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<br />EM 111&-2-1405
<br />31 Aug 59
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<br />during periods of relatively moderate intensities is approximatcl~T uniform in aresl distribution and.
<br />that thp rpgimpn of runoff thprpfrom may be estimated by application of a unit hydrograph derived
<br />from minor flood f{'('ords. However, during the mogt intense 12-hour'pC"riod of rainfall, it is reasonabl{"
<br />to assume that the depth of rainfall in the lowPf basin, or nesr t.h(' prinf'ipal strl'sm ehaJUu'ls, may h("
<br />grr-att'r than the avC'rage over t}}(> entirp drainage area, and that tlw (~on("('ntration of runoff may be
<br />higher, both by ft"Rson of a critical distribution of rainfall and ineff'RSed hydraulic r-fficiC'nciC'S (~hara('ter-
<br />i8ti(" of higher stages in tributary streR.ms. On tbe basis of results obtained by comparing unit hydro-
<br />graphs dprivNl from minor and major floods, as dis('ussed in paragraph 24", it is usually reasonable to
<br />assume that a unit h,,~drograph applied t.o the maximum 12-holJr rainfall exc'('SS valu(' of a design storm
<br />should have a ppak discharge ordinate approximately 25 to 50 pereent higher than a unit hydrograph
<br />reprf'sentativ(~ of runoff from rainfall of moderate intellsity and uniform areal distribution, Ttw differ-
<br />('Iwe may be greater in certain instances,
<br />d, Dpeisions regarding the modifications that should be made in a unit hydrograph derived from
<br />hydrologic re('ords, in order to assure a conservative estimate' of dC'sign flood diseharg('S, are primarily
<br />t1pp('Il(I('nt upon jutlgmt"nt. Tbe charadt'r and scope of basic data, tht' purpose of tlw estima,te and the
<br />importance of conS('rvativp n'Sults are matt('rs to be considered. \Vlu'" d('v('loping" a spillway design
<br />floou for.a rest'rvoir projf'ct, it is usually desirable to determine the amount of ill('f(~ase in the maximum
<br />resprvoir level that would ["esult from various diff('renef's in the ('OI)('pntration of runoff from tlw u('sign
<br />storm before fins] decisions regarding sf'leC'tion of unit hydrographs art' att('mpted"
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<br />26. UNIT HYDGROGRAPHS FOR COMPUTATION OF INFLOW INTO MAJOR RESERVOIRS.
<br />a. The formation of a long r('servoir in a natural drainage basin may matprially alter the regimen of
<br />flood runoff by syn('hronizing high rates of runoff originating above the head of tilt' f('s('rvoi.. wit.h maxi-
<br />mum rates from 8r('as C"ontributing laterally to the reservoir, Undf'r natura1 riv('r conditions, runoff
<br />from t}w upp('r portion of a basin is rctardt'd by vallf'Y storag(' and normal fridional rt'Sistanee as it
<br />passC'S through the rps('rvoir reach, the resultant \,f')oeit,v ('QI'ft'sponding to that indieatt'd by ~[anllillg's
<br />formula for flow in op('n channf'ls. Howpv('r, aftt'l" a deep reservoir has b('f'lI formed by (OoBstruction
<br />of a dam, inflow n('ar th(' Uppt'f en<l of the' J'f's('rvoir mov('s through the pool largely h.," u pro( _~s.<; of
<br />translation, with long-wave vplo('ities subjeet to momentum eontrol, ('qual approximatt'ly to ,'!pl, in
<br />whieh d is thp dppth of flow in fept and II is th,. a('('('leration dn" t<> gravity (:l2,2). ~~stimall's of the time
<br />Tt'quired for flood WRV('S to travers(' natural river ehanTwJs within tlH' limits of s('vt'l'al propos('d rcsprvoirs
<br />have ranged from a few hours to approximatf'ly 1~ da~'s, wlH'rl'as the timp requirpt! for inflow into tilt'
<br />upper end of the full rt'servoir to becomp effedive at the point of r('servoir outflow would range from
<br />practically zero to a fe,\;- hours for comparahlp storms. Changes in the synchroniz.ation of runol[ from
<br />various portions of 8 drainage basin may be surh as to produce rates of inflow into a full f('servoir thnt
<br />8,r(' suhstantially higher than \\'ou)d o('eur at tht' dam site under natural rivpr ("onditions, although in
<br />some cases the differpnees may be negligible.
<br />b. The critical rate of inflow into a full rrsprvoir during tlH' Sjiinwa~T d(.sign storm m~lY he eon\'clI-
<br />iently estimated by the fonowin~ method, whi(Oh eon forms approximately with a proc('dur(' devdopt'd
<br />in the Little Ropk Distriet, Corps of En~ineers, Department of the Army:
<br />The drainagE' art's ront,rihuting to tht' rcsr-rvoil' is dividt'd into suha.rt'8s in th(' mannr-r illus-
<br />trated in plate Xo. \:l, figure e.
<br />l"nit hydrographs are elt'rivr-d for the f('spec:live subareas, using such hydrologic: records as
<br />art' availablf., supplt'm{~llted by syntheti(' unit hyorograph computations. The ralt' of runoff
<br />from tbe r(,sf'ryoir surface is taken ('qual to the' rate of rainfall.
<br />The lime required for flood waters entering- the upper {'nel of the reservoir to become effective
<br />in raising the reservoir level at the spillway site is estimat(~d hy a.....<;uming the velocity of translation
<br />as equal to ,;yd. If the maximum translation time is of significant length, the time required
<br />for runoff from each stream that f"ontributes directly to the reservoir t.o beeomc effeetive in raising
<br />the reservoir level at the spillway site is eBtima-led, assuming that the time of t.ravel is proportional
<br />to distance,
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