Imagine a sponge. Swipe it over a wet surface and it will draw up water; squeeze it and the water will trickle out.
Now imagine a city made of sponges, or spongelike surfaces, able to soak up rainwater, overflowing rivers or ocean storm surges and release stored water during droughts.
Engineers, architects, urban planners and officials around the world are seeking ways to retrofit or reconstruct cities to better deal with water — basically, to act more like sponges. While water management has always been an essential service in cities, climate change, combined with urban expansion into wetlands and floodplains, is making flooding and drought worse at the same time.
Extreme precipitation is becoming more frequent and intense, and droughts more likely and severe. Impermeable paved surfaces like concrete and asphalt trap water without giving it a place to run off to, and they prevent accumulation of groundwater to be used later for drinking, cooking and cleaning. (On top of that, concrete production contributes significantly to climate change, accounting for about 8 percent of global greenhouse gas emissions.) Many cities are also coastal and low-lying, so they face an additional front in the war against water as sea levels rise.
So, around the world officials are moving away from the traditional, hard infrastructure of flood barriers, concrete walls, culverts and sewer systems, and toward solutions that mimic nature. They are building green roofs and parks; restoring wetlands, swales and rivers; digging storage ponds; and more. Such projects — called by various names, including sponge cities, porous cities or blue-green infrastructure — also improve city dwellers’ quality of life.
Instead of dealing with water by trying to get rid of it quickly, sponge cities slow water, absorb rain and halt runoff, a major source of pollution in urban waterways.
“Climate change will mean that any storm or drought or natural disaster will easily surpass what we have designed,” said Yu Kongjian, a landscape architect at Peking University and founder of the firm Turenscape, who popularized the term “sponge city.” “We are too dependent on this infrastructure, so whenever a natural disaster happens it will be overwhelmingly destructive.”
Mr. Yu said that the sponge development philosophy was not a new concept, but rather a way to live in cities that used their natural features, considered their climate and applied locally appropriate solutions.
China is under pressure to find a way to deal with water. In July, about 300 people died in floods in central China, mainly in Zhengzhou, including 14 who drowned in the subway; an estimated one million people were displaced. In 2012, 77 people died in Beijing when intense rainfall hit the city.
In 2015, the Chinese government announced ambitious plans for a sponge country. By 2030, 80 percent of its cities are to be able to accumulate and recycle 70 percent of rainwater. Localities have established their own targets; for example, Zhengzhou aims to make 90 percent of its core urban area spongy by 2030.
China is more equipped to apply ambitious solutions to flooding because of its centralized government and financing. China also has been adept at large-scale urban construction, having built 600 cities in recent decades to accommodate its growing urban population.
But the flooding problem is hardly confined to China. In 2021, New York experienced record rainfall during two storms, flooding subways, highways and basement apartments, and killing 13 people. Cities all over the world face these kinds of tragedies. Here are a few examples of how other cities are employing sponge solutions.
Wuhan, now a household name because of the virus that causes Covid-19, used to be known as “the city of 100 lakes,” with more than 100 lakes and ponds. Paving and construction, however, decreased the area covered by lakes and ponds by more than 60 percent over the last century.
Sitting where the Yangtze, Han and some smaller rivers meet, Wuhan has long experienced flooding, particularly during monsoon season. Now, with fewer places for the water to go, channelized rivers and concrete have turned floodplains into basins.
In 2016, rainstorms killed 14 people in the city and caused about $360 million in damage. The disaster added urgency to existing plans to make Wuhan a pilot sponge city. About 390 infrastructure projects costing nearly $2 billion have since been built in nearly 15 square miles of Wuhan, primarily in the districts of Qingshan and Sixin.
One of those is the Yangtze River Beach Park. Where embankments once lined the river, the project added gentle slopes of vegetation and permeable surfaces for more than four miles; 45,000 trees, 125 square miles of shrubbery and 150 square miles of grass; 15 soccer fields; and seven swimming pools. The park now sequesters 725 tons of carbon dioxide a year, reduces temperatures by more than 5 degrees Fahrenheit compared with the rest of the city and has more than doubled the value of the land, according to a study.
The study also found that these solutions cost about $600 million less than traditional hard infrastructure projects and provide protection against a 30-year storm rather than a 1-in-10-year storm, one expert told The Guardian.
In the 1990s, engineers in the Augustenborg neighborhood of Malmo began separating stormwater from wastewater to avoid overwhelming the sewer system in the event of a big storm, largely by allowing rainwater to run through open channels. By doing so, Augustenborg has managed to avoid disastrous flooding.
Pipes run off buildings to open gutters, which connect to concrete canals large enough to hold water from a 1-in-25-year flood. Those canals connect with ponds. Concrete “stones” along the bottom slow down the flow and create small vortices, providing oxygen for plants to grow. The water then passes through a marshy area to another pond, and eventually to pipes that keep the water away from the sewage system.
Augustenborg has fared much better than other parts of Malmo during flooding, according to Per-Arne Nilsson, a senior strategist in Malmo’s environmental department.
This project took shape in Augustenborg partly because the land and buildings are city-owned, but also because the neighborhood was in decline economically. City officials saw the sponge project as a way to deal with water, but also to enhance land values, offsetting disinvestment, and to provide services and benefits to people who were historically neglected, Mr. Nilsson added.
In 2011, Philadelphia began work on its “Green City, Clean Waters” plan, a 25-year, $4.5 billion project to reduce the city’s 13 billion gallons of runoff and expand storm-water treatment using elements of green and traditional infrastructure. The project came about after an order from the Environmental Protection Agency, enforcing the federal Clean Water Act.
A decade into the project, the city has “greened” 2,200 acres, reducing the runoff by three billion gallons. It is on track to meet its target of reducing runoff by 85 percent, or eight billion gallons, by 2036. Its strategies include rain gardens, green roofs, restored wetlands and other green infrastructure.
The holistic approach was deliberate, officials said. “One of our biggest lessons over the last 10 years is that you don’t take on a project like this just to address stormwater,” said Jessica K. Brooks, the director of the water department’s green stormwater infrastructure implementation unit. Rather, they look at the whole city and how its component parts work together.
One key is bringing residents on board to help, she said.
And like Malmo, they are also using the project to address justice and equality issues.
“Wherever we are working, we tailor it to that neighborhood so the benefits we’re getting are what that neighborhood needs,” Ms. Brooks said, “and that’s because equity is such an important part of what we are doing.”