Closing the floodgates

Another flood barrier could be needed by 2050 to protect London from rising water levels. Hard questions about where it should go, how it will work and who will pay for it are already looming.

The clock is ticking on one of the biggest engineering challenges facing the nation: protecting London and south-east England from catastrophic flooding. The Thames Barrier has done the job for almost 20 years, but government and engineers alike will soon have to make hard decisions over the future defences for the UK’s most populous and economically important region – and a second barrier may be inevitable.

The stakes could not be higher, as events of almost exactly half a century ago prove. On the morning of 30 January 1953 an atmospheric depression was forming over the Atlantic, a little to the south of Iceland. Later in the day this unremarkable feature headed eastwards. It rounded the tip of northern Scotland on the following day and moved down the east coast.

Unusually, however, the pressure continued to drop, and gale-force winds blew up. The combined effect, on that winter’s night of 31 January produced a tidal surge of almost 3m. The storm smashed through what existed of sea defences protecting Suffolk and Essex, and the rising waters surged up the Thames Estuary, destroying 100m of sea wall and flooding 1,000 houses. Businesses, factories and oil refineries were also flooded. In total 307 people lost their lives. When the surge hit the Netherlands more than 1,800 people died.

The floods eventually passed, but concern remained about rising sea levels and the potentially increasing magnitude of any future freak events. A decision was taken in 1966 to build the Thames Barrier at Woolwich Reach. It opened in 1984 and was designed to protect London against the ‘one in 1,000-year event’ until 2030.

Questions are now being asked about what happens after that date. Tide levels are continuing to rise, and the south-eastern corner of the country is sinking at about 2mm per year. There are engineers within the Environment Agency (the body responsible for the barrier and co-ordinating the UK’s flood defences) and its consultants who now believe that it could be necessary to build a second barrier further downstream some time this century – maybe as early as 2050.

This might sound a long way off. But given that the existing barrier had a lead time of 30 years, took eight years to build and was completed way over budget, time and money could be in short supply if a new barrier is deemed necessary soon after the guarantee on the existing one runs out. It must also be taken into account that a second barrier, possibly stretching from Sheerness, Kent, to Shoeburyness in Essex – more than five miles – would be a much larger and costlier project.

There are concerns about how such a project would be funded. With the UK’s poor track record of privately funded infrastructure initiatives it could be too risky to hand such an important contract over to PFI, that is if we want to be sure that a second barrier is completed on time and offers a reliable level of sea defence. Other large-scale barrier projects in Venice and St Petersburg have run into similar problems (see sidebar).

The EA is drawing up a plan to be published in 2007 that will outline what needs to be done to protect against the one in 1,000-year event up to 2100. It is this document that could recommend a second barrier, according to insiders. As part of the Flood Risk Management Project engineering consultant Atkins is carrying out an assessment of the existing barrier to determine which, if any part of it, is likely to fail before 2030 and beyond. It will be possible to over-rotate the gates of the existing barrier to gain a few extra feet of defence, but according to Geoff Ballard, a consultant for Atkins, this and other remedial works to the river walls may not be enough.

‘It may not be the most sensible option to raise the existing defences,’ he said. A second barrier in the Thames Estuary of similar design to that proposed for Venice, that would ‘cream off the peak’ of a tidal surge, is something the UK ‘could go for’, he said.

As development continues along the banks of the Thames (more than 20,000 houses are planned in the flood plain of Thamesmead and other locations, according to EA figures) the need for improved flood protection will increase.

The EA’s Flood Risk Management project, which is being managed from the Thames Barrier, will seek to take a consistent approach to protecting Kent, Essex and the Thames Estuary. Martin Earlam, team leader for Flood Defence Improvements at the barrier, said. The plans will represent a complicated equation that must take account of housing development and environmental issues, as well as tide level rise.

A combination of two things causes this: global warming, and another trend known as post-glacial recovery. The latter refers to the retreat northwards of the glaciers at the end of the last Ice Age. As the thaw spread across the south-east the land level began to rise, while the north of England and Scotland, still heavy with snow and ice, began to fall. Ever since the total disappearance of the ice from the north the balance has been slowly redressed with the south-east sinking, in London by the equivalent of 2mm a year.

The solution to the increasing risk of flooding must be cost effective, manage risk to an acceptable level, have minimal impact on property and business activities and be flexible enough to respond to changing conditions within the time frame. It must also be 100 per cent reliable.

There is a view that if global warming outstrips predictions, the point when action must be taken could come sooner rather than later. Necessary work to the existing barrier or the need for a new one would be flagged up very quickly, said Earlam. As soon as any of the parameters, such as post-glacial recovery or tide level rise, start to deviate from the projections made in the plan, a decision could be taken swiftly to bring forward the second barrier or let it go for longer, added Earlam.

