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<br />cross sections used for North Fork Gunnison River were taken from <br />topographic maps (Reference 10), two cross sections were field <br />surveyed. All bridges, dams and culverts were field surveyed <br />to obtain elevation data and structural geometry. The below water <br />sections of all cross sections were obtained by field measurement. <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 is 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 factors (Manning I s "n") used in the hydraulic computations <br />for the study streams were chosen by engineering judgment and <br />based on field observations of the flooding sources and floodplain <br />areas. Roughness values for the main channel of the streams studied <br />ranged from 0.035 to 0.045, and from 0.040 to 0.070 for overbank <br />floodplain areas. <br /> <br />Flood profiles were drawn showing computed water-surface elevations <br />to an accuracy of 0.5 foot for floods of the selected recurrence <br />intervals (Exhibit 1). <br /> <br />The results obtained from the HEC-2 computer were adjusted in <br />the vicinity of bridges to more accurately represent actual flooding <br />conditions (Reference 11). Starting water-surface elevations <br />for Minnesota Creek were calculated using the slope-area method. <br />For the North Fork Gunnison River, the starting water-surface <br />elevations were obtained from a previous u.s. Army Corps of Engineers <br />report (Reference 12). <br /> <br />The hydraulic analyses for this study were based on unobstructed <br />flow. The flood elevations shown on the flood profiles are, thus, <br />considered valid only if the hydraulic structures stay in place, <br />remain unobstructed, 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 used in the study <br />are shown on the maps. <br /> <br />4.0 FLOOD PLAIN MANAGEMENT APPLICATIONS <br /> <br />The National Flood Insurance Program encourages State and local governments <br />to adopt sound flood plain management programs. Therefore, each Flood <br />Insurance Study includes a flood boundary map designed to assist com~uni- <br />ties in developing sound flood plain management measures. <br /> <br />10 <br />