Check out the episode:

You can find the shownotes through this link.

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Are you interested in the concept of sponge cities?

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Our summary today works with the white paper titled Creating sponge cities to tackle surface water flooding from 2025, by Mark Coates and Neal Edmondson, published on the Infrastructure Policy Advancement website.

This is a great preparation to our next interview with Mark Coates in episode 344 talking about infrastructure and sponge cities, among others.

Since we are investigating the future of cities, I thought it would be interesting to see how the infrastructure can alleviate some of the challenges during flooding in urban areas. This white paper advocates for sponge cities as a solution to ever frequently occurring floods, based on the implementation of innovative, nature based sustainable drainage systems.

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Welcome to today’s What is The Future For Cities podcast and its Research episode; my name is Fanni, and today we will introduce a research by summarising it. The episode really is just a short summary of the original investigation, and, in case it is interesting enough, I would encourage everyone to check out the whole documentation. This conversation was produced and generated with Notebook LM as two hosts dissecting the whole research.

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Speaker 1: Today we’re diving into something that’s literally flooding our streets and newsfeeds surface water. Flooding,

Speaker 2: sometimes called pluvial flooding.

Speaker 1: Yeah, not rivers, bursting banks. This is the rain itself, especially in cities just having nowhere to go.

Speaker 2: It’s a problem that’s really picking up speed, isn’t it? You’ve got climate change bringing these heavier, more intense downpours

Speaker 1: because warmer air holds more moisture, right? Yeah. Like 7% more per degree.

Speaker 2: Precisely.

Speaker 1: Yeah.

Speaker 2: And at the same time, our towns and cities keep getting paved over. More concrete, more tarmac. Less ground to actually soak the water up.

Speaker 1: It’s a perfect storm, creating chaos pretty much everywhere.

Speaker 2: Yeah, it really impacts communities infrastructure. It’s a global challenge,

Speaker 1: and to get under the skid of this, we’ve been digging into a really interesting white paper. It’s called Creating Sponge Cities to tackle surface water flooding. What really grafted me was how it links the problem directly to these really practical, innovative solutions. Okay, let’s start with the basics. P fluvial flooding. What’s actually going on when one of these events hits a city?

Speaker 2: The core issue is pretty simple. Really. You get a massive amount of rain falling very quickly.

Speaker 1: Like a cloud burst.

Speaker 2: Exactly. And the existing drainage systems, the pipes under the street, they just get overwhelmed. The sheer volume is too

Speaker 1: much. Is it just the volume or is it also where the water’s landing?

Speaker 2: That’s the other key part because so much of our urban environment is impermeable roofs, roads, car parks. The water can’t soak into the ground like it would naturally,

Speaker 1: so it all just runs off straight into the drains.

Speaker 2: Pretty much funnelling huge amounts of water into systems that weren’t designed for that kind of sudden peak load. That combination is what causes the worst of the flooding,

Speaker 1: and we’ve seen again and again recently, this isn’t just puddles on the road. The impacts can be. Absolutely massive.

Speaker 2: Oh, definitely far reaching transport. Just shut down roads, impassable, train lines flooded,

Speaker 1: even airports affected.

Speaker 2: And then there’s the damage to property, homes, businesses, the cost is enormous. People lose belongings, businesses can’t operate,

Speaker 1: and tragically it can be even worse. The white paper mentions Hurricane Ida in New York, people drowning in basement flats.

Speaker 2: That was awful. It really shows the human cost. These aren’t just economic events.

Speaker 1: The paper gives some specific examples too. London, July, 2021. That seemed pretty bad.

Speaker 2: It was a couple of really intense downpours hit the city hard. I remember seeing pictures of tube stations flooded. Whips cross hospital had to evacuate patients because of the power went out ‘

Speaker 1: cause of the flooding

Speaker 2: and the Docklands light railway.

Some parts were under quite deep water. The insurance claims alone, I think Sedgwick estimated something like 280 million.

Speaker 1: Wow. And New York with Ida, that was August, 2021. Another brutal example. Absolutely.

Speaker 2: The rainfall rate was apparently almost double what the city’s sewers could cope with. Just couldn’t get the water away fast enough.

Speaker 1: Leading to those flash floods and yeah, the fatalities we mentioned and the overall cost.

Speaker 2: Huge. Estimated somewhere between 16 and $24 billion across the US Northeast. Just staggering numbers

Speaker 1: and it’s not just a US UK thing. Copenhagen had a major one back in 2011. That seems to have really shifted their thinking.

Speaker 2: Oh, definitely. That was a wake up call for them. They got something like 150 millimetres of rain in just two hours.

Speaker 1: That’s incredible.

Speaker 2: Yeah. Parts of the city were under a meter of water. The damage was estimated at what? Around 6 billion Danish kroner. Which is about 690 million or so. Huge impact, especially on businesses.

Speaker 1: Seeing these events pile up really drives home the need for different solutions. The paper also flags how many properties in England are already considered at high risk.

