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3.0 ENGINEERING METHODS For floOding sources studied in detail in the community, standard hydrologic and hydraulic study methods were used to determine the flood hazard data required for this stul1y, n:Jod events of the magnitude which are expected to be equalled or exceeded once on the average during any 10-, 50-, 100-, and SOO-year floods, have-a-IO, 2, I, and 0,2 percent chance, respectively, of being equalled or exceeded during any year, Although the recurrence interval represents the long 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 IOO-year flood (1 percent chance of annual occurrence) in any 50-year period is approximately 40 percent (4 in 10), and, for any 90-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 county 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 floods of the selected recurrence intervals for each stream studied in detail affecting the comnunity. Peak di scharges for the various recurrence interval s for TOlnichi Creek and the Gunnison River were taken from data prepared by ECI for the Gunnison River Basin at Gunnison, Colorado (Reference 1), Streamflow data from 51 measuring stations located within the Gunnison River Basin and in nearby basins were used in this analysis, The source of this data is the official U,S. Geological Survey Water Supply Papers (Reference 3), The peak flow figures for the four different frequenCies used in the hydrauliC analysis of this study were those obtained from a Regional Log-Pearson Type III Analysis (Reference 4). A list of discharges is shown in Table 1 for various frequency intervals, Table 1 - Summary of Discharges Dra i nage Area (sq, mi.) 10-year Peak Discharge (cfs) 50-year IOU-year 500-year Flood Source and Location Gunnison River Confl uence with Ante lope Creek Tomichi Creek At Gold Basin Road 1,570 5,762 7,967 8,930 11 .256 1,020 1,501 1,904 2,050 2,340 6