FLOOD03740

• County: Statewide
• Basin: Statewide
• Title: Adjustment of Peak Discharge Rates for Urbanization
• Date: 2/1/1979
• Prepared For: US
• Prepared By: COE
• Floodplain - Doc Type: Educational/Technical/Reference Information

and 2 refer to different points in time.) Loss rates were developed as functions
of the percent of man-made impervious areas; and contributing drainage areas
were modified over time because of changes in drainage patterns due to highway
construction and channel realignment.
Since three storm events and their resulting Ilood hydrographs were utilized
in the calibration process, it was a relatively simple matter to use these events
to reconstruct flood hydrographs for prior land.use conditions in the basin.
Modifying the necessary parameters, as previously cited, the hydrologic responses
to the three storm events were simulated for past conditions. The results for
one event are shown in Fig. I and the peak discharge rates for all three events
are plotted versus time in Fig. 2(a).
Because of the wide spatial and temporal variation in precipitation aod in
the loss rate functions adopted for the specific storm events analyzed, the rate
of change of peak discharge with urbanization, as rellected by the slope of
the curves in Fig. 2(a), could he significantly dirferent for other rainfall events
(distributions) producing nearly the same peak runoff under existing conditions.
Since the curves are approximately parallel, representative (synthetic) rainrall

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FIG. 3.-Rlte of Ching I of Peak Dischlrge with Time

events were developed to determine the adjustment of peak discharge rates
for urbanization.
An analysis of the rainfall estimates contained in Ref. 8 for the study area
indicates that a general and systematic method exists for simulating tbe rainfall
pattern for numerous events given any single event. By taking ratios of the
rainfanamounts for various durations (3O-min, l-hr, 3-hr, 6-hr, 12-hr, and 24-hr)
of a specific recurrence interval, the rainfall amounts for respective durations
of different recurrence intervals can be determined within reasonable limits.
As an example of this procedure, a 2-yr storm pattern can be developed from
a 50-yr pattern according to

R,
(P,), = (P,)..- . .
R"

. . . . . . . . . . . . . . . . . . . . . (3)

in which P = storm rainfall amount for a specified duration, in inches; i =
duration interval, in hours; R = total storm rainfall for a 24.hr period, in inches;

3