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FLOOD10571
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FLOOD10571
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Last modified
11/23/2009 2:01:50 PM
Creation date
6/11/2007 11:16:21 AM
Metadata
Fields
Template:
Floodplain Documents
County
Gilpin
Community
Black Hawk
Title
FIS - Black Hawk
Date
4/16/1984
Prepared For
Black Hawk
Prepared By
FEMA
Floodplain - Doc Type
Current FEMA Regulatory Floodplain Information
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<br />The water-surface elevations were computed through use of the <br />U.S. Army Corps of Engineers HEC-2 standard step-backwater computer <br />program (Reference 11) and supplemented by hand calculations where <br />required. <br /> <br />Cross sections for the backwater analyses of North Clear Creek, <br />Gregory Gulch, and Chase Gulch were field surveyed at close intervals <br />above and below bridges and culverts in order to compute the signif- <br />icant backwater effects of these structures. <br /> <br />Topographic data for channel cross sections were obtained from <br />topographic maps supplemented with field survey data (Reference 12). <br /> <br />Locations of selected cross sections used in the hydraulic analyses <br />are shown on the Flood Profiles (Exhibit 1) and on the Flood Boundary <br />and Floodway Map (Exhibit 2). <br /> <br />Channel roughness factors (Manning's "nil) for these computations <br />were assigned on the basis of field inspection of the flood plain <br />areas. The channel "nil values ranged from 0.022 to 0.058, and <br />flood plain values ranged from 0.025 to 0.080. <br /> <br />Starting water-surface elevations for each stream were determined <br />using critical depth. <br /> <br />A unique situation exists on Gregory and Chase Gulches where the <br />natural stream channels are largely obliterated and the water <br />in a portion of each study reach flows in a subsurface conduit. <br />When the culvert at each location reaches its capacity, the roadway <br />becomes the flood channel. <br /> <br />On Chase Gulch, the circular 36-inch pipe was judged unreliable <br />for carrying any floodwater; therefore, computations were conducted <br />in the normal manner. <br /> <br />On Gregory Gulch, the culvert is as large as 8-foot square and <br />judged to flow at one-half its capacity with inlet submerged to <br />street level. Flood computations were then made by subtracting <br />the culvert flow, placing the remaining flow in the street, and <br />continuing in normal computation fashion. <br /> <br />There are two locations on North Clear Creek where lateral flow <br />diversion occurs. Divided flow occurs between cross sections <br />Wand Z and between cross sections AV and BA. Separate water- <br />surface profiles were not computed. <br /> <br />The hydraulic analyses for streams studied by approximate methods <br />were. taken from a report prepared by the Colorado Water Conservation <br />Board (Reference 2). <br /> <br />13 <br />
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