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Causes of Flooding

A number of conditions cause or contribute to flooding. Learn about some of them here and find out What You Can Do to Help.

Extreme Weather and Climate Change

New York City faces increasing risks from the impacts of global climate change. In 2015, the NYC Panel on Climate Change (NPCC) released updated projections for the region in its publication Building the Knowledge Base for Climate Resiliency. According to its findings, by 2050, NYC will experience:

  • Increased average temperatures (4.1 to 5.7° F)
  • Increased precipitation (4 to 11 percent)
  • Rising sea levels (11 to 21 inches)
  • Increased frequency and intensity of coastal flooding
  • Average number of days above 90°F is expected to at least double
  • Increased frequency and intensity of hurricanes across the North Atlantic Basin

Increased Precipitation

New York City experiences significant precipitation throughout the year, with relatively little variation from month to month in a typical year. Annual average precipitation ranges between approximately 43 and 50 inches, depending on the location within the City. Precipitation has increased at a rate of approximately 0.8 inches per decade from 1900 to 2013 in Central Park. Such changes in climate patterns may result in more frequent localized flooding over time.

Sea Level Rise and Tidal Impacts

In coastal areas, rising sea levels can exacerbate local flooding conditions, causing tidal water to enter the sewer system and reducing the speed at which water can drain from inland sources. Sea level rise in New York City has averaged 1.2 inches per decade since 1900, nearly twice the observed global rate.

Blocked Catch Basin Grates

Another serious cause of flooding is the blocking of catch basin grates in our streets. This occurs when rainwater—especially during very intense storms—scours streets and sidewalks like a fire hose, pushing debris like leaves, gum wrappers and restaurant menus on to the catch basin where it molds itself into a sort of mat. Such debris can block the grate so completely that water cannot enter the storm sewers. Instead, it pools around the basin, causing flooding even before the sewer is full. You can call 311 or submit an online form to report a blocked or clogged catch basin.

Surcharged Sewers

Sewers can also become overtaxed during intense rain events when the sheer volume of stormwater and wastewater entering the system fills them to capacity, leaving no space for excess water to enter. In this condition, described as a sewer being surcharged, the excess stormwater remains above ground. Unless absorbed by green spaces or channeled to a body of water, this flooding can flow off the street into below-grade areas such as driveways, patios and basements.

Day to day, DEP works to keep our system up and running efficiently. We clean and maintain our sewer and drainage systems to keep them in a state of good repair. At the same time, we plan and build both traditional infrastructure and implement best management practices for managing stormwater and wastewater.

Green Infrastructure