2020-03-24_PERMIT FILE - C1981019 (107)

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Exhibit 7 Streeter Pond The location of Streeter Pond is presented on Map 12. The profiles of the associated permanent ditches, Streeter Gulch Ditch and Buckskin Draw Ditch, are presented on Maps 33 and 33A respectively. Channel designs of these two permanent drainage channels are provided in Exhibit 7, Item 14, Part E. This exhibit contains the SEDCAD model for the 10-year, 24-hour and the 25-year, 24-hour storm events for the Streeter Pond. Exh. 7-14SP, Figure 1 provides the breakdown of the drainage areas within the Streeter Pond watershed, and the corresponding hydrologic conditions for each subwatershed. A summary of the design parameters for the Streeter Pond can be found on Exhibit 7 Table 6. The hydrologic analysis presented herein, is the postmining condition for the Streeter Pond watershed. The final postmining topographic surface and the final locations of the permanent drainage channels as presented on Exh. 7-14SP, Figure 1 were used to model the watershed for the post-mining condition. Three in-stream stock ponds were included in the permanent drainage channel in the Streeter Gulch Ditch to maximize the sediment control and peak flows as the reclaimed watershed progresses toward bond release. These stock ponds are constructed as the final grading of the reclaimed lands and construction of the channels proceeds as shown on Map 29. These stock ponds will be constructed in accordance with design requirements shown on Figure 2.05-6 in Volume 1. The following pages present the SEDCAD model runs for the above described postmining condition. Two design storms were evaluated; the 10-year, 24-hour storm to assess expected effluent quality, and the 25-year, 24-hour storm to assess the capacity of the emergency spillway to safely discharge the design storm. 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 (Volume 2D). Soil erodibility factors and other parameters used in estimating sediment concentration in runoff are also drawn from that exhibit. For the post mining case, a 10-year, 24-hour storm produces 16.82 acre feet of runoff into the Streeter Pond. Settleable solids released from the pond are 0.0 ml/1. The 25-year, 24-hour storm does not engage the emergency spillway at the 6,588 elevation. The following items are also found in this exhibit: Figures: • Exh. 7-14SP, Figure 1, Streeter Pond Watershed Areas Postmining SEDCAD Model output results: • SEDCAD Model Post Mine Case, 10 Year-24 Hour storm • SEDCAD Model Post Mine Case, 25 Year-24 Hour storm Exh. 7-SP-1 Revision Date: 12/20/19 Revision No.: TR-135