2024-08-22_PERMIT FILE - C1981019A

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RULE 4 PERFORMANCE STANDARDS Rule 4 Performance Standards 4-12 Revision Date: 8/15/22 Revision No.: TR-155 area exemption.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) 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.