Drainage & Hydrology

Rational Method — Peak Runoff

Rational Method peak runoff calculator (Q = CiA) for drainage areas up to 200 acres, with composite C from multiple land use types.

Drainage & Hydrology · ASCE / FHWA

Rational Method — Peak Runoff

The Rational Method is the standard approach for estimating peak discharge from small urban and rural watersheds. It relates peak runoff to drainage area, surface cover, and rainfall intensity through a single, computationally direct equation that has been the basis of storm sewer, inlet, and culvert sizing for over a century.

The method assumes steady-state rainfall over the entire drainage area for a duration equal to the time of concentration (T_c). It is applicable for homogeneous watersheds up to approximately 200 acres. For larger areas or where detention, complex land cover, or distributed timing effects are significant, the SCS/NRCS Curve Number method (TR-55) should be used instead.

Rational Method (Imperial)

Q = C × i × A Q = peak discharge (cfs) C = runoff coefficient (dimensionless) i = rainfall intensity (in/hr) A = drainage area (acres)

Composite C — Mixed Land Use

C = Σ(Cⱼ × Aⱼ) / Aₜₒₜ₋ₓ Weighted average of runoff coefficients by sub-area

Reference: ASCE Manual of Engineering Practice No. 37; FHWA HEC-22 Urban Drainage Design Manual; most state DOT stormwater design manuals. Rainfall intensity from NOAA Atlas 14 (hdsc.nws.noaa.gov/atlas14) or local IDF curves.

Rational Method Calculator

Imperial units — acres, in/hr, cfs

Drainage Area A (acres)

Rational Method is most reliable for A ≤ 200 acres

Design Storm Return Period

For labeling — obtain matching i from local IDF curves for this return period

Rainfall Intensity i (in/hr)

From NOAA Atlas 14 or local IDF curve at T_c and selected return period

Runoff Coefficient C

C Input Method

Land Use Lookup (auto-fills C below)

Runoff Coefficient C (0.05 – 0.95)

Edit directly or use lookup above

Rational Method Results

Peak Discharge (Q)—

Design Storm—

Runoff Coefficient (C)—

Rainfall Intensity (i)—

Drainage Area (A)—

Q in GPM—

Reference only. Obtain rainfall intensity from NOAA Atlas 14 or local IDF curves for your specific location and return period. Compute T_c using Kirpich or TR-55 methods. Requires PE review before use in any design document.

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Rational Method — Peak Runoff