By Emily Mei Lau
In the fall of 2010, New York City released the NYC Green Infrastructure Plan designed to mitigate stormwater runoff, which can overload treatment plants and send sewage into local waterways. Despite New York’s constantly modernizing cityscape, it operates with an outdated sewer and storm runoff system.
Pipes and other infrastructure collecting surface runoff during storm events are the same ones that transport sewage to the wastewater treatment plants. During times of heavy rain or snow, the treatment plants are unable to process the high volume of these “combined sewer overflows” and divert the excess water—along with raw sewage—directly into local waterbodies such as the Hudson River. Riverkeeper, an organization advocating for clean water in New York, estimates that such storm events happen about once a week and lead to over 27 billion gallons of raw sewage and polluted stormwater in local waterbodies over the course of a year.
In an effort to curb the overflows, New York City turned to green infrastructure: right-of-way bioswales, green roofs and rain gardens, among other practices. These measures help decrease stormwater runoff by increasing pervious areas and introducing water-loving plants that can absorb some of the water and encourage evaporation. Under the management of the Department of Environmental Protection (DEP), the city proposed to spend up to $1.5 billion over the next 20 years to implement these strategies. They have focused their efforts primarily on specific areas in Brooklyn, Queens and the Bronx. As of February 2016, the DEP reported that 4,469 assets have been constructed, are in construction, or are awaiting construction, and that 90 percent of these are bioswales.
Above: an interactive map of green infrastructure projects from the New York City Department of Environmental Protection; view it online here.
What are Right of Way Bioswales?
These bioswales run along streets and sidewalks and are similar to tree pits on the sidewalk, but wider and with specific design modifications to assist with stormwater capture. A standard bioswale will have a tree guard, a three-sided fence protecting the planted area, and a thick curb on the street side. This curb has an inlet and outlet, allowing water to enter during storm events, and exit if it cannot retain the volume of water flowing into it. It has sandy soil designed for fast water infiltration, and a stone base beneath the planted area to temporarily store the water and allow for slower infiltration to the soil below.
In the years following the announcement of the NYC Green Infrastructure Plan, bioswales began to pop up throughout the city adding greenery to the sea of gray pavement. But how effective are they in reducing combined sewer overflows? What are some of the challenges associated with implementing them? Are they worth the money and effort?
Measuring Bioswale Performance
To address these questions and explain the function of bioswales in greater detail, the Gowanus Canal Conservancy, based in Brooklyn, recently hosted a talk, “Flow, Filter and Foliage: Measure Bioswale Performance.” The Earth Institute’s Suzanne Lipton moderated the event, and the speakers were Sarah Bruner, who is pursuing a master’s degree focusing on urban ecology from Columbia University; Nandan Shetty, a PhD candidate researching bioswale performance at Columbia University; and Walter Yerk, a PhD candidate in the Department of Civil, Architectural & Environmental Engineering at Drexel University.
Yerk focused on the interception loss of specific species of shrubs. Interception loss refers to the water that remains on the surface of leaves, later to be evaporated, or the water that the plant itself absorbs—essentially the rainfall that does not reach the ground because of the plant. In the context of bioswales, the higher the interception loss of a plant, the better. Yerk detailed the process of calculating interception loss through measurable data of gross precipitation, throughfall and stemflow, and he showed select results for four kinds of shrubs: sweetspire, red-twig dogwood, oakleaf hydrangea and cherry laurel. His data supported that the cherry laurel, a native plant of the Middle East, had the highest interception rates.
His findings raised the question if it would be better to stick with locally native plants, as the NYC Green Infrastructure Plan encourages, or to look for plants with the highest interception loss. All three panelists agreed that the native area of a species is a secondary concern; the real problem is finding plants that are both drought and flood-tolerant and able to survive with low maintenance on the busy streets of New York.
Bruner talked about the inner workings of plant water use. She focused on stomatal conductance, the rate of carbon dioxide entering and water vapor exiting the stomata of a leaf, to see the times of the day when plants are the most active. With photos of bioswales taken by a thermographic camera, she pointed to specific plants, such as the New England aster, that were retaining and using water longer than the surrounding plants since moist areas are cooler than dry areas. Bruner also noted that plants help combat the problems of urban heat island effect through transpiration, further adding to the benefits of bioswales in the city.
Since the palette of a bioswale is never comprised of a singular species, we also must consider the interaction between plants living in such close proximity to one another, Bruner said. Two or more species can occupy the same part of a habitat but coexist harmoniously because of their differing needs. Thus, when selecting a mix of plant species for a bioswale, the individual performance of the species is important, but so is its ability to coexist well with other plants.
Shetty steered the conversation more toward the performance of bioswales. Just recently implemented, bioswales have shown a 20-75 percent stormwater retention capability, and with such a wide range of results, it is difficult to make any conclusive statements about its broader efficacy at this point.
One of the main problems Shetty discussed was added nitrogen in the groundwater as a result of the bioswales. Since the best soil type for fast draining is sandy, oftentimes compost is added to help fertilize the plants. Yet the excess chemicals from the compost seep into the ground with the rainfall and pollute the groundwater. While research is still ongoing for encouraging denitrification in bioswales, Shetty pointed out that the reduction in combined sewer overflow from bioswales would lead to an overall greater reduction of nitrogen. Still, he argued we should find a method to prevent any further pollution of NYC groundwater—even though NYC gets its drinking water from upstate.
The panelists shed light on the biological aspects of bioswales, but said less about the noticeable or quantifiable impact the bioswales have on combined sewer overflows. For such a newly implemented practice, it still might be too early to tell. The DEP’s Green Infrastructure Performance Metrics Report from June 2016 estimates that the initiative, which currently mainly consists of bioswales, reduced the annual overflows by 507 million gallons. In spite of these encouraging results, there is still a dauntingly long way to go before meeting the city’s ambitious goal of reducing 1.5 billion gallons of overflows by 2030 through these green infrastructure practices.
As bioswales continue to transform the streets of New York, the panelists hope that the public will take notice and interest in these initiatives, because they rely on public attention and participation. People should avoid littering and walking on the plants, and should notify the DEP when weeding or other maintenance is needed. If you see an issue with a bioswale near you, call 311 or email email@example.com. The DEP specifies to include “right-of-way bioswale” or “green infrastructure” in your report, as well as the location of the bioswale.
Emily Mei Lau is an intern with the Earth Institute. She is an undergraduate student at Columbia majoring in English, with a concentration in sustainable development.
The student work depicted in the photos above is part of the Coastal Science, Engineering and Education for Sustainability project operated out of the Urban Design Lab. The National Science Foundation-funded project, a collaboration among several Earth Institute centers and Columbia schools, studies how New York City urban green infrastructure can mitigate coastal zone pollution.