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

e

e

e

EM 1110-2-1406
31 Au. 69

C.640
q.=-
t.

(3)

. t'R~I.+0.25(IR-I,)
640('.
q"R=~
I.R

(4)

(5)

q"t"
q"R=-'---
I.R

(5a)

Q.~q. A

(6)

c, The distance to the center of area may be taken as the mea..n distance off a cumulative ares-
distancp, curve Of" {>Stimated as the distance to the eenter of gravity of the drainage area above a particular
station obtained in tht' lollowing manner:
Trace tllP outline of the drainage basin on a piece of cardboard and trim to shape.
Suspend the cardboard before a plumb-bob by means of a pin near the edge of the card-
board and draw a verticalliu(', In a similar manner, draw a second line at approximately a 900
angle to the first. The intersection of the two lines is the center of gravity of the area. Transfer
the point to the original map for uS(" in determining LtD.,
d. The distance 1~C4 is measured along tht' principal stream channel to a point approximately
opposite tJu-" center of ar('s, TIU" distan(~e L is equal to Lr,a, plus the remaining distance to the upper
basin limit, following approximately the principal stream channel.
f. The average value of the product 640 C. is approximately 400, as determined by Snyder, for
basins in the fairly mountainous Appalachian Highlands, and the corresponding average value of
(1, is 2.0. Anal~"srs of unit hydrographs for various sections of the country have indicated an extreme
range in vahll's of 640 e. and (', of from 600 and 0.4, respectively, for southern California to 200 and 8.0,
respcctivel~', for sections of states bordering on the eastern Gulf of :\1exico. If hydrologic records are
availahlp for a rrprespntative portion of the drainage area under study, or for nearby basins of similar
,'harad,'risties, the coefficients 640 e. and C, should be computed therefrom. Special field observations
to dptl'rmilH' HI(' lag may be practi('l1ble and desirable in many cases. If the drainage area is formed
by two or morl" important tributaries of different characteristics, unit hydrographs should ~e computed
separat..!.. for eal'h and the ,,'suits combined to obtain a graph for the total area. Reference is made to
Snydl.r's ol"ig-illal papl'r 7 for additional details rf"garding the derivation and use of the equations given
ahov(',

20. CONCENTRATION OF RUNOFF NEAR PEAK. a. A study of unit hydrographs for a large
lIumlU'r of drainagp basins has rev('aled an approximate relationship between the peak discharge rate
Bnd till' width of unit hydrographs at ordinates exceeding approximately 50 perc,ent of the maximum,
Rl'SU!tS of thp stud.,' are shown in plate '"\0.7. Curves Nos. W-50 and W-75 in plate No, 7 were drawn
to pn\'(.lop tl... majority of values of unit hydrograph widths measured at discha;.ge ordinates equal to
50 and 75 P('("('('llt of the peak ordinate, respeetive].y, a.s obta.ined from the study of a large number
of unit hydrographs for drainagl' basins of various configurations and runoff characteristics,
b. If ,,'liable rainfall-runoff records are not available for deriving unit hydrographs for a particular
clrailla~e an'a, Curv('S Nos. W-50 and W-75 of plate 1'\0. 7 may be used in determining conservative
widths for syntheti,' unit h~'drographs having various peaks. In the event unit hydrograph data are
availahll' for ("('presentative portions of a basin, and it is desired to modify sueh unit hydrographs to
r('prl'S('nt mon' ("ollsPr\',ativ(' runoff ratf'S, the known width values may be plotted on plate No, 7 and
curves applil'abll' to HI(' specific hasin may be drawn throu~h thes!' points paranel to Curves Nos. W-50
and W-7~, us illustrated by ('urves A-A' and B-B', respectively.

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