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SECTION 3 Retention Basin The Compost Facility retention basin has a total capacity of 760,600 cf(approx. 5.7 million gallons) and 598,700 cf (approx. 4.5 million gallons) not including the 2-ft freeboard. Compost runoff water sheet flows to perimeter ditches which direct the runoff to two pond inlet pipes/culverts. The inverts of the pipe inlets at the pond are at elevations 31.71 and 32.66-ft for the north and south inlets, respectively. The pond is designed to have sufficient storage capacity to maintain the runoff from the 25-yr, 24 hr storm below the elevation of the pipe inverts, however one way valves are placed on the pipe ends as an added precaution. Retention basin sizing calculations are provided in Appendix B. The Compost Facility retention basin will be located in the current storm water pond area, directly west of the compost facility. The retention basin will be lined with a 60-mil single sided textured HDPE geomembrane underlain by a geosynthetic clay liner. The retention basin will encompass approximately 2-acres and will extend approximately 240 feet in the north-south dimension and 460 feet in the east-west dimension. The crest of the retention basin is at elevation 35-ft. The base of the retention basin ranges from elevation 21-ft in the southern corner to 26-ft in the northeast. A pan lysimeter is located under the lining system in the southwest corner of the basin. The retention basin containment system consists of a composite liner placed as follows, from bottom to top: • Prepared subgrade; • Geosynthetic Clay Liner(GCL); • 60-mil, single-sided textured HDPE geomembrane liner(textured side down); The GCL will be placed by deploying GCL panels and overlapping panel seams. The GCL panels will be anchored in the same trench as the HDPE geomembrane. Manufacturer-recommended procedures for proper installation of the GCL are included in the Specifications. Inspection and testing of the GCL material and installation are described in the CQA Manual included in Appendix D. The GCL will be covered with a 60-mil thick, single-sided, textured HDPE geomembrane (textured side against GCL) placed concurrently with GCL deployment. By covering all deployed GCL with HDPE geomembrane, the GCL will be effectively protected from possible moisture damage. However, if the GCL becomes saturated during construction, it will be removed and replaced in accordance with the Specifications (Appendix C). The termination edge of the GCL and geomembrane will be placed in perimeter anchor trenches. The 60-mil single-sided textured HDPE geomembrane will be installed in accordance with the Specifications (Appendix Q. Extensive CQA will be performed on the geomembrane to identify and COMPOST FACILITY STORM WATER IMPROVEMENTS 3-1