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Pond Pack- predevelopment target calculation using Modified Rational Method does not equal ciA. what am I missing?

In modified rational method, the result for the allowable flow (pre-development) does not - cIA.  Summary shows:

C= 0.500

I= 10.200

A=2.080

Flow = 10.70 cfs should be 10.608 cfs.

what gives?

  • Hello Charles,

    The different is due to the units - your intensity (I) is likely in inches per hour and your A is in acres, yielding a unit of acre-in/hr. Many engineers approximate this as CFS since it is so close, but PondPack converts to the actual unit. So, you would need to account for the unit conversion factor of 1.008 in your hand calculation to achieve the same results.

    See related article in our wiki:

    Rational Method (Q=CIA) hand calculations different from model results


    Regards,

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

    Answer Verified By: charles wirtanen