On the rise: The Thames in 2100

2100 Thames flood simulation - see which areas are hit

Could London face a flood like New Orleans? How long will the Thames Barrier last? And what is being planned for the future? BBC News's John Walton asked experts what the rest of this century has in store for the capital and its rising river.

London and the whole Thames Estuary have an uneasy relationship with water.

There's either too little of it, or potentially way, way too much - it's a situation that climate change only promises to make worse.

The same environmental forces that make droughts increasingly likely in southern England are also responsible for raising sea levels, which in turn increase the chances of flooding.

'A nasty reminder'

New Orleans was devastated by floods in 2005

Looking decades ahead, Sarah Lavery, of the Environment Agency, sounds a warning: "There is a very small probability of the Thames flooding, but if it does the consequences are enormous.

"New Orleans is a rather nasty reminder to us all of the difference between the probability of something happening and the consequences if it does."

Ms Lavery, who works on flood risk management for the Thames Estuary 2100 project, points out that should a flood hit the capital it could cause up to £80bn of damage. That is, if nothing is done to upgrade current infrastructure around the Thames Barrier, and further down river.

About a sixth of London's population, well over a million people are at risk from flooding, according to the project. Kent and Essex also face massive disruption by the end of the century, if things are left as they stand now. See the map above for more details.

The barrier will get busier as the years pass

In the meantime, the Thames Barrier is already feeling the effects of global warming. When it first came on stream in the 1980s it was raised on average every couple of years. In 2003 it was raised 19 times.

But planning for the worst is already under way. Sarah Lavery is working on the basis that the Thames Barrier will offer "good service" up until 2030, and beyond. But steps have to be taken now to develop defences that will protect London from flooding from 2030 up until the end of this century.

Ms Lavery suggests a "ballpark" figure of £4bn over the next 20 years is needed to improve flood management and ensure that London and the Thames Estuary as a whole have a sustainable future.

The ongoing work to guard against future flooding is based on the projection that by 2080 sea levels will rise 26 to 86cm.

But she is not alone in making plans for London's future. Proposals exist for thousands of new homes to be constructed east of the capital in the Thames Gateway. Doesn't the possibility of rising sea levels driven by global warming make this a bad idea?

"The Thames Gateway is steaming ahead. If you can build anywhere else that isn't a flood plain you should do it," Ms Lavery says. "However we do accept there may be a need, in south-east England, to start encroaching into tidal flood-plains. But you should always look at the risks."

Risk and the river

Speaking for the Campaign to Protect Rural England Nigel Kersey says developers should avoid building in the greenbelt if brownfield sites in the Thames Gateway can be made safe.

"Flood risk is obviously an issue, but it needs to be handled pragmatically - in order to bring those urban brownfield sites back into use."

The Association of British Insurers predicts rising sea levels will translate into rising costs. Minimising risk is a high priority for them.

Spokeswoman Kelly Ostler acknowledges: "There will always be some uncertainty in our estimates of climate change, the size of the risks means that society must take sensible steps to prepare."

What, when and who?

So, if preparation is the key, what plans for minimising flooding do Sarah Lavery and the Thames Estuary 2100 project have in store for us?

"It's too early to say what we are building, when we are building it and who pays for it," she says. "But in terms of planning for 2030 I think we are moving quickly enough."

The project is preparing to publish its proposals in 2008, and although time and tide wait for no one, Sarah Lavery is confident they are both on her side, for now.