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3.2 Hydraulic Analyses Analyses of the hydraulic characteristics of flooding from the sources studied were carried out to provide estimates of the eleva- tions of floods of the selected recurrence intervals. Water-surface elevations of floods of the selected recurrence inter- vals were computed through use of the U.S. Army Corps of Engineers HEC-2 step-backwater computer program (Reference 5). Cross sections for the backwater analyses were obtained from several sources. For the Fraser River and Vasquez Creek reaches previously studied by Worrall, the cross section data were used without modifi- cation (Reference 1). For the reach of the Fraser River through Beaver's village, cross sections from the developer's study were used in the upper reaches (Reference 6). Additional cross sections used for the Fraser River were taken from field surveys (Reference 7). For Cooper Creek, Mary Jane Creek, Leland Creek, and Little Vasquez Creek, digitized sections from aerial photographs flown in June 1981 were used where photography was available. Where photography was not available, field surveyed cross sections were utilized (Reference 7). All bridges, dams, and culverts were field surveyed to obtain elevation data and structural geometry. LOcations of selected cross sections used in the hydraulic analyses are shown on the Flood Profiles (Exhibit 1). For stream segments for which a floodway was computed (Section 4.2), selected cross section locations are also shown on the Flood Boundary and Floodway Map (Exhibit 2). Roughness factors (Manning's "n") used in the hydraulic computations were chosen by engineering judgment and based on field observations of the streams and flood plain areas. They were also checked using a regression developed by the Colorado Water Conservation Board (Reference 9). The regression was developed for mountain streams under flood conditions. Values from the regression are for a bank- full situation and, therefore, include the effect of the dense bank vegetation commonly found in this area. Values of roughness factors computed using the regression ranged from 0.06 to 0.075. These are higher than normally associated with a channel roughness factor of this type of stteam, but it is explained by the fact that the regression includes the full effect of bank vegetation whereas the roughness factor is generally selected for only the characteristics of the bed of the main channel. The impact of the vegetation is increased because of the relatively narrow, 20- to 40- foot stream channels in the study area. The regression was used as a guideline; standard procedures (Reference 10) were used for the final selection. Photographic documentation of channels and overbanks was used ex~ensivelyin this process. Roughness values for the main channel of the Fraser River and its tributaries 15