Laserfiche WebLink
<br />DRAINAGE CRITERIA MANUAL <br /> <br />RUNOFF <br /> <br />. <br /> <br />v = 0.47 ft/sec. <br /> <br />The travel time can then be calculated using this velocity and 2100 <br />feet of travel length. <br /> <br />t - L = 2100 ft. <br />t - 60V (60 sec/min)(0.47 ft/sec) <br /> <br />tt = 75 minutes <br /> <br />Step 4: Combine ti and tt to find the estimated time of concentration tc' <br /> <br />tc = ti + tt <br />tc = 31 + 75 = 106 minutes <br /> <br />(Equation 3-2) <br /> <br />. <br /> <br />3.4.2 Time of Concentration In Urbanized Basins <br />Overland flow in urbanized basins can occur from the back of the lot to <br />the street, in parking lots, in greenbelt area, or within park areas. It can <br />be calculated using the procedure described in Section 3.4.1 except the travel <br />time tt to the first design point or inlet is estimated using the "Paved Area <br />(Sheet Flow) & Shallow Gutter Flow" line in Figure 3-2 and the over land flow <br />distance should not exceed 300 feet. Also, the time of concentration at the <br />first design point in an urbanized basins using this procedure should not <br />exceed the time of concentration calculated using Equation 3-4. Equation 3-4 <br />was developed using the rainfall/runoff data collected in the Denver region <br />and, in essence, represents regional "calibration" of the Rational Method. <br /> <br />t = <br />c <br /> <br />L <br />180 <br /> <br />+ 10 <br /> <br />(3-4) <br /> <br />In which tc = time of concentration at the first design point <br />in an urban watershed (minutes) <br />L = watershed length (feet) <br /> <br />. <br /> <br />Normally, Equation 3-4 will result in a lesser time of concentration at <br />the first design point and will govern in an urbanized watershed. For <br />subsequent design points, the time of concentration is calculated by <br />accumulating the travel times in downstream drainageway reaches. The minimum <br /> <br />5~1-84 <br />