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PondPack Routing Analysis and Water Elevations?

Hey Everyone!

I am currently trying to design a storage pond in Pondpack V10, and am running into a couple of problems.

First, the output generated by the model and displayed in the "outlet design" tab of the pondmaker worksheet is very different from the output generated by running a full routing of the flows. Specifically, the estimated storage is much lower than the actual storage required, and the interpolated flows of my outlet are much lower than the peak outflows generated by the model. This is making sizing my pond very difficult.

Also, I was wondering if there is any way to change options on the maximum water elevation (in the routing analysis tab) to display more than 1 significant figure. Right now, working in SI units, the maximum water elevation is displayed to the nearest 10 cm, which seems very high. As a result, I have a 50-year storm at an elevation of 60.0 m which passes the freeboard requirement (0.3m) and a 100-year storm at an elevation of 60.0 m (same elevation!) which fails. I'm guessing this is a rounding issue, but would like to know of a way to resolve the problem.

 Thanks in advance!

Jeff

  • Hello Jeff,

    This is more of a technical support request - in the future, I would recommend sending an email to Support@bentley.com.

    The "estimated storage" value in the outlet design tab was generated under the assumption that the pond would be releasing water at the rate equal to your predeveloped peak (your "target".) So, if the outlet structure you used in your model during routing allows water to peak at a different rate, then the storage will indeed be different. For example, a routed peak that is less than the target will require more storage, because it is being constricted more than you really need. So, you need to test different outlet structures until the interpolated flow is close to the pre-developed peak. 

    Then, since the estimated storage was just an estimate (based on several different methods, which you can read about in the help) you must actually compute the network and route the flows for accurate peak flow and water surface elevation. If your outlet trial was close to the target, then they should be pretty close.

    On top of all this, you need to be very careful to follow the Pondmaker process in the correct order, or the results will be misleading. A helpful quick-reference guide is included in the documentation.

    If this doesn't address the problem you are having, please zip and email the model to technical support. It seems odd that the routed volume and peak are both higher than the estimated storage, so I suspect a step may have been skipped or different outlet structures are being compared. Examining the model makes it much easier to determine the exact problem.

     

    Regarding the display precision - In the current version of Pondpack, I do not believe there is a way to change it. Note that if you want to see the actual values for freeboard dept, instead of "PASS" or "FAIL", go to the Pond Design Options tab and select "display numerical values" for the "tolerance checking" dropdown. If this doesn't help, again, please send your model to support.

    Note that the latest version of Pondpack is 10.1, which is available for download on our website, if your company has SELECT. I believe there were some improvements in the Pondmaker routine in this version, specifically with SI units. So, the upgrade may be worth it if you do not already have it.

    Regards,

    Jesse Dringoli

    Bentley Technical Support


    Regards,

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

  • I should clarify that the "storage" in the routing tab of the pondmaker worksheet is the volume at the maximum water surface elevation. So, it could be possible to have a peak flow that is higher than the interpolated flow with a "maximum storage" that is also greater than the estimated storage, because the higher peak flows will occur at higher elevations (even if the peak only occurs for a small amount of time, like in the case of a sharp hydrograph peak). For certain pond/outlet structures, it may be more difficult to get your routing results close to the estimates. For example, if the flows on the outlet rating curve vary greatly around the elevation where the interpolated water surface occured, a small change in the routing (which is bound to occur) can results in a greatly different peak flow/volume.

    Basically the estimated storage is only an estimate. If your hydrographs have sharp peaks, the estimates may be less reliable when actually routing the flow. A model that is like this will likely have large differences between the different estimated storage types (curvilinear, lower boundary, upper boundary, etc.) To check this, make sure all methods are selected in the estimated storage tab of the pond node properties and then click the report button (white paper icon.) The first report should show you the estimates for each method, for each return event. If these values vary greatly for a particular return event, it's probably an indication that a good estimate is difficult to make with the given inflow hydrographs and target flows, so adjustments may be necessary in the routing stage. You can read more about how the different estimated storage methods work in the help documentation.

    I hope this helps. Maybe if another forum contributor has had experience with this type of situation, they can share their advice.

    Regards,

    Jesse

     

     


    Regards,

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

  • Jesse,

    Thank you for the reply. I have uploaded my model to the Bently ftp site, in order to get some clarification. That being said, I believe you are right as my input storm hydrogrpahs are user-defined Chicago Storm Distributions, which inherently have very sharp peaks. My routing analysis shows that my actual outlet flows are very close to the pre-development rate (within 5%), however there is still a large difference in water surface elevation, storage, and peak outflow. I think the storm distribution is the reason for the large difference.

    I am running pondpack V10.1, and have switched the tolerance check to display the freeboard depth instead of the "pass" or "fail".

    Thanks again!

    Jeff