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. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . I. . . . Hvdraulic Analvsis Water surface profiles for stream reaches studied by detailed methods and not revised in the 2001 PIS were developed using the U.S. Army Corps of Engineers HEC-2 step-backwater computer model. Cross section data were obtained from aerial survey information supplemented by field surveys of below-water portions of cross sections. Water surface profiles for the stream reaches revised by the 200 I PIS were developed using the Boss River Modeling System (RMS). Boss RMS is an Autocad-based program designed to simplify the input of V,S. Army Corps of Engineers HEC-2 step-backwater computer model data. Starting water surface elevations for the revised reaches were taken to be either normal or critical depth as appropriate. Hydraulic roughness coefficients were estimated from field investigations. Cross section information for the revised river reaches was developed using 1994, 2-foot contour interval topographic mapping, provided by the City of Durango. Hydraulic methods used for the approximate floodplain delineations shown on the La Plata County FIRM panels are not documented by FEMA in the PIS. For future studies, an approximate lOO-year flood elevation at a particular location on the river may be obtained by using the lOO-year peak flow value provided in this report (Table 2) in conjunction with cross section information and an acceptable hydraulic analysis. Hydraulic analyses for determining lOO-year water surface elevations for approximate floodplains may be performed using FEMA's Quick-2 computer program, the Corps of Engineers' HEC-2 computer program, Flowmaster, or other acceptable hydraulic methods. Channel and floodplain cross-sections may be obtained from detailed topographic mapping or from field surveys. Cross-sections obtained from USGS quadrangle maps are not advised. Floodplain Mappin!! The 2001 PIS for La Plata County includes stream reaches with both detailed and approximate floodplain mapping. For reaches studied by approximate methods, the PIS does not include flood profi les, base flood elevations, or sufficient detail for county and municipal officials, engineers, developers, and land owners to make floodplain determinations with any certainty. Flow rates and methodologies have been included in this HA to assist with these determinations and the Colorado Water Conservation Board can be contacted to request technical assistance for approximate floodplain analyses as needed. 10