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":'";,~- Cross sections used in the backwater analyses streams were developed photogrammetrica11y photography flown in October 1981 (Reference 13). dams, and culverts were field surveyed to obtain and geometry.. for the study using aerial All bridges, elevation data " Locations of selected cross sections used in the hydraulic analyses are shown on the Flood Profiles (Exhibit 1). For stream segments for which a floodway was computed (Section 4.2), selected cross section locations are also shown on the Flood Boundary and F100dway Map (Exhibit 2). Roughness factors (Manning's "n") used 1n the hydrau1 ic computations for the detailed-study streams were chosen by engineering judgment and based on field observations of the flooding sou'rces and floodplain areas. Roughness values for the main channel of the detailed-study streams ranged from 0.02 to 0.10; floodplain roughness values ranged from 0.04 to 0.13. Starting water-surface elevations were determined by the slope- area method, critical depth, Or elevations at confluences if the timing of the peaks coincided. The hydraulic analyses for this study were based on partially obstructed flow, as defined by the City of Boulder and FEMA through field inspection. The flood elevations shown on the profiles are thus considered valid only if hydraulic structures remain unobstructed, operate properly, and do not fail. All elevations are referenced to the National Geodetic Vertical Datum of 1929 (NGVD). Elevation reference marks used in this study are shown on the maps. 4.0 FLOODPLAIN MANAGEMENT APPLICATIONS The NFlP encourages State and local governments to adopt sound flood- plain management programs. Therefore, each Flood Insurance Study produces maps designed to assist communities in developing floodplain management measures. 4.1 Floodplain Boundaries :. To provide a national standard wi thout regional di scriminat ion, the 1 percent annual chance (100-year) flood has been adopted by FEMA as the base flood for floodplain management purposes. The 0.2 percent annual chance (SOO-year) flood is employed to indicate additional areas of flood risk in the community. For each stream studied by detailed methods, the 100- and 500-year floodplain boundaries have been delineated using the flood elevations determined at each cross section. Between cross sections, the boundaries were interpolated using topographic maps at a scale of 1:2,400, with a contour interval of 2 feet (Reference 13). 15 r~;;':::~~.a_- '"",!,