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<br />e <br /> <br />e <br /> <br />e <br /> <br />EM 1110-2--1405 <br />31 Aug 59 <br /> <br />d" Three general methods of developing quantitative design storm estimates are in ("ommon U8(,- <br />Method I. Computation of a maximum rainfall depth-duration relation for the size of a,'ea in- <br />volved, based on rainfall data for ,8 large numb('f of storms that arc consiclPf('d possibl(' of oCCUrreIl('.(' <br />in the given region, and the development of a hy('tograph to fppr('sent the critical sequen('c of rainfall <br />quantitips corrrsponding to the adopt!"d dc'pHI-duration (',UI"V(', <br />Method II. Transposition of the isohyetal pattern of an a('tuaI storm to a critical position DV('r <br />th(' givPJl drainage basin, without major changes in pattern or ('hranoIogy of rainfall increments, <br />Method I I I. .'\1odified transposition method, in which it is assumcd that the direction and/or <br />rat(' of translation of rainfall ZOllPS during the rceord storm might have diffC'fCd in sueh a mannpf as to <br />have fesultrd in a more critical s('queru'e and coneC'ntration of rainfall increments over an area com- <br />parabl(' to the basin under stuely, the modifications assum("d being predicated 011 metl'orologi('ul studips. <br />e. Method I is most directly applicable in deriving spillway design storm e-slimates for b""ins <br />l('ss than a few thousand square miles in arC'a. This procl'dure may be used in preparing similar estima.tes <br />for larger basins, hut greatrr approximations are r{'quired in estimating thp volume and areal dist.ribution <br />of ra.infall in suece-ssiv(' intervals of t.he storm. ~tethod 1I is applicable to all size areas, but it is mos t <br />usrful in studif's p{,l'taining to basins having an area l:\feater than a fe-\\' thousand square miles, in which <br />variations in intllllsit,Y and are-al distribution of rainfall during sure('s.~ive int('rvals of 'illC' storm have <br />major {'ffpcts on infiltration losses and the eOJu'('ntration of rUlloff. Thl' use of the mpthod is limited to <br />sttHlif's in whieh data 81"(' availablf' for a storm of I'f'('ord that is ("onsitlered suitablf', possibly with minol' <br />modifications, as d{'sign storm ('ritpriu. ~Il'thod I I I is usuully requil'l'cJ only in studies pe-rtaining to <br />]nl'~p drainagp basins. <br /> <br />33. METHOD I, MAXIMUM RAINFALL DEPTH-DURATION DATA AND RAINFALL EXCESS. <br />11. III pn'parillg ('stimatps of maximum probabh' rainfall rut('s for basins If>SS than u (C'w thousand square- <br />milt.s in an'a, it is usually rp{L"ionanlp to assume- that rainfall quantitirs ('qual t.o the maximum averagr <br />dt'pths, obsprvpd ill various storms of I'('('ord in a partieular n>gioll, may (-'v('ntuall,\" occur OVf'r an.'" <br />~iv{'n al'f'a within it, provided tll('r(' is 110 appr{'('iablf' diffe-rf'lu'(' in topography bf'twecn the rC'spe-etive <br />points, or tlH' basin is not extJ't'ml'ly irrpgular in shap('. Thl' maximum avt'rage dl'pths of rainfall over <br />all arps. corrf'sponding to HlC' hasin und(.1' stud,,"' mo.:,? bl' eomputed for various pe-riods of lime, using <br />maRs-rainfall ('urv{'S d('vrlop<"d ill t1H' malllu'r OUtliIlf'd in paragraph 6 as a basis for dl;'trrmining t hp <br />qURntitips of rainfall ill the f('sp('etiv(> p{'riods,17 Examplps of maximum rainfall d{'pth-duration curv('s <br />UI'f' sllown,in plate :\"0. 1.5. ('urv{'S A and B wen' baspd 011 maximum depth-duration data for, two major <br />storms and 8.1'(' int{'rul(.d to in"hull' allowall('l's f('f{'rfl'd to in paragr"aph:11. Curve C \Vas drawn to <br />(lllv{'lop all vnlu('s r('pn's(mtt'd by (~urv('s A a,nd B. <br />b. 'I'll(' ra.illfall de-pth-duration r<'latioll rr-quir('d to produce the maximum flood discharge in a <br />druirUlg't' basin is part.iall.v drtt'rmilH'd by thl' amount of natural and artificial st.orage, capacity available <br />foJ' modulatillg tl1<' rat<. of runoff from iJlt('ns(' rainfall. In a drainag(' basin having reh,tively small <br />vallt'.\" storug(' eapaf'ities, t.hp p('ak rat<' of runoff from rainfall corresponding to Curve B of plate No. 15 <br />may ('x('{.(>(1 tll(. p{'ak rat<. that would f('sult from tll(' grf'atf'r volume but less intense rainfall represented <br />hy ('urv(' A, In ba.."iins charactf'riz('d by largr vallry st.orage capacities, the reverse might be true. The <br />('rpation of an artifidal I'e-o('rvoil" in a nat.ural drainag(~ basin may e-ither increase or decrease the storage <br />eapaeity that would atfect flood runoff rates immediately below the dam, the amount of change depend- <br />ing, to all important ('xt<~nt. upon the- typt' and size- of spillway adopted. Inasmuch as an envelopinK <br />rainfall depth-duration curv(~ represents the greatest volume and highest intensities represented in an.Y <br />of thr basic storms, a design storm estimate based thereon will assure a flood dischargf' estimate more <br />conservat.ive than anyone of the basic storms, regardless of storage-discharge characteristics of the <br />natural basin or artificial rese-rvoirs. <br />c. Although hypothetical hydrographs derived from rainfall estimates based on an enveloping <br />df'pth-duration ('urve are somewhat more eonservat.ive than would be obtained from a depth-duration <br />curve for any individual storm, the difference may be smaH. In basins characterized by rapid ('on- <br /> <br />23 <br />