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Energy Grade Line Drops Going Upstream

Hi all,

I'm working with SewerGEMS for the first time ever and am having an issue with the EGL dropping upstream (or jumping downstream) at a manhole structure. Can anyone explain this? For reference there is a large basin discharging roughly 80 cfs into this same manhole at this period in time. I would appreciate any help.

Thank you,

Kevin

  • Hi Jesse,

    I went back and looked at my stage storage output from AutoCAD Civil 3D and it looks like I missed elevation 25, which messed up my Elevation-Volume table for pond PO-2. I have attached the updated stage storage table that I used to update the pond model. I'm still seeing discrepancies between the two methods, the only change I made between the two batch computes was changing Elevation-Volume to Elevation-Area for pond PO-2. I've attached screen grabs showing the maximum ponding volume results of the two methods. Any idea what might explain this?

    Also regarding basin E-4 that you mentioned, I am fairly confident in my delineation. We have a printout from a GIS model from the client showing the network of the nearby collection system. Pairing that with the contour data helped me delineate the basin. The client hasn't provided the actual GIS data so we're not holding our breath. 

    Thanks again,

    Kevin

    PDF

  • Hi Kevin,

    Thanks for the clarification. I've taken a look at the differences in results between the E-A and E-V cases and found that, although the max storage result field appears to be significantly different between the two cases, other results such as pond outflow do not have as large of a difference. I set up a separate scenario with a new child physical alternative to observe the differences between E-A and E-V for the 100 year event, so that the pond outflow can be graphed to visualize this. You can also see the mass balance for the pond in the hydraulic reviewer, which indicates only a very small differences.

    So, I believe that the difference in results may indicate that the model is very sensitive, so a small change like this exhibits a relatively large change in some results. This can happen more often in models with sharp hydrograph peaks and certain pond geometries where conditions change quickly. This is also especially the case with the Implicit solver, which tends to require smaller timesteps and calculation option adjustment to achieve stable results with ponds. On the other hand, the Explicit (SWMM) solver tends to handle pond models better. I noticed if I use the Explicit solver in your model with a timestep of 0.025 hours, the storage results are almost identical between the E-A and E-V cases. I would recommend using the Explicit solver with this pond model.


    Regards,

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

  • Hi Jesse,

    Thanks for your detailed response. I've been working on other projects and am only now getting around to this. When you say you modeled it with a "timestep of 0.025 hours" are you referring to the Routing Time Step or the SWMM Ouput Increment? They are currently set at 5.0 sec and 0.250 hours, respectively.

    Thanks again,

    Kevin

  • When you say you modeled it with a "timestep of 0.025 hours" are you referring to the Routing Time Step or the SWMM Ouput Increment? They are currently set at 5.0 sec and 0.250 hours, respectively.

    I had set the SWMM Output increment to 0.025 hours in my testing.


    Regards,

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

  • Thanks Jesse. Can you provide any insight into the different routing methods available within the Explicit numerical solver? There is Dynamic Wave, Kinematic Wave, and Uniform Flow. I want to be sure the model is taking into account the backwater effects from downstream.

    Regards,

    Kevin