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EXHIBIT 7, ITEM 20, PART C <br />WEST TAYLOR SEDIMENTATION POND <br />The location of the West Taylor Sedimentation Pond is presented on Map 41A. The profiles of the <br />associated permanent ditches (West Taylor East and West Tributary Ditches, and East Taylor East and <br />West Tributary Ditches) are presented on Map 3313. Channel design of these permanent drainage <br />channels is provided in Exhibit 7, Item 20, Part E. Exhibit 7, Item 20, Figure 1C -1 shows the breakdown <br />of drainage areas and hydrologic conditions for this sediment pond, and Figure 1C -2 provides a diagram <br />of the structures and networking. A design drawing for the impoundment is presented on Figure 1C -3. <br />To minimize the size of the structure needed, a variety of control features will be utilized. Five short-term <br />sediment traps will be utilized along the edge of the topsoil replacement limit. It is expected that these <br />sediment traps will be relocated as the topsoil replacement progresses. Thus the life of these structures <br />will generally be less than one year. These sediment traps will be subgrade (excavated by dozing) and will <br />have a maximum capacity of 2.0 acre -feet. The sediment traps will typically be rectangular in shape with <br />a maximum depth of 10 feet and a maximum of 211: IV side slopes. They will be evacuated by pumping <br />when needed. The design also includes the construction of six permanent stock ponds in the re- <br />established permanent channels in the post- mining topography, as shown on Figures 1C-4, and 1C -5. <br />Each will have a maximum capacity of 0.50 acre -feet and will be similar in nature to those currently <br />constructed. <br />The following pages present the results of two SEDCAD runs for the worst case hydrologic conditions <br />under two distinct mining time frames. The first set of SEDCAD models, which were developed as part <br />of TR -88 described in more detail below, present the conditions when the majority of the watershed is <br />intercepted by the active pit, but the reclaimed lands closest to the sediment pond are in the early stages of <br />10 revegetation. The second set, developed pursuant to TR -81, describes the conditions upon completion of <br />mining with the majority of the disturbed portions of the watershed now with several years of vegetation <br />growth, but the entire watershed reporting to the pond. At this stage the most recent reclamation at the <br />headwaters of the watershed in the initial stage of reclamation, that is, topsoiled and seeded. In each case, <br />the first SEDCAD run provides the results of the 10 year, 24 hour design storm and demonstrates the <br />system will meet the applicable settleable solids standard. The second model run in each case <br />demonstrates that the sediment pond's emergency spillway is capable of safely passing the spillway <br />design flood, in this case the 25 year, 24 hour storm. <br />Table 4 of the introductory text of Exhibit 7, Item 20 provides a summary of the design parameters for <br />this sediment pond. <br />Revisions Pursuant to TR -88 <br />There are no physical changes to the West Taylor Sediment Pond pursuant to TR -88. However, an <br />intermediate case has been evaluated for the condition wherein the base of the West Taylor Fill has been <br />constructed but has not yet been seeded. In addition, some active mining areas within the watershed also <br />report to the pond. At the same time, about two- thirds of the overall tributary watershed is intercepted by <br />the active pit and does not report to the sediment pond. Because the watershed is greatly reduced in area <br />for this condition, it is by inspection not the critical case with respect to spillway hydraulics. However, <br />from a sedimentology perspective it still could be the critical case. This intermediate active mining <br />condition, including tributary areas and associated runoff Curve numbers (CN) for this scenario, is <br />depicted on Figures 1C -4 and 1C -5 See pages 7- 20C -83 and 7- 20C -84). <br />The SEDCAD modeling was redone (see page Exh. 7- 20C -85) using the CNs as specified in TR -73, with <br />disturbed lands close to the sediment pond but a smaller direct watershed than included in the post- mining <br />analysis. A normally dry temporary impoundment approximately 0.5 acre feet in size immediately <br />Exh. 7 -20C -1 Revision Date: 12/31/10 <br />Revision No.: TR -81 <br />