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9.2 Second Revision This study was revised on May 21, 2001, to incorporate detailed flood hazard information along the Animas River from approximately 2.09 miles downstream of U.S. Highway 155/160 to approximately 0.39 mile upstream of La Posta Road and from approximately 0.58 mile upstream of 32nd Street to approximately 3.56 miles upstream of 32nd Street, and along Lightner Creek from the confluence with the Animas River to approximately 3,525 feet upstream of U.S. Highway 160. The hydraulic analyses for this restudy were performed by Foothill Engineering Consultants, Inc., for the Federal Emergency Management Agency (FEMA) , under Contract No. EM~-93-C-4150. This work was completed in May 1998. The floodway determination and boundary delineations for Lightner Creek and the upper portion of the Animas River from approximately 0.58 mile upstream of 32nd Street to approximately 3.56 miles upstream of 32nd Street were completed by Michael Baker Jr., Inc., for FEMA, in February 1999. The peak discharge-frequency relationships for the studied streams were taken from previous Flood Insurance Studies for the City of Durango, Colorado (Reference 18), and La Plata County, Colorado (Reference 19). Water-surface elevations for the 10-, 50-, 100-, and 500-year flood events were computed using the BOSS River Modeling System (RMS) (Reference 20). BOSS RMS is an Autocad-based program designed to simplify the input processing of the USACE HEC-2 computer program (Reference 21). Topographic maps with 2-foot contour intervals were acquired from the City of Durango, in the form of Autocad drawing files. These maps were created from aerial photography flown April 4, 1994. Bridge inspection and construction drawings were available for the bridges over the Animas River. These drawings were used as input data for the HEC-2 models. A limited number of surveyed cross sections of the Animas River were provided by the City of Durango. These sections were used to supplement the Autocad map and bridge data. Field investigations were conducted to visually confirm the topographic mapping, verify culvert and bridge geometry, and estimate roughness coefficients (Manning's "nH values) for the studied streams. Roughness coefficients for the main channel depended on channel condition and obstructions. Roughness coefficients for the floodplain depended on vegetation, irregularity, obstructions, and meandering (Reference 22). The ranges of Manning's "nil values used for each stream are shown in the table in Section 3.2, "Hydraulic Analyses." The starting water-surface elevations were calculated using normal depth or critical depth as appropriate. Using BOSS RMS, the cross sections were located to effectively represent changes in flow, conveyance, surface characteristics, and structures. 38