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Vyrnwy Dam
Llanwddyn, Vyrnwy, Powys, Wales, UK
Vyrnwy Dam
associated engineer
George Frederick Deacon
Thomas Hawksley
date  July 1881 - May 1891, opened 14th July 1892
era  Victorian  |  category  Dam/Reservoir  |  reference  SJ017192
ICE reference number  HEW 214
photo  © David Purchase and licensed for reuse under this Creative Commons Licence
The first high masonry gravity dam in Britain, and also the first such dam to act as a weir, dispensing with the need for a separate spillway. The dam was also the first to incorporate a drainage system to combat uplift pressure. It was constructed as part of an extensive water engineering system to deliver water from mid Wales to Liverpool. Now Grade I listed, the neo-Baroque style structure is a testament to Welsh industrial architecture.
By 1867, Liverpoolís rapid growth and population increase had resulted in the cityís corporation cutting water supplies and new sources were required urgently. In May 1874, Thomas Hawksley (1807-93) and John Frederick La Trobe Bateman (1810-89), the foremost British water engineers of the 19th century, considered six potential new sources but none were suitable.
In 1877 and August 1878, George Frederick Deacon (1843-1909), then Liverpoolís borough engineer, submitted two reports on creating a reservoir in the valley of the Afon Vyrnwy, a tributary of the River Severn. A third report was prepared in June 1879.
Hawksley and Deacon were joint engineers on the parliamentary plans for the scheme to create Lake Vyrnwy (Llyn Efyrnwy) by damming the valley. On 6th August 1880, the Liverpool Corporation Waterworks Act authorising the works received royal assent. Deacon became the corporationís water engineer, a post he held between 1880 and 1890.
Liverpool Corporation purchased 9,713 hectares of the catchment area surrounding the site to control the purity of the water supply. In March 1881, the corporation appointed Hawksley as engineer-in-chief for the Vyrnwy project. Construction began soon afterwards and Edward James Herbert (1818-91, 3rd Earl of Powis) laid the first stone on 14th July 1881.
The ground conditions were ideal for the construction of a gravity dam, as it is located on a buried rock bar in a glacial valley of moraine. Hawksley and Deacon adhered to the two fundamental principles required, weight and watertightness, and achieved them here in spectacular style.
Apparently it was Hawksley who suggested using masonry ó local grey Silurian slate ó instead of earth for constructing the dam. An earthen dam was estimated to cost £311,410 and a masonry dam more than twice as much at £650,527. However, the extra cost was judged favourably as ensuring greater structural reliability and providing additional reservoir capacity.
The dam is 358m long, with a maximum height in the centre of 44.2m from foundation to crest. The crest acts the same way as the lip of a weir ó overflowing water cascades over the damís downstream face, which is angled at 56.3 degrees to the vertical, or a slope of 1.5 to 1. The steeper upstream (submerged) face of the dam slopes at 7.27 to 1, or 82.2 degrees to the vertical. In the mid section, the base of the dam is about 38.7m wide.
The body of the structure consists of irregularly shaped stone blocks, each up to 10 tonnes in weight. During construction, the courses were closely bedded in concrete mortar, and the spaces between blocks filled with more mortar, forced through with smaller stones.
A level 6m wide roadway is carried across the crest supported on 31 arches of equal 7.3m span. The tops of the parapet walls on either side of the carriageway are 49m above the damís foundations.
An important feature of the design was to provide a drainage system that prevented water pressure building up and potentially overturning the structure through uplift. A network of drainage tunnels built into the foundations, and two transverse discharge tunnels through the base of the dam, relieve pressure using hydraulically operated valves. The discharge tunnels release compensation water into the Afon Vyrnwy so that the river does not dry up.
The tunnel portals, at the east and west ends of the dam, are flanked by large steps. Above the tunnels, from crest to parapet, rise two groups of four towers with corbelled pyramidal stone roofs. These groups are positioned at either end of the 19-arch principal overflow section. The towers allow internal access throughout the dam via tunnels and tapering circular shafts with cast iron staircases.
In 1885, Hawksley resigned over the corporationís concerns about rising costs, and the ambiguity of his position relative to Deacon.
By November 1888, the dam was substantially complete and the valves were closed for impounding the waters of the new Lake Vyrnwy. In 1889, the lake was stocked with 400,000 brown trout brought from Loch Leven in central Scotland. The downstream (south) side of the dam bears two inscriptions ó OPUS INCHOATUM MDCCCLXXX (work begun 1880) and OPUS ABSOLUTUM MDCCCXC (work completed 1890).
