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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31 Aug 59

as the character of basic data permitted. The topography of the basins considered varied from rolling
slopes to relatively steep hills several hundred feet in height above the principal stream channels.
c. With but a few exceptions, it was found that unit hydrographs required to reproduce the major
flood hydrographs had peak discharge ordinates consistently higher than those computed from records
of minor floods in which the areal distribution of rainfall was approximately uniform. In the majority
of basins considered, the peak ordinates of unit hydrographs derived from major flood hydrographs,
representing runoff volumes greater than approxima.tely 5 inches depth from the drainage area, \"'err
25 to 50 percent higher than values computed from records of minor floods, in which the runoff was
from one to two inches. The variations were greater in a few instances. The differences were not
proportional to the volumes of flood runoff, but apparently were the result of a number of factors, some
of which had greater influences during certain floods than in others.
d. The following probably represent the principal reasons for the observed differences between
unit hydrographs derived from minor and major flood hydrographs referred to above:
Differences in areal distribution of rainfall-The minor flood rises analyzed resulted from rainfall
of approximately uniform areal distribution. Precipitation during the major floods usually covered
the entire drainage area, but in most instances the rainfall intensity and accumulated amounts varied
over the area. If the volume of rainfall excess during the major storm were proportionately heavier
in the lower portion of the basin, or near the principal stream channels, the concentration of runoff
would be higher than represented by the unit hydrographs deri ved from the minor flood rises.
DijJerence8 in hydraulic relation8- During minor flood rises, the hydraulic gradients in natural
streams are usually relatively low, because of the series of pools that exist ill the channels. As the
stage increases during major floods, the pools tend to drown out and, in basins Jess than several
hundred square miles in area, the channel conveyance is usually increased substantially. It is to be
expected thcreJore that unit hydrographs derived from major flood hydrographs for small streams
would have higher peak discharge ordinates than those derived from minor floods. In large basins in
which high quantities of overbank storage occur at flood stages, the channel conveyance may decrease
with rises in stage but the higher cffieiencics of flow in tributary streams U"nd to counteract such
decreases.

25. SELECTION OF UNIT HYDROGRAPHS FOR DESIGN-FLOOD COMPUTATIONS. a. In
the majority of hydrologic studies pertaining to the determination of maximum probable flood hydro-
graphs, reliable data for the determination of unit hydrographs are .limited to reeords of relatively
minor flood occurre.nct's. For reasons discussed in paragraph 24, it is usually justifiable to assume
that a unit hydrograph applicable to the most intense periods of rainfall during a design storm would
have a higher peak discharge ordinate, and would represent a higher concentrat;on of runoff, than
might be indicated by unit. hydrographs derived from minor floods. If adequate and reliable hydrologic
records are available for floods that resulted from rainfall intensities and areal distributions reasonably
comparable to those to b" expected during the design storm, uncertainties regarding the proper unit
hydrograph values for use in estimating the design flood runoff are substantially reduced, However,
it is fJ'{'quentIy necessary to modify unit hydrographs derived from available hydrologic records to
represent higher rates of runoff, in order that their use in computing the design flood discharges may
RRsurc conservative results.
b. The general procedure outlined in paragraph 23 is suitable for modifying a given unit hydro-
graph to represent a higher peak discharge. The method was followed in arbitrarily modifying unit
hydrograph No, I-A, plate No. 13, figure b, to obtain ]l;os. I,B and I~C, which have peak ordinates
25 and 50 percent higher, respectively, than ]1;0. I-A. The widths of each of the three unit hydrographs
at ordinates equal to 50 and 75 pereent of the maxima, were obtained from Curves A-A' and B-B',
of plate ,]1;0. 7.
c. The unit hydrographs selected for estimating rates of runoff from successive unit periods of
the design storm should be applicable to the areal distributions of rainfall that are assumed to occur
during the rf'sprrtive periods. As a g.?neral rule, it is ordinarily satislador,v to consider that rainfall

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