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Sediment and runoff from Mine 2 enter the storm sewer via a 12-inch <br /> diameter Cu3 anchored to the existing concrete wall. This 12-inch diameter <br /> s <br /> OT is to be constructed on top of the slope to the road where the pipe is <br /> buried to cross the road to a 6-foot deep, 4-foot- ID concrete manhole. <br /> This concrete manhole will also serve as a surge chamber for pressurized <br /> pipe flows in the event that the maximum open channel flow (5 cf s) is <br /> exceeded. The required minimum flow is 2 cfs. for a 10-year storm. <br /> The concrete outlet structure for the storm sewer is at station <br /> 105 + 00 (in accordance with haul road centerline) . This structure is <br /> designed with rip-rap to dissipate flow energy developed in the 10% graded <br /> ditch invert. The 4-foot wide by 2-foot deep ditch will convey sediment <br /> and runoff to the sedimentation pond. <br /> SEDUHP.TTATION POND <br /> One sedimentation pond (Pond 1, Plate 2) has been designed to <br /> store all sediment from 3 disturbed areas (see Appendix) generated over <br /> a 3-year period as well as all runoff developed during a 10-year, 24-hour, <br /> Type II Storm. (Plates 1, 2, and 3) <br /> The only outlet designed for this pond is an emergency spillway that <br /> is an ungated 18-inch diameter CIT, . <br /> This pond has a total designed volume of 2.7 acre feet. Approximately <br /> 0.3 acre foot is for sediment, 0.4 acre feet for the 10-year flood storage <br /> and the remaining l.1 acre feet for mine discharge and 25-year flood routing. <br /> NOTE : Pad 3 Two ponds were constructed in series with a <br /> total capacity of 3 .0 AF . Mine #2 lower coal bin <br /> pad (1984) go into the new #2 mine ponds 0 . 61 A.F . <br /> -6- <br /> 30 <br />