The stage-II diversion tunnel of the Nurek hydroelectric scheme is designed in conjunction with the stage-I and stage-In tunnels to pass flood flows during construction of the first-phase dams and the hydroelectric station building, and to release irrigation water downstream of the hydropower complex during the reservoir filling and lowering period within the range of levels below the sill of the intake portal of the stage-III diversion tunnel.Depending on the hydrologic conditions the tunnel has had two operating periods: The first was an unregulated period . when it came imo operation with the rise in upstream water level at the commencement of a flood; the velocity of the flow in the tunnel during this period did not exceed 15-16 m/sec. The second was the regulated period , when the head on the gates reached 110 m and the velocity below the gates ranged up to 42 m/sec. To ensure free-flow conditions in the tunnel, the chambe~ housing the regulating gates is provided with an air-supply conduit with a cross-sectional area of 9 m z.With regard to the above-mentioned operating conditions, the lining in the section upstream of the gate chamber, where the tunnel is under press=c and velocities do not exceed 20 m/sec, was designed to be of Mark-200 concrete; in the free-flow section downstream of the gate chamber, where the velocities range from 42 m/sec (at the start) to 32 m/sec (at the end). the lining within the limits of the wetted perimeter is constructed of Mark-400 concrete, with a slump of 2-4 cm for the invert, and the upper part is of Mark-300 concrete. In order to prevent cavitation in the tunnel or keep it to a minimum, strictrequirements were set for the quality of the lining surface (see Table 1).

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