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10-, 50-, 100-, or 500-year period (recurrence interval) have been selected as l1aving speciaL significance for floodplain management and for flood insurance rates. These events, commonly termed the 10-, 50-, 100-, and 500-year floods. have a 10, 2, 1. and 0.2 percent cl1ance, respectively, of being equaled or exceeded during any year. Although tile recurrence interval represents the long-term, averaee period between floods of a specific magnitude, rare fLoods couLd occur at short intervals or even witl1in the same year. The risk of experiencing a rare flood increases when periods greater than L year are considered. For example, the risk of having a flood which equals or exceeds the lOO-year flood (1 percent c-hance of annuaL exceedence) in any 50-year period is approximately 40 percent (4 in 10); for any 90-year period, the risk increases to approximately 60 percent (6 in 10). The anaLyses reported herein reflect flooding potentials based on conditions existin?; 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 the peak discharge-frequency relationships for each flooding source studied in detail affecting the community. The hydrologic analysis for Clear Creek was performed by the COE, Omaha District (Reference 23). Their analysis was based on fully developed basin conditions. DiSCharge records for the Clear Creek stream gages, at Golden and Derby were analyzed using methods presented in Bulletin No. 17 published by the U. S. Water Res'ources Council (Reference 35). The results of these analyses were used to calibrate to the following runoff models: the Massachusetts Institute of Technology Catchment Model (MITCAT) (Reference 36), and the U.S. Environmental Protection Agency (EPA) Storm Water Management Model (Reference 37). MITeAT was used to model the 400- square-mile mountainous area upstream from Golden; the Storm Water Management Model was used to model the lower l75-square~miLe plains basin. Rainfall values used in the models were obtained from the Precioitation-Freouencv Atlas of the Western United States. Volume II. Colorado, published by the National Oceanic and Atmospheric Administration in 1973 (Reference 38). The hydrologic analysis for Bear Creek was performed by the COE, Omaha District (Reference 24). The Bear Creek Dam and Reservoir intercepts fLows from 239 square miles of the total 261-square-miLe drainage basin. The remaining square-mile drainage area below Bear Creek Oam still has the potentiaL of generating damaging runoff flows during a cloudburst event over the Lower basin. No applicabLe runoff records are avaiLable through this reach because of the recent tonstruction of the dam. Therefore, discharges Ear the 22-square-mile drainage area beLow the dam were developed by using the EPA Storm Water ~anagement Model (Reference 37) with modifications by the ~issouri River Division of the COE. These discharges were computed assuming full basin development. 16