Okuyoshino Pumped Storage Power Station

The Okuyoshino Pumped Storage Power Station (奥吉野発電所) is located 15 km north of Totsukawa in Nara Prefecture, Japan. Using the pumped-storage hydroelectric method, the power plant has an installed capacity of 1206 MW. To accomplish power generation, the power station shifts water between two reservoirs, the lower Asahi Reservoir and the upper Seto Reservoir. Construction on both the Asahi and Seto Dams began in 1971 and was complete in 1978. The power station was commissioned in 1980. Due to heavy sediment and turbidity in the Seto Reservoir, caused by logging and landslides upstream, a sediment bypass tunnel was constructed between 1992 and 1998.

Asahi Dam
The lower reservoir is created by the Asahi Dam which is a 86.1 m tall and 199.41 m long arch dam on the Asahi River of the Shingu River system. Its catchment area covers an area of 39.2 km2 and the surface of the reservoir covers 52 ha. The lower reservoir's storage capacity is 1685000000 m3 of which 1250000000 m3 is active (or usable) for pumping up to the lower reservoir.

Seto Dam
Creating the upper reservoir in a valley above the lower is the Seto Dam. It is a 110.5 m tall and 342.8 m long rock-fill embankment dam with 3740000 m3 of fill. Its catchment area covers a much smaller area of 2.9 km2 and its surface covers 52 ha. The upper reservoir has a storage capacity of 1692000000 m3 of which 1250000000 m3 is useful for power generation down at the power station.

During periods of low demand when electricity is cheap, the power station pumps water from the lower reservoir to the upper. When energy demand is high, the water is released back down to the power station through the same tunnels to generate electricity. Additionally, the six 201 MW Francis pump-turbine-generators are reversible and serve to both pump water and generate electricity. The pumping and generation process is repeated as needed and although the power station consumes more electricity pumping than it does generating, pumping occurs when electricity is cheap and generating when it is expensive; making the power station economical. The difference in elevation between the two reservoirs affords a hydraulic head of 505 m.

Sediment bypass tunnel
To allow sediment to pass the lower Seto Reservoir, a bypass tunnel was constructed. The tunnel itself is hood-shaped and 2350 m long. It passes through rock on the north side of the reservoir. The intake for the tunnel is controlled by a 13.5 m tall and 45 m long weir located 2.5 m upstream of the dam. The weir is used to divert sediment-laden river water into the tunnel or to let it flow into the reservoir. The tunnel can divert a maximum of 140 m3/s of water and discharges downstream of the Seto Dam.