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7-6f.3C) to show that the as-built configuration can safely pass the 25-year, <br />29-hour discharge. The channel utility modeling shows a peak flow depth of <br />approximately 1.1 feet, which would be approximately 3.5 feet below the top <br />of the embankment (7,020.0 feet). If the principal spillway were to clog, or <br />become inoperable for some other reason, the modeling shows that the <br />emergency spillway is of sufficient size to safely pass the peak discharge <br />from the 25-year, 29-hour storm event. Pond 006 is constructed and operated <br />in compliance with Rule 9.05.9(2)(c)(ii). <br />Pond 008 Sedimentology Demonstration <br />SCC has revised the SEDCAD4 demonstration to update the volume of allowable <br />sediment storage in Pond 008. With the cessation of mining operations and <br />subsequent revegetation of disturbed areas, the annual sediment yield to the pond <br />will decrease significantly and, consequently, it is not necessary to maintain the <br />excessive sediment storage capacity which was used to capture the large volumes of <br />sediment generated from the disturbed and barren areas during coal resource <br />removal operations. <br />The watershed area for Pond 008 is presented on Exhibit 7-22, Pond 004 S 008 <br />Watershed Eoundary. The structure elevations input into the model are based on a <br />physical survey conducted in September 2005. The stage/storage inputs utilized <br />data presented by SCC in the as-built report pr eviously approved as past o£ <br />Technical Revision No. 37 (TR-37). <br />SEDCAD4 uses an Annual R factor for a set number of storage years (Y) to determine <br />if the pond capacity is adequate to store the calculated sediment yield from the <br />design storm. The approved Annual R factor is 10, which is appropriate for <br />reclaimed and revegetated landscapes. In previous SEDCAD4 modeling exercises <br />(i.e., TR-37) the storage year value (Y) was artificially set at two (2) years. <br />Use of this value resulted in low sediment storage capacity because the model <br />suggested that the pond needed a large amount of capacity to store. water. This <br />minimal sediment storage capacity caused SCC to evaluate the need to clean the <br />pond on a yearly basis. <br />The updated SEDCAD4 demonstration shows that with the revised number of storage <br />years (270 years), the effluent from Pond 008 meets the requirements of Rule <br />4.05.2(7). in addition, the updated SEDCAD4 demonstra tions shows that the pond <br />satisfies the operational criteria of Rule 4.05.9 (2)(b) and Rule <br />9.05.9(2)(c) (iii). <br />TR-42 9 Revised 09/07 <br />