We are thinking to use HEC RAS for flow routing, either for flooding, forecast flow, or for planed water release (flow scenarios) from an upstream dam. use a higher minimum flow value).Ī further question about unsteady flow analysis. Providing a good "cushion" at the front end of a dam breach wave will help as well (i.e. The Mixed Flow option is a great tool for minimizing the effects of large accelerations at the front end of a dam breach wave. 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.
![hec ras unsteady flow hec ras unsteady flow](https://media.cheggcdn.com/media%2F4d8%2F4d85602a-395f-4278-80a3-7557b02f1d70%2FphpDe6utZ.png)
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. Stabilizing a Dynamic Unsteady HEC-RAS Model. A great summary of potential causes for instability in HEC-RAS models, especially those with low stages. Investigating Instabilities with HEC-RAS Unsteady Flow Modeling for Regulated Rivers at Low Flow Stages, Sharkey 2014.Master’s Thesis by Jennifer Kay Sharkey.You can access it from the main HEC-RAS window under Help…User’s Manual.
Hec ras unsteady flow 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 unsteady flow 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. 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.
![hec ras unsteady flow hec ras unsteady flow](http://www.appsolutelydigital.com/ModelPrimer/images/image-46.png)
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.
![hec ras unsteady flow hec ras unsteady flow](https://www.kleinschmidtgroup.com/wp-content/uploads/attachments/attachment-7266-1.png)
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”