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I I I I I I I I I I I I I I I I I I I I. ,~ / ;~5~ Introduction' and Ba~ion Dry Creek drains a watershed of some 62 square miles generally north and west of the City of Fort Collins (referred to as the "Upper Basin"). The watershed is unique in that it is intersected by 11 irrigation ditches which capture the daily and frequent flood flows originating in the watershed above each ditch. Two onstream reservoirs, Dry Creek Reservoir and Douglas Reservoir also capture and store Dry Creek flows. Hydrologic studies dating back to 1979 have estimated the peak rates of flood flow for various frequencies for the Dry Creek Basin. The most recent studies conducted at the request of the Omaha District US Army Corps of Engineers, estimated the peak flow rate of a 100 year storm at Willox Lane to be 5,000 cubic feet per second. Dry Creek has not experienced serious flooding below the Larimer and Weld Canal since the early 1900's. Flooding in Dry Creek was documented in 1904 and 1924. References to the flood in 1924 are vague, and may have referred to a breach of the Larimer and Weld Canal. The absence of flooding may be attributed to the network of irrigation canals and storage areas created by manmade impoundments and road embankments. It is thought that the potential for breach of the irrigation canals may considerably worsen downstream flooding during an extreme rainfall event. The very intense, prolonged rainfall event that struck Fort Collins and caused considerable damage and flooding to Spring Creek in July 1997 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. A Dry Creek flood would inundate a significant developed area of northeast Fort Collins and unincorporated Larimer County. One area of significant damages is the area north of Lincoln Avenue and south of East Vine Drive, generally east of the City of Fort Collins. This area is almost entirely within the unincorporated area of the County. It includes several industrial commercial subdivisions as well as the Downtown Fort Collins Airport. Correspondingly, a Dry Creek flood would inundate a sizeable area of the North College Avenue Corridor extending north of East Vine about one mile. This area is mostly within the City of Fort Collins. In addition to facilities to address the hazard posed by flood waters from the upstream basin, there is a need for facilities to provide for stormwater originating within the area bounded by East Vine on the south, College A venue on the west, Timberline Road on the east, and the watershed boundary to the north (approximately Country Club Road). This area, (referred to as the "Local Basin") drains an area of some 5 square miles. At the time Timberline was extended from Colorado State Highway 14 north in 1996, a channel was constructed alongside the new street to provide for the conveyance of floodwaters from Dry Creek and the Local Basin into the Cache La Poudre River. A Dry Creek Major Drainageway Planning Study was prepared for the City of Fort Collins, Larimer County and the Colorado Water Conservation Board by Gingery Associates, Inc. (GAl) in 1980. That study included a hydrological study of the Dry Creek Watershed, a flood hazard study of the area downstream of the Dry Creek crossing of Colorado State Highway 287 north of Fort Collins, and a study of improvement alternatives for eliminating the flood hazard downstream of the intersection of Will ox Lane and Dry Creek.