2021-04-05_PERMIT FILE - C1981019A (17)

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Appendix Exh. 7-14S Section 16 Pond, Temporary, and Permanent Channels The location of the Section 16 Pond is presented on Map 12. The profiles of the associated permanent channels associated with the Section 16 Pond watershed are presented on Map 33. These channels consist of the Section 16 Fill Ditch and Trib F, and their design information is presented in this appendix. The as-built configuration for the Section 16 Pond is presented on Figure Exh. 7-145-1, and Figure Exh. 7-145-2 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. Section 16 Pond The following pages present the results of the SEDCADTM models for the hydrologic conditions under the post mining condition. At this stage the oldest reclamation (most Phase III released) is on the eastern extent of the reclaimed Section 16 Pit, and the younger (topsoil and seeded) reclamation is the western reaches of the Section 16 Pond watershed. The SEDCADTM model herein provides the results of the 10 year 24 hour design storm and demonstrates the Section 16 Pond will meet the applicable settleable solids standard under this modeled storm event. The second SEDCADTM model demonstrates that the Section 16 Pond emergency spillway elevation is capable of containing the 25 year 24 hour storm. For the post mining case at the Section 16 Pond, the 10 year 24 hour storm produces 1.6- acre feet of runoff, and the seattlable solids discharge is 0.0 ml/l. The 25 year 24 hour storm event peaks at the 7,751.4' elevation, which is below the emergency spillway elevation of 7,754.5. West and East Section 16 Ditches (Temporary) The East Section 16 Ditch is designed to be trapezoidal in cross section with a 2-foot bottom width, left side slope ratio of 1 H:1 V, a right side slopes ratio of 7H:1 V, and depth of at least 0.5 feet. The West Section 16 Ditch is a vegetated channel. Page Exh. 7-145-2 Revision Date: 1/20/21 Revision No.: TR-145