The Atbashi hydroelectric station with an installed capacity of 40,000 kW was put into operation in early 1970; the total capacity of its reservoir is 9.58 million m s, the useful capacity is 4.34 million m 3 at the normal pool level (NPL) of 154 m, and the dead storage level (DSL) is 145 ms The structures of the hydrostation include [i]: a gravel--pebble earth dam with a height of 79 m with a grouting gallery and grout curtain (Fig. !), operating tunnel spillway Qdes = 330 m3/sec, intake and power tunnel d = 5 m, I = 300 m, powerhouse with four units, grouting adits in the rock walls located at two levels ( Fig. 2), access adits, and a diversion tunnel. The hydrostation is located in the high-mountain region of Central Tien Shan on the Atbashi River, a left-bank tributary of the Na~/n River.The region of the hydrostation site has complex toBographic and engineering-geologic conditions: the foundation and bank abutments of the dam are composed of marmorized limestones whose permeability is characterized by permeability coefficient 0.2-0.01 m/day.The numerous fractures in the limestones are mainly of a tectonic origin. The bank release fractures with an open width to 50 cm, usually filled with sand--clay material or calcite, extend tens and hundreds of meters~The developed ancient karst has cavities from several millimeters to tens of meters.The groundwater level is 15-18 m below the water level in the river. The seismicity of the region is intensity 8. The climate is markedly continental: the mean annual air temperature is +2.5~ with an absolute maximum of +34 ~ and minimum of --38 ~ The mean annual discharge of the Atbashi River is 32.8 m3/sec, the sediment load varies from 0.I to 5.8 million m31yr, and the volume of runoff of shuga reaches 5.6 million m3/yr. At the present-day level the minimum storage of the reservoir for daily and weekly regulation needed for the Atbashi hydrostation to eliminate the weekly nonuniformity of the load is 1o7 million m 3. Since the volume of sediment and shuga [spongy ice lumps] far exceeds the indicated quantity, for providing operation of the hydrostation it was necessary to specify --for the first time in our country for a reservoir on a mountain river --deep (more than 40 m) annual flushing with complete drawdown of the reservoir during recession of the flood and brief stopping of the hydrostation.Such flushing was planned in stages with alternation of a partial rise of the water level in the reservoir and discharge of the clarified water for washing out the deposits formed in the lower pool during flushing into the Naryn River (a distance of about 4 km).Periodic deep flushings are supplemented by constant flushings in the su~er and fall by discharging surplus water at the 145-m DSL.For deep flushing, the inlet structure of the operating tunnel spillways has a number of special features (Fig. i): the sill of the radial gate is located 42 m below the NPL and the three-level intake openings, in the event of clogging of the lower opening by sediments, make it possible to flush ...
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