Tuesday, July 7, 2009

New Features 4.0.1 Part 3...Modified Puls Hydrologic Routing

Written by Chris Goodell, P.E., D. WRE | WEST Consultants
Copyright © RASModel.com. 2009. All rights reserved.

This will be a very useful feature for those pesky dam breach unsteady flow models (or any very dynamic unsteady flow model) with steep sections of reaches and low depths. By designating a portion of your reach to be solved using the Modified Puls Hydrologic routing technique, you are removing a common problem with unsteady flow solution scheme used-it is very sensitive in steep, shallow reaches, especially during the rising limb of the breach hydrograph.

In the previous version of RAS, we could sometimes model short steep reaches with inline structures, but that always felt a little like cheating to me. Plus, what discharge coefficient do we use? For longer steep reaches, we would have to run a separate HEC-HMS model of the affected reach, then import those results to steady flow RAS to map it. That was kind of painful, but occasionally necessary. Now all of it is built into HEC-RAS, so switching to a different model is no longer required.

First thing you have to do is set up a steady flow run with multiple profiles (flows). These flows should encompass the full range of expected flows in your unsteady flow run, from low initial flows to the peak of the flood event. I found that this feature did not work when it had to extrapolate. Keep in mind that the more steady flows you include, the better storage/outflow relationship can be established.

In the geometry editor, go to Options...Hydrologic Unsteady Routing, and you'll get the following window. Here you simply select regions where you want to apply the modified puls routing and then import the storage-discharge relationship (RC's) from the steady flow profiles for each region. Each cross section within a region will be treated as an individual Modified Puls "Reservoir". Make sure you select the "Use Modified Puls Routing" checkbox, and then you can compute the unsteady flow run.



7 comments:

  1. Chris,

    Allowing different routing techniques along the reach is a great help, no reason to go fully dynamic on a 3% reach. Isn't one of the limitations of the modified puls that it should not be used for steep hydrographs? That would pretty much rule out using it for dambreaks.

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  2. Simon-

    My understanding is the reason Mod. Puls is not good for quickly rising hydrographs is because it is essentially level pool routing (which does not allow for hydraulic gradients withing the "pools". RAS, however, will set up a pool at each cross section within the range you select to run Mod Puls. So...if your cross sections are spaced very tightly, and you have a small enough computational time step, I think it should work okay for steep hydrographs. Please provide some insight if there are other reasons not to use Modified Puls in RAS.

    Thanks for the comment.

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  3. where to get the hec-ras 4.0.1beta package now?

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  4. I would call up HEC (or email). Tell them you are doing research and would like to test the new Modified Puls routing option. They're usually happy to have more testers and would be likely to send you the beta version of 4.0.1. No guarantees though.

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  5. Is there a definite rule as to what counts as a steep reach? 1%? 3%? Or a question of if the unsteady simulation does not work then use Mod Puls on the troublesome sections (which do tend to be the steeper parts of the reach especially when doing DB analysis)?


    Thanks

    Neil

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  6. This is great information, thanks’ for share!

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  7. I have heard from multiple sources that anything over 2% is fair game for modified puls. For 2% and less, the slope/flow is probably not the primary reason for model instability.

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