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Design and Operations Plan <br />Western Gravel E & P Waste Disposal Facility <br />to produce 2 Y2 inches of rainfall A NRCS Type II rainfall distribution was used for drainage calculations <br />and an SCS curve number of 87 was used for the fmal facility surfaces and run -on from off -site undisturbed <br />areas. Run -on was calculated using a 210 acrea area. Calculations used to design these control structures <br />and maps showing the run -on watershed are provided in Appendix I. <br />3.5.2 Run -on Control/Operational Drainage Structures/Permanent Run -on Structures <br />Clean surface run -on water from the south of the facility will be diverted around active cells by a series of <br />channels constructed along the south, east, and west property boundaries located south of the White River. <br />The channels will divert the clean water into na tural drainages that discharge to the south and into the White <br />River. The channels will be constructed using excavated ditches (channels), soil berms, or a combination of <br />channels and berms. Culvert pipes will also be required at several points where the run -on must cross under <br />the access road that runs along the southern edge of the property and is servicing the Phase 1 and Phase 2 <br />gravel pits. The channels, ditches, and culverts have been designed to carry a peak discharge of 4.87 cubic <br />feet per second (cfs). The corrugated metal pipe (CMP) culverts must be a minimum diameter of 18- inches. <br />Details for run -on control channels are provided on Figure 9, Detail 9D. <br />In order to provide access for long -term post - closure monitoring and maintenance, channels, culvert pipes, <br />and the access road will be left in place following Phase 2 mining completion . In addition, run -on controls <br />shall remain following closure in order to help minimize the erosion potential and manage stormwater run - <br />on. <br />3.5.3 Run -off Control/Permanent Drainage Structures <br />All run -off generated in the cell vicinities will be managed using a series of permanent run -off control <br />channels constructed around the cells and facility. The run -off control channels will direct flow discharging <br />from the facility and outside the cells, during and after construction, into storm water retention ponds. Since <br />run -off will not come into contact with cell wastes, run -off is not considered leachate. Run -off control <br />channel details are shown on Figure 9, Detail 9C. <br />Pond 1, located at the west side of Cell A, has been designed to contain run -off from Cell A and a future cell <br />to be located west of Pond 1. The pond has been sized to contain 1.2 acre -feet of run -off plus two feet of <br />freeboard. The surface area of Cell A following closure will be 6.8 acres. Therefore, the run -off control <br />channel along the east and north sides of Cell A has been designed to convey run -off from half of the <br />completed cell (3.4 acres) to Pond 1 and the channel located along the south and west sides of Cell A has <br />been designed to convey the other half of run -off (3.4 acres) to Pond 1. Run -off from Cell A following <br />closure has been calculated as 0.7 acre -feet and run -off from the future cell to be located west of Pond 1 will <br />be 0.23 acre -feet following closure. The volume of total run -off from the two cells following closure will be <br />0.93 acre -feet. <br />Pond side slopes are designed and will be constructed with 3(H) to 1(V) slope configurations and 10 foot <br />wide crest. The pond embankments will be non jurisdictional dam structures designed and constructed in <br />accordance with the State of Colorado - Division of Water Resources Rules and Regulations For Dam Safety <br />and Dam Construction (2007). Pond 1 details are shown on Figure 9, Details 9A and 9B. <br />A future pond will be constructed at the northeast corner of the COD and will be designed to contain run -off <br />generated from the three future cells located in the eastern half of the COD. Design details for this pond will <br />be provided to the CDPHE for review and approval prior to construction and the D &O plan will be modified <br />accordingly. The retention ponds will be designed to contain 100 percent of run -off generated by the cells. <br />Hence, the facility is considered non - discharging and technically does not require a storm water permit. <br />However, in order to ascertain that the facility remains in compliance with the clean water act, a stormwater <br />NWCC, Inc. <br />October 24, 2011 <br />Page 30 of 51 <br />