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A9r. ddike l3arlay <br />Colorndo Division afillinrrolt m+d Geolo3ry~ <br />dlmrG 26, 2004 <br />Pn;e d <br />The proposed localized change in postmining topography results in stecpcning (from approximately <br />4.9:1 to 35:1) of an 800' section of the slope along the north highwall, but generally maintains the <br />overall gradient of postmining Channel YPM-7. The new 3.5:1 slope is well below the natural angle <br />of repose for spoil materials, is compatible with the objective of long-term slope stability and existing <br />natural slopes in the area, and meets the applicable general grading requirements of Rule 4.14.2(1)(6). <br />SCC recognizes that post-reclamation natural settlement has the potential to result in limited exposure <br />of the upper portion of the Valoca Pit north highwall. The alternative to the current backBelling and <br />grading approach in this area would have been backsloping and grading the final highwall from a <br />point well above the highwall limit. This would have resulted in additional surface disturbance of the <br />steep areas to the north. Backfilhng and final grading in this area will result in covering and <br />elimination of the highwall, in compliance with Rule 4.74.2(])(6). <br />SCC proposes to address the settlement/highwall exposure concern by monitoring this area for <br />settlement, surface cracks, and any highwall exposures. It is anticipated that any surface cracks or <br />highwall exposures that may occur will be localized and of limited extent and can be repaired by <br />minor regrading and reseeding. The description of backfilhng and grading activities (Tab 20) has been <br />revised to reflect these considerations. <br />Comment 4: Portminmo cbmurel YPM-7 fear been relocated and its prbftle Gar changed Please provide a rrvired <br />.S'EllCAD design for this charmed and update Figure 73-12.3 and Table 20-2. Pleare add a completion date far <br />YPM-7 to Exhibit 19-7A. Sime YPM-7 is nehvorked to YPihl-6, Pleare also provide a revised design for YPM-6, <br />if appropriate. <br />Resn~onse: Based on the revised postmining topographic configuration and postmining channel <br />locations, as shown by Map 20-2, postmining Topography Map, the designs for postmining Channels <br />YPM-6 and YPM-7 have been reviewed. The contributing watershed area for YPM-7 has decreased <br />slightly, overall gradient has not changed significantly, and maximum gradient is still within the slope <br />design limits for this channel shown on Figure 13-12.3. Given these considerations, no change in <br />channel design information, other than updates to "fable 20-2 are appropriate or required. If <br />anything, the design for channel YPM-7 is now more conservative (oversized). Channel YPM-7 will <br />be completed in conjunction with final grading in the north end of the Valora Pit, as shown on <br />Exhibit 19-1B. <br />Channel YPM-6 has been designed to handle all flows from channels YPM-1, YPM-2, YPM-5, and <br />YPM-7. "1'he minor reduction in watershed area Fox YPM-7 resulting from it's relocation does not <br />affect the YPM-6 watershed area, since it already includes the subwatershed areas for channels YPM- <br />1, YPM-2, YPM-5, YPM-6, and YPM-7. The channel design for YPM-6 is consistent from top to <br />bottom (refer to Figure 13-12.3), so the change in the location where YPM-7 enters the YPM-6 <br />channel does not affect the YPM-6 channel design. <br />Comment 5: To be consistent with existing postnnne channel designs at the Yaast Mine, please provide the <br />ydralogic inputparameter table for YPM-13 showing information. for curve number selection <br />Response: The hydrologic input parameters arc provided for insenion in the postmining Channel <br />YPM-13 design documentation <br />