Stormwater Detention Volume (Modified Rational)
The detention storage a site needs to hold its peak runoff down to the allowable release rate.
Example
You enter
- Developed runoff coefficient C 0.85
- Design rainfall intensity (in/hr) 3
- Drainage area (acres) 2
- Allowable release (cfs) 1
- Storm duration (min) 30
You get
- Peak inflow 5.1 cfs
- Required storage 7380
Details, formula, and sources
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.
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.
The Rational-method inflow and the (inflow minus release) x duration storage are standard published hydrology results.
Estimate. AHJ and licensed professional govern.
Field names used by the API: runoff_c, intensity_in_hr, area_ac, q_allow_cfs, duration_min, q_in_cfs, storage_cf
- Rational inflow Q_in = C i A (cfs, with A in acres)Rational method
- Storage (Q_in - Q_allow) x duration x 60 (ft^3)Modified Rational method
- Critical duration search the IDF curve for the largest volume; routing is more exactscope of this tile