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<br />3.0 ENGINEERING METHODS <br /> <br />For the flooding sources studied in detail in the community, <br />standard hydrologic and hydraulic study methods were used <br />to determine the flood hazard data required for this study. <br />Flood events of a magnitude which are expected to be equalled <br />or exceeded once on the average during any 10-, 50-, <br />100-, or 500-year period (recurrence interval) have been <br />selected as having special significance for flood plain <br />management and for flood insurance premium rates. These <br />events, commonly termed the 10-, 50-, 100-, and 500-year <br />floods, have a 10, 2, 1, and 0.2 percent chance, respectively, <br />of being equalled or exceeded during any year. Although <br />the recurrence interval represents the long term average <br />period between floods of a specific magnitude, rare floods <br />~ could occur at short intervals or even within the same <br />year. The risk of experiencing a rare flood increases when <br />periods greater than 1 year are considered. For example, <br />the risk of having a flood which equals or exceeds the 100- <br />year flood (1 percent chance of annual occurrence) in any <br />50-year period is approximately 40 percent (4 in 10), and <br />for any 90-year period, the risk increases to approximately <br />60 percent (6 in 10). The analyses reported here reflect <br />flooding potentials based on conditions existing in the <br />community at the time of completion of this study. Maps <br />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 <br />peak discharge-frequency relationships for floods of <br />the selected recurrence intervals for each stream <br />studied in detail in the community. <br /> <br />Recorded flood information for the streams studied with- <br />in the community is non-existent. Good records do exist <br />for the South Platte River; but with the construction <br />of the Chatfield Dam, the recorded information is not <br />applicable. <br /> <br />As a result, synthetically derived hydrographs were <br />computed to determine potential flood magnitudes. These <br />hydrographs reflect the effects of precipitation, ground <br />cover, slope, area, and other physical characteristics <br />of the drainage basins. To assure correctness of the <br />synthetic data, recorded flood data in adjacent areas <br />were also analyzed. <br /> <br />. <br /> <br />10 <br />