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<br />analysls were obtained from topographic aerial survey photographs <br />flown in May, 1980 (Reference 6), coupled with appropriate field <br />verification. All bridges and culverts were measured in the field to <br />obta in elevation data and structural geometry. Locati ons of selected <br />cross sections used in the hydraul i c analyses are shown on the F1 000 <br />Profiles, and on the Flood Boundary and Floodway Map, <br /> <br />Roughness factors (Mannings "n" val ues) were determined by field <br />inspection. Ground level photography aided in the selection of rough- <br />ness factors and aerial photography was used to determine the 1 imits <br />of each roughness zone. The roughness val ue for the main channel of <br />the Rio Grande was 0.03 with flood plain val ues ranging from 0.02 to <br />0.05 for all floods. <br /> <br />Flood profiles were drawn Showing computed water-surface elevations to <br />an accuracy of 0.5 foot for floods of the selected recurrence inter- <br />vals. The flood profiles presented in this study are based on the <br />effects of flow through unobstructed hydraul ic structures and area, <br />thus, considered valid only if these structures do not fail and remain <br />unobstructed. Starting water surface elevations were detennined by <br />using the slope-area method. <br /> <br />Horizontal control for the aerial mapping is based on Colorado State <br />Plane Coordinate System South Zone 1929 USC & GS Sea Level Datum. <br />Elevation 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 governments <br />to adopt sound f100d pl a i n management programs. Therefore, each Flood <br />Insurance Study includes a flood boundary map designed to assist commun- <br />ities in developing sound flood plain management measures. <br /> <br />4.1 Flood Boundaries <br /> <br />In order to provide a national standard without regional discrimina- <br />ti on, the lOa-year flood has been adopted by FEMA a s the ba se flood <br />for purposes of flood plain management measures. The SOD-year flood <br />is employed to indicate additional areas of flood risk in the com- <br />munity. For each stream studied in detail, the boundaries for the <br />100- and SOO-year floods have been delineated using the flood ele- <br />vations determined at each cross section: Between cross sections, the <br />boundaries were interpolated using topographic maps at scale of <br />1:2400, with a contour interval of 2 feet (Reference 6). In cases <br />where the 100-and SOO-year flood boundaries are close together, only <br />the 100-year flood boundary has been shown. <br /> <br />F1 ood boundaries for the 100- and SOD-year floods are shown on the <br />Flood Boundary and Floodway Map. Small areas I'lithin the flood bound- <br />aries may not lie above the flood elevations and, therefore, may not <br />be subject to flooding. Owing to limitations of the map scale, such <br />areas are not shown. <br /> <br />7 <br />