China is Building Sponge Cities to Fix Its Flood Problem

It's raining again in China. None of us like the rain, but here it's gone from mild annoyance to a serious problem. Once a largely rural country, China now has more than 100 cities with populations of over a million people. And all that building on former farmland has led to a major downside. Flooding. But instead of constructing big barriers like others have done, China is turning back to nature for the solution, upgrading its cities so that they welcome the water rather than hold it back. Get yourself comfortable and prepare to soak up China's sponge cities. China's home to more large cities than any other nation. 60% of its 1.4 billion citizens now reside in urban areas and creating the space for them to live, work and play has meant replacing huge areas of green space with sprawling buildings and infrastructure. That's a lot of land that would have previously absorbed rainwater being replaced with surfaces that do the opposite, and all in a country that regularly experiences heavy storms. Little surprise then that floodings become such a serious problem, even in places with good drainage. In 2020, over just two months, more than 400 rivers flooded across China, many of them worse than ever before, and at least 50 million people were impacted. On top of that, water shortages are occurring in some cities where water runs off the streets too quickly to enter water systems that are already overstretched. The response from the Chinese government was to mop up this mess, and convert many of these areas into so-called sponge cities. This is where areas are redesigned to cope with and process rainwater in a few different ways, soaking it up, storing it and then releasing it when needed, a bit like, you guessed it, a sponge. Now, this usually involves combining what's known as grey and green infrastructure, so things like advanced drainage and water treatment systems, alongside natural and semi-natural areas. The US, Russia and India have all dabbled in sponge cities, but China's programme is on an entirely different scale. Brace yourself for quite a few numbers. 16 cities were chosen to be part of the scheme when it launched in 2015, and another 14 joined a year later. The aim is to have 80% of these 30 cities absorbing and reusing 70% of rainwater by 2030. Hopefully you've followed all that. That means massive changes to land use, surfaces, underground infrastructure and more across the world's most populated country. Although it sounds like a lot of work that needs to be done in not much time, this is China we're talking about, so of course they've already made huge progress. Chongqing, which has a population of over 16 million, can now rather grandly call itself one of China's leading sponge cities. Here, a new park in the Tongnan District helped the area quickly recover from the worst storm for 100 years in 2020. Up until 2019, just a small concrete barrier separated the Fu River from land that was becoming increasingly vulnerable to flooding. Now, the barrier's gone and when levels rise, water is free to flow into this incredible maze of constructed wetlands. Built over 99 hectares, it's an impressive network of native plants, trees, ponds, islands and elevated walkways, making it more than somewhere for excess water to go. People want to be there too, soaking up their surroundings. Chongqing was also the first sponge city to be given a smart digital control system, which lets authorities monitor its sewer and stormwater networks in real-time. With this, they can control how water flows into the natural environment more precisely, analyse how well the whole system performs in extreme weather, and then use the data to prevent future floods. To the east of the country, on the banks of the Yangtze River, Zhenjiang has also gone big with sponges. As well as rain gardens and creeks that drain and retain rainwater, the city has been equipped with a system of bioswales, which stretch for several kilometres in total. These specially engineered channels with sloped sides funnel stormwater inside, where it filters down slowly into the ground beneath, often with debris and pollutants removed as part of the process. Go for a stroll in any decent sponge city and you'll also find permeable roads, paths and cycleways, all of which allow water to soak directly into the material instead of forcing it into drainage systems that can become quickly overwhelmed. Polyurethane binders are combined with gravel or stones to create a surface with similar properties to concrete or tar, but with tiny cavities that let water through. Around 4,000 litres of water per square metre can be absorbed each hour, and there are now networks of these across China, from Tian'an City to Hangzhou, which is set to host the 2022 Asian Games. Now, that event is actually relevant to this video because this site for the tournament was designed as one big sponge city project, a fantastic place to soak up the sporting atmosphere. Previously a flat piece of land, the area is being transformed into a 116-acre eco-park spread across a rolling hillscape. Rainwater will be collected, filtered and reused for water features, plumbing, heating and cooling, according to Architectonics, which designed the masterplan. But despite all this work, China's sponge cities haven't been able to prevent every flood from occurring, including one notable recent example. In July 2021, the city of Zhengzhou had almost a year's worth of rainfall in just four days, leading to extreme flooding that tragically killed hundreds of people. Zhengzhou spent over $7BN upgrading hundreds of flood-prone areas as well as the drainage system. Unfortunately, these improvements were completely overpowered, leading many to But Chinese officials responded by pointing out the severity of the storm, claiming that no city would have been able to avoid taking heavy flood damage in such a situation. Experts also say that sponge city projects are only really meant to protect against light to moderate storms and floods, which kind of brings us to the main point. Schemes like these do their job in most scenarios. There are loads of different options to suit a variety of cities and they look great too. But with severe events like the one in Zhengzhou expected to become more common, we'll need more than this to stop further tragedies from occurring. Even with that said, for most cities worried about water levels, introducing a sponge strategy of their own or something similar is likely to put them on a much firmer footing in the future. 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