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<br />fudraulk Analvsis <br />W2:ter surface profiles for stream reaches studied by detailed methods were developed using the <br />U.S. Army Corps of Engineers HEC-2 step-backwater computer model. Cross section data were <br />obtained from aerial survey information supplemented by field surveys of below.water pOJ1ions <br />of cross sections. <br /> <br />W2,ter surface profile:s for the reach of the Animas Rivl~r revised by the 1995 FIS were also <br />developed using the U.S. Army Corps of Engineers HEC'-2 step-backwater computer model. <br />Starting water surface elevations and hydraulic roughness coefficients were taken from the <br />original 1981 PIS. Revised cross section information was developed using 1"=200', 2-foot <br />contour interval topographic mapping, produced by (ioff Engineering and Surveying and dated <br />November 10, 1992. <br /> <br />Hydraulic methods used for the approximate floodplain delineations shown 011 the La Plata <br />CO'Jnty F[RM panels are not documented by FEMA in the FIS.. For future studies, an <br />approximate 100-year flood elevation at a particular J ocatioll on the river may be obtained by <br />using the 100-year peak flow value provided in this report Crable 2) in conjunction with cross <br />section in'ormation and an acceptable hydraulic analysis. Hydraulic analyses for determining <br />100-year water surface elevations for approximate 1l0odplains 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 floodpIain cross--sections may be obtained <br />fron detailed topographic mapping or from field surveys, Cross-sections obtained from USGS <br />quadrangle maps are not advised. <br /> <br />FI~odolain Maooine: <br />The 1995 PIS for La Plata 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 10 assist with these determinations <br />and the Colorado Water Conservation Board can be contacted to request technical assistance for <br />approximae floodpla:in analyses as needed. <br /> <br />10 <br />