2018-01-31_PERMIT FILE - C1981019A (8)

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W EXHIBIT 7, ITEM 20, PART C WEST TAYLOR SEDIMENTATION POND The location of the West Taylor Sedimentation Pond is presented on Map 41A. The profiles of the associated permanent ditches (West Taylor East and West Tributary Ditches) are presented on Map 33B. While this exhibit contains channel design information in the SEDCAD model the actual channel design for the two post mine channels for this watershed can be found in Exhibit 7, Item 20, Part E. A design drawing for the impoundment is presented on Figure IC -3. For the active mining case, Figure IC -4 shows the breakdown of drainage areas and hydrologic conditions for this sediment pond. For the post mining case, Figure 1C-5 shows the breakdown of drainage areas and hydrologic conditions for this sediment pond. Table 4 of the introductory text of Exhibit 7, Item 20 provides a summary of the design parameters for this sediment pond. The following pages present the results of two SEDCAD models for the worst case hydrologic conditions under two distinct mining time frames. The first set of SEDCAD models, present the conditions when the majority of the watershed is intercepted by the active pit, but the reclaimed lands closest to the sediment pond are in the early stages of revegetation. The second set, describes the conditions upon completion of mining with the majority of the disturbed portions of the watershed now with several years of vegetation growth, but the entire watershed reporting to the pond. At this stage the most recent reclamation is near the northern end of the pit in the early stage of reclamation, that is, topsoiled and seeded. In each case, the first SEDCAD model provides the results of the 10 year, 24 hour design storm and demonstrates the system will meet the applicable settleable solids standard. The second SECCAD model in each case demonstrates that the sediment pond's emergency spillway is capable of safely passing the spillway design flood, in this case the 25 year, 24 hour storm. The final postmining topographic surface and the final locations of the permanent drainage channels as presented on Map 41A were used to model the watershed for the post -mining condition. Two in -stream stock ponds were included in the permanent drainage channels (West Taylor West Tributary and West Taylor East Tributary) 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 to the northend of the final pit. 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 computer runs for the above described two distinct mining phases. Two design storms were evaluated for the active mining case; a 10 year, 24 hour storm to assess expected effluent quality, and a 25 year, 24 hour storm to assess the capacity of the emergency spillway to safely discharge the design storm. The 10 year, 24 hour storm was also evaluated for the post mining case to assess expected effluent quality, and a 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 active mining scenario, approximately 85% of the watershed is intercepted by the active pit, and the 10 year, 24 hour storm produces 0.52 acre feet of runoff at the West Taylor Sedimentation pond. As summarized in Table 4, peak reservoir stage while passing this storm is at elevation 7471.16, about 0.16 feet above the overflow of the primary (drop inlet) spillway and 4.2 feet below the invert of the emergency spillway. Settleable solids released from the pond in the downstream "null" modeling node Exh. 7-20C-1 Revision Date: 11/15/17 Revision No.: TR -117