Laserfiche WebLink
i <br /> f Chapter 3: Time of concentration and travel time <br /> f - <br /> Travel time (To is the time it takes water to travel management system. Slope will tend to increase <br /> from one location to another in a watershed. Tt is a when channels are straightened and decrease when <br /> component of time of concentration (Tc), which is the overland flow is directed through storm sewers, <br /> time for runoff to travel from the hydraulically most street gutters, and diversions. <br /> distant point of the watershed to a point of interest <br /> within the watershed. Tc is computed by summing all <br /> the travel times for consecutive components of the Computation of travel time and <br /> drainage conveyance system. time of concentration <br /> Tc influences the shape and peak of the runoff <br /> 1 Water moves through a watershed as sheet flow, <br /> hydrograph. Urbanization usually decreases Tc, g <br /> "thereby increasing the peak discharge. But Te can be shallow concentrated flow, open channel flow, or <br /> increased as a result of(a)ponding behind small or some combination of these. The type that occurs is a <br /> inadequate drainage systems, including storm drain function of the conveyance system and is best <br /> `. inlets and road culverts, or (b) reduction of land slope determined by field inspection. <br /> through grading. <br /> Travel time (TL) is the ratio of flow length to flow <br /> velocity: <br /> Factors affecting time of concentration <br /> and travel time __ L 3600 V <br /> TL [Eq. 3-11 <br /> Surface roughness where <br /> One of the most significant effects of urban Tt = travel time (hr), <br /> development on flow velocity is less retardance to L = flow length (ft), <br /> flow. That is, undeveloped areas with very slow and V = average velocity (ft/s), and <br /> shallow overland flow through vegetation become 3600 = conversion factor from seconds to hours. <br /> modified by urban development: the flow is then <br /> delivered to streets, gutters, and storm severs that Time of concentration (Te) is the sum of Tt values for <br /> transport runoff do more rapidly. Travel the various consecutive flow segments: <br /> time through the watershed is generally decreased. <br /> Tc = Tti +Tt2 + ... Ttm [Eq. 3-21 <br /> Channel shape and flow patterns where <br /> : In small non-urban watersheds, much of the travel Tc = time of concentration (hr) and <br /> time results from overland flow in upstream areas. m = number of flow segments. <br /> Typically, urbanization reduces overland flow lengths <br /> by conveying storm runoff into a channel as soon as <br /> possible. Since channel designs have efficient <br /> hydraulic characteristics, runoff flow velocity <br /> increases and travel time decreases. <br /> Slope <br /> Slopes may be increased or decreased by <br /> urbanization, 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-TR-53, Second Ed., June 1986) 3-1 <br />