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contractors typically use a combination of plastic sheeting, plywood , and roofing tar to address the first two concerns — leakage around and between the weir boards — with varying degrees of success . The discussions also confirmed that the third concern — the difficulty of accessing the weirs and adjusting and securing the weir boards — has forced contractors to resort to a variety of means, none of which promote safe and efficient weir operations . Contractors typically use the limited access provided by the existing fixed ladders to install or remove weir boards from above, and use a johnboat or similar small vessel positioned directly in the discharge flow to install or remove weir boards from below . From these precarious positions , the contractors then typically secure the boards by pounding in wooden wedges or shims . The remainder of this section specifically addresses these three issues by presenting a conceptual design for minor modifications to the weir system proposed for installation at the planned IR- 14 containment facility. All site-specific elements of the proposed modifications (e. g., elevations) refer to the preliminary weir design as presented in the site ' s Management Plan (Taylor et al . , May 1999) . 2 . 3 Conceptual Weir Modifications Figure 2 . 1 presents the basic elements of the proposed weir modifications . Notably, these revisions require no dismantling, and only minor modification , of the weir system ' s basic structure. Each of the proposed modifications is presented here as a preliminary conceptual design only. The Project Engineer remains responsible to fully design, specify, and incorporate the proposed modifications into the bid package for the site ' s first use. The selected contractor must then complete the proposed weir modifications before the initiation of dredging as part of required site preparation activities . As shown , the proposed modifications include three basic elements . First, to address the issue of leakage around the ends of the weir boards , the modifications include wooden extensions, fabricated from dimensional pressure-treated (PT) lumber, installed vertically against the inside (downstream) flange of each vertical I-beam channel that holds the weir boards . These flange extensions will provide a smooth, wide surface against which the downstream face of the weir boards will seat. Although providing an imperfect seal, the width of these mating surfaces will greatly reduce the likelihood of leakage around the ends of the weir boards. Equally important, the flange extensions will also provide a means to secure each weir board in position without the use of wedges that can become dislodged by changes in water level . Second, to address the issue of leakage between boards , the modifications also extend to the weir boards themselves . The contractor must still supply all required weir boards, but with the boards modified - 4 -