The Blue-Green Cities Project

What can cities do to reduce the risk of flooding? Professor Nigel Wright, from Leeds University, explains the concept of the Blue-Green cities project to Chris Smith...

Nigel - With the river flooding, you're looking at rain that's fallen upstream, gone into the river and then threatens the city. But of course, the rainfall occurrs in the city as well and with all the surfaces we're putting in, impermeable surfaces that the rain can't pass through. That rain that falls in the city has to go somewhere and that's what creates a problem with what we call surface water flooding in urban areas.

Chris - So, this would be the point that when rain used to fall down and hit the ground, it would soak into the ground like a giant sponge and there would be a latency between the rain arriving on the surface and then making its way into the river whereas if I come along and put roads and concrete and surface coatings on the ground, then that water doesn't soak in. It's going to run off into a drain and that drain is very quickly going to transmit it to a river.

Nigel - Well, that and then we've designed the drains for a certain level of rainfall, but with more of these car parks, for supermarkets, people covering up their front gardens. That increases the amount of water going into the pipes and the pipes reach capacity, and then the water can't get into the drainage system and just spreads out around people's houses.

Chris - What does it actually do to groundwater though when people do build say, a new big housing development on a piece of countryside which was previously just open land? What happens to the water table around the development?

Nigel - Well, if that water is no longer seeping into the ground then the water table is likely to drop to an extent.

Chris - So, does this mean that the buildings are all going to sink and so therefore, that in fact, the land is going to drop a bit and therefore, they might be at more risk of flooding later?

Nigel - I don't think that's not a problem in most of the UK, though I know it's a problem where I used to live in the Netherlands and therefore, it may well be a problem in the area where you are, particularly if you have peat. If that starts to dry out then the land surface can sink. So yes, that's another problem with covering over the surfaces which is why we're actually quite keen for people to put down surfaces that the water can actually seep through.

Chris - Some cities do do this though. I've lived in Australia in the past and walking around in Sydney, you will see verges adjacent to pavements completely grassed and this means that when the rain hits the pavement and boy, does it hit the pavement sometimes, it can run to the side of the pavement and it then soaks in. Is this the sort of thing you're getting at?

Nigel - Yes, that's it and then there are places in the UK and in other countries that already do this. One thing that was done a few years ago by some councils is to say that if you want to cover your front garden to put your car on, if you do it with an impermeable surface, you need planning permission, but you fit a permeable surface, then you don't. So obviously, people will go for the permeable surface and it's surprising across a large city, how much difference that can make.

Chris - Isn't it a bit late for most of our cities though? Have we not now got a problem that we've made and now, we've got to try and undo it rather than just regulate people making new developments?

Nigel - We certainly need to work with the new developments and legislation was put in place in 2010 that's yet to be implemented to say that all new developments must use these what we call sustainable systems where the surfaces are more permeable and the water is kept at source. But for the other areas, they're always being renewed. We're renewing road surfaces, we're looking at pavements all the time, and we can look at how we can replace them in a way that holds the water back at the source.

Chris - What can you tell us about the whole point of the Blue-Green City project? How did it get started and what's it trying to do?

Nigel - What we're looking at there is the sort of things we might do to decrease the risk of flooding in urban areas, but also, saying that a lot of those have other benefits beyond flooding. So, if we're introducing more grass and green spaces within cities, there might actually be multiple benefits so we could do these things for many reasons. And we're trying to work how we can quantify these new methods because of course, developers are keen not to spend a lot more money. They like to stick to what they've done before. So, what we wanted to do is try and demonstrate and give them some confidence that these measures can work.

Chris - Have you got some numbers you can put on this yet? Can you actually quantify, this would be the direct benefit to the city for instance?

Nigel - That's what we're working towards. We want to get those numbers because in all these debates on flooding, people have many different ideas as to what we could do to improve the situation. But what we want to come up with is some hard figures. You heard earlier the question of, should we dredge or should we store water in the uplands? These are all good ideas, but what we're trying to do is put some numbers on that. So, we have colleagues at Cambridge University in particular who are working on calculating the economic benefits of these measures.

Chris - Rather than just re-inventing the wheel here in Britain, what can we learn from looking at other countries and how they solve this problem? You mentioned the Netherlands earlier. I mean, they must know more about this than anybody.

Nigel - They do. They're particularly focused on flooding from the coast, from the sea, and flooding from rivers, and they do have flooding in urban areas with problems you get with heavy rainfall. But in most Dutch cities in the residential areas, there's no drainage from the street into the drainage system. All the streets are made with block paving, so the water seeps down between the blocks and goes into the sand underneath and will eventually go into the drainage system, but it takes a lot longer.  In my house, when I lived there, there was no drainage in the garden. So if it rained heavily, I could have 5, 10 cm of rain collecting in the garden. But it didn't affect the house and eventually, that would seep out into the system. So, it's not really about storing the water at source. It's just holding it back for a bit. That means you don't get the peak that will flood the streets and people's basements.

Chris - That's presumably the same aspiration when people plant these green rooves where you have a big thick layer of various plant species growing on a roof which acts like a big sponge when the rain falls.

Nigel - Yup, that's it. Green rooves, water butts and even different sorts of plants in the garden. There was a discussion earlier about how farming practices can increase runoff and we can even help a bit by making sure there's a lot of vegetation cover in our gardens that will hold the water at source.

Chris - It doesn't sound like it might go down well with all planners though, Nigel.

Nigel - No, it doesn't. But if we start this process and we start to do some of it then it makes a difference. And if as I was explaining with the people covering their front gardens, that can make a big difference if you get 20% or 30% of the people adopting these measures.