The two methods
Kirpich (1940): T꜀ = 0.0195 · L0.77 · S−0.385 (minutes, L in m, S in m/m). Derived from small agricultural watersheds with defined channels; tends to run fast — short T꜀, conservative (higher) intensities. The default for natural and channelised rural catchments, including wadi tributaries.
FAA (1970): T꜀ = 1.8 · (1.1 − C) · Lft0.5 / S%1/3 (the calculator converts units internally). Developed for airfield drainage — essentially sheet flow over uniform surfaces. The standard pick for paved urban sub-catchments, car parks and roofs-to-gutter paths.
Which value to design with
- Natural / channelised catchment: Kirpich.
- Predominantly paved overland flow: FAA.
- Mixed paths: segment the flow path and sum travel times — or take the smaller T꜀ for a conservative peak. If the spread between methods exceeds ~40%, the catchment probably deserves a segmented calculation.
- Always apply the code minimum (commonly 10 min in MOMRAH and Abu Dhabi DMT practice) — short computed times produce unrealistically extreme intensities from IDF curves.
T꜀ sets the storm duration for intensity lookup — in the Gulf that lookup is its own minefield; see rainfall design criteria for Saudi Arabia. Then feed Q = CiA in the Rational Method calculator and capacity-check with Manning's equation.