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<br />Water-surface elevations were computed using the COE HEC-2 computer <br />model (Reference 2) and supplemented by hand calculations where <br />required, Cross sections for the backwater analyses of Eureka and <br />Gregory Gulches were taken from a topographic map for the City of <br />Central City with a scale of 1:1,200 and a contour interval of <br />2 feet (Reference 7). Cul vert elevations and sizes of openings <br />were field surveyed, Locations of selected cross sections used in <br />the hydraulic analyses are shown on the Flood Profiles (Exhibit 1) <br />and on the Flood Insurance Rate Map (Exhibit 2). Starting water- <br />surface elevations for Gregory and Eureka Gulches at the downstream <br />boundary of the study reach were determined by critical depth <br />calculations. <br /> <br />Channel and overbank roughness factors (Manning I s "n") used in the <br />hydraulic computations were chosen using V,S. Department of <br />Transportation Design Charts for Open Channel Flow (Reference 3), <br />Roughness coefficients for the study reach ranged from 0.02 to 0.08 <br />in the channel and overbank areas, <br /> <br />Cul verts along the study reach were analyzed using the Federal <br />Highway Administration guidelines for Hydraulic Design of Highway <br />Culverts (Reference 6). Analyses of flume capacities were <br />performed by Manning's equation, and lower limits of roughness <br />factors (Manning I s "n") were determined by the Darcy equation and <br />Moody diagrams to account for turbulence and viscosities. <br /> <br />The hydraulic analyses for thi s study were based on unobstructed <br />flow, The flood elevations shown on the profiles are thus <br />considered valid only if hydraulic structures remain unobstructed, <br />operate properly, and do not fail. <br /> <br />All elevations are referenced to the National Geodetic Vertical <br />Datum of 1929 (NGVD). Elevation reference marks and the <br />descriptions of the marks used in this study are shown on the maps. <br /> <br />4.0 FLOODPLAIN MANAGEMENT APPLICATIONS <br /> <br />The NFIP encourages State and local governments to adopt sound <br />floodplain management programs. Therefore, each Flood Insurance Study <br />provides 100-year flood elevations and delineations of the 100- and 500- <br />year floodplain boundaries and 100-year floodway to assist communities <br />in developing floodplain management measures. <br /> <br />4.1 Floodplain Boundaries <br /> <br />To provide a national standard without regional discrimination, the <br />1 percent annual chance (lOO-year) flood has been adopted by FEMA <br />as the base flood for floodplain management purposes. The 0.2 <br />percent annual chance (SOO-year) flood is employed to indicate <br />additional areas of flood risk in the community, For each stream <br />studied by detailed methods, the 100- and SOO-year floodplain <br />boundaries have been delineated using the flood elevations <br />determined at each cross section. Between cross sections, the <br /> <br />6 <br />