How do flash floods happen in cities?

Brian May, the guitarist from the band Queen, posted videos this week on social media of an overflow of sewage that had risen through his basement. Stinking black sludge flooded the floor, destroying most of the childhood photos and memorabilia that were stored there. And many others in his area of London suffered similar - or worse - fates. The cause? Well, Brian May blames a rise in construction of mega-basements - so-called ‘iceberg basements’ that, like icebergs, feature many stories hidden below the surface. Supposedly, these structures stop groundwater from properly draining. But is it true? Phil Sansom spoke to flood forecasting expert Linda Speight from the University of Reading to find out what actually happened...

琳达,在伦敦,周一,有一个真正heavy rainfall event, kind of the average rainfall that's normal for the whole of July fell in just a couple of hours. The impact of that caused flooding across the area. Really lots of different areas of London were showing videos of flooded streets, flooded basements, flooded rainfall stations. I believe that's the expected outcome from that amount of rainfall in such a short amount of time.

Phil - Of course, the consequence for people like Brian May is that stuff that's in basements can be damaged or destroyed.

Linda - Yeah. Brian May, unfortunately, this week, suffered the experience that many people across the country suffer when flooding happens and it's a really devastating and emotional time that your property and your personal belongings and things that hold value for you as an individual get destroyed in the floodwater.

Phil - And Brian blames it on, in Kensington where he lives, the so-called epidemic of people building huge mega basements many stories down beneath their houses. Is that a reasonable thing to blame it on?

Linda - I think when this kind of flooding happens people are really keen to find somebody or something to blame. And there clearly has been a lot of basement conversions and basement development across London. And the reason for this is because there's no other space to build in the area, and the same is true of rainfall. So when it rains a lot in London, there's nowhere else for the water to go. There's lots of hard surfaces, and when the rain hits hard surfaces, it can't soak into the ground like it would do if it was falling in the countryside or on fields and grass and woodland areas. So that means, unfortunately, where people have built basements, they've actually built something that's below ground level and the water wants to try and get to the lowest place possible, so it flows into the basements.

Phil - So is it these big basements that are letting this water build-up and creating the hard surfaces that prevent it from draining properly?

Linda - It's not necessarily the basements, it's just development in urban areas in general. I wouldn't like to say that basements have made any difference in this event, that amount of rainfall would have caused flooding anyway. They may or may not have contributed to it, but they're unlikely to have been a significant factor. It's more generally the fact that there is no space for water in our cities. And when it rains a lot, like we saw last week, the water naturally has to flood because there's nowhere else for it to go.

Phil - So even if more and more basements aren't the problem here, are these kinds of floods still getting more common?

Linda - Anecdotally, it would seem that we are experiencing more of these heavy rainfall events in the last few years. And the climate change predictions seem to be backing up this statement, saying that as the atmosphere warms, then these kinds of events will get more frequent in the future. And the reason for that being warmer air can hold more water. That means that when it rains, there's more moisture in the atmosphere and therefore more rainfall falls very quickly, and we see these heavy, intense rainfall events increasing in the future.

Phil - And how often is that likely to overwhelm a city's drainage like we've just seen?

Linda - Cities in the UK have what we call combined sewer systems. That means that both our dirty water and the rainfall flow into the same sewer system. The problem is that the way we've designed those systems uses historical rainfall data. And as I've already said, we're seeing more and more of these events, and more intense rainfall is predicted in the future. So what was once a once in 30 year event may soon become a 1 in 20 year, 1 in 10 year event, and the sewer systems are likely to get overwhelmed more frequently.

Phil - I mean, what do we do? Because these sewer systems are built beneath a city. It's not as if you can just lift it up and enlarge everything and then put it all back down again.

Linda - Nope. It's probably not practical to redesign our sewer systems, though we might be able to do that in small areas, we're not going to be able to do that across whole cities across the whole country. So what we need to do is actually make people more aware that they are at risk of flooding. And as you can see from Brian May story, it's really devastating when it does happen to you. So in the UK, we're quite good now at forecasting these types of events - the MET office did issue a heavy rainfall warning for the flooding on Monday. And what we need people to understand is when they see these kinds of warnings, they need to start taking action. That might be that they don't want to keep their best possessions down in their basements where the water's naturally liked to go. They could also think about installing flood resilient defenses onto their properties - small gates that they put across their doorways, or making their property, in general, more resilient. Moving their plug sockets higher up, putting damp-proof courses in, and things like that. So there's lots of proactive ways people can take action themselves to help protect themselves from the impacts of flooding.