2024-11-01_PERMIT FILE - C1981019A

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Exh. 7-14S3-1 Revision Date: 9/18/24 Revision No.: TR-164 Appendix Exh. 7-14S3 Upper and Lower Section 3 Ponds The location of the Upper and Lower Section 3 Ponds are presented on Map 12. These two ponds will eventually replace the Streeter Pond as the primary sediment controls structures in the Streeter Ditch watershed. The profiles of the associated permanent channels associated with the Upper and Lower Section 3 Ponds 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 design configuration for the Upper and Lower Section 3 Ponds are presented on Figure Exh. 7-14S3-1, and Figure Exh. 7-14S3-2 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 SEDCAD TM. For channels to be protected by a vegetative liner, the permissible velocities are also determined using SEDCADTM routines. Upper and Lower Section 3 Ponds The following pages present the results of the SEDCADTM models for the post mining condition. At this stage the oldest reclamation is on the southern extent of the reclaimed West Pit, and the younger (topsoil and seeded) reclamation is the northern reaches of the Upper and Lower Section 3 Ponds watershed. The SEDCADTM model herein provides the results of the 100-year, 24-hour design storm and demonstrates the Lower Section 3 Pond will fully contain the modeled event. 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, the 100-year, 24-hour storm produces 106.83 acre-feet of runoff into the Upper Section 3 Pond. The storm event peaks in the Lower Section 3 Pond at 7,146.5’, which is below the emergency spillway elevation (7,147’).