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<br />The floodflow-frequency curves [or North Fork Gunnison River W€ce <br />based primarily on streamflow data collected at the U.S. Geological <br />Survey gage located near Somerset and secondarily on values adopted <br />from other gages in the area. The peak flows recorded since the <br />regulation of flow by paonia Reservpir were also used in the deter- <br />mination of the peak flows for the river~ The present condition <br />peak-flow frequency curves for the ungaged sites on North Fork <br />Gunnison River were developed by applying the difference between <br />the preregulated and present conditions for the gage located near <br />Somerset to the preregulated peak-flow frequency curves for the <br />ungaged sites. <br /> <br />Peak discharge-drainage area relationships for North Fork Gunnison <br />River are shown in Table 1. <br /> <br />3.2 Uydraulic Analyses <br /> <br />Analyses of the hydraulic characteristics of flooding from the <br />sources studied were carried out to provide estimates of the eleva- <br />tions of floods of the selected recurrence intervals. <br /> <br />Water-surface elevations for 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 9). <br /> <br />Cross sections used in the backwater analyses were obtained from a <br />U.S. Army Corps of Engineers Flood Hazard Information report (Refer- <br />ence 7). These cross sections were developed from field survey <br />data furnished to the U.S. Army Corps of Engineers by the Colorado <br />Water Conservation Board. The below-water sections of all cross <br />sections were obtained by field measurement. All bridges, dams, <br />and culverts were field surveyed to obtain elevation data and <br />structural geometry. Several small private driveway bridges which <br />were not considered sturdy enough to withstand peak floods were <br />assumed to be washed out. The results obtained from the HEC-2 <br />computer program were adjusted in the vicinity of bridges to more <br />accurately represent actual flooding conditions (Reference 10). <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 floodway was computed (Section 4.2), selected cross <br />section locations are also shown on the Flood Boundary and Floodway <br />Map (Exhibit 2). <br /> <br />Roughness factors (Manning's "n") used in the hydraulic computations <br />for the study streams were chosen by engineering judgment and based <br />on field observations of the flooding sources and flood plain areas. <br />The roughness values used for North Fork Gunnison River varied <br />from 0.04 to 0.07 in the channel, and from 0.04 to 0.11 in the <br />overbanks. <br /> <br />7 <br />