Flow optimization failed to converge

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Flow optimization failed to converge

Arthur B
Hello !

I need to calculate the volume in different sections of a river for several flow values.
HEC-RAS has no problem for low values, but concerning high values (more than the 100-year-flood) there is an error : " Flow optimization failed to converge ".

I've seen on this article ( http://hecrasmodel.blogspot.fr/2009/02/how-to-best-model-junction.html) that it may come from xs near the junction but mine are as close as possible to the junctions.

I tried to change the maximum iteration in split flow from 30 to 60 but I've got the same error. ( However, volume values calculated by HEC-RAS has changed !)

I'm lost ... does anybody know what can be the cause of this error ?

Thanks in advance.

Arthur
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Re: Flow optimization failed to converge

Chris G.
Administrator
You can always try a different initial flow distribution.  See what RAS is heading towards while going through the optimization.  If it looks like the optimization runs are converging, but just run out of iteration attempts before it could get there, then use the flow distribution of the last iteration attempt, and then rerun RAS.  If the optimization runs are diverging, or going unstable (i.e. solution errors are oscillating out of control, rather than minimizing for each iteration), then you may just have a geometry that is not conducive to split flow optimization.  It could be that one of the channels is so much smaller than the other, or the ground points on one of the splits are so high, that the channel wants to go dry. In that case, perhaps a pilot channel would help.  

If nothing seems to work, you can always use a stepdown scheme hotstart to let RAS determine the distribution of flow.  http://hecrasmodel.blogspot.com/2010/12/how-to-create-hotstart-file-in-hec-ras.html

good luck
Chris G.
@RASModel
www.therassolution.com
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Re: Flow optimization failed to converge

Arthur B
Thank you Chris. I'll try that and see if it works.
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Re: Flow optimization failed to converge

Scott La Vanne
In reply to this post by Chris G.
Chris,

How do you know whether a steady-flow optimization is diverging or converging? Is there an option to review the iterations?

Thanks

Scott

Scott LaVanne

Water Resources Engineer

WBK Engineering, LLC

View Scott LaVanne's LinkedIn profile View Scott LaVanne's profile

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Re: Flow optimization failed to converge

Kevin
Scott,

I know for Steady State the following grabs the flows for you:

1. Open the Steady Flow Data window
2. Click options
3. Click Initial Split Flow Optimization (LS and pumps)
4. Highlight the cross sections that have laterals or pumps
5. Click Get Split Flows from Output

There may be a similar tool for unsteady.

- Kevin
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Re: Flow optimization failed to converge

Derek
In reply to this post by Scott La Vanne
did you find out how to check the values from the iteration steps?

thanks!