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What is the best type of control structure to model an overflow pipe with a vertical drop?

We have a composite system for storm water network(see below screenshot)

I would like to model an overflow pipe(see below screen shot) by considering a start control structure at the outlet pipe.

My question is, What is the best type of control structure to model an overflow pipe with a vertical drop?

Parents
  • Mohthesham,

    Can you clarify where the overflow pipe is? See below comment in red:

    As water first reaches the overflow pipe, it may "spill" into it and act as weir flow, and then when the depth is high enough it would act as orifice flow. However, it may be acceptable to simply assume orifice flow and model the conduit control structure as an orifice with the Orifice Type set to Bottom Orifice. See more here: Using Start and Stop Control Structures for conduits and channels


    Regards,

    Jesse Dringoli
    Technical Support Manager, OpenFlows
    Bentley Communities Site Administrator
    Bentley Systems, Inc.

    Answer Verified By: Mohthesham Mohammed 

  • Hi Jesse,

    Good day!

    In continuation to the above, please see the below screenshot for the results in one of the branches. The overflow pipe drops are now modelled as orifice(bottom outlet).

    Appreciate, if you can elaborate on if the behavior of HGL and EGL are normal. How is it possible for the HGL rise above the EGL?

    regards,

    Mohthesham Hussain Mohammed

  • Mohthesham,

    Which product and solver are you using? For example the GVF-Rational (StormCAD) solver, or the Implicit or explicit (SWMM) dynamic solver in SewerGEMS.

    If you're using the Implicit or Explicit solver, ensure that the continuity error is low (see troubleshooting here and here), ensure you are looking beyond time zero (use the Time Browser to advance forward in time) and try setting the "Output Options" field for your conduits to "Detailed Results".

    If this does not help, please provide a copy of the model for review along with steps to reproduce: Sharing Hydraulic Model Files on the OpenFlows Forum


    Regards,

    Jesse Dringoli
    Technical Support Manager, OpenFlows
    Bentley Communities Site Administrator
    Bentley Systems, Inc.

  • Hi Jesse,

    Good day!

    Modelling the overflow pipe with start control structure set to orifice(bottom outlet) is giving unfavorable results for HGL as per our trailing discussion.

    However, we have got acceptable results when we set the same to weir(v-notch 60 deg.) plus allowing it to use orifice equation when surcharged.

    My question is , how to ensure the width of weir is more or less equal to the overflow pipe rather than the software considering it to be full width of the chamber? Can it be justified with the angle of the V-notch as per below screenshot?

    Appreciate your expertise support!

    regards,

    Mohthesham Hussain Mohammed

Reply
  • Hi Jesse,

    Good day!

    Modelling the overflow pipe with start control structure set to orifice(bottom outlet) is giving unfavorable results for HGL as per our trailing discussion.

    However, we have got acceptable results when we set the same to weir(v-notch 60 deg.) plus allowing it to use orifice equation when surcharged.

    My question is , how to ensure the width of weir is more or less equal to the overflow pipe rather than the software considering it to be full width of the chamber? Can it be justified with the angle of the V-notch as per below screenshot?

    Appreciate your expertise support!

    regards,

    Mohthesham Hussain Mohammed

Children
  • It is possible that the results appear better because the weir may allow more flow through and help prevent flooding/overflow. I would recommend modeling them as orifices if the control structure is actually shaped like an orifice, so you can simulate the hydraulics as closely as possible to the real system, then use the guidance in the following articles to stabilize the model results if needed:

    Troubleshooting unstable SewerGEMS and CivilStorm results using the implicit solver

    Troubleshooting unstable SewerGEMS and CivilStorm model results using the Explicit SWMM Solver

    If you are using the Implicit solver, changing to the Explicit (SWMM) may be needed as the SWMM solver tends to work better with conduit control structures. You may need to set the Routing Step to one second and set the SWMM Output increment to something smaller than the default.

    If you are actually using a different solver, please confirm and provide a copy of the model so we can look into the issue with the control structure set as an orifice.

    My question is , how to ensure the width of weir is more or less equal to the overflow pipe rather than the software considering it to be full width of the chamber? Can it be justified with the angle of the V-notch as per below screenshot?

    You could enter the orifice bottom elevation as the weir crest elevation and the orifice top elevation as the weir top elevation (with the surcharge option selected). This will limit the weir rating curve to be within the top and bottom of the orifice and cause it to use the orifice equation above the top, which should be fairly similar to the behavior of an orifice. Again though it may be best to model it as an orifice and solve the problem with the results.


    Regards,

    Jesse Dringoli
    Technical Support Manager, OpenFlows
    Bentley Communities Site Administrator
    Bentley Systems, Inc.