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 111~2-1405
31 Aug 59

Hydrographs, paragraph 26. The difference between the provisional spillway-design-flood hydro-
graphs for natural river conditions above the damsite and inflow into the reservoir, respectively, reflect
t he effect of the reservoir in modifying the regimen of runoff. (See Hydrograph A, plate X o. 16.)
A group ~f hypothetical hydrograp,,^, representing greater concentrations of runoff but hating the same
mtume u.' the prorisional spillway desifln flood inflow hydrograph. These arbitrarily modified hydrographs
are used to determine the amount of increase in maximum reservoir level that would result from possible
variations in the concentration of runoff, and are of value in estimating the factor of safety inhere-Ilt in
the freeboard storage of a reservoir. (See Hydrographs Band C, of plate Xo. 16,)
A Spillway Design Flood Inflow Hydrograph, or the hydrograph finally accepted as rfpresenting the
critical t'olume and concentration oj flood runoff into the reservoir under the most extreme conditions considered
reasonably possible. The spillway design flood inflow hydrograph is assumed to reflect all factors of
safety necessary to assure a safe f"stimate of the maximum reservoir level that would ()btain with thE:"
adopted spillway capacity and method of operation.
b. The steps followed in computing hypothetical hydrographs for use in estimating spillway reo
quirements for reservoirs in drainage basins less than a few thousand square miles in area are illustrated
b:v eomputations pertaining to a reservoir project in the Saluda River Basin, S, C.
The hydrographs and related data for six major flood rises and three minor rises were anal~-zed to
determine minimum infiltration indices for the basin and to obtain unit hydrographs corresponding to
rcpresentativf' rainfall excess distributions. (See plates Nos. 1 to 6, and 11, and related discussions.)
Thp relative magnitudc and the meteorological characteristics of sevpral major storms in the region
WCfr investigated to determine maximum probable rainfall quantities for th(' 1,100 squart'-mile drainage
area involved. ~Iaximum rainfall depth-duration Curve C of plate 1\0. 15 was selected as the design-
storm criteria. Six-hour rainfall values of the design storm are shown in the hyetographs in plate
1\0. 16.
An initial loss of 0.4 inch and an infiltration index of 0.05 inch per hour wen' adopted as representa-
tive of minimum values likely to prevail during the spillway design flood. The computed design stonn
rainfall excess quantities are indicated in the hyetographs in plate 1\0. 16.
Unit Hydrograph 5 of plate Xo. 13, figure a, was derived to represent runoff from unit rainfall
quantities above the damsite undrr natural rivE'r conditions, assuming rainfall distributions comparable
to those recorded during the October 1-7, 1929, flood rise (plate 1\0. 3). The Pro,-isional Spillway
Design Flood Hydrograph for natural river conditions above the damsite was eomputed by application
of unit hydrograph No.5, plate No. 1:1, figure a, to the spillway design storm rainfall excess quantities.
(See plate No. 14.)
The rate- of inflow into a full reservoir from subareas Nos. 2 and 3 of plate No. 13, figure c, was
computed hy application of unit hydrographs Nos. 2 and 3 of plate 1\0. 13, figure b, respectively, to
the entire series of 6-hour rainfall excess quantities of the design storm. The individual hydrogl'Rphs
and th,. adopted total for the two subareas are shown on plate No. 16, figure a. Beeause of the relatively
small runoff volumes involved, the adopted-total hydrograph shown was assumed to represent eritical
runoff from subareas Kos. 2 and 3 during the design storm, regardless of modifications in assumptions
ppftaining to otlwr portions of the drainage basin.
H~'dl'Ograph X was computed by applying unit hydrograph 1\0. I-A, plate Xo. 13, figure b, to
S\J('('("ssivr 6-hour rainfall ("xeess incr("ments of the design storm, with exception of the two maximum
6-hour values, whic'h werf' omittC'd. Hydrograph X was assumed to represent the critical rate of runoff
from all rainfall ('xr(~ss incremf'nts of the design storm over subarra ~o. I, plate No. 13, figure <', other
than the two maximum 6-hour quantities,
Hydrographs 1\os. I-A, I-B, and I-C of plate 1\0. 16, figure b, were computed by applying unit
h~-drogJ'fiphs 1\os. I-A, I-B, and I-C of plate 1\0. 13, figure b, to the two maximum 6-hour rainfall
,'xcess vallll's of the design storm, and adding the respective partial hydrographs obtained thereby to
hydrogl'Rph X, H)'drograph I-A of plate 1\0. 16, figure b, was chosen to represent the Provisional
Spilhray Design Flood runoff for subarea 1\0. 1 of plate 1\0. I:l, figure c, and hydrographs Nos. I-B

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