Lake Vyrnwy - The Straining Tower and Aqueduct Aka The Vyrnwy Large Diameter Trunk Main (LDTM)

The Straining Tower and Aqueduct Aka The Vyrnwy Large Diameter Trunk Main (LDTM)

Approximately 1,000 metres (3,300 ft) 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 is Gothic and built during the same time as the dam. The tower as a whole is 63 metres (207 ft) tall, 15 metres (49 ft) of which is underwater. The other 48 metres (157 ft) is above water, and is topped with a pointed copper clad roof, which makes it look light green.

The sixty-eight miles of aqueduct bring water from Lake Vyrnwy to Liverpool, and are part of extensive works that also involve Britain's first high masonry dam at Vyrnwy.

The aqueduct originally consisted of two pipelines, made largely of cast iron. To help maintenance work on the 9 ft (2.7 m) 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 mains piping.

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 (1.1 m) in diameter delivering up to 50 million imperial gallons (230,000 m3) per day into reservoirs at Prescot, east of Liverpool.

The aqueduct carrying water away from Lake Vyrnwy to Liverpool 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 a number of 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. In terms of the history of roads in the Tanat Valley, it is interesting to note that complaints were made about damage to local roads during the construction of the Lake Vyrnwy reservoir.

The Vyrnwy Large Diameter Trunk Main (LDTM) is currently undergoing a major refurbishment project by United Utilities.