Speaker 2: That’s right. Based on environment agency mapping. The National Infrastructure Commission reckons its hundreds of thousands already

Speaker 1: and that number’s only going one way, isn’t it? With climate change, more development.

Speaker 2: Exactly. Projections show it increasing significantly by the middle of the century. More intense rain, more hard surfaces being built. It all adds up.

Speaker 1: The UK, particularly England, we have this extra complication with combined sewers, don’t we?

Speaker 2: A lot of older systems mix rainwater runoff with sewage in the same pipes,

Speaker 1: so when you get heavy rain,

Speaker 2: the system gets overwhelmed and to prevent sewage backing up into homes is designed to overflow into rivers or the sea. These are the combined sewer overflows CSOs.

Speaker 1: Discharging raw sewage and rainwater, yes.

Speaker 2: Happened hundreds of thousands of times in 2022. Apparently there’s a government plan to reduce it, but it needs huge investment.

Speaker 1: But managing surface water better could help with CSOs too.

Speaker 2: Absolutely. If you can stop so much rainwater getting into those combined sewers in the first place, you reduce the pressure on the system and cut down the number of spills. It’s a direct link.

Speaker 1: The white paper quotes Sir James Bevin, when he was head of the Environment Agency. He called the 2007 floods, a wake up call and said, surface water flooding threatens more people and properties in England than any other flood risk.

Speaker 2: That’s a powerful statement, and you hear similar things from the water company, is Steve Wilson at Welsh Water apparently identified it as their single biggest challenge because of climate change.

Speaker 1: Okay, so the challenge is clear and it’s growing. Let’s pivot to the solutions this Sponge City id. What does that actually involve?

Speaker 2: It’s fundamentally about changing our mindset instead of just building bigger pipes to try and get waterway faster,

Speaker 1: which often just moves the problem downstream anyway. Yes,

Speaker 2: the Sponge City approach is about designing urban areas to actually absorb, hold and manage rainfall. More like a natural landscape would

Speaker 1: Soaking it up locally.

Speaker 2: Exactly. Holding it, filtering it, releasing it slowly, making the city itself act like a giant sponge. In places like Copenhagen, New York, they’re really leading the way on this.

Speaker 1: Copenhagen’s work sounds particularly comprehensive, especially after that 2011 flood. They were already cleaning up their harbor, weren’t they?

Speaker 2: They were, and the flood really spurred them into action on surface water. They developed this huge citywide plan, the cloudburst management plan,

Speaker 1: over 250 projects. The paper said,

Speaker 2: yeah, a massive undertaking. It includes some traditional pipe upgrades, yes, but also big storm water tunnels in dense areas. But crucially, it’s all joined up.

Speaker 1: Joined up, how

Speaker 2: they use really detailed hydraulic modelling for the whole city. So when they plan an intervention in one area, they calculate the knock on effects elsewhere to make sure they’re not just shifting the flood risk. It’s properly integrated.

Speaker 1: And a big part of it is the sustainable drainage systems said ds. The paper calls them products, swales, rain gardens, that sort of thing.

Speaker 2: That’s right. These are techniques designed to mimic natural drainage. Swales are grassy channels, rain gardens or planted depressions. They catch rainwater near where it falls

Speaker 1: and let it soak in or slow it down.

Speaker 2: Both really. They allow infiltration into the ground, which is great for groundwater. Or they hold the water temporarily and release it slowly into the drains, taking the peak off the flow.

Speaker 1: And in Copenhagen, they seem to make them do double duty.

Speaker 2: That’s the really clever bit. A rain garden isn’t just drainage, it’s also a green space. Improving the look of the street, providing habitat, maybe even a place for people to sit, getting multiple benefits

Speaker 1: like that Tess Plaids example. Turning a car park into a park with a rain garden.

Speaker 2: Exactly. Took an old paved area, made it green, pleasant, and functional for drainage. It manages runoff from nearby buildings while being a community asset

Speaker 1: in San Guild’s. Plaids the roundabout, they redesigned

Speaker 2: similar idea, made the road smaller, planted loads of trees and plants, created sunken areas designed to hold water during heavy rain,

Speaker 1: and it actually boosted property values nearby

Speaker 2: and it became a bit of a showcase project. Lots of international visitors coming to see it.

Speaker 1: Then there’s ENG of Park and sounds like a huge project with underground tanks.

Speaker 2: It’s their biggest surface climate project. They built massive underground reservoirs to store storm water over 22,000 cubic meters.

Speaker 1: Wow. What do they do with the water?

Speaker 2: They actually reuse it for irrigating the park cleaning streets. The sports pitch in the park is also designed to hold water temporarily, and they have automated gates to manage flows during really extreme events. It’s very high tech nature.

Speaker 1: Even Karen’s mind, an existing park got a makeover.

Speaker 2: Yeah, they retrofitted it with groundwater ponds, raised paths to guide water, even a rice field, all designed to slow down and manage storm water before it hits the main system.

