Lake Vyrnwy - The Dam

The Dam

Dr George Deacon (1843–1909) began the design of the Vyrnwy Dam in 1879 at the age of 36. In 1890, following Vyrnwy, he founded an engineering practice in London which subsequently became Sir Alexander Binnie Son & Deacon, then Mr Binnie and Partners. Its present-day successor is now Black & Veatch.

Dr Deacon was instructed to prepare the Parliamentary Plans for the scheme in 1879. The dam construction started in 1881 and was completed seven years later in 1888. It was the first large stone-built dam in the United Kingdom, and is built partly out of great blocks of Welsh slate. When built it cost £620,000, which today is around £22,000,000. The dam is 44 metres (144 ft) high from the bottom of the valley, and 39 metres (128 ft) thick at the base; it is 357 metres (1,171 ft) long and has a road bridge running along the top. It is decorated with over 25 arches and two small towers (each with four corner turrets) rising 4 metres (13 ft) above the road surface.

Vyrnwy was the first dam to carry water over its crest instead of in a channel at the side. At the bottom of the dam is a body of water known as the Stilling Basin, this is necessary to absorb the energy when the water flows over the crest and into the valley, and stops the water from eroding the foundations of the dam.

Underneath the West Tower is a building known as the Power House, which contains an electrical generator driven by water leaving the reservoir. Before mains electricity arrived in the 1960s this was Llanwddyn's only source of power.

The West and East Towers release compensation water by huge valves, which are controlled by Severn Trent Water. This water is purely for the River Vyrnwy, which would otherwise dry out unless in flood. Depending on the Water Levels downstream the reservoir could release anything from 25 to 45 megalitres (5,500,000 to 9,900,000 imp gal) of compensation water per day. Only a few hundred metres downstream is a weir, which the Environment Agency use to measure the daily amount of compensation water. This weir also holds back enough water to create the stilling basin.

Earlier dams in Britain had been built by making great earth embankments to hold back the water. This new type of stone dam would change the face of the Welsh landscape over the coming years. The next stone dams to be built in Wales on an even bigger scale than Vyrnwy were those built in the Elan Valley. 1