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<br />3.2 Hydraulic Analyses <br /> <br />Analyses of the hydraul ie characteri sties of the fl ooding sources <br />studied in detail in the cOD1l1unity were carried 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 e1 evations for the 10-, 50-, 100- and 500-year fl oods <br />for the Rio Grande were computed using the Corps of Engineers <br />HEC-2 Water Surface Profiles Program (Reference 7). Cross sections <br />for this analysis were obtained from aerial photographs flown in <br />May, 1980 (Reference 8), coupled with appropriate field verifica- <br />tion. All bridges and culverts were measured in the field to <br />obtain 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 (Mannings "n" valves) were determined by fiel d <br />inspection. Ground level photography aided in the selection of <br />roughness factors and aerial photography was used to determine the <br />limits of each roughness zone. A roughness value of D.034 was used <br />for the main channel of the Rio Grande with flood plain val ues <br />ranging from .025 to .045 for all floods. <br /> <br />Flood profiles were drawn showing computed water-surface elevations <br />to an accuracy of 0.5 foot for fl oods of the sel ected recurrence <br />intervals. The flood profiles presented in this study are based on <br />the effects of fl ow through unobstructed hydraul ic structures and <br />area, thus, considered valid only if these structures do not fail <br />and remain unobstructed. Starting water surface elevations were <br />determined using the slope-area method. <br /> <br />During the study of this reach of the Rio Grande, the Flood Hazard <br />Boundary Map (FHBM) for Del Norte, Colorado was reviewed (Reference <br />9). This map shows a much larger portion of the Town of Del Norte <br />affected by flooding. Due to more accurate mapping information in <br />the present study areas downstream of U.S. Highway 112 were <br />determined to be outside the 100-year and the 500-year flood <br />boundaries. <br /> <br />Horizontal control is based on Colorado State Plane Coordinate <br />System South Zone 1929 USC & GS Sea Level Datum. Elevation <br />reference marks used in this study are shown on the maps. <br /> <br />7 <br />