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EXHIBIT 7, ITEM 20, PART A <br />SECTION 28 SEDIMENTATION POND <br />The location of the Section 28 Sedimentation Pond is presented on Map 41A. The profiles of the <br />associated permanent ditches (Trib. A, Trib. C and Trib. D) are presented on Map 3313. Channel <br />designs of these permanent drainage channels are provided in Exhibit 7, Item 20, Part E. Figure <br />IA -3 presents the original design drawing. Exhibit 7, Item 20, Figure IA -4 shows the <br />breakdown of drainage areas and hydrologic conditions for this sediment pond, and Figure 1 A-5 <br />provides a diagram of the structures and subwatersheds. <br />Section 28 Sedimentation Pond was initially submitted with PR -02 in March 2007. Changes in <br />the limits of mining activity, essentially pulling the southeastern limit of disturbance back to the <br />northwest about one quarter mile, and revisions to the PMT pursuant to TR -81 resulted in <br />changes to the watershed that warranted a review of the pond hydrology and hydraulics for TR - <br />81. Revisions to the PMT pursuant to TR -105 have required yet another review of the hydrology <br />and hydraulics, although the TR -105 change represents simply a small reduction (— 4%) in <br />overall watershed area as compared to the previously approved TR -81 scenario. <br />Runoff Curve numbers assigned to the undisturbed and reclaimed lands in various stages of <br />reclamation have been selected in accordance with Table 1 in the Introductory Text for Exhibit 7 <br />(Vol. 2D). Soil erodibility factors and other parameters used in estimating sediment <br />concentration in runoff are also drawn from that exhibit. <br />The following pages present the results of two SEDCAD runs for the worst case hydrologic <br />condition, which is taken as the entire tributary watershed reporting but reclamation in various <br />stages of revegetation. The first run provides the results of the 10 year, 24 hour design storm. <br />The 10 year, 24 hour event generates about 19.7 cfs of peak runoff into the pond and a total <br />runoff volume of about 2.1 acre-feet. Settleable solids discharged from the pond during the 10 <br />year 24 hour storm are 0.0 ml/1, well below the allowable standard. The second model run <br />documents the results of the 100 year, 24 hour design event, which has been selected to evaluate <br />the emergency spillway. However, the peak elevation of the pond surface while routing the 100 <br />year, 24 hour storm does not rise high enough to engage the emergency spillway. <br />Table 4 of the introductory text of Exhibit 7, Item 20 provides a summary of the design <br />parameters for this sediment pond. <br />Exh. 7-20A-1 Revision Date: 02/15/15 <br />Revision No.: TR -105 <br />