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<br />. <br />. <br />. <br />. <br />. <br />. Hvdraulic Analvsis <br />. Water surface profiles for the Uncompahgre River (the only stream reach studied by detailed <br />, . methods) were developed using the U.S. Army Corps of Engineers HEC-2 step-backwater <br />. computer model. Cross section data were obtained from field surveys and aerial photography <br />. dated 1975. Starting water surface elevations were determined using normal depth methods. <br />. The hydraulic analyses are based on unobstructed flow through hydraulic structures, Manning n <br />. values used in the analysis were based on field observations and engineering judgement but <br />. values are not documented in the PIS. <br />. <br />. Hydraulic methods used for the approximate floodplain delineation for Cottonwood Creek are <br />. not documented by FEMA in the PIS, For future studies, an approximate IOO-year flood <br />. elevation at a particular location on the creek may be obtained by using the IOO-year peak flow <br />. value provided in this report (Table 2) in conjunction with cross section information and an <br />. acceptable hydraulic analysis. Hydraulic analyses for determining laO-year water surface <br />. elevations for approximate floodplains may be performed using FEMA's Quick-2 computer <br />. program, the Corps of Engineers' HEC-2 computer program, F1owmaster, or other acceptable <br />. hydraulic methods. Channel and floodplain cross-sections may be obtained from detailed <br />. topographic mapping or from field surveys. Cross-sections obtained from USGS quadrangle <br />. maps are not advised. <br />. <br />. Floodulain Mauuine <br />. The 1985 PIS for the Town of Ridgway includes stream reaches with both detailed and <br />. approximate floodplain mapping. For reaches studied by approximate methods, the PIS does not <br />. include flood profiles, base flood elevations, or sufficient detail for county and municipal <br />. officials, engineers, developers, and land owners to make floodplain determinations with any <br />. certainty. Flow rates and methodologies have been included in this HA to assist with these <br />. determinations and the Colorado Water Conservation Board can be contacted to request technical <br />. assistance for approximate floodplain analyses as needed. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br /> <br />I: <br /> <br />. 7 <br />. <br />... <br />