When full, Lake Vyrnwy contains 59.7 million cubic metres of water. It is 251.5m above sea level and its water surface covers 453.7 hectares. It is 7.6km long and varies in width from around 400m to 1km, with a maximum depth of 25.6m. More than 30 watercourses flow into it, including streams, waterfalls and rivers. The six rivers entering the lake, clockwise from west of the dam, are the Hirddu, Eunant, Eiddew, Nadroedd, Cedig and Dolau Gwynion.
The creation of the reservoir submerged the existing village of Llanwddyn (SH998213), some 200m south of where Afon Cedig enters the lake. Its inhabitants were moved to new locations south of the dam in present-day Llanwddyn.
Water is extracted from the lake through a straining tower (SJ012201) to the north west of the dam, entering the Vyrnwy Aqueduct which conveys it to Liverpool. The tower, designed by Deacon, is 52m high with some 35m visible above water level. It is connected to the shore by a narrow stone bridge of four segmental arches. At the shore end are steps to road level and a pair of wrought iron gates set between stone pillars.
The main tower is of stone-faced mass concrete, circular to machicolated parapet level and of smaller octagonal section above. The gallery level was constructed using an early prototype of reinforced concrete with reinforcement consisting of steel wire wound on cast iron chairs. The domed ceiling of the engine room, though of mass concrete, is supported on wrought iron ribs. A small circular stair tower is corbelled to the south east side. Both towers have steep conical roofs of copper sheeting over timber frames.
Below water level, the base of the tower houses three strainers almost 8m tall and adjustable inlet pipes that can be raised or lowered externally. The tower entrance is at bridge level, where a washing floor is provided for raising the strainers for spray cleaning. A spiral staircase leads up to the engine room. In 1889, a two-cylinder hydraulic engine was installed to operate the three hydraulic rams for lifting the strainers and the two rams to move the inlet pipes, which are controlled by winches in the gallery. Power for the engine comes from a hill reservoir with 35m head.
In May 1891, water from Lake Vyrnwy was first sent to Liverpool along the Vyrnwy Aqueduct. Prince Arthur, Duke of Connaught and Strathearn (1850-1942), formally opened the works on 14th July 1892. The success of the venture relied on a workforce of some 1,000 labourers, a tramway from Hendre (SJ017204) to the dam and a fleet of steam cranes. The whole scheme including dam and the almost 110km of aqueduct between Vyrnwy and Liverpool cost around £2.1m to construct.
In 1895, Deacon presented a paper on the project and was awarded the George Stephenson medal and a Telford premium by the Institution of Civil Engineers.
A stone obelisk (SJ020194) was erected north of the east end of the dam in memory of the 44 workers who died during construction of the Vyrnwy project ó 10 were killed in site accidents and 34 lost their lives in the course of the works. The memorial is 8m tall and stands on a double plinth about 2m square.
Three bronze plaques (SJ019193) have been set in a granite slab at the east end of the dam. They commemorate various stages in the projectís construction and improvement, from 1881 to 1938.
In November 1993, the dam, the straining tower and its approach bridge were all Grade I listed for their national importance as an outstanding achievement of Victorian water engineering. The dam was cited as one of the most impressive examples of industrial architecture in Wales and the tower for its exceptional completeness in structure and equipment.
In February 2003, the original water gauge house (SJ017191) at the west end of the damís downstream face received Grade II listing. At the same time, the memorial obelisk and the three bronze plaques were also Grade II listed for group value.
Lake Vyrnwy remains well used for fishing, canoeing, sailing and windsurfing. A dedicated RSPB nature reserve covers 6,475 hectares and a perimeter road measuring 18.9km encircles the water.
Contractor: direct labour
RCAHMW_NPRN 32442, 32443, 309728, 309973, 309980, 309981
Research: ECPK
"Deacon, George Frederick (1843-1909)" by W.F. Spear, rev. Anita McConnell, in Oxford Dictionary of National Biography, Oxford University Press, 2004
"Hawksley, Thomas (1807-1893)" by T.H. Beare, rev. Mike Chrimes, in Oxford Dictionary of National Biography, Oxford University Press, 2004
"Obituary. George Frederick Deacon, 1843-1909", in Minutes of ICE Proceedings, London, Vol.177, pp.284-287, January 1909
"Lake Vyrnwy and the Vyrnwy Water Supply to Liverpool", supplement to The Engineer, London, 15th July 1892
"Liverpool Water Supply and the Vyrnwy Lake and Masonry Dam", in The Engineer, pp.34-35, London, 14th January 1887
reference sources   CEH Wales

Vyrnwy Dam