2025-06-30_PERMIT FILE - C1981019 (5)

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RULE 4 PERFORMANCE STANDARDS Rule 4 Performance Standards 4-12 Revision Date: 4/9/25 Revision No.: MR-261 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) or fully contain the 100-year, 24-hour event. As such, the general operation of the ponds that are designed to discharge will be a passive discharge system where water is allowed to discharge automatically as necessary. Ponds designed to the 100-year, 24-hour storm event will fully contain the storm event and be managed as non-discharging structures. 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 Volumes 2D and 2E Exhibit 7. 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 ponds associate with the Collom Pit be found in Exhibit 7, Item 23. For compliance purposes, in systems that incorporate ponds in a series, CDPS effluent quality parameters will only apply to the last pond in the series that directly discharges into the receiving stream or drainage. Out of pit designed ponds internal to the last pond in the series will be inspected and maintained in the same manner as the “compliance” pond to ensure proper sediment control and design performance. The sumps along Haul Road A are required to have flow measurement devices at the outflow of each sump. Each sump therefore will have a flume installed at the outflow to measure any flow discharged from each sump. It was deemed necessary to install a french drain structure to monitor the flows from the East Taylor Spring. The East Taylor Spring is just above (south) of the East Taylor Pond. A clean washed gravel filter was installed to route water to an underground-perforated pipe. The perforated pipe then routes from the seep water to the surface to a parshall flume for flow measurements. At the outlet of the parshall flume, the water is then routed through another twelve-inch solid pipe, which is on the ground surface directly adjacent (west) to the Taylor Trib Ditch. The pipe ends at the inlet of the East Taylor Pond were the spring water is discharged into the East Taylor Pond. The location and a design for this french drain system is presented on Figure 4.05.7-1. 4.05.8 Acid-forming and Toxic-forming Spoil Acid forming materials do not exist within the overburden to be removed by the mining operations. A discussion on the overburden at the Colowyo operation has been conducted as set forth in Section 2.04.6. A discussion of the overburden monitoring plan is set forth in Section 2.05.