The bioswales of New York: A city plan to make more tree-stands and less sewage runoff

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The bioswale (Department of Environmental Protection)
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Look out for "bioswales": Measuring about five feet wide and 20 feet long, these small patches of plants, trees and rocks will soon be making appearance all over New York as part the city's sustainable-planning efforts.

The bioswale, or bioretention swale, is essentially a street tree, but recontextualized with plants and low curbs, the better to absorb rainfall.

If other investments in the city’s green infrastructure plan like solar panels and green roofs are more celebrated causes among urban planners and sustainability advocates, swales will be doing their work more quietly, at the street level. They'll be anchored by mostly locally grown plants with pastoral-tinged names like Swamp Milkweed, New England Aster and Summersweet Clethra. Their job is to prevent city sewers from overflowing by absorbing rain.

“I love bioswales,” said Nette Compton, a landscape architect who heads a small but growing green infrastructure division within the city parks department.

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That division is helping allocate funding for green infrastructure as part of the city’s $187 million effort to retrofit New York over the next five years.

It's an incremental part of a plan to prepare the city for the almost unimaginably large potential challenges presented by climate change, fuel shortages and old, degrading infrastructure.

Rethinking the city's pavement, in that context, is not revolutionary. But it is helpful, and doable in the relatively short term.

"Bioswales are as close to a natural system as we can get on a New York City street," she said.

Though the project is being run by the D.E.P., the initiative is part of a collaboration of city agencies including the Parks Department, which will maintain the plots, and the Department of Transportation. The D.E.P. still has to work with the other departments to find and test sites as well as get permits approved by the necessary agencies.

“A significant portion of the $187 million will be allocated for the construction of bioswales, but the exact budgeting for them will be determined as needed together with our partner agencies,” said D.E.P. spokesperson Corey Chambliss in an email.

The swale isn’t new to the urban landscape. Portland, the original "green" city, installed them along the bank of the Willamette River in 1996. And the definitions of swale vary widely enough in many places that they’re interchangeable with retention ponds. But New York might be the first to pick up on them and adapt the bioswales to a more pure urban use. 

When the city’s design commission approved the “Right-of-way Bioswale” design last month, it helped clear a path for city agencies to build the green plots across the city. Chambliss estimated the city would have roughly 100 across the city by the end of 2012.

The city already has about 20 experimental and pilot swales, according to D.E.P. head Carter Strickland. But the commission’s recent approval is a bureaucratic step allowing them to become a standard planning element on city sidewalks.

“To get up to scale, which is what we’re trying to do, we need to adopt a standard bioswale,” Strickland said to the commission.

Compton said each swale costs about $13,000 to install, but the Parks Department expects the costs may go down as it can build and order supplies to scale.

"When you look at the finished product, you don't see a lot of the work that's going on underground,” she said. “So on the surface there's a lot of similarities, but we're excavating over twice as deep as we typically do for a street tree. The pits are much larger than a typical street tree."

Every year, an average of 50 inches of rain falls within the city limits, and in some built-up sections of New York, there isn't enough dirt to absorb it all. So the swale’s larger footprint is meant to help absorb the water that otherwise flows into New York’s old sewer system. In heavy rains, all wastewater and stormwater combines in one connection and goes into the water.

Right now, according to the D.E.P., there are 422 outfalls like these where untreated runoff can get into waterways, most are in Manhattan, but priority areas include low lying parts of the outer boroughs that flow into the Jamaica Bay, the Bronx River and the Newtown Creek.

But that’s why some of the city’s first swales went near the Gowanus Canal back in November. The Parks department expects four bioswales to keep 7,200 gallons of water out of the canal. Again, this is a tiny amount relative to the size of the overall problem of the city's sewage run-off. But the idea is that it's something, and that solutions like this are, to an extent, scaleable.

In some of these low-lying areas where they're particularly needed, there may be one or two per block. The bioswales will generally go just uphill from the outfalls, taking in all the water from light rains and up to one quarter of the water in a relatively strong one-inch storm. (No word yet on 100-year floods.)

The swales also mark a change in the city’s tree-buying habits, according to Compton. Previously the city would work through contractors to pick trees from nurseries and, if they liked a patch of sweetgums, they were sidewalk-bound.

But with more needs, the city can be pickier, choosing trees that work better in certain neighborhoods. For the bioswales and some other trees, the city is dealing directly with contract growers. The parks department dictates the particular tree and goes right to a nursery to grow it.

Strickland said during the hearing that the total green infrastructure investment will also raise property values and save energy costs.

“We estimate we’ll raise $400 million a year in additional value,” he said.

Many of the trees were picked to withstand the city’s temperature and weather extremes, especially the salt, sediment and snow. Still, the natural environment is unfortunately prone to organic plagues like bugs and diseases. New York has dealt with tree pests like the Asian Longhorn Beetles and the threat of Emerald Ash Borer. Compton said she grew up in New Haven, Connecticut, the “Elm City” without any elms.

“They planted elms everywhere, every street, all over the place," she said. "And then Dutch Elm disease came in and basically wiped out the entire street tree population of the entire city."

To avoid outbreaks possible in clustered areas of trees, different varieties of trees are being spaced out around various parts of the city, and the bioswales give the trees and plants inside enough room to live.

"It's just like with people, the first line of defense in not getting sick is to be healthy in the first place,” she said. “So when the trees are healthy, when the trees are thriving, they're much more resistant to disease."