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

e

.

.

tit

.

~

e

EM 1110--2,1405
31 Aug 59

Hydrographs, paragraph 26. The difference between the provisional spillway,design,lIood hydro,
graphs for natural river conditions above the damsite and inflow into the reservoir, respectively. reflect
the effect of the reservoir in modifying the regimen of runoff. (See Hydrograph A, plate Xo. 16.)
A I/roup ~f hypothetical hydeol/raph8 repre8entinl/I/reater concentrations oj runoff but haring the same
ra/ume a.' the prori8iom:tl 8pillway desil/n flood inflow hydrograph. These arbitrarily modified hydrographs
are used to determine the amount of inereasc 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 inherent in
the freeboard storage of a reservoir. (See Hydrographs Band C of plate :\'0. 16.)
A Spillway Design Flood Inflow Hydrol/raph, or the hydrol/raph finally accepted as "presenting the
critical wlume and concentration oj flood runoff into the reservoir under the most extreme conditions con8idered
reasonably p088ible. The spillway design lIood inflow hydrograph is assumed to reflect all factors of
safety necessary to assure a safe estimate of the maximum reservoir level that would obt,ain with the
adopted spillway capacity and method of operation.
b. The steps followed in computing hypothetical hydrographs for nse in estimating spillwa~' re-
quirf'ments for reservoirs in drainage basins less than a fe\"" thousand square mile,s in area are illustrated
b~~ computations pertaining to a resrrvoir project in the Saluda River Basin, S. ('.
The hydrographs and related data for six major flood rises and three minor rises \\,pre analyze.d to
determine minimum infiltration indices for tlIp basin and to obtain unit hydrographs corresponding to
represpntative rainfall rxeess distributions. (See plates K os. 1 to 6, and 11, and related discussions.)
The relative magnitude' and the meteorological characteristics of sevpral major storms in the region
werr investigated to dpterminp maximum probablr rainfall quantities for t1lP 1,100 squarp-mile drainage
area involved. ~laximum rainfall depth-duration Curve C of plate Ko. 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 were adopted as representa,
tive of minimum values likely to prevail during the spillway design flood. The computed desigll storm
rainfall exeess quantities are indieated in the hyetographs ill plate 1\0. 16.
Unit Hydrograph 5 of plate f\o. 13, figure a, was de.rived to rrpresent runoff from unit rainfall
quantities above the damsite under natural river conditions, assuming rainfall distributions comparable
to those recorded during the Octob,'r 1~7, 1929, flood rise (plate 1\0. 3). The Provisional Spillway
Design Flood Hydrograph for natural river conditions above the damsite was computed by application
of unit hydrograph 1\0. 5. plate No. 1.1, figure a, to the spillway design storm rainfall excess quantities.
(See plate 1\0. 14.)
The rate of inllow into a full reservoir from subareas I\os. 2 and 3 of plate 1'\0. 13, figure c, was
e.omputed by 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 hydrographs
and the adopted total for the two subareas are shown on plate No. 16, figure a. Because of the relativel~'
small runoff volumes involved, the adopted-total hydrograph shown was assumed to represent critical
runoff from subareas Kos. 2 and 3 during the design storm, regardless of modifi{'ations in assumptions
pertaining to other portions of the drainage basin.
Hydrograph X was computed by applying unit hydrograph 1\0. I-A, plate No. 13, figure b, to
SU('('(-'ssive 6-hour rainfall excess incrrments of the design storm, with exception of the two maximum
6-hour vuhws, whi{'h were omittrd. Hydrograph X was assumed to l'epresent the eritieal ratr of runoff
from all rainfall ('xress increments of the design storm over subarra No.1, plate No. 13, figure (', other
than ill<' two maximum 6-l1our quantities.
H~'drographs Nos. I~A. I,B, and 1-(; of plate 1\0. 16. figure b, were computed by applying unit
hydrographs I\os. 1'..1, I-B, and 1,(' of plate No. 13, figure b, to the two maximum 6-hour rainfall
ex('rss vahH's of ttH' desig'Tl storm, and adding ill{' respective partial h.ydrographs obtained thereby to
hydrograph X. Hydrograph I~A of plate ]'1;0. 16, figure b, was ('hosen to represent the Provisional
Spilhyay Design Flood runoff for subarea 1\0. I of plate ]'1;0. 1:1, figure c, and h~'drographs I\os. I,B

19