Causes of Flooding
A number of conditions cause or contribute to flooding. Learn about some of them here.
Extreme Weather and Climate Change
Intense storms can overwhelm the capacity of the sewer system. In 2007 alone, New York City was hit by at least three significant storms that far exceeded the capacity of the system. In one case, on August 8, 2007, National Weather Services gauges in the metropolitan area measured up to 3.3 inches of rain in a two-hour period, and DEP's rain gauge in Bay Ridge measured a peak five-minute intensity of nearly 4.5 inches per hour. For the year, annual rainfall exceeded projections by several inches.
Fortunately, this type of event remains relatively rare, but New York City is facing the challenge of more frequent extreme weather events and flooding. Average rainfall in the City has increased nearly 10%, or more than four inches per year, in the last century -- and climate projections indicate the potential for increasingly frequent intense storms. In 2003, DEP formed a Climate Change Task Force to analyze these projections and consider their impact as we plan, engineer and design new processes and system upgrades.
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.
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.