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<br />I\lethod and Date <br /> <br />ummary of time of conceutration formulas <br /> <br />Formula for Ie (min) <br /> <br />Remarks <br /> <br />Kicpich (1940) <br /> <br />California <br />Culverts <br />Practice <br />(1942) <br /> <br />Izzard (1946) <br /> <br />Federal Aviation <br />Administration <br />(1970) <br /> <br />te = 0.0078Lo.775-0.385 <br />L = length of channeVditch <br />from headwater to oUllet, <br />ft <br />5 = average watershed slope, <br />ftlft <br /> <br />T, = 6O(l1.9L'fH)O 385 <br />L = length of longest <br />watercourse, mi <br />H = elevation difference <br />between divide and <br />outlet, ft <br /> <br />4 L025(0.OOO7i + e)L"]] <br />t, = 50.333jO.667 <br />i = rainfall intensity, inlh <br />c = retardance coefficient <br />L = length of flow path. ft <br />S = slope of flow path, ftlft <br /> <br />t[ = 1.8( 1.1 - OLO.50/sO.333 <br />C = rational method runoff <br />coefficient <br />L = length of overland flow, ft <br />5 = surface slope, % <br /> <br />Developed from SCS data for seven rural basins in Tennessee <br />with well-defined channel and steep slopes (3% to 10%); for <br />overland flow on concrete or asphalt surfaces multiply t[ by <br />0.4; for concrete channels multiply by 0.2; no adjustments for <br />overland flow on bare soil or flow in roadside ditches. <br /> <br />Essenti.dly the Kirpich fonnula; developed from small moun- <br />tainous basins in California (U. S. Bureau of Reclamation, <br />1973, pp. 67-71). <br /> <br />Developed in laboratory experiments hy Bureau of Public <br />Roads for overland flow on roadway and turf surfaces; val- <br />ues of the retardance coefficient range from 0.0070 for very <br />smooth pavement to 0.012 for concrete pavement to 0.06 for <br />dense turf; solution requires iteration; product i times L should <br />be s 500. <br /> <br />Developed from air field drainage data assembled by the Corps <br />of Engineers; method is intended for use on airtield drainage <br />problems, but has been used frequently for overland flow in <br />urban basins. <br /> <br />Kinematic wave <br />formulas <br />Morgali and <br />Linsley <br />(1965) <br />Aron and <br />Erborge (1973) <br /> <br />SCS lag <br />equation <br />(1973) <br /> <br />SCS average <br />velocity <br />charts (1975, 1986) <br /> <br />t = 0 94Lo.6nO.6 <br />C (i0.4S0.3) <br />L = length of overland flow, ft <br />n = Manning roughness <br />coefficient <br />i = rainfall intensity inlh <br />S = average overland slope <br />ftIft <br /> <br />100 I.08[(1000/CN) - 9J".7 <br />t, = 1900 ;<" <br />L = hydraulic length of <br />watershed (longest flow <br />path), ft <br />eN = SCS runoff curve number <br />S = average watershed slope, % <br /> <br />I = -L~!: <br />, 60 V <br />L = length of flow path, ft <br />V = average velocity in feet <br />per second from Fig. 3-1 <br />of TR 55 for various <br />surfaces <br /> <br />Overland flow equation developed from kinematic wave anal~ <br />ysis of surface runoff from developed surfaces; method <br />requires iteration since both i (rainfall intensity) and tc are <br />unknown; superposition of intensity-duration-frequency curve <br />gives direct graphical solution for tc <br /> <br />Equation developed by SCS from agricultural watershed data; <br />it has been adapted to small urban basins under 2000 acres; <br />found generally good where area is completely paved; for <br />mixed areas it tends to overestimate; adjustment factors are <br />applied to correct for channel improvement and impervious <br />area; the equation assumes that tc = 1.67 x basin lag. <br /> <br />Overland flow charts in Fig. 3-1 ofTR 55 show average veloc- <br />ity as function of watercourse slope and surface cover. (See <br />also Table 5.7.1) <br /> <br />Source; Kibler, 1982, Copyright by the American Geophysical Union. <br />