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measures were considered in the hydrologic or hydraulic analyses conducted herein; however, if these measures were achieved, they would greatly reduce, and in most cases eliminate, flood hazards from storms of frequencies as low as the SOD-year recurrence interval. The City of Florence adopted a flood plain ordinance in December 1978. This ordiance accepts the Federal Insurance Administration Flood Hazard Boundary Map of February 11, 1977 and prohibits further development within these areas designated as flood plain. 3.0 ENGINEERING METHODS For flooding sources studied in detail in the conununity, standard hydrologic and hydraulic study methods were used to determine the flood hazard data required for this study. Flood events of the magnitude which are expected to be equaled or exceeded once on the average during any 10-, 50-, 100-, or 500-year period 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 long-term averaqe 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 that equals or exceeds the lOa-year flood (1 percent chance of annual occurrence) in any 50-year period is approximately 40 percent (4 in 10), for any gO-year period, the risk increases to approximately 60 percent (6 in 10). The analyses reported here 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 floods of the selected recurrence interval for each stream studied in detail in the corrununity. Peak discharges of various recurrence intervals for Coal Creek and Oak Creek were determined by CDM. To predict the peak 2-, 25-, and 100-year flow rates for these creeks a synthetic unit hydro graph was developed, using the basin characteristics presented in the Arkansas River Basin, Florence, Colorado, Desiqn Memorandum No.1, prepared by the U. S. Army Corps of Engineers (Reference 3). A SCS Type II-A storm distribution was applied to each basin developed from rainfall data contained in Reference 4, to calculate the peak discharges. The peak discharges for 2-, 25-, and lOa-year events were then plotted on logarithmic probability graph paper and peak discharges for the 10-, 50-, and SOQ-year frequencies were estimated. - 5 -