Stormwater Detention Volume (Modified Rational)
Q_in = C i A, storage = (Q_in - Q_allow) x duration x 60. C 0.85, i 3 in/hr, 2 ac, Q_allow 1.0 cfs, 30 min -> 5.10 cfs in, 7,380 ft^3 (0.169 acre-ft); a longer 60 min / 2 in/hr storm needs 8,640 ft^3, so the critical duration must be searched. The routing and outlet govern.
Formula and source
Q_in = C i A (cfs); storage = (Q_in - Q_allow) x duration x 60 (ft^3); acre-ft = ft^3 / 43560.
The Modified Rational method for detention storage volume, standard stormwater-management practice, by name.
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