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<br />DRAINAGE CRITERIA MANUAL <br /> <br />RUNOFF <br /> <br />concentration needs to be considered in combination with the travel time (tt) . <br />which is calculated using the hydraulic properties of the swale, ditch, or <br />channel. For preliminary work, the travel time (tt) can be estimated with the <br />help of Figure 3-2. The time of concentration is then the sum of the initial <br />flow time (ti) and the travel time (tt). Should the calculations reSult in a <br />tc of less than 10-minutes, it is recommended that a minimum value of <br />10-minutes be used for non-urban watersheds. The process of calculating the <br />preliminary time of concentration in non-urbanized basin is illustrated in the <br />following example: <br /> <br />Example. Time of Concentration In Non-Urban Basin <br /> <br />Given: A 100 acre non-urbanized rangeland watershed having clayey type <br />soils. Watershed has a length of 2500 feet and an average slope of <br />1.0 percent. Uppermost 400 feet of watershed has an average land <br />slope of 2 percent. <br /> <br />Required: Estimate the Time of Concentration <br /> <br />. <br /> <br />Solution: <br />Step 1: <br /> <br />Fi rst fi nd the <br />5-year storm. <br /> <br />runoff coefficient for clay type soil grassland for a <br />From Table 3-1: <br /> <br />C5 = 0.10 <br /> <br />Step 2: Find the initial (overland) flow time for the uppermost 400 feet of <br /> <br />the watershed. From Figure 3-1, t. for the 400 foot sub-watershed <br />1 <br />length, slope and c5 is <br /> <br />t. = 31 minutes <br />1 <br /> <br />Step 3: Find the travel time for the remalnlng 2100 feet of the watershed <br />length. From Figure 3-2, the average travel velocity for the given <br />watershed, one percent slope, and "Fallow or Minimum Tillage <br />Cultivation" curve is <br /> <br />. <br /> <br />5-1-84 <br />