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Longdon-on-Tern Aqueduct
Longdon-on-Tern, Shropshire, UK
Longdon-on-Tern Aqueduct
associated engineer
Thomas Telford
date  1796
UK era  Georgian  |  category  Aqueduct  |  reference  SJ616156
ICE reference number  HEW 280
photo  © Chris Allen and licensed for reuse under this Creative Commons Licence
Thomas Telford’s aqueduct over the River Tern in east Shropshire is the world’s oldest (and almost its first) cast iron canal aqueduct. It is thought to be the prototype for his magnificent Pontcysyllte Aqueduct. Constructed to carry the Shrewsbury Canal, it is now an isolated structure, though highly deserving of its status as a Grade I listed building and scheduled ancient monument.
In 1793, the building of the Shrewsbury Canal was authorised, for a route from Trench to Shrewsbury. Chief engineer for the canal was Josiah Clowes (1735-94) but he died unexpectedly before the project was completed. On 28th February 1795, Thomas Telford (1757-1834), then Shropshire County Surveyor, was appointed his successor.
The original canal aqueduct at Longdon had been designed by Clowes. It crossed the River Tern (a tributary of the River Severn) about 8km north west of the town of Wellington. The aqueduct was of sandstone and brick, and its arches were still under construction when it was demolished by a flood on 10-12th February 1795.
Telford, perhaps influenced by local ironmasters such as William Reynolds (1758-1803) of Ketley Ironworks, decided to use cast iron to rebuild the aqueduct. This was a pioneering step and it enabled the structure to be lighter and quicker to erect. Longdon-on-Tern Aqueduct is also likely to have been the prototype for the 307m Pontcysyllte Aqueduct (completed 1805) on the Llangollen Canal at Cefn-mawr in Wales.
By mid March 1795, Telford’s aqueduct plans for Longdon had been approved by the canal company, providing the structure cost less than £2,000. Clowes' heavy masonry abutments, each with two flood arches, were retained and the aqueduct was reconstructed in cast iron.
Its rectangular open trough is 2.7m wide, 910mm deep and 57m long overall — too narrow for boats to pass. It was fabricated at Ketley Ironworks from flanged cast iron plates joined by wrought iron nuts and bolts. The vertical sides are composed of wedge shaped plates.
The channel is supported on three intermediate piers, dividing the length into four spans of 14.5m. Each cast iron pier is set on a masonry base and consists of a pair of slender three-legged cruciform sections (one vertical and two inclined) with plain diagonal cross braces between.
The towpath is carried on cantilevered brackets on the south side of the trough, level with its base. Plain cast iron railings run along the outer edge of the towpath.
Longdon-onTern was the first navigable aqueduct to be designed in cast iron but not the first to be operational. In February 1796, just one month before Longdon, a much smaller aqueduct opened in Derby, for Telford was not the only engineer exploring cast iron's possibilities.
The Derby Canal was under construction at the same time and its engineer, Benjamin Outram (1764-1805), decided on a cast iron trough for the Holmes Aqueduct. It was much smaller than the Longdon Aqueduct, with a single span of 13.4m. However, it suffered a series of structural problems and was demolished in 1970-1, to make way for new road infrastructure.
In 1944, the Shrewsbury Canal closed. Longdon-on-Tern Aqueduct was dewatered in the 1950s. Though it remains in its original position, the canal embankment that served it has been levelled.
In March 1971, the aqueduct was given Grade I listing for its engineering heritage importance. It is also a scheduled ancient monument.
Cast iron trough: William Reynolds, Ketley Ironworks
Research: ECPK
reference sources   CEH W&WTTBDCE1

Longdon-on-Tern Aqueduct