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The worst floods in the Greeley area occurred in 1876, 1884, 1904, 1923, 1947, and 1965. All of these events, except the 1947 flood, were caused by intense rainfali and/or snowmelt in the upper Poudre basin. The storm producing the flooding in 1947 was centered over the Windsor area (Reference 6). 2.4 Flood Protection Measures No structural flood-protection measures exist or planned for the reach of the Cache La Poudre River Town of Windsor. are currently affecting the 3.0 ENGINEERING METHODS For the flooding sources studied by detailed methods 1n the community, standard hydrologic and hydraulic study methods were used to determine the flood hazard data required for this study. Flood events of a magnitude which are expected to be equaled or exceeded once on the average during any 10-, 50-, 100-, or 500-year period (recurrence interval) have been selected as having 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 chance, respectively, of being equaled or exceeded during any year. Although the recurrence interval represents the 10ng- term, average period between floods of a specific magnitude, rare floods could occur at short intervals or even within the same year. The risk of experiencing a rare flood increases when periods greater than 1 year are considered. For example, the risk of having a flood which equals or exceeds the 100-year flood (1 percent chance of annual exceedence) 1n any 50-year period is approximately 40 percent (4 1n 10); for any 90-year period, the risk increases '0 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 each flooding source studied by detailed methods affecting the community. The hydrology used in this study were developed by the COE, Omaha District (Reference 1). The COE hydrology estimates were prepared using HEC-l, a generalized rainfall-runoff computer model developed by the COE's Hydrologic Engineering Center (Reference 4). This routing method was verified using historical data for the 1976 and 1983 floods on the Cache La Poudre River and is considered valid to use for the routing of the hypothetical 100-year flood (Reference 1). The results of the COE hydrologic analysis were reviewed and accepted by the community, FEMA, and the CWCB. 4