(As part of the data-gathering exercise the Environment Agency proposes to survey the entire Thames Estuary region with Light Detection and Ranging (LIDAR) equipment, commonly known as laser radar.

This will give an exact representation of the topography and indications of land usage.)

So far the EA has done ‘scoping’ studies to assess what work needs to be done and its proposal is currently awaiting approval from Defra to proceed with drawing up the plan.

One of the first mentions of a second barrier is made in Thames Barrier, a book by Ray Horner, the original project manager of the Woolwich defence. He speculates about a barrage stretching between Sheerness and Shoeburyness, an idea that still has currency among those responsible for the estuary defences today. Earlam said there are trade-offs to be made between size and location, and how much wall raising would need to be done from the new barrier site outwards.

The further out the barrier is located the larger and more expensive it would become. But, said Earlam, the further out it goes the more property it would be able to protect, and the construction project would cause less damage to the riverbanks. ‘If the whole of the Thames Estuary was designated a Site of Special Scientific Interest (SSSI), for example, it would pose major difficulties in terms of trying to raise walls. You would have vessels and cranes driving all over this SSSI, whereas if you could place your barrier further out into the Thames Estuary this would not happen.’

There are also some benefits to be gained in terms of the structure itself, the further out into the estuary it is located. ‘Ray Horner proposed in his book that the next barrier would be a Sheerness to Shoeburyness-sized barrage. This is an enormous length, but it has less tidal range to cope with. The tidal range we have here is 21ft, whereas out in the Thames Estuary it is about a third of that. This must be significant, but I think width of barrier will be the primary factor. Navigation will be a major consideration and also the effect on the geomorphology of the estuary in terms of siltation and effects on the flow.’

Earlam agreed with Ballard that a second barrier could be used in conjunction with the existing one, to combat the larger tidal surges. ‘It could be that we keep this barrier to protect against a second-division flood and have a barrier out in the estuary to protect against a premiership flood, and you work the two in combination.’

Cost and a means of funding will also be a concern if a new barrier is required, agreed Earlam. The current asset value of the existing barrier is £1bn, which works out at £2m per metre over 500m. At that price a Sheerness to Shoeburyness barrage would come in at about £17bn, a figure that nobody would be prepared to spend, said Earlam: ‘A barrier like this [at Woolwich] would not be suitable for Sheerness-Shoeburyness. So we would have to look at something that was more economic, such as the Venice barrier.’

The Venice barrier is a system of hollow gates that lie flat on the seabed. To raise them sea water is pumped out of the tanks within the gates, and their buoyancy lifts them into an upright position. Earlam’s team has been asked to advise the Venice project on the operation of the barrage. While such a system would be cheaper than the Thames Barrier, and would allow greater navigation in the estuary, they have not yet been proven on a large scale.

‘They [the gates] have not been used on that scale anywhere else in the world. It’s one of those technology advances where if it works in Venice and it’s shown to be reliable we could potentially use it here,’ said Earlam.

If such a system were required today, however, Earlam said that we do not have enough PFI experience yet to risk handing such an important project over to the private sector. The contract to protect the capital and a large part of the south-east against tidal flooding would demand a low level of risk that has not yet been achieved in other PFI infrastructure projects.

‘However, by the time we have to make those investment decisions [for a second Thames barrier] there will be lots of experience of PFI. The public sector may have built up enough experience to come out with a low-risk solution. The appropriate financing and also construction management should be based upon low risk. Look at hospitals that are over budget, and how there are issues over the Channel Tunnel Rail Link – and they are not doing as important a job as protecting London from tidal flooding. Of course they are doing very important jobs, but if a hospital is late or part of it does not work, another part of the system can compensate. We don’t have that in terms of the barrier.’

Earlam said the Flood Risk Management Project will also consider what alternatives to a second barrier are offered by expected advances in technology. ‘Within 97 years technology may have advanced so that there could be another means of flood protection. Rather than building hard defences there could be a means of waterproofing properties or putting them on hydraulic rams that move up and down. If you look at how technology has moved on in the past 100 years, just in terms of the simple stuff, if the same thing happens within flood defence, which I assume it will, we would be foolish not to take those into account.’

One novel idea might be to use offshore wind turbines to ‘blow’ tidal surges away from the land. ‘You could reverse a wind farm to create a blow or a wind from west to east to blow the water out. This is real pie-in-the-sky thinking but wind has a big effect – it modifies the tide in terms of making it higher or lower. If you could have a low-level breeze to blow the top bit of water away or hold it back with air rather than a physical structure, it may be more environmentally sustainable than creating something that is going to restrict silt movement and navigation.’

In the 30 years between the great flood of 1953 and completion of the first barrier the government was prepared to accept a certain amount of risk that London could go under in that time. If we don’t build a second barrier some time this century then the UK must be prepared to do the same again and insure itself more adequately against this event.

