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|Lake Vyrnwy/Llyn Efyrnwy|
View overlooking Lake Vyrnwy showing the full extent of the lake
|Primary inflows||River Vyrnwy and other small streams|
|Primary outflows||River Vyrnwy|
|Managing agency||Severn Trent Water|
|Max. length||7.64 kilometres (4.75 mi)|
|Max. width||0.80 kilometres (0.5 mi)|
|Surface area||4.54 square kilometres (1,121 acres)|
|Max. depth||26 metres (84 ft)|
|Water volume||59.7 gigalitres (13.125×109 imp gal)|
|Shore length1||19 kilometres (12 mi)|
|1 Shore length is not a well-defined measure.|
Lake Vyrnwy (Welsh: Llyn Efyrnwy, pronounced [ɛˈvərnʊɨ] or Llyn Llanwddyn) is a reservoir in Powys, Wales, built in the 1880s for Liverpool Corporation Waterworks to supply Liverpool with fresh water. It flooded the head of the Vyrnwy (Welsh: Afon Efyrnwy) valley and submerged the village of Llanwddyn.
The Lake Vyrnwy Nature Reserve and Estate that surrounds the lake is jointly managed by the Royal Society for the Protection of Birds (RSPB) and Severn Trent Water and is a popular destination 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.
George Deacon (1843–1909) was instructed to prepare the parliamentary plans for the Vyrnwy Dam in 1879. The valley was chosen because of its size, favourable geology and its source, the River Vyrnwy, has a large water catchment area. The river is 63.9 kilometres (39.7 mi). It now flows from the dam into Shropshire where it converges with the River Severn near the village of Melverley on the Welsh border. The ouflow is into the Bristol Channel.
Thomas Hawksley was appointed as engineer-in-chief and prepared the design for a stone dam. Dam construction started in 1881 and was completed in 1888. George Deacon was appointed as joint engineer and Hawksley resigned in 1885 because of this conflict. It was the first large stone-built dam in the United Kingdom, and is built partly out of great blocks of Welsh slate. When built it cost £620,000 (equivalent to £70 million in 2016).
The dam is 45 metres (146 ft) high from the bottom of the valley, and 37 metres (120 ft) thick at the base; it is 355 metres (1,165 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 4.3 metres (14 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 a stilling basin; this is necessary to absorb the energy when the water flows over the crest and into the valley, and to stop the water eroding the foundations of the dam.
A power house located under the west tower contains an electrical generator driven by water leaving the reservoir. Before mains electricity arrived in the 1960s this was the area'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 flows into for the River Vyrnwy, which would otherwise dry out unless in flood. Depending on the water levels downstream the reservoir can release anything from 25 to 45 megalitres (5,500,000 to 9,900,000 imp gal) of compensation water per day. This flow is measured by the Environment Agency at a weir a few hundred metres downstream.
Earlier dams in Britain had been built using 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.
As the reservoir is part of the tributary of the River Severn, responsibility for the Vyrnwy Dam and associated structures falls to Severn Trent Water. The rights to the water abstraction are currently with United Utilities for drinking water supply to Liverpool.
Straining tower and aqueduct
Approximately 1,200 metres (0.75 mi) from the dam is the reservoir's straining tower. Standing only 30 metres (98 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 47 metres (154 ft) tall, 32 metres (104 ft) of which is above top water level, and is topped with a pointed copper-clad roof, coloured light green.
The water from Lake Vyrnwy is carried 109 kilometres (68 mi) to Liverpool in the Vyrnwy Large Diameter Trunk Main (LDTM) aqueduct. This originally consisted of two pipelines, made largely of cast iron. To help maintenance work on the 2.7 metres (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 aqueducts were constructed 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 aqueducts are largely underground 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 was undertaken in 1978-81. The current provision relies on three pipes 1.1 metres (42 in) in diameter delivering up to 230,000 cubic metres (50×106 imp gal) per day into reservoirs at Prescot, east of Liverpool. In 2013 United Utilities commenced a major refurbishment of the LDTM aqueduct, which was scheduled for completion in 2020.
The reservoir is Severn Trent Water's largest. When full, it is 26 metres (84 ft) deep, contains 59.7 gigalitres (13.125×109 imp gal), and covers an area of 4.54 square kilometres (1,121 acres), the equivalent of around 600 football pitches. The lake has a perimeter of 19 kilometres (12 mi) with a road that goes all the way around it. Its length is 7.64 kilometres (4.75 mi). On a clear day the lake, along with many others in North Wales, can be seen from space.
311 brooks, 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.
The lake continues to supply Liverpool with drinking water.
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 peregrine 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.
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.
Activities in the area include sailing, hiking on Glyndŵr's Way, rock climbing, cycling, walking and horse riding. The Lake Vyrnwy Half Marathon is conducted annually. The RSPB have laid out seven waymarked trails ranging from 2 to 9 kilometres (1 to 5.5 mi) in a range of habitats.
The site was once home to the tallest tree in the UK, a Douglas Fir 63.79 metres (209 ft) high. This was damaged in stormy weather and had to be felled. A nearby Douglas Fir is now, at 60.62 metres (199 ft), the tallest tree in Wales.
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