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The passage of major floods through incomplete large-scale high-head hydraulic facillties, and the scheduled development of stage construction of the reservoir as the structure is being raised, are serious engineering problems and require the timely completion of intake structures at each stage of construction. These circumstances were fully manifested during the passage of the Vakhsh flood in 1978 at the incomplete Nurek hydroelectric plant, in which the design head reaches 300 m and the maximum flood flows amount to 3600 mS/sec.Four intake tunnels arranged in stages according to reservoir depth were constructed in rock excavations on the left bank of the Nurek hydro project to pass the Vakhsh flows in 1978. The first three tunnels, I, If, and Ill, are construction tunnels, while the fourth is an operating h~h-flow spillway:with a deep intake. In addition to this, a similar spillway with a surface intake should be completed by 1980. The first four tunnels are accommodating high flows through the slope of the left bank, while the surface intake spillway is being excavated in the stage-Ill construction tunnel beyond the segmented-gate chamber (Fig. i).The stage I construction tunnel was intended for the passage of Vakhsh River flows during the initial period of construction of the hydroelectric plant. All remainln 8 tunnels would ensure filling of the reservoir after startup of the plantWs units, and for these purposes were equipped with segmented regulating gates designed for a capacity of 2000 mS/sec through two gates in each tunnel under a head of 100-120 m.Passage of maximum flood flows and the stepwlse impoundage in the reservoir during the construction period were to be provided for by the complete readiness of the next two tunnels --I and If, II and III, Ill and the deep spillway tunnel--by the next flood. Passage of flood flows during the plant's operation is provided for by two spillways and by the working units.The readiness of the basic structures of the Nurek plant in 1977 made it possible to maintain the level of the reservoir at an elevation of 858 m. This made possible the operation of the first six hydraulic units, of which three were equipped with temporary rotors designed for a reduced head with water supplied to them from a temporary intake.The complete readiness of the stage-III construction tunnel and the deep submerged spillway for passage of the 1978 flood and the further filling of the reservoir should have been provided by the planned constructlon schedule for 1978, and all units at the plant should be placed in service with basic rotors and water supplied from the permanent plant intake. In conformity with this, the total reservoir volume required to ensure the planned generation of electricity and to satisfy the demand for water in 1978 should be 8 kmS; this determined the required level of the reservoir--885 m.According to plan, passage of the 1978 flood and the impoundage of this volume of water in the reservoir should be realized in two stages:i. For water levels up to elevation 858 m...

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