Reservoirs of Wales

There are a large number of reservoirs in Wales reflecting the need for the supply of water for both industry and for consumption, both within the country itself and in neighbouring England. A number also provide hydroelectricity and many old reservoirs also provided motive power for industries, especially for the processing of minerals such as metal ores and slate.

Wales has a relatively high rainfall and significant areas of mountainous topography which are sparsely populated which ensures that water is a plentiful natural resource. Much of the south and east of England has significantly lower rainfall^[1] and a high population density which puts significant strain on the water supply infrastructure there. Collecting water in deep valleys with high river flows is far more efficient that trying to collect sparse rainfall in a generally flat landscape. Because of this engineers have viewed Wales as a potential supply of water. The rapidly growing populations of the industrial centres of Britain in the industrial revolution made the need for high volume supplies of clean water even more important and Wales was seen as the ideal source. Such engineering and economic views paid scant regard to the social, cultural and linguistic impacts that the constructions of large reservoirs in Wales would create.

Some reservoirs have been formed by the raising of dams on water courses, others are natural water bodies, the levels of which have been raised to various degrees both for direct supply of water or else for regulation of supply with water being extracted from the river some way downstream. Many reservoirs which were constructed for one purpose e.g. industry, may now serve different or additional purposes e.g amenity or, as in many cases, have been abandoned entirely. Reservoirs range in size with many under 1 acre in area serving local needs, up to Llyn Trawsfynydd which extends to 1,128 acres (456 ha). Bala Lake (Welsh Llyn Tegid) is a natural lake which at 1,192 acres (482 ha),^[2] slightly exceeds Llyn Trawsfynydd in size, but whose height has been slightly raised.

Water supply to England[]

Most Welsh lakes supply the nation itself with water, however, there are lakes and reservoirs which supply England. In the early 1960s, 70 people were forced to leave their homes in Capel Celyn before the whole valley was drowned, and the Llyn Celyn reservoir built in order to supply water to Liverpool, England, 60 miles (95 km) away. Birmingham Corporation had one hundred occupants moved out of their homes in the Elan Valley, Powys, in order to build the reservoirs.^[3][4] Wales has its own water company, Dŵr Cymru, but two water companies operate across the border namely Severn Trent Water and United Utilities.

Reservoirs in Snowdonia[]

Local water supply[]

Within Snowdonia, numerous existing natural lakes were co-opted to provide local water supplies during the late 19th and early 20th centuries. Within the Carneddau, Llyn Dulyn and Llyn Melynllyn were dammed in 1881 and 1887 respectively, to provide water to Llandudno whilst Llyn Anafon was dammed in 1930 to do likewise for coastal communities to the north. Llyn Cowlyd has supplied the towns of Conwy and Colwyn Bay with water since the level of the natural lake was raised by a dam in 1897. Bala and surrounding villages have been supplied by the artificially raised waters of Llyn Arenig Fawr. Elsewhere Llyn Elsi has been supplying water to Betws-y-Coed since its damming in 1914 whilst supplies to the village and to nearby Llanrwst are supplemented by water from Llyn Conwy. Barmouth is served by Llyn Bodlyn. The height of Llyn Cwellyn was raised as it took on the role of supplying water to parts of Gwynedd and Anglesey.

HEP schemes[]

The establishment of an aluminium works at Dolgarrog led to construction of Dolgarrog Power Station which was supplied with water from the damming of the natural lake of Llyn Eigiau and the smaller Coedty Reservoir downstream of it. The latter was constructed in 1924 but the failure of the Eigiau dam in 1925 caused its destruction. It was rebuilt in 1926 and modified in 1956. Water was also sourced for this HEP scheme from Llyn Cowlyd, itself supplemented by a leat from a dammed Ffynnon Llugwy. The latter has provided a water supply for Bangor and east Anglesey since the mid 1970s.

The extensive mire of Gors Goch was flooded by the raising of a dam between 1924 and 1928 to form Llyn Trawsfynydd to enable HEP at Maentwrog. Between 1965 and 1991 the reservoir was also used as a source of cooling water for the Trawsfynydd nuclear power station. The UK’s first major pumped storage scheme was developed at Ffestiniog in 1963. It uses , a natural tarn lake enlarged by the construction of a dam as an upper reservoir and Tanygrisiau reservoir as a lower reservoir. Llyn Stwlan had been previously enlarged in 1898.

The development during the 1970’s of the Dinorwig HEP scheme involved the damming of Marchlyn Mawr as a top water storage facility and the use of the originally natural ribbon lake of Llyn Peris as the lower reservoir for the scheme. Its capacity was increased by the removal of slate debris which had been dumped into it over decades during the operation of the Dinorwic quarry.

On a smaller scale, the damming of Llyn Bodgynydd and Llyn Glangors served to provide power to the former Pandora lead mine just as Llyn Parc’s waters supplied power to Aberllyn lead and zinc mine. Llyn Mair provided both a drinking supply and a source of hydro-electric power for Plas Tan-y-Bwlch from the 1890’s.

Reservoirs in South Wales[]

The growth in the size of towns in industrialising South Wales during the latter part of the 19th and early part of the 20th centuries necessitated the construction of numerous reservoirs for domestic water supply and for industrial use.

