2023-12-14_PERMIT FILE - C1981019 (17)

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Exh. 7-23I-2 Revision Date: 10/23/23 Revision No.: TR-161 on Map 33C. The temporary channels that route water to the Section 25 Pond are presented on 41B. Eight post mine channels will be constructed in the Section 25 watershed. In the 10-Year 24-Hour and 25-Year 24-Hour modeled events, Lower and Middle Bobcat Draw are not included as they will be reconstructed within the footprints of the Middle and Section 36 Ponds respectively when both are reclaimed. A separate hydrology model demonstration is included for the post mine channel design and is discussion later in this exhibit. The as-built configuration for the Middle Pond is presented on Figure 7-23I-1, the as-built configuration for the Section 36 Pond is presented on Figure 7-23I-2, the as-built configuration for the Section 25 Pond is presented on Figure 7-23I-3, and the as-built configuration for the Coal Stockpile Pond is presented in Exhibit 7, Item 23 Part M, Figure 7-26M-1. Figure 7-23I-4 provides the breakdown of drainage areas and hydrologic conditions for all areas in the watershed reporting to these four sediment ponds. The Middle and Section 36 Ponds are part of the Section 25 Pond sediment control system and are utilized to reduced peak flows and provide additional sediment storage if need. All discharges from the Middle and Section 36 Ponds flow directly to the Section 25 Pond through a natural channel, and the Section 25 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 25 watershed has been reclaimed with the newest or younger (topsoil and seeded) reclamation near the northern eastern reaches of the Upper Bobcat Draw. The 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 25 Pond will meet the applicable settleable solids standard under this modeled storm event. The second SEDCADTM model demonstrates that the Section 25 Pond emergency spillway elevation is capable of containing the 25-year 24-hour storm. In summary, for the post mining case at the Section 25 Pond, the 10-year 24-hour storm produces 9.5 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 6,875.07 elevation, which is right at the emergency spillway elevation of 6,875.0’ Permanent Post Mine Channels The Section 25 Pond watershed is comprised of eight post-mining channels which are Little Collom Gulch, Sage Draw, Dusky Draw, Grouse Draw, Sharpie Draw, Lower, Middle, and Upper Bobcat Draws respectively. The locations of these post mine channels are presented on Map 41B and Figure 7-23I-5, and the channel profiles are presented on Map 33C. Little Collom Gulch The area contributory to Little Collom Gulch is greater than one square mile, and the lower portion of the channel passes over a permanent valley fill; therefore, the modeled storm event required by rule is the 100-year 24-hour event. The entire length of Little Collom Gulch will be riprap lined. Four stock ponds LCG-1, LGC-2, LCG-3, and LCG-4 will be constructed inline in the channel to