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sedimentology computer model was used to calculate the 10 -year, 24 -hour and 25 -year, 24 -hour storm <br />event runoff for sizing the 1986 pond configuration. The evaluation indicated that the principal <br />spillway needed a modification to satisfy the performance requirements. It was determined that a 6- <br />inch diameter pipe was necessary to increase detention time and maintain an adequate storage volume <br />above the peak stage for pit pumpage. <br />The 1986 pond evaluation was certified by Mr. Ronald J. Gehrke, a registered professional engineer in <br />the State of Colorado (P.E. No. 19959) experienced in the design and construction of earthen <br />structures. The certified as -built drawing for the 1986 pond evaluation is found in the Seneca H Mine <br />permit as Exhibit 7 -12, Pond #004. <br />2005 Principal Spillway Design Analysis — In response to the deteriorated condition of the principal <br />spillway observed in April 2005, SCC prepared a design and replacement plan for a new principal <br />spillway in June 2005 as part of TR -37. <br />The design evaluation for the replacement principal spillway is found in the Seneca 11 Mine permit in <br />Appendix 7 -6c. The SEDCAD4 hydrologic computer model was used for the replacement spillway <br />design. Model inputs were based on current watershed conditions (i.e., reclaimed and revegetated); <br />the design pond configuration was based on a survey conducted during June 2005. The design <br />drawing for the replacement principal spillway is found in the Seneca II Mine permit as Exhibit 7 -21, <br />Pond 004 Principal Spillway Re- design. Both the replacement spillway design analysis and drawing <br />were certified by Mr. John Pellicer, a registered professional engineer in the State of Colorado <br />experienced in the design and construction of earthen structures. <br />2005 Principal Spillway Replacement — The 36 -inch CMP barrel was removed and replaced with an fl- <br />inch plastic corrugated pipe. Bedding material was backfilled in the trench to the appropriate elevation <br />(an invert elevation of 6,948.2 feet) and after the pipe was installed, the trench material was compacted <br />back around the pipe. It should be noted that the replacement of the principal spillway did not affect <br />or include the existing emergency spillway. <br />An as -built survey was completed in September 2005. Using the as -built survey data, SEDCAD4 was <br />used to check the replacement spillway as -built configuration and show that the pond meets or exceeds <br />the performance requirements. The survey indicated a permanent pool elevation of 6,948.2 ft, with a <br />storage capacity of 7.6 acre -feet (ac -ft). The SEDCAD4 modeling indicated that runoff volume and <br />peak inflow for the 10 -year, 24 -hour storm event are 2.10 ac -ft and 4.25 cubic feet per second (cfs), <br />respectively, with a calculated peak stage of 6,949.0 feet. The corresponding peak discharge from the <br />10 -year, 24 -hour storm event was estimated as 0.7 cfs. The SEDCAD4 results also indicated the <br />average discharge settleable solids concentration during the peak 24 -hour period will be 0.0 milliliters <br />per liter (ml/1). The Water Quality Control Commission (WQCC) settleable solids effluent <br />• concentration limit is 0.5 ml /1. The SEDCAD4 modeling demonstrated there is sufficient storage <br />capacity available, and that the settleable solids effluent concentration is less than the applicable water <br />2.05.3 -E2.C3 11/09 <br />