Cardiff Corporation Waterworks opened both Llanishen Reservoir and Lisvane Reservoir in 1886. It later commissioned the construction of three reservoirs in Cwm Taf to supply water to the capital. Beacons Reservoir was the first to take shape, between 1893–97, Cantref Reservoir was built between 1886–92 and the damming of the largest of the three, Llwyn-on Reservoir began in 1911 with the supply of water starting in the mid 1920s.^[5]

Cray Reservoir was constructed between 1898-1908 to supply water to Swansea and the city council completed work on Usk Reservoir in 1955. Nearby Neath began to take water from Ystradfellte Reservoir in 1914 (1907) on completion of the rural district council’s work on the dam on the Afon Dringarth. Newport Corporation built Talybont Reservoir between 1931-1939.

Llandegfedd Reservoir began supplying water in 1965 to Cardiff, Newport, Pontypool and other areas nearby. It draws its water from the River Usk, being pumped up from a weir above Usk.

Grwyne Fawr Reservoir, the only reservoir within the Black Mountains massif was completed by Abertillery and District Water Board in 1928, serving to supply water to various towns in the traditional county of Monmouthshire.^[6] A holding reservoir in Cwmtillery was constructed as a part of the scheme. Blaenycwm Reservoir to the northwest of Brynmawr, was originally constructed as a water supply for Nantyglo Ironworks but enlarged for public supply during the 20th century.^[7]

Llyn Brianne was constructed in the late 1960s to provide additional water supply to South Wales via the Felindre water treatment works. The original intention was to supplement supplies of water to Swansea, Neath and Port Talbot^[8] but a subsequent extension of the trunk mains provides water up to west Cardiff.

River regulation[]

Rivers are an effective and cheap means of moving water from high ground to lowland areas where much of the population lives. River regulation reservoirs make use of that opportunity to store winter rainfall and release it, as needed , during drier parts of the year. This makes increased volumes of water available for abstraction down-stream, sometimes right down at the tidal limit as in the case of the River Dee. It also provides flexibility in where the downstream abstractions are made depending on demand on a day-by-day basis.

River regulation reservoirs may also play a role in winter flood mitigation. Operational rules may prescribe a maximum water height at the end of Autumn to ensure that the reservoir can absorb any extreme winter rainfall events in the uplands thus mitigating high flows downstream, Llyn Clywedog performs this function on the River Severn. River regulation reservoirs are marginally cheaper to construct since there is no requirement for a take-off tower or associated large volume pipe-work or aqueducts piercing the dam wall. However a number of such reservoirs have installed hydro-electric generating sets to extract power from the released water. Examples of river regulation reservoirs include Llyn Brianne on the River Tywi and Llyn Celyn and Llyn Brenig on the River Dee. Proposals in the past have also included complex inter-river transfer schemes with, in one case, the enlargement of Craig Goch reservoir,^[9] the release of water into the River Severn^[10] via the Afon Elan and then a pumped transfer into the headwaters of the River Thames.^[11]

Despite the benefits of such regulation schemes, they have been shown to have a number of adverse environmental impacts. When releases are made they are typically from the reservoir bottom water drawing from the hypolimnion which is often very cold, acidic, iron rich and anoxic.^[12] The change in flow rate can also be very rapid which may be a serious hazard for water users who may have no warning of a very sudden increase in flow. Aquatic animals have also been shown to be adversely affected with spawning behaviour and smoltification of salmon affected below Llyn Brianne for example.

There may also be unexpected benefits. The operational rules for Llyn Celyn include a specific allocation of releases for recreational purposes to support competitive whitewater sports in the Afon Tryweryn below the reservoir. Canolfan Tryweryn , the national white-water centre for Wales was constructed on the river and maintains the infrastructure of overhead gates etc for all white-water sports which also benefit from the routine releases of water to support river flow, mostly in the summer months.

Notable reservoirs[]

• Llyn Trawsfynydd is the only reservoir or lake in the United Kingdom to have provided a source of cooling water for a nuclear power station
• Llyn Brianne has the tallest dam in the UK
• Llyn Clywedog has the tallest concrete dam in the UK

Safety and failures[]

It was, in part, the failure of the Llyn Eigiau dam, and consequent collapse of the dam of a second reservoir (Coedty) downstream, in the then Caernarvonshire (modern Gwynedd) in 1925 with the loss of 16 lives in the village of Dolgarrog which led to the enactment of the Reservoirs (Safety Provisions) Act in 1930. Under this legislation, all reservoir owners in Wales and England were required to have them inspected by a suitably qualified civil engineer, if the capacity exceeded five million gallons (22,700 cubic metres). The placed further duties upon both owners and local authorities in respect of the safety of those with a capacity of more than 25,000 cubic metres. The Environment Agency assumed safety responsibilities in September 2004^[13] and in 2013 the duties were transferred from the Environment Agency Wales to Natural Resources Wales.

References[]

• The Lakes of Eryri, by Geraint Roberts, Gwasg Carreg Gwalch, 1985

See also[]

• List of lakes of Wales