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<br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <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 for below water sections was obtained from field surveys <br />while topography for overbank areas was determined from USGS 1:24,000 topographic mapping. <br />The 1985 PIS states that Manning's n values used in the analysis were based on "field <br />investigation" but is not clear on whether this simply means site visits combined with <br />engineering judgement or if actual water surface elevation surveys were carried out. Manning n <br />values used in the analysis ranged from 0.051 to 0.072 for the main channel and 0.080 to 0.10 for <br />overbank areas. <br /> <br />Hydraulic methods used for approximate floodplain delineations are not documented by FEMA <br />in the PIS. For future studies, an approximate l00-year flood elevation at a particular location <br />may be obtained by using the 100-year peak flow values recommended in this report in <br />conjunction with cross section information and an acceptable hydraulic analysis. Hydraulic <br />analyses for determining 100-year water surface elevations for approximate floodplains may be <br />performed using FEMA's Quick-2 computer program, the Corps of Engineers' HEC-2 computer <br />program, Flowmaster, or other acceptable hydraulic methods. Channel and floodplain cross- <br />sections may be obtained from detailed topographic mapping or from field surveys. Cross- <br />sections obtained from USGS quadrangle maps are not advised. <br /> <br />Floodplain Mappinl!: <br />The 1985 PIS for the Ouray County includes stream reaches with both detailed and approximate <br />floodplain mapping. For reaches studied by approximate methods, the PIS does not include <br />flood profiles, base flood elevations, or sufficient detail for county and municipal officials, <br />engineers, developers, and land owners to make floodplain determinations with any certainty. <br />Flow rates and methodologies have been included in this HA to assist with these determinations <br />and the Colorado Water Conservation Board may be contacted to request technical assistance <br />with approximate floodplain analyses as needed. <br /> <br />9 <br /> <br />- <br />