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<br />Water-surface elevations of floods of the selected recurrence inter- <br />vals were computed using the COE HEC-2 step-backwater computer <br />program (Reference 3). <br /> <br />Orthophoto and digitized cross sections used for this study were <br />compiled by Nelson, Haley, Patterson and Quirk, Inc., in November <br />1975 (Reference 4). <br /> <br />Structural and elevation data of bridges and culverts were obtained <br />during field surveys. <br /> <br />Water-surface elevations determined using the methods described <br />above are shown on the Flood Profiles (Exhibit 1) . <br /> <br />Shallow flooding is often characterized by highly unpredictable <br />flow directions. Where such conditions exist in La Veta, the entire <br />area susceptible to this unpredictable flow has been delineated as <br />a zone of equal risk. Small-scale topographic variations have been <br />averaged across inundated areas in determining flood depths. <br /> <br />No flood profiles were prepared for these areas of shallow flooding. <br /> <br />The hydraulic analyses for this study were based on unobstructed <br />flow. The flood elevations shown on the profiles are thus considered <br />valid only if hydraulic structures remain unobstructed, operate <br />properly, and do not fail. <br /> <br />All elevations are referenced to the National Geodetic Vertical <br />Datum of 1929 (NGVD). Elevation reference marks used in this study <br />are shown on the mapsl descriptions are presented in Elevation <br />Reference Marks (Exhibit 3). <br /> <br />4 .0 FLOOD PLAIN MANAGEMENT APPLICATIONS <br /> <br />The NFIP encourages State and local governments to adopt sound flood <br />plain management programs. Therefore, each Flood Insurance Study provides <br />laO-year flood elevations and delineations of the 100- and SOO-year flood <br />plain boundaries and laO-year floodway to assist communities in developing <br />flood plain management measures. <br /> <br />4.1 Flood Boundaries <br /> <br />To provide a national standard without regional discrimination, <br />the 1 percent annual chance (lOa-year) flood has been adopted by <br />FEMA as the base flood for flood plain management purposes. The <br />0.2 percent annual chance (SOO-year) flood is employed to indicate <br />additional areas of flood risk in the OOIIDDunity. For each stream <br />studied in detail, the 100- and SOO-year flood plain boundaries <br />have been delineated using the flood elevations determined at each <br /> <br />9 <br />