SH6644 : Constructing the dam at Llyn Stwlan, 1959
taken 65 years ago, near to Tanygrisiau, Gwynedd, Wales
The top reservoir of Ffestiniog Pumped Storage Scheme, which is held in place by Stwlan Dam, a highly visible local landmark (when it's not foggy).
Ffestiniog Power Station, commissioned in 1963, was the first major pumped storage system in the UK and has a maximum output of 360 MW. The upper reservoir, Llyn Stwlan, is 300 m above the lower reservoir at Tanygrisiau. The capacity of the reservoirs is 2,000,000 cu m. The power station, at the lower reservoir, has four turbines which can each generate 90MW within 60 seconds of a demand, with a water flow of 29 cu m/s. In periods of low demand or excess generating capacity, the generators can operate as motors and drive pumps to return water to the upper reservoir. Each pump can deliver 22 cu m/s of water at 75 MW.
The building of the lower reservoir flooded the route of the Ffestiniog Railway which had been closed since 1946. However, the line was progressively reopened from Porthmadog from 1955. The railway was reopened to Blaenau Ffestiniog in 1982, which required a new tunnel and deviation around the reservoir and power station.