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Claewen Dam
Rhayader, Powys, Wales, UK
Claewen Dam
associated engineer
Sir William Halcrow & Partners
date  1946 - October 1952
era  Modern  |  category  Dam/Reservoir  |  reference  SN867636
ICE reference number  HEW 186
photo  © Pete Walker and licensed for reuse under this Creative Commons Licence
Claerwen Dam was the tallest mass concrete gravity dam in Britain at its date of completion. Its appearance harmonises with the earlier dams of the Elan Valley, though its reservoir storage capacity is far greater. Now Grade II listed, it supplies water to the city of Birmingham.
In 1892, the Birmingham Corporation Water Act authorised the construction of impounding reservoirs in the Elan and Claerwen Valleys south west of Rhayader in central Wales. The first four reservoirs and their dams ó Caban Coch (SN925646), Garreg Ddu (SN910640), Penygarreg (SN902674) and Craig Goch (SN897694), all in the Elan Valley ó were opened on 21st July 1904 by King Edward VII. Water from the reservoirs flows 117.5km under gravity through the Elan Aqueduct to the city.
Birminghamís rapidly increasing population and a severe drought in 1937 highlighted the need for greater storage capacity for its water supply. By early 1939, proposals for a large new dam in the upper Claerwen Valley were well advanced. Plans were shelved during World War II (1939-45), and construction commenced in 1946.
Claerwen Dam, designed by Sir William Halcrow & Partners, appears to be of similar style to the older dams in the neighbouring Elan Valley. Though constructed of concrete, it is faced with dressed stone on the downstream (south) side and upper portion of the upstream side. The masonry work resulted in considerable extra cost. The remainder of the damís upstream side is faced with blue engineering bricks.
The dam is a mass concrete gravity structure, curved in plan and 355.7m long. Its crest rises 56m above the bed of the River Claerwen.
Stone parapet walls flank the viaduct above the stepped downstream face of the dam. The roadway is supported on 13 elliptical arches ó six spans of 12.2m on either side of a central spillway spanning 18.3m. A viewing platform is constructed over the central arch above the 165m long spillway.
The reservoir impounded behind the dam holds almost as much water as the combined capacity of all the Elan Valley reservoirs together. With a top water level 368.8m above sea level, the reservoir covers 263 hectares. Its capacity is 48.3 million cu m at maximum water depth.
Water is released from the reservoir via either of two 1.2m diameter pipes, each side of the damís base, which discharge into the river. Overspill water flows down the damís face like a waterfall, into a stilling pool, which slows the velocity and provides an aesthetic appearance.
A workforce of some 470 people constructed the dam in six years. More than 100 stonemasons from Italy were employed, because most skilled British workers were restoring the Palace of Westminster (Houses of Parliament), London, at the time. Men and materials were brought to site by road from the railway at Rhayader.
On 23rd October 1952, Queen Elizabeth II opened the dam, during one of her first official engagements as head of state.
In July 1995, Claerwen Dam was Grade II listed. In November 2000, the valve house and single-lane reinforced concrete bridge over the spillway, both to the south of the dam, were also listed at Grade II.
In September 2014, BBC2 Top Gear television presenter Richard Hammond drove a Land Rover Defender up the downstream face of the dam, supported only by a winch cable. As the vehicle was not allowed to be hauled over the parapet, it had to be reversed back down the dam! The programme aired on 15th February 2015.
Contractor: Edmund Nuttall, Sons & Co
RCAHMW_NPRN 261844, 277198
Research: ECPK
reference sources   CEH Wales

Claewen Dam