Llyn Brianne Dam
Rhandirmwyn, west of Llanwrtyd Wells, Powys, Wales, UK
Binnie & Partners
October 1968 - 1972
ICE reference number
photo courtesy PHEW, ICE
Llyn Brianne Dam regulates the River Tywi in mid Wales and impounds water supplies for south Wales. It is of rock fill with a clay core, and believed to be the tallest of its type in Europe and the highest in Britain. The picture shows the damís spillway. A hydroelectric power station was added 25 years after the dam was completed.
Llyn Brianne Dam and the reservoir it retains are situated in the mountainous Powys countryside west of Llanwrtyd Wells, some 19km north of Llandovery. They were constructed during a period of increasing water shortages in west Wales. Despite the need for it, the scheme attracted many objections before a public enquiry sanctioned its construction under the West Glamorgan Water Board (Llyn Brianne) Order 1968.
The dam is located at the south west of the reservoir in a narrow V-shaped gorge of mudstone. It regulates flow in the River Tywi to ensure a dependable supply. Water from Llyn Brianne is abstracted 64km downstream at the Nantgaredig Intake Works (SN487204) in Carmarthenshire. From here a 1.68m diameter steel pipeline conveys it 26.5km to the Lower Lliw Reservoir (SN652035), which in turn supplies the nearby Felindre Water Treatment Works (SN644023), north of Swansea. Treated water is supplied to Swansea, Neath and Port Talbot.
Originally, the dam was 90m high from river bed level. Its crest is almost 275m long and 9.1m wide, with an access track along the top. It is constructed of rock fill with a tapering central boulder clay core, 6m wide at the crest, contained between filter walls. The terraced downstream (south) side of the dam slopes at 1 in 1.75, while the submerged upstream side slopes at 1 in 2 and its rock fill is protected by a layer of armour stone. Suitable clay and rock were sourced locally during construction.
A concrete spillway weir (SN793485) at the east end of the dam discharges water into a channel 24.4m wide, feeding the River Tywi. When the spillway is in use, a continuous discharge of water takes place at the foot of the dam through an upward facing jet with a spectacular spray, oxygenating the water.
Two reinforced concrete bridges with raking piers cross the spillway channel. The upper bridge carries road traffic across the dam, from the car park to the crest, and the lower bridge provides access to the control house below the dam.
A road runs the 9.7km length of the reservoir and visitors can drive to the bottom of the spillway to view the water jet at the base of the dam. It has become significant visitor attraction.
Construction began in October 1968. During November 1971, the dam reached its maximum height and, in February 1972, the dam was plugged. In October 1972, the first water was released into the river. On the 15th May 1973, Princess Alexandra officially opened the scheme.
Movement in the rock fill of the downstream shoulder of the dam was monitored during construction. It was compared calculations from a retrospective analysis using finite element techniques, informed by data from laboratory tests on samples of rock fill. The results showed broad agreement, confirming the design parameters.
To meet future increases in water demand, the dam has been designed to allow for the raising of the crest by up to 18m. Provision has also been made for the pumped transfer of water from Llyn Brianne to the lower reaches of the River Wye and westwards to the River Usk.
Concerns have been raised about the possible detrimental effects to fish stocks (mainly salmon and sea trout) from the particularly cold and somewhat acidic waters of the reservoir. Occasional dosing with lime has been carried out to reduce the acidity.
On 27th May 1994, planning approval was granted for the addition of hydroelectric power generating capability at Llyn Brianne. The plant was designed by Hyder Consulting (part of Welsh Water).
As part of the project, in 1995-6, the crest and spillway of the dam were raised by one metre, to 91m, making it the tallest dam in Britain (280.4m above sea level). The top of the reinforced concrete spillway was remodelled and the dam was heightened by extending its core in concrete and increasing the flanking rock fill shoulders. This increased the reservoirís capacity by 6 percent, from 60,916 million litres to 64,553 million litres. The water surface covers some 210 hectares.
A hydroelectric generating station (SN791482) was constructed to the east of the foot of the spillway, near the control house. The station is equipped with three Francis turbines. A single turbine handles the average summer flow of 6.8 million litres of water per day, and operates year round. During periods of high flow in winter when the reservoir is full, usually from November to March, the two larger turbines come into operation. Together the three turbines can generate 4.3MW of electricity, which is enough for a small town.
On 15th January 1997, the Hyder Industrial Hydroelectric Station opened. Its electricity is supplied to the National Grid at Llandovery via a subterranean cable laid beside the Rhandirmwyn to Llandovery road.
Architect: Adrian Montagu
Rock fill testing: Imperial College, London
Resident engineer: D.E. Evans
Deputy resident engineer: J.J. Long
Contractor: George Wimpey & Co Ltd
RCAHMW_NPRN 403965, 402255, 413851
"Constructional deformations in a rockfill dam" by A.D.M. Penman and J.A. Charles, in Ground Engineering, pp.24-36, March 1973