FLOOD09933

• 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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EM 11lll-2-1405
31 Aug 59

16. UNIT HYDROGRAPHS FROM ISOLATED LNIT STORMS. The most direct method of deriving
a unit hydrograph involves thf' analysis of records of runoff resulting from an isolat('d unit storm that
produces reasonably uniform rainfall-excess rates for a period approximatply equal to the desired unit-
rainfall duration. The following general procedure is suitable for the computations (see plate .:\"0. 6):
a. Prepare a map (plate :\0. 6, fig. A) showing an outline of the hasin, the lor:ation of stream gaging
stations and precipitation stations in and near the oasin.
b. Construct a nct,...'ork of Theissen polygons (par. 14b) covering the basin lIndpl' study.
c. fnspcet precipitation records (\Vcathcr Bureau Climatological Data) to determine approximately
the date of occurrence of periods of intense rainfall over t.he hasin that appear to ha ve bef'Il reasonably
well isolated from other periods.
d. Refer to stream flow records for the basin under study to oeterminf' approximately the volume
of runoff from each of the rainfall periods considered in step c. Select for further study the h.vdrographs
that represpnt at least one or two inehcs of runoff.
e. Prepare mass rainfall curves for pre('ipitation stations in and near ill{' hasin for each of the periods
selected in step d. (Sep plate 1\0. 6, fig. E, and par. 6.)
.f. Plot disdlarge hydrographs for each of the periods select",] in step d. (See plate :\0. 6, fig. D.)
y. Study the data obtained in steps e andf and select for final study those that arc most satisfactory
for the purpos(~ involved.
h. :\10dify the obsrrved hydrog-raphs as rf'quired to exclude runoff from extraneous rainfall, and
estimate the base flow. Subtract th" base flow from the total hydrographs of runoff resulting from the
respeet.ivr unit storms to obtain thC' hydrographs of surface runoff. (See platf' Xo. 6, fig. D and par. 11.)
i. :\Ieasure the volume under the hydrographs of surface runoff (by planimetering or eomputation),
('ompute rainfall-ex('ess quantities and plot the data in the form of hyetographs. (See plate "lo. n,
fig. C and par. 14b.) Althongh the rainfall,rx('ess data aI'<' not directly involved in the computation of
unit h,vdrographs from hydrographs resulting from unit. storms1 these data aI'(~ necessary to indicate the
arf'al distribution and intensity eharacterist.ics of the runoff-producing rainfall that may have had an
important effect. on thp regimen of runoff.
). Divide the hydrographs of surface runoff resulting from each of t he unit storms by the volumes
under the rrsppdivc surface-runoff hydrographs, pxpresseu in inches runoff :rom the drainage area, to
obtain unit, ~l.\'(lrographs. (See plate :\0. 6, fig. D.)
k. If the durations of rainfall excess during the various storms differ appreciably from the unit
duration adopted for general use, the computed unit hydrographs may be adjusted to the desired unit
duration 10 the manner sug-grsted in paragraph 21 c.

17. UNIT HYDROGRAPHS FROM MAJOR FLOOD RECORDS. a. The applicability of a unit
hydro~raph for usp in (~omputing tlip r('gimpn of runoff from an estimat(~d maximum probable rainfall
ov('r a hasin ma.\' }H' partiaIl,v verified by ('pproducing an observpd major flood hydrograph by applying
tll{' unit hydrog'ruph to ruinfall-rxccss inc('(lm('nts of tilt, relatC'd storm. The procedure requires a eareful
analysis of rainfull-rllTlofl' data in order to (h.t('rminf' major variations in UH' ureal distribution and
intpTlsit,v of rainfall-('x{'C'ss during successive' unit periods of the actual storm. Illustrations of data
rpqui('('d arC' pn's('ntpd in plat(.s .\'os. 1 to 41 and 11. A ullit hydrograph dprived from n unit storm, or
n trial graph d('vC'loped by synthptic methods, is first applied to th(' eomputrJ rainfall-ex{'css values to
obtain a hypothdieal hydrograph for eompal'ison with the ob:...p['vpd. :\Iotlifirft,tions in the lag and shape
of thp unit hydrograph llrf' madc fiS rrquin'el, following the S-cun'c procedure discussed in paragraph
21, to ohtain II r(,llsollably clasp agreement brtw('pll tflf' actual and C'omputpd hydrographs. In the event
mRjor diffpf'{'JH'PS <,xist ill Uw art'al distribution of rainfllll-rx('pss durillg succ:pssive unit periods of the
storm, diff('n'llt unit hydrographs may bp applied to tll(' n'spp(.tivp rainfall-l'xccs:'\ Vllluc,:-.
b. The' pn)('pdu{'r dif'('usspd ill tlH' pr('('editlg paragraph is 01H' ffiPthod of drriying uniL hydrographs.
Tlw J'('sults obtnine'd ordillllrily JJlay 1)(' (,oIlsidpl'f'd more I"l'li.lhb. for dpriying desitPl floods than those
obtnillPd by llllal,\'si~ or minor rUllorr h,nlrographs I'psulting fl'om utlit storms, aithough botll methods

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