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PondPack Differential between Peak Flow from catchment and peak flow to pond in

I have a differential between my peak flow from my catchment and the peak flow to my pond (in). Shouldn't these be the same number? What would be the cause for this? See attached results summary to visualize what I am asking.

 

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  • Does the catchment connect to a channel first and then a pond? (in which case the flow could be attenuated)

    Have you taken a look at a graph of the catchment flow along with the pond inflow?

    Can you provide a screenshot of the plan view and/or a copy of the model? See: Sharing Hydraulic Model Files on the OpenFlows Forum


    Regards,

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

  • I uploaded the file. It is called "Model Share". 

  • Thanks for sharing the model. If you graph pond inflow along with catchment flow, you will see the problem is that the peak is very sharp and the timestep is not small enough to capture enough detail. Meaning, the real peak is missed because of the timestep. If for example you change the Output Increment (in the Calculation Options) to one minute instead of three minutes, the peak flow will match as expected. See the first item under the Solution section of this article.

    I think there is a deeper issue here as you would not typically want to have a hydrograph with such small volume. It appears that you have configured the Modified Rational Method to have the predevelopment and postdevelopment information the same (same Tc, CN and area). PondPack expects the postdevelopment conditions to generate a higher flow than the predevelopment peak, so that it can properly determine a critical storm duration and resulting Modified Rational Hydrograph. I recommend reading the PondPack section of the following article to understand the expectations of this runoff method: Understanding the Modified Rational Method

    Also see the section near the bottom, entitled "What if this is an existing system where the pre/post conditions are the same, or what if I don't care about the predevelopment peak flow?"


    Regards,

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

Reply
  • Thanks for sharing the model. If you graph pond inflow along with catchment flow, you will see the problem is that the peak is very sharp and the timestep is not small enough to capture enough detail. Meaning, the real peak is missed because of the timestep. If for example you change the Output Increment (in the Calculation Options) to one minute instead of three minutes, the peak flow will match as expected. See the first item under the Solution section of this article.

    I think there is a deeper issue here as you would not typically want to have a hydrograph with such small volume. It appears that you have configured the Modified Rational Method to have the predevelopment and postdevelopment information the same (same Tc, CN and area). PondPack expects the postdevelopment conditions to generate a higher flow than the predevelopment peak, so that it can properly determine a critical storm duration and resulting Modified Rational Hydrograph. I recommend reading the PondPack section of the following article to understand the expectations of this runoff method: Understanding the Modified Rational Method

    Also see the section near the bottom, entitled "What if this is an existing system where the pre/post conditions are the same, or what if I don't care about the predevelopment peak flow?"


    Regards,

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

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