2021-04-05_PERMIT FILE - C1981019A (18)

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Appendix Exh. 7-14SP Streeter Pond and Permanent Channels The location of the Streeter Pond is presented on Map 12. The profiles of the associated permanent channels associated with the Streeter Pond watershed are presented on Map 33. These channels consist of the Streeter and Buckskin Ditches, and their design information is presented in this appendix. The as-built configuration for the Streeter Pond is presented on Figure Exh. 7-14SP-1, and Figure Exh. 7-14SP-2 provides the breakdown of drainage areas and hydrologic conditions for this sediment pond. 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. Streeter 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 northern extent of the reclaimed West Pit, and the younger (topsoil and seeded) reclamation is the southern reaches of the Streeter Pond watershed near the convergence of the West and South Taylor Pits. The SEDCADTM model herein provides the results of the 10 year 24 hour design storm and demonstrates the Streeter Pond will meet the applicable settleable solids standard under this modeled storm event. The second SEDCADTM model demonstrates that the East 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. Three in-stream stock ponds (SD-1, SD-2, and SD-3) are included in the permanent channels to decrease peak flows from the modeled storm event, and to provide a water source to support the post-mining land use. A typical design for these stock ponds is presented on Figure 2.05-6 in Volume 1; however, each stock pond will ultimately be design and constructed to account for local ground conditions present in the post mine topography at the time of construction. In summary, for the post mining case at the Streeter Pond, the 10 year 24 hour storm produces 2.60 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 6,587.9 elevation, which is below the emergency spillway elevation of 6,588.0' Exh. 7-14SP-1 Revision Date: 1/20/21 Revision No.: TR-145