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Over the last hundred years, shaft spillways have become widely used in hydraulic engineering practice due to their undeniable advantages: high discharge capacity, maximal water consumption per one cubic meter of concrete, point structure compactness. The modern theory of hydraulic calculations was formed based on works on the study of the operation of a circular spillway with a sharp edge carried out by Wagner in 1954. Although numerous hydraulic studies have not proved many of the statements Wagner's calculation methodology was based on, the materials of his studies have been presented in special hydraulic literature for hydraulic calculations up to date. The accepted design conditions of the drainage surface of the spillway shaft, which is round-cylindrical in the cross-section and in the form of a convex parabolic line in the longitudinal section, do not correspond to the possibilities of work. As a result, the drainage surface is replaced by a system of truncated cones, the joints of which cause flow separation from the drainage surface and the formation of vacuum zones. It results in approximate hydraulic calculations. To eliminate these disadvantages of hydraulic calculations and bring design developments in line with technological possibilities of works, it is possible to replace a round-cylindrical cross-section of the inner surface of the shaft with a polygonal one. In this case, the drainage surface will consist of a system of longitudinal wedges with one-dimensional curvature, for which the formwork can be made of flat sheets. This paper presents materials on the calculation and design of a shaft spillway of a hydroelectric complex with a dodecagonal cross-section for Algeria, and four shafts with an octagonal cross-section and two shafts with a tetragonal cross-section for Syria. The receiving hopper head is round-cylindrical, and the drainage surface is elliptical, which allows adapting the drainage surface to any initial design parameters of shaft spillways.

The current practice of hydraulic engineering construction applies four methods for damping extra energy of excess water discharge: throwing out the stream into the tailwater; using twisting of the stream in the water passageway, in a water-well, and on a multistage drop; and energy damping in a vertical shaft. The cheapest method is energy damping by throwing the jet into the downstream reservoir. The main disadvantages limiting the use of this method include the need for appropriate topographic conditions in the tailrace basin, strong rocks in the river channel, and the possibility of water throwing at a large distance from the hydropower project, which restricts the use of this method. Extra energy damping with the use of twisting flow in the water passageway requires complex structures, providing for a rotational movement of water and forming a deep vacuum in the outlet conduit. The breaking of vacuum requires a large amount of air, which causes dynamic modes, and its removal complicates the design of energy damping structures. The most reliable way of energy damping is damping in a stilling well. But the disadvantages of water-sucking wells are low efficiency accompanied by an uneven distribution of specific discharges in the inlet section and a sharp increase in the cost at high-pressure hydroelectric installations. The use of multistage cross-fall drastically facilitates the operation of a stilling well and serves as its supplement. Energy damping in a vertical shaft is easy to use. Still, it results in trapping a large amount of air at all operating modes, which has limited its practical use in hydraulic engineering construction. However, the damping of flow energy in a vertical shaft except for possible air supply has shown high efficiency of operation in combination with a diversion water pipe with a reverse slope. A variant of such design of flow energy damping in a vacuum vertical shaft has been developed and studied as applied to conditions of the Rogun hydropower project. Model studies of the design on the scale of 1:100 showed high efficiency and reliability in all possible ranges of discharged flow rates.

The article considers a variant with the operation of spillway No. 2 of the Boguchanskaya HPP as the main spillway structure for the period of temporary operation of the hydroelectric complex. In the process of developing options for skipping construction costs, a calculation was made of the throughput with a free overflow through a flat threshold at the level of 167.0m at the FSL=175m and forcing the UVB to the level of 185.0 m with the damping of the flow energy in the water well. The task of the calculations is to obtain the discharge characteristics of the considered spillway option with the calculated flood hydrograph with a probability of 0.2%. At the same time, based on the throughput capacity of the spillway No. 1, the dependence of the turbine flow rate and the water depth in the downstream, as well as the transformation of the flood of a given probability, the estimated flow rate is 5500 m3/s, the maximum head H=18 m (determining) at a minimum threshold level of 167.0 m. Weir refers to weirs with a wide threshold. The throughput capacity of the spillway No. 2, operating in the free overflow mode, was determined. The change in the lateral compression coefficient depending on the head for the middle and outer working spans is considered. The outlines of free-falling jets behind the culverts operating in the free overflow mode are obtained, which makes it possible to refine the outlines of the side walls of the spillway.

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