Tuesday, May 19, 2015

My Unsteady HEC-RAS Model is Unstable…Why?

Written by Christopher Goodell, P.E., D.WRE  |  WEST Consultants
Copyright © The RAS Solution 2015.  All rights reserved.

This question (or some variation of it) comes up quite a bit on The RAS Solution:  “I have an unsteady flow model.  When I run it, it goes completely unstable.  What is causing this?”  Many times a screen shot of the computation window with the dreaded “red bar” is attached.  


While the person asking the question genuinely needs help, what he/she doesn't understand is that it is impossible for anyone to troubleshoot his/her model with this limited amount of information or without the model data files.  There are an almost infinite number of possible reasons the model crashed.  

If I could sum up my approach to troubleshooting unsteady HEC-RAS models in one sentence, it would be
“HEC-RAS likes things to change gradually”

If your model is crashing, look for places or times where something is changing drastically.  It could be a sudden and significant change in the size and shape of the main channel from one cross section to the next.  It could be a sudden increase/decrease in flow.  It could be a sudden increase/decrease in stage.  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.  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.  Making methodical, logical, and beneficial changes to your model is a much better approach.  By methodical, I mean understand what you are doing, and why it can improve numerical stability.  Also understand what accuracy (if any) you are sacrificing to achieve the increased stability.  Keep in mind, stabilizing your model may require more than one “change”.  This is why it is important to understand the theory behind the computations in HEC-RAS.  That understanding will allow you to make informed and intelligent decisions on what techniques to use to stabilize your model. 

Here are some very helpful references that deal directly with how to troubleshoot HEC-RAS unsteady flow models that are unstable and/or crashing.  If you’re having trouble with your unsteady flow model, please carefully read through these references.  And consider taking an HEC-RAS training course if you can.  The “Unsteady Flow” and “Dam Breach” classes both cover techniques for troubleshooting an unsteady flow HEC-RAS model. 
  • HEC-RAS User’s Manual Chapter 8, Performing an Unsteady Flow Analysis.  Particularly the section on Model Accuracy, Stability, and Sensitivity.  This manual (along with the Hydraulic Reference Manual) comes with the installation of the HEC-RAS software.  You can access it from the main HEC-RAS window under Help…User’s Manual.



 Stabilizing a Dynamic Unsteady HEC-RAS Model.  Post on The RAS Solution.  Steps taken to stabilize a HEC-RAS model, along with the dataset used.  http://hecrasmodel.blogspot.com/2013/10/stabilizing-dynamic-unsteady-hec-ras.html



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