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

Zero overflow volume in manholes that show True in the "is ever overflowing"?

SewerGEMS: Why I have zero overflow volume (Hydraulic Reviewer manhole table) in manholes that show "True" in the "Is ever overflowing" field? I'm using the explicit SWMM solver and I don't have bolted covers. Thank you very much.

Parents Reply Children
  • Hi Sushma,

    We have "No storage" in the "Surface storage type" field for all of our manholes.

    All the outfalls are "Free outfall" type

    I see flooding in SWMM calculation summary report, but it is not equal to the overflow volume reported in the Hydraulic Reviewer:

    The model is uploaded here: https://drive.google.com/file/d/11UH8ob_L3i-ZmMBSg7XsZd_y65QBi0DC/view?usp=sharing (I tried to upload it into the message but it showed some kind of error). The scenario I'm reviewing is Escenario11_Usuarios30_IDF2021_TR10_HuffLocal.

    Thank you very much! 

     

  • Hi Ana Maria,

    Thank you for providing a copy of the model. From a quick glance at manhole 6061407 (the first one on the list), it shows the maximum HGL does reach the rim elevation, yet the graph of HGL does not show this. Looking at the "Time to maximum hydraulic grade" result field, the max HGL happens at 642.252 minutes, which is between the reporting timesteps of 640 and 645 minutes (because you have the Output Increment set to 5 minutes yet the model calculates every 0.1 seconds). So, it could be that the HGL spikes up above the rim elevation for a very short period of time which happens between the reporting timesteps so you don't see it in graph view (and some other places). 

    I will try re-running the model with a shorter Output increment and will get back to you.


    Regards,

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

  • I did this with a report timestep of 1 minute and I got the same results... maybe the report timestep should be even shorter?

  • Yes I tried with 0.5 minutes and although the sum of overflow in the hydraulic viewer then matched what was shown in the calculation summary, I did not see it in the graph for the same manhole. I suspect you'll need to use an even smaller timestep, or perhaps just consider it to be a momentary overflow. I will investigate further on Monday.


    Regards,

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

  • I tested this with the Routing Step set to 1.0 seconds and was able to observe the spike in HGL that is causing the momentary overflow:

    I see that with the increased routing step, the flow continuity error increases though. It appears that this momentary spike in HGL and overflow is caused by instability. The following article has further guidance on how to address this. You may need a combination of the reduced Routing Step as well as other adjustments and a close look at the data input (which can also cause stability problems): Troubleshooting unstable SewerGEMS and CivilStorm model results using the Explicit SWMM Solver

    I will plan to take a deeper look at the model to see if I can locate any specific recommendations to improve stability and avoid these "spikes", and will let you know if I find anything.


    Regards,

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