Detention Pond by Rational Method

A Google search for rational method detention basin turned up quite a few resources. This one looks like a good explanation:
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"Is it the only lesson of history that mankind is unteachable?"
--Winston S. Churchill

Just an FYI, that does not exist. Rational Method computes peak discharge and does not provide hydrograph which is needed for routing. There is a "Modified" Rational Method that is out there that tries to accomplish the hydrograph issue.

While I would switch over to using SCS instead of rational, there is a crude way to get close.

Rv = 0.05 + 0.9 * IA
Where: Rv = Runoff coefficient (unitless)
IA = Impervious fraction (unitless)

DV = 3630 * RD * RV * A
DV = Design volume (cu ft)
RD = Design storm depth (in)
A = Drainage area (ac)

Again, I would strongly suggest using another method if you are trying to design a detention pond.