2021-04-05_PERMIT FILE - C1981019A (19)

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Exhibit 7, Item 20, Part B West Taylor Pond and Channels The location of the West Taylor Pond is presented on Map 12. The profiles of the associated permanent channels associated with the West Taylor Pond watershed are presented on Map 33B. These channels consist of the West Fork Taylor Ditch, East Fork Taylor Ditch, Trib 1 through Trib 6 ditches and the terrace ditches that will be constructed across the West Taylor Fill. The permanent channel design information for all channels is presented in this exhibit. The as-built configuration for the West Taylor Pond is presented on Figure Exh. 7-2013-1, and Figure Exh. 720B-2 provides the breakdown of drainage areas and hydrologic conditions for this sediment pond in the post mining condition. Exhibit 7, Item 14 in Volume 2D describes the hydrologic methodology used in sediment pond and post mine channel assumptions. Runoff curve numbers assigned to the undisturbed and reclaimed lands in various stages of reclamation have been selected in accordance with Table 1 in the Introductory Text for Exhibit 7 in Volume 2D. For channels protected by a riprap liner, selection of minimum riprap size is done using the Simons/OSM method in SEDCADTM. For channels to be protected by a vegetative liner, the permissible velocities are also determined using SEDCADTM routines. West Taylor Pond The following pages present the results of the SEDCADTM models for the post mining condition. At this stage the oldest reclamation is on the eastern and southern extent of the reclaimed South Taylor Pit, and the younger (topsoil and seeded) reclamation is the northern reaches of the West Taylor Pond watershed very near the West Taylor Pond. The SEDCADTM model herein provides the results of the 10 year 24 hour design storm and demonstrates the West Taylor Pond will meet the applicable settleable solids standard under this modeled storm event. The second SEDCADTM model demonstrates that the West Taylor Pond emergency spillway elevation is capable of containing the 25 year 24 hour storm. The final post mining topographic surface and the final locations of the permanent drainage channels as presented on Map 12 were used to model the watershed for the post-mining condition. Four in-stream stock ponds (WFSP-1, WFSP-2, EFSP-1, and, EFSP-2) are included in the permanent channels to decrease peak flows from the modeled storm events, and to provide a water source to support the post-mining land use. A typical design for of each stock pond in the West Taylor Pond watershed are provided on Figure 2.05-6 in Volume 1. However, depending on the ground conditions encountered during reclamation, these stockponds may be constructed similar to ETD-1 (please see Figure Exh. 7-20ET-3) in the East Taylor watershed. In summary, the post mining case at the West Taylor Pond, the 10 year 24 hour storm produces 5.10 acre feet of runoff, and the seattlable solids discharge is 0.2 ml/l. The 25 year 24 hour storm event peaks at the 7,473.71' elevation, which is well below the emergency spill way elevation of 7,475.0'. Exh. 7-20B-1 Revision Date: 2/17/21 Revision No.: TR-145