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How It Works
Time of concentration (tc) is how long runoff takes to travel from the most distant point in a watershed to the outlet. The Kirpich formula estimates it from travel length and slope. A shorter tc produces a higher peak flow rate for the same storm, which directly affects culvert, sewer, and detention basin sizing.
Example Problem
A small watershed has a maximum flow path of 2,000 ft and a slope of 0.04 (4%). What is the time of concentration?
- tc = 0.0078 × 20000.77 × 0.04-0.385
- tc = 0.0078 × 417.5 × 4.217
- tc ≈ 13.7 minutes
Frequently Asked Questions
What is time of concentration used for?
It sets the duration for selecting rainfall intensity from IDF curves. In the Rational Method (Q = CiA), tc determines the design storm intensity, which drives peak flow calculations for drainage design.
When is the Kirpich formula appropriate?
It works best for small rural watersheds (under a few hundred acres) with well-defined channels. For urban or mixed-use sites, use the NRCS TR-55 method, which segments the flow path into sheet flow, shallow concentrated flow, and channel flow.
How does urbanization affect time of concentration?
Impervious surfaces (roads, roofs) reduce tc by increasing runoff velocity and eliminating infiltration. A 5-acre rural site might have tc of 20 minutes; the same site developed could drop to 8–10 minutes, significantly increasing peak flow.
Related Calculators
- Manning Equation Calculator — estimate channel flow velocity for tc calculations.
- Gutter Design Calculator — size roadway gutters using the peak flow from tc analysis.
- French Drain Design Calculator — design subsurface drainage for the computed runoff.
- Rainwater Collection Calculator — estimate runoff volume from rainfall intensity and duration.
- Time Converter — convert time of concentration between minutes, hours, and seconds.