Huge skies. That’s what you get on the Norfolk Broads. Huge skies and flat landscapes. Not, perhaps, as flat as the Cambridgeshire Fens which seem almost concave, but nevertheless flat enough for you to see for miles. In the distance there can be sails moving slowly through the fields amongst the remains of drainage windmills. Those holiday makers intrepid enough to venture from the safe haven of the North Rivers to the wilder Southern Rivers – the Yare and the Waveney – will come across several iconic structures.
The Broadland swing bridges were built to accommodate tall trading vessels bound for Norwich or Beccles in Suffolk. Over the years several lines closed and with them three of the bridges – Aldeby and Haddiscoe over the Waveney and the huge Breydon Water swing bridge. But despite the closures, Network Rail still owns four swing bridges in the Anglia area, Trowse, Reedham, Somerleyton and Oulton Broad – all of which must be able to open for river traffic – a requirement of the original acts of parliament.
Challenging mixture of disciplines
Bob Chatten is Network Rail’s electrification and plant maintenance engineer and is responsible for all the swing bridges. They are a challenging mixture of disciplines. They carry track that has to return to a safe state after each bridge opening. The basic load carrying structures are conventional bridges, the lifting and swinging components are mechanical plant. They are also linked into the signalling system and are operated by signallers who have to be trained to open and close them. The Trowse structure also carries overhead line electrification – a unique feature in the UK.
Swing bridges can be either very simple structures with fiendishly complex controls or they can be complex structures whose operation is simplicity itself.
Trowse is the youngest and simplest structure and is just outside Norwich station. It spans the river Wensum just before it flows into the Yare – the river that flows out to sea at Yarmouth. It’s several years since the last coastal freighter berthed in the centre of Norwich. Holiday and leisure craft are now all that need to have the bridge swung.
The present structure was built in 1986 at the time of the electrification of the Ipswich to Norwich section of the Great Eastern main line. It seemed like a good idea to restrict it to a single track – hindsight is a wonderful luxury! It’s a compact structure. The bridge swings on a pivot at the river’s edge. To reduce encroachment into the river bank it has a short landward span with a massive counterweight slung underneath to counter-balance the river span.
Like all of the swing bridges, when they are open to rail traffic they sit solidly on both landward bearings and the pivot pier.
This poses the question – how are they freed from these bearings so that they can be swung with a minimum of effort. The answer is that they have to be jacked up clear of their bearings.
The Norwich structure uses brute force. The whole bridge is lifted at the pivot, chocked and then swung clear of the river.
Up to 2008 all was well. Then came a violent thunderstorm and a lightning strike completely obliterated the electronic control system of the hydraulics. The bridge was stuck – shut to the river. Bearing in mind that the original electronics were of 1980s vintage it proved very difficult to carry out like-for-like repairs. In fact, the damage was so great that the whole of the control gear had to be rebuilt from scratch.
In conjunction with Converteam UK Ltd it has taken several years of trial and error to build a new control system. There is very little time during the night both to experiment with settings and to return the bridge to a serviceable condition before the early morning London trains are due.
There are a large number of stages involved in opening a bridge with overhead line electrification – which has to be isolated – that is linked into colour light signalling and that carries around eight trains an hour. The hydraulic jacks have to operate to millimetre accuracy and all have to be detected.
Once the structure is airborne and clear of encumbrances it can be spun by hydraulic motors. The jacks and transducers are located in the counterweight swing pit and controlled from the operating room beneath the signalbox. The sequences are controlled by banks of computers.
With a completely refurbished control system the bridge is close to being fully operational. The bridge isolates itself, lifts clear of the bearings and swings smoothly and silently.
It is easy to misinterpret the structures of the old swingbridges at Reedham, Somerleyton and Oulton Broad. Dominating the skyline are slender steel sections that rise above the central piers giving the impression that they are involved in carrying the trains. They’re not of course, they’re much too slender. A Victorian Broadland swingbridge consists of two separate elements within the moving bridge. There are the track/train carrying structures both of which are over the river – one span over the navigable section and one over the non-navigable section. There is also a cradle (the visibly dominant bit) that carries these two spans when the bridge is open to river traffic.
To complicate matters slightly, the track carrying structures have supports at their midpoints in the cradle so that they can pivot a small amount vertically when their ends are free.
To free the track span from the landward bearings, heavy steel chocks are drawn out from under the span ends on the central pier allowing them to drop slightly as they pivot about their mid-point supports in the cradle. Thus the landward ends rise clear of their bearings.
Once clear, the bridge is hauled round by a steel hawser that is powered by a very simple and ancient clutch operated winding gear. It is the skill of the signaller that ensures that the hawser is operated smoothly so ensuring a gentle landing when the bridge is hauled back into position. To the river-user the bridge moves completely silently and remarkably quickly.
No repair manual
These structures, although simple, can pose tricky engineering challenges. There’s no manual on how to replace worn parts. Several generations can pass between major repairs and so there’ll be nobody to pass on knowledge. When the central bearing on Somerleyton swing bridge started to seize it required some very careful, first principals engineering to coax the bridge back into life. There are jockey wheels around the perimeter of the central pier to steady the swing operation. When it was found that some of these were floating above the steel tracking ring this too indicated that all was not well with the bearing alignment. For such large structures Bob says that they work on extremely fine tolerances.
At the time of writing, Kier with subcontractors Sonic Rail Services were on site at Somerleyton gently nurturing the new bearing during it’s long commissioning period, and also putting in some strategic improvements. There will be ancillary manual plant to haul the bridge back into position if the aging winding gear becomes overloaded. Of course, this isn’t straightforward on a river bank sitting on vast quantities of silt.
Historic seawater pumps
The Oulton Broad swing bridge which is on the seaward side of the Oulton Broad lock is rarely seen by the general run-of-the-mill Broadland holiday makers. If they’ve got that far they’re definitely out of bounds. This is a slightly smaller structure than the others but operates on similar principles. In October it is scheduled to have its historic seawater hydraulic pumps and its equally ancient winding gear replaced by modern equivalents. This follows a very successful Civil’s refurbishment a couple of years ago.
Whole life management strategy
In the winter months the bridges at Somerleyton and Reedham are operated fairly infrequently. Only truly hardy sailors venture on the Southern rivers at this time of year. But a new holiday season has just started and they will be left open to river traffic, only closing just before a train is due. By the end of this financial year, Network Rail will have finished all essential planned works. In parallel though, they will be carrying out a review to decide on the whole life management strategy for Reedham and Somerleyton. But for now these extraordinary structures will continue to open and close silently over the tidal Broadland waters whatever the weather.