BioCycle February 2006, Vol. 47, No. 2, p. 38
The impact of green roofs on mitigating urban storm water runoff for individual buildings or designated drainage basins can be evaluated using modeling system.
Leslie Hoffman

New York City is not renowned for its rain, at least not to the same extent as Seattle. But contrary to popular belief, with an average annual precipitation over the last decade of 46.7 inches, the Big Apple received seven inches more than the western metropolis, according to the National Oceanic and Atmospheric Administration. Some of the rain is absorbed by parks and pervious surfaces, but most storm water runs off of roofs and along the streets, sweeps up pollutants, and flows into underground sewer pipes.
New York City, like many municipalities, has a combined sewer system that transports storm water and sewage. About half the time it rains in New York City, the volume of this mixture of raw sewage and polluted storm water exceeds the system’s capacity. In these instances, which occur on average once a week, the toxic effluent is diverted from wastewater treatment plants and released directly into the city’s surface waters, violating the Clean Water Act.
The overflow from New York City’s 450 outfalls contains bacteria, pathogens, nutrients, untreated industrial wastes, toxic pollutants including oil and pesticides, waste-water solids and debris. Overflows not only cause serious water quality problems, they also pollute private property and public places, such as beaches and parks. Steady rain falling at a rate of as little as one-quarter inch per hour can trigger hundreds of these combined sewer overflow (CSO) events. In an average year, approximately 40 billion gallons of this untreated waste-water pours into nearby waters.
Reducing sewage overflow is a priority for the New York City Department of Environmental Protection (DEP). And the solution might lie overhead – specifically, on rooftops. In 2004, the DEP engaged Earth Pledge to develop a storm water modeling system to determine the effectiveness of green roofs at reducing storm water runoff thus reducing incidences and volume of CSOs. The versatility of the model makes it a valuable tool for anyone working on wastewater issues. Using the Earth Pledge Green Roof Storm Water Modeling System, architects, engineers, building managers and homeowners are able to calculate storm water retention and detention of a green roof on a specific building. Meanwhile, urban planners, policy makers and politicians can evaluate the aggregate macroimpacts of green roofs on a given sewershed.
Green roofs incorporate a waterproof membrane topped with lightweight growing medium and vegetation. Internationally, green roofs are used to address a number of environmental issues. Not only are they an effective tool for storm water runoff abatement, green roofs also mitigate the Urban Heat Island Effect, reduce cooling requirements for individual buildings and promote biodiversity. They are used most extensively in Germany, where it is estimated that 14 percent of all flat roofs are now green, according to a 2003 study.
In New York City, the green roof movement is gaining momentum. Dozens of green roofs have been built in recent years and more are under construction and being planned. The greater the number of green roofs, the greater the potential for reducing CSO events. More precise knowledge about these impacts will enable designers, developers and government officials to make decisions that have the desired outcome.
Impact Of Green Roofs On Storm Water Runoff
The green roof model was created to determine the impact of green roofs on mitigating storm water runoff. It simulates and evaluates the impacts of green roofs for individual buildings as well as a designated drainage basin. The application allows users to model the storm water retention and detention capabilities of a particular green roof based on design criteria including growing medium type and depth, vegetation and roof size. The user can review how the ability to retain storm water changes depending on design inputs; the system also compares the storm water impact of green roofs and conventional roofs.
For instance, variations in the depth of the growing medium will have large impacts on the storm water retention capabilities of a green roof. On a 2,000 square foot roof, an increase in the growing medium depth from four inches (Figure 1) to six inches (Figure 2), using a five-year design storm and keeping all other specifications identical, will increase the retention capacity of the roof by 46 percent from 2,333 gallons to 3,402 gallons. The increase in growing medium depth also will impact the detention capabilities of the green roof, delaying the storm water discharge from the fifth to the sixth hour of the storm.
The macromodel shows the effectiveness of a network of green roofs on reducing volume and number of incidences of CSOs – currently for a lower Manhattan subbasin of New York City’s Newtown Creek drainage basin. Once the user has run the micro model for a particular design specification, the macromodel can be used to select buildings for potential green roof development. Through a Geographic Information Systems interface, the user highlights buildings within the subbasin area for greening. The model then evaluates the percent reduction on incidents and volume of CSOs retained by these potential green roofs. Over time, the model will be built out for the rest of New York City and other cities in North America.
