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. . . . . . . . eek .0 . . . . .reek and ''It en .,m t:~ . the end . . .on) . Iyon) ng al . end IShiPS ods Hydraulic Analyses Analyses of the hydraulic characteristics of streams in the county were carried out to provide estimates of the elevations of floods of the selected recurrence intervals along each stream studied in the county. Users should be aware that flood elevations shown on the FIRM represent rounded whole-foot elevations and may not exactly reflect the elevations shown on the Flood Profiles or in the Floodway Data tables in the FIS report. Flood elevations shown on the FIRM are primarily intended for flood insurance rating purposes. For construction and/or floodplain management purposes, users are cautioned to uSe the flood elevation data presented in this FIS in conjunction with the data shown on the FIRM. Water-surface profiles of the selected recurrence intervals were developed using the U.S. Army Corps of Engineers HEC-2 step-backwater computer model (Raference 7). Cross section data for streams in the area were obtained from aerial photographs (Reference 8); the below-water sections were obtained by field measurement. Cross section data for the Animas River and Hermosa Creek near Hermosa were taken directly from the U.S. Army, Corps of Engineers Flood Hazard Information report (Reference 6). All bridges and culverts were measured to obtain elevation data and structural geometry. Locations of selected cross sections used in the hydraulic analyses are shown on the Flood Profiles (Exhibit 1). For stream segments for which a floodway is computed (Section 4.2), selected cross section locations are also shown on the Flood Boundary and Floodway Map (Exhibit 2). Roughness coefficients (Manning's "n") were estimated by field inspection at each cross section and checked for reasonableness against a recognized standard text (Reference 9). The channel and overbank values are shown below. . . (r~m .o1r . .ls at ! !_ ervoi _ s of of "ps . 'e . . and . 'reek, .cito . . . . . Manning's "n" Stream Channel Overbank Animas River Hermosa Creek Junction Creek Dry Gulch Creek Lightner Creek Los Pinos River at Bayfield Vallecito Creek Grimes Creek 0.034-0.050 0.035 0.025-0.070 0.024-0.040 0.040-0.100 0.035-0.045 0.050 0.050 0.050-0.080 0.060 0.030-0.120 0.050 0.040-0.150 0.060-0.070 0.080 0.080 Starting water-surface elevations for the Animas River, Junction Creek, and Dry Gulch were taken from the Flood Hazard Information report (Reference 2). Hermosa Creek starting water-surface elevations were taken from the Animas River at their confluence. All other starting water-surface elevations were determined by the slope-area method. Flood profiles were drawn showing computed water-surface elevations to an accuracy of 0.5 foot for floods of the selected recurrence intervals (Exhibit 1). 9