FLOOD11746 - Laserfiche WebLink

Home Browse Search

Major drainageway planning reports have been completed for Little Dry Creek, Sand Creek, and Big Dry Creek (References 30, 34, and 35). These reports designate various structural measures and nonstructural actions that would be appropriate to alleviate potential flood damage along these streams. 3.0 ENGINEERING METHODS For the flooding sources studied in detail in the community,standard hydrologic arid hydraulic study methods were used to determine the flood hazard data required for this study. Flood events of a magnitude which are expected to be equaled or exceeded once on the average during any 10-, 50-, 100-, or 500-year period (recurrence interval) have been c selected as having special significance for floodplain management and for flood insurance rates. These events; commonly termed the 10-, 50-, 100-, and 500-year floods, have a 10, 2, 1, and 0.2 percent chance, respectively, of being equaled or exceeded during any year. Although the recurrence interval represents the long-term average period between floods of a specific magnitude, rare floods could occur at short intervals or even within the same year. The risk of experiEmcing a rare flood increases when periods greater than 1 year are considered. For example, the risk of having a flood which equals or excE!eds the 100-year flood (l percent chance of annual exceedence) in any 50-year period is approximately 40 percent (4 in 10), and, for any 90-year period, the risk increases to approximately 60 percent (6 in 10). The analyses reported herein reflect flooding potentials bBlsed on conditions existing in the community at the time of completion of this study. Maps and flood elevations will be amended periodically to reflect future changes. 3.1 Hydrologic Analyses Hydrologic analyses were carried out to establish the peak discharge-frequency relationships for each flooding source studied in detail affecting the geographic area of Arapahoe County. . Recorded flood information for the majority of the streams studied by detailed methods within Arapahoe County is nonexistent. Good records do exist for the South Platte River and Cherry Creek. Due to the construction of Chatfield Dam, the recorded inform.:ltion on the South Platte River is not applicable. As a result, synthetically derived hydrographs were computed to dt~termine potential flood magnitudes for those streams with relatively small drainage basins in the Denver metropolitan area. These hydrographs reflect the effects of precipitation, ground cover, slope, drainage area, and other physical characteristics of the drainage basins. The synthetic hydrograph method was used on Big Dry Creek, Piney Creek, Cottonwood Creek, Murphy Creek, Cherry Creek, and South Platte River. Where available, hydrologic data were compared with other studies completed in the area (References 30, 34, and 35). For the large drainage basins to the east of metropolitan area, flood magnitudes for the selected the Denver frequencies 11