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<br />Major drainageway planning reports have been completed for Little <br />Dry Creek, Sand Creek, and Big Dry Creek (References 30, 34, and <br />35). These reports designate various structural measures and <br />nonstructural actions that would be appropriate to alleviate <br />potential flood damage along these streams. <br /> <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 equaled or exceeded once on the average during any <br />10-, 50-, 100-, or SOO-year period (recurrence interval) have been <br />selected as having special significance for floodplain management and <br />for flood insurance rates. These events, commonly termed the 10-, 50-, <br />100-, and SOO-year floods, have ala, 2, 1, and 0.2 percent chance, <br />respect i vely, of being equaled or exceeded during any year. Al though <br />the recurrence interval represents the long-term average period between <br />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 are considered. For <br />example, the risk of having a flood which equals or exceeds the lOa-year <br />flood (l percent chance of annual exceedence) in any SO-year 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 analyses <br />reported herein reflect flooding 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 <br />discharge-frequency relationships for each flooding source studied <br />in detail affecting the geographic area of Arapahoe County. <br /> <br />~ <br /> <br />Recorded flood information for the majority of the streams studied <br />by detailed methods within Arapahoe County is nonexistent. Good <br />records do exist for the South Platte River and Cherry Creek. Due <br />to the construction of Chatfield Dam, the recorded information on <br />the South Platte River is not applicable. As a result, <br />synthetically derived hydrographs were computed to determine <br />potential flood magnitudes for those streams with relatively small <br />drainage basins in the Denver metropolitan area. These hydrographs <br />reflect the effects of precipitation, ground cover, slope, drainage <br />area, and other physical characteristics of the drainage basins. <br />The synthetic hydrograph method was used on Big Dry Creek, Piney <br />Creek, Cottonwood Creek, Murphy Creek, Cherry Creek, and South <br />Platte River. Where available, hydrologic data were compared with <br />other studies completed in the area (References 30, 34, and 35). <br /> <br />For the large drainage basins to <br />metropolitan area, flood magnitudes <br /> <br />the <br />for the <br /> <br />east of <br />selected <br /> <br />the Denver <br />frequencies <br /> <br />11 <br />