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<br />For the streams studied by detailed methods, the profiles are <br />used to determine the depth of water in the channel and adjacent <br />flooded areas, as they were calculated using the HEC-2 computer <br />program. <br /> <br />The la-year flood would be contained within stream channels, <br />while the 50-year flood would closely follow the pattern of the <br />lOa-year flood for the West Dolores River, Dolores River, and <br />Lost Canyon Creek. In some sheetflow areas, broad overland <br />flooding less than 2-feet on the average; and characterized by <br />unpredictable flow paths, water-surface elevations are <br />independent of those along adjacent streamways, and are affected <br />principally by obstructions and local topography 1n the area <br />flooded (Reference 1). <br /> <br />Cross sections used in the hydraulic analysis for Mancos River <br />and Chicken Creek were obtained by both photogrammetric and <br />field-survey methods. All bridges and other significant <br />structures that might divert or restrict floodflows were <br />surveyed. Because the digitized cross sections show the water- <br />surface elevation in each stream on the date of photography, the <br />cross sections were adjusted by field measurements to reflect the <br />streambed elevations and an area beneath the mapped water surface <br />for a discharge of 300 cfs in the stream at the time of <br />photography. Floodplain roughness factors (Manning's "n") used <br />in the hydraulic computations were selected based on field <br />inspection of the actual floodplain conditions. Manning's "nt' <br />values for Mancos River ranged from 0.035 to 0.045 in the <br />channel, and 0.060 to 0.100 in the overbanks. For Chicken Creek, <br />Manning's "n" values ranged from 0.030 to 0.050 in the channel, <br />and 0.040 to 0.070 in the overbanks. <br /> <br />Flood profiles were drawn showing <br />elevations for floods of the selected <br /> <br />the computed water-surface <br />recurrence intervals. <br /> <br />The hydraulic analyses for this Flood Insurance Study are based <br />on unobstructed flow. The flood elevations shown on the profiles <br />are thus considered valid only if hydraulic structures remain <br />unobstructed, operate properly, and do not fail. <br /> <br />All elevations are referenced to the National Geodetic Vertical <br />Datum (NGVD) of 1929. <br /> <br />4.0 FLOODPLAIN MANACEMENT APPLICATIONS <br /> <br />The NFIP encourages state and local governments to adopt sound flood- <br />plain management programs. Therefore, each Flood Insurance Study <br />includes a floodplain boundary map designed to assist communities 1n <br />developing sound floodplain management measures. <br /> <br />8 <br />