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<br />3.2 Hydraulic Analyses <br /> <br />Analyses of the hydraulic characteristics of the flooding sourcps <br />studied in detail in the community were carripd out to provide <br />estimates of the elevations of floods of the selected recurrence <br />intervals along each of these flood sources. <br /> <br />Water surface elevations for the 10-, 50-, 100- and 500-year floods <br />for the Gunnison River and Tomichi Creek were computed using the <br />Corps of Engineers HEC-2 Water Surface Profiles Program (Refer- <br />ence 5). Cross sections for this analysis were digitized from <br />orthophoto contour maps flown in May, 1978 (Reference 6) and May <br />1980 (Reference 7), coupled with appropriate field verification. <br />All bridges and culverts were measured in the field to obtain <br />elevation data and structural geometry. <br /> <br />Locations of selected cross sections used in the hydraulic analyses <br />are shown on the Flood Profiles, For stream segments for which a <br />floodway is computed (Section 4.2), selected cross section <br />locations are also shown on the Flood Boundary and Floodway Map. <br /> <br />Roughness factors (Manni ngs "n") were determi ned by fi el d <br />inspection. Ground level phot09raphy aided in the selection of <br />roughness factors and orthophoto contour maps (Reference 6 & 7) was <br />used to determine the limits of each roughness zone. Roughness <br />values for the main channel of the Gunnison River and Tomichi Creek <br />ranged from 0.030 to 0.039 with flood plain values ranging from <br />0.032 to 0.085 for all floods. <br /> <br />Flood profiles were drawn showing computed water-surface elevations <br />to an accuracy of 0.5 foot for floods of the selected recurrence <br />intervals. <br /> <br />The flood profiles presented in this study are based on the effects <br />of flow through unobstructed hydraulic structures and area, thus, <br />considered valid only if these structures do not fail and remain <br />unobstructed. Hydraulic analyses were conducted in Reference 1 <br />and reviewed by the study contractor. Starting water surface <br />elevations for both streams were determined by the slope area <br />method. <br /> <br />Horizontal control is based on Colorado State Plane Coordinate <br />System South Zone 1929 USC & GS Sea Level Dltum. Elevation <br />reference marks used in this study are shown on the maps. <br /> <br />4.0 FLOOD PLAIN MANAGEMENT APPLICATIONS <br /> <br />The National Flood Insurance Program encourages state and local <br />governments to adopt sound flood plain management programs. Therefore, <br />each Flood Insurance Study includes a flood boundary map designed to <br />assist communities in developing sound flood plain management measures. <br /> <br />7 <br />