2023-12-14_PERMIT FILE - C1981019 (18)

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Exh. 7-23J-2 Revision Date: 10/23/23 Revision No.: TR-161 The as-built configuration for the Section 26 Pond is presented on Figure 7-23J-1, and the as-built configuration for the Sidehill Pond is presented on Figure 7-23J-2. Figure 7-23J-3 provides the breakdown of drainage areas and hydrologic conditions for all areas in the watershed reporting to these two sediment ponds. The Sidehill Pond as part of the Section 26 Pond sediment control system is utilized to reduced peak flows and provide additional sediment storage if need. All discharges from the Sidehill Pond flow directly to the Section 26 Pond through the D-1 Ditch, and the Section 26 Pond is the final discharge location. The following pages present the results of the SEDCADTM models for the post mining condition. At this stage all areas that have been disturbed by mining activities in the Section 26 watershed has been reclaimed with the reclamation areas having three plus years growth in the majority of the Section 26 Pond watershed. The farthest northern portion of the watershed is relatively undisturbed. The SEDCADTM model herein provides the results of the 10-year 24-hour design storm and demonstrates the Section 26 Pond will meet the applicable settleable solids standard under this modeled storm event. The second SEDCADTM model demonstrates that the Section 26 Pond emergency spillway elevation is capable of containing the 25-year 24-hour storm. In summary, for the post mining case at the Section 26 Pond, the 10-year 24-hour storm produces 0.3 acre feet of runoff, and the peak settleable solids concentration is 0.00 ml/l. The 25-year 24- hour storm event peaks at the 6,983.1 elevation, which below the emergency spillway elevation of 6,986.0’ Section 26 Pond Permanent Post Mine Channels The Section 26 Pond watershed is comprised of two post-mining channels which include Bear Draw and Lower Bear Draw. The locations of these post mine channels are presented on Map 41B and Figure 7-23J-4, and the channel profiles are presented on Map 33C. The area contributory to Bear Draw and Little Bear Draw are less than one square mile; therefore, the modeled storm event required by rule is the 10-year 24-hour event. However, for built in conservatism, both channels will are modeled against the 25-year 24-hour storm event. The upper reaches of Bear Draw will be a vegetative lined while the lowest most portion will be riprap lined. Lower Bear Draw will be protected with riprap over the entire length of the channel. Two stock ponds, BD-1 and BD-2 will be constructed inline in the Bear Draw to reduce peak flows from larger storm events and provide a water source to support the post mine land use. Lower Bear Draw will be constructed inside the reclaimed footprint of the Section 26 Pond. Therefore, in the 25-Year 24-Hour model, it is assumed the entire watersheds reporting to both the Sidehill and Section 26 Ponds has been approved for Phase II bond release and both ponds are no longer necessary for sediment control.