Telford1

Thomas Telford’s Bridges

Over the River Severn

10th November 2000

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Among the many splendid arch bridges over the River Severn are some excellent designs by Thomas Telford. He lived from 1757 to 1834. His career in civil engineering dates from around 1785. He designed magnificent canal aqueducts and bridges, probably the most famous being the Menai Strait suspension bridge, with a span of 580 feet, a record in 1826.

Over Bridge near Gloucester

These pictures show the old bridge at Over, west of Gloucester, which was used from 1829 to 1974. This 150 foot span, designed by Telford after the previous bridge was damaged by ice in 1818, uses the cornes de vache technique, which may improve the flow in times of flood, previously used by Perronet for a bridge over the River Seine at Neuilly. It also gives the illusion of a flatter arch. The actual profile of the arch along its centre-line is not changed by this technique – what is changed is the sharpness of the transition from the full width of the river in spate to the narrow arch. With the horn shapes cut out there is some hope of reducing turbulence as the water is funnelled into the opening.

The crown of the arch dropped by about ten inches, about 25 cm, when the centring was removed. This is shown in two of the pictures. The line of thrust is still within the deep voussoirs, and so the bridge continued to stand, and it is still there, over 170 years later. The ground here is composed of soft alluvial material, and in fact Gloucester, like nearby Tewkesbury and Worcester, has been built entirely on or near the left side of the River Severn, where the ground is better. In fact Tewkesbury is not very close to the Severn at all. The first recorded bridge in the neighbourhood of Over was recorded in Domesday Book as existing in 1089. Another bridge was started in the sixteenth century, and it was this one that was damaged by ice carried down by the river.

In one picture the new steel replacement bridge can be seen just to the north of Telford’s bridge.

The pictures show the Over bridge at a time of high flood. We can imagine that the shape of the arch with cornes de vache might make the flow a little easier when the water reaches the arch, especially as the water flows fastest at the surface, firstly because it experiences little friction, and secondly because it is being forced through the narrowing arch. In these pictures the water has begun to encroach upon the arch, as the third picture shows. The first two pictures show the vertical chamfers that the cornes de vache provide.

Severn bridges upstream have much less clearance, and are submerged more often. The diagram below shows a rough idea of the horizontal cross section of the bridge at a normal water level and at very high flood level. Perhaps Telford wisely provided the clearance because the Severn estuary has a very high tidal range. Flooding can be severe when spring tides coincide with flood water coming down river, though of course the incoming tide reduces the speed of the water. But when the tide goes out the high level spurs the water to higher speeds than before.

The Mythe Bridge near Tewkesbury

Near the junction of the Rivers Severn and Avon close to Tewkesbury another Telford bridge, this time of cast iron, carries the A438 over the Severn at The Mythe. Some pictures are shown.

The road narrows to one lane, with traffic lights permitting flow in only one direction at a time.

The road signs refer to a weak bridge with a weight limit of 17 tonnes. But of course when Telford completed this bridge in 1826 it was only intended for horses and carts and pedestrians. Seventeen tonnes is a large load that would have been considered absurd at the time. The bridge is of cast iron construction with triangulated trusses, crossing the river in a single span. There is now a pronounced dip in the middle, seen in the picture of the roadway, showing that the abutments must have moved slightly further apart, just as they have at Over.

At each side of the main arch there are several very narrow pointed arches, presumably to allow flood water through. Why did Telford not use a simple round arch at each end in order to reduce the resistance to flow? Perhaps these narrow arches were easier to build in a place where side-thrust was not desirable. Perhaps they are not for flood water at all.

Apparently Telford was not the original designer at the site – a Mr Moneypenny is said to have started construction, before the contract was transferred to Telford.

Holt Bridge or Holt Fleet Bridge

Holt Fleet bridge is rather similar to the Mythe bridge. It was built between 1826 and 1828. In 1928 it was strengthened. A curved sheet of concrete was made around all the lower ribs, and concrete as also added around upper ones. Given the techniques at the time, this was probably all that could be done, while allowing Telford’s basic structure to show through. It was a sensitive piece of repair work.

The third picture shows a detail. The concrete encases the lower ribs over the full span and width. But the only upper ribs that are encased are the outer ones, except near the abutments, where all are encased.

Unlike the Mythe bridge, Holt Fleet bridge has two semicircular arches at each end, built in old red sandstone.

Visit Severn bridges book for details of a very useful book by Chris Witts, who has spent much of his working life on the River Severn. This book, called "A Century of Bridges", includes information, a grid reference, and a drawing by the author for every bridge, even including notes about demolished bridges and some history. A Century of Bridges, ISBN 0 9532711 0 2, is published by River Severn Publications, Gloucester.

Another very interesting book by the same author is called "Along the Severn from Source to Sea", ISBN 1 873877 31 5. This book is published by Reardon Publications, Leckhampton, Glos.

Link to Atlas of Thomas Telford’s Bridges –

http://www.umist.ac.uk/~civhist/sources/atlas.htm

Biographies of Severn bridge designers –

http://www.severnbore.ndirect.co.uk/design.htm

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