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SewerGEMS Runoff Volume Calculation Differences

Hi All,

I have what I hope is a simple question about how the volume of runoff for a site modeled in sewergems is calculated. When I run the scenario for the desired storm in SewerGEMS, the calculation executive summary shows that there is a total inflow volume of 13.3 ac-ft into the site (See image 1). However, when I go into the catchment table for the same scenario, and add up the Volume (Total Runoff) column for the catchments, the result is 13.7 ac-ft. If my interpretation of these two calculations is correct, then it looks like the model is saying that more runoff is leaving the site than stormwater that entered the site (which doesn't make sense to me). If I could have someone explain to me what is happening here that would be very helpful. Thanks!

  • Hello Joshua,

    From the calculation summary you can see that;

    Total Inflow = Total Overflow + Total Outflow = 1.4 + 11.9 ac-ft = 13.3 ac-ft

    In the catchment flextable try and change the tolerance of the "Volume (Total Runoff)" field to 2. I checked this with a sample model and based on the tolerance the sum is little skewed and may appear different. You can right click on the column, go to Units and Formatting > Display Precision - change to 2.

    More details on display precision can be found here:Changing the display precision (decimal places) or format for an attribute

    Now see if the results are similar. It should show a total outflow of 11.9 ac-ft as 1.4 ac-ft is the overflow volume.

    Let me know if this helps.


    Regards,

    Yashodhan Joshi

  • Thank you for the reply. By changing the "tolerance" of the Volume (Total Runoff) column are you meaning just update the precision from 1 decimal place to 2 decimal places? When I change the precision for the column from 1 to 2, The sum of the runoff volumes is 13.87ac-ft. The total inflow volume is still 13.3 ac-ft, so the data still does not align.

    A secondary question I now have is about the "Total System Overflow Volume". Can you define exactly what this term means? Is this flow that is lost from the model, and if so, how can I see what elements are overflowing so that I can make sure everything is captured? This term seems to be misleading to me because when I look at all possible places where flow could overflow (channels, manholes, inlets, ponds) none of these elements show any overflow at any timestep. However, I am receiving warnings that "one or more elements are flooding". I don't understand what this is telling me?

  • Hi Joshua,

    For the inflow volume problem - the inflow volume in the calculation summary accounts for all flow entering the system from external sources. This includes but is not limited to catchment runoff volume. Check other elements to see if there are for example inflows or other loads added directly to nodes in the model. It may help to open and review the tabs of the Inflow Control Center and Sanitary Load Control Center. If you need further help with this, please provide a copy of the model for review: Sharing Hydraulic Model Files on the OpenFlows Forum

    For the overflow volume question - overflow refers to flow that is lost from the system when the water level rises above the rim elevation of catchbasins, ponds, wetwells, unbolted manholes, etc. To see per-element details of not only the overflow volume but also the other components of the mass balance, use the Hydraulic Reviewer.


    Regards,

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