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<br />On Cedar Creek, water-surface elevations were computed using the <br />U.S Army Corps of Engineers HEC-2 step-backwater computer program <br />(Reference 9), supplemented with hand calculations based on culvert <br />rating curves (Reference 2). <br /> <br />Cross sections for the Uncompahgre River were obtained from aerial <br />photogrammetry (Reference 10). All below-water sections, as well <br />as all bridges, dams, and culverts, were measured and surveyed in <br />the field. <br /> <br />The cross sections for Montrose Arroyo and Cedar Creek were obtained <br />from the CWCB. These cross sections were derived from aerial photos <br />flown in June 1977 (as cited in Reference 2). All bridges, dams, <br />culverts, and below-water sections were measured and surveyed in <br />the field. <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 Fl00d- <br />way Map (Exhibit 2). <br /> <br />Roughness factors (Manning's "n" values) used in the hydraulic <br />computations for the Montrose study reaches were chosen based on <br />engineering judgment and field observations of the flooding sources <br />and flood plain areas. <br /> <br />Table 3 gives a summary of the Manning's "n" values used in this <br />report. <br /> <br />The results obtained from the HEC-2 computer program were adjusted <br />in the vicinity of bridges to more accurately represent actual <br />flooding conditions (Reference 11). For the Uncompahgre River, <br />the starting water-surface elevations were obtained from the Mon- <br />trose County Flood Hazard Information Report (Reference 1). The <br />starting water-surface elevations for Montrose Arroyo and Cedar <br />Creek were obtained from a previous study by the CWCB (as cited in <br />Reference 2). <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 />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 assumed to stay <br />in place remain unobstructed, operate properly, and do not fail. <br /> <br />13 <br />