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<br />3.2 Hydraulic Analyses <br /> <br />Analyses of the hydraulic characteristics of streams in the county were <br />carried out to provide estimates of the elevations of floods of the selected <br />recurrence intervals along each stream studied in the county. Users should be <br />aware that flood elevations shown on the FIRM represent rounded whole-foot <br />elevations and may not exactly reflect the elevations shown on the Flood <br />Profiles or in the F100dway Data tables in the FIS report. Flood elevations <br />shown on the FIRM are primarily intended for flood insurance rating purposes. <br />For construction and/or floodplain management purposes, users are cautioned <br />to use the flood elevation data presented in this FrS in conjunction with the <br />data shown on the FIRM. <br /> <br />Water-surface profiles of the selected recurrence intervals were developed <br />using the U.s. Army Corps of Engineers HEC-2 step-backwater computer model <br />(Reference 7). <br /> <br />Cross section data for streams in the area were obtained from aerial <br />photographs (Reference 8); the below-water sections were obtained by field <br />measurement. Cross section data for the Animas River and Hermosa Creek near <br />Hermosa were taken directly from the u.s. Army' Corps of Engineers Flood <br />Hazard Information report (Reference 6). All bridges and culverts were <br />measured to obtain elevation data and structural geometry. <br /> <br />Locations of selected cross sections used in the hydraulic analyses are shown <br />on the Flood Profiles (Exhibit 1). For stream segments for which a floodway <br />is computed (Section 4.2), selected cross section locations are also shown on <br />the Flood Boundary and Floodway Map (Exhibit 2). <br /> <br />Roughness coefficients (Manning's .'n") were estimated by field inspection at <br />each cross section and checked for reasonableness against a recognized <br />standard text (Reference 9). The channel and overbank values are shown <br />below. <br /> <br />Manning's "nil <br /> <br />Stream <br /> <br />Channel <br /> <br />Overbank <br /> <br />Animas River <br />Hermosa Creek <br />Junction Creek <br />Dry Gulch Creek <br />Lightner Creek <br />Los Pinos River at Bayfield <br />Vallecito Creek <br />Grimes Creek <br /> <br />0.034-0.050 <br />0.035 <br />0.025-0.070 <br />0.024-0.040 <br />0.040-0.100 <br />0.035-0.045 <br />0.050 <br />0.050 <br /> <br />0.050-0.080 <br />0.060 <br />0.030-0.120 <br />0.050 <br />0.040-0.150 <br />0.060-0.070 <br />0.080 <br />0.080 <br /> <br />Starting water-surface elevations for the Animas River, Junction Creek, and <br />Dry Gulch were taken from the Flood Hazard Information report (Reference 2). <br />Hermosa Creek starting water-surface elevations were taken from the Animas <br />River at their confluence. All other starting water-surface elevations were <br />determined by the Slope-area method. <br /> <br />Flood profiles were drawn showing computed water-surface elevations to an <br />accuracy of 0.5 foot for floods of the selected recurrence intervals <br />(Exhibit 1). <br /> <br />9 <br />