2015-09-09_PERMIT FILE - C1981019A

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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 3313. Channel design of these permanent drainage channels is provided in Exhibit 7, Item 20, Part E. A design drawing for the impoundment is presented on Figure 1C-3. For the active mining case, Figure 1C-4 shows the breakdown of drainage areas and hydrologic conditions for this sediment pond, and Figure 1C- 5 provides a diagram of the structures and networking. For the post mining case, Figure 1C-6 shows the breakdown of drainage areas and hydrologic conditions for this sediment pond, and Figure 1 C-7 provides a diagram of the structures and networking. 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 runs for the worst case hydrologic conditions under two distinct mining time frames. The first set of SEDCAD models, which were developed as part of TR -105 described in more detail below, 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, developed pursuant to TR -105, 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 at the headwaters of the watershed is in the early stage of reclamation, that is, 1-2 year growth. In each case, the first SEDCAD run provides the results of the 10 year, 24 hour design storm and demonstrates the system will meet the applicable settleable solids standard. The second model run 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. Revisions Pursuant to TR -105 The final postmining topographic surface associated with TR -105 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 tanks were included in the permanent drainage channels (West Taylor West Tributary and West Taylor East Tributary) to maximize the sediment control as the reclaimed watershed progresses toward bond release. These stock tanks will be constructed as the final grading of the reclaimed lands and construction of the channels proceeds to the south. These stock tanks 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. 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 (Vol. 2D). Soil erodibility factors and other parameters used in estimating sediment concentration in runoff are also drawn from that exhibit. For the intermediate (active mining) scenario, with approximately 70% of the watershed intercepted by the active mine pit, the 10 year, 24 hour storm produces 3.4 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 7471.74, about 0.74 feet above the overflow of the primary (drop inlet) spillway and 4.26 feet below the invert of the emergency spillway. Settleable solids released from the pond in the downstream Exh. 7-20C-1 Revision Date: 06/15/15 Revision No.: TR -105