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captured flow vs capacity

StormCAD Connect (latest version)

How is inlet capacity related to captured flow? I have an inlet with a capture curve. Its capacity shows less than a Neenah curb inlet capacity even though they are the same size. And as its capacity is less, the spread is more.


  • Hello Brian,

    If you click on the field in the properties window, you will see a description at the bottom (make the bottom section larger as needed) See: Where can I find the definition of a property?

    For the "Capacity (inlet)" result field, this shows the flow corresponding to the maximum design spread, which you would enter in the Design alternative (Inlets tab). In your example screenshot it shows 1.69 CFS for the Capacity (inlet), which seems to correspond go about 2.4 inches of depth from the gutter depth vs. captured flow curve on the left side of the screenshot. That 2.4 inches should correspond to a spread (based on the gutter slope) equal to the max spread value in your design alternative. This is a reference point that the program uses as the basis for the selection of inlet sizes during automated design, though that only applies to catalog inlets. See "designing inlets on grade" and "designing inlets in sag in this article.

    Note that the "Gutter depth. vs captured flow" inlet type is only for in-sag (100% capture) inlets. It looks at the total approach surface flow (total flow coming to the inlet) and determines the corresponding depth from the curve you enter, then determines the spread based on that depth and the gutter slope. See more here: Modeling Custom Inlets Using Inlet Capture Curves

    With that being said, the screenshot you included seems to show that the approach flow (and captured flow, since it assumes 100% efficiency) are 5.26 CFS, which seems to correspond to a depth of around 5.5 inches on the curve shown on the left side of the screenshot, yet the computed gutter depth is shown as 8.1 inches. You may need to check the other properties of the inlet (not shown in the screenshot) to ensure that it is indeed set to use that catalog inlet, and that there are no other problems with the input interfering.

    If this still does not help, please provide a copy of the model for review, and reply back with the file name and steps to reproduce: Sharing Hydraulic Model Files on the OpenFlows Forum


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