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<br />At the outset of the hydrologic analysis, an attempt was made to develop separate <br />regression equations for the Yampa River and its tributaries. Because a <br />representative number of gages could not be located on the Yampa River, the gage <br />data were aggregated to develop a single equation, given below: <br /> <br />c <br />QT = bA <br /> <br />where Q = discharge (cfs), A = drainage area (square miles), and b and c are <br />regression coefficients. The coefficients for the 10-, 50-, 100-, and 500-year storm <br />return periods are shown below. <br /> <br />Return Period T (Years) <br />b <br /> <br />c <br /> <br />10 <br />117 . 84 <br />0.6362 <br /> <br />50 <br />153.11 <br />0.6248 <br /> <br />100 <br />167.33 <br />0.6211 <br /> <br />500 <br />199.9 <br />0.6128 <br /> <br />It should also be noted that no stream gage information is available for Burgess <br />Creek and that the regression equation above was also used to determine the peak <br />flows along Burgess Creek. <br /> <br />A hydrology report that describes the hydrologic analysis in greater detail was <br />submitted to representatives of the local community and to FEMA. The hydrology <br />report was approved by all agencies involved (Reference 14). <br /> <br />ARCVIEW Geographic Information System (GIS) software was used to <br />overlay all the information related to hydrology and to generate digital work <br />maps. ARCVIEW coverages that were either developed for this project or <br />incorporated to assist in the hydrologic analysis included the following: <br /> <br />. Routt County boundary and city limits <br />. Planimetric base map data (roads, railroads, streams, etc.) <br />. Drainage areas <br />. Digital USGS Quadrangle Maps with 40-foot contours <br />. Digital aerial photography <br />. Locations of USGS stream gaging stations <br /> <br />Drainage areas were delineated using digital USGS topographic maps (Reference 15), <br />and then digitized using ARCVIEW GIS software. The digitized drainage areas were <br />then compared to drainage areas tabulated by the USGS. A discrepancy between the <br />new digitized areas and the USGS tabulation was observed. Ten USGS subareas <br />along the Bear River (an upstream tributary of the Yampa River) with a total drainage <br />area of 49.41 square miles were double counted. When this double-counted area was <br />subtracted from the tabulation, the comparison of the revised USGS areas and TC&B <br />areas was much closer. The USGS was alerted regarding this discrepancy in drainage <br />area calculations and concurred that the USGS tabulations contained an error. <br /> <br />25 <br />