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<br />The inlet time can be estimated by calculating the various overla:r.d <br />distances and flow velocities taken f"om the most remote point. 1\'0 <br />common errors are (1) to assume velocities that are too small for the <br />areas near the collectors, and (2) to not review the runoff from only a <br />part of the basin which is sometimes greater than te,at computed for the <br />whole basin. This error is most often encot:ntered in long basins, or <br />a basin "here the upper portion contains grassy park land and the lower <br />is developed urban land. Often the reTIote areas have 1'10" that is very <br />shallow, .3.nd in this case the velocities cannot be calculated by Ilchannelll <br />equations, but special overland flow analysis must be considered (see <br />Section 4.4). Fig. 4.2.4-1 can be used to eelp estimate time of surface <br />flow. <br /> <br />Fig. 4.2.4-1 is a solution of the follo"ing equation: <br />1.87(1.1-CCf)Dl/2 <br />-- <br />Sl/3 <br /> <br />Tc = <br /> <br />(Eq., 4.2,1,-1) <br /> <br />"here: <br /> <br />Te = time of concentration, in min. <br /> <br /> <br />S = slope of basin, in percent <br /> <br /> <br />CCf = Rational Method runoff coefficient (see Sec. 4.2.6) <br /> <br /> <br />D length of basin, in feet <br /> <br />The overland flo" path perpendicular to the contours is not neces- <br />sarily th,~ best decision "hen studying a proposed developmE,nt, sinee the <br />land \rill be graded, and s"ales "ill often intercept the na,tural contour <br />and conduct the "ater to the streets, thus cutting down on the time of <br />concentration. <br /> <br />4.2.5 Intensity <br /> <br />The intensity, I, is the average rainfall rate in inches per hour <br />for the p"riod of maximum rainfall of a given frequency having a duration <br />equal to the time of concentration. After the d,esign storm frequency <br />has been selected, a graph should be m<lde that sho"s rainfall intensity <br />versus time. The procedure for obtaic,ing the local data and dra"ing <br />the graph is explained in Section 3 of this'lanua l. <br /> <br />4.2.6 Runoff coefficient <br /> <br />In the Rational Method, the runoff coefficient, C, is the variable <br />least suseeptible to precise detercnination, "nd requires jud~el1t and <br />understanding on the part of the e:!1gineer. Although its use in the <br />formula implies a fixed ratio for 'my given drainage area, in reality <br />this is not the case. The coefficIent repre.sents th," integrated effects <br />of infiltration, detention storage, evaporatIon, retention, flow routing, <br />and intereeptiol "hich all affect the time distribution and peak rate <br />of runoff. Table 4.2.6-1 presents recommended range:s for C values. <br /> <br />LCS-j.,'M Manual <br /> <br />4.2-..-3 <br /> <br />April 1979 <br />