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

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EM 1110-2-1405
31 Aug 59

16. UNIT HYDROGRAPHS FROM ISOLATED UNIT STORMS. The most direct method of deriving
a unit hydrograph involves thr- analysis of records of runoff resulting from an isolated unit storm that
produces reasonahly uniform rainfall-excess rates for a period approximaU'Iy equal to tlw desirrd unit-
rainfall duration. The following gl>llcral procedure is suit-able for the C'omputation~ (see plate ::\0. 6):
a. Prepare a map (plate r\o, 6, fig. A) showing an outline of the basin, the lo(;ation of stream gaging
stations and precipitation stattons in and near the basin.
fJ. Construct a network of Theissen polygons (par. 14b) cov{'ring the basin under study.
c. fnspeet precipitation records (\oVeather Bureau Climatological Data) to determine approximately
the date of oecurrenee of periods of intense rainfall over the basin that appear to ha\'e bf'C'n reasonably
well isolated from other periods.
d. Refer to stream flow records for t.he basin under study
of runoff from each of the rainfall periods eonsidered in stpp c.
t ha.t represent at least one or two inches of runoff.
e. Prepare mass rainfall curves for precipitation stations in and near the basin for each of the periods
seleeted in step d, (See plate :'\0. 6, fig, B, and par. 6.)
.f. Plot discharge hy(lrographs for each of the periods selected in step d. (See plate 1\0. 6, fig. D.)
g. Study the data obtained in steps e and] and select for final study thos" that arc most satisfactory
for the purpose involved.
h. ~1odif.v the observed hydrographs as rcquircd to f'xcludf' runoff from extraneous rainfall, and
estimate the base flow. Subtract the base flow from the total hydrographs of runoff resulting from the
respective unit storms to obtain the hydrographs of sllrface runoff. (S",' plate ,",0, 6, fig. D and par. 11.)
i. :\feasure the volume under the hydrographs of surface runoff (by planimetering or computation),
mmpute rainfall-excess quantities and plot the data in the form of hyetographs, (See plate No, 0,
fig, C and par. 14b,) Although the rainfall.excess data are not directly involved in the computation of
unit hydrographs from hydrographs resulting from unit storms, these data arc necessary to indi('ate the
arf'al distribution and intensity characteristics of the runoff-producing rainfall that may havf' had an
important e{feet on thp regimen of runoff.
). J}ivirle thp. hydrographs of surface runoff resulting from parh of the unit. storms by the volumes
llndf'r the ff'spf'd.ive surface-runoff h~Tdrographs, pxpressNI in inches runoff from the drainage area, to
obtain unit hydrographs. (Sce plate :\'0.6, fig. D.)
k. If the durations of rainfall (>xcess during the various storms differ appreciably from the unit
duration adopted for gencraluse, the computed unit hydrographs may be adjusted to the desired unit
duration in Ow mannpr stlggrsted in paragraph 21 c.

to dctcrminp. approximately the volume
Sl'lect for further study the h,'drographs

17. UNIT HYDROGRAPHS FROM MAJOR FLOOD RECORDS. a. The applicability of a unit
hydro~I'aph foJ' liSP in (Oomputing' thC' f<'gimI'Il of runoff from an estimatc(l maximum probable rainfall
ovpr a hasin mU.\T 1)(' partially vprifipd by [,(,pl'oducing an ohsprvrd major flood hydrograph by applying
I ~H' unit h,nlrograph to rainfall-excess inc['pnH'nts of the rE~latcd storm. The procC'durc requires a careful
:lnal.,"sis of raillfuJI~l'lInofr data in ordpr to detC'rminp majol' variations in tlw areal distribution and
intpnsit.y of rainfall-('x('{'SS during Hucc('ssivp unit pcrio(ls of the actual storm. Illustrations of data
r('quin'd arp fH'('S('Iltpd in platps :\08,1 to 4, and 11, A ullit hydrograph dprin_'d from a unit storm', or
fl Il'inl g-raph dpvplo[wd by synthetic mpthods, is first applied to tlw computC'u rainfall-ex{'('s':5 values to
obtllin a h.\"pothptieul hydrograph for comparison with the obseI'n~d. :\Iodificutions in the lag and shape
of thp unit hydro~raph arp madC' as I'('quired, following the 5-(,)1I'l"e procedure diseu5scd in paragraph
21, to ohtaill a I'f'Hsonubl.," dos(' agn'('mC'nt brtwppn the fletual and compute'd hydrographs. In the event
major diff(')"('Tlc('S ('xis! ill the arpul distribution of l'ainfall~C'x('Pss during successive unit periods of t.he
storm, difl'(,t'('nt unit hydrographs may be' applied to t1H' l'l'Slwf'livp raillfall-('\':C(,5~ valuC's.
b. Till' procedure dis('uss('d ill th(' prN'('(ling paragraph is 011i' nwthod of d('dying; unil h.\~drographs.
Till' l"('f::.ults ohtaiTlpd ordillllril.\' may }w ('ollsidprcd morp !'l'liahl/' for d('rivill~ d('si~Jl fhods than those
ohtllilH'd by llllUlysi-.; of minol' rllllofl' hydrographs {"('suIting fl'Om unit storms, alLhollgh hoth mrlhods

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