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<br />I <br />- <br />il <br />il <br />il <br />I <br />I <br />- <br />TI- <br />I- <br />II <br />il <br />:1 <br />11 <br />'I <br />I <br />,I <br />II <br />II <br /> <br />PART III <br /> <br />RUNOFF <br /> <br />A. Rational Method <br /> <br />'The Rational Method shall be used to analyze peak flows <br />from drainage basins containing less than 20 acres. This <br />method, developed in the late 1800's has been shown to yield <br />satisfactory results in small basins when properly applied. <br />The formula this method uses to relate runoff to rainfall <br />intensity is: <br /> <br />Q = CiA <br />where <br />Q = peak runoff, cubic feet per second, cfs <br />C = runoff coefficient representing drainage area <br /> characteristics <br /> <br />, <br />i = average intensity of rainfall for the duration <br />_r",-quire_d for__the runoff to become established <br />and flow from the entire basin, inches per hour <br /> <br />A = size of drainage basin, acres <br /> <br />S_u-9.'Lested .values of the runoff coefficient "c" for <br />different land uses are tabulated in Table III-I. <br /> <br />Where sufficient information exists to determine a more <br />precise coefficient based on actual character of surface, <br />Table 111-2 shall be used to determine a composite value of <br />"c" for the basin. <br /> <br />With these surface type coefficients, the composite <br />runoff coefficent is then calculated using: <br /> <br />C = <br /> <br />n <br />L <br />i=l <br /> <br />CiAi <br /> <br />where <br /> <br />At <br /> <br />C = the composite runoff coefficient <br /> <br />C, <br />1 <br /> <br />= individual runoff coefficient corresponding to <br />surface type <br /> <br />A, <br />1 <br /> <br />= area of surface type corresponding to Ci. <br /> <br />At <br /> <br />= total drainage are2 for which composite runoff <br />coefficient is applicable <br /> <br />n = total number of surface types in drainage area <br /> <br />13 <br />