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in . from the common, downstream face (Figure 2 . 1b) . For the spline itself, '/ -in . aluminum is the recommended first-choice option , with ''/z -in. 5 -ply , marine-grade plywood ( actual thickness — 12mm, or 15/32 in. ) the lower-cost, second-choice alternative. Other materials , such as plastics or composites may be acceptable , but must be submitted to the Project Engineer for review and approval . Each spline shall be fabricated from the approved material, 2 -in . wide and of an equal length as the weir boards into which each will be inserted . Despite its greater cost compared to marine plywood, aluminum remains the preferred alternative. Unlike marine plywood or other wood-based materials (e . g., wood/plastic composites) , aluminum will not adsorb water, expand , and potentially lock the spline into the slot . As a result , the slot to accept the spline can be milled to a closer tolerance . The addition of an approved , non-toxic lubricant will improve the effectiveness of the sea] , limit corrosion, and insure that the spline will not lock in place . Other non-wood materials (e. g. , plastics) may also prove acceptable, but would likely require a thickness greater than '/ in. to provide sufficient shear strength . Marine plywood remains the second-choice alternative. Its primary advantage, in addition to its relatively low cost, is that additional splines can be easily fabricated on-site as needed . Milling the slot with sufficient tolerance to allow for the plywood ' s inevitable expansion will reduce the likelihood that the spline will lock into position . The addition of a non-toxic lubricant such as a heavy coating of beeswax will reduce the plywood ' s tendency to swell and lock itself into the slot. 2 . 3 . 2 . 1 Weir Board Installation During dredging, each weir board will be added to the stack as follows . Because water typically will flow over the weir crest , the weir operator must first divert the flow to the two remaining weir stacks by driving a section of 1/2 -in. marine-grade plywood between inner face of the upstream I-beam flange and the weir boards . To adequately divert the flow , this section of plywood must span the full distance between the webs of the opposing I-beam channels into which the weir board will be placed, and extend sufficiently above the weir crest such that little or no flow continues over the weir. If necessary to prevent hydrostatic pressure from bending the plywood , the operator may reinforce the plywood ' s upper edge by securing a metal channel to its downstream face . The weir operator must then clear excess water from the slot in the upper surface of the top board in the stack . [Note: the above procedure applies only during active dredging as water flows over the weir crest. Obviously, diverting the flow will not be required when the contractor prepares to start dredging and the weir operator initially installs weir boards to set the initial ponding depth (Section 3. 2), or after the completion of decanting as he closes off the weirs as part of his demobilization procedures (Section 3. 4). ] Next , the operator must apply a thick coating of the - 9 -