Max Tributary Drainage Area for an Allowable Flow
The largest tributary catchment an inlet, pipe, or outlet rated for an allowable flow can accept.
Example
You enter
- Allowable outlet/inlet flow (cfs) 2
- Surface asphalt
- Rainfall intensity (in/hr) 2
You get
- Max area (acres) 1.05263
- Max tributary area 45852.6
Checked against published worked examples from: USEPA / NRCS.
Details, formula, and sources
ScopeA_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 or tableA_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.
Source and editionRational method by name (public hydrology), solved for the area. C-values from public engineering practice.
Free accessRainfall intensity / IDF curves free at hdsc.nws.noaa.gov (NOAA Atlas 14).
What governsEstimate. AHJ and licensed plumber / gas fitter govern. Verify against the IPC / IFGC edition adopted in your jurisdiction.
API field namesallowable_flow_cfs, surface, rainfall_in_per_hr, max_area_acres, max_area_ft2
- C-values data/plumbing/runoff-coefficients.json keyed to surfacepublic engineering practice
- Small uniform catchment Rational method valid for < ~200-acre uniform catchments; peaks at Tcrational method assumption