Video By DW Planet A | Copenhagen is redesigning its cityscape with green and grey infrastructure to absorb rainfall, reduce flood risk, and adapt to climate change.
💧 The Shift Toward Water-Wise Urban Design
As the climate warms, cities are experiencing more intense rainfall and longer dry spells. Traditional urban infrastructure, dominated by concrete, asphalt, and storm drains, is no longer enough. Flash floods overwhelm outdated systems while droughts strain limited water supplies. To meet these twin challenges, a new model is emerging: sponge cities. These are urban environments redesigned to absorb, hold, and reuse water — just as nature does.
🌧️ Why Flooding Is Getting Worse
Urban areas are typically covered in hard surfaces that prevent water from soaking into the ground. When it rains, runoff flows quickly across streets and rooftops, heading straight into overburdened drainage systems. As a result, even moderate storms can trigger floods. Since 1995, floods have killed more than 157,000 people worldwide and remain the most deadly natural disaster globally.
Meanwhile, many cities lack access to clean water during dry periods. The sponge city concept addresses both problems at once. It captures rainwater where it falls, filters it through natural systems, and stores it for later use.
🧽 What Makes a City a Sponge?
Sponge cities integrate green infrastructure design across every layer of urban space. Key features include:
- Porous pavement that allows water to seep into the ground
- Green roofs that retain and slow down rainfall
- Tree-lined streets and urban wetlands that absorb runoff
- Parks and plazas that double as temporary holding basins during storms
This nature-based infrastructure allows cities to manage up to four times more rainwater than traditional systems and cut urban flooding by about 50 percent. It also brings added benefits: cleaner air, cooler streets, better biodiversity, and improved mental health through more green spaces.
🛠️ The Role of Grey Infrastructure
Sponge cities are not purely natural. They rely on a well-designed foundation of grey infrastructure, the engineered systems that move and store water below ground.
For example, Copenhagen has built 18 kilometers of underground tunnels, each 3 meters wide, that redirect excess rainwater from parks and streets to the harbor. These tunnels can carry up to 20,000 liters per second and also store water for use during dry periods. This dual system of green on the surface and grey below ensures everyday efficiency and climate resilience.
Grey infrastructure also includes upgraded storm drains, pumps, and retention tanks. These systems help control flow, prevent backups, and allow for the safe reuse of water. The key is not to replace grey infrastructure, but to integrate it with green design in a coordinated way.
🌍 China Leads, Others Follow
China is a global leader in sponge city development. Inspired by architect Kongjian Yu and ancient Chinese irrigation systems, the country is building 30 sponge cities by 2030. The target is to capture and reuse 70 percent of urban rainfall. The cost is enormous—at least one trillion US dollars nationwide, with individual cities investing billions over a decade or more.
Zhengzhou, one of China’s flagship projects, demonstrated the stakes in 2021. A massive storm hit, and the city flooded heavily. Although it had begun sponge upgrades, only a small portion had been implemented. The resulting flood damage cost as much as the full sponge city buildout would have. The lesson was clear: partial implementation is not enough.
Other countries, including Germany, the United States, and Indonesia, are now piloting sponge city principles tailored to their own climate and infrastructure.
Video By Freethink | Explores how sponge cities mimic nature to reduce urban flooding, improve water reuse, and build long-term climate resilience.
💰 A Long-Term Investment
Building a sponge city requires full commitment. These systems only work when green infrastructure, grey systems, and governance are aligned. That means investing across departments and maintaining long-term political will. While expensive, the payoff is often greater than the cost of repeated flood damage, water shortages, and public health risks.
🔄 A Smarter Urban Future
Sponge cities do more than prevent disaster. They make cities healthier, more livable, and more sustainable. As climate impacts grow more severe, this model offers a blueprint for cities that want to adapt, thrive, and reconnect with the natural systems they once paved over.
