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(Goff Engineering and Surveying, Inc., 1992). Roughness coefficients (Manning's n) in the Goff model <br />ranged from 0.08 for the channel to 0.3 and 0.5 for the overbanks. <br />The Sandco Reach of the Animas River was modeled by Southwest Hydro-Logic for the 100-year <br />flood in an eazlier report using the U.S. Army of Corps of Engineers HEC-RAS computer program <br />(Wright, 2004). The plan view aerial photo of the Sandco Reach and locations of cross sections are shown <br />on figure 2. The HEC-RAS modeling results simulated steady-state conditions for 24,600 ft3/sec through <br />the Sandco Reach, and roughness coefficients ranged from 0.035 for the channel to 0.1 for the overbanks <br />(typical of Rocky Mountain river channels, see Barnes, 1967). The modeling results indicated that water- <br />surface elevations for 24,600 ft3/sec would be contained within the banks of the Animas River through the <br />Sandco reach. <br />The HEC-RAS model was rerun by Southwest Hydro-Logic for this letter to make sure the results <br />were correct, and the results were compared to the Goff 1992 results and the 2002 LOMR results. The <br />Sandco Reach station SC-7 and the Goff river station 77.760 are estimated to be a similaz cross section <br />location, and the LOMR water-surface elevation of 6,559 crosses the river at approximately the same <br />location (figure 3). Figure 4 shows the comparison ofthe results. <br />The Southwest Hydro-Logic HEC-RAS model did not include river reaches upstream from the <br />Sandco Reach, and it did not include river reaches downstream from the Sandco Reach where flood <br />backwater effects from other constrictions might affect water-surface elevations. <br />References Cited <br />Barnes, H.H., Jr.,1967, Roughness characteristics of natural channels: U.S. Geological Survey <br />Professional Paper 1849, 219 p. <br />Goff Engineering and Surveying, Inc., 1992, Floodway and flood elevations, modifications, MP 76.012 to <br />MP 78.940, La Plata County, Colorado. <br />Wright, W.G., 2004, Hydrologic study of the Sandco Reach, Animas River valley, La Plata County, <br />Colorado, 2004: Southwest Hydro-Logic Hydrological Studies Report 2004-1, 24 p. <br />Respectfully Submitted, <br />GUrk /l,~/~ /CJic,~ ~ <br />0 <br />Winfield G. Wright ,~~~~~p~d±~ <br />Professional Hydrologist ~~~pes9~!~u~: <br />Civil Engineer tS`; , <br />Owner ~f to14 <br />€a WiniieldG. <br />,~, im Wright <br />