Pontcysyllte Aqueduct

Year:1805

Duration:10 years

Cost:£47,000 (£3.4m today)

Country: River Dee, Wales

What did this project achieve?

Build an aqueduct to carry canal boats and people across the Dee valley

Pontcysyllte Aqueduct is the longest and highest aqueduct in the UK. Crossing the Dee Valley, it's a grade 1 listed building and part of a World Heritage Site.

The aqueduct is 1,007ft (307m) long and 126ft (38m) above the river. Designed by engineer Thomas Telford, it was one of his first major projects.

The project was viewed as ambitious and ground-breaking when work started in 1795. Until then aqueducts had been masonry structures with a waterway contained by lining the stone basin with clay. They were rarely higher than 35ft (10m) above the river or road below.

Telford's plans used a cast-iron trough to carry water and boats across the valley. The structure would be more than 3 times the height of existing aqueducts.

The aqueduct was built during the 20 years of so-called 'canal mania' between 1790 and 1810. Over 1,180 miles (1,900km) of canal were completed over this period.

Difference the aqueduct has made

Pontcysyllte Aqueduct was built to carry what's now known as the Llangollen canal across the river Dee valley.

The canal connected the industries of north east Wales with the canal network leading south to the English midlands.

The crossing made it easier for goods to get from Wales to England. It helped boost the local economy and opened the region to new markets.

How the aqueduct was built

Telford's design for the aqueduct saw workers build 18 slender stone piers [columns] 125ft (38m) high. To save weight and material the piers were hollow from about 70ft (21m).

The canal itself was carried in a trough made of cast iron plates. The trough was 12ft (3.7m) wide and 5.5ft(1.7m) deep.

The overall width at the top of the aqueduct was enough for a canal boat and a towpath for pedestrians.

Telford gave each of the 18 spans 4 cast iron supporting arches. He designed the arches in the same way as a typical stone arch. This made good use of the strength of the cast iron in compression – i.e. when subjected to weight.

Iron castings for the project came from the nearby Plas Kynaston Foundry. Project structural engineer and ironmaker William Hazledine built the foundry hoping to get the contract for the aqueduct.

Once work was complete engineers filled the aqueduct with water and left it for six months to check the structure was watertight.

"​‌

[A] most impressive work of art.

Sir Walter Scott

Novelist and poet, on Pontcysyllte Aqueduct

Fascinating facts

Llangollen canal has become one of the most popular routes in the UK for narrow boat holidaymakers because of its aqueducts and scenery.

The area has become known for its wildlife – otters have been spotted in the canal.

Every 5 years the aqueduct is closed and a plug pulled out to drain water into the river Dee below. This is to allow maintenance of the trough.

People who made it happen

  • Designer: Thomas Telford, first ICE president
  • Consulting engineer: William Jessop
  • Structural engineer: William Hazledine, who also cast ironwork for the bridge

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