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The entrance to the eirexgency spillway is located 4.5 feet above the <br />principal spillway orifice. There will be no flow through the emergency <br />spillway channel as a result of the runoff from a 10-year, 24-hour <br />precipitation event or less. <br />The peak stage from the 100-year, 24-hour precipitation event is at <br />elevation 7017.5; the top of embanlanent elevation is 7019.5. This allows <br />for 2.0 feet of freeboartii. This is based on worst case, i.e., that the <br />orifice and perforated riser ~ gets plugged and all flow must go through <br />the emergency spillway. <br />Sediment Pond 008's capacity will be attributed partially to the <br />embarilonent and partially to the pond excavation. During passage of the <br />10-year, 24-hour storm, the spillway does not discharge sediment <br />downstream nor is there any flow through the emergency spillway. The <br />embarilanent is constructed to be stable and provide adequate freeboard. <br />Therefore, based on the design, this structure oong~lies with the <br />• appropriate regulations for a sedimentation structure. Peabody Coal <br />Cm~a~~y will periodically inspect this structure for sediment storage, <br />structural integrity, and hazardous conditions. Maintenance and remedial <br />work will be performed in accordance with the approved mine permit. <br />Construction Plan <br />The existing 24-inch CMP principal-emergency spillway will be modified. <br />The existing embarila~nt will not be modified. The existing principal <br />spillway orifice will be reduced from 6.0 inches to 4.7 inches in <br />diameter. The top of the 24-inch perforated drop inlet pipe will remain <br />at elevation 7015.0 (see Dd~i.bit 7-17B) . <br />The trapezoidal emergency spillway will be constructed at elevation <br />7016.0, as shown on Ddzibit 7-17B. The emergency spillway will be <br />undercut to allow for placement of geotextile fabric, bedding, and riprap <br />material. All joints between rolls of geotextile fabric shall consist of <br />18 inches minimum overlap. All overlaps shall be made such that upstream <br />• geotextile overlaps dantistream geotextile in spillway area. Riprap <br />7-6f-17 Revised 08/21/91 <br />