Wednesday, May 20, 2009

Flow/Conveyance Distribution

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

In HEC-RAS conveyance for a given cross section is divided into the main channel, left overbank and right overbank. The distribution of conveyance is controlled by geometric features of the cross section itself, such as the terrain, bank station locations, ineffective flow triggers, and Manning’s n values. The latter three are typically subjective in how they are defined at any given location. It is important for the modeler to define these parameters to accurately define the hydraulics in the cross section, but also to maintain numerical stability for unsteady flow modeling.

A convenient way to check for appropriately defined conveyance distribution is a quick scan of Standard Table 2, in the Profile Output Table. Standard Table 2 displays the flow in the left overbank, main channel and right overbank (Q Left, Q Channel, Q Right). The modeler should view this table and look for cross sections that show a sudden change in distribution. Any sudden change is a good indication of poorly placed bank stations or ineffective flow triggers.
The attached table illustrates how Standard Table 2 can be used to locate areas of sudden changes in flow or conveyance distribution.


5 comments:

  1. Is there any way to input rating curve when discharge and stage data are not following an increasing or decreasing trend but rather changes unpredictably with each other.

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  2. Is there any way to input rating curve to hec ras when stage and discharge are not following an increasing or decreasing trend but rather changes unpredictably with each other with time?

    ReplyDelete
    Replies
    1. Are you talking about inputting observed data? Why would you want to input a rating curve?

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  3. Hi Chris,

    I'm modeling in steady flow downstream with boundary conditions (Known W. S.), but there is a sudden change in distribution (as you said) on the first interpolated cross section after cross section downstream. And then follows a consistent pattern.

    How to solve this? It would be a geometry error?

    ReplyDelete
    Replies
    1. Check your ineffective flow areas. Make sure they are all either on or off for a given profile. Also, make sure your bank stations are placed properly.

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