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FLOOD10335 (2)
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Last modified
11/23/2009 12:41:32 PM
Creation date
5/9/2007 12:10:05 PM
Metadata
Fields
Template:
Floodplain Documents
Designation Number
589
County
Adams
Community
Adams County
Stream Name
Clear Creek
Title
FHAD - Clear Creek (Adams County)
Date
12/1/2005
Prepared For
Adams County, CDOT, UDFCD
Prepared By
Ayres Associates
Floodplain - Doc Type
Floodplain Report/Masterplan
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<br />I <br /> <br />I <br /> <br />Hydraulic analyses were completed to determine the water-surface elevations for the 10- and 100-year <br />storm events. The elevations were computed using the Corps of Engineers' HEC-RAS computer program <br /> <br />3 <br /> <br />AWES <br />~2.35;:JC:A7ES <br /> <br />The 1 O-year flood magnitude has also been computed at various design points along Clear Creek. This <br />flood magnitude represents an event that can cause damage but would not inundate the area characteristic <br /> <br />I <br /> <br />Hydraulic AnalysIs <br /> <br />The District, the CWCB, and the Federal Emergency Management Agency (FEMA) designate the <br />flood event as the flood magnitude for regulatory floodplains and flood improvement purposes. In <br />Colorado, the 1 OO-year floodplain represents an area of State interest as defined in House Bill 1 041 <br /> <br />OO-year <br /> <br />'I <br /> <br />Discharge-probability profiles for the 10- <br />37. <br /> <br />The flood discharges used in this report were computed and supplied by the Omaha District of the Army <br />Corps of Engineers (USACE 1978B). Peak discharge-frequency relationships were established for floods <br />of 10-, 50-, 100-, and 500-year recurrence intervals and are based on future development conditions <br />projected for the basin. <br /> <br />50- <br /> <br />00- <br /> <br />and 500-year frequency storms are provided in Figure <br /> <br />1 <br /> <br />The 1 OO-year flood event, which can be expected to occur at any time In a given area, based upon <br />recorded historical precipitation and other valid data, has a 1 % chance of being equaled or exceeded <br />during anyone year. <br /> <br />'I <br /> <br />!I <br /> <br />Hydrologic AnalYSIS <br /> <br />Floods of the same and greater magnitude as those that have occurred in the past will occur in the future. <br />With development in the basin, flooding will occur more frequently. The discharges reported in Table 1 for <br />the 1 OO-year flood frequency represent the relative extent and impact of this flood event. The discharge <br />information is usable not only for floodplain regulation but also for planning, engineering, and floodplain <br />management. <br /> <br />\1 <br /> <br />HYDROLOGIC AND HYDRAULIC DETERMINATIONS <br /> <br />'I <br /> <br />There have been three previous floodplain studies of Clear Creek. In 1966, the U.S. Army Corps of <br />Engineers (USACE) publiShed a Flood Plain Information Report for Clear Creek (USACE 1978A). In 1979 <br />Gingery Associates, Inc., produced the Clear Creek Flood Hazard Area Delineation Study for the District, <br />which was included in the most recent Flood Insurance Study for Adams County (FEMA 1995). The flood <br />profiles presented in this study differ from those previously published due to revised flood discharges <br />resulting from split flows and the availability of more recent topographic mapping <br /> <br />Flood Frequency and Discharge <br /> <br />A detailed <br /> <br />FUTURE FLOODS <br /> <br />technical appendix supporting <br /> <br />the hydraUlic calculations is available at the District <br /> <br />offices <br /> <br />(I <br /> <br />\1 <br /> <br />FLOOD RELATED STUDIES <br /> <br />The 1 OO-year flood elevations computed as part of this report are different from those published in the 1979 <br />FHAD due to the revised discharges (based on the split flows occurring from Sheridan Boulevard <br />downstream to Pecos Street) and more recent topographic mapping as well as changed topographic <br />features <br /> <br />\1 <br /> <br />II <br /> <br />Flood of July 23-26, 1965: On July 23 and 24, during severe storms over the headwaters of Clear Creek <br />and Tucker Gulch, 4.5 inches of rain was reported to have fallen in Tucker Gulch in an hour, which caused <br />flash flooding in Golden. Flooding, however, extended only a short distance downstream. In Golden, flood <br />waters from Tucker Gulch spread over about 17 blocks and caused an estimated $112,000 damage to 69 <br />residences, three commercial enterprises, three railroad bridges, four street bridges, and utility lines. At <br />Georgetown, debris blocked the channel and diverted the waters down a street, thereby causing extensive <br />washing of the surface and the flooding of several basements. <br /> <br />The computed flood profiles for the 1 OO-year and 1 O-year flood flows are plotted on the Flood Hazard Area <br />Delineation profile sheets included in the Drawings - Profiles section of this report. Also, the computed <br />flood elevations for Clear Creek 1 OO-year and 1 O-year flood flows are tabulated at each reference point in <br />Table 1. <br /> <br />II <br /> <br />Flood of June 1956: Unusually heavy snowmelt runoff resulted in the failure of the Georgetown Dam <br />located about 1 mile downstream from Georgetown. The peak discharge passing the gage above Golden <br />was 5,250 cfs. By the time the crest reached the gaging station near the mouth of Clear Creek, it was <br />reduced to 2,880 cfs. <br /> <br />Manning's "n' values were also adjusted to reflect non-typical conditions such as overflow areas with <br />multiple buildings and in some cases, floodplain areas downstream of roads to maintain flow continuity with <br />maximum "n' values of 0.175. <br /> <br />11 <br /> <br />:1 <br /> <br />Flood of August 1888: This flood resulted from cloudbursts on the eastern slope of the Front Range of the <br />Rocky Mountains. A discharge of 8,700 cubic feet per second (cfs) was reported at the mouth of Clear <br />Creek canyon. This is the largest measured discharge in the history of this gaging station, which is located <br />1.5 miles upstream from Golden. <br /> <br />. <br />. <br /> <br />Estimates of channel and overbank roughness factors were made after a field investigation of the study <br />area. Typical roughness or Manning's 'n' values used in this study are <br /> <br />Channel - 0.030 to 0.050 <br />Overbank - 0.040 to 0.090 <br /> <br />II <br /> <br />1 <br /> <br />Historically, flooding in the Clear Creek basin has been relatively infrequent. Since 1864, twelve floods <br />have been reported on Clear Creek and its tributaries. No lives have been reported lost due to flood <br />related causes in the Clear Creek basin. The following descriptions of the floods of August 1888, June <br />1956, and July 1965 are typical of the information currently available (Gingery 1979). <br /> <br />(USACE 2001 A). Valley cross sections for the length of Clear Creek in Adams County were derived from <br />the digital elevation model (OEM) and filtered to reduce the number of points in the cross section. The <br />resulting cross sections were manually checked against the 1 "=1 00' scale, 2-foot contour interval <br />topographic mapping that is included with this report. Cross sections and profiles were based on <br />topographic mapping provided by Landmark Mapping, L TO, 1995, updated in 1998. The Mapping was <br />supplemented by field survey of the drop structures under 1-25 and of the newly filled areas east of Pecos <br />Street. <br /> <br />I <br /> <br />I <br /> <br />FLOOD HISTORY <br />
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