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of 6,950.5 feet (see Addendum 7-6c.13). The emergency spillway <br />• configuration was checked using the SEDCAD4 Channel Utility (see Addendum <br />7-6c.1C) to show that the as-built configuration can safely pass the 25- <br />year, 24-hour discharge. The channel utility shows a flow depth of <br />approximately 1.2 feet, which would be approximately 1.8 feet below the top <br />of the embankment (6,953.0 feet). If the principal spillway were to clog, <br />or become inoperable for some other reason, the SEDCAD4 modeling shows that <br />the emergency spillway can safely pass the peak discharge from the 25-year, <br />24-hour storm event. Pond 004 is constructed and operated in compliance <br />with Rule 4.05.9(2)(c)(ii). <br />Pond 004 Sedimentology Demonstration <br />SCC revised the SEDCAD4 demonstration as part of TR-42 to update the volume of <br />allowable sediment storage in Pond 004. With the cessation of mining <br />operations and subsequent revegetation of disturbed areas, the annual sediment <br />yield to the pond has decreased significantly and, consequently, it is not <br />necessary to maintain the excessive sediment storage capacity needed to capture <br />the large volumes of sediment generated from the disturbed and barren areas <br />during coal resource removal operations. <br />• <br />The watershed area for Pond 004 is presented on Exhibit 7-22, Pond 004 & 008 <br />Watershed Boundary. The area is based on the resurveyed drainage divide <br />separating Pond 004 and Pond 008 completed in August 2009. The structure <br />elevations input into the model are based on a physical survey conducted in <br />September 2005. The stage/storage inputs utilized data presented by SCC in the <br />as-built report previously approved as part of Technical Revision No. 37 (TR- <br />37). <br />SEDCAD4 uses an Annual R factor for a set number of storage years (Y) to <br />determine if the pond capacity is adequate to store the calculated sediment <br />yield from the design storm. The approved Annual R factor is 10, which is <br />appropriate for reclaimed and revegetated landscapes. In previous SEDCAD4 <br />modeling exercises (i.e., TR-37) the storage year value (Y) was artificially <br />set at two (2) years. Use of this value resulted in low sediment storage <br />capacity because the model suggested that the pond needed a large amount of <br />capacity to store water. This minimal sediment storage capacity caused SCC to <br />evaluate the need to clean the pond on a yearly basis. <br />• <br />TR-51 <br />4 Revised 09/09 <br />I