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<br />.. <br />. <br />. <br />. <br />. <br />. <br />. Hydraulic Analysis <br />. Water surface profiles for streams studied by detailed methods were developed using the U.S. <br />. Army Corps of Engineers HEC-2 step-backwater computer model and cross section data from <br />. topographic mapping and field surveys. Manning's n values were based on engineering <br />. judgement and ranged from 0.035 to 0.08 for channel areas and 0.045 to 0.15 for overbank areas. <br /> <br />. <br />. The 1988 PIS states that hydraulic analysis of approximate study areas were based on normal <br />. depth computations and information obtained from available topographic mapping and field <br />. surveys. No information on the source of flow rates used in the analyses is provided. For future <br />. studies, an approximate 100-year flood elevation at a particular location may be obtained by <br />. using the 100-year peak flow values recommended in this report in conjunction with cross <br />. section information and an acceptable hydraulic analysis. Hydraulic analyses for determining <br />. lOO-year water surface elevations for approximate floodplains may be performed using FEMA's <br />. Quick-2 computer program, the Corps of Engineers' HEC-2 computer program, Flowmaster, or <br />. other acceptable hydraulic methods. Channel and floodplain cross-sections may be obtained <br />. from detailed topographic mapping or from field surveys. Cross-sections obtained from USGS <br />. quadrangle maps are not advised. <br />. <br />. Floodplain Mappinl!: <br />. The 1988 PIS for Pitkin 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 can be contacted to request technical assistance for <br />I . approximate floodplain analyses as needed. <br />. <br />. <br />. <br /> <br />I: <br />. <br />, <br />. <br />. <br />. <br />. <br />. <br />. <br />I. <br />'. <br />I. <br />'. <br />. <br />. <br />. <br />. <br />. <br />. 13 <br />. <br />