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<br />EM 1110-2-1405 <br />31 Aug 59 <br /> <br />(lJ,")03~9.9 <br /> <br />e <br /> <br />tp~(',(LJ,,")O,3~2.8x9,9=27.7 hours <br /> <br />27.7 I <br />t,~ 5.5 ~5.1 lOUrs <br /> <br />tR~6 hours (dl'Sired) <br /> <br />t'R~27.7+0.25(6~5.1) <br /> <br />= 28 hours <br /> <br />640('. 370 fl' <br />qPR=-~-28 =13.2 e. .s. sq. mI. <br />tpR <br /> <br />Q.R~12,800 c.e.s. (Hee plate No. 12.) <br /> <br />f. Values of 11'", and 11'" corrl'Sponding to the synthetic unit hydrograph peak valne computed <br />for the 970 square-mile area above the dam site wcre read from Curves A-A' allll B-B' of plate No.7, <br />and thc rl'Spective values were indicatcd by points plottl'd on each side of the unit hydrograph peak <br />approximately as shown in plate .'\0. 12, <br />g. A tentative synthetic unit hydrograph was sketchNl through the {'stimated peak discharge, <br />and the plotted valul'S of W'" and W", terminating temporarily at approximately point X indicated in <br />plate No. 12. <br />h. A provisional S-curoe hydrograph corresponding to tentativ(' unit hydrog-raph .No, I-A was <br />computed to point X and projected forward as a smooth curvp until t,h(' maximum ordinate was fC'---3chp<!. <br />The computation of thc trial unit hydrograph was then completed, as indicated in plate No. 10 part (a). <br />Minor adjustmcnts were made in the provisional unit hydrographs and S-curf' hydrograph until the most <br />logical forms of both were obtained, as indicated by the final curves of plat<- No. 12. The final compu- <br />ta.tions are shown in plate .Ko. 10, part (b), Tn making th{" adjustmC'nts, it is eonv('uicnt to work from <br />the right end of the unit hydrograph where the corrcct S-curre value is known, and work backwards <br />toward point X by assuming values of unit hydrograph ordinat('s that appt'ar fPRsollab]p and computing <br />the corresponding S-curve values. The adjustm('ut IH"Cpssary to make th(~ two portions of the S-curup <br />meet near point X can easily be made. <br /> <br />e <br /> <br />24. COMPARISON OF UNIT HYDROGRAPHS DERIVED FROM MAJOR AND MINOR FLOOD <br />HYDROGRAPHS. a. The definition of the unit hydrograph implil'S that ordinatcs of any hydrograph <br />resulting from a quantity of runoff-producing rainfall of unit duration would he equal to oorresponding <br />ordinates of a unit hydrograph for the same areal distribution of rainfall, multiplied by the ratio of <br />rainfall-excess values, However, the relation infprred by thp definition is only approximately corr(lct, <br />and may be appreciably in eITor if the conditions affpct.ing runoff differ greatly during floods of various <br />magnitudes in the basin under study. <br />b. In an effort to determinc the probable dcgrec of aceuraey inherent in the use of unit hydrographs <br />de-rived from rerords of minor floods in estimating the critical ratps of runoff from maximum prohahle <br />storms, hydrologic data for minor and major floods of record in a larg-p number of basins have hewlI <br />analyzed. Minor floods were selectl'd that resulted from rainfall of relatively uniform areal distribution, <br />The volumes of rainfall and rainfall excess during successive 6-hou-r periods of storms causing Inajor <br />floods in each basin werc computcd and unit hydrographs wcre developed thd would rcproduce the <br />observed hydrograph when applied to the known rainfall execs.. values. :\fost of t he major f100dsinvest i- <br />gated were the result of one or more periods of intense rainfall of approximatp,Jy 12 hours duration, <br />supplpmented by periods of lighter precipitation, The samp ~en("ral pro('('clnrC"s 'n'n', followed in I h(' <br />analysis of records and in the computation of unit hydrographs for both minor find major floods, insofar <br /> <br />14 <br /> <br />e <br />