Earth Pledge’s research on green roofs goes beyond the modeling system at the Earth Pledge Green Roof Research Station at Silvercup Studios in Long Island City, Queens. Silvercup Studios, home to “Sex and the City” and “The Sopranos,” has installed a 35,000 square foot green roof – the largest in New York City. Using green and conventional roof replica platforms, Earth Pledge is monitoring and comparing their thermal performance and storm water management capabilities to quantify the value of the impact of green roofs on energy usage and storm water overflow. This data will also help to test and refine the green roof model. Additionally, on a nearby building owned by Gratz Industries, Earth Pledge is installing the city’s first in-situ monitoring of 5,000 square feet of green roof and an adjacent control.
CSO Challenges and Abatement
New York City isn’t the only municipality with storm water and CSO issues. According to the EPA, as of November 2004, there were approximately 836 permits in the U.S. for combined sewer systems. The communities in 32 states, primarily in the Northeast and Midwest, affected by CSOs have a combined population of an estimated 46 million people. A significant number of these communities have not implemented minimum controls or developed long-term CSO control plans.
Of those cities that are tackling the problem, several have expanded existing sewer systems to accommodate CSOs; however, this solution has not entirely eradicated CSOs in some municipalities, and it can cause additional environmental pollution. The high cost of separating sewer systems to prevent CSOs has prohibited many cities from taking action.
Milwaukee, for instance, undertook a $2.2 billion CSO abatement project that relies on collector systems that divert flow from the existing sewers into deep tunnels for temporary storage. The deep tunnels lie 300 feet beneath the surface, are up to 32 feet wide and run for nearly 20 miles. According to a 2004 NRDC report, in the decade since the Deep Tunnel was completed, the volume of CSOs has increased and 12 billion gallons of raw sewage and storm water have been dumped into nearby rivers and Lake Michigan, which is a drinking water source.
After Milwaukee and other cities that address the CSO problem by ensuring sewer systems and wastewater treatment centers have the capacity to absorb storm water, the next level of concern is likely cutting down on the volume of storm water being treated. Though a city may no longer release raw sewage into nearby surface waters, rainwater is being treated in the same rigorous, expensive process as sewage.
This practice increases the volume of chlorinated, treated water released into surface waters. The wastewater and storm water are both disinfected with a type of chlorine, such as sodium hypochlorite. Chlorine lowers the pH of water, making it more acidic, which can create a toxic environment for fish and decrease biodiversity. In these instances, city officials and designers could use the green roof model to see how a system of green roofs could potentially decrease the amount of storm water mixing with sewage, saving the city money and decreasing water pollution.
Alternative Approach In Minneapolis
Minneapolis has taken an alternate approach to attain its goal of eliminating CSOs by 2008. The city has been separating sewer and storm water pipes since the 1960s and today more than 95 percent of the sewers have been separated. The remaining sewers will be separated, but they are the most difficult and time-consuming and were therefore left for last.
In addition to separating the systems, in 2003 Minneapolis developed a five-year plan to eliminate CSOs. Under a city council ordinance, every land parcel and building will be inspected to check for prohibited connections to the sanitary sewer system; for instance, teams make sure no rain gutters empty into sewer pipes. In addition, Minneapolis has created the Storm Water Credit program whereby residents and building owners can get a credit of up to 100 percent of their storm water utility fee if they use best management practices to address storm water quantity and quality, including green roofs, rain gardens and swales.
Residents must have their application certified by a state licensed engineer or landscape architect to verify that their approach can handle rain events of varying intensity and duration. A tool such as the green roof model could be used to help verify that a proposed green roof does in fact manage storm water to policy specifications.
As more municipalities begin to tackle their storm water and CSO problems, government officials and designers are seeking alternatives to conventional, capital-intensive solutions. The Earth Pledge Green Roof Storm Water Modeling System can help municipalities understand that green roofs are more than an increasingly popular trend. By quantifying the amount of storm water that could be prevented from either being treated or being released into waterways with raw sewage, the green roof model shows how these elegant oases amid miles of concrete can help alleviate CSO and storm water problems.