2023-05-26_PERMIT FILE - C1981019 (2)

Home Browse Search

Exh. 7-14PP-1 Revision Date: 1/30/23 Revision No.: TR-158 Appendix Exh. 7-14PP Prospect Pond The location of Prospect Pond is presented on Map 12, and the as-built configuration of the pond is presented on Figure Exh. 7-14PP-1. The profiles of the associated permanent channels associated with the Prospect Pond watershed are presented on Map 33. These channels consist of Prospect Ditch, North Tributary East Pit Ditch, EP3 Inlet and Outlet Ditch, and the Final East Pit Ditch. The design information for these post mine channels is presented in this appendix. The Prospect Pond watershed is also comprised of six post-mine stock ponds including PD1, PD2, NTEP1, EP1, EP2, and EP3. The design and as-built information for all these stock ponds can be found on Figure Exh. 7-14PP-3. 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. Prospect 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 extent of the reclaimed East Pit, and the younger (topsoil and seeded) reclamation is the far western reaches of the Prospect Pond watershed. The SEDCADTM model herein provides the results of the 10-year, 24-hour and 25-year, 24-hour design storms and demonstrates the Prospect Pond can adequately handle both modeled storm events. 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. The six in-stream stock ponds 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. Several specific items to note for the model for Prospect Pond. First, the curve numbers for sub- watersheds contributing the Section 16 Fill Ditch and Prospect Collection Ditch had to be artificially inflated to produce flows, since the approved curve numbers for the corresponding land conditions in those two watersheds did not produce enough flows for SEDCADTM to model them appropriately. Second, EP1 stock pond is designed with two emergency spillways and one straight pipe and water is routed to two different locations in the model. Due to the limitations of the routing routines in SEDCADTM, the EP1 stock pond (Structure #24) is modeled as two stock ponds with half toeh full capacity be applied to EP1 and to a dummy EP1 stock pond. A dummy EP1 stock pond (Structure #34) was included with an artificial subwatershed to ensure the model