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SwerGEMS volume difference

I have noticed the following two cases in the SewerGEMS Regarding volume: 

Case 1:

The total reported system outflow volume in the calculation summary is not matching the summation of the volume out at the outfalls as marked in the screenshot below

Case2:

The Statistic "Sum" Volume (total runoff) is not equal to the total runoff for the whole catchments (when I copy to excel and do summation), also comparing to the total Inflow in the calculation summary.

From Statistic and calculation summary = 9,927.7 m3

From Excel = 9936.8 m3

The model is uploaded to the secured link, please check scenario "DM 10YR 1440MIN"

Regards,

Parents
  • Hello Mohamad,

    For Case1, a timestep of 0.5 minutes gave me the total volume as 9919.8 m3 in Calculation Summary and 9919.6 m3 from the sum of both outfalls. A smaller timestep can help you in this case to get both the values same or very close to each other.

    See this article for troubleshooting Implicit Solver results: Troubleshooting unstable SewerGEMS and CivilStorm results using the implicit solver

    For Case2, I kept the timestep as 0.5 minutes only and the results match in this case too. The total runoff from the catchments by copying the runoff column and adding up the volume in Excel gives a sum of 9927.65 m3 and the Statistics show the sum as 9927.66 m3. With an improved timestep and higher display precision (2 in this case) the results would match up. See this article for increasing display precision: Changing the display precision (decimal places) or format for an attribute

    Hope this helps.


    Regards,

    Yashodhan Joshi

    Answer Verified By: Mohamad Azzam 

  • Hello Yashodhan,

    That works, and I did check the Troubleshooting post.

    after that change in the calculation option, I Run the scenario "DM 5YR 1440 MIN" the Total Inflow volume is Less than the Total system Outflow Volume, is that possible? 

    Regards,

    Mohamad

  • Hello Yashodhan,

    The above screen shot from Eng. Azzam shows that the continuity  error is 0%, then how there is difference between inflow & outflow volume.

    Also if there is continuity error then outflow volume should be slightly lesser than the inflow volume - please explain.

    Regards

    Ganesh Bala S

    Regards

    Ganesh Bala S

    Design Engineer

    Khatib&Alami Engineering Consultants Pvt. Ltd.

  • Ganesh,

    When rounding to one decimal place, the mass balance error can still be 0.0% while the inflow and outflow volume (no overflow or volume change in this case) can still be slightly off. This article explains how the continuity error (CE) is calculated. In this case:

    CE = (6417.3 - 6419.0 - 0.0 - 0.0) / 6417.3
    CE = -1.7 / 6417.3
    CE = -0.0002649 (-0.026%, rounding to 0.0%)

    The slight differences seen in this particular modeling case indicate that the results are quite stable and well within the typical range of what would be acceptable, especially considering the margin of uncertainty with many other physical parameters and with the actual built system.


    Regards,

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

  • Hello Ganesh,

    If you change the display precision of the continuity error to 3 you would see a value of 0.026%. 


    Regards,

    Yashodhan Joshi

  • CE = (6417.3 - 6419.0 - 0.0 - 0.0) / 6417.3

    The question is: if the Ti "Total Inflow Volume"=6417.3 m3 how does the model generate a bigger value To "Total Outflow Volume"= 6419.0 which is bigger? the Continuity Error is calculated later based on those parameters.

    i.e. In which cases can be total outflow volume is greater than the total inflow volume? or what is making that happened?

    Regards,

    Mohamad

  • The question is: if the Ti "Total Inflow Volume"=6417.3 m3 how does the model generate a bigger value To "Total Outflow Volume"= 6419.0 which is bigger? the Continuity Error is calculated later based on those parameters.

    i.e. In which cases can be total outflow volume is greater than the total inflow volume? or what is making that happened?

    This is in the nature of dynamic modeling - the dynamic numerical solvers need to trade off stability with performance and the results will never be perfectly accurate. Typically a continuity error less than 5% (and the discrepancy in volumes resulting from such mass balance error) is acceptable for the purposes that models are used for (also mentioned here), especially when you consider the degree of uncertainty with other modeling parameters (exact roughness, elevations, etc) and the margin of error in the actual built system. To quote my colleague Dr. Tom Walski: "worrying about accuracy at this level is like worrying about whether your bill at McDonald's is $4.8956 or $4.8963."

    As Yashodhan had mentioned, if you adjust the calculation options a bit more per the troubleshooting article, you can get the inflow and outflow volume even closer. However as I mentioned above, you may accept the discrepancy as being within normal tolerances for the purposes of your analysis.


    Regards,

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

    Answer Verified By: Mohamad Azzam 

Reply
  • The question is: if the Ti "Total Inflow Volume"=6417.3 m3 how does the model generate a bigger value To "Total Outflow Volume"= 6419.0 which is bigger? the Continuity Error is calculated later based on those parameters.

    i.e. In which cases can be total outflow volume is greater than the total inflow volume? or what is making that happened?

    This is in the nature of dynamic modeling - the dynamic numerical solvers need to trade off stability with performance and the results will never be perfectly accurate. Typically a continuity error less than 5% (and the discrepancy in volumes resulting from such mass balance error) is acceptable for the purposes that models are used for (also mentioned here), especially when you consider the degree of uncertainty with other modeling parameters (exact roughness, elevations, etc) and the margin of error in the actual built system. To quote my colleague Dr. Tom Walski: "worrying about accuracy at this level is like worrying about whether your bill at McDonald's is $4.8956 or $4.8963."

    As Yashodhan had mentioned, if you adjust the calculation options a bit more per the troubleshooting article, you can get the inflow and outflow volume even closer. However as I mentioned above, you may accept the discrepancy as being within normal tolerances for the purposes of your analysis.


    Regards,

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

    Answer Verified By: Mohamad Azzam 

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