2009-10-06_PERMIT FILE - C1980005 (2)

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SCC corrected the Pond 008 watershed boundary in response to field • observations made during a Phase II bond release inspection in August 2009. The field observations noted that landscape areas which had been included in the Pond 004 watershed area actually drained to Pond 008. An August 2009 survey of the two watersheds corrected the drainage divide and shows the contributing watershed to Pond 008 is 302 acres. Approximately 262 acres were disturbed by mining activities and have been reclaimed and revegetated. The As-Built Report was revised in August 2009 to reflect the corrected contributing watershed area. There were no modifications to the pond configuration, embankment or spillway structures. New Principal Spillway Design Analysis In response to the deteriorated condition of the principal spillway noted in April 2005, SCC prepared a redesign and replacement plan for a new principal spillway in June 2005 as part of TR-39. The pond was designed to meet or exceed the design and performance requirements of the Regulations of the Colorado Mined Land Reclamation Board for Coal Mining and adhere to the following specifications: • 1) Provide storage and treatment for the runoff to the pond from a 10- year, 24-hour storm event to assure compliance with receiving water quality standards in accordance with Rule 4.05.6(3)(a). 2) Provide storage for the 10-year, 24-hour storm event without discharging through the emergency spillway in accordance with Rule 4.05.9(2)(b). 3) Provide an emergency spillway capable of passing runoff from the 25- year, 24-hour storm event in accordance with Rule 4.05.9(2)(c)(ii). The design elevation for the new principal spillway is found at the front of Appendix 7-6e, while the design drawing for the replacement principal spillway is presented as Exhibit 7-17D, Pond 008 Principal Spillway Redesign. The SEDCAD4 hydrologic computer model was used for the redesign of the principal spillway. Model inputs were based on reclaimed and revegetated conditions; the design pond configuration was based on a survey conducted during June 2005. Results of the 2005 pond design SEDCAD4 modeling are included in the replacement plan found at the front of Appendix 7-6e. The 2005 pond design was approved by the Colorado Division of Reclamation, Mining and Safety (CDRMS) as part of tR-39 in July 2005; the design was • revised as part of TR-42 and approved by CDRMS in September 2007. The TR- 42 SEDCAD4 demonstration is the model design basis for the August 2009 SEDCAD4 demonstration. The only changes between the TR-42 SEDCAD TR-51 2 Revised 09/09