However, it seems likely that the 2007 plan will recommend a second barrier. Should this happen, Earlam is confident that action would be taken in time to see the project through. The reasoning behind the report produced by Prof Herman Bondi of Cambridge University, which recommended the first barrier in 1966, will still apply. If you weigh what would be protected by the barrier against the cost and difficulty of building it, there can be no other decision.

It is unlikely that anybody will be able to counter that argument. It will hold for as long as London continues to be a capital city or until all its business and important assets are moved away from the flood plain.

Sidebar: European schemes: the environmental and economic barriers – Julia Pierce

Over the past two decades flood barriers have been built throughout Europe. Many use technologies that could be applied to a second Thames Barrier, depending on its location. But their construction has provoked controversy from both an environmental and economic perspective – in some cases causing severe delays.

In Venice a combination of local subsidence and rising water levels has caused a rise in sea levels of 23cm since 1900. Climate change projections estimate that levels will continue to increase by up to 60cm by 2100. Once storm surges are taken into account this could make the city uninhabitable.

As a result the Venice flood defence scheme, called MOSE, was approved by Italian premier Silvio Berlusconi in December 2001. MOSE involves the construction of 79 gates at three lagoon inlets. When the water level rises 87cm above mean sea level, air is injected into the hollow gates causing them to rise and block sea water from entering the lagoon. The barrier will take eight years to complete, at a cost of more than £1.6bn.

But scientists, such as Paolo Antonio Pirazzoli of the French Centre National de la Recherche Scientifique (CNRS), have questioned whether the design will be sufficient to protect the city from unusually high rainfall, tidal surges and subsidence. If the barrier is forced to remain closed over long periods environmentalists are also concerned that the lagoon’s ecosystem may suffer as toxins build up and are unable to be flushed into the Adriatic.

‘Controlling flooding of Venice is not just an engineering problem,’ said Dr Caroline Fletcher, Venice research fellow at the University of Cambridge’s Coastal Research Unit. ‘It is important to prevent damage to the city, but the impact on local water quality and the lagoon’s salt marshes and sediment must be considered as well. People are very divided, even though the government has decided to go ahead with the project.’

Environmental concerns have also blighted the construction of a 25km barrier across St Petersburg’s Neva Bay. The project began in 1980, but came to an abrupt halt in 1990 owing to a lack of funds and concerns over pollution in the bay, leaving the barrage 60-70 per cent complete.

The design consists of 11 sets of large steel gates designed to be able to close through over 1m of ice, as well as six water-exchange complexes to allow free passage of water, plus two navigation passages for shipping. ‘The design would be suitable for use in an area like London though it would depend on land availability on either side of the site,’ said Peter Hunter, technical director at Jacobs Engineering, who headed the design team for the Cardiff Bay barrage and helped produce a feasibility study on the St Petersburg design.

A decision to complete the barrier was taken, at an estimated cost of around £290m. While concerns persist over possible deterioration of water quality in the bay once the barrier is in place, engineers say this can be remedied by controlling discharge from local industries and improving sewage treatment.

In 2000 the Russian government approached the European Bank for a £154m loan to fund the remainder of the project. The bank is now selecting an external organisation that will review the barrier’s design before funding is granted.

‘The St Petersburg barrier is the equivalent of building a barrage stretching from Herne Bay in Kent up to the River Crouch in Essex,’ said Hunter. ‘The scale of the barrier is huge. One set is of a similar type to one built in Rotterdam a few years ago. These sector or radial gates are similar to canal lock gates where water is higher on one side of the barrier than the other and spokes leading into a hub reduce pressure on the system as the gate moves. As with central London, though a devastating flood has not occurred for some time, its risk is rising and something must be done.’

With around half of its land lying below sea level, of all Europe’s countries the Netherlands is most prone to devastating floods. While the area has invested millions of pounds in building conventional defences such as barrages and dikes, some groups have begun to voice fears that if sea levels continue to rise, in years to come the cost of preserving the current land mass may become unsustainable. In a worst-case scenario, surrendering large amounts of land to the sea would create a large number of displaced persons in an area already with a high population, as well as requiring the relocation of vital infrastructure such as Schiphol International Airport.

In an attempt to work with rather than in spite of the region’s natural conditions, the government has been experimenting with building floating roads. Companies offering floating houses are reporting growing waiting lists. These are constructed on concrete pontoons filled with steel-reinforced polystyrene, which are anchored to the land and steel piles using cables. Some have concrete bunkers underneath them that can store floodwater diverted from neighbouring areas. Plans for floating greenhouses have also been proposed, and officials are considering constructing an entire floating city at Haarlemmermeer near Amsterdam by 2010. If the Dutch example succeeds there may be valuable lessons for London’s planners.