could very well make it worse and get you nowhere-and waste a lot of time. Whatever steps you take to try to stabilize your model, make sure you are confident that the steps you are taking will improve stability without giving up more accuracy then you’re willing to sacrifice. It could be a sudden increase/decrease in stage. It could be a sudden increase/decrease in flow.
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It could be a sudden and significant change in the size and shape of the main channel from one cross section to the next. If your model is crashing, look for places or times where something is changing drastically. “HEC-RAS likes things to change gradually”