#combinedseweroverflows

More than 27 billion gallons of raw sewage and polluted stormwater discharge out of 450 combined sewage overflows (“CSOs”) into New York Harbor alone each year. As the largest urban area in the United States, New York City (NYC), NY, faces substantial challenges in managing stormwater runoff. Currently, there are more than 450 CSO outfalls distributed throughout New York City. The city has been making efforts to reduce CSOs for many years through a variety of approaches including treatment plant upgrades, maximizing the wet-weather flow that is conveyed and treated at the treatment plants, improved CSO regulators, inline storage, and other storage and detention options. Recently, the city has undertaken an innovative initiative to supplement these activities with green infrastructure in order to better manage stormwater runoff. Using natural processes such as infiltration and evapotranspiration, as well as rainwater recycling, green infrastructure source controls reduce and delay the flow of stormwater to the combined sewer system.

Last Month, New York City Department of Environmental Protection (NYC DEP) Departments of Transportation, Parks and Recreation, and Design and Construction, elected officials, environmental advocates, and community groups to announce a major expansion of New York City’s nationally recognized Green Infrastructure Program. These different groups aim to develop standard designs, specifications and procedures for building green infrastructure installations. In the coming months, the City will accelerate ongoing construction to build approximately 2,000 specially engineered curbside gardens, called bioswales in Brooklyn, Queens and the Bronx. Bioswales are landscape elements designed to remove silt and pollution from surface runoff water. They consist of a swaled drainage course with gently sloped sides (less than six percent) and filled with vegetation, compost and/or riprap.

When construction is completed, the 2,000 new bioswales will have the capacity to collect and absorb more than 4 million gallons of stormwater when it rains. NYC DEP engineers estimate that the new bioswales will capture more than 200 million gallons of stormwater each year, and will improve the health of the Bronx River, Flushing Bay, Gowanus Canal, Jamaica Bay and Newtown Creek. Currently, 255 bioswales have already been constructed and there are plans to add thousands more over the next several years. The implementation of green infrastructure has become a critical aspect of the NYC Department of Environmental Protection’s (DEP’s) efforts to address combined sewer overflows within the city, with plans to use green infrastructure to manage runoff from 10% of impervious surfaces within combined sewer areas by 2030

Stormwater in New York City flows over impervious surfaces into roof drains or catch basins in the streets, and from there into the sewers.  These impervious surfaces cover approximately 72% of New York City’s 305 square miles in land area and generate a significant amount of stormwater.  Stormwater can pose challenges to the City in the form of combined sewer overflows and flooding and during significant storm events and post snowstorm melting events carries stormwater with pollution, silt, oils, and whatever else is in the sewers into the rivers of New York City. By softening the impervious urban landscape and naturally absorbing rainwater that would otherwise drain into the combined sewer system and very often into rivers will aid in improving the health of our waterways. It is estimated that New York City will have to spend up to 36 billion dollars over the next 20 years to remedy the stormwater outfall into the rivers of New York City. Bioswales vary in size, have curb cuts that allow stormwater to enter and overflow, and are designed in a way that allows them to manage between 1,300 and 3,000 gallons each during a storm according to NYC DEP engineers in the sustainability division.

The layers of stone and engineered soil contain void spaces that store the stormwater and promote infiltration, therefore reducing pollution outfall to rivers and in some cases removal of pollutants to the environment. The addition of hardy plants encourages infiltration through root growth and increases the capacity of each bioswale through evapotranspiration.

While there appears to be very little Permeable paving or projects in the near future in NYC, there is one in Far Rockaway, Queens that comes to mind. It is in the Far Rockaway section of Queens, permeable pavement was installed within a DOT park-and-ride facility. This source control pilot consists of a porous asphalt section, a permeable pavement section constructed from crushed glass, and a standard asphalt section for comparison purposes. The porous asphalt and crushed glass pavement cover approximately 6,400 square feet and 4,200 square feet, respectively.

Investing in permeable paving systems will be a game changer in reducing CSO outfall into NYC rivers, bays, and channels. Permeable pavement can be introduced in newly built or rebuilt parking decks and parking lots to more permeable surfaces would greatly increase stormwater infiltration and reduce combined sewer outfall to the rivers. Permeable Pavement is a range of sustainable materials, systems, and techniques for permeable surfaces using a base and subbase that allow the movement of stormwater through the surface. While some porous paving materials appear nearly indistinguishable from nonporous materials, their environmental effects are qualitatively different.

Organizations now have the capacity to create ultra-lightweight soil out of the waste stream in order to establish ecological and agricultural systems on rooftops, providing a means for storage during stormwater events that later evaporate leaving roof material in place and the building of detention facilities currently being built to retain stormwater during significant precipitation events only to be discharged later during dry days so current wastewater treatment facilities can handle volumes without discharging to the rivers. What about parks? Why aren't more of these surfaces natural materials or sustainably designed? This all being said, bioswales could be a important small piece in the effort to eliminate combined sewer outfall into NYC rivers. So compared to other cities, I believe NYC is leading the way.

The following photo in series captures permeable paving in Far Rockaway pilot shortly after a storm event, showing the crushed glass pavement in the foreground, conventional asphalt in the middle, and porous asphalt in the background.

The following bioswales appeared to be in good condition, but some random bioswales were not being kept up well by the NYC Parks Department.

“The gardens appeared suddenly along an industrial corridor of Brownsville, Brooklyn—one, two, a half-dozen—as if airlifted from a culde-sac upstate… In what officials have billed as one of the most ambitious programs of its kind in the United States, New York City has, with little fanfare, embarked on a roughly 20-year, $2.4 billion project intended to protect local waterways, relying in large measure on “curbside gardens” that capture and retain storm-water runoff.”

References:

1. Stormwater Source Control in New York City; 

John McLaughlin, is director of the New York City Department of Environmental Protection’s Office of Ecological Services.

Julie Stein, is director of the NYCDEP Wet Weather Planning & Water Quality Policy.

Sandeep Mehrotra, P.E., is a vice president with Hazen and Sawyer.

William Leo, P.E., is a senior vice president with HDR-HydroQual.

Matthew Jones, P.E., Ph.D., is a principal engineer with Hazen and Sawyer.

2. www.thegaiainstitute.org

3. http://www.nyc.gov/html/dep/html/home/home.shtml

4. Green Infrastructure Report: http://www.nyc.gov/html/dep/pdf/green_infrastructure/gi_annual_report_2014.pdf