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• assure effective drainage and provide for maintenance of drainage in the <br />event that the primary subdrain is blocked by any vertical offset <br />resulting from subsidence in the refuse stockpile area. <br />As the refuse pile advances, EFCI will extend both existing primary and <br />secondary subdrains in sequence with topsoil removal operations. subdrain <br />specifications and construction will be consistent with design <br />recommendations presented in Exhibit 18, Geotechnical Investigations. <br />These specifications were also utilized in constructing the existing rock <br />subdrains. The rock subdrain extensions will be established with a <br />minimum gradient of 2 percent. subdrain material will consist of durable <br />rock fills and will be wrapped with TYPAR 3401 filter cloth or equivalent <br />to prevent clogging. <br />Surface Water Collection and Drainage Ditches <br />In order to protect the hydrologic balance and assure that all drainage <br />from disturbed areas associated with refuse disposal operations is <br />collected and routed through sedimentation ponds in accordance with <br />applicable regulations, a system of perimeter collection and diversion <br />ditches was designed. Existing perimeter collection and diversion ditches <br />are shown on the Mine Area Surface and Groundwater Hydrology Map, (Map <br />12) and the Drainage and Sediment Control Map, (Map 20). Detailed designs <br />for the drainage and sediment control structures associated with the <br />permanent coal refuse stockpile are presented in Exhibit 19, Drainage and <br />Sediment Control Plan. All permanent collection and diversion ditches <br />for the coal refuse stockpile were redesigned and reconstructed in 1989 <br />to provide sufficient capacity to pass peak flows from the 100-year, 24- <br />hour storm event. All drainage from disturbed refuse stockpile areas is <br />routed to existing Sedimentation Pond No. 4 located immediately northeast <br />of the refuse stockpile area. <br />Detailed descriptions of the surface drainage control system can be found <br />• Sections 2.05.3, Operation Plan - Permit Area, 2.05.6, Mitigation of <br />Surface Coal Mining Impacts, and 4.05, Hydrologic Balance. Specific <br />design information and discussion of design methodology are presented in <br />Exhibit 19, Drainage and Sediment Control Plan. <br />4.10-4 <br />