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Manholes and catch basins are 'Overflowing' even though the hydraulic grade line isn't above the street elevation at any time step.

Hi,

I'm annotating catch basins to see if they are overflowing. To my surprise, all of them are overflowing at all steps - even after the storm event. A particular hydraulic profile I am looking at (where overflowing elements are) shows the hydraulic grade line below the street elevation at all steps. In the help 'theory' section, it says:

'The overflow occurs when water elevation at the manhole reaches above defined highest street elevation.'

If my profile indicates the HGL (elevation of water in open channel) to be below the manhole/catch basin rim at most locations/time steps, why does the program say that I have flooding at every time step?

Below is the profile i mentioned (at a snapshot during storm event):

  • Hello Francesco,

    Please provide the model files as explained in this article.

     

    Regards,

    Craig Calvin

    Bentley Technical Support

  • Please also provide the following:

    1) Exact version of SewerGEMS that you're using (the latest is 10.01.00.70)

    2) Exact field that you're looking at - "Flow (Overflow)"? "Is Overflowing"? "Is Ever Overflowing"? Something else?


    Regards,

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

  • Just Uploaded

    1) Version 10.01.00.70

    2) Field: Is Overflowing?

  • Thanks for sending the model. The reason for the "Is Overflowing?" field is showing as "True" when the subsurface network is not flooded is because this field also triggers when there is no gutter downstream of an on-grade inlet to capture inlet bypass flow and the flow is lost to the system. You will see several user notifications about this:

    "The associated 'On Grade' inlet does not have a bypass gutter.  The bypass flow will exit the system."

    If you graph "Flow (Overflow)" and "Flow (Bypass)" for one of the catchbasins in question, you will see that the overflow is equal to the bypass flow. Because the bypass flow has nowhere to go (no downstream gutter link), the flow is lost to the system in the same manner as overflow and is counted in the Overflow result figure (and mass balance). Hence, the "is overflowing" is triggering because of the positive value for "Flow (Overflow)".

    The solution to this is to lay out gutters downstream of the offending catchbasins. I have created the following new article in our Wiki about this situation:

    Overflow reported at catchbasin even though HGL below rim elevation


    Regards,

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

    Answer Verified By: Francesco Martinez 

  • Jesse,

    would you please add model example to your Wiki post, the Wiki is really very helpful but it does need model example supporting the text.

    Thank you .................. Mohamad