FLOOD03848

• County: Denver
• County: Jefferson
• Community: Denver County
• Stream Name: South Platte River
• Basin: South Platte
• Title: Stream Stability Investigation South Platte River
• Date: 11/1/1983
• Prepared For: UDFCD Denver
• Prepared By: Michael Stevens
• Floodplain - Doc Type: Floodplain Report/Masterplan

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21

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augmented by snowmelt from the mountains. Lesser floods are caused by snow-
melt or isolated rainstorms which cover only a small portion of the catch-
ment.

annual floods is 4900 cubic feet per second. The mean of the logs of the
peak diSCharges is 3500 cubic feet per second.
Prlor to the closure Of the flood control dams. the lOO-year flood
was approximately 30,000 cubic feet per second in Denver. Since the com.
pletlon of the Chatfield and Bear Creek Dams, the lOO-year flood in Denver
is expected to be lower. To detennine the new f~quency distribution would
require the routing of these floods with known return periods listed 1n
Table 5 through the reservoirs. The combined outflows would have the same
returnperlod as the peak flows. In 1974, the Corps of Engineers estilll1lted
a peak of 15,000 cubic feet per second as the 100.year flood with dams.
The Urban Drainage and Flood COntrol District has updated this estilll1l tein
1983 and predicts a lOa-year peak flow of 22,000 cubic feet per second at
the Denver gage.
The main channel can handle 16,000 cubic feet per second in Denver
before appreciable overbank flooding occurs. The operating rules for the
three flood control reservoirs call for the maintenance of the flow in the
South Platte River at Henderson to a value less or equal to 5000 cubic feet
per second. The Corps of Engineers monitors 11 gaging stations and
operates the outlet gates in accordance with the reservoir levels and the
knowledge of the incoming discharges.
Henderson. The annual maximum instantaneous discharges at the U.S.
Geological Survey's streamgaging station at Henderson are 1isted in Table 6.
The sequence of peaks is plotted ~n Figure 7. The drainage area for the
station is 4,713 square miles.
Flood peaks in the South Platte River at Henderson are different
than at Denver. There is a large amount of floodplain storage between the
two sites which decreases the magnitude of peaks between Denver and
Henderson (Gingery Associates, Inc., 1977). Also there are the additions
of flows from Clear Creek and Sand Creek which increase the magnitude of
peaks downstream from Denver. The mean of the annual flood peaks at
Henderson is 5590 cubic feet per second which is nearly 700 cubic feet per
second JI'Ore than at Denver. The mean of the logs of the peak discharges is
3940 cubic feet per second.
The largest peak occurred in the South Platte River at Henderson
in May 1973. Themagnitude..as 33,000 cubic feet per second, almost double
that at Denver.
The second largest peak ..as the June 1965 flood when the flow reached
29,600 cubic feet per second. This value is much less than the peak of
40,300 cubic feet per second which passed Denver on the same day.

The flood records for the South Platte River reflect the influence
of man. During the snowmelt months, May and June, water is stored in the
mountain reservoirs and diverted from the rivers into storage on the high
plains. Small annual peaks can be decreased 9reatly by diversions. Since
the construction of the flood control reservoirs on Bear Creek, Cherry
Creek and the South Platte at Chatfield, the frequency of occurrence of
flooding is expected to decrease. The source of major floods in the South
Platte will be severe rainfall on the urban area and on the catchments of
Clear Creek and Sand Creek.
Appreciable or severe flooding has occurred in the Denver area in
1844,1864,1876,1894,1912,1921,1933,1935,1942,1949,1965,1969 and
1973 (Gingery Associates. Inc., 1977; Hatthai, 1969).
Denver. The annual maximum instantaneous discharges at the U.S.
Geolojical Survey's streamgage in Denver (catchment area = 3,804 square
miles are listed in Table 5 and plotted in Figure 7.
The largest flood was the June 1965 disaster. Fourteen inches of
rain fell on one tributary in seven hours. Historical information indicates
that the 1965 peak discharge 40,300 cubic feet per second at Denver ..as
the greateit since at least 1844 (U.S. Department of Housing and Urban
Development, 1976). The entire flow in Cherry Creek upstream from the
Cherry Creek Dam was stored in the reservoir. The peak inflow to the
reservoir was estimated as 10,100 cubic feet per second (Corps of Engineers,
1974). Without the Cherry Creek Dam, the flood peak at Denver would have
been larger. Immediately upstream at littleton, the peak discharge for this
June flood was estimated at 110,000 cubic feet per second.
The fourth largest flood peak, caused by rainfall on the high
plains and foothills and snow in the mountains, occurred in May 1973. The
peak of 18.500 cubic feet per second at Denver is that quoted in many reports,
for example, Corps of Engineers (1973). The U.S. Geological Survey lists
17,600 cubic feet per second for this peak.
A superficial search of the flood records indicated that all the
largest floods at Denver ..ere the result mainly of rainfall. Snowmelt con.
trlbuted to some peaks but not toa great extent. AsshowninTab1e5,at
least 60 percent of the peaks were completely or mostly the result of rain.
Other floods not designated "rain" may have been indeed caused by rain. More
study is required to dete~ine th~ causes of the floods for which there are
no remarks in Table 5.
The flood frequency curve for the Denver streamgage on the South
Platte River is shown in Figure8,forthePeriod1895to1974,theyea r
prior to the closure of the Chatfield Dam. There are 77 annual peaks.
Clearly the floods are from a mixed distr1buti?n. A compouryd 109:nonnal
distribution has been fitted to the plotted pOlnts. The arlthmetlc mean of

A flood of 14,800 cubic feet per second occurred at Henderson in
1957 ..hen none occurred at Denver. Conversely, there was no flood recorded
at Henderson in 1933, when a peak of 22,000 cubic feet per second was
recorded at Denver.