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Overflow in SewerGEMS

I  would to ask about the screenshot below; why the Hydraulic Grade Line does not go down below the ground level in spite of the flow "overflow" goes down during the simulation (9 hours), the expectation is the HGL level to dropdown.

Regards,

Mohamad Azzam

  • Hello Mohamad,

    The issue is likely related to some instability in the calculations. The following link has information on this: Zero overflow at manhole despite HGL above rim. Adjusting the LPI coefficient may help with the results. There are other steps you can review as well.

    If that doesn't help, some other adjustments in the calculation options may help with the issue. The See Also section in the link above as some additional information on adjustments you can make

    If this doesn't help, we will likely need to see a copy of the model files

    Regards,

    Scott

  • Hello Scott,

    The model is uploaded to the shared file link.

    The model is not implicit and does not have any bolted manhole , no gullies and no gutters.

    It is built based on an explicit solver, would you please let us know what steps need to solve the instability issue you have mentioned?

    Thank you,

    Mohamad

  • Hello Mohamad,

    This is related to the surface storage used at the manhole. When using Ponded Area surface storage, a constant surface area is assumed and no overflow will occur at any depth. With the Ponded Area surface storage type, positive values for "Flow (Overflow)" indicate subsurface flooding flow that exits the rim elevation and enters the ponded area. This volume is not lost and can later recede back into the manhole. This link has more information.

    Based on this, this appears to be working as expected. As a further test, if you set the manhole to use no surface storage and recompute the model, you will see the hydraulic grade reach the rim elevation and Flow (Overflow) results while the hydraulic grade is at the rim elevation.

    Regards,

    Scott

    Answer Verified By: Mohamad Azzam