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PondPack V8i - Orifices in series??

Can PondPack model orifices in series?  I have a scenario where I have an existing 7" orifice plate mounted to an 18" culvert to control the outflow from a detention basin.  I now need to restrict the flow for a 1-yr event even more with a 4" orifice at the pond discharge elevation (46.86ft).  I want to allow all events more than a 1-yr to use the existing 18" culvert, so I put in a weir at an elevation slightly above the 1-yr water surface (50.25ft).  All water overtopping this weir can then re-enter the 18" culvert.  Somewhere I need to still control the release with a 7" orifice plate.  I have tried modeling the 7" orifice plate at two different spots: at the pond discharge, and at the downstream pipe of the outlet structure containing the weir.

The attached image shows what I would like to model.  PondPack enters some endless loop when analyzing this outlet structure.  It seems that when I enter two orifices in my outlet structure they both are draining the pond simultaneously (in parallel) rather than in series where one orifice's discharge would then go through the other orifice.

 Any suggestions on how to model this situation?

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  • Hello Josh,

    Yes, Pondpack can model orifices in series. Since you had also submitted your question to technical support, I took a look at your model and diagram and found the problem.Basically you need to have the 7" orifice and "upstream culvert" in series, with the 4" orifice and manhole weir in parallel, but downstream of it. Pondpack cannot handle this because you can only have one downstream ID associate with each outlet structure element. The "upstream culvert" would need to have both the manhole weir and the 4" orifice as the downstream ID. With the way you have it in your model, no outlet structure element has the manhole weir as its downstream ID, so it's as if the manhole weir is exposed to the pond headwater.

    The outlet structure calculation is not going into an endless loop but is actually taking a very long time to iterate and settle on a solution. If you increase the tolerance in both culverts to 0.1, the calculation will eventually complete, but as mentioned above, it's not an accurate depiction of what you need to model.

    My suggestion for modeling this case is to represent the upstream side of the manhole as a second pond. Basically the outlet structure of the real pond would be the 7" orifice and culvert in series, discharging to the second pond. The second pond would have the dimensions of the upstream chamber of the manhole and its outlet structure would consist of the weir and 4" orifice in parallel, each with the culvert as the downstream ID. With this configuration, a small calculation timestep and headwater increment is recommended.

    Another option would be to take a look at our CivilStorm package, which is designed to model more complex situations. Dynamic routing can occur through conduits, which can have control structures on either end.

    I hope this helps.

    Regards,

    Jesse Dringoli

    Bentley Technical Support


    Regards,

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

Reply
  • Hello Josh,

    Yes, Pondpack can model orifices in series. Since you had also submitted your question to technical support, I took a look at your model and diagram and found the problem.Basically you need to have the 7" orifice and "upstream culvert" in series, with the 4" orifice and manhole weir in parallel, but downstream of it. Pondpack cannot handle this because you can only have one downstream ID associate with each outlet structure element. The "upstream culvert" would need to have both the manhole weir and the 4" orifice as the downstream ID. With the way you have it in your model, no outlet structure element has the manhole weir as its downstream ID, so it's as if the manhole weir is exposed to the pond headwater.

    The outlet structure calculation is not going into an endless loop but is actually taking a very long time to iterate and settle on a solution. If you increase the tolerance in both culverts to 0.1, the calculation will eventually complete, but as mentioned above, it's not an accurate depiction of what you need to model.

    My suggestion for modeling this case is to represent the upstream side of the manhole as a second pond. Basically the outlet structure of the real pond would be the 7" orifice and culvert in series, discharging to the second pond. The second pond would have the dimensions of the upstream chamber of the manhole and its outlet structure would consist of the weir and 4" orifice in parallel, each with the culvert as the downstream ID. With this configuration, a small calculation timestep and headwater increment is recommended.

    Another option would be to take a look at our CivilStorm package, which is designed to model more complex situations. Dynamic routing can occur through conduits, which can have control structures on either end.

    I hope this helps.

    Regards,

    Jesse Dringoli

    Bentley Technical Support


    Regards,

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

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