Thursday, August 14, 2008

Pontcysyllte Aqueduct

At Pontcysyllte (Welsh for “connecting bridge”) the Llangollen Canal crosses the River Dee at a height of 120 feet (36 meters) by means of a breathtaking aqueduct that marches more than 1,000 feet (306 meters) over the valley on nineteen slender, tapering (and partly hollow) masonry piers. Built between 1795 and 1805, the graceful structure remains the highest navigable canal aqueduct ever built. Besides its own weight it supports 1,680 tons (1,524 tonnes) of water. Pontcysyllte Aqueduct has been correctly identified as “one of the heroic monuments of the Industrial Revolution,” and the novelist Walter Scott asserted that it was the finest work of art he had ever seen.

Part of Britain’s 2,000-mile (3,200-kilometer) canal network, the spectacular Shropshire Union Canal—really a collection of canals built by various companies at different times—stretches 67 miles (107 kilometers) from the English Midlands town of Wolverhampton to the River Mersey. The original ambitious plan to link the Mersey and Severn Rivers was never achieved. The first stage, the Chester Canal from the River Dee in Chester to the town of Nantwich, was completed in 1779. Between 1796 and 1806 the Ellesmere Canal (later renamed the Llangollen Canal), fed largely by the Dee at the Horseshoe Falls, was built to connect Wrexham’s ironworks and collieries with Chester and Shrewsbury. Joining the Chester Canal to Ellesmere Port in the Mersey Estuary, it reached as far as Llantisilio near Llangollen in North Wales. It is probably the most beautiful canal in Britain.

At Pontcysyllte the Llangollen Canal is channeled over the Dee Valley in a 12-foot-wide (3.6-meter) trough made of cast-iron plate and supported by four arched iron ribs spanning 44 feet (13.6 meters) between the tall piers. The water level comes to within a few inches of the top of the trough, and because a towpath is cantilevered over the surface, the navigable width for long boats and barges is reduced to about 7 feet (2 meters). There is a safety railing on the towpath side, but on the opposite side there is a sheer drop to the valley floor, 120 feet (36 meters) below.
The courageous decision to use a relatively unknown material followed hard upon Thomas Farnolls Pritchard’s graceful Ironbridge at Coal-brookdale, Shropshire. Tradition continues to attribute the design of the Pontcysyllte Aqueduct to the Scots engineer Thomas Telford, who indeed claimed most of the credit, but more recent evidence suggests that, since he was working under the direction of William Jessup, the latter played a major part in the project. The financial backers of the Llangollen Canal approached Jessup in 1791, seeking an engineer “of approved character and experience.” Jessup prepared the earliest working drawings, he gave expert evidence before the parliamentary committee, and he first proposed an aqueduct with an iron trough. Many consider Jessup to be the greatest canal and river navigation expert of his day.
The 70-foot-high (21-meter), 400-foot-long (120-meter) aqueduct at Chirk (also with a cast-iron trough but supported with conventional masonry); the 1,200-foot (360-meter) Darkie Tunnel; and the Whitehurst Tunnel, all built by Jessup and Telford between 1796 and 1801, also form part of the Llangollen Canal. The spoil from their excavations was used to construct the huge earth embankments of the Pontcysyllte Aqueduct. The structure has been listed for many years as a historic monument and it was among thirty-five sites nominated by the British government in April 1999 for inclusion on the UNESCO World Heritage List.

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