This discussion has been locked.
You can no longer post new replies to this discussion. If you have a question you can start a new discussion

SewerGEMS Max Discharge Higher than Physically Possible

I have a gravity/pressure combination SewerGEMS model that has multiple inflows that total 15.15 cfs and the model shows at the outfall the discharge is 15.15 cfs. This is physically impossible because the line is only 24" and should only be able to carry 11.9 cfs. Why isn't the model showing that there is a restriction in the flow due to the pipe size?

  • 11.9 cfs corresponds to 3.8 ft/s which doesn't sound line a restriction. What is you basis for saying that the flow is physically impossible?

    Even at 15.15 cfs, the velocity is only 4.8 ft/s which is high but not excessive.

    If given the pipe slope, the pipe can only carry 11.9 cfs when flowing full, the hydraulic grade slope will increase when the flow exceeds that value.. If that's the case, the model would show flows backing up until the level hits a low manhole rim and the manhole would overflow.

  • A good way to check what is going on, is to plot the profile for a few pipes upstream of the outfall.

  • Tom,

    We used FlowMaster to check the model and found given the slope it should have a discharge of 11.90 cfs.  Below is the profile. The upper reach (0+00 to 35+00) is 18" with 9.9 cfs and then 5.25 cfs is added at approximately 35+00 where it changes to 24". From approximately 35+00 to 72+50 is the reach that from flowmaster should be 11.90. I can't find anywhere in the FlexTables that states that flow is lower in this reach.

    Thank you for your help!

  • Hi Jackie,

    Can you provide a screenshot of the FlowMaster setup? I can see from the profile that many pipes between 35+00 and 72+50 are flowing under pressure (more than full) and they also have multiple different slopes. If the 11.9 figure you are citing is the full flow capacity based on normal depth (flow if normal depth is equal to pipe top), that could explain why you see more flow. Tom had mentioned this in his earlier post and this article explains a bit more (the concept is similar for Full Flow Capacity in FlowMaster).


    Regards,

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

  • Jesse,

    Here is the screen shot of FlowMaster:

    What still isn't clear is how my conduit table does not line up with my outfall. Here a small shot of the conduit table:

    This shows that the Capacity (Full Flow) is less than Flow (Maximum) which makes sense based on the slopes and diameters of the pipes. But that does not match the outfall which shows a Flow (Total Out) of 15.15.