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C~ SEDIMENTATION POND HYDRAULIC CALCULATIONS SEDIMENTATION POND N0. 4 Sedimentation Pond No. 4 is designed to contain the surface runoff and sediment load generated by the 10-year, 24-hour storm event at the coal refuse storage area. Runoff from the refuse pile is collected around the pile's perimeter and routed to Sedimentation Pond No. 4 via refuse collection ditches as shown on Figure 1, Refuse Disposal Area Drainage Plan. • Pond No. 4 employs three (3) separate discharge structures designed to safely pass the 25-year, 24-hour storm event. Two (2) are horizontal 30-inch CMP culverts with a 2H:1V slope at their outlets. The culverts are located at the west and east corners of the embankment. The 2H:1V slope at their outlets is sufficient to develop critical flow at the outlet which dictates inlet control for determining spillway capacities. The third structure is a 30-inch CMP drop inlet structure located in the center of the embankment. The method outlined in "Engineering Design Manual - Coal Refuse Disposal Facilities" was used to determine flow versus depth of water surface above the inlet for this structure. The outlet to the two (2) 30-inch CMP culvert spillways has been lined with 10-inch minus riprap (d 50 = 6.7 inches), and utilizes hard durable rock with diameters ranging from 4.5 to 10 inches. The riprap has successfully prevented slope erosion from occurring. The 30-inch CMP drop inlet structure located in the center of the embankment discharges to a naturally riprapped drainage channel. • Pond No. 4 utilizes a 10-inch gated outlet for dewatering purpose. Dewatering is performed to insure that adequate design storm storage capacity exists shortly after other storm events occur. Dewatering is performed only after 24 to 36 hours of detention time is achieved to allow for settling of suspended solids. -17R- Revised - February, 1990