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RUNOFF` DRAINAGE CRITERIA MANUAL (V. 1) <br />L = length of overland flow (500 ft maximum for non-urban land uses, 300 ft maximum for urban <br />land;uses) <br />S = average basin slope (ft/ft) <br />Equation`RO-3'is adequate for distances up to 500 feet. Note that, in some urban watersheds, the <br />overland flow time may be eery small because flows quickly channelize. <br />2.4:2 Overland Travel Time <br />For catchments with overland and.:channelized flow, the time of concentration needs to be considered in <br />combination with the overland'.travel time, t,, which is-calculated using the hydraulic properties of the <br />swale, ditch, or channel. For preliminary work, the overland travel time, t,, can be estimated with the help <br />of ;.Figure RO-1 or the following equation (Guo 1999): <br />V = +? os <br />vSW (RO74) <br />in which: <br />V velocity (ft/sec) <br />C, conveyance coefficient (from Table RO-2); <br />Sw = watercourse slope (ft/ft) <br />Table RO-2-Conveyance Coefficient, C. <br />Type of Land Surface Conveyance Coefficient, C,, <br />Heavy meadow 2.5 <br />Tillage/field 5 <br />Short pasture and lawns 7 <br />Nearly bare ground 10 <br />Grassed waterway 15 <br />Paved areas and shallow paved swales 20 <br />The time of concentration, i, is then the sum of the initial flow time, t,, and the travel time, t,, as per <br />Equation RO-2. <br />2.4.3 First Design Point Time of Concentration in Urban Catchments <br />Using this procedure, the time of concentration at the first design point (i.e., initial'flowtime, .r;) in an <br />urbanized catchment should not exceed the time of concentration calculated using Equation .R0=5. <br />RO-6 6/2002 <br />Urban Drainage and Flood Control District