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<br />~;M 1110.2-1405
<br />31 Aug 59
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<br />and 1-,(' \\"('1'(' lIspd to ff'fleet mon~ s('v('n~ COIH'entratiolls of runoff from t}H:' most inteI\se' 12-hour pc-Tinl
<br />uf thp dr-sigH storm~ with riD ehallgp in assumptions fE'ga.rding othpJ' pC'riods of t Il(' d('sig-Il storm,
<br />Oil tll(' basis of datu ohtailH'd from studipg of floods of [e-("ord ill tlIp busin, ttlf' h8s(' flow during
<br />the d('s;gn storm was estimated as 2 se('ond-fppt per square milf', or 2,200 sP('ond-fpt't from tll(' rntirp
<br />dnlinage area,
<br />Hydrographs Xos. A, B, a.nd C of plate Ko. 16, figUff' <', W{'I'P eomput('d by a,dding h.nlrogl'aphs
<br />Nos, I-A, I-B and I-C, alternately, to the adopted.total h~'drol(J'aph for subareas 1'\ os, 2 and :l (pIal<'
<br />No. 16, figure a), and the assumpd hase flow of 2,200 second-feet, taking into eOllsidpratioll the propp['
<br />limp relation of each hydrograph to the design storm values.
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<br />STORM RAINFALL ESTIMATES FOR COMPUTATION OF MAJOR FLOODS
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<br />30, GENERAL. a. The methods of deriving storm rainfall estimates as discussed herein apply primaril~'
<br />to the clt'lr-rmination of rainfall criteria for use in computing hypothdi('ul hydl'O~raphs representing major
<br />flood runoff, The estimates may be referred to in general h'rms as "rainfall criteria," or as "design-
<br />storm raillfall" when the pstimate is to be used in computing a flood hydrograph for fixing thE' design
<br />('upacity of a specific project. In either easel the estimate may involyp a single pt'riod of intense rainfall,
<br />01' any S<'ries of rainfall events that may be expected to occur o\'(~1' the area, whi('_hev('J' \vould produ('e
<br />(Oritical run-off for assumt'd conditions,
<br />b. For clarity in prescntation, the following discussions frlate sp(~cifi('.a.lly to critical design-stOl'm
<br />f'stimatps for usp in determining design floods for spillways of dams, or for otll('l' important, hydrauli{'
<br />~lrudtlfeS in which a high dpgrt'c of security against failul'f' is mandatory. D('sign st.orm t'stimates for
<br />projt~(.t5 wfin-auting a lower degre,e of protection may be derived in similar rnallll('l", px('cpt t.hat less
<br />S('Yf'rt' ba.sic ussumptions will be applied.
<br />('. In (h~tf'rmining the criti{~aI design storm rainfall estimatp for a part.icular draillagf' area, it is a
<br />llt'ceSSRI'Y to ronsidf'l' the size, configuration and runoff characteristics of the hasin, as well as met.col'ologi- .-
<br />('aI ('haracteristi('s of major storms in the region. In some dralna~(' basins, v('ry intpTlse st,orms of
<br />n'lati\"ely short duration produee erHical discharges whereas, in others, storms of less inlpnsit,v and
<br />longPI' duration result in the most scvere floods. As a gClH'ru1 rule, meteorological ('auditions that
<br />n~~;ult in the most intense rainfall rates over small areas differ from those thaI ('a use maximum precipita-
<br />tions ov('1' large a,rt'RS. (For a concise treatise all meteorolog~', see reference 12.)
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<br />31. COMPREHENSIVE DESIGN STORM INVESTIGATIONS. a. A rational determination of
<br />('rilieal design storm critpria for a particular drainagp basin requires a. comprehensi';e study of major
<br />storms of record 23 in the rr-gion and an evaluation of the effpcts of local conditions in the project area
<br />upon rainfall and runoff rates. The comprphensivc studies may follow approximately the ou dine
<br />h.;wn below:
<br />Anal~.ze precipitation data and synoptic situations of major storms of record in a region
<br />surrounding tllf' basin uno.pr stud.y, in ord('r to drtel'mine characterist-ic eombinations of ml'lc-
<br />OTological conditions that TPSUlt in various rainfall patterns ann duration-depth-area relations.
<br />On the basis of an anaJ~'sis of air-mass properties and synopti(' situations prevailing during
<br />the record storms, estimate thr- amount of inerNtsC in rainfall guuntitiC's thnt would hay!' resulted
<br />if conditions during thp actual storm had })(,(,11 as critical as U108(' considprcd probable of O(~("tlIT('rH'(,
<br />in ill<' region.
<br />Estimat{' thp modifications in mpteorologif'al conditions that \\1ould have b(~('n n'qllired for
<br />('a :'\ of tlH' 1'('eord storms tll IJuv(' oc("urred ovrr thr dl"ainagf' basin und('1' stud,v, ('onsidpring
<br />tOpt ..:-raphieal fpatul'f''S and locations of thC' rcspectiyc arras involve(].
<br />Tnking- inlo a('count tII(' iner('as~ ill rainfall fjuantitips that might ha.vr rcstIlt!'d from morr
<br />;":(:'\'('1"{' m('tPoro}ogiea1 ('orulitions dUT"in~ t 1](' r~eord storms, and t1H' lldjustml'llt.s 1l('(~{lssnr.\" to
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