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! ! I , Locationsl of selected cross sections used in the hydraulic analyses are shown, on the Flood Profiles (Exhibit l). For stream segments for which a floodway is computed (Section 4.2), selected cross section locations are also shown on the Floodway Map (Exhibit 2). Hydraulic roughness coefficients (Manning's "n" values) were either computed from stream gage data or determined by field observation. The coeff~cients used ranged from 0.035 to 0.070 for channels and from 0.050 to 0.180 for overbank areas. Flood proriles were developed showing computed water-surface eleva- I tions to an accuracy of 0.5 foot for floods of the selected recur- rence intkrvals (Exhibit 1). Starting water-surface elevations were determined by the slope-area method. All elevations are referenced to the National Geodetic Vertica.l I Datum of ~929 (NGVD). Elevation reference marks used in the study are shownion the maps. The hydraulic analyses reflect unobstructed flow conditions. Flood elevation$ shown on the profiles are thus considered valid only if , hydraulic I structures remain unobstructed. For streams studied by approximate methods, the elevation of the 100-year flood was established by professional engineering judgment, , giving co~sideration to field observations and data available in reports by the U.S. Army Corps of Engineers (References 2, 3, 4, and 5) . I 4.0 ! FLOOD PLAIN MANAGEMENT APPLICATIONS A prime purpose of the National Flood Insurance Program is to encourage state and 10ca4 governments to adopt sound flood plain management p~o- grams. Each F~ood Insurance Study, therefore, includes a flood boundary map designed tq assist communities in developing sound flood plain management mea~ures. , , 4.1 Flood Boundaries , I I In order ~o provide a national standard without regional discrimina- tion, the IIOO-year flood has been adopted by the Federal Insurance Administr~tion as the base flood for purposes of flood plain manage- ment measulres. The 500-year flood is employed to indicate additional areas of fllood risk in the community. For the streams studied by detailed m~thods in the Grand Junction area, the boundaries of the 100- and 500-year floods were delineated by transferring flood ele- , vations frpm the computed water-surface profiles to topographic maps at a scale of 1:2400, with a contour interval of 2 feet (Refer- ence 12). In the Gateway area, topographic maps at a scale of 1:24,000, with a contour interval of 20 feet (Reference 13), were used. 19 ,..........-