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10 . Insert the 5/ 16 - in . x 3 -in. stainless steel bolts through the C-channel flange, the spacer or bushing, and the flange extension and securely tighten into position with matching washer and nut. 11 . Following installation , ensure that the bolt does not project beyond the upstream face of the flange extension and prevent the weir boards from seating firmly against the face of the extension. 12 . Confirm that the face of the extension remains parallel to the axis of the weir crest to maximize the mating surfaces and provide the optimal seal . 2. 3 . 2 Weir Boards and Splines The contractor will be responsible for providing and fabricating — from standard (PT) dimensional lumber — all weir boards required for the minimum 13 . 9 -ft range of weir adjustment for each of the three weir stacks . To adequately resist the greater hydrostatic forces near the bottom of the weir stack, the contractor must use (nominal) 6x6 boards (finished dimensions – 5 '/� in . x 5 '/� in.) within the lowermost 4 ft of the weir stack (that is , below +2 . 8 ft NGVD) with (nominal ) 4x6 boards (finished dimensions – 3 %x in . x 5 % in . ) for only the upper 10 ft of the weir stack (that is, above +2 , 8 ft NGVD) . In addition to reducing cost and weight, using 4x6 weir boards (3 '/z in . finished vertical dimension) above +2 . 8 ft will improve the accuracy and resolution of the weir height adjustments . To further reduce cost and weight the contractor may use (nominal) 4x4 weir boards (finished dimensions – 3 . 5 in . x 3 . 5 in .) , but only within the uppermost 3 ft of the weir stack (that is , above + 9 . 7 ft NGVD) (Figure 2 . 1 b) . To maximize the mating surface between the flange extension and the weir boards, each weir board must be of sufficient length to extend to within 1 in . of the web of both opposing I -beam channels . No weir boards can be used that when fully saturated would bind within the channels . For the Site IR- 14 installation , a weir board length of 52 in . should meet this requirement based on a 53 . 5 -in. design separation between opposing webs . However, the FIND cannot guarantee that the I-beam channels remain plumb and parallel throughout their length . The contractor remains responsible to verify the actual field dimensions and provide weir boards that will fall within the required tolerance throughout the full height of the weir stack. As stated earlier, to prevent between board leakage and to strengthen the weir stack, a spline will tie each weir board to the adjacent members above and below . To provide a keyway for the spline, the contractor must mill a matching dado or slot on opposite faces of each weir board, each slot set back 1 '/� - 8 -