2007-10-22_PERMIT FILE - C1981014A (5)

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Generally, the discharge structures are designed to provide EFCI with the flexibility to control sedimentation pond discharge. This flexibility is important because it allows EFCI to match sedimentation pond discharge to natural streamflow patterns and, where appropriate for major storm events, allows the sedimentation ponds to function as flood control structures, with discharge deferred until the runoff peak has passed and elevated streamflows have declined to reasonable levels. Inflow to each sedimentation pond will routed through the pond and discharged through the outlet structures. Because of variances in pond capacity, configuration, and characteristics of the natural receiving drainages at the discharge point, discharge structures for each pond are somewhat different. However, all principal spillway structures are designed to pass the peak flows resulting from a 10-year, 24-hour storm event with no discharge through the emergency spillway. Similarly, emergency spillways are designed to pass the peak discharge resulting from a 25-year, 24-hour storm event with a minimum of one (1) foot of freeboard ~ between the design flow and the crest of the pond embankment. • The principal spillway for Sedimentation Pond No. 1 has a 12 inch diameter gated decant line with an invert at approximately 6,114 feet. The principal spillway discharges to a riprapped apron at the outflow point. The emergency spillway for this pond is a riprapped trapezoidal channel with a five (5) foot bottom width and an invert elevation of 6121.5. The maximum water level for the 25-year, 24-hour storm event is 6122.4 feet and total freeboard is approximately 2.6 feet. Discharge from Sedimentation Pond No. 4 is controlled by a principal spillway consisting of a 10 inch diameter CMP with a 10 inch gated inlet The principal spillway is set at an invert elevation of 6231 feet and discharges to a riprapped apron at the outflow. The emergency spillway for Pond 4 consists of three (3) 30 inch CMP culverts. The outside culverts located at the east and west ends of the pond embankment are straight culverts set at an invert elevation of 6241.5 feet. The center culvert has a drop inlet structure which is set at a invert elevation of 6241.3 feet. The three (3) culverts comprising the Pond 4 emergency spillway provide for effective dewatering of the pond for flows resulting from any storm 4.05-16 R Revised - September, 1991