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  • 1.  HEC-RAS - Modeling Lateral Weirs Over Roadway Cross-sections

    Posted 05-11-2017 02:25 PM
    We are working with an unsteady HEC-RAS model created by someone else for a small, flat, highly urbanized watershed.

    Channel by-pass flow comes out of the main channel, flows through several blocks of neighborhood in an ill-defined path, and then re-enters the main channel some distance downstream.

    The original modeler used a lateral weir to depict the long stretch through which flow leaves the channel. The lateral weir crosses several roadways (which have bridges or culverts).

    The model does not appear to be correctly handling the flows coming in and out of the channel, all of the time.

    Can you use lateral weirs in HEC-RAS across roadway cross-sections? The HEC-RAS manual does not seem to address this issue.

    If not, what is a solution other than removing the lateral weirs in the model and using a different modeling technique to represent the bypass flow?

    Karen Kabbes P.E., D.WRE, ENV SP, F.ASCE
    Kabbes Engineering, Inc.
    Barrington IL

  • 2.  RE: HEC-RAS - Modeling Lateral Weirs Over Roadway Cross-sections

    Posted 05-12-2017 09:46 AM
    In general, for a 1-D RAS model you should limit lateral weirs to upstream or downstream of the bridge/culvert cross-sections.  In 4.1 and earlier, RAS would pop up with an error if the geometry had a lateral structure and internal structure overlapping.  That error is curiously gone in version 5.0.3.  It appears the 5.0.3 solver now simply ignores the lateral weir at the bridge/culvert:  flow in the channel is identical upstream and downstream at a bridge even for the case of a lateral weir that is set well below the water surface.

    Good practice would be to split the lateral weir such that it does not extend into the reach between the bridge cross-sections.

    What's on the floodplain side of the lateral weir?  It can be connected to another reach, level pool, downstream XS or the water dumped out of the model.  As long as you're not removing the water from the model, the lateral weir could be impacted by a tailwater condition for a flat reach.

    I'd suggest reviewing the WS vs lateral weir crest profiles, reducing the lateral weir to only the primary overtopping points and then trying to get the model working for that case.  Alternatively, you could model the floodplain (downstream side of the lateral weir) as another reach or in 2D if more detail is desired.  Or if you have a good idea how much flow is bypassing then you could hard code that into the flow file.

    Patrick Flanagan P.E., M.ASCE
    Principal Engineer
    Indicator Engineering PLLC
    Seattle WA

  • 3.  RE: HEC-RAS - Modeling Lateral Weirs Over Roadway Cross-sections

    Posted 05-12-2017 11:14 AM
    Hi Karen!

    Patrick is correct in that lateral structures should not cross roadways. The simplest solution would be to split up the single long lateral structure into multiple shorter lateral structures in between each road crossing. It wasn't clear from your message how the flow in the overbank is being modeled - sounds like a good application for a 2D area to me. One other thing to check is the weir coefficients for the lateral structures, as these should often be different than your typical value for in-line weirs depending on the configuration. There is some guidance from HEC on appropriate values to use.


    Martin Teal P.E., HG, D.WRE, F.ASCE
    Vice President
    WEST Consultants, Inc.
    San Diego CA

  • 4.  RE: HEC-RAS - Modeling Lateral Weirs Over Roadway Cross-sections

    Posted 05-16-2017 09:41 AM

    I saw your post on this.  I went to my experts and Andy Yung offers this response -

    "I believe an early version of HEC-RAS (I think it was version 3.0.1) did allow lateral weirs to be defined across roadway sections.  However versions after that do not allow this to occur.  I have had this situation before in which I had a really long lateral weir with a bridge in the middle.  My solution was to break the lateral spillway into two separate spillways (one upstream and one downstream of the bridge).  That seemed to work well for the scenario I was working with.  I hope this helps."

    Charles Penland P.E., M.ASCE
    Senior Principal
    Walter P Moore
    Houston TX

  • 5.  RE: HEC-RAS - Modeling Lateral Weirs Over Roadway Cross-sections

    Posted 05-17-2017 11:35 AM

    Using separate weirs as Mr Penland indicated also worked for us in our models of canal networks in southern Florida.

    Mark Wilsnack P.E., D.WRE, F.EWRI, F.ASCE
    South Florida Water Management District
    West Palm Beach FL

  • 6.  RE: HEC-RAS - Modeling Lateral Weirs Over Roadway Cross-sections

    Posted 05-17-2017 12:52 PM
    hello sir i need help in HEC RAS 2d working does any one of you have any expertise in that?

    Moazzam Rind S.M.ASCE