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4�> Spillway flows initiate erosion of spillway channel undermining riprap <br />subgrade initiating headcut <br />4�> Headcut erosion migrates upstream to spillway control section <br />4�> Headcut undermines subgrade of concrete spillway control <br />section removing support for the structure. See Figure 2 <br />below <br />4� Spillway control section collapses initiating breach of <br />the abutment <br />4�> Reservoir drains through breach section <br />Figure 2 - Headcut Undermining Spillway Control Section. Source: RMA, 2013 <br />Contributing factors to the failure of the dam include: <br />• The soil at the site consists of fine silty sand that does not provide erosion resistance <br />• Lack of robust filter between the riprap and the underlying abutment soils to prevent <br />migration of soil through the riprap <br />• Lack of cut -off structures in spillway channel to arrest /retard headcut migration <br />• Lack of support for concrete spillway control section to resist undermining <br />A map of the floodplain downstream of the dam is provided in Figure 3 below. This map is <br />based on an aerial photograph taken September 14, 2013 and shows the abandoned railroad <br />embankment impounding water along with several other areas of significant surface storage. <br />