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<br />Flood elevations were computed through use of the U.S. Army Corps of <br />Engineers HEC-2 computer program (Reference 6). Cross sections for <br />backwater analyses were located at close intervals upstream and <br />downstream from bridges and culverts in order to establish the <br />significant backwater effects of the structures. Cross sections were <br />furnished by the State of Colorado or developed in whole or in part <br />from topographic data furnished by the Routt County Planning Commis- <br />sion at a scale of 1:2400, and with a contour interval of 2 feet <br />(Reference 7), Dales Y. Foster, Inc. at a scale of 1:4800, and with <br />a contour interval of 5 feet (Reference 8), and by the State of <br />Colorado at scales of 1:2400 and 1:4800, with a contour interval of <br />2 feet (Reference 9). Field surveys and observations were used as <br />needed to supplement topographic data available. Selected cross <br />sections are shown on the maps. <br /> <br />Roughness factors (Manning's "nil) for hydraulic computations were <br />assigned on the basis of stream gage data and field inspection of <br />flood plain areas. The factors used for the Yampa- River were 0.05 <br />for overbank areas and from 0.03 to 0.043 for the channel. For the <br />tributary creeks, the factors ranged from 0.035 to 0.09 for overbank <br />areas, and from 0.04 to 0.07 for channels. <br /> <br />Flood profiles showing computed water-surface elevations to an <br />accuracy of 0.5 foot for floods of the selected recurrence intervals <br />are shown on the Flood Profiles (Exhibit 1). Starting elevations <br />were taken from the flood elevations of the streams to which they are <br />tributary, or were developed by the slope-area method. All elevations <br />are measured from NGVD. <br /> <br />Elevation reference marks used in conjunction with this study com- <br />prised twenty-four U.S. Geological Survey, U.S. Army Corps of Engi- <br />neers, State of Colorado, and Town of Steamboat Springs bench marks. <br />Descriptions of elevation reference marks used in the study are shown <br />on the maps. <br /> <br />The hydraulic analyses reflect unobstructed flow. Flood elevations <br />shown on the profiles are thus considered valid only if bridges and <br />cuI verts remain unobstructed by flood-bol.-ne debris, ice, or sediment. <br /> <br />For streams studied by approximate methods, the approximate elevation <br />of the 100-year flood, which is used in determining flood boundaries, <br />was established by professional engineering judgment, giving considera- <br />tion to available historical flood data, minor hydrologic and hydraulic <br />analyses, correlation with similar basins studied in detail, and <br />field observations. <br /> <br />23 <br />