At hydroelectric plants, different types of wooden structure~ are built in zones where the water level varies, with the result that the wood rots and the useful life of these structures is reduced. Considerable quantities of wood are used each year for repair work of which a sizable portion is irretrievably lost.At the Vuoksinsk cascade, steel was successfully used instead of wood for the drift barrier, the scaffolds under the turbine runners, and formwork for the reconstruction of deteriorated concrete. This measure proved to be economical as regards the initial expenses and permitted reducing the maintenance personnel. Drift Barrier on Steel BuoysA barrier supported on steel buoys, which was built according to a design prepared by the operating personnel of the cascade, has been operating successfully since 1967 at the Lesogorsk hydroelectric plant (Fig. 1). The barrier sections consist of welded steel frames supported on five steel buoys of 1.2-m diameter and 0.6-m 3 volume (Fig. 2). A wooden deck on the frame serves as the operating bridge along the entire barrier.

To close the outlets of draft tubes, measuring 13.0 x 6.2 m, a spoked gate consisting of 30 metal spokes weighing 1 ton each was originally used. The spokes were installed and removed by divers,by means of temporary lifting tackle with a winch and cantilever beam. The placement and removal of the spokes required considerable manual labor and time, tied up equipment, and did not provide optimal working conditions with respect to safety.Based on the favorable experience gained in the use of a floating emergency gated" we decided to use the floating gate~: for closing the outlets of the draft tubes. The gate (Fig. 1) is a flat, rectangular, welded pontoon measuring 13.4 x 6.6 m and weighing 40.9 tons. The gate consists of a system of metal beams around the contour of the gate, load-support props with a water-retaining lining which form a box, and two cylindrical ballast compartments (floats) with a diameter of 1600 mln. The buoyancy and maneuverability of the gate are secured by the boxshape of its structure and by the floats, and when the box is submerged (when placing and raising) by floats with a displacement reserve of 29%.