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HGL lower for larger rainfall event in CivilStorm

james Eason
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I have a Civil Storm stormwater outfall model setup to run the Implicit Dynamic Wave Solver. My areas are entered in with SCS CN data and my rainfall is entered as time-depth. I am running the 10-, 25-, 50-, and 100-year storm through the system and the main goal is to get a flood elevation at a low point in the system. There is an HGL increase in the HGL at the low point between the 10- and 25-year storm but not from the 25- to 50- or 100-year storm events. In fact the HGL actually decreases from 25- to 50- and from 50- to 100- The model volumes, volumes loss, continuity errors (0.2% max), and all of the other outputs for the individual scenarios seem to make sense. 

There is a diversion in the system where the main outfall splits, which this solver ignores but felt like I should at least mention it. The lowest two structures in the system are part of a tributary system and not the main outfall. These are the only structures in this part of the model that the HGl exceeds the rim elevation for the 10- and 25-year storms. In the 50- and 100-year storms there is another structure in the system that over tops. All of the structures that overtop have stage-storage ponding area above the rim as well. 

I am wondering if the structure in the main outfall that is overtopping in the bigger storms is giving me the issues that I am seeing in the model. I have double checked all of the background data and I don't see anything that looks wrong but I am not 100% sure what could be causing this. I also have a peak flow model in a separate file with the same system and it seems to be working and giving "logical" results. (I.e. the HGL increases with the increased rainfall). 

This is about the best I can describe the model and issues I am having. Any help would be greatly appreciated. I almost feel like it is something simple but I have exhausted everything I know to look for. 

Thanks in advance, 

James E. 

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  • 0 Offline

    We took a look at your model in the latest build of CivilStorm (08.11.05.58) with the latest cumulative patch applied.

    Below is the data at a junction upstream fro the O-1 outfall.  As you can see the hydraulic grade increases between each scenario.

    This is a graph of the outfall O-1 in the different scenarios.  This is a slight difference between each scenario.  Is this what you are seeing?

    These values appear to be correct considering the flow and the size of the pipes.

    Terry Foster
    Technical Support
    Hydraulics and Hydrology product line
    Bentley Systems, Inc.


  • 0 Offline in reply to tlfoster56

    Terry, This is similar to what I am getting. My major concern was between structures EJB-2B and EDI-2D. Below is the 25-year storm HGL.

    The 50-year output for this outfall looks like this:

    This is the area that concerned me since the HGL actually lowered during the larger storms. If you step through this outfall in the 50- and 100-year storms it appears that it almost has two peaks. This one at time 12 hrs. and then another small peak about an hr later. Is this what the model is showing on your end? If the model seems valid on your end that makes me feel a little better about it but it doesn't follow conventional logic. 

    On a side note, Is there anyway to change the time step in the graphs (especially the engineering graphs) besides just getting as close to the peak as possible, hitting plan, and then hitting pause when the peak hits. I feel like there should be some easier way to do this. 

    Thanks for your help. 

  • 0 Offline in reply to james Eason
    Sir, you can change the time step by going to the time browser (Analysis > Time Browser). There you can change the increment and if you right-click on Time from Start you can change it from minutes to hours or whatever you want. In the time browser, you can click on the time that you want to look at and the profile will jump to that time.

    I do see the peak at 12.05 hours, but I do not see a second peak as you mentioned. I see that the model is worked on in the latest build of CivilStorm. Do you have the latest patch applied?

    Terry Foster
    Technical Support
    Hydraulics and Hydrology product line
    Bentley Systems, Inc.


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  • 0 Offline in reply to james Eason
    Sir, you can change the time step by going to the time browser (Analysis > Time Browser). There you can change the increment and if you right-click on Time from Start you can change it from minutes to hours or whatever you want. In the time browser, you can click on the time that you want to look at and the profile will jump to that time.

    I do see the peak at 12.05 hours, but I do not see a second peak as you mentioned. I see that the model is worked on in the latest build of CivilStorm. Do you have the latest patch applied?

    Terry Foster
    Technical Support
    Hydraulics and Hydrology product line
    Bentley Systems, Inc.


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