This article is adapted from Erica Gies's book 'Water Always Wins: Thriving in an Age of Drought and Deluge', available at slowwater.world.
Yu is at the forefront of a movement to restore the ebb and flow of water in urban environments. His landscape architecture firm Turenscape, which he co-founded in 1998, creates flexible spaces for water to spread out and seep underground, both to prevent flooding and to store it for later use.
His vision is to heal the natural hydrology that we have disrupted by limiting rivers with dams, placing buildings or car parks where water wants to flow, or erecting dams that, to varying degrees, dried up 333 rivers in the Yangtze River area (China). By trying to solve each problem separately (floods on the one hand, water scarcity on the other), the 20th century approach to water management has undermined itself. "Drainage is separate from water supply; flood control is separate from drought resilience," he wrote in 2016 for a paper he presented at a symposium at Harvard University (USA).
Since the early 18th century, we have filled or drained up to 87 % of the world's wetlands, which would otherwise absorb and release water flexibly. It is a key reason why urban flooding is more frequent around the world: as populations grow and cities expand, developers pave over floodplains and farmland, clear forests and channel rivers, leaving stormwater that once seeped into the ground with nowhere to go. Land lost to city building has doubled worldwide since 1992. When a city increases the area of roads, pavements or car parks by 1 %, stormwater runoff increases the magnitude of annual flooding in nearby waterways by 3.3 %.
In densely populated cities, only about 20 % of rainfall infiltrates into the ground, the rest goes down drains and pipes, which Yu sees as crazy in water-scarce places.
We have already started to see the impact on the water cycle. The summer and autumn of 2021 brought deadly floods to the US, Germany, Belgium, India, Thailand and the Philippines. At the same time, drought, crop failures and wildfires ravaged the western United States, Syria, Guatemala, Greece and Siberia. Global economic losses from floods rose from an average of EUR 441 million per year in the 1980s to EUR 67,097 million in 2020.
In terms of drought, more than 2 billion people worldwide already live with severe or high water insecurity. Researchers predict that as global warming continues, two-thirds of the world's population, more than 5.25 billion people, will experience worsening and more frequent drought conditions.
These recent disasters have shown many people the truth of what climate scientists have been saying for years: climate change is a water-related change.
Sponge cities are part of a global movement that goes by various names: green infrastructure in Europe, low-impact development in the United States, water-sensitive urban design in Australia, natural infrastructure in Peru, nature-based solutions in Canada. In contrast to industrial management, where people confine water with dams, canals and asphalt and push it out of the ground as fast as possible, these newer approaches seek to reclaim the natural tendency of water to remain in wetlands and floodplains.
Because of this common denominator, I have come to think of them collectively as the "Slow Water" movement. As in the Slow Food movement, solutions are tailored to the local ecology, climate and people. The most ambitious Slow Water projects involve conserving or restoring wetlands, river floodplains and mountain forests, simultaneously safeguarding carbon storage and protecting the homes of endangered plants and animals. But there are also small urban projects, nestled between buildings or in narrow corridors along streets.
It has turned Turenscape into an empire, with 600 employees in three offices. The company has more than 640 completed or ongoing projects in 250 Chinese cities and 10 other countries. Turenscape also publishes Landscape Architecture Frontiers magazine, in both Chinese and English, and supports master's, doctoral and post-doctoral students researching hydrology or ecology, or measuring the effectiveness of completed projects.
When planning a project, designers must first find out what the water did before the city was built. In a large white room in Turenscape's offices, young men and women sit at desks separated by a jungle of plants, focused intently on that question. They build models of how water behaves within the built environment, taking into account ecology, geology, hydrology and site-specific culture - a kind of computational geography. Such data allows Yu and other Slow Water practitioners to model how the reorganisation of land and available space affects the way water flows and slows down in various ways.
Although Xi's sponge cities initiative is based on principles that reflect his own ideas, Yu fears that, in some cases, China is getting it wrong. The country has sometimes used standard solutions for other programmes, Yu points out, but for sponge cities to succeed, each project must be site-specific. In his own words, "each patient needs a different solution".
Monsoon rains in China have been intense in recent summers, challenging not only existing infrastructure such as dams (several of which have failed or were on the verge of failure), but also fledgling sponge cities. In the summer of 2021, one pilot project city with a population of around seven million, Zhengzhou (China), suffered significant flooding when more than twenty centimetres of rain fell in an hour and nearly 300 people died, leading some to doubt whether sponge cities work well.
Zevenbergen points out that designs may need to be modified to better suit local needs. But the most important factor may be that the interventions are simply not ambitious enough. Absorbing 70 % of rainfall in 13 square kilometres of a city that spans 7,500 square kilometres will not prevent flooding. Slow Water projects work best when they can absorb water across the landscape, so planners need to think beyond the urban territory. A city is part of a larger watershed. Restoring upstream space in natural river floodplains can reduce downstream levels.
Yu is also working hard on this. He is creating a landscape master plan for the whole of China. In his office, he showed me a series of maps documenting China's elevation, watersheds, flood paths, biodiversity, desertification, ecological security, soil erosion and cultural heritage. As urbanisation spreads, as estuaries and deltas silt up, as water begins to move differently across landscapes and urban areas, Yu identifies the places where his projects will have the greatest impact.
