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• minimum freeboard of 0.3 feet. Permanent diversions are designed to pass <br />the peak flows from a 100-year, 24-hour storm event with similar <br />freeboard requirements. A detailed discussion of diversion structures is <br />presented in Section 2.05.3 Operation Plan - Permit Area, under the <br />subtitle "Ponds Impoundments and Diversions". In addition, detailed <br />calculations for diversion structures are presented in Exhibit 19, <br />Drainage and Sediment Control Plan. <br />In conjunction with mine development, facility installation, and ongoing <br />coal refuse placement, Dorchester, and subsequently EFCI, graded and <br />maintained all disturbed areas to provide for effective drainage and <br />minimize potential erosion. Where appropriate, EFCI utilizes localized <br />grading measures to effectively control and route drainage and minimize <br />flow velocities. Examples would include the yard area drainage system in <br />the mine facilities area, temporary sumps and ditches in active coal <br />refuse placement areas, and the drainage control berms utilized to isolate <br />disturbed area drainage in the Southfield loadout area. <br />• Localized drainage control and grading measures are supplemented by EFCI's <br />extensive drainage and sediment control network consisting of collection <br />ditches which intercept disturbed area runoff and route it to <br />sedimentation ponds, the ponds themselves, and associated discharge <br />structures. The drainage and sedimentation control network, described in <br />detail in Section 2.05.3, Operation Plan - Permit Area, under the subtitle <br />"Ponds Impoundments and Diversions", serves to collect and retain <br />disturbed area runoff allowing settlement of suspended solids and any <br />necessary treatment prior to release to natural drainages. Sedimentation <br />ponds are operated under applicable provisions of EFCI's NPDES discharge <br />permits. Compliance with the terms of these permits insures that <br />downstream water quality impacts are minimized. <br />The three (3) existing sedimentation ponds in the mine area and the two <br />(2) ponds at the loadout area, along with associated collection ditches, <br />spillways, and discharge structures provide effective drainage and <br />. sedimentation control for both existing and proposed future surface <br />disturbance. These existing sedimentation ponds were designed using <br />standard USDA-SCS methods to contain the runoff resulting from a 10-year, <br />2.05.6-17 <br />