The New York City Departments of Environmental Protection (DEP) and Design and Construction (DDC) completed a $7.3 million project to construct approximately 300 specially designed rain gardens in the Queens neighborhoods of Sunnyside, Maspeth, and Ridgewood. DEP and DDC also announced the addition of approximately 115 specially designed rain gardens to the northern Queens neighborhood of Flushing.
Each rain garden has the capacity to collect and absorb as much as 2,500 gallons of stormwater when it rains. Modeling shows that, together, the 415 rain gardens will capture an estimated 53 million gallons of stormwater each year, easing pressure on the sewer system and reducing overflows into Newtown Creek, Flushing Creek, and Flushing Bay. DEP funded the projects and DDC managed construction.
Continued construction of rain gardens and other green infrastructure, including in city parks, schools, and public housing, will work in conjunction with more traditional upgrades to the sewer system to improve the ecological health of the waterways. This includes the $349 million Combined Sewer Overflow retention tank that went online in 2007 and more than $40 million in sewer upgrades to increase flow to the Tallman Island Wastewater Treatment Plant.
Ongoing projects include a $132 million sewer separation project in College Point that will reduce overflows by nearly 50 million gallons a year and permanently close three combined sewer outfalls into Flushing Bay. In addition, a $33 million project is making significant modification to key junction points in the area’s sewer network, which will prevent approximately 225 million gallons of combined sewer overflow each year, according to DEP. The agency is also investing $34 million to dredge Flushing Bay and build wetlands.
DEP has developed standard designs, specifications, and procedures for building green infrastructure in the streets and sidewalks of New York City. The rain gardens are built in city sidewalks and do not result in the loss of any parking spaces. They resemble standard street tree pits, except that they vary in size and have curb cuts that allow stormwater to enter and overflow if it becomes saturated.
In partnership with the Departments of Transportation and Parks and Recreation, DEP conducts an extensive site-selection process that includes geotechnical investigations and surveys. During construction, the rain gardens are excavated to a depth of 5 feet and are then backfilled with layers of stone and engineered soil. These layers contain void spaces that store the stormwater and promote infiltration. The addition of hardy plants further encourages infiltration through root growth and increases the capacity of the rain garden through evapotranspiration.
The rain gardens are designed so that all the stormwater is absorbed in less than 48 hours and dedicated maintenance crews ensure that they are functioning properly, including removing any trash that may have accumulated and pruning the trees and plants. The crews are active seven days a week, visiting each rain garden approximately once a week. Additional crews will continue to be added as the program expands.
DEP primarily builds rain gardens in neighborhoods that are serviced by combined sewers. Within these neighborhoods, locations for the rain gardens are initially chosen by DEP engineers who, armed with maps of the local sewer systems, walk the streets and identify sidewalk locations that are upstream of a catch basin and have the room necessary to accommodate a garden.
This initial group of potential locations is then reviewed by the Department of Transportation to ensure that they meet all necessary pedestrian and vehicle clearance requirements and the Department of Parks and Recreation, which provides guidance on trees and planting plans.
Soil samples are then taken from the approved locations to ensure they can absorb the necessary amount of stormwater. The extensive survey and testing ensures that each site functions as designed. The locations that meet all these requirements will then be approved for construction.
New York City, like other older urban communities, is largely serviced by a combined sewer system where stormwater and wastewater are carried through a single sewer line to treatment plants. The city’s 14 treatment plants can manage and treat to federal Clean Water Act standards all the wastewater produced in New York City on a dry weather day, or about 1.3 billion gallons, on average.
On a rainy day they have the capacity to clean more than twice the dry weather flows. However, during intense precipitation events, stormwater that falls on the city’s impervious surfaces exceeds that capacity and overflows can be discharged into local waterways. If the overflows were not discharged, the city’s treatment plants would be flooded and severely damaged and wastewater could backup into homes and businesses.
During the last decade, the city has invested more than $10 billion in upgrades to wastewater treatment plants and related efforts to reduce combined sewer overflows. Although testing confirms that the water in New York Harbor is cleaner today than it has been in more than a century, overflows remain the city’s primary harbor water quality challenge.
As traditional “grey” infrastructure upgrades became increasingly expensive, the NYC Green Infrastructure Plan was launched. An alternative approach to improving harbor water quality, it combines traditional infrastructure upgrades and the integration of green infrastructure to capture and retain stormwater runoff before it can enter the sewer system and contribute to overflows.
New York City and New York State entered into a Modified Consent Order that formalized the city’s inclusion of green infrastructure as an important component of its plan to reduce combined sewer overflows into local waterways and improve the ecological health and cleanliness of New York City harbor water.
Information provided by New York City Department of Environmental Protection (http://nyc.gov/dep).