In large urban areas like New York City, where most streets, sidewalks and rooftops are made up of impervious surfaces, stormwater runoff can cause huge problems. Past incidents including flooded subway and tunnels during Hurricane Sandy and water pollution in Hudson River raised awarenesses of rainfall runoff in the City. Unlike natural, undeveloped areas where water is usually absorbed by soil and plants, impervious surfaces such as roofs, streets, or parking lots in urban areas are not capable of soaking water in, which leads to an accumulation of stormwater on streets and in drainage systems that can eventually cause major flooding. In addition, stormwater runoff also plays a large role in water pollution. Runoff carries trash, bacteria, heavy metals, and other pollutants from the surface into natural bodies of water. Gutters and storm sewers were built in order to solve the problems of both flooding and water pollution however, they are not always 100% efficient in preventing flooding in the New York City Area.
Sewer system construction in New York City started in the 19th century after a series of deadly cholera outbreaks and sanitation problems. Before wastewater treatment plants were built, the waste carried in pipes simply flowed out into the surrounding waterways for decades. Today, approximately 60% of New York City’s sewer system is categorized as a combined sewer system, meaning that it transports sanitary waste from households, businesses, and stormwater accumulated over streets. The other 40% of the sewer system is separated, meaning that specific pipes carry sewage directly to water treatment plants and separate storm sewers carry stormwater runoff to nearby water bodies. Today, around 6,600 miles of mains and pipes exist in NYC’s underground, transporting 1.3 billion gallons of wastewater to wastewater treatment plants every day. However, in recent years, the capacity of sewer system has reached to a point where combined sewage overflow happens more often after rain.
For the past few decades, global warming has led to overall changes in climate and sea level rise, which has influenced not only living beings but has also affected many sewer systems. Heavy precipitation events are increasing in occurrence, causing combined sewage overflow (CSO). In a well-functioning sewer system. all the waste in a combined system flows into treatment plants. However, when sudden high-intensity rainfall occurs, the pipes become overwhelmed and spill waste that should have been treated at a treatment plant into nearby water bodies. According to New York City Department of Environmental Protection, about 40 billion gallons of untreated overflow sewage spills out every year. “Oil, garbage, dog poop and everything in the gutters comes blasting out of the system, and then we tell our kids to go swimming in it,” says Terry Backer of Long Island Soundkeeper, an environmental group.
Hurricane Sandy was one of the biggest natural disasters to ever hit New York City. According to a report from Climate Central, 11 billion gallons of untreated and partially treated sewage flowed into rivers, bays, and canals. In addition, due to the malfunction of sewer system in the city, widespread flooding occurred, including the flooding of New York City’s transit system and several public parks. The city’s transport system was shut down for nearly a week after Hurricane Sandy, causing billions of dollars in damage. Thus, the city government has been trying to come up with solutions for combined sewage overflow. However, since it is impractical to replace combined sewage system with new separate pipes by ripping up miles of street, Green Infrastructure was introduced to the city of New York.
Green infrastructure is an approach to water management that can protect, restore, and simulate the natural water cycle. It is more effective and economical and enhances safety and quality of life for residents. Green Infrastructure combines the natural environment with engineered systems to treat stormwater and creates wonderful environmental and economic benefits. Methods such as rainwater harvesting systems, bioswales, and green roofs are implemented in the city to not only increase the retention time of stormwater before entering sewage systems but also add some green landscapes to the harsh concrete jungle.
Rainwater Harvesting System
Rainwater harvesting means to collect and store rainwater in large, durable containers. According to GrowNYC, a sustainability organization in New York City, there are over 140 rainwater harvesting systems that are capable of collecting more than 1.5 million gallons of rainwater a year from roofs or shade structures. The harvested rainwater can not only make water collection more convenient for gardeners but also reduce the demand on public water supply system. More importantly, the system helps mitigate stormwater runoff that can overload sewage system and pollute the waters around the city.
A bioswale is an area designed to capture and convey rainwater, allowing it to infiltrate into the ground slowly over a 24 to 48 hour period. Slopes on both sides are planted with native species which also help to prevent erosion. By increasing the retention time before the stormwater enters the sewer system, the chance of combined sewage overflow can be reduced.
A green roof is a roof that is planted with vegetation and greenery. Green roofs are important for two main reasons – they can hold rainwater reduce the temperature of the roof. Some green roofs are used as urban farms where they grow vegetables for market. In addition, the purpose of green roofs is to increase the retention time of storm water before it enters the sewage system that would otherwise cause combined sewage overflow. It is crucial to know the weight-bearing capacity of the roof in order to estimate how much water that the rooftop could hold.
In New York City, The Department of Environmental Protection conducts a Green Infrastructure Program where it partners with local agencies on constructing and maintaining a variety of sustainable green infrastructure practices such as green roofs and rain gardens. They promote the importance of preserving water by collecting and managing stormwater runoff from streets, sidewalks, and rooftops through soil, stones, and vegetation. This process prevents stormwater runoff from entering the City’s sewer system that would cause combined sewage overflow. An interactive map from the website shows a large amount of green infrastructure construction in Manhattan and Brooklyn. Not only is green infrastructure capable of controlling water quality and flow, but it also beautifies city streets and neighborhoods while improving air quality.
With a consensus of sustainability and self-help, residents of Gotham city are capable of protecting themselves through the application of green infrastructure. If everyone could make a little effort in day-to-day lives, superheroes are no longer demanded to protect our home.