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Zero overflow volume in manholes that show True in the "is ever overflowing"?

SewerGEMS: Why I have zero overflow volume (Hydraulic Reviewer manhole table) in manholes that show "True" in the "Is ever overflowing" field? I'm using the explicit SWMM solver and I don't have bolted covers. Thank you very much.

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  • Hello Ana, 

    Are you using any surface storage like ponded area for manholes? Is it a free outfall or tailwater elevation is defined? 

    In case of explicit solver overflow is determined by the total inflows minus the total outflows which is based on the enforced node elevation. 

    Do you see any flooding in SWMM calculation summary reports as below? 

    How do the dynamic solvers calculate overflow from a catchbasin or manhole? 

    If required you may upload model files for our review, Sharing model files.

    Regards,

    Sushma Choure

    Bentley Technical Suppport

  • Hi Sushma,

    We have "No storage" in the "Surface storage type" field for all of our manholes.

    All the outfalls are "Free outfall" type

    I see flooding in SWMM calculation summary report, but it is not equal to the overflow volume reported in the Hydraulic Reviewer:

    The model is uploaded here: https://drive.google.com/file/d/11UH8ob_L3i-ZmMBSg7XsZd_y65QBi0DC/view?usp=sharing (I tried to upload it into the message but it showed some kind of error). The scenario I'm reviewing is Escenario11_Usuarios30_IDF2021_TR10_HuffLocal.

    Thank you very much! 

     

  • I tested this with the Routing Step set to 1.0 seconds and was able to observe the spike in HGL that is causing the momentary overflow:

    I see that with the increased routing step, the flow continuity error increases though. It appears that this momentary spike in HGL and overflow is caused by instability. The following article has further guidance on how to address this. You may need a combination of the reduced Routing Step as well as other adjustments and a close look at the data input (which can also cause stability problems): Troubleshooting unstable SewerGEMS and CivilStorm model results using the Explicit SWMM Solver

    I will plan to take a deeper look at the model to see if I can locate any specific recommendations to improve stability and avoid these "spikes", and will let you know if I find anything.


    Regards,

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

  • Ana Maria,

    Try the following calculation options:

    • Routing step: 1.0 seconds
    • SWMM Output Increment: 0.5 minutes
    • Surcharge Method: Slot
    • Max trials per timestep: 20

    With this, the %non-convergence is 3%, the Flow Continuity Error is 0.4% and the overflow volume in the calculation summary matches pretty close to the sum of the overflow volume in the hydraulic reviewer. Also, I was only able to find a few manholes with "is ever overflowing?" set to True, without overflow showing in the overflow graph (8576963, 6066226, 6066059, 6060055, 6065070)


    Regards,

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

  • I was able to further improve the results by reducing the SWMM Output Increment to 0.1 minutes. With this, I was able to view positive values in the overflow graph for all elements with "is overflowing ever?" set to True. 

    I have updated this article based on the outcome of this conversation.


    Regards,

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

  • Hi Jesse.  I tried the calculation options you proposed and the results are very good.  The surcharge method "Slot" is the Preissman Slot?  What is the difference between Preissman Slot method and Extran method?  

    Thank you very much!!!!! Very helpful advice!!!

  • Hello Ana, 

    The Preissmann slot method is used for simulating pressure or surcharged flows for gravity conduits, by adapting the conceptualization of pressurized flow to fit a free surface model. The slot extends vertically from pipe crown to infinity and over the entire length the pipe, and the width of the slot is usually 1% of the characteristic pipe dimension (diameter for a circular pipe) but not large than 0.02 ft. Read more about this in help documentation - search for "Pressure (Surcharged) Flow and Overflow (Street Flooding)".

    EXTRAN is the original SWMM method to simulate pipes under pressurized conditions. When the nodes at both ends of a pipe have a water stage higher than the top of the pipe at that node, EXTRAN switches to equations for pressurized pipe flows.

    Also you can read more about the comparison in this below discussion on SWMM forum. 

    Surchage method

    Regards,

    Sushma Choure

    Bentley Technical Suppport

Reply
  • Hello Ana, 

    The Preissmann slot method is used for simulating pressure or surcharged flows for gravity conduits, by adapting the conceptualization of pressurized flow to fit a free surface model. The slot extends vertically from pipe crown to infinity and over the entire length the pipe, and the width of the slot is usually 1% of the characteristic pipe dimension (diameter for a circular pipe) but not large than 0.02 ft. Read more about this in help documentation - search for "Pressure (Surcharged) Flow and Overflow (Street Flooding)".

    EXTRAN is the original SWMM method to simulate pipes under pressurized conditions. When the nodes at both ends of a pipe have a water stage higher than the top of the pipe at that node, EXTRAN switches to equations for pressurized pipe flows.

    Also you can read more about the comparison in this below discussion on SWMM forum. 

    Surchage method

    Regards,

    Sushma Choure

    Bentley Technical Suppport

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