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Is it okay to have a negative value for Total system overflow volume?

Hi,

Using implicit I am getting negative value for Total system overflow volume. Is this acceptable? Thanks

Regards,

Jan

  • Hello Jan, 

    It could be due to large continuity error, please check user notifications that shall guide you to any problems in the model. 

    Generally continuity error below 5 % is acceptable, please look at troubleshooting steps mentioned in below wiki to reduce the continuity error. 

    What is the continuity error and what's a good percentage for it?  

    Troubleshooting unstable SewerGEMS and CivilStorm results using the implicit solver 

    Regards,

    Sushma Choure

    Bentley Technical Suppport

  • Hi Sushma

    I've tried changing parameters but the negative volume is still there. There is a pump within the system. Maybe the pump is the reason why there is negative volume? Thanks

    Iteration No. Remarks Total inflow volume Total system overflow volume continuity error
    1 Parameters=Default Values 79,658.90 -14,858.10 60.5
    2 change NR weighting coeff=0.99 79,682.40 -15,694.20 1.7
    3 set default NR; computational distance=5 ft 79,623.10 -14,884.60 58.8
    4 set default computational distance; calc time step=0.5 min 79,473.60 -15,467.10 40.9
    5 keep calc time step=0.5 min; change NR weighting coeff=0.99 79,463.70 -13,315.90 53.1
    6 keep calc time step=0.5 min; set default NR; computational distance=5 ft 79,498.70 -13,241.20 51.7
    7 increase NR iteration=20 79,496.20 -13,317.10 63.4
    8 increase LPI coeff=10 79,300.90 -17,471.60 7.6
    9 Parameters=Default Values; computational distance=10ft 79,666.60 -14,300.70 35.9
    10 Parameters=Default Values; computational distance=10ft; NR weighting coeff=0.99 79,645.40 -15,213.70 6.9

    Default parameters:

    Output increment-3 min

    Calc timestep-1.5min

    LPI coefficient-1.0

    NR weighting coefficient-10

    Relaxation WC-0.66

    Computational distance-15.2m

    Regards,

    Jan

  • Jan,

    We will check with our lead developer, but here are a few other thoughts in the meantime:

    • Are you using a recent version? The latest is 10.03.01.08. You can find your version number either under File > Help > About or Help > About.
    • Do you see any user notifications that may indicate other problems in the system? If so you may need to take care of those first.
    • Consider trying the Explicit (SWMM) solver to see if the results and mass balance are improved. If complex pumping is involved, it may not be as stable. You may need to try a smaller Routing Step (calculation options) such as 1 second and an output increment of 0.01 hours. You can read more about the differences and strong points of these solvers in this article.


    Regards,

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