FLOOD05393

• County: Statewide
• Basin: Statewide
• Title: Flood-Hydrograph Analyses and Computations
• Date: 8/31/1959
• Prepared By: US Army Corps of Engineers
• Floodplain - Doc Type: Educational/Technical/Reference Information

EM 1110-2-\(05
31 Aug 59

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should be u5ed as a check. The fact that a uuit h~'drograph repcoduces a particular major flood h~'dro-
graph closely, does not ll{'('('ssarily assure that application of thl' unit hydrograph to dt'sign storm
rainfall-ex('('s~ quantities will indicate the critical ratE"S of runoff. Thr- rainfall distribution, Intpl1sit."
and SE'quence may be' Bueh as to cause a substantially higher ral(' of runoff during tlw design storm than
is indicated by the unit hydrogruph appli('able to the actual storm. How{'V('I', if the abilit.'-" to Rccount
for major flood runoff rates by rational analysis can be demonstrate-d, grpatcr confid('Jl('c ma.'-~ be plac('d
in the fPsUlts obtairll'd in applying the same methods to hig-hpI' rainfall vahlC's.

18. SYNTHETIC UNIT HYDROGRAPHS. a. In the majority of important h~'drologic studies,
synthetic unit hydrographs are requirrd eit.hf'r as a substitutr for df'rivations from h,vdrologic I't'cortlst
or as. a means of correlating and suppiemf'nting ob3erved data. SevC'ral methods of computing s.'Tn_
the tic unit. hydrogl'aphs have bf'en presented in t{'chnical publications. :\tIost. of these fD.dhods \\oT('["e
developed to serve special purposes, and may not constitutl" the most. suitahlC' proc('durr for crrtain
other uses. For installCf', in flood forecastingt the [H'pd for speed in cal('ulatiolls ina." justify approxi-
mations that are not warranted in rstimating dpsign flood hydrographs, In pstimating critical hydro-
graphs of runoff, {'onditions favorable to high concrntrations of runoff must br aSSllffi{ld, wlwl'l'as tlw
assumption of averagf' condit.ions may hf' more l'rasonahle in oUwr pl'oblf'ms. Thl' prO('pdlln' olltliTWd
in the following paragraphs is intended primarily for use in estimatin~ (,I'itieal ratt'S of runoff from
major storms, although the general met hods an' adaptable to ot"er probl"m5. It i5 not praet.ieable to
eliminate thE' need for judgmrnt and rxpHience in such studies.
b. In developing unit hydrographs for use in estimating critical hydrograph;; of runofft ("ollsprvativc
determinations of the peak discharget the degree of eoncentration of runoff iwar thl' peak, and the
Hlag" time arE' of prima.ry importancE'. The shape of the rising and rt'('('ssion sidps, and the Ipngth of
base of the unit hydrograph arf' usually of s('{'.ondary importance if tht' thrpe components PlluffiPratrd
above are fixf'd,

19, SNYDER'S SYNTHETIC UNIT HYDROGRAPH RELATIONS. a. The {'mpirieal rclatioJls .-
presented b~' Franklin F. Snyder i have provcn to be particularly llsl'ful ill tlw study of runoff .-
charactpristics of drainage areas when' strram-flow r('cords are not available, as w('lI ll.:4 in modifying 01'
supplementing available runoff rp('ords to st'rve speeific purposes. Tht' follo\villg tprms an' ww<l in
the equations:

lp=lag time from midpoint of unit rainfall durationt lr, to peak of unit h~"drogr8_pht in hOUI':4.

tr=unit rainfall duration equal to .~~5' in hours.

tR=unit rainfall duration oth('r than standard unitt tr, adopted in specific study, in hours.
tpR=lag time from midpoint of unit rainfall duration, tR, to peak of unit hydrograph, in hours.
qp=peak rate of discharge of unit hydrograph for unit rainfall duration, trt in e.f.s./sq. mi.
qpR=peak rate of discharge of unit. hydrograph for unit rainfall duration, lR, in c.f.s"/sq. mt.
Q,=peak rate of dis{'harge of unit hydrograph, in cJ,s.
A = drainagt' arPR in square- miles.
l-'ca=river mileage from the station to cpnter of gravit.'T of the drainage area,
L=river mileagr from the given station to the upstream limits of the drainag-f' arf'a.
01 and rp=copffieie-nts dept'nding upon units and drainage basin ('haraeteristies.

b. The following equations lire the most frequ('lltlv used:
f,=C,(rl".)O,3

t,
t'=5.5

10

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(2)

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