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<br />between floods of a specific magnitude, rare floods could occur at short <br />intervals or even within the same year. The risk of experiencing a rare <br />flood increases when periods greater than 1 year arG considc~rcd. For <br />example, the risk of having a flood which equals or exceeds Ule 100-year <br />flood (1 percent chance of annual occurrence) in any 50-yoar period is <br />approximately 40 percent (4 in 10), and, for any 90-year period, the <br />risk increases to approximately 60 percent (6 in 10). The dnalyses <br />reported here reflect floodinq potentials based on conditions existing <br />in the community at the time of completion of this study. Maps and <br />flood elevations will be amended periodically to reflect future changes. <br /> <br />3.1 Hydrologic Analyses <br /> <br />Hydrologic analyses were carried out to establish the peak discharge- <br />frequency relationships for floods of the selected recurrence <br />intervals for the stream studied in detail in the community. <br /> <br />The hydrologic analysis for this study is covered in a technical <br />addendum entitled, Hydrology Report, Flood Insurance Study, Eaqle <br />County, Colorado (Unincorporated Areas), submitted to th0- Federal <br />Insurance Administration and the Colorado Water Conservation Board <br />(Reference S). All of the gage records within Eagle County were <br />analyzed using the standard log-Pearson Type III distribution as <br />outlined iu u.s. Water Resources Council BulleLin 17 (Heference 6). <br />The analysis was completed for both snowmelt events and rainfall <br />events. Separation of the snowmelt and rainfall events was performed <br />using u.s. Geological Survey gage data. <br /> <br />The data showed that, except for small basins (less than 10 square <br />miles), the snowmelt events yielded higher discharge peaks than the <br />rainfall events for the flow frequencies used in this study. Using <br />u.s. Geological Survey snowmelt event gage records, which cover <br />approximately 12 years of record, regression equations were developed <br />to predict 10-, 50-, 100-, and SOO-year frequency discharges as a <br />function of drainage area. The equations were of the form: <br /> <br />. I (') o. 7 <br />Dlsclarge = constant x Dralnage Area <br /> <br />The discharge-drainage area curves were then modified to include the <br />effect of rainfall events on smaller drainage areas using a synthetic <br />hydrograph approach. <br /> <br />The 10-, SO-, 100-, and SOO-year frequency discharges were obtained <br />for the gage location on the Eagle River at.:. Red Cliff (drainage area, <br />72 square miles), and the discharge-drainage area relationship cited <br />earlier was used to predict discharge for the Eagle River at Minturn <br />(drainage area, 260 square miles) . <br /> <br />Peak discharge-drainage area relationships for the Eagle River are <br />shown in Table 1. <br /> <br />5 <br />