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im:ITascs when periods grcalcr Ihal! I year arc considered. For example, lhe risk of having a no",! Ihal equals or ex~eeds the IOO'year nood (I-per~enl ~han~e or annual ex~eeden~e) in any 50-year period is approximately 40 percent (4 in 10); for any 90-year period, the risk in~reases to approximately 60 percent (6 in 10). The analyses reported herein reflect flooding potentials based on conditions existing in the community at the time of completion of this study. Maps and flood elevations will be amended periodically to reflect future changes. 3.1 Hydrologic Analyses Hydrologic analyses were carried out to establish peak discharge-frequency relationships for the 10-, 50-, 100-, and 500-year recurrence interval floods for each flooding source studied by detailed methods affecting the community. The peak-flood discharges on the Cache La Poudre River were revised in 1988 by the USACE (Reference I) and utilized in the previous restudy for the Cache La Poudre River downstream of Horsetooth Road. The USACE hydrology was developed using the HEC-I hydrograph computer modeL The appropriateness of this routing model method was verified using historical data for the 1976 and 1983 floods on the Cache La Poudre River. Discharges for Spring Creek were computed using the U.S. Enviromnental Protection Agency's (EPA's) Storm Water Management Model (SWMM) based on a 2-hour storm duration (Reference 20). The level of urban development for each subbasin was determined by field inspection and was modeled for existing basin development for 1988. The updated hydrology utilized 97 catchment areas. The individual flood hydrographs were combined at various design points along each stream segment to determine peak discharges at designated design points. The percent-impervious estimates for each catchment were based on the 1988 level of urbanization. Values recommended by the Urban Drainage and Flood Control District (UDFeD) and the EPA were used for infiltration, overland-flow roughness coefficients, and surface-storage depths for both pervious and impervious areas (References 20 and 21). Rainfall information was taken from the 1973 NOAA Atlas. The Spring Creek channel is a tributary to the Cache La Poudre River immediately east of the City of Fort Collins. The channel is generally deep and well defined, with a relatively wide, gentle overbank area throughout much of its length. The majority of the channel has been maintained as an open drainageway with several culvert crossings at roadways and railroads. Between 1983 and 1993, several major channel-improvement projects were performed by the City of Fort Collins. These projects are identified as Hillpond, Johnson Mobile Home Park, Spring Creek Professional Park, Stuart Street and Stover Street, and Prospect Road and Timberline Road, As a result of these improvements, many structures were being removed from the floodplain. Four irrigation canals cross the Spring Creek basin and channel. The Pleasant Valley and Lake Canal crosses Spring Creek south of Drake Road and west of Taft Hill Road. The New Mercer Canal and Larimer County Canal NO.2 cross Spring Creek in Rolland Moore Park north of Drake Road between Shields Street and Taft Hill Road. Arthur's Ditch crosses Spring Creek between Drake Road and Prospect Road west of the BNRR and College A venue. All of the canals intercept storm- water runoff. With the exception of the Pleasant Valley and Lake Canal, each canal has a structure at its intersection with Spring Creek that allows excess storm water to spill past the canal. Typically, these structures consist of a manually operated gate for control of low flows and an emergency side-overflow weir for larger flows. The City of Fort 8