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<br />High-intensity, heavy rains occurred at three locations in the Plum <br />Creek basin on the afternoon of June 16, 1965. Over 12 inches <br />fell near Castle Rock and over 14 inches fell near Palmer Lake and <br />near Larkspur in approximately 4 hours. East and West Plum Creeks <br />crested at 126,000 and 36,800 cfs, respectively, during the after- <br />noon. The unit runoff above the site on East Plum Creek just <br />downstream from Castle Rock was 1,170 cfs per square mile for a <br />drainage area of approximately 108 square miles. Western tribu- <br />taries of West Plum Creek and all tributaries of Plum Creek down- <br />stream of Sedalia were out of the high rainfall areas and contrib- <br />uted little or no runoff during the flood. <br /> <br />The combination of steep slopes, sand and gravel streambed, and <br />relatively open and straiqht reaches of Plum Creek near Louviers <br />was conducive to high velocities and standing waves. The computed <br />mean velocities in seven cross sections surveyed after the flood <br />were near 15 cfs, which implies maximum velocities of approxi- <br />mately 20 to 22 cfs. The amount of scour and fill, the size of <br />the cottonwood trees that were uprooted or bent over, and the <br />matted condition of the debris on trees confirmed the computed <br />velocities. <br /> <br />The damage in rural areas of the Plum Creek basin was extensive. <br />The heavy runoff deposited all kinds of debris, from sand to huge <br />boulders and trees, on fields and pastures. Road embankments were <br />severely eroded and bridges on County, State, and Interstate <br />highways were destroyed. Large-cut banks, particularly along East <br />Plum Creek, were left after land had been washed away. Much of <br />the Town of Castle Rock was inundated, and service to approxi- <br />mately 100 telephones in the town was disrupted. Seven homes, a <br />church, the Grange Hall, and the lower part of the main street in <br />Sedalia vanished during the flood. <br /> <br />The Denver and Rio Grande Western Railroad between Denver and <br />Palmer Lake, built in 1871-72, had never been damaged as exten- <br />sively as it was in 1965. Five bridges, many culverts, and about <br />4 miles of track were damaged. The track was out of service for <br />ap- proximately 6 weeks after the flood. Repairs to the <br />facilities cost $468,000. The Atchison, Topeka, and Santa Fe <br />Railway also follows the South Platte River and Plum Creek, and <br />repairs, pri- marily to one bridge, cost approximately $500,000. <br />Although the plum Creek gaging station near Louviers was <br />destroyed, observa- tions indicated that the flow increased from <br />approximately 150 to 154,000 cfs in less than 3 hours. The <br />recurrence interval of this flood was estimated as greater than <br />500 years. Prior to the 1965 event, the maximum discharge was <br />7,700 cfs, in August 1945 (Reference 5). <br /> <br />11 <br />