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DRAINAGE CRITERIA MANUAL (V. 1) RUNOFF <br />2.4 Time of Concentration <br />One of the basic assumptions underlying the Rational Method is that runoff is a function of the average <br />rainfall rate during the time required for water to flow from the most remote part of the drainage area <br />under consideration to the design point. However, in practice, the time of concentration can be an <br />empirical value that results in reasonable and acceptable peak flow calculations. The time of <br />concentration relationships recommended in this Manual are based in part on the rainfall -runoff data <br />collected in the Denver metropolitan area and are designed to work with the runoff coefficients also <br />recommended in this Manual. As a result, these recommendations need to be used with a great deal of <br />caution whenever working in areas that may differ significantly from the climate or topography found in <br />the Denver region. <br />For urban areas, the time of concentration, t consists of an initial time or overland flow time, t,, plus the <br />travel time, t,, in the storm sewer, paved gutter, roadside drainage ditch, or drainage channel. For non- <br />urban areas, the time of concentration consists of an overland flow time, t,, plus the time of travel in a <br />defined form, such as a swale, channel, or drainageway. The travel portion, t,, of the time of <br />concentration can be estimated from the hydraulic properties of the storm sewer, gutter, swale, ditch, or <br />drainageway. Initial time, on the other hand, will vary with surface slope, depression storage, surface <br />cover, antecedent rainfall, and infiltration capacity of the soil, as well as distance of surface flow. The <br />time of concentration is represented by Equation RO -2 for both urban and non -urban areas: <br />t� =t + <br />which: <br />= time of concentration (minutes) <br />= initial or overland flow time (minutes) <br />= travel time in the ditch, channel, gutter, storm sewer, etc. (minutes) <br />24.1 Initial Flow Time <br />The initial or overland flow time, t„ may be calculated using equation RO -3: <br />0.3951.1— C WE <br />ti 15°33 <br />in which: <br />t, = initial or overland flow time (minutes) <br />C5 = runoff coefficient for 5 -year frequency (from Table RO -5) <br />2007 -01 <br />Urban Drainage and Flood Control District <br />(RO -2) <br />(RO -3) <br />RO -5 <br />