2011-12-07_REVISION - M1986015 (8)

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submittal. Therefore the channel section and profile slope are sized to perform successfully and carry the 100 year CFS without channel failure due to erosion. Constructing the channel to a 100 year rainfall event equates to only a 1% failure risk each year. A 1% risk is a significantly low chance of channel failure. We believe that the channel as designed provides more than adequate protection against failure. 9.d. Refer to the Memo from Lyman Henn of December, 5, 2011. Exh E — Amendment to the Phase 1 Reclamation Plan 10.a. The Amendment 3 Exh E map of the Phase 1 area shows the contours as they exist now, that is post mining and pre reclamation. Prior to mining, the bottomland consisted of low terraces as one moved away from the River. The terrace inclines were small (3 to 8 feet in height) and the terrace flats between the inclines were very flat (0.2% to 0.4% slope). The gradient of the land along and parallel to the River is from 0.16% to 0.2% slope. The reclamation of the Phase 1 area is to be four separate sediment basins. The slurry fines pumped into the basins will settle horizontally so the slope inside each basin is flat. The elevations of the filled basins will be highest in the northern section (Basin 4) and be slightly lower in the southern section (Basin 2). Page 2 of Exh E states that the land will "have a general slope to the southeast with the northwestern end being about 4 to 5 feet high than the southeastern corner ". Based on our recent monitoring well data, we estimate that the final elevation of the filled Basin 4 will be 4602 feet and the filled Basin 1 will be 4598 feet. So generally speaking, there is a small slope to the southeast while the final land form is the terraced basins. The channels between the basins will allow non - infiltrated precipitation to flow toward Basin 1 and on to the River. 10.b. The interbasin channels between the basins (see the enclosed Exh F -1A - Phase 1 Detail - Revised) will allow non - infiltrated precipitation to flow through the basins to the River. Thus the chances of long term ponding of water around the small cottonwood trees and basal stem rot are reduced. 11. Topsoil will not be distributed on the Sediment Basins in Phase 1. Standard reclamation practice is to allow plants to naturally degenerate on the silt filled basins. 12.a. Since the proposed contours are difficult to see, we have included spot elevations of the final reclaimed surface. Existing elevations along the edge of the basins are noted as ".000" and final elevations of the basins are noted as "x000 ". We have included elevations for top of berm, filled silt basin, and estimated groundwater. 12.b. Channels between the sediment basins in Phase 1 are included on Exh F -1A. The channels will be concrete rubble choked with road base and depressed one foot into the berms to allow vehicle movement along the top of the dikes. See the plan note for more detail. 3