How ‘sponge cities’ are redesigning themselves for extreme rain

In California, and around the world, climate change is making extreme rain more likely. Here’s how some cities are adapting.

When a heavy storm dumped 5 1/2 inches of rain on San Francisco on New Year’s Eve—more than the city usually gets in the entire month of December—streets started to flood. In one small Japanese restaurant in the Mission District, employees had to crawl out of a window as the kitchen filled up with water. Outside, a parklet with wooden benches for outdoor dining started to float away, held back only because it was chained to the ground.

At the same time that California is in the middle of the worst drought in centuries, climate change is making extreme rainfall more likely. “Atmospheric rivers,” long jets of air that can carry 15 times as much water as the Mississippi River, aren’t new on the West Coast. But as global warming continues, the number of intense storms from atmospheric rivers may nearly double by 2100.

The current parade of storms is dumping trillions of gallons of water on the state. Most of it isn’t being captured for later use, despite the drought. It’s also causing floods, a problem compounded in some areas by sea level rise and sinking land. But better urban design could help. Globally, there are cities that are pioneering solutions.

Image: courtesy SLA Copenhagen

In Copenhagen, one project is creating reservoirs in a city park—in areas that people can use when it’s not raining—and a new channel will lead extra water to city lakes. On nearby streets in the neighborhood, rain gardens help soak up water to avoid overloading the sewer system. In other neighborhoods, sunken gardens double as rainwater storage.

In a similar “water square” in the Dutch city of Rotterdam, a sunken basketball court can store nearly half a million gallons of water in a storm. In Bangkok, a recently built park was designed with a slope that sends water into large storage tanks underground.

“Typically in urban environments you have rooftops, you have paved areas, and all of that covers up dirt and vegetation that used to be able to act as a sponge,” says Michael Kiparsky, director of the Wheeler Water Institute at the University of California, Berkeley. Green roofs, rain gardens, and other interventions can help slow down water and begin to “undo this vast scale alteration to the hydrologic cycle,” he says. The approach is sometimes called the “sponge city” model.

Green infrastructure is becoming standard in many new developments, says Geeti Silwal, a principal in urban design at Perkins&Will. “We’re thinking about ‘de-paving,’ thinking about how we create larger areas of tree canopy. . . . All of that is intrinsically embedded in our projects,” she says. In a project in Palo Alto, a creek is embedded in a concrete channel; the firm is working on a design to return it to a natural state, creating both a bigger area for water to soak into the ground and new access to nature for people living nearby.

Other projects have already demonstrated how much they can help. In New York City, when rain from Hurricane Ida and Tropical Storm Henri flooded some basement apartments, the area in front of one apartment building stayed dry. A “living street,” built with permeable pavement that can suck up water, helped prevent flooding, along with rain gardens around the building.

In a 2021 report, the architects who designed the building shared a rendering of how an intersection covered in pavement could be transformed with the same principles: A parking lot could become a park, bike lanes could be bordered by rain gardens, and the street could be lined with trees to capture large volumes of water via their roots.

San Francisco has multiple recently built green infrastructure projects. Along 14 blocks of Sunset Boulevard, rain gardens capture water running off the street. Holloway Avenue has eight blocks of permeable pavement and bioretention planters (curbside planters designed to capture water). Some other streets have added green “bulb-outs”—curb extensions at intersections that make it easier for people to cross the street and that create extra space for plants and water storage. In a city park, a stream that was put in an underground channel in the past will be “daylighted,” bringing it back to the surface where it can capture rain that otherwise would have flowed into the sewer system and out to the ocean.

“Green infrastructure has an important role to play in helping San Francisco adapt to climate change,” says Joseph Sweiss, spokesperson for the San Francisco Public Utilities Commission. It can also boost biodiversity, and trees can add shade to fight the urban heat island effect. It also provides a buffer for the sewer system by reducing runoff; like many older cities, San Francisco has a combined sewer system, meaning that it collects wastewater and rainwater together, and pipes can easily be overwhelmed.

Green infrastructure solutions are “flexible, additive, and can be modified and adapted over time as our climate changes,” Sweiss says. “Sewer improvements are also part of the solution to adapting to climate change, but it is much more challenging to increase the size of a new underground pipe once it has already been put in the ground.” Some green projects can store water for future use in irrigation or flushing toilets.

Still, the city’s new projects are happening at a relatively small scale. “What we’ve seen now is a lot of good pilot projects and a sprinkling of these projects in a lot of urban areas. That’s great. People are seeing that those things can work,” says Dave Smith, principal at Water Innovation Services, who spent three decades working on water issues at the Environmental Protection Agency. “But what is challenging is making this kind of the standard operating procedure wherever anybody’s doing new paving, new street development, new land development, so that it becomes more of a standard practice.”

On the street where the Japanese restaurant flooded, the city is working on increasing the size of sewer pipes; Sweiss says that green infrastructure could help in that area, though in heavy storms, it wouldn’t be enough on its own.

City agencies are often reluctant to adopt new approaches. Funding is another challenge, including the money needed to maintain a distributed system of green infrastructure after it’s built. (San Francisco relies in part on a “Rain Guardians” program, which tasks volunteers with maintaining rain gardens.) And while better stormwater design may become more common in new development, it’s harder to incorporate in the miles of paved urban areas that already exist.

“There really needs to be systematic investment and reinvestment in our communities, and how we manage water in them,” Smith says. “And the problem is, our cities are largely built out. So there are not that many opportunities each year, in most cities, to go redo how water is managed on a portion of the land in that city.”

Then there’s the fundamental challenge of where development has happened. In San Francisco, as in other cities, some buildings were constructed in floodplains, and on top of former streams, wetlands, and ponds (or, in some cases, on landfill inside the San Francisco Bay). In heavy storms, the water returns. Climate change is exacerbating the problem not just because of extreme rainfall but because rising sea levels are backing up drains, making it harder to get rid of large volumes of water.

In some neighborhoods, green infrastructure and improvements to traditional sewers can’t necessarily prevent flooding during extreme storms. It’s possible that some people may eventually need to move from the hardest-hit areas; San Francisco has previously considered buying some homes that repeatedly fill with sewage water in storms. Many cities are grappling with the same possibility, whether because of flooding from rain or sea level rise or both.

“I think the whole pattern of development throughout the last 100 years has been to assume that we can manage water with hard engineering measures, pipes, and pumps and tanks,” says Thaddeus Pawlowski, managing director of the Center for Resilient Cities and Landscapes at Columbia University. “With climate change, we know that that’s impossible now. There are places where development happened where people will have to move. And that’s really painful, because people have their whole lives and sometimes intergenerational wealth tied up.”

Cities need better tools to help people move, Pawlowski says, and to build more housing in safe places. And there’s no single solution to help cities prepare for more rain, though multiple solutions can make a difference. “There’s physical stuff like permeable pavement, bioswales, more trees,” he says. “But then there’s operational stuff that’s also really important, like helping people understand their risk, buying flood insurance, having a good sheltering program. We need all of the above.”

Image Credit: SLA Copenhagen


January 10, 2023

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