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SEDIMENTATION POND N0. 4 <br />Sedimentation Pond No. 4 is designed to contain the surface runoff and <br />sediment load generated by the 10-year, 24-hour storm event at the coal <br />refuse storage area. Runoff from the refuse pile is collected around the <br />pile's perimeter and routed to Sedimentation Pond No. 4 via refuse <br />collection ditches as shown on the Drainage and Sediment Control Map, (Map <br />20). <br />Pond No. 4 employs three (3) separate discharge structures designed to <br />safely pass the 25-year, 24-hour storm event. Two (2) are horizontal 30- <br />inch CMP culverts with a 2H:1V slope at their outlets. The culverts are <br />located at the west and east corners of the embankment as shown on the <br />Sedimentation Pond No. 4 Design Map, (Map 23). The 2H:1V slope at their <br />outlets is sufficient to develop critical flow at the outlet which <br />dictates inlet control for determining spillway capacities. The third <br />structure is a 30-inch CMP drop inlet structure located in the center of <br />the embankment as shown on the Sedimentation Pond No. 4 Design Map, (Map <br />• 23). The method outlined in "Engineering Design Manual - Coal Refuse <br />Disposal Facilities" was used to determine flow versus depth of water <br />surface above the inlet for this structure. <br />The outlets of the two (2) 30-inch CMP culvert spillways have been armored <br />with 10-inch minus riprap (Dso = 6.7 inches), and utilizes hard durable <br />rock with diameters ranging from 4.5 to 10 inches. The riprap has <br />successfully prevented slope erosion from occurring. The 30-inch CMP drop <br />inlet structure located in the center of the embankment discharges to a <br />naturally riprapped drainage channel. <br />Pond No. 4 utilizes a 10-inch CMP with a 10-inch gated inlet for <br />dewatering purposes. Dewatering is performed to insure that adequate <br />design storm storage capacity exists shortly after each storm in case a <br />subsequent storm event occurs. Dewatering is performed only after 24 to <br />36 hours of detention time is achieved to allow for settling of suspended <br />• solids. <br />~ ~ ll <br />24 R Revised - September, 1991 <br />