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Llyn Brenig
north of Cerrigydrudion, Conwy, Wales, UK
associated engineer
Binnie & Partners
date  1973 - June 1976
UK era  Modern  |  category  Dam/Reservoir  |  reference  SH971543
Llyn Brenig is a man-made lake in north Wales. It was constructed to regulate the amount of water flowing into the River Dee, which lies to the south, and was created by damming the Afon Brenig. The reservoir also collects water from a 2,200 hectare catchment area. It is the fourth largest lake in Wales.
The construction of Llyn Brenig was promoted by the Dee & Clwyd River Authority and financed by the Welsh Water Authority. It provides water for the River Dee regulation system, a wider scheme supplying water to large areas of Liverpool and the Wirral, which also uses Llyn Tegid and Llyn Celyn in Snowdonia. Llyn Brenig’s neighbouring reservoir at Llyn Alwen, to the west, is retained by a concrete dam (SH955529) and was completed in 1921 to supply drinking water to Birkenhead via the River Dee.
The Afon Brenig is a tributary of the Afon Alwen, joining it (SH973527) near Pentre-llyn-cymmer in Conwy. The Afon Alwen flows some 20km south eastwards and discharges into the Afon Dyfrdwy (SJ060425), or River Dee, south west of Corwen in Denbighshire.
The Brenig Valley is partially blocked by two large drumlins (rounded oval hills formed by glacial action on unconsolidated sediments) overlying Silurian mudstone and drift clays. The dam constructed to create Llyn Brenig was sited to exploit the natural advantage of the topography, and incorporated the drumlins into its structure, reducing the quantity of material required.
The dam is an embankment of mudstone with a clay core. The downstream (south) face has a gradient that varies from 1 in 1.5 near the crest to 1 in 4 at the toe, while the upstream face slopes at 1 in 2. Both faces include a pair of berms, giving them a stepped appearance.
The dam’s core of rolled boulder clay is laid to a slope of 1 in 1.25, and is inclined upwards towards the downstream face. This reduces the risk of the core developing tensile stresses from differential settlement. Its horizontal thickness is about 10m at the crest and some 14m at the toe. The clay is sandwiched between protective sand and gravel filters. A grout curtain forms a cut-off from the base of the core down into the bed rock below.
The dam is 1.2km long, with a crest level 380m above sea level. The top water level is 3m lower, and the maximum height to the spillway is 47m. A draw off tower (SH976542) is located north of the dam, on the east side of the lake. The dam cost £12.2m and was completed in June 1976. A plaque (SH974541) commemorates its inauguration on 21st December 1976. The resulting reservoir took three years to fill. It is up to 45m deep and covers an area of 372 hectares. It has a maximum capacity of 61,525,000 cu m.
Archaeological work undertaken at some 50 sites before completion of the dam revealed possible Mesolithic and Neolithic seasonal use, Bronze Age cairns and burials, and Medieval and later settlements, some of which were submerged by the waters.
A range of instruments had been incorporated into the dam during its construction, enabling ongoing monitoring of reservoir levels, rainfall, piezometric pressure, total pressure, settlement and movement. More than 30,000 separate readings were taken in the period 1976-96.
Llyn Brenig isn't the only reservoir in the catchment area, which can't supply enough water to fill it in a single annual hydrological cycle. If it were to be emptied, it could take up to 10 years to refill. So, it is only used for water supply during drought conditions, or if Llyn Tegid and Llyn Celyn are predicted to have insufficient water to maintain flow in the River Dee.
The reservoir is well stocked with rainbow, brown and tiger trout, and is used exclusively for fly fishing. In 1990, it was the only Welsh reservoir selected to host the World Fly Fishing Championships.
Llyn Brenig has a visitor centre (SH967547) on the west shore, north of the dam, with displays on the archaeological finds and on the construction of the dam. It is the start point for pedestrian and cycle paths around the lake. Llyn Brenig Sailing Club, to the north of the visitor centre, is the highest sailing club in the UK.
From summer 2012 to May 2013, the visitor centre was enlarged and modernised with funding from Welsh Water, Denbighshire County Council and the European Regional Development Fund. The site is open to the public year round. It is managed by Dwr Cymru Welsh Water.
Contractor: Fairclough Civil Engineering Ltd
Research: ECPK
"Observation analysis at Llyn Brenig", by Andrew Thomas in The Prospect for Reservoirs in the 21st Century: Proceedings of the Tenth Conference of the BDS Held at the University of Wales, Bangor on 9-12 September 1998, Thomas Telford Ltd, London, 1998
"Dam and Reservoir Construction in Glaciated Valleys" by M.S. Money in Glacial Geology: An Introduction for Engineers and Earth Scientists, edited by N. Eyles, Pergamon Press Ltd, Oxford, 1985
reference sources   CEH Wales

Llyn Brenig