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<br />intercepts and stores runoff from approximately 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 squaTe 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 />Spling 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,
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<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).
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<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),
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<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 />N ational Weather Service,
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<br />3.0 ENGINEERING METHODS
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<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 500-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
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