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FLOOD07708
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Last modified
1/25/2010 7:12:34 PM
Creation date
10/5/2006 3:09:51 AM
Metadata
Fields
Template:
Floodplain Documents
Designation Number
317
County
Boulder
Community
Unincorporated Boulder County
Stream Name
Bullhead Gulch and Tributaries
Basin
South Platte
Title
Flood Hazard Area Delineation - Bullhead Gulch and Tributaries
Date
7/1/1991
Designation Date
11/1/1991
Floodplain - Doc Type
Floodplain Report/Masterplan
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<br />..;;--- <br /> <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br /> <br />The locations of selected cross-sections used in this repon are shown on the plan and profile <br /> <br />drawings located in the Appendix. The velocities associated with these cross sections are mean <br /> <br />channel velocities. All cross sections were taken left to right facing downstream with a staning <br /> <br />left station of 1000. In the event that a staning station shown on the map is different than 1000, <br /> <br />the station has been so noted, <br /> <br />The 10- and l00-year staning water surface elevations on Bullhead Gulch were estimated using <br />the slope area method, On the tributaries to Bullhead Gulch affected by the backwater from <br />Bullhead Gulch, the staning water surface elevations were based on the backwater flood elevation <br />at the time to peak on the tributary. <br /> <br />The two floodways developed for this study are based on equal conveyance reduction for each <br />side of the floodplain for a maximum allowable energy gradeline increase of one-half foot and <br />one-foot. The one-halffoot rise standard reflects the floodplain regulation of the Urban Drainage <br />and Flood Control District. The one-foot rise floodway reflects the minimum National Flood <br />Insurance Program (NFIP) criteria for defining floodways and is consistent with City of Lafayette <br />and City of Louisville regulations. UDFCD does not object to local government decisions to <br />adopt the less restrictive one-foot rise floodway definition, The floodway is that ponion of the <br />lOO-year floodplain which is required to pass the lOO-year flood event without raising the flood <br />profIle more than an acceptable amount and also represents the ponion of the floodplain that is <br />most hazardous to personal safety and welfare. <br /> <br />The lOO-year floodplains developed for this study have been delineated using the flood elevations <br />determined at the cross section locations, Between cross sections, the boundaries have been <br />interpolated using the flood contour method outlined in the CWCB' s "Floodplain Designation and <br />Approval" manual (CWCB, 1988), and the topographic mapping. <br /> <br />The lOO-year floodplain and floodway data developed for this repon are shown on the Flood <br />Plain and Floodway Reference Data Table 8. The floodway boundaries must be interpreted by <br />the user based on the data presented in these tables. <br /> <br />4,3 FLOOD DAMAGES <br /> <br />The floodplain downstream of the Leyner-Cottonwood Ditch to the point that Bullhead Gulch <br />crosses Highway 287 was defmed as a "Shallow Flooding" zone and no water surface profiles <br />or floodway definition have been presented in this repon. The thalweg is undefined in this reach <br />and the berm along the Leyner-Cottonwood Ditch cannot be relied upon to withstand the forces <br />of a lOO-year flood. The uncenainty regarding the location of a breach in this berm has <br />necessitated the use of a "Shallow Flooding" designation. <br /> <br />Major floods along the existing Bullhead Gulch floodplain could cause some damage in the form <br />of bank erosion, roadway washouts, and limited structure damage. Several residential and <br />commercial buildings are located within the l00-year floodplain, The irrigation ditches can <br />capture the storm runoff from minor storm events and a part of the l00-year storm runoff. <br />Damage and breaching of the irrigation ditches is very likely during a major flood. <br /> <br />The width of the lOO-year floodplain along Bullhead Gulch varies from 67 ft to 1845 ft. The <br />average floodplain width is about 1000 ft downstream of the Leyner-Cottonwood Ditch and about <br />200 ft upstream of the Leyner-Cottonwood Ditch. Channel velocities range from 1.3 ft per <br />second (fps) to over 14 fps, and average 5 to 7 fps, The Bullhead Gulch longitudinal channel <br />slope varies from 1.6 percent to 1.9 percent in the upper reaches and 0.3 percent to 1.2 percent <br />in the lower reaches. <br /> <br />FLOOD HAZARD AREA DELINEATION <br />BULUlEAD GULCH AND TIUBUTARIES <br /> <br />FINAL <br />7131191 <br /> <br />12 <br />
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