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quality standard. Therefore, the 2005 spillway construction and, subsequent pond operation, meets the <br />intentions of Rule 4.05.6(3)(a). <br />The calculated peak stage from the 10 -year, 24 -hour storm event is 6,949.0 feet. This elevation is 1.2 <br />feet below the emergency spillway invert elevation (6,950.2 feet). This demonstrates that no flow will <br />pass through the emergency spillway during the 10 -year, 24 -hour storm event. Therefore, the pond as <br />constructed and operated meets the intentions of Rule 4.05.9(2)(b). <br />The modeled 25 -year, 24 -hour storm event peak inflow is 16.5 cfs, while the calculated peak discharge <br />is 7.2 cfs with a corresponding peak stage of 6,950.5 feet. The peak discharge and stage assume that <br />the principal spillway is inoperable (e.g., no discharge through a principal spillway). The emergency <br />spillway configuration was checked using the SEDCAD4 Channel Utility to show that the as -built <br />configuration can safely pass the 25 -year, 24 -hour discharge. The channel utility shows a flow depth <br />of approximately 1.2 feet, which would be approximately 1.8 feet below the top of the embankment <br />(6,953.0 feet). The SEDCAD4 modeling shows that the emergency spillway can safely pass the peak <br />discharge from the 25 -year, 24 -hour storm event even if the principal spillway is inoperable. <br />Consequently, Pond 004 is constructed and operated in compliance with Rule 4.05.9(2)(c)(ii). <br />The as -built configuration following replacement of the principal spillway is found in the Seneca II <br />Mine permit as Exhibit 7 -12A, Pond 004 Principal Spillway Replacement As- Built. The replacement <br />• spillway as -built drawing was certified by Mr. Robert J. Shevling, Colorado registered Professional <br />Engineer No. 373 82. <br />2007 Pond 004 Sedimentology Demonstration - SCC revised the SEDCAD4 demonstration in 2007 to <br />update the volume of allowable sediment storage in Pond 004. With the cessation of mining <br />operations and subsequent revegetation of disturbed areas, the annual sediment yield to the pond has <br />decreased significantly and, consequently, it is not necessary to maintain the excessive sediment <br />storage capacity which was used to capture the large volumes of sediment generated from the <br />disturbed and barren areas during coal resource removal operations. <br />The as- built, permanent pond, storage capacity evaluation, approved by CDRMS as part of Seneca H <br />Arsine Technical Revision No. 42 (TR -42) in September 2007, was supervised and certified by Mr. <br />David A. Hettinger, Colorado Professional Engineer, No. 18406. <br />2009 Pond 004 Watershed Area Adjustment - SCC corrected the Pond 004 watershed boundary in <br />response to field observations made during a Phase II bond release inspection in August 2009. The <br />field observations noted that landscape elements which had been included in the Pond 004 watershed <br />area actually drained to Pond 008. The only changes between the TR -42 SEDCAD demonstration and <br />18 the August 2009 SEDCAD demonstration are changes to the watershed areas to reflect the corrected <br />watershed boundary. <br />2.05.3 -E2.C4 11/09 <br />