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FLOOD09933
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Last modified
1/26/2010 10:11:09 AM
Creation date
10/5/2006 4:46:02 AM
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Floodplain Documents
County
Statewide
Basin
Statewide
Title
Engineering and Design Flood-Hydrograph Analyses and Computations
Date
8/31/1959
Prepared For
US
Prepared By
US Army Corps of Engineers
Floodplain - Doc Type
Educational/Technical/Reference Information
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<br />EM 1110--2,1405 <br />31 Aug 59 <br /> <br />should he used as a check. The fact that a unit hydrograph reproduces a particular major flood hydro- <br />graph rlosely, docs not necessarily assure that application of the unit hydrograph to design storm <br />rainfall-excess quantities will inclicate the critical ratrs of runoff. Thr rainfall distribution, intt'llsity <br />and sequence may bi' such as to CRUSt' a substantially higher rah' of runoff during tll(' design storm thull <br />is indicated by the unit hydrograph appli('abk to the aetual stornl. How('vC'r, if tlw abilil.\" to ftt'('.Qunt <br />for major flood runoff ratC's by rational analysis can br demonstrat<'d, greater confidpll(,c ma.'. he phH'PO <br />in the results obtained in applying the same methods to higher rainfall values. <br /> <br />18. SYNTHETIC UNIT HYDROGRAPHS. a. In the majority of important h~'drologic studies, <br />synthetic unit hydrographs are requirrd eithrf as a substitute for d{'rivations from hydrologic records, <br />or as a means of correlating and supplementing Ob3f'fVrd data. Several methods of computing- s~Tn- <br />thetic unit hydragr8phs have been presented in tpchnical puhlieations. ~'Iost of these mt'thods \",pre <br />developed to serve special purposes, and may not constitute> the most suitabh' pro('('dur(' for ('.l'rtain <br />other uses. For instance, in flood forecasting, the 1H'('(1 for spepd in calculations ma~' justif.\" approxi- <br />mations that are not \\>'urranted in {'stimating dpsign flood h:vdrographs. In estimating critical hydro- <br />graphs of runoff, conditions favorable to high concentrat.ions of runoff must br as.3uillPd, whprl'us tlw <br />aS3umption of av{'rag'e conditions may b{' more l'f'asonahlC' in otllC'I' probl('mr'l. Tlw proel'clure outlirll'd <br />in the following paragraphs is intended primarily for usc' in pstimating eritiefl.l rates of rUlloff from <br />major storms, although the gr..n{'ral methods an' adaptabl(' to atlwr prohh'rns. It i", not practicable t.o <br />eliminate the n('ed for judgmC'nt and rxperienc(' in such studirs. <br />b. Tn developing unit hydrographs far usr in ('stimating eritical h,vdrogrl1phs of runoff, COIlSPITativc <br />determinations of th(' peak dischargp, the degree of concentration of runoff IH'al' thp p('ak, llnd the <br />"lag" time are of primary importanee. The shape of the rising and rp(,t~ssion sidps, and the Ipngt.h of <br />base of the unit hydrograph arC' usually of secondary importance if Hw thrC'e eompotwllts f'numt'ratrd <br />above arc fixed. <br /> <br />19. SNYDER'S SYNTHETIC UNIT HYDROGRAPH RELATIONS. a. The ,'mpirical relations <br />prrsentrd b.\' Franklin F. Snyder 7 have prov'('n to ,be purtieularly usdul in thl' st.ud.\' of runoff <br />charartf'l'istics of drainage areas where strf'am-flow records are lIot availablE', as wpll as in modifying' or <br />supplemE'nting available runoff r('cords to serve spe('ific purposes. TIH~ following tt'rms arp used ill <br />thr ('quations: <br /> <br />tll=lag time from midpoint of unit rainfall duration, t" to peak of unit h.vdrogra.ph, in hOlll's. <br /> <br />t,= unit rainfall duration equal to ;~5' in hours. <br /> <br />tR=unit rainfall duration othrr than standard unit, tr, adopted in sperific stud.y, in hours. <br />tpR=lag time from midpoint of unit rainfall duration, tR, t.o peak of unit hydrograph, in hours. <br />qp=peak rate of discharge of unit hydrograph for unit rainfall duration, t" in e.r.s.jsq. m\. <br />qpR~peak rate of discharge of unit hydrograph for unit rainfall duration, t., in e.r.s./sq. mi. <br />Qp=peak rate of discharge of unit hydrograph, in e.l.s. <br />A=drainagr area in square miles. <br />/'ra=river mileage from the station to center of gravit~. of the drainage area. <br />L=river milt'agr from the given station to the upstream limits of the drainag(' art'a. <br />rt and rp=roeffirients df'pf'nding upon units and {lrainagc basin characteristics. <br /> <br />b. The following equations are the most f"'quentlv used; <br />1,=<;,(I.L,,)0.3 <br /> <br />tp <br />1'=5.5 <br /> <br />10 <br /> <br />e <br /> <br />. <br /> <br />e <br /> <br />. <br /> <br />(I) <br /> <br />(2) <br /> <br />e <br />
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