The Great Barrier Feat
Despite being one of the most iconic landmarks in the city, it seems that few people understand how the Thames Barrier actually works. Do those things swivel round? Are they giant vacuum cleaners sucking out water? BBC London went to find out.
It is certainly more than just a shiny tourist attraction. London and the Thames Estuary are vulnerable to flooding. In 1928 central London was flooded and 14 people died. Then, in the great North Sea flood of 1953, which killed nearly 2,000 people in the Netherlands, the Thames and River Lea burst its banks causing damage to over 1100 homes in East London and killing 300 people.
Mercifully, central London had a lucky escape, but it was decided that something needed to be done to protect the city from future floods. Had the 1953 flood reached central London, the loss of lives and damage caused to critical infrastructure would have been catastrophic. A flood defence system became a necessity.
A government committee recommended that a barrier be built across the Thames that could control the tides. Several sites were proposed, before it was decided that Woolwich, because of the relative straightness of its banks, was the best location.
The structure would consist of 10 movable gates across the width of the river, with each of the circular gates being supported between concrete pillars. Work started on the £500m project in 1974 and took ten years to complete. Now one of the most recognisable features of London, its striking shape is a true marriage of both form and function.
The Environment Agency
Today, the Thames Barrier is managed by the Thames region of the Environment Agency. At their offices right next to the Barrier, the technical support team leader, Steve East, explained to BBC London how it works and what would happen if it didn't.
"Effectively it puts a wall of steel right across the river," says Steve. "The barriers are about 61m across and by using 10 gates it blocks off the incoming tide from coming into the capital. The piers hold the gates and house the operating machinery."
"Before the tidal defences were built there was a contingency plan involved – sounding air raid sirens, moving buses onto higher ground and tube stations closing. So the life of London was disrupted. Whereas, nowadays, if there was a threat of tidal flooding the barrier comes into action and London can go about its normal business, without even being aware that the barrier is being used."
How it works
The Environment Agency runs computer models forecasting the height of the tides for the next 36 hours. Other factors taken into account are the weather conditions and the wind directions. If the tides are predicted to be close to the closure levels, then the controller will make a firm decision to close the barrier.
A gate in its closed position
"Then, we can start to bring our procedures into operation," says Steve East. "That involves, if it is 3am in the morning, calling our team at home - they've got two hours to come in and respond - and we start working through a great big thick set of procedures which are basically communications procedures, talking to people internally and externally, doing checks on the structure and moving it into position."
"While we could move a large gate in about 8 minutes or so, we don't actually do that. We use the smaller gates and bring them into the closure position and then we finally bring the large, rising 61m gates about half way, so we slow the tide of the river, which is kinder to the barrier and kinder to the bed of the river. We slow that flow before we finally bring the barrier to closure position. It probably takes about a couple of hours to undertake the whole process."
An open gate
Once the barrier has been closed a differential is created (as much as 4 metres) between the tides flowing upstream and downstream. Once the equilibrium has been restored, the barrier is re-opened.
When the question is put to Steve East why the barrier can't be closed all the time to save everyone the trouble (and the 3am wake-up calls) he politely points out the ecological damage this would cause, the disruption to river life and the fact that this would be illegal.
Since it began operating, the barrier has been closed on over a hundred occasions. Before 1990, the barrier only needed to be closed once or twice every year. Since then the average has risen to about four or five times a year. In 2003 the barrier was closed for 14 consecutive tides. The Environment Agency admits to a 6.5mm rise on the Thames in the mean tide each year. All this is evidence of climate change, surely?
"We are finding that it is far too early to say whether what we are seeing in relation to the sea level rise on the Thames is related to climate changes," says Rachael Hill, the Technical Strategy Manager of the Environment Agency's Thames Estuary 2100 project (TE2100).
"Because sea level rises is one of those things which you can predict way in advance and our predecessors who designed and built the Thames Barrier actually planned for that sea level rise, so the use of the barrier today is difficult to connect it with any sort of possible climate change impact. That is not to say that we are not planning for those future changes. That is something we are looking at."
It will be Rachael's job, along with her colleagues, to manage the flood risk for the next hundred years based on four possible scenarios – from mild effects of climate change to the worst possible. They aim to identify 'what needs to done, where it needs to be done and when it needs to be done' to ensure that the risk of floods are minimised and that if floods do occur then the consequences are manageable. The TE2100 project will be launching an online public consultation initiative this autumn and their recommendations will be put forward to the government in 2009.
Not like the movies
Severe floods in both the north and south of England in 2007 highlighted the scale of the threat. More than ever before, flooding has become a political issue and one that can grab the headlines.
It has even caught the imagination of movie-makers. The film, Flood, is a disaster move that sees London being overwhelmed by giant waves. Despite being a complete work of fiction – 'it couldn't possibly happen, it's never going to happen' – the Environment Agency cooperated with the filmmakers and welcomes its release.
"We have shown that if you model the worst weather conditions that have ever been recorded in the North Sea and mix that with the highest ever tides that have ever been recorded, you cannot combine those factors and create a surge that would defeat our defences," explains Steve East.
Perhaps it speaks volumes about the current systems of flood defences in the Thames Estuary that the Environment Agency is so relaxed about a film that purports to show the very devastation they are working so hard to avoid.
"We are really confident when we say that London has a world-class level of flood protection. But it is right that we are looking ahead, think of future generations and looking ahead to the next 100 years."
last updated: 15/05/2008 at 11:44