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FLOOD01901
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Last modified
11/23/2009 12:58:05 PM
Creation date
10/4/2006 10:26:49 PM
Metadata
Fields
Template:
Floodplain Documents
County
Larimer
Community
Larimer County Unincorporated
Title
Flood Insurance Study - Larimer County Unincorporated Areas, Volume 1 of 3
Date
3/23/1999
Prepared For
Larimer County
Prepared By
FEMA
Floodplain - Doc Type
Historic FEMA Regulatory Floodplain Information
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<br />3.1 Hydrologic Analyses <br /> <br />Hydrologic analyses were carried out to establish the peak <br />discharge- frequency relationships for each flooding source studied <br />in detail affecting the community. <br /> <br />The discharges on the Big Thompson River, the North Fork Big <br />Thompson River, and their major tributaries were developed based <br />upon a combination of gage records and regression equations <br />contained in the Colorado Water Conservation Board's Technical <br />Manual No.1, prepared by the U.S. Geological Survey <br />(Reference 10). The locations of the stream gages analyzed and <br />their respective years of record are as follows: Drake gage <br />located on the Big Thompson River (47 years), Big Thompson River <br />below Lake Estes (17 years), Big Thompson River above Lake Estes <br />(27 years), and North Fork Big Thompson River at Drake (30 years). <br />The gage records were analyzed using the log-Pearson Type III <br />distribution as recommended in the U.S. Water Resources Council <br />Bulletin 17 (Reference 11), and the discharges were adjusted as <br />recommended in Technical Manual No.1 (Reference 10). <br /> <br />The Storm Water Hanagement Model (SWKH) (Reference 12) was used to <br />determine discharges on drainage basins that are representative of <br />the smaller Big Thompson River and North Fork Big Thompson River <br />tributaries. These discharges were plotted on semi-log paper to <br />develop discharge-drainage area curves for the region. These <br />curves were entered with the drainage areas of the smaller <br />tributaries, and the appropriate discharges were tabulated. <br /> <br />Discharges for the lower portion of the Big Thompson River near <br />Loveland were provided by the COE in their Flood Plain Information <br />report published in 1971 (Reference 13). <br /> <br />The discharges for Buckhorn Creek, Redstone Creek, and Black Canyon <br />Creek were calculated based upon the regression equations found in <br />Technical Hanual No.1 (Reference 10). Parameters needed for the <br />regression equations were taken from U.S. Geological Survey <br />topographic maps at a scale of 1:250,000 (Reference 14), SCS County <br />Soils Haps (Reference 15) and County Land Use Maps (Reference 16). <br />For Buckhorn Creek and Redstone Creek, discharges for each design <br />paint were calculated for the portions of the basin above 7,500 <br />feet and below 7,500 feet, and the largest discharge at each point <br />was used. <br /> <br />The SCS performed the hydrologic analyses on Boxelder Creek in the <br />vicinity of the Town of Wellington while compiling their Watershed <br />Work Plan (Reference 8). Precipitation values were taken from <br />Nat~onal Weather Service Technical Papers 40 and 49 (Reference 8). <br />Soil information was taken from an unpublished detailed soil survey <br />completed by the SCS for Larimer County. The SCS Technical Release <br />No. 20 (Reference 17) was used to develop and route hydrographs <br />based upon the information gathered. <br /> <br />12 <br />
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