Businessweek + Green
The Sponge Megacity
São Paulo is planting hundreds of absorbent rain gardens in parks and along roadsides as it braces for a wetter, warmer future.
This is the second story in Climate-Proofing Cities, a series that explores how cities around the world are adapting to the impacts of global warming. Read more on Beira, Mozambique’s early warning system for cyclones.
Clipboard in hand, architect Lara Freitas surveys a line of car-size cavities dug into a street in southern São Paulo. Every square meter in this high-rise megacity is valuable real estate, but Freitas isn’t planning to raise another glass-and-steel tower. Rather, she’s gouging the concrete to make space for shrubbery—a green palisade to guard against the ravages of climate change.
“What we are creating is sponges for the neighborhood,” says Freitas, who’s also an elected councilor in one of the city’s 32 districts. “We take these spaces and return them back to nature.”
The “sponges” are more widely known as rain gardens, areas of greenery that soak up and filter stormwater runoff, relieving pressure on sewer systems. As cities in Brazil and around the world are pummeled by more intense downpours and beset by more frequent flooding, rain gardens have become part of the answer to an increasingly common urban problem: how to deal with all that water.
In São Paulo, which sprawls over an area twice the size of New York City, curbside gardens with leafy bushes have been built on side streets to stop water pooling. They’ve been installed along busy corridors, including the 23 of May Avenue, where the median now has prairie grass poking over its barriers. Existing parks on slopes and at the bottom of hills are being revamped, dug deeper and replanted with flora that can absorb more of the water flowing down from higher ground.
Gardens have also been squeezed into cramped spaces prone to flooding—such as concrete stairwells, where they’re often staggered next to steps—and wedged into car-crammed streets in the form of vagas verdes, or green parking spaces, which may feature a small tree or a few bushes.
Well before it became Brazil’s financial capital, São Paulo was known as terra da garoa, or the land of drizzle. An explosion of concrete and people in the second half of the 20th century—residents now total about 12 million—raised urban temperatures, and as the atmosphere has warmed, a local climate once characterized by light rains has shifted. Powerful storms ripped through the region in March and last November, pounding it with rain and causing blackouts and several deaths.
The number of days of rainfall in São Paulo is expected to double by the end of the century if, as projected, global temperatures increase by 2C or more from preindustrial times. Rainfall of about 30 millimeters per square meter per day can lead to flooding. Before 1950 there was no record of rainfall of 50mm or more in a single day, according to Brazil’s National Institute for Space Research; now that happens two to five times a year. “This is how climate change manifests in São Paulo,” says Lincoln Alves, a researcher at the institute.
The struggle to manage water is as old as the city itself. Cutting through São Paulo are several hundred waterways, ranging from large rivers such as the Tietê and Pinheiros—both of which were redirected to help generate electricity—to small canals. Many have been paved over to make way for infrastructure, while formal and informal housing settlements have sprung up on flood plains.
Grassroots activists started to install rain gardens about a decade ago, when a drought in southeast Brazil left reservoirs at critically low levels, leading to restrictions on water use. When storms brought torrential rains, the city’s channels and waterways were overwhelmed, resulting in a contradictory situation in which paulistanos had no running water even as their homes and streets flooded.
Taking a cue from New York City, which has installed thousands of rain gardens, São Paulo’s city hall launched its own official program and turbocharged it. There were just 23 gardens in 2017; now the number has risen to 337, and it’s expected to reach 400 by yearend.
Plots are dug about 1.2 meters (3.9 feet) deep into streets or sidewalks, then filled with rubble and compost-rich soil, along with plants that sit just below the surrounding pavement. The mini-gardens can help cool local air temperatures, while the porous layers of soil and rock make the gardens highly absorbent. City planners are quick to cite other positive, harder-to-measure effects a dose of green can bring to an urban community, such as improved mental health.
Rain gardens alone can’t soak up the water brought on by extreme weather. For massive flooding, “there really is no silver bullet,” says Thaddeus Pawlowski, managing director of the Center for Resilient Cities and Landscapes at Columbia University. The danger of intense rain became acute in late April and May, when Brazil’s Rio Grande do Sul state was lashed by record precipitation, causing flooding that left more than 170 people dead and over half a million displaced. That’s raised concerns across the nation about cities’ ability to cope with climate disasters.
But even less severe floods can make streets impassable and close subway stations, upending the daily routines of millions and threatening homes. As floods become increasingly common, “rain gardens are part of a whole system of actions that will make a city more able to cope,” Pawlowski says.
Costs of installation vary depending on the site and size, but a garden of up to 100 square meters costs about 87,000 reais ($16,500) to build, says Luiz Jamil Akel, an architect who advises the mayor’s office on rain gardens. Carolina Lafemina, the deputy secretary of city districts, whose office is in charge of the gardens, says they pose virtually no additional cost to the city, given that many of the personnel needed for their upkeep have already been contracted for other duties. “We already had the resources. It was just a matter of training our people,” she says.
That’s helped the rain garden initiative to grow quickly. The city is also teaming up with activists and local leaders such as Freitas, who’s overseeing a project to install gardens across a seven-block area in the Vila Mariana neighborhood.
The rollout hasn’t been seamless. As in New York, some residents complain that the gardens are magnets for trash. They also doubt their long-term viability in a city plagued by crime, homelessness and a seemingly endless list of other problems. Officials concede that keeping up with maintenance has been a challenge. In Pinheiros, an affluent district of west São Paulo, one of the city’s first rain gardens, a gas station turned public plaza, is now strewn with plastic bottles and cups. Palm trees overlook graffiti-covered benches and unkempt bushes. Squatters have set up camp nearby, which locals say scares people off. “It’s basically a port-a-potty,” says Ivan de Abreu, who works in a shoe repair shop close to the garden.
Even so, rain gardens are catching on across Brazil. Belo Horizonte, the country’s third-largest city, and Curitiba, the capital of Paraná state, have begun installing them.
And for city dwellers such as Rosana Santos, a nurse who lives near the rain gardens going up in Vila Mariana, they offer a respite to the encroaching gray. “Every day there are more and more buildings going up. What’s left for the rest of us?” she asks. “This is at least a breath of fresh air.”