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Last modified
1/26/2010 10:08:28 AM
Creation date
10/5/2006 4:09:15 AM
Metadata
Fields
Template:
Floodplain Documents
Designation Number
530
County
Adams
Community
Unincorporated Adams County
Basin
South Platte
Title
FIS - Adams County and Incorporated Areas Volume I of III
Date
8/1/1995
Designation Date
9/1/2000
Floodplain - Doc Type
Floodplain Report/Masterplan
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<br />3.0 ENGINEERING METHODS <br /> <br />For the flooding sources studied by detailed methods in the community, <br />standard hydrologic and hydraulic study methods were used to determine <br />the flood hazard data required for this study. Flood events of a <br />magni tude which are expected to be equaled or exceeded once on the <br />average during any lO-, 50-, lOO-, or 500-year period (recurrence <br />interval) have been selected as having special significance for <br />floodplain management and for flood insurance rates. These events, <br />commonly termed the lO-, 50-, lOO-, and 500-year floods, have a lO, 2, 1 <br />and 0.2 percent chance, respectively, of being equaled or exceeded <br />during any year. Although the recurrence interval represents the long- <br />term, average period between floods of a specific magnitude, rare floods <br />could occur at short intervals or even within the same year. The risk of <br />experiencing a rare flood increases when periods greater than l year are <br />considered. For example, the risk of having a flood which equals or <br />exceeds the 100-year flood (l percent chance of annual exceedence) in <br />any SO-year period is approximately 40 percent (4 in LOll for any <br />90-year period, the risk increases to approximately 60 percent (6 in <br />lO). The analyses reported herein reflect flooding potentials based on <br />conditions existing in the community at the time of completion of this <br />study. Maps and flood elevations will be amended periodically to <br />reflect future changes. <br /> <br />3.1 Hydrologic Analyses <br /> <br />Hydrologic analyses were carried out to establish peak discharge- <br />frequency relationships for each flooding source studied by <br />detailed methods affecting the community. <br /> <br />Bilt Drv Creek (Below Interstate 25). Box <br />Creek. Little Comanche Creek. Little Drv <br />Northfield Creek. and the South Platte River <br /> <br />Elder Creek. Comanche <br />Creek. Niver Creek. <br /> <br />Data used ln the hydrologic analyses include <br />developed storm runoff records, and information <br />study of the South Platte River (Reference 21). <br /> <br />synthetically <br />f rom the USACE <br /> <br />The discharge profiles used in this study were provided by the <br />USACE, Omaha District, on June 17, 1975, and July 18, 1975 <br />(Reference 21). These discharge profiles were keyed to the lOO- <br />year flood values computed from the Denver and Henderson gaging <br />records, which were adjusted for the regulating effects of <br />Chatfield, Mt. Carbon, and Cherry Creek Reservoirs. The Denver <br />gage operated from May to October 1889, from June to October 1890, <br />and from July la95 to the present. The Henderson gage has been in <br />operation since 1926. Values above and below these gaging stations <br />were based on the USACE analysis using their hydrologic computer <br />model for the South Platte River (Reference 21). This study was <br />revised to use the final versions of the discharge profiles, <br />covering the segment downstream of the Henderson Gaging Station <br />(Reference 22). <br /> <br />24 <br />
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