New York City has its nicknames: the Empire City, Fun City, the city that never sleeps. Now, because of a partnership between New York and Copenhagen, another might join the list: Sponge City.
New York, city officials said, needs to do better at dealing with weather phenomena that are becoming more common — cloudbursts, which are especially intense rainstorms that dump enormous amounts of water in a short time. Climate change means cloudbursts are likely to happen more frequently.
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So officials have spent three years studying how Copenhagen coped with heavy storm water runoff after a deluge in 2011. A Danish official called it a thousand-year weather event.
The storm drenched Copenhagen with six inches of rain in two hours. Afterward, officials considered ways of making the city more absorbent with design changes, like planting grass to replace asphalt (because asphalt does not absorb rainwater) or lowering playgrounds and basketball courts so they hold water in a storm.
Then in 2012, Hurricane Sandy flooded 51 square miles in New York, about 17 percent of the city’s total land mass, according to city statistics.
When New York officials learned that Copenhagen had developed a master plan to deal with storms and runoff, the two very different cities formed a partnership. Copenhagen’s population is less than 10 percent of New York’s, and Copenhagen covers far less land than do the five boroughs.
“Yours is much, much bigger, but the principle is the same,” said Lykke Leonardsen, a Copenhagen official involved in the partnership. “The idea of creating a new type of infrastructure for the management of storm water is a way of making sure that you do not experience an unwanted flood from sewer water and storm water, because then you’re not just talking about a nuisance but a health issue.”
Officials from both cities decided they needed open space that can, in effect, absorb water like sponges, or at least slow runoff gushing through populated areas during or after a storm. Finding such spaces is a tall order in urban areas, but “sponges” help to keep water out of the sewer system when sewers are overwhelmed in a storm.
“The obvious thing is, why don’t you build bigger sewers,” Vincent Sapienza, the commissioner of New York’s Department of Environmental Protection, said in an interview. “One is, they cost a fabulous amount of money to do, and two, on many residential streets, there’s no room for bigger sewers.”
Ms. Leonardsen said Copenhagen’s experience showed that turning to green infrastructure and solutions like sponge areas had economic advantages.
“We found that instead of digging down in underground reservoirs and expanding the sewer system,” she said, “this was much cheaper.”
After the 2011 cloudburst, Copenhagen began 300 projects to drive storm water away from populated areas and manage flooding better. “Copenhagen showed you can take it a step further by creating spaces where you can store larger volumes of water,” said Alan Cohn, a managing director in the environmental agency’s Bureau of Environmental Planning and Analysis.
Adding green space or replacing asphalt with grass could increase the spongelike properties of a neighborhood. And when sewers are overwhelmed by a rainwater runoff, he said, the goal should be “flooding by design” — that is, designing the landscape so water goes where it can be stored temporarily if it cannot be absorbed into the ground.
Designing a basketball court like an amphitheater, with steps leading down, could accomplish that.
On an appropriately recent rainy day, officials from the two cities, along with environmental experts and officials from other cities, gathered at the Center for Architecture in Greenwich Village for a discussion of what could be accomplished through international collaboration.
Susanne DesRoches, a deputy director of the Mayor’s Office of Recovery and Resiliency, said the project with Copenhagen had been “a huge success.” Mr. Sapienza said the partnership would continue and expand to include other cities.
Other officials said it was important to share ideas because each city tends to formulate plans in its own way.
“There’s no cookbook for how to make cities resilient,” Ms. Leonardsen said. “It’s new for us, and we all have to figure it out.”
In 2016, the second year of the partnership, New York began a cloudburst study in southeastern Queens, where storm water drains into Jamaica Bay. Now in the planning stages is a pilot program at the South Jamaica Houses, a public-housing project that dates to when Fiorello H. La Guardia was mayor.
A second pilot-project area is in St. Albans, Queens, which also sustains heavy flooding.
Southeastern Queens is shaped somewhat like a bowl and sits at a low elevation with inadequate sewer infrastructure, officials said, so the city is committing $1.9 million to reduce flooding and improve street conditions there. The money will go to 45 infrastructure projects to be completed over the next 10 years.
Cynthia Rosenzweig, co-chairwoman of the New York City Panel on Climate Change and a professor at Barnard College, said municipalities across the country needed to think bigger.
“In Europe, they take a larger approach,” she said. “Here, we take a jack-o’-lantern approach,” concentrating on limited projects that are the equivalent of the eyes or the mouth on a Halloween pumpkin. “We need to scale up to the neighborhood level and beyond.”