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FLOOD00734
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Last modified
11/23/2009 1:21:32 PM
Creation date
10/4/2006 9:26:44 PM
Metadata
Fields
Template:
Floodplain Documents
Designation Number
213
County
Adams
Community
Thornton
Stream Name
Brantner Gulch and Tributaries
Title
Flood Hazard Area Delineation - Brantner Gulch and Tributaries
Date
1/1/1983
Designation Date
5/1/1983
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 />23 <br /> <br />VI. HYDRAULIC ANALYSIS <br /> <br />At the upstream end of Regional Park Tributaries VI and <br /> <br /> <br />VII, stock ponds provide minimal detention ponding. The delay <br /> <br /> <br />of peak flow does not appear to significantly affect the resultant <br /> <br /> <br />flow at the confluence with Brantner Gulch, however. <br /> <br /> <br />The most significant influence on flows in any reach of <br /> <br /> <br />Brantner Gulch are the dams and reservoirs in Reach A west of <br /> <br /> <br />Colorado Boulevard. Eastlake Dam No. 2 will be retained and the <br /> <br /> <br />reservoir area will normally remain dry to allow for a greenbelt <br /> <br /> <br />park area, possibly with recreational facilities. Eastlake Dam <br /> <br /> <br />No. 3 will be retained as a combined dam and recreation reservoir <br /> <br /> <br />with a permanent minimum pool elevation. <br /> <br /> <br />In March of 1982, the Thornton City Council made a decision <br /> <br /> <br />to retain Eastlake Dam No. 2 as a flood detention dam. Although <br /> <br /> <br />there is still a need for improvements to the dam, Brantner Gulch <br /> <br /> <br />flood plain limits are shown assuming that the dam will remain <br /> <br /> <br />in place and be effective in reducing flood peaks in the area. <br /> <br />After development of the hydrology described in the pre- <br /> <br /> <br />ceding section, hydraulic analyses were made to determine the <br /> <br /> <br />water surface elevations which would occur in the two-year, <br /> <br /> <br />five-year, ten-year and 100-year flood. The principal tool used <br /> <br /> <br />in the analyses was a computer program developed by the U.S. <br /> <br /> <br />Army corps of Engineers entitled "HEC-2, Water Surface Profiles" <br /> <br /> <br />(Ref. 2). Landmark Mapping Company, in addition to preparing <br /> <br /> <br />the topographic maps, provided digitized cross sections of the <br /> <br /> <br />channels. These were supplemented by sections taken directly <br /> <br /> <br />from the maps. The maps and sections were used first in the <br /> <br /> <br />computation of the water surface profiles, and second in the <br /> <br /> <br />description of the flooded outlines for the lOO-year flood. <br /> <br /> <br /> <br />Estimates of the channel roughness coefficients were made <br /> <br /> <br />based upon field observations. Values of Manning's "n" used in <br /> <br /> <br />this report were as low as 0.02 for streets and parking lots, <br /> <br /> <br />and as high as 0.07 for portions of the channel encroached upon by <br /> <br /> <br />structures which inhibited the flow pattern. In general, areas <br /> <br /> <br />of natural channel section with natural over-banks were assigned <br /> <br /> <br />"n" values of approximately 0.030 and 0.035, respectively. A <br /> <br /> <br />detailed, technical appendix of hydraulic calculations is <br /> <br /> <br />available at the offices of the Urban Drainage and Flood Control <br /> <br /> <br />District. <br />
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