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stormwater Runoff in NYC

NYC is densely developed.  72% of our entire landmass is impervious. This means that during wet weather, our hard surfaces such as roads, sidewalks, buildings, parking lots, playgrounds, etc., generate large volumes of stormwater runoff and we don't have enough permeable land to soak it all up. So where does it all go?  Most of the city's stormwater runoff flows into our sewer systems via storm drains on our streets, and some of it flows directly into our local waterways from properties along our coastline.  Stormwater runoff picks up oil, chemicals, litter, and all manner of contaminants as it travels through our streets and overland on its way into the  city's sewer systems.   

In NYC we have two types of sewer systems that manage our stormwater runoff: Here is a map of the two systems and below are general descriptions of the two systems and how they function. 


Combined Sewer System (CSS):


  • The CSS transports all of NYC's wastewater from our homes and buildings throughout the city and the stormwater runoff from approximately 60% of the city's impervious area. During dry weather our wastewater goes to 14 different treatment plants across the city where it is filtered and cleaned before being discharged into our waterways.​


  • During wet weather, sometimes even when it only rains one tenth of an inch per hour, stormwater runoff overwhelms the CSS and causes discharges of raw untreated wastewater (from our toilets, sinks, showers, etc.) mixed with polluted stormwater from our streets, into our local waterways. These discharge events are called combined sewer overflows (CSO). On average these events discharge ~20 billion gallons per year of CSO directly into our waterways.


  • CSO discharges are currently the largest source of ongoing pollution in our waterways, stormwater runoff from our impervious surfaces is the root cause of the CSO's. Older cities throughout the country and around the world are still using this antiquated type of sewer system. As our populations grow and we keep building more and more structures and paving over our last remaining swaths of undeveloped land, we are taxing our old systems well beyond their capacity. 

  • NYC is currently developing and implementing eleven CSO Long Term Control Plans that aim to reduce CSO pollution in our waterways by the year 2030 through a series of green and grey infrastructure solutions. Visit our CSO LTCP page to learn more about the plans. SWIM Coalition members in every borough have been following these plans closely to make sure they provide vital public input on the city's plans to clean up their local waterways.

 Separate Storm Sewer System (MS4):

  • The MS4 System receives stormwater runoff from 35 % of the city's landmass and discharges it directly into our waterways without any treatment before it enters the local waters. 

  • We know the annual volume of discharges from this system is extremely high. For instance, on average, the Bronx River is estimated to receive ~1.4 billion gallons of polluted stormwater annually from the MS4 outfalls in the Bronx, and Coney Island Creek receives ~1.5 billion gallons per year. NYC is just getting started on calculating stormwater volumes for every waterway. 


  • As of spring 2020, the NYC Dept. of Environmental Protection is mapping all of the impervious surfaces on all privately owned property as well as municipal property under their jurisdiction so that an accurate measurement of the stormwater runoff volume generated by our impervious surfaces can be calculated and thereby better managed. 

  • Incentives for private property owners to manage their stormwater runoff on-site ( so that it never enters the sewer system) are coming to the fore in 2020 and 2021. These incentive programs in combination with a new stormwater rule will help reduce stormwater runoff citywide.  SWIM is closely following and providing public input on these programs and keeping stakeholders citywide  informed about these advancements. See our blog post here on these programs. 

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