Lake Vyrnwy

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Lake Vyrnwy Nature Reserve and Estate is an area of land in Montgomeryshire, surrounding the Victorian reservoir of Lake Vyrnwy. Its stone-built dam, built in the 1880s, was the first of its kind in the world. The nature reserve and the area around it are jointly managed by the Royal Society for the Protection of Birds, and Severn Trent Water. It was built for the purpose of supplying Liverpool and the surrounding areas with fresh water. It flooded the head of the Vyrnwy valley and submerged the small village of Llanwddyn. Today it is a popular retreat, for people in the Midlands for days out, and also for ornithologists, cyclists, and hikers. The reserve is designated as a national nature reserve, a Site of Special Scientific Interest, a Special Protection Area, and a Special Area of Conservation.

The dam

The dam under construction, circa 1885
The Dam looking East, showing compensation water being released from the reservoir
Rhiwargor Falls on the Lake Vyrnwy Estate

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 slate. When built it cost £620,000, equivalent to £60,000,000 in 2014. The dam is 144 ft high from the bottom of the valley, and 128 ft thick at the base; it is 1,170 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 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 5,500,000 to 9,900,000 imperial gallons of compensation water per day. Only a few hundred yards 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]

Straining tower and aqueduct

A sideview of the dam (2009)

Approximately 3,300 ft from the dam is the reservoir's straining tower. Standing only 100 ft from the shore, its purpose is to filter or strain out material in the water with a fine metal mesh, before the water flows along the aqueduct to Liverpool. Its architecture represents Gothic revival, built at the same time as the dam. The tower as a whole is 207 ft tall, 50 ft of which is underwater. It is topped with a pointed copper-clad roof, which makes it look light green.

The Gothic revival straining tower

The sixty-eight-miles aqueduct carrying water from Lake Vyrnwy to Liverpool is officially named the Vyrnwy Large Diameter Trunk Main (LDTM). It originally consisted of two pipelines, made largely of cast iron. To help maintenance work on the 9 ft diameter cast-iron tunnel which took the aqueduct under the Mersey, riveted steel piping was also used. This was an early use of the material which was to become the norm for trunk water-main piping.

The original aqueduct was constructed across the valley from the reservoir between 1881-92. It crosses the valley floor near Penybontfawr and then runs north of Llanrhaeadr-ym-Mochnant and Efail-rhyd on the north-east of the Tanat Valley. The aqueduct is largely hidden from view although there are some visible surface features including air valves, the Cileos valve house, the Parc-uchaf balancing reservoirs, and a deep cutting to the west of Llanrhaeadr-ym-Mochnant.

Brick and concrete-lined tunnels carried pipes at Hirnant, Cynynion and Llanforda, and a fourth later added at Aber so that the Hirnant tunnel could be made accessible for maintenance. The first section of a third pipeline was laid in 1926-38 using bituminous-coated steel. To increase capacity, a fourth pipeline was added in 1946.

Re-organisation of the pipe crossings beneath the Mersey and the Manchester Ship Canal were undertaken in 1978-81. The current provision relies on three pipes 42 inches in diameter delivering up to 50 million imperial gallons per day into reservoirs at Prescot, east of Liverpool. As of 2013 the LDTM is currently undergoing a major refurbishment project by United Utilities, scheduled for completion in 2020.[2]

The lake/reservoir

The reservoir is Severn Trent Water's largest. When full, it is 84 feet deep, contains 13.125×109 imp gal, and covers an area of 1,120 acres the equivalent of around 600 football pitches. The lake has a circumference of 11 miles with a road that goes all the way around it. Its length is 4.8 miles. On a clear day the lake, along with many others in North Wales, can be seen from space. 311 streams, waterfalls and rivers flow into the lake and are named after the mountains or hillsides they flow from. Some are no more than a trickle, while others cascade down the mountains. The main ones, clockwise from the west side of the dam, are named Afon Hirddu, Eunant, Afon Eiddew, Afon Naedroedd, Afon Cedig, and Afon Y Dolau Gwynionew. On the northern edge of the lake is a small hamlet called Rhiwargor where the rivers Afon Eiddew and Afon Naedroedd meet. Up the valley of Afon Eiddew is an impressive waterfall, one of the largest surrounding the lake. Known locally as Pistyll Rhyd-y-meincau, it is commonly known as Rhiwargor waterfall. In 1889, shortly after completion, the lake was stocked with 400,000 Loch Leven trout. The lake continues to supply Liverpool with fresh water. It is the water source used in the manufacture of Bombay Sapphire gin.

Nature reserve and conservation

Lake Vyrnwy is a designated Nature Reserve. The RSPB has several bird hides around the lake, where a number of rare species of birds are known to be breeding, including the Peregrin Falcon, the Pied Flycatcher the Redstart the Siskin and the Wood Warbler. Every spring they host a Dawn Chorus tour.

Around 90 species of bird have been recorded as breeding on the reserve, and six species of bat, including the pipistrelle and brown long-eared bat. Butterfly species include Purple Hairstreaks, commas and peacocks. Dragonflies include Golden Ringed, Common Hawker and Four spotted chaser.

Heather Moorland that grows on the mountains around the lake is now being restored. This restoration of heather moorland is becoming increasingly common in Britain. In most moorlands, Heather is usually burnt, cut, and the seeds collected to be sowed where the heather has gone. Burning at the Lake Vyrnwy moorland is no longer carried out, as the burning of heather can have negative consequences for water management (namely water colouration). Management of the moorland helps improve the habitat for Red Grouse and the Short-eared Owl. Sheep, cattle and ponies also graze on the heather. The livestock is managed by tenant farmers who farm the moorland in accordance with Organic Agriculture.

Broadleaf trees are being planted to replace coniferous trees, and man-made features such as hedgerows and dry-stone walls are also being restored, and wildflower areas are being restored to help insects, birds, and other wildlife.


Wood sculpture

Llanwddyn has had since 1995 a sculpture park in the valley below the dam, containing many wooden carved works. There are large wooden picnic benches in the shape of leaves and trees on the west side of the lake at Llechwedd Ddu. Near the Old Village on the beach is a sculpture of dolphins which, when the lake rises in a flood, appear to be jumping out of the water. Several totems are carved into standing trees and re-erected fallen trunks.[3]


Activities in the area include sailing, hiking on Glyndŵr's Way, rock climbing, cycling and horse riding. The Vyrnwy Half Marathon is conducted annually.

Largest Tree

The site was, until recently, home to the tallest tree in England and Wales, a Douglas Fir 209 ft 3 in high. This tree was damaged in stormy weather and had to be felled. A nearby Douglas Fir has now inherited the title of 'Tallest tree in Wales', at 199 ft tall.[4]


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