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StormCAD - HGL vs Depth in Properties Table

On StormCAD's profile view, the HGL for a pipe/conduit shows that the pipe is flowing full, and the HGL is around 0.5ft below the ground elevation. But the Conduit Properties table for this pipe shows that the Depth is 0.86Ft. It is an 18" pipe. I need to show the Q,V,D values for this pipe in my plan set, what D value should I use? 0.86 Ft will mean that the flow is contained within the pipe, but the HGL will show that the pipe is flowing full.

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  • Just to reiterate the important part of Sushma's response - the "Depth" results that you have highlighted in your screenshot are the Normal Depth and Critical Depth. These are not the actual computed depth but rather reference values for the Mannings-based normal depth and critical depth. You can read more about normal depth and critical depth in the theory section of the help or in hydraulics theory books.

    The actual computed depth is seen in the "Results (Profile)" section of the properties, where you will see "Depth (In)" and "Depth (Out)". These are based on a Gradually Varied Flow (GVF) backwater analysis, where conditions in downstream pipes are accounted for, so the flow is not necessarily at normal or critical depth. In your case if the highlighted pipe essentially had a free outfall (no downstream influence) and was sufficiently long enough to reach "normal" conditions, the water depth would be equal to the "Depth (Normal)" figure but since there is downstream surcharging accounted for, the actual depth is higher. This article has more information on this subject.


    Regards,

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

Reply
  • Just to reiterate the important part of Sushma's response - the "Depth" results that you have highlighted in your screenshot are the Normal Depth and Critical Depth. These are not the actual computed depth but rather reference values for the Mannings-based normal depth and critical depth. You can read more about normal depth and critical depth in the theory section of the help or in hydraulics theory books.

    The actual computed depth is seen in the "Results (Profile)" section of the properties, where you will see "Depth (In)" and "Depth (Out)". These are based on a Gradually Varied Flow (GVF) backwater analysis, where conditions in downstream pipes are accounted for, so the flow is not necessarily at normal or critical depth. In your case if the highlighted pipe essentially had a free outfall (no downstream influence) and was sufficiently long enough to reach "normal" conditions, the water depth would be equal to the "Depth (Normal)" figure but since there is downstream surcharging accounted for, the actual depth is higher. This article has more information on this subject.


    Regards,

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

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