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II II 11 II II II I I I I I I I II :1 il 11 II II Lower Big Dry Creek Hydrologic Study meander patterns. Bank erosion, as evidenced by exposed sediment, was present on the almost vertical banks that are typical of outside bends in a meandering channel. This condition prevails throughout the creek. As sediment is eroded in the upstream portion of the creek, erosive potential is actually reduced in the downstream portion of the creek as the eroded bank sediment is carried in suspension, thereby reducing the tractive forces of the flowing water and subsequent bank line erosion. . Although erosion of the bare stream banks appears to be a dramatic loss of topsoil, the erosion of most bank .lines actually appeared to be minor. In particular, the ?latural erosion process within open space areas did not appear to be excessive. There were, however, a few cases of more accelerated rates of erosion either caused by fallen trees or by movement of rip rap, which had originally been placed along the bank, into the active stream. . Extent of bank erosion was estimated based on review of an aerial video and is summarized in {Table 3] below. The overview indicates that at a macro-level, Big Dry Creek is experiencing relatively low rates of erosion, as evidenced by roughly 80 percent of the banks being vegetated. Of the remaining 20 percent of banks that are denuded to some extent, 13 percent show low erosion potential, 6 percent show moderate erosion potential and only 1 percent show heavy erosion potential. It should be noted that most of the areas showing heavy erosion potential are concentrated in the area between Highway 36 and Sheridan, where the steep cut banks are located. Review of aerial photos and geologic data in this area indicate that these banks are actually relatively stable, even though they are steep and unvegetated. 971-179.092 June 2005 Wright Water Engineers, Inc. Page 22