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<, <br />1 <br />Chapter 3: Time of concentration and travel time <br />Travel time (T~ is the time it takes water to travel <br />from one location to another in a watershed. Tt is a <br />component of time of concentration (T~, which is the <br />time for runoff to travel from the hydraulically most <br />distant point of the watershed to a point of interest. <br />within the watershed. T~ is computed by summing all <br />the travel times for consecutive components of the <br />drainage conveyance system. <br />T~ influences the shape and peak of the runoff <br />hydrograph. Urbanization usually decreases T~, <br />thereby increasing the peak discharge. But Tsan be <br />increased as a result of (a) ponding behind small or <br />inadequate drainage systems, including storm drain <br />hilets and road culverts, or (b) reduction of )and slope <br />through grading. <br />Factors affecting time of concentration <br />and travel time <br />Surface roughness <br />One of the most significant effects of urban <br />development on flow veloaty is less retardance to <br />flow. That is, undeveloped areas with very slow and <br />shallow overland flow through vegetation become <br />modified by urban development: the flow is then <br />delivered to streets, gutters, snd storm sewers that <br />transport runoff downstream more rapidly. Travel <br />time through the watershed is.generally decreased. <br />where <br />L lEq.8•l] <br />Tc =3600 V <br />Tt =travel time (hr), <br />L =flow length (ft), <br />V =average veloaty (fds}, and <br />3600 =conversion factor from seconds to hours. <br />Time of conrentration ff~ is the sum of Tc values for <br />the various consecutive flow segments: <br />Tr = Tct + Ttp + ... T~ lEq. 3.2] <br />Channel shape and flow patterns <br />In small non•urban watersheds, much of the travel <br />time results from overland flow in upstream areas. <br />Typically, urbanization reduces overland flon• lengths <br />by conveying storm runoffinto a channel as soon as <br />possble. Since channel designs have effiaent <br />hydraulic characteristics, runoff flow velocity <br />increases and travel time decreases. <br />where <br />T~ =time of concentration (hr) and <br />m =number of flow segments. <br />Slope <br />Slopes may be increased or decreased by <br />urbanvation, depending on the extent of site grading <br />or the extent to which storm sewers and street <br />ditches are used in the design of the water <br />(210-VI-TRS5, Second Ed., Tune 1986) <br />management system. Slope wi71 tend to increase <br />when channels are straightened and decrease when <br />overland flow is directed through storm sewers, <br />street gutters, and diversions. <br />Computation of travel time and <br />time of concentration <br />Travel time tT~ is the ratio of flow length to flow <br />velocity: <br />Water moves through a watershed as sheet flow, <br />shallow conrentrated flow, open channel flow, or <br />some combination of these. The type that occurs is a <br />function of the conveyanre system and is best <br />determined by 5eld inspection. <br />3-1 <br />