Raising the bar on Old Farmer Thames

0
162

The Thames Barrier is the world’s second largest movable flood barrier. It was constructed between 1974 and 1984 at Woolwich Reach to provide flood control of the River Thames.
Flooding of the Thames has long caused a problem for London. Samuel Pepys knew it well. On 7 December 1663, his diary notes: “There was last night the greatest tide that ever was remembered in England to have been in this river, all Whitehall having been drowned.” Fourteen died in the 1928 floods and 307 in 1953.

When a low pressure system in the Atlantic Ocean tracks eastwards past the north of Scotland, the resulting storm surge can be driven into the shallow waters of the North Sea and, thence, funnelled into the narrows of the English Channel and the Thames Estuary.
Dangerously high water levels occur in the Thames Estuary when such a surge coincides with a spring tide. The barrier’s job is to control such levels.

The barrier occupies a 523m-wide stretch of the river. Nine concrete piers and two abutments divide the river into four 60m and two 31m navigable passages, plus four small non-navigable channels.
The openings are spanned by rotating flood gates made of 38mm thick steel, and having circular segmented cross sections. They fill with water when submerged and empty when raised to allow for underspill. For maintenance they can be rotated 180°. 

The four central gates are 67m long and 10.6m high, weighing-in at 3500 tonnes. The outer navigable gates are 30m wide.
The barrier is raised only for the duration of the high tide. It is lowered at ebb to release upstream water backed up behind it.
Prior to 1990, the number of barrier closures averaged one to two per year. Since then the number has increased to four per year, but in 2003 the barrier was closed on 14 consecutive tides.

On 9 November 2007, the barrier was closed twice following a massive storm surge from the North Sea comparable to that which produced the devastating floods in 1953.
A near catastrophic incident occurred on 27 October 1997. 

The dredger, MV Sand Kite, collided with one of the Thames Barrier’s piers. As she sank on top of one of the barrier’s gates, the ship dumped her 3300-tonne load of aggregate. Initially the gate could not be closed as it was covered in a thick layer of gravel.

If flooding had occurred before the vessel was refloated, the damage was estimated to have been around £13 billion.
The concept of the rotating gates was devised by Charles Draper. The barrier was designed by Rendel, Palmer and Tritton for the Greater London Council and tested at HR Wallingford Ltd.

The site at Woolwich was chosen because of the relative straightness of the banks, and because the underlying river chalk was strong enough to support the barrier.
Work began in 1974, with construction undertaken by a Costain/Hollandsche Beton Maatschappij/Tarmac Construction consortium. It was largely complete by 1982, and officially opened on 8 May 1984.

The total construction cost was about £534m.