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NYC Rooftop Stormwater System Captures 60% of Runoff, Avoiding Sewer Overflows

New York, NY, USA

At a Glance

To combat the vulnerability of its sewer system to stormwater overflows, New York City tested innovative runoff capture programs on building rooftops. Both blue and green roof systems offer sustainability benefits and runoff capture with green roofs capturing at least 60% of runoff.

Problem Addressed

Due to a combination of its sewer system and the high percentage of impervious surfaces within the city, New York City is susceptible to having stormwater overflow with as little as ½ inch of cumulative rain. The city utilizes a combined sewer system that handles both sewage and stormwater simultaneously; a concession that makes building infrastructure a cheaper process. The added variable of the high rate of impervious surfaces within the city means that rain simply washes pollution into nearby storm drains and either carries it to nearby bodies of water or overflows.

Because of the obvious environmental consequences from stormwater sweeping pollution to bodies of water and stormwater overflows, the city needed to find a novel solution to absorb more of its rainfall.

New York City used/is using rooftop stormwater capture systems to address this/these challenge(s).

Solution(s) Used

Recognizing the environmental challenges posed by impervious surfaces, NYDEP began to proactively test blue-roof concepts in 2010 alongside more traditional green-roof installations.

Blue roofs are non-vegetated mechanisms that passively collect stormwater. Weirs all along the roof can create temporary ponding and the gradual release of stormwater, preventing overflow. Unlike blue roofs, green roofs consist of a vegetative layer that grows in a specially-designed soil, which sits on top of a drainage layer. Green roofs are more costly than conventional roofs but they are capable of actively collecting and retaining large amounts of stormwater. Both models can provide sustainability benefits.

Many of the pilot program installations were on rooftops, as rooftops account for between 10 to 20 percent of Manhattan’s overall built surface area and tend to be underutilized. One of the main installations was on the rooftop of PS 118, an elementary school in Queens.

The roof of PS 118 in Queens is divided into 3 sections- a blue roof, a green roof, and an uncontrolled section. Each section is around 3,200 square feet and is managed and tested by the Department of Environmental Protection. Data from the study at PS 118 was collected to compare the stormwater management performance of green, blue, and control roofs during a three-year monitoring period. Because all three surfaces were on the same building, the pilot compared costs and benefits under similar environmental conditions.

The study found that the green roof pilot captured at least 60% runoff for 1-inch or smaller storms where the blue roof system of check dams was not as efficient with volume retention measured between 0% and 80%. Initial monitoring results indicated that both source control types provided significant peak runoff reduction, for low-intensity storms, but that green roofs may provide slightly better runoff control benefits than the blue roof.

Outcomes

  1. The use of rooftop installations led to a reduction in water runoff, preventing stormwater overflows in NYC
  2. Through the use of runoff capture systems, rooftops in NYC, a traditionally underutilized space, can reduce the strain on the city's combined sewer system.
  3. While both systems provide runoff reduction, green roofs may provide better runoff control benefits.
  4. Sustainability benefits from blue roofs include rooftop cooling and sustainability benefits from green roofs include absorbing air and noise pollution.
  5. As a result of the pilot project, New York City created the One NYC initiative in 2015—an extensive measure to create sustainable rooftops all over the city

Who Should Consider?

Cities or towns looking to activate underutilized rooftops in building resilience against stormwater overflows

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