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<br />acceptable considering the inherent inaccuracies of the analysis <br />and the short period of record. The regional analysis was, <br />therefore, adopted for use in Clear Creek County for both gaged <br />and ungaged areas. <br /> <br />. <br /> <br />Due to either size, operating criteria or use, reservoirs and <br />lakes within Clear Creek County were assumed not to have any <br />effect on .flood flows within the County. <br /> <br />A study done by -the University of Colorado (Reference 7) defining <br />the Georgetown drainage basin and examining the effects of snow- <br />melt on flooding was reviewed for information pertinent to this <br />study. Basic data and a general understanding of the basin <br />characteristics were gained from this review although the <br />methodologies were not incorporated into the analysis because <br />of a general lack of data to establish the required frequency <br />relationships. <br /> <br />Drainage area-peak discharge relationships as developed -for Clear <br />Creek County and adapted to the streams under study are shown on <br />Fi gure 2. <br /> <br />3.2 Hydraulic Analysis <br /> <br />Analyses of the hydraul ic characteristics of streams in the <br />County were carried out to provide estimates of the elevations <br />of floods of the selected recurrence intervals along each stream <br />studied in detai I. <br /> <br />Water surface elevations of floods of the selected recurrence <br />intervals were computed through use of the U. S. Army Corps of <br />Engineers HEC-2 step-backwater computer program (Reference 12-14) <br />and supplemented by manual calculations. Manual calculations were <br />primarily used to determine the division of flood flows at bridge <br />sections and to verify water surface elevations near critical <br />depth. Cross sections for the backwater analyses were field- <br />surveyed and were located at close intervals above and below <br />bridges and culverts in order to compute the significant back- <br />water effects of these structures in the urbanized areas. Loca- <br />tions of selected cross sections used in the hydraul ic analyses <br />are shown on the water surface profi les (Panels OlP - 50 Pl. <br /> <br />Channel roughness factors (Manning's "n") for these computations <br />were assigned on the basis of field inspection of flood plain <br />areas, comparison with streams of known roughness (Reference I) <br />and from computations relating to the USGS stream gage at Lawson. <br /> <br />. <br /> <br />Flood profi les were drawn showing computed water-surface elevations <br />to an accuracy of 0.5 foot for floods of the selected recurrence <br />intervals (Figure 2). Starting elevations for hydraul ic computations <br /> <br />10 <br />