Monday, December 8, 2008

Profiles that intersect-Not Good!

Written by Chris Goodell, P.E., D. WRE | WEST Consultants
Copyright © RASModel.com. 2008. All rights reserved.

Have you ever plotted out your profiles (10-year, 50-year, 100-year, whatever) only to see two profiles intersecting? It's maddening, because you know it's not possible (unless you have something like a levee breach). But if there's not physical explanation, there must be a numerical fix for it. Sometimes, just adding in a few more cross sections in the reach where the intersection occurs will fix the problem.








5 comments:

  1. What if adding a cross-section doesn't help and the profiles still cross?

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  2. First I would check to see if the profiles crossing are because of some numerical error, in which case you would have to determine what is causing it. For example, if there is a very large and unrealistic head loss on one profile causing them to cross. This could be from improperly defined ineffective flow triggers, bad htab parameters, or poor definition of flow conveyance (i.e. n value break points). It's also very common to see profiles intersect just upstream or through a bridge/culvert. Usually this is due to a flow regime switch (low flow, to pressure flow to pressure/weir flow). A check of the bridge modeling approach and/or coefficients used may help to cure this problem. Other than that, it's really hard to say without seeing your model.

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  3. Are crossing energy gradelines also a cause for concern?

    I have a situation where a 25-yr flow over tops a bridge roadway and is therefore effective in a 1000' wide overbank; while the 10-yr flow does not over top the roadway and is therefore NOT effective in the overbank (in the obstructed area where the full XS of flow is not effective). The result is much lower velocities for the 25-yr flow vs the 10-yr. This is causing the 10-yr to have significantly higher energy than the 25-yr. Is this hydraulically kosher?

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    Replies
    1. Not really kosher typically. Sounds like the overbanks are taking too much percentage of the overall cross section conveyance. I would suggest raising your n values in the overbanks until the energy levels look more reasonable.

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    2. Thank you for the quick response Mr. Goodell!

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