Around 72 percent of New York City’s land is covered in an impervious layer of concrete, living up to its hype as the “concrete jungle” that Alicia Keys and Jay-Z sang about in “Empire State of Mind.” This city might be “where dreams are made,” but unfortunately a lot of sewage and pollution are made here, too. And thanks to all of these impermeable surfaces, heavy rains often wash untreated sewage and urban run-off into nearby rivers.
In an effort to reduce environmental and health problems related to stormwater runoff, in 2011, the New York City Department of Environmental Protection planned on spending $1.5 billion to turn 8,000 acres of ‘concrete jungle’ into green spaces. These measures could include creating curbside gardens or bioswales, expanding green roofs, and planting street trees. But how do urban planners decide which of these types of green infrastructure is best for a project or community?
Scientists know that green infrastructure like parks and wetlands help to retain water in the soil and filter out pollutants from run-off. However, while reviewing the existing literature, a team of researchers at Columbia University observed that the added benefits of green infrastructure have not been as thoroughly studied. The team made an effort to fill this gap by delving deeper into the social, cultural, and environmental services of 14 kinds of green infrastructure, using New York City as a case study. The research was published last week in the journal Ambio.
It has been a major challenge for urban planners to compare the different types of green infrastructure and what kind of ecosystem benefits they might offer, said study co-author Ben Orlove, co-director of Columbia University’s Center for Research on Environmental Decisions and senior research scientist at the International Research Institute for Climate and Society. “Now they can turn to our study to compare them,” he added.
The researchers surveyed experts from various academic backgrounds to evaluate the social and ecological benefits of having parks, wetlands, community gardens, green roofs, street trees, bioswales, vine canopies and other kinds of green infrastructure.
A pool of 46 experts participated in workshops held by Orlove and his colleagues for four months. These experts included engineers, urban sustainability researchers, anthropologists, economists, psychologists, and graduate students from Columbia University’s School of International and Public Affairs, who took an advanced course on climate change adaptation.
During these workshops, the experts were first given a 30-minute-long presentation that introduced the concepts of green infrastructure and ecosystem services. The experts were then given an hour to fill out a matrix listing the infrastructure types and potential benefits and were asked to rank how much each infrastructure type contributed to each potential benefit, on a scale of minus 3 to plus 3. The experts were allowed to discuss their judgments and opinions on the linkages between 22 ecosystem services and 14 types of green infrastructure.
After compiling, calculating, and studying the academic experts’ assessments of these different types of green infrastructure and their multiple benefits, Orlove and his colleagues found that large green spaces like parks and wetlands have the most co-benefits.
Parks and wetlands are almost ideal for maintaining groundwater levels and supporting habitats. These huge green spaces are also effective in raising awareness of conservation-related issues. In addition, parks purify air, support the insects that pollinate plants, contribute to social interaction among residents, and boost local tourism, while wetlands provide the co-benefits of improving water quality and soil formation. By controlling and preventing erosion, wetlands can also help in reducing the severity of natural disasters such as floods.
The study finds that community gardens offer many co-benefits, too. Like parks, they are seen as aesthetically appealing and offer soothing spaces for people to interact and feel more connected to nature. Across these types of green infrastructure, the top cultural services they offer to citizens are science and education. Community gardens’ fruits and vegetables could improve food production and supply for residents.
Similarly, green roofs are also highly beneficial for creating awareness about the importance of smaller green spaces and can go a long way in boosting pollination. The study offers further insight that green roofs with a deeper depth of soil offer far more co-benefits than their shallower counterparts.
When it comes to even smaller types of green infrastructure — like bioswales, rain gardens, retention ponds, cisterns, and rain barrels, and permeable paving — their biggest benefit is reducing run-off during heavy rainfall events.
In New York City, rain gardens and retention ponds also help to maintain groundwater levels, although on a lower scale than parks and wetlands. Academic experts also valued them lower for other ecosystem services like supporting local habitats and improving water quality.
On the other hand, the researchers noted some negative effects of certain types of green infrastructure for New Yorkers. Only three types of green infrastructure were found to be counter-productive in one category: pest control. This includes vacant land, retention ponds, and cisterns and rain barrels.
Amy Motzny, the co-lead author of the study and a research scientist at Columbia University’s Urban Design Lab, said vacant land was perceived by academic experts to have the fewest benefits as compared other types of urban green infrastructure. The experts believed vacant land often encouraged illicit activities and were a source of noise.
“This does not necessarily mean that vacant land doesn’t have ecosystem service benefits — just that there is a negative perception associated with this type of space,” said Motzny. She further explained that, at least in a New York City context, many of the experts reported vacant land to be a source of pests such as rats and mosquitoes.
“Unlike other forms of planned or intentional green space, these areas are often neglected and lacking a level of maintenance and care that lead to more positive associations,” added Motzny.
While the study highlights that the co-benefits of different types of green infrastructure far outweigh any health or environmental concerns, the researchers say they do not intend it to serve as a prescriptive tool which would provide specific recommendations. Rather, they suggest that their study could help urban planners make better-informed choices about which types of green infrastructure could most benefit their communities.