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<br />aerial photography. The flow in the river at the time of the <br />photography was obtained from USGS gaging station records. <br /> <br />Bridges and culverts on the Crystal River within the study area <br />were field surveyed to 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 (Exhibit 1). For stream segments <br />for which a floodway was computed (Section 4.2), selected cross <br />section locations are also shown on the Flood Boundary and Floodway <br />Map (Exhibit 2). <br /> <br />Roughness coefficients (Manning's On") used in the hydraulic computa- <br />tions were based on field investigation. The roughness factors <br />used for the channel ranged from 0.035 to 0.0401 overbank factors <br />ranged from 0.050 to 0.090. <br /> <br />Flood profiles were drawn showing elevations to an accuracy of <br />0.5 foot for the floods of the selected recurrence intervals (Ex- <br />hibit 1). <br /> <br />The starting water-surface elevation for the Crystal River was <br />based on the flood elevation on the Roaring Fork River at the <br />confluence. <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 <br />study are shown on the maps. <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 <br />produces maps designed to assist communities in developing flood <br />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 (lOO-year) flood has been adopted <br />by FEMA as the base flood for flood plain management purposes. <br />The 0.2 percent annual chance (SOO-year) flood is employed to <br />indicate additional areas of flood risk in the community. For <br />each stream studied in detail, the 100- and SOO-year flood plain <br />boundaries have been delineated using the flood elevations determined <br /> <br />7 <br />