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<br />intercepls and sl()rcs runoff from approximalely 250 acres of drainage area. making Ihis <br />area essentially non-contributing to the Spring Creek channel. The Horsetooth Reservoir <br />is owned by the USBR and operated by the Northern Colorado Water Conservancy <br />District. It was completed in the early 1950s as a water-supply reservoir, but has also <br />helped reduce flooding problems in Spring Creek by reducing the tributary drainage area. <br />The total drainage area for Spring Creek is approximately 30 square miles, Downstream <br />of the Horsetooth Reservoir, the drainage area is approximately 12 square miles, The <br />outlet works for the Horsetooth Reservoir are located on the north end of the reservoir, <br />discharging to a water-supply canal. The outlet and canal are not within the boundaries <br />of the Spring Creek basin. Off-stream detention ponds are located throughout the Spring <br />Creek basin, In most cases, these ponds were designed to control local runoff and, in <br />general, appear to have little effect on flows along Spring Creek. However, the <br />Rossborough, Woodwest, Fairbrooke, and Colorado State University Animal Medical <br />Center ponds have relatively larger storage areas and may have some effect on flows in <br />Spring Creek, These ponds are controlled and operated by the City for flood-control <br />purposes and were included in the hydrologic analyses. Additionally, several man-made <br />embankments for road and railroad crossings within the Spring Creek basin result in some <br />flood attenuation. These crossings include the Taft Hill Roadway, Overland Rail <br />Roadway, and Burlington Northern Railroad (BNRR) west of College Avenue, and the <br />C&SRR west of Timberline Road. These areas were considered for detention in the <br />hydrologic analysis. <br /> <br />In 1970, the U.S. Department of Agriculture, Natural Resources Conservation Service <br />(NRCS) (formerly the Soil Conservation Service) proposed the construction of five <br />floodwater-detention structures in the Boxelder Creek basin. Two of these structures <br />were completed when the discharges were computed for the detailed analysis for Boxelder <br />Creek. There are nO flood-control structures located in the Cooper Slough basin <br />(Reference 2), <br /> <br />Dry Creek has several lakes and storage reservoirs that reduce the contributing drainage <br />area by approximately 13 percent, although they are not really flood-control structures. <br />Douglas Reservoir, located outside the City of Fort Collins, is also an irrigation reservoir <br />and reduces peak-flood discharges by an estimated 15 to 20 percent (Reference 19), <br /> <br />The City of Fort Collins is provided some protection from floods through flood warning <br />and forecasting by the National Oceanic and Atmospheric Administration (NOAA), <br />National Weather Service. <br /> <br />3,0 ENGINEERING METHODS <br /> <br />For the flooding sources studied by detailed methods in the community, standard hydrologic and <br />hydraulic study methods were used to determine the flood-hazard data required for this study. <br />Flood events of a magnitude that are expected to be equaled or exceeded once on the average <br />during any 10-, 50-, 100-, or SOD-year period (recurrence interval) have been selected as having <br />special significance for floodplain management and flood insurance rates. These events, <br />commonly termed the 10-, 50-, 100., and 500-year floods, have a 10-, 2-, 1-, and O.2-percent <br />chance, respectively, of being equaled or exceeded during any year. Although the recurrence <br />interval represents the long-term, average period between floods of specific magnitude, rare floods <br />could occur at short intervals or even within the same year, The risk of experiencing a rare flood <br /> <br />7 <br />