2019-04-16_REVISION - C1981019 (3)

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RULE 4 PERFORMANCE STANDARDS Rule 4 Performance Standards 4-12 Revision Date: 4/16/19 Revision No.: MR-200 All ponds will be designed so that the minimum elevation at the top of the settled embankment is at least one foot above the elevation of the water surface in the pond with the emergency spillway flowing at design depth. Colowyo will design, construct and maintain the sedimentation ponds to prevent short circuiting to the extent possible. As a general rule, the inflow to the ponds will be at the opposite end from the outflow area. The constructed height of the sedimentation pond embankment will be designed to allow for settling. During construction, a registered professional engineer will ensure that the appropriate embankment height is accomplished. For all sedimentation ponds, the entire embankment, including the surrounding areas disturbed by construction, will be seeded after the embankment is completed. The active upstream side of the embankment where water will be impounded will be riprapped or otherwise stabilized, where necessary. Areas in which revegetation is not successful or, where rills and gullies develop, will be repaired and revegetated. Sedimentation ponds that are subsequently approved as part of the post-mining land use will remain in place after the termination of mining activities. Colowyo will inspect the condition of each pond and submit a quarterly report. None of Colowyo’s ponds meet the size criteria of 30 CFR 77.216(a)(1989). A small area exemption for sediment control will be utilized for sediment control for the remaining reclamation area in the Gulch A watershed that has not achieved Phase II or Phase III bond release. This small area exemption allows for a grass filter to be utilized for sediment control for the small area exemption, and for Colowyo to reclaim the Gulch A Pond and it’s associated diversion ditches. Please see Volume 2C, Item 18 for the small area exemption demonstration (grass filter) for the Gulch A watershed. 4.05.7 Discharge Structures The sedimentation ponds at Colowyo are designed to treat the 10-year, 24-hour storm event in accordance with Rule 4.05.6(3)(a). As such, the general operation of the ponds will be a passive discharge system where water is allowed to discharge automatically as necessary. Colowyo will sample discharges as appropriate to remain in compliance with applicable CDPS Permit requirements. Pond dewatering through a manual headgate may be performed as necessary to lower the water level depending on operational requirements. Manual dewatering of ponds will meet applicable CDPS Permit standards. Discharge from sedimentation ponds will be controlled by energy dissipaters and flow check devices where necessary. All embankment ponds utilize separate principal and emergency spillways with the emergency spillway located at a minimum of 1 foot above the elevation of the maximum water surface during the discharge of the 10-year, 24- hour storm event through the principal spillway. The principal spillways are designed for the 10- year, 24-hour storm event and the emergency spillways are designed to pass the 25-year, 24-hour storm event in accordance with Rule 4.05.9(2)(c)(i). The design requirements for existing ponds can be found on each of the pond as-built drawings or in Exhibit 7, Item 15 of the existing permit document. All embankment sedimentation ponds will provide a non-clogging dewatering device or conduit spillway to remove water storage from inflow. Design requirements for all sediment