Discharge erosion can be reduced by use of deflectors, and a large amount of air delivered beyond the weir bulkhead.During construction of the Bureya hydroproject, eight bottom discharges (BD, Fig. 1) consisting of diffusor pressurized pipes with a 5.5´6-m radial gate at the outlet and nonpressurized discharge pipes with a terminal spring board were used for the passage of flows. The discharges were designed to operate for a period of three years under a maximum head of 117 m and at velocities of up to 46 m/sec within the bounds of their entire run during the final year of their service.Based on model tests, and utilizing data derived from laboratory and field studies for similar structures at the Krasnoyarsk and Zeya HPPs, the air duct to the ceiling of the discharge beyond the gate has a cross-sectional area of 14 m 2 [1], while under the jet falling from the shelf, air is conducted through two pipes 0.9 m in diameter at the junction between the pressurized and nonpressurized sections. Experience gained with cavitation damages sustained by discharges at the Krasnoyarsk hydroproject, beyond the shelf of which air was conducted through a pipe 0.6 m in diameter was defined more precisely. Admission to the nonpressurized section is relatively low; the air in the discharges also falls toward the discharge flow; this is visually noted during their operation.The discharges in question were constructed in 1996 -2002; here, the quality of the work had been lowered due to funding delays. The following damages to these structures were revealed during an inspection conducted prior to the start of their use:-undulations, shelves, and discontinuities in interblock and intercolumn joints, caused by inaccurate assembly of formwork, and its unsatisfactory bracing. Individual local irregularities, the height of which reached 5, and in certain cases, even 10 cm; -zones of poorly worked concrete along the intercolumn and interblock joints, in which individual crushed stones were not bound with cement mortar near the surface of the walls, and local depressions reached 5 -6 cm; and, -an undulating surface of the walls of the No. 8 BD due to deflections in panel forms (approximately 3 cm over a length of 50 cm).The presence on the walls of a significant number of different irregularities, including projections with a height of up to 10 cm and a 1:1 slope of the faces, pits up to 10 cm deep with planform dimensions of up to 20´50 cm, etc., was noted during inspection as soon as the discharges had been placed in service. FOL 263.4 NOL 256.0 MOL 236.0 139.0 14.0 17.0 155.0 135.4 138.0 130.0 (135.0) 134.0 161.0 R = 1 8 .0 1 0 .0 1 :0 .7 265.0 Fig. 1. Section through spillway dam of Bureya project: FOL, frontal backwater level; NOL, normal backwater level; MOL, minimum operating level.
The performance of the lower pool during operation of a spillway is discussed.The spillway dam of the Bureya hydroproject ( Fig. 1) consists of eight spans each 12 m wide, which are situated at the head of the spillway with bulkheads each 3 m thick, is designed for the passage of flows in conjunction with the HPP: Q 0.1% = 10,400 + 2100 = 12,500 m 3 /sec, and Q 0.01% = = 11,200 + 2100 = 13,300 m 3 /sec with a pool differential of 117 m. The pools are merged by a discharge of flow from the dam over a distance of 200 m in a diversion channel beyond a 140-m-wide spillway where the overall width of the river is 250 m, and the average depth 15 m in the case of maximumflow passage.During passage of construction flows through eight bottom openings, local washouts of the diversion channel reached 20 m at a distance of 80 m from the dam, washouts directly at its attachment amounted to 7 m, and up to 10 m near the attachment to the abutment, whereupon undermining of stabilizing slabs was also noted. Deposition of scouring products in the form of a ridge (bar) up to 6 m high with a crest elevation of approximately 136.5 m, partitioned the channel; this led to an increase in the level at the HPP as compared with the natural conditions of up to 3 m at a passing flow rate of 500 m 3 /sec.The decision to slope the area of impingement of the discharge flow of beyond the service spillway from the left bank, along which a dirt road passes, and from the split abutment between the diversion channels of the spillway and HPP, was made in connection with the negative experience acquired with scouring of the stabilization. For this purpose, diverting springboards common to two spans are being built beyond extreme spans 1 and 2, and 7 and 8 of the spillway dam. This will solve the indicated problem, but will result in a concentration of flows in the median section of the channel, and here, to an increase in the depth of scouring, especially during the passage of spring floods in the case of fully opened basic gates.The following should be obtained from laboratory investigations for cases where different flow rates are passed:-a prediction of development of local washouts of the channel, the formation of a bar, and the hydrostatic head on the HPP;-the velocity distribution of the flow, and the height of the waves in the section with a length of up to 900 m from the axis of the dam; -information on the effect of the jet dropping directly onto the abutment and left bank, an estimate of the amount of water impinging on the abutment and terminal section of the powerhouse, and a determination of the effectiveness of the wall protecting the left-bank abutment and platform from splash; and, -information on the influence exerted by the flow on the combined stabilization of the right bank, which consists of a concrete lining and rocks of no specified size.
A considerable part of the investigations performed by the B. E. Vedeneev All-Union Scientific-Research Institute of Hydraulic Engineering (VNIIG) to substantiate the design of the Chirkey hydroelectric station [1] consisted of work in the laboratory, designing the main spillway structures-the dam (Figs. 1 and 2) and tunnel (Figs. 1 and 3) variants of the operating spillway and the temporary level II spillway (Fig. ld) --on a three-dimensional model (scale 1 : 50).It was necessary to solve the complex problem of providing satisfactory conditions for connecting the pools in the case of discharges from 50 to 2900 mS/see passing the high-head hydrodevelopment located in a narrow, tortuous canyon with steep slopes up to 300 m high. In this case the design of the spillway had to be such as to eliminate intense scouring of the bottom in the immediate vicinity of the structure, which was dangerous for the stability of the dam, and to keep a considerable part of the spillway discharge from striking the canyon slopes, which were composed of Cretaceous fracturedlimestones [2]. Inthe latter case undermining of the slopes and their collapse would almost certainly obstruct the channel and cause the groundwater to rise under the powerhouse.Another important problem was to work out the design of spillways whlch would provide their sufficiently reliable use under different discharge condRions, to determine the optimal parameters for a number of elements of these designs, and to establish the hydraulic characteristics of the flow within those limits. These problems were solved for the dam variant of the spillway, consisting of three pressure pipes and chutes on a special supporting structure (Figs. I and 2). However, this variant was rejected since it was impossible to construct the powerhouse and spillway simuitaneously. We will examine only the results of investigations that were used for the further development of the elements of the adopted tunnel variant of the spillway.
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