Max Tributary Drainage Area for an Allowable Flow
The inverse of the Rational method tile: the largest tributary catchment an inlet, pipe, or outlet rated for an allowable flow can accept, A_acres = Q / (C x i). A 2-cfs outlet on asphalt (C 0.95) at 2 in/hr accepts up to ~45,850 ft^2 (1.05 ac); a rougher surface or lower intensity allows more. A design aid; the AHJ and the civil engineer govern.
Formula and source
A_acres = Q / (C × i); A_ft2 = A_acres × 43560. The Rational method Q = C × i × A solved for the area, C from bundled runoff coefficients per surface.
Rational method by name (public hydrology), solved for the area. C-values from public engineering practice.
Audience
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