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<br />3.0 ENGINEERING METHODS <br /> <br />For the flooding Sources studied in detail in the community, standard <br />hydrologic and hydraulic study methods were used to determine the flood <br />hazard data required for this study. Flood events of a magnitude which <br />are expected to be equalled or exceeded ~ on the average during any <br />10-, 50-, lOO-, or SOQ-year period (recurrence interval) have been selected <br />as having special significance for flood plain management and for flood <br />insurance premium rates. These events, commonly termed the 10-, 50-, <br />100-, and 500-year floods, have a lO, 2, 1, and 0.2 percent chance, <br />respectively, of being equalled or exceeded during any year. Although <br />the recurrence interval represents the long term average period between <br />floods of a specific magnitude, rare floods could occur at short intervals <br />or even within the same year. The risk of experiencing a rare flood <br />increases when periods greater than 1 year are considered. For example, <br />the risk of having a flood which equalS or exceeds the lOO-year flood <br />(1 percent chance of annual occurrence) in any 50-year period is approxi- <br />mately 40 percent (4 in 10), and, for any 90-year period, the risk in- <br />creases to approximately 60 percent (6 in lO). The analyses reported <br />here reflect flooding potentials based on conditions existing in the <br />community at the time of completion of this study. Maps and flood eleva- <br />tions will be amended periodically to reflect future changes. <br /> <br />3.l 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 flooding source studied in detail affecting <br />the community. <br /> <br />The hydrologic analysis for this study is covered in detail in <br />a Flood Insurance Study for Eagle County, Colorado (Reference 5). <br />The analysis was completed for both snowmelt events and rainfall <br />events. <br /> <br />Separation of the snowmelt and rainfall events was completed using <br />U.S. Geological Survey data. The data showed that except for <br />small basins (less than approximately 10 square miles), the snowmelt <br />events yielded higher discharge peaks than the rainfall events <br />for the flow frequencies used in this report. Using snowmelt <br />event gage records, regression equations were developed to predict <br />10-, 50-, lOO-, and 500-year frequency discharges as a function <br />of drainage area. The equations were of the form: <br /> <br />. h . 0.7 <br />D1SC arge = constant x (dralnage area) <br /> <br />The discharge-drainage area curves were then modified to include <br />the effect of rainfall events on smaller drainage areas using a <br />synthetic hydrograph approach. <br /> <br />7 <br />