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<br />phones in town. The Denver and Rio Grande western Railroad between <br />Denver and Palmer Lake, built in 1871-72, had never been damaged <br />as extensively as it was in 1965. Five bridges, many culverts, <br />and approximately 4 miles of track were damaged. Repairs to Rio <br />Grande Western Railroad facilities cost $468,000. The Atchison, <br />Topeka & Santa Fe Railway also follows the South Platte River and <br />Plum Creek, and repairs, primarily to one bridge, cost approxi- <br />mately $500,000. The Denver and Rio Grande Western Railroad track <br />was out of service for approximately 6 weeks after the flood. <br /> <br />Although the Plum Creek gaging station near Louviers was destroyed, <br />observations indicated that the flow increased from approximately <br />150 to 154,000 cfs in less than 3 hours. The recurrence interval <br />of this flood was estimated to be greater than 500 years. Prior <br />to the 1965 event, the maximum discharge was 7,700 cfs, in August <br />1945 (Reference 8). No floods of consequence have occurred since <br />1965. <br /> <br />Aerial photographs of the 1965 flood are shown in Figures 2 through <br />4 (Reference 9). <br /> <br />2.4 Flood Protection Measures <br /> <br />Crib walls have been constructed in several locations to prevent <br />erosion during periods of high runoff. A wall built upstream from <br />the Interstate Highway 25 bridge over East Plum Creek protects the <br />north bank of the confluence of East Plum Creek and Sellers Gulch. <br />A short length of wall on the east bank of East Plum Creek north <br />of Wolfensberger Road protects the embankment of Interstate Highway <br />25. <br /> <br />Since 1971, the Town of Castle Rock has enforced a zoning ordinance <br />establishing major flood channels in which no structure other than <br />those for flood control may be built, unless approved by the town <br />trustees. Until major flood channels are established, construction <br />is prohibited within 100 feet of the channel centerline of any <br />stream. <br /> <br />3.0 ENGINEERING METHODS <br /> <br />For the flooding sources studied by detailed methods in the community, <br />standard hydrologic and hydraulic study methods were used to determine <br />the flood hazard data required for this study. Flood events of a magni- <br />tude which are expected to be equaled or exceeded once on the average <br />during any 10-, 50-, 100-, or 500-year period (recurrence interval) have <br />been selected as having special significance for flood plain management <br />and for flood insurance rates. These events, commonly termed the 10-, <br />50-, 100-, and 500-year floods, have a 10, 2, 1, and 0.2 percent chance, <br />respectively, of being equaled or exceeded during any year. Although <br /> <br />6 <br />