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By Mary Nunn, RLA & Nandan Shetty, NYC Parks Green Infrastructure Unit
New York City is the densest city in America and as a result, largely impervious. The impacts of this are numerous, including combined sewer overflows, flooding, and damage of infrastructure and property. According to the Natural Resource Defense Council, the 100-year flood will occur as frequently as every 15 to 35 years in New York by the 2080s. Traditional wastewater infrastructure, such as overflow systems and treatment plants, comes at a high cost both financially and environmentally. In contrast, a green approach to addressing these problems — including green roofs, parkland bioretention systems, stormwater greenstreets, and right-of-way bioswales — supplies a myriad of social, economic, and environmental benefits in addition to managing runoff.
In New York City we are currently constructing hundreds of green infrastructure sites in the city’s most polluted “sewersheds”. The road to implementation remains perhaps one of the most challenging in the country, given the city’s degree of urbanization, physical and political complexity, and aging infrastructure. Given this, we have developed a university partnership model that aligns us with academics who are similarly motivated and interested in understanding these considerable challenges. Together, we undertand that green infrastructure is a new technology with many variables and unknowns. Our joint research challenge is to monitor performance, so that stormwater capture is quantified, cost effectiveness is known, and construction details and designs are constantly improved.
Our academic partners at Drexel University have used live tracking to monitor the performance of several constructed sites. At the stormwater greenstreet located on Nashville Boulevard between 116th Avenue and 209th Street in Queens (Nashville), 100% of stormwater runoff entered local catch basins and ultimately the combined sewer system prior to installation in 2011.
Over our 2012 monitoring season (April - November), we found that 21 out of 24 storm events were 100% retained within the site. During only three storm events, ponding inside the greenstreet caused brief overflows to the local catch basin. On an annual basis, the site’s performance suggests 74% - 86% retention of all rainfall over its catchment area, dependent upon annual precipitation variations. Furthermore, our data suggests that the Nashville site can retain 100% of the flow directed to it during all storms with less than 1.6 inches of rainfall.
In addition, Nashville was closely monitored during both Hurricane Irene and Superstorm Sandy, and it captured much more stormwater runoff than anticipated. Although the site was sized for a 5:1 ratio of catchment area to planting area, during Superstorm Sandy, inflow from the street was approximately 31 times direct precipitation on the site. Given the location of the site at a low point of the neighborhood, the increased ratio most likely occurred due to clogged drainage upslope. In total, approximately 40,000 gallons of water deposited by Superstorm Sandy either infiltrated into the site or evaporated.
We know that green infrastructure works, but there is much more to be gained by fostering a constant university partnership, especially given the scale of investment in these systems. A “design - build - research” feedback loop is requisite to monitor and learn how we can continue to improve performance-based green space.
Mary Nunn, RLA is a Landscape Architect with the NYC Parks Green Infrastructure Unit. At Parks, she has worked on a variety of projects citywide with an emphasis on sustainable design and stormwater management. Currently, she is responsible for the project management and design of green infrastructure systems in the Bronx River and Hutchinson River sewersheds.
Nandan Shetty is a PhD candidate at Columbia University, and has been working at NYC Parks Green Infrastructure Unit since 2008. Nandan received a MS from Columbia University in Civil Engineering in 2013 and a BE from Dartmouth College in Environmental Engineering in 2008.
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