The Great Floods of the winter of 1794-5 gave Thomas Telford a big opportunity. The River Dee was proving to be a major obstacle to the engineers of the Ellesmere Canal (as it was known then). In his original plans, William Jessop included two tunnels, one 4,600 yards long, to enable the Dee to be crossed at a low level.
In January 1794 William Turner of Whitchurch produced his plan for a three arch masonry aqueduct for the Dee crossing at Pontcysyllte. Canal engineers had from the first experienced difficulty in constructing masonry aqueducts as the sidewalls had to resist the weight of water and puddled clay bursting the sides. The bigger problem was that the combined weight of the masonry and puddled clay limited the height of an aqueduct.
Although the new plan now proposed got rid of Jessop’s long tunnels, this masonry aqueduct would need to be at a low level because of the weight factor. This meant carrying the canal down into and out of the valley by a flight of locks; a wasteful and costly arrangement. Telford persuaded the canal committee to postpone a decision on Turner’s aqueduct and give him until the end of March to come up with an alternative.
Telford proposed forming the bed of the canal of cast iron plates bolted together and securely bonded into the masonry on either side. These plates would form a watertight bed and so get rid of the need for clay puddle thus reducing weight.
The death of Josiah Clowes, the engineer of the Shrewsbury Canal, in February 1795 led to Thomas Telford’s appointment as his replacement. Clowes’ masonry bridge at Longdon on Tern had been swept away in the great floods of the winter of 1794-5 leaving just the abutments. Telford used this opportunity to build a trough constructed of cast iron supplied by William Reynolds of Ketley, proving that his proposal was sound despite the activities of William Turner who denounced Telford’s scheme as impracticable.
William Hazledine purpose-built the Plas Kynaston Foundry at nearby Cefn Mawr, in order to provide many of the cast iron sections for the Pontcysyllte aqueduct. Once the stone piers had been constructed, Hazledine’s cast iron supporting arches and water trough sections could be manoeuvred into place and bolted together. Telford ensured that the trough was watertight by developing a strange combination of Welsh flannel and a lead, iron and sugar concoction.
Whilst the construction of the Pontcysyllte aqueduct is usually credited to Thomas Telford, he collaborated closely with William Jessop. Jessop advised an additional arch to make eight spans of 55 feet giving more support. Other modifications to Telford’s original plan included extending the width of the supports, and widening the trough from 9ft to 11ft 10in. The latter change enabled the towpath to be carried over the trough instead of beside it as at Longdon. This allows water to escape past a moving boat rather than spilling over the side. Interestingly, a suspended towpath (built in wood) featured in Berwick Tunnel on the Shrewbury Canal close to Berwick Wharf near Attingham Estate.
By Sean McKeown