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I I I I I I I I I I I I I I I I I I I SECTIDNONE Introduction This report summarizes the engineering analysis and preliminary designs related to the Dry Creek Flood Control Plan. Several design meetings were held with representatives of Larimer County Engineering Department, City of Fort Collins, Larimer and Weld Irrigation Company, and URS Greiner Woodward Clyde (URSGWC) in which these design alternatives and recommendations were discussed. 1.1 HISTORIC CONDITIONS Dry Creek has reportedly not experienced serious flooding below the Larimer and Weld Canal since the early 1950's, The very large rainfall event of July 1997 that struck Fort Collins and caused considerable flooding and damage to Spring Creek did not cause significant flooding in the Dry Creek Watershed. This is partly due to the fact that the storm was generally centered over the Spring Creek watershed and was very localized. However, the absence of serious flooding may also be attributed to a network of irrigation canals and storage areas created by man-made impoundments and road embankments. Flooding in Dry Creek was documented in 1904 and 1924. Because of the series of irrigation canals which cross the Dry Creek channel, the channel below the Larimer and Weld Canal is often "dry" and in fact essentially disappears near the crossing of College Avenue. It is believed that storage impoundments and canals can likely mitigate flooding for events up to approximately a 10-year frequency return period event. However, for flood frequencies greater than a 10-year recurrence interval, a significant flood hazard downstream is apparent. In fact, combined with the potential for irrigation canals to breach during extreme flood conditions, the downstream hazards could be worse. Estimates by Larimer County and the City of Fort Collins indicate that over 800 structures exist in the Dry Creek regulatory 100-year floodplain. Estimated damages (in 1990 dollars) resulting from a 100-year flood event are in excess of$9 million dollars. 1.2 BACKGROUND This investigation builds on the previous reports related to the Dry Creek watershed. The following reports were used in our investigation: · "Dry Creek Hydrology Reconnaissance Study," Prepared for the U.S. Army Corps of Engineers, Omaha District by Resource Consultants and Engineers, Inc" October, 1992. · "Flood Risk Mitigation Plan, Dry Creek Floodplain, Fort Collins, Colorado." Prepared for Colorado Water Conservation Board by City of Fort Collins, Stormwater Utility and Larimer County Engineering Department, July 1995. · "Flood Insurance Study, City of Fort Collins, Colorado, Larimer County." Issued by the Federal Emergency Management Agency (FEMA). Effective Date: March 18, 1996. · "Major Drainageway Planning, Dry Creek, Fort Collins, Larimer County, Colorado," Gingery Associates, Inc., April 1980. T:IProjecls16824660Dry CrkDiversionLarimerCountylSub_OOI12.0 Word ProclFlood Control Plan.Rpl.doc 04/24100(11 :14 AM) 1- ]