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2.3 Principal Flood Problems Major floods have occurred on the South Platte River and Sand Creek since 1844. During that period, 11 floods have occurred on the South Platte and 10 notable floods have occurred on Sand Creek. The most significant floods of recent times on the South Platte River occurred in 1912, 1921, 1935, 1942, 1965, and 1973 for which discharges of .13,000 cubic feet per second (cfs), 8,790 cfs, 22,000 cfs, 12,320 cfs, 10,200 cfs, 40,300 cfs, and 33,000 cfs, respectively, wer.e recorded at the Denver gage. The floods of 1896, 1912, 1917, 1921, 1933,. and 1938 on Sand Creek were not recorded in regards to specific discharges. Measurements were made to estimate the discharges. for the following years: 1948, 10,500 cfs at the mouth; 1957, 25,500 cfs. at Yosemite Street; 1965, 18,900 cfs just downstream of Toll Gate. Creek; and 1973, 5,630 cfs at East 49th Drive. The major cause of floods on the South Platte River and Sand Creek are cloudbursts or intensive rainstorms which normally occur during the period of May through August. The South Platte River flooding also is aggravated by snow melt on the tributary streams during the rainstorm period. There are two areas of shallow sheet flow within the community, both of which are along Sand Creek. The upstream area is on the southwest side of Sand Creek between the corporate limits and 49th Drive. In this area, the ground slopes away from the channel and the flow cannot return. The second area is under Interstate 270 at the two railroad underpasses to the northwest. During a large flood event, the flood waters will pass under the interstate bridge and flow along the low ground away from the channel. In 1965, a unique combination of orographic effects and meteoro- logical conditions in the South Platte River basin caused the worst flooding in the region's recorded history. Severe thunderstorms commenced over the headwaters of Plum Creek and Cherry Creek on June 16 and moved northeasterly down the creeks and augmenting peak flows. More than 14 inches of rain was recorded at Palmer Lake in 4 hours. Overnight, westerly winds moved the storm front to a position over Kiowa and 8ijou Creek basins to meet with thunderstorms forming just south of Agate. Here 5.25 inches fell in 45 minutes. The net result was 6 persons drowned, 2 other deaths caused by flood-related activities, and estimated damages of $500 million in the South Platte River basin, of which $300 million occurred in the Denver area. Scenes of the 1965 flood in Commerce City are shown in Figures 2 through 9. The pictures were taken from photographic u_nx_ec_o_~d_s__of the Denver Post and Hotchkiss, Inc. 4