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<br />II 1-6
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<br />September 1933 Flood
<br />The flood of September 1933 originated from intense rains on Plum Creek and
<br />on Big and Little Dry Creeks. The flood at Denver WdS a flastl flood with a
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<br />June 1965 Flood
<br />This flood was caused by severe thunderstorms over Plum Creek and Cherry
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<br />Creek which later joined other thunderstorms over the Kiowa and Bijou Creek
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<br />peak discharge of 22,000 cfs and a volume of 36,000 acre-feet, The de9ree
<br />of effectiveness of the channel improvement through Denver was demonstrated
<br />by this flood, when the maximum discharge of 22,000 cfs passed throu9h the
<br />improved reach with only minor overtopping and relatively small damages.
<br />However, rural damages from this flood were quite high.
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<br />basins. This combination of events resulted in flows of 110,000 cfs at
<br />Littleton and 40,300 cfs at Denver. These flows caused $300 million in
<br />damages in the Denver metropolitan region alone.
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<br />May 1942 Flood
<br />The May 1942 flood, which had a peak discharge of 9,720 cfs at Littleton and
<br />10,200 cfs at Denver, caused extensive rural damages above and below the
<br />channelized reach. The sustained hi9h flows experienced durin9 the 1942
<br />flood caused destruction of five bridges and damages to three additional
<br />bridges in the channelized reach, and caused raveling of bank riprap and
<br />erosion of levee embankments in this reach even though the flood discharge
<br />caused only minor overtoppin9 of the channel banks in Denver. The 1942
<br />flood also breached the a9ricultural levees in the reach north of Denver and
<br />flooded about 350 acres of truck crops, causing heavy damages.
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<br />May 1973 Flood
<br />A snowpack of up to 170 percent of normal combined with early warm tempera-
<br />tures at high elevations and prolonged rainfall caused the May 1973 flood.
<br />The flood had a peak at Henderson of 33,000 cfs, about double the peak at
<br />Denver of 18,500 cfs. This flood caused extensive agricultural damage due
<br />to erosion and sediment. Sources indicate that the base flow in the South
<br />Platte from snowmelt was approximately 1,000 cfs, indicating that the major.
<br />ity of the flooding was attributed to runoff in the metropolitan area.
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<br />June 1949 Flood
<br />The June 1949 flood was caused by general rain and snowmelt. Although the
<br />discharge at Littleton was only 5,980 cfs, the flood resulted in extensive
<br />rural damages upstream of Denver.
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<br />HYDRAULIC ANALYSIS
<br />For reaches 3, 4, 5, 6, and 7 which extend from Oxford Avenue to Sand Creek,
<br />a hydraulic analysis was performed to determine the water surface elevations
<br />for the 10., 50., and 100.year flood events. Hydraulic computations were
<br />not performed for the fi rst two reaches of thi s study from Chatfi el d Dam to
<br />Oxford Avenue. Reach 2 from Columbine Valley to Oxford Avenue is part of
<br />the Army Corps of Engineers channel project and they have developed the
<br />100.year profile for this reach. For Reach 8, from Sand Creek to Baseline
<br />Road in Brighton, the 10., 50- and 100.year flood profile were obtained from
<br />the 1977 flood insurance study,
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<br />May 1957 Flood
<br />The May 1957 flood was a flash flood which originated on Sand Creek from
<br />intense local rains and caused peak discharges of 24,000 cfs on the South
<br />Platte River just below the mouth of Sand Creek and 14,800 cfs at the
<br />Henderson gaging station, This flood caused high rural damages in the reach
<br />from Sand Creek to Brighton. It illustrates that flood discharges from the
<br />tributary streams in the metropolitan area can cause high discharges on the
<br />South Platte River either independently or in combination with high dis-
<br />char"ges from lhe upper' OctSlrl.
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<br />For Reaches 3, 4, 5, 6, and 7 flood profiles were computed using the U,S,
<br />Army Corps of Engineers HEC-2 water surface profile computer program,
<br />Valley cross sections were developed using digitized cross sections obtained
<br />from aeri al photography flown on April 16, 1983, Since there was a signifi-
<br />cant flow in the South Platte River on that day, the channel bottom eleva,
<br />tion of these cross sections was actually the water surface elevation en
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<br />that day, rather than the actual streaTi thalweg. Because it was believed
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<br />that in certain cases as much as 2 feet of difference could exist between
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