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4.0 Drainage Revisions <br />Site conditions at the RPEE and changes in conditions resulting from the Haul Road HR -1 slide have <br />resulted in changes to the management plan for clearwater diversion and sediment -laden runoff water <br />in these areas. This section presents the revised plans. <br />4.1 Refuse Pile Surface Drainage Revisions <br />It was determined during construction that the east slope of the draw in which the RPEE is situated <br />allows only enough room to for a ditch to divert clearwater from running onto the RPEE area. While <br />the 2010 design in Exhibit 82 routed runoff water drainage from the pile surface to both the east and <br />west limits of the refuse pile then into sediment control drainage ditches, the revised design routes all <br />runoff water to the west edge of the pile and into a sediment control drainage ditch. Runoff water <br />drainage will be routed along haul roads and benches as previously designed and approved. The <br />hydrologic model has been revised to develop updated flows for sediment ditch design. Model inputs <br />and output is included in Appendix B. Revised permit drawings are included in Appendix C. <br />Drawing C -13 shows the revised drainage plan for the RPEE. <br />4.2 Sediment Ditch Design Revision <br />The sediment ditch to be constructed just outside the west limits of the refuse pile will be protected <br />from erosion with riprap and filter materials. The width of the ditch will be increased from the <br />design in Exhibit 82 (approved in 2010) to accommodate the additional flow from the entire refuse <br />pile surface for the 100 -year, 24 -hour rain storm that results from routing all sediment drainage to the <br />west side of the pile. Appendix C, Drawing C -13 shows the revised sediment ditch design. <br />Sediment ditch design north of the rock buttress down to the general location where it turns west <br />toward the sediment pond remains largely unchanged from the location previously approved in <br />Exhibit 82. The profile slope of the first 400 feet (approximately) of the ditch is milder than initially <br />planned due to conditions uncovered during construction. Riprap in this area has been re- designed to <br />economize on dimensions and riprap quantities. The design for other portions of riprap -lined <br />sediment ditch remains unchanged. <br />After repairs conducted to repair the Haul Road HR -1 slide area, cross - sectional area for construction <br />of the sediment ditch in this area is less than indicated by the topographic survey obtained for the <br />approved 2010 design. To accommodate the reduced cross - section, the ditch has been re- designed <br />with a corrugated metal pipe arch (CMPA) half -pipe flume to convey the sediment drainage while <br />P Mpls 06 CO 26 06261003 Haul Road Slide Repair WorkFiles Report MCC_Slump_ Repair - Instrumentation- DrainRev_201 -05 -25 docx <br />17 <br />