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 1110--2,1405
31 Aug 59

Unit hydrographs for the subareas are plotted on the same sbeet in the time relation deter,
mined in the preceding step. (See plate :\0. 13, fig. b.) For convenience in use, unit hydrographs
for the several minor subareas immediately adjacent to the reservoir are combinpd into a single
unit hydrograph applicable to tbe sum of Hlf' areas involved. Tn order tbat the c!fcets of different
areal distributions of rainfall during- the spillway design storm may be evaluated, unit hydrographs
for the various subareas should be retainrd in separate form or combined in such manner as Lo
permit the eomputation of runoff separately for (a) principal tribut.aries, (h) minor subarpss
immediately adjacent to the I'psrfvoir, and (c) the reservoir surface.
Hydrographs of runoff from the various subareas, or groups of subareas, corresponding to
the spillway design storm rainfa.ll excess quantities, are computed and combined in proper time
relation to obtain a composite hydrograph of reservoir inflow. (See plate :\0. 16.) Hypothetical
hydrographs corresponding to several possihle distributions of rainfall may be computed in a
similar manner in order to determine the critical values.

COMPUTATION OF HYPOTHETICAL HYDROGRAPH BY UNIT
HYDRO GRAPH METHOD

27. GENERAL. a. The most eonvenient method of estimating the regimen of runoff in drainage
basins less than a few thousand square miles in area usually involves an application of the unit. hydro-
graph method, with certain supplementary computations in certain instances. In estimating the ordd
of runoff from drainage areas grC'ater than a fC\v thousand square miles, a more rational approach consists
in pstimating runoff from principal tributaries individually by the unit hydrograph method and com-
bining these tributary flows by means of flood rout.ing procedures. Principles and methods of flood
routing are discussed in E)'l 1110~2-1408, Routing of Floods Through River Channels. (See also
hibliographical referenees 8, 9, and 10.)
b. A thorough understanding of the principles of the unit hydrograph method discussed in para-
graphs 15 to 26 is nccessary for reliable results in the computations of bypothetieal hydrographs to
represent runoff from adoptc(] design storm rainfall values. The areal rlistribution and intensity of
rainfall excess during suceessivc unit intervals of the design storm must be considered in selecting unit
hydrographs applicable to anti('ipated conditions. The following generalities should be observed:
I n very large drainage basins, it is improbable that rainfall in eXcess of infiltration losses will
C'ntirely cover the drainage basin during an:,,~ part.icular unit interval of the design storm. Con-
scqurntlYl the areal distribution of rainfall excess during each unit time interval should be estimated,
and unit hydrograpbs applicable to each time interval should be selected by general comparison
of thc areal distribution of rainfall excess during the design storm interval with the distribution
during the storm used in computing the unit hydrograph. (See par. 15b.)
During a convective type storm in which rainfall is relatively concentrated, the effective
eontributing drainage area is usually less for a given volume of rainfall excess than during a frontal,
type storm affecting a larger area. The volume of rainfall excess resnlting from rainfall concentrated
over tbe smaller areas will normally be greater than may be expeeted from the same volume of
rainfall having a wider areal distribution, other conditions being the same. This is particularly
true where infiltration capacities are relatively high, as generally prevails in semiarid regions (and
in practically all basins during dry periods). In moderate size or small drainage areas having rela-
tin-Iy low infiltration capacit.ies, rainfall excess may be assumed to cover all portions of the basin
during unusually intense storms, but in all large basins, and in those, exceeding a few hundred
square miles in regions where the infiltration index exceeds approximatel.y 0.15 inch per hour, the
areal distribution of rainfall during successive intervals of the design storm should be estimated,
or otherwise allowed for, in estimating rainfa.ll excess and in selecting the unit hydrograph to con-
form with the rainfall excess distribution. (See pars. 32b and 35.)
t:-. Special attention is invitt'd to paragraph 2;), Sl'}{'ction of rnit Hydrographs for Design-Flood
Compu.ations.

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