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StormCAD inlet surging

I am trying to model a combined surface and sub-surface drainage system and the main software I have to use is StormCAD. I know CivilStorm can model both of these systems simultaneously but I can't purchase this software at this time. What I am wanting to know is if I can achieve this in StormCAD.

Basically I have a model already created in StormCAD and when I view the HGL at various inlets it is way over the top of the grate of the inlets. Now I know b/c some downstream pipes lack sufficient capacity the HGL is back-calculated up the pipe system until it surges out of the far upstream inlets. What I am wanting to determine is how much water is coming out of these inlets. I already looked at the help menus for StormCAD and it says the HGL here is innaccurate so I can't depend on it for any calculations. What I have tried as a work around is to start with an EGL downstream and adding friction losses and assuming an HGL at the upstream top of grate elevation I can achieve a velocity and from that get the flow that should be in the pipe and subtract that from its capacity to get how much is coming out of the inlet. What I am worried about however is the value I am using for the headloss. I have tried to calculate my own headloss but I get nothing close to what StormCAD does and I am worried that if I use the value it report for the conduit I am analyzing, it will be based off a too small velocity.

Please help anyone. I know CivilStorm is the no-brainer solution to this but I can't afford to purchase this software at this time.

Thanks for any help.

Paul

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  • Paul,

    As you point out, StormCAD isn't designed for this kind of calculation.

    I don't quite follow the workaround you described, but I think it is similar to what I would do if I were forced to use StormCAD for this type of calculation. Basically what I would do is replace all of the catchment runoff flows with manually entered flow values (like 'Flow (Additional Subsurface)' or 'Flow (Known)'). Then I would tweak those flows, increasing them until the HGL just reaches the rim elevation at an inlet. The difference between the manually entered flow value (when the HGL just reaches the inlet rim elevation) and the flow values due to catchment runoff computed by StormCAD would be a very rough approximation of the overflow rate.

    Unfortunately, though, the problems with this approach are numerous. For example, StormCAD computes peak flows based on the rational method. It is much better to compute overflow rates using a hydrograph method rather than the rational method. Also, if you are interested in runoff volume, you MUST use a hydrograph method.

    I would really recommend CivilStorm. If you can't buy it, you could talk to your account manager about getting it on a short-term subscription.

    Regards,

    Mal Sharkey
    Bentley

          

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  • Paul,

    As you point out, StormCAD isn't designed for this kind of calculation.

    I don't quite follow the workaround you described, but I think it is similar to what I would do if I were forced to use StormCAD for this type of calculation. Basically what I would do is replace all of the catchment runoff flows with manually entered flow values (like 'Flow (Additional Subsurface)' or 'Flow (Known)'). Then I would tweak those flows, increasing them until the HGL just reaches the rim elevation at an inlet. The difference between the manually entered flow value (when the HGL just reaches the inlet rim elevation) and the flow values due to catchment runoff computed by StormCAD would be a very rough approximation of the overflow rate.

    Unfortunately, though, the problems with this approach are numerous. For example, StormCAD computes peak flows based on the rational method. It is much better to compute overflow rates using a hydrograph method rather than the rational method. Also, if you are interested in runoff volume, you MUST use a hydrograph method.

    I would really recommend CivilStorm. If you can't buy it, you could talk to your account manager about getting it on a short-term subscription.

    Regards,

    Mal Sharkey
    Bentley

          

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