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<br />DRAINAGE CRITERIA MANUAL <br /> <br />RUNOFF <br /> <br />The travel times are then accumulated in a downstream direction to ... <br />calculate the time of concentration at each successive downstream <br />design points. <br /> <br />3.5 Intensity <br />The intensity, I, is the average rainfall rate in inches per hour for the <br />period of maximum rainfall of a given frequency having a duration equal to the <br />time of concentration. <br />After the des i gn storm frequency has been selected, a graph shoul d be <br />made showing rainfall intensity versus time. The procedure for obtaining the <br />local data and drawings the graph is explained and illustrated by Example 4.2 <br />of the RAINFALL part of this Manual. <br /> <br />3.6 Runoff Coefficient <br />The runoff coefficient, C, represents the integrated effects of <br />infiltration, evaporation, retention, flow routing, and interception, all <br />which effect the time distribution and peak rate of runoff. Its determination <br />requires judgement and understanding on the part of the engineer (10). Table <br />3-1 presents the recommended values of C for the various recurrence frequency <br />storms (42). The values are presented for different surface characteristics <br />as well as for different aggregate land uses. <br />Table 3-1 provides runoff coefficients that vary with recurrence <br />frequency. The coefficients were developed using the available rainfall and <br />runoff information in the Denver region and were designed to work in <br />conjunction with the time of concentration recommendations in 3.4. Use of <br />these coefficients and procedure outside of the semi-arid cl imates found in <br />areas such as the Denver regi on may not be val i d. However, because the <br />coefficients vary with frequency, no further adjustments are needed for large <br />storms as was described in the earlier versions of the Manual. <br />See the STORM SEWERS part of this Manual for further discussion of the <br />use of the Rational Method. <br /> <br />3.7 Application of the Rational Method <br />The first step in applying the Rational Method is to obtain a good <br />topograpic map and define the boundaries of all of the relevant drainage <br />basins. Basins to be defined include all basins tributary to the area of <br /> <br />5-1-84 <br /> <br />. <br /> <br />. <br />