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FLOOD09087
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Last modified
1/26/2010 10:07:57 AM
Creation date
10/5/2006 4:04:42 AM
Metadata
Fields
Template:
Floodplain Documents
Designation Number
371
County
Boulder
Community
Lafayette
Basin
South Platte
Title
Flood Insurance Study - Lafayette, CO, Boulder County
Date
9/1/1979
Designation Date
9/1/1994
Floodplain - Doc Type
Floodplain Report/Masterplan
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<br />This report is based on the data generated for the October 1976 <br />study of Coal Creek and Rock Creek, performed by the u.s. Soil Con- <br />servation Service (Reference 2). Since there are no stream flow <br />records for Coal Creek and Rock Creek, the u.S. Soil Conservation <br />Service used synthetic rainfall-runoff procedures to determine the <br />flows for various frequency storms. Analyses were based on a storm <br />duration of 24 hours, Type 11, and llA distribution, as described in <br />the u.S. Soil Conservation Service National Engineering Handbook, <br />Section 4 (Reference 3). The amount of rainfall was obtained from <br />the precipitation ~requency atlas (Reference 4), and areal adjust- <br />ment was applied to convert the point precipitation values to aver- <br />age precipitation over the watershed area. The studies were con- <br />ducted using runoff computations based on information regarding the <br />type and location of existing and planned land use provided by the <br />u.S. Department of Agriculture, Soil Conservation Service (Reference <br />5). Hydrologic soil cover complexes and associated Runoff <br />Curve Numbers were extracted from the u.S. Soil Conservation <br />Service reports (Reference 5) and field checked. Values of <br />10-, 50-, 100-, and 500-year peak discharges were obtained <br />using the u.S. Soil Conservation Service computer programs <br />WSP2 and TR20 (References 6 and 7, respectively). <br /> <br />. <br /> <br />Peak discharge-drainage area relationships for Coal Creek and Rock <br />Creek are shown in Table 1. <br /> <br />3.2 Hydraulic Analyses <br /> <br />Analyses of the hydraulic characteristics of streams in the com- <br />munity were carried out to provide estimates of the elevations of <br />"floods of the selected recurrence intervals along each stream studied <br />in the community. <br /> <br />Cross section data were taken from 1974 photography and mapping of <br />the study area (Reference 2). All bridges and culverts were surveyed <br />to obtain their elevation and size. The normal openings of the <br />bridges and culverts were used to determine flood data at road <br />crossings. Typical Manning's "n" values for the channel and flood <br />plain range from 0.085 to 0.120 (Reference 2). starting water- <br />surface elevations were determined by the u.S. Soil Conservation <br />Service (Reference 2). Using stream slope at a starting valley <br />section, water-surface profiles for the 10-, 50-, 100-, and 500- <br />year floods were developed using the u.s. Soil Conservation Service <br />WSP2 and TR20 computer programs (References 6 and 7, respectively). <br /> <br />Locations of selected cross sections used in the hydraulic analyses <br />are sho~n on the Flood Profiles (Exhibit 1). For stream segments <br />for which a floodway is computed (Section 4.2), selected cross <br />section locations are also shown on the Flood Boundary and Floodway <br />Map (Exhibit 2). <br /> <br />Flood profiles were drawn showing computed water-surface elevations <br />to an accuracy of 0.5 foot for floods of the selected recurrence <br />intervals (Exhibit 1). <br /> <br />6 <br /> <br />, <br />
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