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Using Curve Number Method to determine required Detention Storage Volume

Using Curve Number Method to determine required Detention Storage Volume

Using Curve Number Method to determine required Detention Storage Volume

(OP)
I'm trying to use the Curve Number (CN) Method calculate the required Detention Storage Volume for a pond near the outlet of a 26-ac watershed. I've used NOAA Atlas 14 to find the total rainfall depth (1.83 inches). I'm required to use the Farmer Fletcher 100-year 3-hour Unit Hydrograph for computing required detention volume. I've attempted to do this using the table in the attached JPG. I came up with 125,866 cf of required detention volume. However, I think there must be a flaw in this calculation...

In my example, at t = 90 min, the unit hydrograph yields a total cumulative rainfall of 0.888*1.83 = 1.625 inches. Over the 26.12 acres, this yields a volume of 154,075 cf. Also, at 90 minutes, a release of 0.2 cfs/ac would yield a total release volume of 28,209 cf. Therefore, the volume in the pond should be the difference between the accumulated and released volumes, which is 125,866 cf.

Two problems I'm struggling with...

1) This calculation seems completely independent of CN.
2) The hydrograph is for RAINFALL, NOT RUNOFF. Seems to me that at 90 minutes, not all of the 154,075 cf would have made its way to the detention pond.

Is there a way to incorporate CN? Is there a way to produce a RUNOFF hydrograph? I'd prefer to use my own spreadsheet so that I can follow the calculation from start to finish. (Unlike HEC RAS and other programs where all I can see is my initial input and the final outcome and "hides" the intermediate steps). Any help would be greatly appreciated.

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