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<br />period, the risk increases to approximately 60 percent (6 in 10). The <br />analyses reported here reflect flooding potentials based on conditions <br />existing in the community at the time of completion of this study. <br />Maps and flood elevations will be amended periodically to reflect <br />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 each stream studied in detail in the community. <br /> <br />The hydrologic analyses for this study are covered in a technical <br />addendum for the Eagle County Flood Insurance Study (Reference 5). <br />All gage records within Eagle County were analyzed to determine <br />discharge-frequency relationships using the standard log-Pearson <br />Type III distribution as outlined by the u.s. Water Resources <br />Council (Reference 6). This analysis was undertaken for both <br />snowmelt and rainfall events. <br /> <br />Separation of the snowmelt and rainfall events was completed using <br />U.S. Geological Survey data for unregulated streams with more than <br />10 years of record. The data showed that, except for small drainage <br />basins (less than 10 square miles), snowmelt yielded higher discharge <br />peaks than the rainfall for the flow frequencies used in this <br />report. Using snowmelt gage records, regression equations in the <br />following form were developed to predict the 10-, SO-, 100-, and <br />SOD-year frequency discharges as a function of drainage area <br />(Reference 5). <br /> <br />Discharge <br /> <br />(d . )0.7 <br />constant x ralnage area <br /> <br />The discharge-drainage area curves were then modified to include <br />the effect of rainfall on smaller drainage areas using a synthetic <br />hydro graph approach. <br /> <br />Flood discharges for the 10-, 50-, 100-, and SOD-year floods on <br />Gypsum Creek were calculated using the regression equations directly. <br />Approximate study discharges were also taken from these equations. <br /> <br />Flood discharges for the Eagle River were obtained from the Water <br />Resources Division of the U.S. Geological Survey (Reference 7). <br />The data were based on flows recorded at a gaging station at Gypsum <br />from 1947 to 1978. Frequency-discharge curves were calculated using <br />a log-Pearson Type III analysis (Reference 6). <br /> <br />Peak discharge-drainage area relationships for Gypsum Creek and <br />Eagle River are shown in Table 1. <br /> <br />5 <br />