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You are correct, which is why dam breach models are among the most difficult to stabilize in HEC-RAS. How do you deal with model instability caused by such cases? However, if one is going to model a dam breach or reservoir operation, the outflow will be, no doublet, a dramatic change and decrease of flow and stage in a very short time. I understand your idea about a sudden change and its impact on model stability. 10 Tips for Troubleshooting Unsteady HEC-RAS.Steps taken to stabilize a HEC-RAS model, along with the dataset used.
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Stabilizing a Dynamic Unsteady HEC-RAS Model. A great summary of potential causes for instability in HEC-RAS models, especially those with low stages.
HEC RAS BLOGSPOT MANUAL
This manual (along with the Hydraulic Reference Manual) comes with the installation of the HEC-RAS software. Particularly the section on Model Accuracy, Stability, and Sensitivity. HEC-RAS User’s Manual Chapter 8, Performing an Unsteady Flow Analysis.The “Unsteady Flow” and “Dam Breach” classes both cover techniques for troubleshooting an unsteady flow HEC-RAS model. And consider taking an HEC-RAS training course if you can. If you’re having trouble with your unsteady flow model, please carefully read through these references.
HEC RAS BLOGSPOT 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.
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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. 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. 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”