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FLOOD09933
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Last modified
1/26/2010 10:11:09 AM
Creation date
10/5/2006 4:46:02 AM
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Floodplain Documents
County
Statewide
Basin
Statewide
Title
Engineering and Design Flood-Hydrograph Analyses and Computations
Date
8/31/1959
Prepared For
US
Prepared By
US Army Corps of Engineers
Floodplain - Doc Type
Educational/Technical/Reference Information
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<br />EM 1110,2,1405 <br />31 Aug 59 <br /> <br />(l,L,,)O"=9,9 <br />t,=(',(lJ,,,)o"=2.8X9.9=27.7 hours <br /> <br />27.7 I <br />iT = 5.5 = 5.1 lOur!'. <br /> <br />tR=6 hours (desired) <br /> <br />t'R=27.7 +0.25(6-5.1) <br /> <br />= 28 hours <br /> <br />q,R= 640(',= 32780 ~ 13.2 c.Ls./sq. mi. <br />t,R <br /> <br />Q'R= 12,800 c.Ls. (See plate :\'0. 12.) <br /> <br />f. Yalues of 11'", and 11'" corresponding to the synthetic unit hydrogmph peak value computed <br />for the 970 squarE'-mile area above the dam sitl' were read from Curves A-A' anel B-B' of plat.e Ko. 71 <br />and the ff'speetive valups \!"'ere indicated by points plotted on ('aeh sid() of the unit h,vdrograph peak <br />approximately as shown in plate ~o. 12. <br />g. A tentative synthetic unit hydrograph \vas sketclwd t.hrough the pstimatC'd peak discharge, <br />and the plotted values of lV.so and lV7~' terminating tpmporarily at approximately point X indicated in <br />plate No. 12. <br />h. A provisional S-curDP h,ydrograph corresponding to t.rntative unit hydrograph 1\0. I-A ,vas <br />computed to point X and projected forward as a smooth curvf' until t.hp maximum onlinut.e ~vas rrllchpd. <br />The computation of the trial unit hydrograph \Vas then eomplrtpd, as indieatrll in plate Xo. 10 part. (a). <br />~-finor adjustments were made in the provisional unit. h,vdrographs aIld S-curff hydrograph until the most <br />logical forms of both were obtained, as indicated by the final emvps of platr. ",,0. 12. The final compu. <br />tations are shown in plate Xo. ]0, part (b). III making thp adjustmpnts, it is cOllvf'llient. t.o ,,{ork from <br />the right end of the unit hydrograph \\.here thr correct S-cnn'f value is knO\\rn, and \vork backwards <br />toward point X by assuming values of unit hydrog'l'aph ordinates that nppear reasonable and comput.ing <br />the corresponding S-cnrl:e values. The' adjustment neC(lssal'Y to make thp t.\IlO portions of the S-Cu.rOf <br />meet near poin t X can easily be made. <br /> <br />24. COMPARISON OF UNIT HYDROGRAPHS DERIVED FROM MAJOR AND MINOR FLOOD <br />HYDRO GRAPHS. a, The definition of the unit hydrograph implies that ordinates of any hydrograph <br />resulting from a quantity of runoff,prodneing rainfall of unit duration would be equal to correspondini( <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 inf(lrred by the' definition is only approximately correct, <br />and may be appreciably in error if thf' conditions affecting runoff differ gI'f'atly during floods of various <br />magnitudes in the basin under study. <br />b. In an effort to determine the probable degree of accuracy inherrnt in the use of unit hydrogl'aphs <br />derived from records of minor floods in estimating the crit.ical ratrs of runoff from maximum probahle <br />storms, hydrologic data for minor and major floods of record in a large number of basins have bN'lI <br />analyzed. Minor floods were selected 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 major <br />floods in each basin were computed and unit hydrographs were developed th"t would reproduce the <br />observed hydrograph when applied to the knO\.....n rainfall excrss values. ~'1ost of thp major floods .investi- <br />gated were the result of one or marC' periods of intense rainfall of approximately 12 hours duration, <br />"upplemented by periods of lighter precipitation. The same i(eneral pro('pdnres we!'p followed in the <br />analysis of records and in the computation of unit hydrographs for both minor and major floods, insofar <br /> <br />14 <br /> <br />e <br /> <br />. <br /> <br />. <br /> <br />e <br /> <br />. <br /> <br />~ <br /> <br />e <br />
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