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FLOOD04702
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Last modified
1/25/2010 6:47:00 PM
Creation date
10/5/2006 12:51:13 AM
Metadata
Fields
Template:
Floodplain Documents
Designation Number
347
County
Jefferson
Community
Unincorporated Jefferson County
Stream Name
Upper Lena Gulch
Basin
South Platte
Title
Flood Hazard Area Delineation - Upper Lena Gulch
Date
1/1/1993
Designation Date
9/1/1993
Floodplain - Doc Type
Floodplain Report/Masterplan
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<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 />Flow through structures is often partially blocked during high flood flows due to debris <br /> <br /> <br />accumulation. The obstruction at structures was estimated using a reduction of the cross- <br /> <br /> <br />sectional area of the structure opening. For structures less than 3 feet in diameter, the structure <br /> <br /> <br />was assumed fully blocked and was not modeled. For structures whose minimum dimension is <br /> <br /> <br />between 3 and to feet, a 20 percent reduction in opening area was made. Larger structures <br /> <br /> <br />were assumed to be capable of conveying all debris and were not modified in the model. <br /> <br />section was identified where flows were combined. A profile was then computed along each <br /> <br /> <br />flow path for a range of discharges. The energy at an upstream section where flow was once <br /> <br /> <br />again combined was computed. When the sum of the two discharges having the same <br /> <br /> <br />computed energy head at the upstream section equalled the total discharge, the proper flow <br /> <br /> <br />distribution had been identified. This analysis served to identify the discharge down each flow <br /> <br /> <br />path and the water surface profile along the path. <br /> <br />Major obstructions in the floodplain, such as buildings, were incorporated into the model cross- <br /> <br /> <br />section data. In developed areas where residences were within the expected tOO-year <br /> <br /> <br />floodplain, the obstruction resulting from entire blocks of homes was incorporated in the cross- <br /> <br /> <br />section. This included sections where no building was physically present but where effects <br /> <br /> <br />from buildings at other sections would have an impact. <br /> <br />Overflows left the main channel at several locations and followed an independent flow path <br /> <br /> <br />unrelated to the main channel profile. In these cases, the HEC-2 split flow option was used in <br /> <br /> <br />which an overflow section was defined and overflow rates were computed. The downstream <br /> <br /> <br />discharge in the main channel was reduced and the overflow discharge was diverted into the <br /> <br /> <br />alternate channel. Significant overflows were evaluated and a water surface profile established <br /> <br /> <br />for the diverted flow using a separate HEC-2 analysis. In cases where overflows were minor, no <br /> <br /> <br />specific profile was computed and downstream main channel discharges were not decreased. <br /> <br />In some cases, flow was divided but discharges were related to downstream channel <br /> <br /> <br />conditions rather than the upstream control. At those locations, flow splits were the result of <br /> <br /> <br />downstream conditions which affected the discharge along each flow path. The analysis for <br /> <br /> <br />this condition required the balancing of energy losses along both flow paths. A downstream <br /> <br />12 <br />
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