Question 1

How do I calculate a composite runoff coefficient?

Accepted Answer

Area-weight the C of each surface type: Cw = Σ(Ci · Ai) / ΣAi. Break the drainage area into surfaces of uniform cover (pavement, roof, lawn, woods), assign each a published C, multiply by its area, sum, and divide by total area. The weighted C then goes directly into the Rational Method: Q = Cw·i·A.

Question 2

What C values are typical?

Accepted Answer

Pavement and roofs: 0.90 (commonly 0.85–0.95). Gravel: 0.35–0.70 depending on compaction. Lawns: 0.05–0.35 depending on soil and slope — sandy flat lawns are 0.05–0.10, clayey steep lawns 0.25–0.35. Woods/undeveloped: 0.05–0.25. Composite business districts run 0.70–0.95, residential areas 0.30–0.50. Always check the local drainage manual — many jurisdictions publish mandatory C tables.

Question 3

Should C increase for large storms?

Accepted Answer

Yes. Published C values are calibrated for roughly 2- to 10-year storms. For rarer events, saturation raises the runoff fraction from pervious surfaces, and many manuals (following ASCE and FHWA HEC-22 practice) apply frequency factors: Cf = 1.1 for 25-yr, 1.2 for 50-yr, 1.25 for 100-yr, with the product C·Cf capped at 1.0.

Question 4

When shouldn't I use the Rational Method at all?

Accepted Answer

The Rational Method assumes a steady peak from a uniform-intensity storm of duration equal to the time of concentration; it produces a peak flow only, not a hydrograph or volume. Most manuals cap its use at roughly 100–200 acres. For detention design, volume-based methods (NRCS curve number with a design hyetograph) are required — a peak-only method can't size storage.

Question 5

What is percent impervious and why does this tool report it?

Accepted Answer

Percent impervious is the fraction of the drainage area with C ≥ 0.85 (pavement and roof rows in this tool). Many stormwater ordinances trigger water-quality treatment or detention requirements at impervious-cover thresholds, so it's a number reviewers ask for alongside the weighted C.

Question 6

Is weighting C the same as weighting CN?

Accepted Answer

The arithmetic is identical (area-weighted average), but they belong to different methods: C is the Rational Method's runoff fraction for peak flow; CN is the NRCS retention parameter for runoff depth and volume. They are not interchangeable and there is no exact conversion between them — pick the method your jurisdiction requires and stay in it.

Surface	C
Asphalt / concrete pavement	0.85–0.95
Roofs	0.85–0.95
Gravel roads / parking	0.35–0.70
Lawn, sandy soil, flat (<2%)	0.05–0.10
Lawn, sandy soil, steep (>7%)	0.15–0.20
Lawn, heavy (clay) soil, flat (<2%)	0.13–0.17
Lawn, heavy (clay) soil, steep (>7%)	0.25–0.35
Woods / undeveloped	0.05–0.25
Cultivated fields	0.30–0.60

Composite Runoff Coefficient Calculator

Common runoff coefficients

Worked example

Example — 12-acre commercial site

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Composite Runoff Coefficient Calculator

Common runoff coefficients

Worked example

Example — 12-acre commercial site

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