Time of Concentration (Kirpich)

The Kirpich estimate tc = 0.0078 L^0.77 S^(-0.385) (tc min, L ft, S ft/ft) - the time for runoff to reach the outlet, the duration at which a designer reads the design rainfall intensity off the local IDF curve. A 1,000 ft flow path at 2% slope gives 7.2 min; flatten it to 0.5% and the water takes 12.3 min, a lower intensity and a smaller peak - the flood-control value of grading a site flat. Single-segment estimate, not the TR-55 three-segment sum. A design aid; the engineer of record governs.

tc_min = 0.0078 L^0.77 S^(-0.385) (tc minutes, L feet, S ft/ft); tc_hr = tc_min / 60.

The Kirpich (1940) time-of-concentration equation tc = 0.0078 L^0.77 S^(-0.385), as compiled in the USDA TR-55 and NRCS drainage references, by name.

TR-55 (Urban Hydrology for Small Watersheds) is a free USDA NRCS publication; the Kirpich equation is public.

Estimate. AHJ and licensed professional govern.

Field names used by the API: l_ft, s_slope, tc_min

• Kirpich equation tc = 0.0078 L^0.77 S^(-0.385) on the longest flow path, single segmentKirpich (1940) / TR-55
• Calibration small rural channelized watersheds; a paved surface is often taken at about 0.4xNRCS drainage guidance
• Use sets the design storm duration read off the local IDF curve, not a routed hydrographrational-method practice