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<br />EM 1116-2-1405
<br />31 Aug 5'
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<br />square milt's, providing appropriatl? eonsidt"ration is given to the longer-duration storms that ma.~" 1)('
<br />requir('d to produ('(' ('riti("al flood discharges ill HI(' larger drainage basins.
<br />The grnerali7.{'d l'stima,tes of prohabl(' maximum precipitation may be ut.ilized by District and
<br />Division Offi("('s in tlH' ("onlputatioll of spillway dpsign floods (and eomparahh' determinations) ill nIl
<br />studips that ,\-Tould otJl('rwisC' I"("quin. IIprdimina.ry estimatl"s" by tll(' WC"atll('J' Bur("Ru. In tli(' ('ase of
<br />larg(' dl"ainagt' hasins, for whirh J'P8sOllUble <,xtrapolations from tll(' data pres("nted ill thp J'('feI't'IH'('d
<br />report are not ('onsidpl'pd appropriate, sp('("ial preliminary l'stimntes of probable maximum rainfall will
<br />be ohtailled from the Wcatllf'f Bm"('au through th(" Chif'f of Enginc{'fS in tlH' maIllJ('J" outlinC'd in
<br />the preeeding subparagr.ph.
<br />
<br />32. APPROXIMATE DESIGN STORM ESTIMATES FOR SEASONS WITHOUT SNOW. a, Al-
<br />though spe..ial comprehensive studies are fully justified and highly desirabh' for important projeets, it
<br />is frf'quently n('('('ssary to prepare design storm rainfall estimat,es on the basis of data immediatel,"
<br />availablt'. Certain of the problems involved in the deter"minalion of design storm rainfall estimates,
<br />and th(' ffi('thods most commonly followf'd in their preparat.ion, wht'n th(' J"('sults of eomprrhellsiv{'
<br />meteorological studit's an' not available, are discussed in thi' following- pal'agraphs,
<br />h. Estimat.es of rainfall quantities to be expected during su('("('ssiv(' unit p('riods of a design stor ill
<br />for a drainagp basin less than a few t.housa,nd square mil('s ill U,1.t'U. are usually express(~d us avcl'a~j'
<br />depths ov('r the dr"ainage area., and UlC' eritical s("qurllcc of intt'ns('-rainfall ilHTemcnls is dct.erminpd
<br />arbitrari):\' by tria] appli('ations of unit h.nlrographs. ~ormal variation!:' in the a('(~al dist.ribution of
<br />dt'Sign storm rainfall ov('r small basins ma.v be satisfactorily allowC'd for in !.wh'ding" infiltration indicps
<br />and in d('vPloping unit hydrographs to rrfleet, critical conditions. Howl'\'t'-r, in d('termining t,lIt, mnxirnu'm
<br />prohablr flood for large drainage basins, definite estimates or assumpt.ions J"('gal'ding the al.PILl distrihution
<br />of rainfall during su("cessivr periods of thC' (It'sign storm art' IH'(',('SSUI'Y. Till' prilH'ipnl I'pasons arl' as
<br />follows:
<br />
<br />The' infiltration opportunity during a storm pl'riotl is proportional to tht, art'8. ('o\,('I'(,(] by
<br />rainfall of an intensity grC'atrr thall the infiltration capaeity of t-lIP soil. If pn'cipitlltion during
<br />a particular pC'riod of tilt' df'sign storm should cover olll~' It portion of a dl'a.inRgl' an'lt, tlw infiltra-
<br />tion loss would be 1('55 thaIl would occur if tl1t' sarnt' volunw of rainfall WPt'(, uniforml.\' (Iistribut('d
<br />over the basin, other factors being till:' same,
<br />1'h(' infiltration capacity of a given soil tends to d('rrl'a.sp as the durat.ion of I'a.infall t'XCPSS
<br />increases. Consequ('ntly, the infiltration loss resulting from two SIH'('l':.;sivt' lH'riods of rainfall
<br />locat('d in the same portion of a basin would bt' 1('8.... thall would ()(~('llr if t h(' sam(' voluIn(,s of pn'-
<br />cipitation had bef>Il located in diffen'nt portions of thr drllinagt' an'a.
<br />The position of rainfall cent.ers during su('cessivl' ppriods of u. storm Inay ha,vP a vl'ry g['(lat
<br />('ffect on the regimen of runoff from a. J..,riven volume of rainfall t'X('('SS, part j(~ulurly ill larg'p dnlin !lgt'
<br />basins in whicli a "dele range of variation in the' location of rainfall t'xc'pss volumes duriH~ SlH'('('ssi \"('
<br />ppriods of a dpsign storm is possihltl,
<br />c, Studi('s of fl largp rnmlbt'J' of major st.orms ha.vp r('vcal{.d tllat intl'llst' rainfall eovl'rin!{ Hl't'aS of
<br />s(-'vt'ral thousand square milt,s may occlIr in a wiol' variety of puUprlls, lillt] thai tht, maximum int('nsitil's
<br />may occur Iwar tilt' hpgiJllling, middlp or end of t.he preeipitutioll I)('riod, :\Iountain rallgt's l('ss than a
<br />few thousand ft't't in height aff('('t the fn'qlH'ncy of O('('IlITPnCt' of (,pl'tain rtlinfnll pattprns, hut appnn'lltl,\'
<br />do not predudt' the o('etll'J'PJI('(' of patterns radil'ally differellt. from tin' normal during (',prtaill UIlIISIlUI
<br />storm conditions. Thl' normal dil'l'ctioIl of mov('ment of ail' mass('s ovpr tht' ("plltrallJnit('(l ;";tllt('S has
<br />c8usf'd thl' majority of isohyetal pattf'rns to be oripnted in upproximutt'ly,a sout.hwt'st to Ilort./H'ast
<br />dirretion, but the patterns of st'vf'ral irnportant storms ('ovprillg st'\"pral thousand square Il!ilps huv('
<br />OCCUlTed with their major a.xps at right anglt's to tht' normal dir<,("tion. Appan'ntl.v, a. eOllsid('nt\)]p
<br />rllngC' in assumptions regarding raillfall paltprns and iuh'Ilsi4y variations (',l1n he mad(' in d('V('loping
<br />d(lsign storm ('rih~ria foJ' relatively small basins, without lH'ing irl<'onsist('nt. with m('l(~orol()gi('al ('tLllsPS,
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