Free Web Tool

Stormwater Runoff (Q = CIA)

Peak runoff using the Rational Method with multi-surface areas, weighted runoff coefficient, and Kirpich time-of-concentration helper.

Inputs

mm/hr (use IDF curve for design)

Time of Concentration (Kirpich)

m
m
min

Sub-areas

Runoff coefficient (C) typical values
  • Asphalt / concrete pavement: 0.75 – 0.95
  • Gravel / unpaved roads: 0.40 – 0.60
  • Roofs: 0.75 – 0.95
  • Lawns & parks (heavy soil, flat < 2%): 0.13 – 0.25
  • Lawns (sandy soil, flat < 2%): 0.05 – 0.10
  • Wooded / forested areas: 0.10 – 0.30
  • Cultivated land (sandy / loam): 0.20 – 0.50
Kirpich (1940)
  • SI form: tc = 0.0195 · L0.77 · S−0.385 (minutes; L in m, S in m/m)
  • US form: tc = 0.0078 · L0.77 · S−0.385 (min; L in ft)
  • Use intensity i corresponding to tc from your local IDF curve.

Results

Total Area—
Weighted C—
Time of Concentration tc—
Slope (from H/L)—
Peak Discharge Q—
Q (alt units)—

Sub-area Breakdown

SurfaceCAreaC·A