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<br />Consideration of the gage records provides a means of
<br />recognizing the effects of the water diversion facilities,
<br />including storage reservoirs and power developments located
<br />along and above the study area, on statistical flood magnitudes.
<br />
<br />A tabulation of the flood discharges used in this study
<br />for all the study areas is provided in Tables 6-12 included in
<br />the APPENDIX.
<br />
<br />The stream gage at the canyon mouth provides the longest
<br />record, 47 years including 1976, through an 89 year historic
<br />period beginning in 1888. This record was supplemented ~it~
<br />stream record data compiled by the OSGS on other front range
<br />streams. A total of 655 station years of record of front range
<br />streams was used in this analysis. The flood hydrology used
<br />in the 1971 Big Thompson River Flood Plain Information Report
<br />was also used in this report at the lower study limit (Ref. I,ll).
<br />The estimated flood discharges at selected locations within the
<br />study area and the discharges used in the hydraulic analysis,
<br />arc summarized in Table 4.
<br />
<br />A detailed description and summary of the hydrologic
<br />analyses and computations is available at the offices of the
<br />Colorado Water Conservation Board.
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<br />Hydraulic Analysis
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<br />Table 4
<br />BIG THOr-lPSON RIVER AND TRIBUTARIES
<br />FLOOD FREQUENCY-DISCHARGE
<br />
<br />A hydraulic analysis was completed to determine the flood
<br />elevation for the 10-, 50-, 100- and 500-year flood events on
<br />each of the streams in the study area. This analysis was
<br />completed using the Corps of Engineer's, HEC-2 Water Surface
<br />Profiles Computer Program (Ref. 8). Cross-sections of the river
<br />bottom for use in the hydraulic study were provided by Kucera
<br />& Associates, Photogrammetric Consultants, in the form of
<br />digitized cross-sections and aerial, topographic maps. The
<br />cross-sections and maps are specified to be representative of
<br />the study area as of September 5, 1976.
<br />
<br /> Discharge (cfs)
<br />Location 10-Yr. 50-Yr. lOO-Yr. SOO-Yr.
<br />Big Thompson, west
<br />of Loveland 5,000 13,000 19,000 44,100
<br />Big Thompson, below I
<br />Buckhorn Creek 5,500 14,600 21,600 47,400 I
<br />Big Thompson, mouth of
<br />canyon 4,250 11,500 16,900 38,900 I
<br />Big Thompson, below ,
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<br />Cedar Creek 4,200 11,400 16,800 38,700 I
<br />Big Thompson, above
<br />Cedar Creek 3,750 7,900 10,500 19,300 I
<br />Big Thompson ot Drake I
<br />below North Fork 3,700 7,850 10,400 19,200
<br />above North Fork 2,750 5,700 7,500 13,600
<br />Big Thompson, belol-.' Dry
<br />Gulch near Lake Estes 2,250 3,800 4,700 7,200
<br />North Fork, above Drake 1,500 4,100 6,100 14,100
<br />North Fork, below Devil's
<br />Gulch near Glen Haven 1,450 3,400 4,400 11,500
<br />
<br />Channel and flood plain roughness coefficients (Manning's
<br />"n" values) used in the analysis were selected to be representative
<br />of typical flood plain conditions. The coefficients were
<br />established from information gathered from field inspectilm,
<br />consultation with other agencies including the USGS, Colorado
<br />State University and the Colorado Highway Department as well
<br />as from estimates based on hydraulic relationships and hydraulic
<br />publications (Ref. 9, 10). The roughness factors used in the
<br />analyses are summarized in Table 5.
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