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<br />3.2 Hydraulic Analyses <br /> <br />Analyses of the hydraulic characteristics of streams in the commu- <br />nity were carried out to provide estimates of the elevations of <br />floods of the selected recurrence intervals along each stream <br />studied in the community_ <br /> <br />Water-surface elevations of floods of the selected recurrence <br />intervals for the Blue River, Lehman Gulch, Illinois Gulch, and <br />Sawmill Gulch were computed through use of the the u.S. Army Corps <br />of Engineers HEC-2 step-backwater computer program (Reference 7). ! <br /> <br />Cross section data for the backwater analysis of the Blue River, <br />Lehman Gulch, Illinois Gulch, and Sawmill Gulch were obtained from <br />topographic maps at a scale of 1:1200, with a contour interval of <br />2 feet (Reference 8); from field measurements; and from a previous <br />report (Reference 1). All bridges and culverts were field checked <br />to obtain elevation data and stuctural geometry. <br /> <br />Flooding from the Blue River in the Town of Breckenridge is reduced <br />by an overflow channel downstream of cross section D, outside the <br />corporate limits. The backwater analysis for the Blue River has <br />been computed for the reduced flow resulting from this overflow. <br /> <br />Locations of selected cross sections used in the hydraulic analyses <br />are shown on the Flood Profiles (Exhibit 1). For stream segments <br />for which a floadway is computed (Section 4.2), selected cross <br />section locations are also shown on the Flood Boundary and Floodway <br />Map (Exhibit 2). <br /> <br />The 'stream distances along Blue River are measured from Cemetery <br />Road. The stream distances along any tributary of the Blue River <br />are measured from the confluence with the Blue River. <br /> <br />Channel rouglmess factors (Manning's "n") used in the hydraulic <br />computations were chosen by engineering judgment and based on <br />field observations of the main channel and flood plain areas. <br />Roughness values for the main channel of the Blue River ranged <br />from 0.035 to 0.050, and overbank roughness values ranged from <br />0.035 to 0.050 for all floods. The roughness value for the main <br />channels of Lehman Gulch, Illinois Gulch, and Sawmill Gulch was <br />0.040, and overbank roughness values from 0.040 to 0.100 for all <br />floods. <br /> <br />The starting water-surface elevations for the Blue River were <br />calculated using the slope-area method. Starting water-surface <br />elevations for Lehman Gulch, Illinois Gulch, and Sawmill Gulch <br />were obtained from normal depth calculations. <br /> <br />8 <br />