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<br />e <br /> <br />e <br /> <br />e <br /> <br />E~I 1110,2.1405 <br />31 Aug 59 <br /> <br />e. Plot rainfall and rainfaH-extrss data for rcpnlsentative zones. in the manner illustrated in plate <br />X o. ~L If tributary flood flows aTf' to be combined by flood routing methods, the zones selected may <br />eOJlvpnicntly correspond to the rf'speetivc tributary basins. <br /> <br />35. METHOD Ill. MODIFIED STORM TRANSPOSITIONS, RAINFALL-EXCESS ESTIMATES. <br />a. The problems involv{'d in the detprmination of critical design storm estimates for very 1.l1rgC' drainage <br />urt'us differ somewhat from those pncounlered in pri'paring similar estimates for small basins. As a <br />g(.ncral rulc', the ('rit.i{'ul floods in small basins result primarily from extremely int.ense' small-area stornlS <br />of f('latj'\f(.Jy short duration, whrreus the gTPatcst floods in large ha"<,ins usually f('sult from a sprips of <br />l('s8 inU.nsp, largp-an'a storms. :\ot only is it necessary to compute maximum volumes of rainfall to <br />IH' pxpect.pd ovpr large basins in various periods of time, but the most crit.ical distributions and locations <br />of rainfall quantilips I hat are ('onsidpfed r("asonably probable during sucees.....ive ppriods of the storm <br />must hl' ('stimall'd, (Sl'l' par. :l2 b.) <br />b. (lompn'-h('nsivc mrtporologi('al studiC's similar to those outlinpd in paragraph :-n arc particularly <br />important as a basis fOf dderminirlg spillway design storm estimatcs for large drainage basins that are <br />bolh fpasonablp and conservative. "Vhen approximat(" estiruates are necessary, a procedure similar <br />to the following probably assurrs tbe most reliable results: <br />~~xamifl(~ available reports by Federal, State, and private agencies that contain information <br />f('-gar-ding major storms of record within several hundr('(l milps of the project- und{'r stud~T, and <br />sp!P('t for invpstigation those storms that appcar to have heen capahle of producing major floods <br />ovpr fi.l'pas equal to the given drainage basin, (See hibliographiesl r((ferences 1:J to 17, inclusive, and <br />20 8tHI 21,) I n making such sdeetions, the areal distribution and intensity of rainfall, and condit ions <br />affp('ting infiltqltion lossps, or contributions from melting snow coineident ''\Iith the storm periods <br />as wdl as tile ~olump of !'Uinfall should be taken into consideratioIl. <br />:\lak(' spch pn'-liminu.ry l'ompilations of data as are required to determine more definitely <br />\vhi('h of the tplItativply seleeled storms offer the most suitable basis for a design storm pstima.te <br />for the givPIl project.. <br />Oblaill all isohydal maps, precipitation )'('("o[(ls, mass-rainfall curves, nnd dm'ation-dppth-arl'u <br />data llvnilahlp for UI(' storms splcd('-d for final study. If IIpe('ssary, supplC"ffit'ut tlH'SP dahl by the <br />pl'p)HLratlOll of isohyptal maps and mass rainfall curvps for padl distin('t Jwriod of h('3VY I'ainfnll, <br />eovpring st,orm an'as appr("ciably larg('f than t}I(' basin undp'r study. <br />Hpvipw u.vailahl(' information rf'garding the mpteorologieal eouditions resulting" in tll{' J"C"sp(.(.tiv(-' <br />stOf.lllS, to dptprminp wlwtll('r it is r('asonabl(" to assume that t1i(' movpmC"lIt of flU' zones of hpuvit'st <br />pn'-('ipitation during slH'('('ssiv(' dist.inct prriods of a storm s('rif's might huvc 1)('('11 slH'h as to ('UUS<, R <br />I!I'{'at('J" uecumulation and/or a mort> ('fitieal ('onccntration ovpr an arpa eomparabl(' to t!lt' basill <br />lint/pI' study t.han actually o('('uITPd in tlip record storm. In this eonJl('dlon, a study of tll(' rainfall <br />Iml tprllS, and HIP movem('nt of rainfall ('('nters during sevl'ral major storms of I'(','onl in HIP [('ginn <br />may SPI'V(' as a f('asonablp basis of jud~mpnt. <br />Supprimposp tll(' outlinp of ilH' givPIl draillagC" basin on the isohyptal patt('fns, thnt l"{'I)I'ps('ut <br />t!lp Sll<"('('ssivp rainfall periods of a particular stofm, in positions ('orrf'sponding" to the mOVf'mpnt of <br />minfull ('('IItpI"S llSSUIJiNI ill th(' abovp st('p, It is not npef'ssary that th(' orientation of t11(' basin <br />olltlill(' h(' thr sam I' 011 isoh~'ptal maps, f('prrspnting su('eessivf' periods of rainfall that arf' s('parat("d <br />by illt("rvals of sPv('ral hours, but ,tll(' orientations should 1)(' n'asonably consistl"nt With th(" ussump- <br />t iOllS n'-gurding tlH' mpt corological eaus('s of t IH~ storm. <br />Complptc t1H' eomputations of rainfall and rainfall ('xccss quantities for ('seh transposition <br />j'n thp malllH'I' outlin('(1 in subparagraphs 34b to 34e. <br />Compurp the rainfall-{'x("('SS quantitirs computf"d for the various major storms eonsidC"rC'tl to <br />d('fprmillp tll(' crilieul rainfall serips to bf" adopted as the spillway df'si~n storm for th(" given proj('('t. <br /> <br />FOR THE CHIEF OF ENGINEERS: <br /> <br />W. P. LEBER <br />Colonel, ('orps of Engineers <br />P;xecutire <br /> <br />25 <br />