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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 Revision <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 and then into sediment control drainage ditches, the revised design <br />routes all runoff water to the west edge of the pile and into sediment control drainage ditches and <br />culverts. Runoff water drainage will be routed along haul roads and benches as previously designed <br />and approved. The hydrologic model has been revised to develop updated flows for sediment ditch <br />design. Model inputs and output is included in Appendix B. Revised permit drawings are included in <br />Appendix C. 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 design <br />approved in 2010 to accommodate the additional flow from the entire refuse pile surface for the 100 - <br />year, 24-hour storm that results from routing all sediment drainage to the west side of the pile. A 36 - <br />inch diameter HDPE pipe routes runoff from the RPEE surface past the west end of the buttress down <br />to the upstream end of the main sediment channel at the toe of the rock buttress. This pipe may need <br />to be moved as additional phases of the buttress are constructed. Appendix C, Drawing C-13 shows <br />the revised sediment ditch design. <br />The 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 2010. <br />The profile slope of the first 400 feet (approximately) of the ditch is milder than initially planned due <br />to conditions uncovered during construction. Riprap in this area has been re -designed to economize <br />on dimensions and riprap quantities. The design for other portions of riprap-lined sediment ditch <br />remains unchanged. <br />P:\Mpls\06 CO\26\06261003 Haul Road Slide Repair\WorkFiles\Report\MCC_Slump_Repair-Instrumentation-Drain Rev 2012-12-12.docx 17 <br />