2022-07-12_PERMIT FILE - C1981019A (17)

Home Browse Search

Exh. 7-23I-2 Revision Date: 4/28/22 Revision No.: TR-154 on Map 33C. The temporary channels that route water to the Section 25 and Coal Stockpile Ponds are presented on 41B. Seven 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 Coal Stockpile Pond is total containment pond during reclamation activities with surface water runoff from reclamation and non-disturbed areas being routed to it through the D-5 Ditch. During reclamation the Coal Stockpile Pond will fully contain the 10-year 24-hour and 25-year 24-hour storm events. The Coal Stockpile Pond is necessary to provide sediment control in a fairly large, young, reclaimed area to assist in controlling surface water runoff and add an additional layer of protection to the Section 25 Pond. The D-5 Ditch will constructed prior to reclamation activities commencing in the Coal Stockpile Pond watershed shown on Figure 7-23I-4. The 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 and eastern reaches of the reconstructed Little Collom Gulch. 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 5.4 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 6,874.3 elevation, which below the emergency spillway elevation of 6,875.0’ Temporary Channels Two temporary channels, the D-5 and D-7 Ditches, will be constructed during reclamation activities to route surface water runoff from reclaimed and non-disturbed areas to the Coal