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<br />bridge opening, forcing higher velocities against both the right <br />and left banks. Higher velocities have caused severe erosion along <br />bo'th banks, and is threatening an irriga'tion canal along the right <br />bank, as well as both bridge abutments. Logan County is currently <br />acting as the local sponsor for a Corps of Engineers Section 14 <br />emergency streambank and shoreline protect:ion project t,o protect <br />the bridge abutments. However, th,~ county is concerned about <br />diminished bridge capacity. The following analysis and alternative <br />development address the restoration of t.he channel capaci1:y through <br />the bridge opening. <br /> <br />ANALYSIS <br /> <br />Data Collection" The Omaha District has collected <br />hydrographic survey da'ta of the sandbar and island area. Ten cross <br />sections were surveyed, two downstream and eight upstream from the <br />bridge. Surveyed cross sections are, presented on Plates 2 through <br />11. No other data were collected. <br /> <br />Analvsis. A geomorphic analysis was conducted on the South <br />Platte River channel for a distance 10 miles upst:rea:m and <br />downstream from the bridge. A geomorphic analysis, simply defined, <br />is the identification and description of the alluvial processes <br />taking place in a riverine environment. These studies can range <br />from very detailed (Which may include soil borings, sediment <br />sampling, extensive mapping, etc.) to cursory (which is t:he result <br />of a field inspection). For the purposes of this reconnaissance- <br />level design effort, U.S. Geological Survey (USGS) topographic maps <br />(1951 and 1971) and 1976 aerial photography were used for the <br />analysis. The main objectives of the analysis were to estimate a <br />stable channel width 'that occurs in nature and to qualitatively <br />assess the local alluvial processl~s that influence sediment <br />movement in the reach. For the purposes of this study, a stable <br />channel is defined as one who's top width remained nearly constant <br />and the channel itself was free from sandbar and island formation <br />when comparing the USGS maps and the aerial photography. <br /> <br />The alluvial processes in this reach of the South Platte River <br />are controlled mainly by the irrigation diversions. 'rhe major <br />alluvial process present is erosion of t,he high banks. BE!CaUSe the <br />bed slope of the river is fixed by diversion dams, bed scour is <br /> <br />Section 22 - South Platte River, Merjno, Colorado <br /> <br />Page 2 <br />