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Time of Concentration Calculator
Estimate how long runoff takes to travel from the top of a catchment to your design point — the input that sets the design rainfall intensity in peak-flow methods.
Time of Concentration Calculator
About this tool: Estimates the time of concentration of a defined watercourse with the Kirpich (1940) formula and the SANRAL Drainage Manual defined-watercourse formula. The two are the same relationship in different units, so they should agree closely — a useful cross-check.
Kirpich (1940)
SANRAL Drainage Manual
The formulas
Kirpich: Tc = 0.0195 · L0.77 · S−0.385 — Tc in minutes, L the flow path length in metres, S the average slope in m/m.
SANRAL defined watercourse: Tc = (0.87 · L² / (1000 · Sav))0.385 — Tc in hours, L in kilometres, Sav the average slope in m/m.
Worked example
A watercourse 2 400 m long falling 36 m: S = 36 / 2400 = 0.015 m/m. SANRAL: Tc = (0.87 × 2.4² / (1000 × 0.015))0.385 = (5.011 / 15)0.385 ≈ 0.66 h ≈ 39 minutes. Kirpich with L = 2 400 m and S = 0.015 gives ≈ 39 minutes — the same, as expected.
These formulas assume concentrated flow in a defined channel. For the sheet-flow upper reach of a catchment, and for the pitfalls that skew Tc estimates, see our time of concentration explainer.
Disclaimer
This tool produces screening-level estimates from standard published formulas. It is not a design deliverable, does not account for site-specific conditions, and must not be used as the basis for design, construction, or regulatory submissions without professional review. Delta Hydro Engineers (Pty) Ltd accepts no liability for decisions taken on unverified calculator output.
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