The Clywedog Dam in Wales was built to supply Birmingham and the Midlands with an extra 50 billion litres of water. It also regulates flow into the river Severn protecting against flooding in the winter and maintaining a minimum flow in the summer.
Previously Halcrow had built dams for hydropower creating a generating and distribution system in Scotland that had brought electric power to many people in the Highlands. However, in the 1960s and 70s hydropower could not compete economically with fossil fuel and hydroelectric construction was wound down. The Clywedog Dam gave Halcrow a start in the UK water industry.
Construction of the concrete buttress dam began in 1963 and was completed in 1967. It is the tallest concrete dam in the UK with a height of 72 meters and a length of 230 meters holding back approximately 11,000 million gallons of water.
Its downstream face, with 11 buttresses beneath a 133m long free flow concrete crest spillway, was formed of large pre-stressed concrete beams producing cascades guided into stepped side-spillway channels. The buttress design was checked by three-dimensional computer-aided stress analysis.
The Clywedog reservoir was carefully landscaped and is now popular with tourists offering fishing, scenic walks and sailing.
The dam was a target for Welsh Nationalists and they did indeed destroy a cableway across the Clywedog Valley, causing Halcrow to design measures to prevent further attacks. These included torpedo netting to protect the valve chambers and motion sensors with hotlines to the police in the interior caverns.