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<br />EM 1110--2,1405 <br />31 Aug 59 <br /> <br />during periods of relatively moderate intcnsiti{"s is approximately uniform in areal distribution ann <br />that the regimen of runoff therefrom may be estimated by applieation of a unit hydrograph derived <br />from minor flood re('ords. However, during the mm~t intense 12-hour'period of rainfall, it is reasonahlf' <br />to assume that the depth of rainfall in the lower basin, or Ileal' tll(' prilH'ipal stream ('hanrwls, may 1)(' <br />grrah'l' than the Rveragp on'l' the entin' drainage area, and that the cOI1("('ntration of runoff may be <br />highef, both by I'('Rson of a critical distribution of rainfall and inen'agpd hydraulic efIi('i(.tl('i('s charactc-f- <br />isti(' of higher stages in tributary streams. On tbe basis of results obtained by comparing unit hydro- <br />graphs derived from minor and major Hoods, as discussed in paragraph 24d, it is usually reasonable to <br />assume that a unit hydrograph applied t.o the maximuP-l 12-hour rainfall ex('f'SS valur of a design storm <br />should have a peak diseharge ordinat" approximately 25 to 50 percellt higher thall a unit hydrograph <br />representative of rllnoff from rainfall of modt'ratc intt'llsity and uniform areal distribution_ Ttw diffpr- <br />{'fH'e ma,Y be greater in c<.'rtain instances. <br />d. D('('isions rpgarding the modifications that should be madC' in a unit hydrograph derived from <br />hydrologic rrcords, ill order to aSSllre a conservative' f'stimate of dpsign Rood dischargl's, an' primarily <br />dependput upon judgm<"nt. T}w chara,d{"r and scope of basic data, the purpose of the estimate and the <br />importancf' of consrrvative rpsults are mattf'fS to be considered. \Vhf'1I <h.vdoping a spillway design <br />flood for a res('rvoil' projf'ct, it is usually dpsirable to det<~rminc dl(' amount of illcrease in the maximum <br />rpsrrvoir l('v('1 that would result from various di{fpf<'lIcpS in the cOIH'<'ntratioll of runoff from the design <br />storm b('(ore final decisions rpgarding s('lcrtion of unit hydl'ographs arp llttpmptP<1. <br /> <br />26. UNIT HYDGROGRAPHS FOR COMPUTATION OF INFLOW INTO MAJOR RESERVOIRS. <br />a. Th(, formation of a long rpsefvoir in a natural draillagp basin may mat{,l'ially alt('r the regimen of <br />nood runoff by synchronizing high rat('s of runoff originating above the lu'u.d of the I"<'sprvoir \vith maxi- <br />mum rates from an'as contributing laterally to the reservoir. Undt'1' nuturnl rivpr conditions, runoff <br />from tlw upper portion of a basin is retardpd by vallry storagr and normal fridiollul rt'sistanee as it <br />passt's through thp I'esprvoir reach, the resultant velocity ('OITPsporuling to that indicutp<! by .:\'1anlling's <br />formula for flow in open channels. HowpvpJ', aftH a deep J"l'scrvoir has lW(,1I formpd by ('onstrudion <br />of a dam, inflow nf'ar the upprr pnd of the I'f'sprvoir moves through the pool largel.\' hy a pro< .~ss of <br />translation, with long-wave vrlocit.ies subj<,et to momentum ('ontrol, ('qual approximatPly t.o "!ldJ ill <br />which d is the depth of flow in fed alld !/ is till' ac'('eleratioll due t'l gravity (:l2.2). Estimat('s of the time <br />ft'quired for flood waw's to travrrse natural rivt'r ('hanTlpls within Hw limits of S('v(,l'ul propos('d reservoirs <br />have ranged from a few hours to approximately 1~ days, \lIllH'reRS the timp r('quin'd COl' inflow into tlH~ <br />upper end of the full reservoir to becom<' effretive at the point of r('servoir outflow would range from <br />practically zero to a few hours for compara,hh' storms. Changes in the s~TIl('hroJlization of runoff from <br />various portions of a drainage basin may be such as to produce rates of illflow into a full J"('scrvoir that <br />arc substantially higher than would occur at Uw dam site under natural rivpr eonditiolls, lllthough in <br />some eases the differences may be negligible. <br />b. The critical rate of inflow into a full resprvo!l' during the sj:oillway dpsign storm may b('. eonvclI- <br />ientlyestimated hy the following method, which eon forms approximatcl:,- with a pro('('(lurp developed <br />in the Little Roek Distriet, Corps of Engineers, Dc'partment of the Army: <br />The drainagp- area cont.ributing to the resrrvoir is divided into subar('as in thp mannpr illus- <br />trated in plate :\0. J:l, figul'l' e. <br />Unit hydrographs are d('rivpcl for the rpspedivr subareas, using sue,h hydrologic records as <br />are availablp, supplE"m(~nled by synthctie unit hydrograph computations. The rat<- of runoff <br />from the r('spryoir surfacr is taken equal to thr rate of rainfall. <br />The time required for flood waters entering- the upper PIld of the reservoir to become eITedive <br />in raising the reservoir level at. the spillway site is estimated hy assuming the velocity of translation <br />as equal to ,'gd. If the maximum translation time is of signifieant length, the time required <br />for runoff from each stream that contributes directly to the reservoir to beeome effective in raising <br />the reservoir level at the spillwa.y site is estirnat<'d, assuming that the time of t.ravel is proportional <br />to distance. <br /> <br />16 <br /> <br />e <br /> <br />.. <br /> <br />. <br /> <br />e <br /> <br />. <br /> <br />. <br /> <br />e <br />