The Thames Barrier
Review 2
The Thames has always flooded. Samuel Pepys wrote in his diary for 7 December 1663, “There was last night the greatest tide that ever was remembered in England to have been in this River all Whitehall having been drowned.” The last time that central London flooded was in 1928 when 14 people drowned.
If we had a severe flood in London now, thousands of homes, shops factories, businesses and buildings would be affected it could take London months to get back to normal. It would damage the fresh water and sewer systems as well as disrupting power gas, telephones and vital data services.
Tide levels are slowly rising due to several reasons. Firstly, because of climate change, sea levels are rising and there are also more storms. The British Isles are also tilting. The southeast corner is tipping downwards. London was built on a bed of clay, which is gradually compacting.
This means that tide levels are rising in the Thames Estuary by about 60 cms per century, which is a lot. The geography of the Thames Estuary means that surge tides are a threat. Certain weather conditions cause a trough of low pressure to move across the Atlantic Ocean towards Britain. When this happens, the sea underneath it rises above the normal level, making a hump of water which moves eastwards with the depression. If this depression passes the North of Scotland and turns southwards in the North Sea, very dangerous conditions can be created. When this hump of water coming from the deep part of the ocean reaches the shallow bit of the North Sea, it causes a surge. The surge can be made even higher by strong northerly winds. If this then happens at the same time as a spring tide (these happen twice a month) reaching the narrow part of the Straights of Dover and going into the Thames Estuary, this could cause a real flood danger along most of the tidal Thames. The picture I have drawn shows how spring tides happen.
A second but not so serious threat comes when a depression travels eastwards up the English Channel. This kind of surge is usually smaller than those from Scotland.
Before the barrier could be built there had to be an Act of Parliament. Royal Assent to the Thames Barrier Act giving the power to go ahead was given in 1972.
How the Thames Barrier works
The Thames Barrier has ten separate movable gates. They run end to end across the Thames and when they are shut they seal off part of the upper Thames from the sea.
When a dangerously high tide threatens the rising sector gates are lifted their river- bed position and the four radial gates are brought down into the closed defence position. This makes the gates into one long steel wall from bank to bank.
The barrier is about 520 metres wide. Each of the four openings is 61 metres wide.
Each gate is over 20 metres high. There are two other smaller gates with 31 metre openings. The four radial gates have non- navigable openings next to the riverbanks. The water there is too shallow for boats.
It takes over 80 people to work and look after the barrier and the other flood defences.
It is the barrier controller who decides whether or not to close the barrier this is done by watching the weather and monitoring the tides along the East coast to predict the height of the tides. Dangerous conditions can be forecast up to 36 hours a head.
Working model of the Thames Barrier
If a flood is predicted the barrier is closed 3_4 hours before the peak of the incoming surge tide. Before the barrier is closed, staff have to inform the Port of London Authority who control navigation on the river Thames. They then tell shipping by radio. Notice boards both upstream and downstream of the barrier are lit up. Navigation lights on the barrier itself also warn of its closure. Although the gates only take a few minutes to close, more time is allowed to close it to stop a reflective wave being created. Its cost f6 million pounds a year to operate the barrier and other defences, plus another f5 millions.
It is an amazing barrier.
This is a picture of me and some of my friends when we visited the barrier. We are going to go again to see the barrier when they do the routine maintenance so that we can see it up.
By Richard Dawe.