2014-11-04_PERMIT FILE - C2010089 (4)

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4. Designs for all Ditches in the Permit Area Steps taken in the ditch designs are outlined in Appendix 2.05.3(4) -1, Appendix 2.05.3(4)- 213, and Appendix 2.05.3(4) -8. Designs presented in Appendix 2.05.3(4) -8 for NHN -002 North and South Ditches supersedes designs presented in Appendix 2.05.3(4) -1. 5. Sedimentation Ponds Sediment pond design details are discussed in the Arcadis report (Appendix 2.05.3(4) -1) as well as Appendix 2.05.3(4) -8 for Pond NHN -002. Designs presented in Appendix 2.05.3(4) -8 supersede all Pond NHN -002 designs presented in Appendix 2.05.3(4) -1. In accordance with 4.05.2 and the approved Water Discharge Permit from Colorado Water Quality Control Division, WFC will use Ponds NHN -001, NHN -002 and NHN -003 to prevent, to the extent possible, additional contributions of sediment to stream flow or runoff outside the permit area due to mining disturbance. Please see Appendix 2.05.3(4)- 1 and Appendix 2.05.3(4) -8 for design calculations. In accordance with Rule 4.05.6(3) each sedimentation pond is designed, constructed, and maintained to: a. Provide storage for the runoff or inflow entering the pond as a result of a 10- year, 24 -hour precipitation event and to meet all applicable State and Federal regulations for all effluent limitations before discharge from the permit area. The characteristics of the mine site, reclamation procedures, and on site sediment control practices are also considered in the design. b. Provide adequate sediment storage volume in compliance with the DRMS regulations. c. Install an appropriate non - clogging dewatering device to maintain a 10 -year, 24 -hour storage volume. d. Install appropriate combinations of principal and emergency spillways to safely discharge the runoff from a 25 -year, 24 -hour precipitation or larger event, as required by the DRMS' regulations. The sediment storage capacity of the sedimentation ponds will be periodically monitored to ensure that at least three years of sediment storage is available beneath the principal spillway inlet (based on the sediment storage volume calculated using the M.U.S.L.E. contained within the SEDCAD4 program). When adequate capacity is no longer available, sediment removal or other corrective actions will be taken to restore adequate sediment storage capacity to the pond. Corrective action required to restore adequate sediment storage capacity may include the following: Raise the spillway and redesign the pond in order to restore adequate storage capacity and to comply with the applicable regulations. 2. Excavate the pond area and enlarge the sediment storage capacity. Section 2.05.3(4) Page 4 September 2014 (TR -06)