Speaker 1: It’s impressive scale, but the paper mentioned a potential snag with new Danish national rules.

Speaker 2: Yeah, that’s an interesting point. It seems new national standards for calculating climate adaptation might aim for a slightly lower level of flood protection than Copenhagen was planning for. So

Speaker 1: it could potentially limit their future ambition.

Speaker 2: It could, yeah. It raises questions about how much protection is enough and who decides.

Speaker 1: Yan Rasmussen, the project director there, stressed how vital that Citywide plan was. Yeah. And developing those Iest products. And Christian Nielsen from Ramble noted this global shift towards focusing on surface water.

Speaker 2: Definitely it’s being recognized as the primary flood risk in many cities now, maybe even more so than river flooding in some cases.

Speaker 1: Okay, let’s bring it back to the UK seven Trent Project in Mansfield. Sounds like a really big deal.

Speaker 2: It really does. A 76 million town investment to put Bluegreen infrastructure right across the town’s catchment area. The goal is explicitly to make Mansfield a sponge city,

Speaker 1: protecting 90,000 people. That’s a huge scale.

Speaker 2: It is. It’s a multi-year program. Hundreds of interventions planned.

Speaker 1: What kind of things are they doing?

Speaker 2: A whole mix. Big detention basins for storage bios, whales along roads, permeable paving in car parks, street planters, special tree pits, verge rain gardens, even small planters connected to house down types.

Speaker 1: Trying to catch the rain everywhere. They’re aiming for 30 million liters of storage.

Speaker 2: That’s the figure. Yeah, it’s substantial.

Speaker 1: And what’s unique about Severn Trent leading this? Is it Unusual for a water company?

Speaker 2: Adam Bucher from Severn Trent highlights that in the paper, it seems this is pretty groundbreaking for a UK water company to lead such a catchment wide rollout of nature-based solutions. It came out of their green recovery funding post covid.

Speaker 1: So they’re not just focusing on pipes?

Speaker 2: No. They’ve even developed their own study standards and a special control chamber because the water collected is cleaner than typical storm runoff, and it takes some pressure off the local council for maintenance, which is often a barrier.

Speaker 1: They’re using tech like LIDAR data with Arab’s Terrain software too.

Speaker 2: Yes. Using that detailed 3D mapping to pinpoint the best spots for interventions. Model how effective they’ll be at holding back water and protecting vulnerable areas. They’re also working with experts like Professor Nigel Dunt from Sheffield, known for the gray to green scheme there.

Speaker 1: It’s interesting that while these study s might cost a bit more upfront than just bigger pipes, the extra benefits seem significant.

Speaker 2: That’s crucial. You reduce flood risk, yes, but you also cut sewage overflows, which improves river quality. You create nicer, greener places for people. Boost biodiversity. De Abrahams, the mayor of Mansfield, mentioned the benefits for the local economy too. It’s a much broader value proposition.

Speaker 1: New York City is also betting big on green infrastructure.

Speaker 2: Seems so Mayor Adams committed what? USD 3.5 billion for green infrastructure in Seward areas.

Speaker 1: And the cost benefit analysis looks good.

Speaker 2: Yeah. Ramble and rebuild by design. Did a study suggesting a really strong return, potentially over $2 back for every $1 spent

Speaker 1: through avoided sewer upgrade costs plus greener streets, more trees.

Speaker 2: Exactly. Quality of life improvements, as well as hard infrastructure savings.

Speaker 1: It’s not just these headline projects though. The white paper also talks about needing updated guidance like the CRI, various TIUs manual.

Speaker 2: Yeah. Sharing best practice is vital, so everyone can learn from places like Copenhagen and Mansfield. Keeping the guidance up to date is key.

Speaker 1: And even tackling things like paved over front gardens,

Speaker 2: that’s a real issue. Lots of small paved areas add up to a big reduction in permeable ground across the town. So engaging communities on that, encouraging greener driveways or gardens is also part of the picture.

Speaker 1: It really feels like the key takeaway is needing these integrated plans, using data, using modelling, combining the grey and the green.

Speaker 2: Absolutely. It has to be holistic, using climate projections, digital tools to understand the risks, and then deploying the right mix of solutions, traditional engineering where needed, but really embracing these nature based approaches wherever possible.

Speaker 1: So wrapping this up then surface water flooding is clearly a major growing headache for our towns and cities. No doubt about it. But this shows there’s real momentum behind these innovative, nature-based solutions. The whole Sponge City concept, it feels genuinely promising,

Speaker 2: and the winds go way beyond just stopping floods, don’t they? Greener streets, cleaner rivers, better public health, maybe even higher property values. It’s about creating better places to live fundamentally,

Speaker 1: It definitely makes you think, what if these sponge city ideas really took root everywhere? Consider your own neighbourhood. What small changes could help make it more resilient, more liveable? When those heavy downpours come, is that balance, isn’t it, between the concrete and pipes we’re used to and harnessing the power of green natural solutions. Something to mull over.

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