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StormCAD/ CivilStorm Inlet Over-topping Issues

Hello,

I have a question regarding a project that I am currently working on in both StormCAD and CivilStorm. Some of the background information on the project is as follows:

The project that I am currently working on is approximately a 27-acre site with appropriate stormwater infrastructure proposed on-site.  There are currently 4 different files for the existing and proposed system one for each outfall (8 total files).  The files are set up in such a way that the model doesn’t terminate until the “subject site” is less than 2.5% of the system flow or until the system hits an actual outfall.  Therefore, a large portion of the model is existing piping.  The original modeling for this site was given to me from the city as a StormCAD file.  Since all of the existing information was given to me in StormCAD models, this facilitated creating the 4 proposed models in StormCAD as well.

A couple things I noticed about the existing StormCAD models that were given to me was that a large portion of the existing system was under pressure already.  Additionally, all the inlets were set up as “sag inlets” as opposed to “on grade” with gutter flow as it appears is the likely drainage scenario for the inlets.  

Now that there’s some background info on the files, this brings us to the current issue that I am experiencing-  Currently the proposed StormCAD files are over-topping the existing inlets at several locations. Please note that I am allowed to over-top the inlet structure by 0.5’, but at some locations it is more than that allowable head.  One of the attempts to remedy this was the addition of gutter flow in the proposed models as discussed above.  When this didn’t fully remedy the issue, I did research on the Bentley Forums and saw that StormCAD doesn’t properly model the over-topping. After a discussion with the city engineer, they feel that if we are to use StormCAD we now shouldn’t be over-topping the inlets at all.  This has led me to the Bentley Product CivilStorm to try and analyze the project. After reviewing CivilStorm, it appears that the only way to model the over-topping correctly would be to change the way everything has been analyzed thus far.

At this point I am looking to find out what is the most appropriate way to utilize these programs to take the internal overflow and keep it in the model.   While not redoing all of the modeling if possible, as most of these StormCAD models are existing infrastructure which I have limited information on.

Any and all help or thoughts on this issue would be truly appreciated.

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  • Hello Anthony,

    Though we talked about this on the phone, I wanted to post a response on the forum so other users may benefit as well. We can continue any conversation on the forum or through the service request that was created for this.

    The following link has information on the differences between the solvers: Difference between solvers- GVF-Convex vs GVF-Rational vs Implicit vs Explicit (SWMM).

    You may have already reviewed this. Basically, which solver to use will depend on what the client is after. If they want a peak flow, worst-case analysis, StormCAD may be the way to do. This will assume that the intensity in your IDF curve will be equal to the time of concentration for the catchment, and will likely have a relatively high intensity and thus the greater possibility of overflows. While surface storage is not modeled in StormCAD, the fact that it will overflow will tell you that the system is undersized.

    When there is an overflow at a catch basin or an unbolted manhole, the hydraulic grade will reset to the rim elevation. The following link has additional information about this: Why is the HGL reset to the rim elevation for flooded structures?

    If the user does want to model a longer duration storm and account for surface storage, the Implicit or Explicit solver can be used. The same IDF curve can be used, but since there will be a time element, the Modified Rational Method will be used. The intensity of the storm can be changed by entering a duration in the Global Storm Events dialog. If you were to use the 8 hour duration, the intensity will be much lower compared to the intensity at the catchment time of concentration (which the StormCAD solver assumes), so there is a change that there will be no overflows at all.

    More information on the Modified Rational Method can be found here: Understanding the Modified Rational Method.

    I would talk to the reviewer to confirm what it is they need modeled, since I would expect much different results for an 8 hour storm versus a shorter duration.

    Regards,

    Scott

    Answer Verified By: Sushma Choure 

Reply
  • Hello Anthony,

    Though we talked about this on the phone, I wanted to post a response on the forum so other users may benefit as well. We can continue any conversation on the forum or through the service request that was created for this.

    The following link has information on the differences between the solvers: Difference between solvers- GVF-Convex vs GVF-Rational vs Implicit vs Explicit (SWMM).

    You may have already reviewed this. Basically, which solver to use will depend on what the client is after. If they want a peak flow, worst-case analysis, StormCAD may be the way to do. This will assume that the intensity in your IDF curve will be equal to the time of concentration for the catchment, and will likely have a relatively high intensity and thus the greater possibility of overflows. While surface storage is not modeled in StormCAD, the fact that it will overflow will tell you that the system is undersized.

    When there is an overflow at a catch basin or an unbolted manhole, the hydraulic grade will reset to the rim elevation. The following link has additional information about this: Why is the HGL reset to the rim elevation for flooded structures?

    If the user does want to model a longer duration storm and account for surface storage, the Implicit or Explicit solver can be used. The same IDF curve can be used, but since there will be a time element, the Modified Rational Method will be used. The intensity of the storm can be changed by entering a duration in the Global Storm Events dialog. If you were to use the 8 hour duration, the intensity will be much lower compared to the intensity at the catchment time of concentration (which the StormCAD solver assumes), so there is a change that there will be no overflows at all.

    More information on the Modified Rational Method can be found here: Understanding the Modified Rational Method.

    I would talk to the reviewer to confirm what it is they need modeled, since I would expect much different results for an 8 hour storm versus a shorter duration.

    Regards,

    Scott

    Answer Verified By: Sushma Choure 

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