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StormCAD: Pond will not function, Flow and Depth do not change with different storms

User Notification: Starting water elevation (689.23 ft) is lower than pond's invert(or minimum) elevation (700.00 ft).

I am using StormCAD V8i (SELECT series 3) trying to use it to check the flow out an area where ponding is possible.  My network outfalls into an area where ponding is possible due to an existing 30in CMP culvert that is now draining more area than before due to a new project.  Both the outfall and a very large catchment flow into the pond, 9.26 cfs from the outfall and 79.06 cfs drain from the catchment for the 50 yr storm.  The flow out of the pond is shown as 2.88 cfs, even when I change the storm data the pond will flow at 2.88 while the outfall and catchment change.  The pond depth node is always 0 for every storm I run. 

I used Elevation-Area for the pond volume and i tried to model the pond as dry to begin with by defining the start elevation as 689.23 the invert of the 30in CMP.  The pond invert seems to be set at 700, the highest elevation i entered in the elevation-area table, 700 is also the invert of the outfall. 

Is there a way I can set the Pond Invert? 

 

  • Hello LeeFricke,

    The pond element is new to StormCAD as of SELECTseries 3 and operates a bit differently than it does with our products (or solvers) that route hydrographs. Since StormCAD uses a peak flow, steady state analysis, you are only looking at a single instance in time; no routing or storage effects are involved. So, the assumption made with ponds in StormCAD is that, if you have an outlet structure specified, the pond outflow is based on the flow from the outlet's rating curve, at the specified initial elevation. So for a peak flow analysis, you might decide to estimate a water surface elevation near the top, to be conservative. It then looks up this elevation on the rating curve and the resulting flow is what you see leave the pond. It does not route hydrographs or analyze storage because there is no element of time involved with a steady state. So, this explains why you can get the same flow out of the pond even when the upstream inflow changes (because the initial WSE and outlet rating curve stay the same.)

    I believe there is a slight issue with the wording of the user notification you mentioned - it should be referring to the pond elevation instead of the top. What is the lowest elevation in your elevation-area curve? Could it be that it is something slightly higher than the 689.23 that you're entering as the starting water elevation (such as 689.2342). Regardless, with the starting elevation set to the bottom, you will get zero flow out of the pond, since you will have zero flow on the outlet rating curve at a depth of zero. You will want to select something higher than the bottom, to estimate what the peak pond outflow will be for that particular return event.


    If you use this method, it is worth noting that the "Initial Conditions" alternative stores the initial elevation setting. So, if you need to have a different initial water surface elevation for each return event being analyzed, you'll need to make new initial conditions alternatives and assign them to the respective scenarios, so that the values are retained separately. See the help topic on scenarios and alternatives or quick start lesson #3 for more on this subject.

    If you want to analyze storage in the pond to size it or see what the actual maximum outflow rate is, you will need to use a product that supports hydrograph routing, either SewerGEMS, CivilStorm or PondPack. So for the StormCAD model, you may consider ending the model just before the pond (at an outfall, possibly with an estimated tailwater elevation), then analyze the pond separately in one of the previously mentioned products. Or, if you will be using CivilStorm or SewerGEMS, you can take advantage of the shared file format; you'll be able to directly open the StormCAD model, with the pond included. File exchange and the single file format are one of the main reason why the pond (and some other element types) are available in StormCAD.

    If you'd like to simply assume flow in = flow out for this pond, simply choose "no" for "Has control structure?" in the properties of the pond outlet.

    Here are some related resources that you might find helpful. For future questions or concerns, try out the search bar here in the Hydraulics and Hydrology community to find other Support Solution articles, technotes, FAQs and discussions that may help.

    Pond and Pond Outlet Structure Elements in StormCAD
    StormCAD Vs. CivilStorm
    StormCAD layout question

    Hope this helps,


    Regards,

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

    Answer Verified By: Scott Kampa