Okuyoshino Pumped Storage Power Station

The lower reservoir is created by the Asahi Dam which is a 86.1 metres (282 ft) tall and 199.41 metres (654.2 ft) long arch dam on the Asahi River of the Shingu River system. Its catchment area covers an area of 39.2 square kilometres (15.1 sq mi) and the surface of the reservoir covers 52 hectares (130 acres). The lower reservoir's storage capacity is 1,685,000,000 cubic metres (1,366,000 acre⋅ft) of which 1,250,000,000 cubic metres (1,010,000 acre⋅ft) is active (or usable) for pumping up to the lower reservoir.

Creating the upper reservoir in a valley above the lower is the Seto Dam. It is a 110.5-metre (363 ft) tall and 342.8-metre (1,125 ft) long rock-fill embankment dam with 3,740,000 cubic metres (4,890,000 cu yd) of fill. Its catchment area covers a much smaller area of 2.9 square kilometres (1.1 sq mi) and its surface covers 52 hectares (130 acres). The upper reservoir has a storage capacity of 1,692,000,000 cubic metres (1,372,000 acre⋅ft) of which 1,250,000,000 cubic metres (1,010,000 acre⋅ft) is useful for power generation down at the power station.[3][4][5]

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 megawatts (270,000 hp) Francis pump-turbine-generators are reversible and serve to both pump water and generate electricity.[6] 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 metres (1,657 ft).[3]

To allow sediment to pass the lower Seto Reservoir, a bypass tunnel was constructed. The tunnel itself is hood-shaped and 2,350 metres (7,710 ft) long. It passes through rock on the north side of the reservoir. The intake for the tunnel is controlled by a 13.5 metres (44 ft) tall and 45 metres (148 ft) long weir located 2.5 metres (8 ft 2 in) 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 cubic metres per second (4,900 cu ft/s) of water and discharges downstream of the Seto Dam.[3]