We Asked an Expert What Would Happen if London Completely Flooded

An image released by the Environmental Agency

In his 1962 novel The Drowned World, JG Ballard imagined our planet given over to water. In the last few weeks – as David Cameron has woken every morning to a briefing from the Met Office and many other people have woken to a living room full of swimming human shit – it’s felt as though Britain might just be heading that way.

To find out if there was any real chance of London actually flooding and to find out what would happen if it did, I got in touch with Professor Ian Cluckie of Swansea University. Prof Cluckie is one of the UK’s foremost hydrology and water engineering experts, and once ran a simulation of a breach in London’s defences for the Flood Risk Management Consortium.

So, he seemed like the perfect person to talk to.  

VICE: Prof Cluckie, please tell me about the land London is built on. If there were no flood defences would most of London be under water?
Professor Ian Cluckie: At times, yes. The Thames Barrier protects just under 50-square miles of central London. The River Thames, left to its own devices – if you were a Roman soldier looking at it – would be a very broad, braided, gravelly river. As we built the capital, we’ve contained the river; we’ve compressed it between embankments. The river does what it naturally does. At times, it suffers from big floods coming down the river, rainfall draining the hills around it, including other rivers that drain London.

Okay, so we can’t really “blame” the river, as such.
No. There are rivers that run off Hampstead Heath, for example. Some of them run under the London streets but some you can see, though a lot of people don’t know they’re there. It’s a proper river basin but it happens to be in the middle of the capital. Many of these areas are what you’d call flood plain. Central London would definitely flood if we didn’t protect it. We had flooding in central London in the 1940s, we had the 1953 flood and we had appalling flooding in 1928, when the embankments didn’t really exist, nor did the Barrier, and water went inside the central hall at Westminster.

Wow. So when were the embankments built?
The primary embankments that we benefit from were built in the 19th century. Prior to that, the Thames was basically an open sewer in its own right because the waste of the city went to the river. There were times when it smelt so bad they had to close parliament.

What would have to happen for the Thames Barrier to be breached?
You’d need a real confluence of events: a big storm that produces a tidal surge, wind and rain from the North Sea, as well as flooding from the river and high tides. We’ve recently had high tides and a fluvial flood but we haven’t had the wind blowing down from the North Sea, creating the tidal surge. Because London has so much to protect in terms of assets, that system is designed to fail once in one thousand years. Crudely put, if a conjunction of events occurred and a flood was formed within London that was as rare as once in one thousand and one years, London floods.

Alright, well let’s go with that then and imagine that the flood defences have been breached. Which parts of London would flood first and where would the water go from and to?
The low-lying areas of London would flood: Houses of Parliament, O2 Arena, Southwark, Isle of Dogs, Tower Bridge, Whitechapel, West Ham, Hammersmith. Any of the river valleys that aren’t protected would have a problem. Basically you are thinking of water behind the embankment – behind the wall, which then goes into the low areas. Once you get water onto the lower parts of the flood plain – and London is on the flood plain of the Thames and many other rivers – 50 square miles is at risk.

The type of failure you’d get in London is also quite unique because it’s unlikely that this would happen purely as the consequence of a rainfall flood coming down the river. The massive disaster is when you’ve got everything happening at once: giant drainage floods, spring tides and a surge. The winds needed to generate a three or four metre surge are the same strength – force 10, 11 and 12 – as the winds we have just had in the southwest of the country.

Scary. So what parts of London should people flee to?
Any high land. If you’re sitting in your deck chair looking down from Hampstead Heath, the only thing that will bother you is the rain and some puddles. London has so many high buildings so you just need to get in one of them and go up above the surge level. In extreme cases, that is four metres. The problem with climate change impact is that the extremes will be more problematic because at the same time we are going to have more flooding because of the nature of much larger and more energetic storms. More heat means more energy, more chaos.

Can water bring down a tall building?
No.

Not at all?
Well, you’re just going to mess up the first one or two levels. It isn’t going to come down. Skyscrapers are designed to withstand such high winds that the chance of them toppling over is literally zero, so a flood wouldn’t be capable of this. The only way they could be destroyed is if the flood went over the top of them.

How about the Houses of Parliament?
In 1927, if you stood there you would be standing in what looked like a lake. You have two different types of flooding you could envision in London. If you just get very intense thunderstorms, then little bits of London will go under water and we deal with that. It’s a pain, it’s a sewage problem and it’s a local drainage problem. It’s fairly unpleasant if you’re affected by it – when you get a flood, it’s not just salt water and rainwater; it’s sewage as well, which is a health hazard. In the event of a mega-flood, the curvy nature of the Thames would disappear and almost all the land to the coast would be swamped.

It would crawl out over the city.
Absolutely, and it would be about a metre deep. Low-lying areas would be a problem, like Woolwich. In these areas it’s not only high water levels but high velocities, as well. Low-level water travelling quickly can be a very big problem as it pushes you over. That kills a lot more people than people trying to escape the water depths.

What are the primary responses to flooding: how can security services try to save people from the coming deluge?
You have a colour-coded system that ramps up. Councils colour-coordinate the extent of the damage sustained. From there, you can coordinate what services are needed and where. A big London flood would be a major national emergency, so you would have unlimited access to the army. The warning would come far enough in advance thanks to the Met Office. They have outstanding models that would enable you to guestimate what would hit you.

What about the areas surrounding London?
With a huge flood, it wouldn’t just be London that has the problem; it would be the whole of the Thames estuary. If this event happened you could have several embankment failures across the whole length of the Thames, resulting in a much larger area suffering from problems. This would cause the situation to be much more demanding. So, instead of a specified location being under crisis, where you can simply evacuate people or send in more people to help, you have something much larger.

Presumably another issue would be potential social collapse, looting, etc…
I wouldn’t say that in general we’d expect the rule of law to break down but certainly there would be isolated cases. Overall though, I think you could expect people to help one another in situations like this. In 2007, with the floods in the Lower Severn Area, things were pretty diabolical and the majority of people were helping each other in great difficulty – we didn’t get a social breakdown.

In the aftermath of this devastating flood, where could a new London – one less susceptible to flooding – be built?
The Barrier itself is probably going to last to 2060 or 2070 – that is the current estimate of what we believe it should be able to deal with. Having said that, there is no statistical reason why we shouldn’t suffer a one-in-a-thousand type storm in the next two weeks. You could have a succession of them one after another.

Okay.
I think the reality is that we can protect from increasing risk levels if we are prepared to spend money at extraordinary levels. The problem is that it soon becomes too expensive for any society to acquire such protection, so there has to come a point where you back away, and we are at that point in a number of areas. In the Somerset Levels, is it cheaper to buy up all the houses that have been subject to flooding so you don’t have to protect them? You would just let the water run over the relatively poor lands there. The people there want to be protected but society has to pay for this. It’s a difficult question to answer and from our predictions this question in general isn’t going to get easier to answer in the next 150-200 years.

This problem is going to get a lot worse than we can imagine, so my children and my children’s children are going to have a few headaches to resolve.

Okay Professor, thanks for talking to me.

Follow Oscar on Twitter: @oscarrickettnow