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trench drains, or interceptor drains, are to be installed with their orientation generally <br />perpendicular to the seepage path alignment from Dam No. 3. The drains are shown in <br />plan view on Detail 1 of Drawing No. 3DS-2 and in section view on Detail 2, Section B. <br />The drains have been arranged such that the natural drainage path from each identified <br />seep will intersect at least one, and in most cases two drains. <br />Each drain trench will be about 60 to 70 feet long. The objective is to collect as <br />much seepage as possible in the upper drains to reduce the flow into the lowest trench <br />where pumping might be needed. Detail 3 shows a cross section of the drains. <br />The soils will be removed down to a depth of about five feet deep and a width of <br />about five feet. Drainage fabric will be draped into the trench followed by one ply of 45- <br />mil PVC or EPDM liner and a top cover of drain fabric on top of the liner to provide <br />puncture protection of the liner. The trench will then be filled to original ground surface <br />with 1-inch screened river-gravel or crushed stone. Drain fabric will be laid on top of the <br />gravel before the entire area is backfilled to the original grade. <br />Storm Drainage Management <br />Storm drainage from the west side of the county road will be allowed to bypass <br />the collection system as shown on Detail 5 of Drawing No. 3DS-2. That detail shows the <br />current swale alignment (light blue) and a new alignment further northeast along the <br />edge of the trench drains. The hill slope on the north side of the valley will be cut to a <br />slope of about 3:1 down to the relocated swale position. That will yield about 600 cubic <br />yards of fill that can be placed as top cover over the constructed trench drains. The final <br />slope over the trench drains will be about 8:1 and will cover the existing grade with about <br />2 to 3 feet of soil. <br />The relocated swale will be armored with rock sorted from the excavation. The <br />completed footprint will then be reseeded. A passive treatment wetland swale will be <br />constructed on the upstream side of the pipe alignment to collect unexpected overflows <br />from the siphon vault. Details 3 and 5 of Drawing No. 3DS-2 shows the construction. <br />An emergency overflow pipeline from the dosing vault will discharge into the non- <br />jurisdictional wetlands below the site, in the same manner as the current seepage flow. <br />Wetland vegetation will provide passive treatment for any such emergency overflows. <br />Emergency overflow releases will not discharge to State or Federal Waters. <br />Henderson Operations, Mill Site 9/7/10 <br />No. 3 Dam Seep Mitigation, Design Report Page 7