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Wednesday, 15 November 2017 01:00

Rain gardens in NY City to eliminate the overflows during intense precipitation events

Written by  TheStructuralEngineer.info

New York City, like other older urban communities, is largely serviced by a combined sewer system where storm water and wastewater are carried through a single sewer line to the city’s 14 treatment plants. These facilities can manage and treat all the wastewater produced in the city on a dry weather day (1.3 billion gallons on average) and also have the capacity to clean more than twice the dry weather flows on a rainy day. But during intense precipitation events, the storm water exceeds this capacity and overflows can be discharged into local waterways. In an effort to improve water quality, the city’s Department of Environmental Protection (DEP) is building rain gardens, designed to absorb polluted storm water that would otherwise end up into the sewer system.

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 water quality challenge. On May 2016, the New York City Departments of Environmental Protection (DEP) and Design and Construction (DDC) announced work has begun to build approximately 321 specially designed curbside rain gardens in the Queens neighborhoods of Sunnyside, Maspeth and Ridgewood, a $7.3 million project that was completed last October. DEP (that funds the project) and DDC (that manages construction) also announced the addition of approximately 115 specially designed rain gardens to the northern Queens neighborhood of Flushing, while modeling shows that together, the 415 rain gardens will capture an estimated 200 million L (53 million gallons) of stormwater each year.

Characteristics

Each rain garden has the capacity to collect and absorb as much as 2,500 gallons of storm water when it rains, and according to DEP, this is absorbed in less than 48 hours. They are built in city sidewalks and do not result in the loss of any parking spaces, resembling standard street tree pits, except that they vary in size and have curb cuts that allow storm water to enter and overflow if it becomes saturated.

During construction, the rain gardens are excavated to a depth of 5 feet and are then backfilled with layers of stone and engineered soil, which contain void spaces that store the storm water and promote infiltration. The addition of hardy plants further encourages infiltration through root growth and increases the capacity of the rain garden through evapotranspiration.

Even though rain gardens are generally considered as a ‘green’ infrastructure project, there are controversial voices clamoring that the pits are unsightly and are built without community consultation, with complaints ranging from the aesthetic to the practical (standing water, debris etc). 

Maintenance

To these complaints, the city answers that dedicated maintenance crews (whose number is expected to grow as the program expands) ensure that the rain gardens are functioning properly, by 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.

Picking up the ideal location

In partnership with the Departments of Transportation and Parks and Recreation, DEP conducts an extensive site-selection process that includes geotechnical investigations and surveys, in order to make sure that each site functions as designed.

Rain gardens are primarily built in neighborhoods that are serviced by combined sewer systems and their locations 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 necessary space to accommodate a garden. The outcome of this initial survey is then reviewed by the Department of Transportation to ensure that they meet all pedestrian and vehicle clearance requirements and the Department of Parks and Recreation, which provides guidance on tree selection and planting plans. Soil samples are then taken from the approved locations to ensure they can absorb the necessary amount of storm water. The locations that meet all these requirements will be approved for construction.

How a rain garden works 

A rain garden may look similar to a street tree pit or a small garden, but there are some 

1. Curb inlet - The inlet allows water to flow into the rain garden as it flows down the curb toward the catch basin.

2. Outlet - Larger rain gardens also have an outlet. If the rain garden fills to capacity, water can exit through the outlet and continue into the catch basin on the street corner.

3. Stone Strip - The stone strip allows people to step out of their cars without damaging the plants.

4. Plants - All rain gardens have plants and grasses which have been carefully selected to ensure they can survive on busy New York City streets.

5. Soil - The soil is graded so that water ponds in the center of the rain garden.

6. Tree Guard - All rain gardens have tree guards around them that protect the plants and keep people and dogs from walking inside of it.

7. Tree - DEP plants trees in rain gardens as often as possible. Trees benefit neighborhoods by lowering temperatures in hot summer months, improving air quality, and providing habitat for birds and butterflies.

rain gardens New York 1

Each of the rain gardens will be able to absorb up to 9400 L (2500 gal) of stormwater. Photo credit: NYC DEP

 

rain gardens New York 2

Over the past five years, approximately 3,000 rain gardens have been built throughout New York City, all on public property,  1500 currently are under construction, and thousands more are planned for the next several years.

Watch below an NYC rain garden in action:

 

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