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<br />DRAINAGE CRITERIA MANUAL <br /> <br />RUNOFF <br /> <br />The time from the beginning of unit rainfall to the peak of the unit . <br />hydrograph is determined by: <br /> <br />Tp = 60t + 0.5t <br />p u <br /> <br />(4-2) <br /> <br />In which T = time from beginning of unit rainfall to peak of <br />hydrograph in minutes. <br /> <br />t = time of unit rainfall duration in minutes. <br />u <br /> <br />The unit peak of the unit hydrograph is defined by: <br /> <br />qp = 640 cp <br />tp <br /> <br />(4-3) <br /> <br />In Which qp = peak rate of runoff in cfs per square mile. <br />Cp = coefficient related to peak rate of runoff. <br />Once qp is determined, the peak of the unit hydrograph for the basin is <br />computed by: <br /> <br />. <br /> <br />Qp = qp A <br /> <br />(4-4) <br /> <br />In Which Qp <br />A <br /> <br />= peak of the unit hydrograph in cfs. <br />= area of basin is square miles. <br /> <br />4.4 Unit Storm Duration <br />For most urban studies the unit storm duration, tu' should be 5-minutes. <br />However, the unit duration may be increased for larger basins. It is <br />convenient to have the unit duration incremented in multiples of 5-minutes <br />(i.e. 10- or 15-minutes) with the maximum unit duration recommended at <br />15-minutes for the 1982 version of the CUHP. <br />An acceptable unit storm duration, whenever it is larger than 5-minutes, <br />should not exceed one-third of tp' As an example, if the basin has a tp = <br />35-minutes then an appropriate unit storm duration would be 5-minutes or <br />10-minutes (i.e. less than or equal to 1/3 tp)' <br /> <br />. <br /> <br />5-1-84 <br />