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NYC Hazards: Flash Flooding

Flash Flooding

What is Flash Flooding?

In addition to flooding caused by coastal storms, New York City also experiences flooding from thunderstorms and other intense rainstorms. This is flash flooding.

Flash flooding is the number one cause of deaths associated with thunderstorms, claiming more than 140 lives each year in the United States.

Flash floods can strike any time with little or no warning. City streets can become rivers in seconds. Most flash flooding is caused by slow-moving thunderstorms, thunderstorms repeatedly moving over the same area, or heavy rains from hurricanes and tropical storms. Blocked catch basins, storm drains, water main breaks, and sewer lines can also lead to flooding.
Read more about flash flooding causes from the NYC Department of Environmental Protection

Flash Floods and New York City

Much of New York's infrastructure — particularly low-lying and poor drainage areas — cannot cope with rainfall of more than one inch per hour.

National Weather Service Terms

  • FLASH FLOOD WATCH: Issued to indicate current or developing hydrologic conditions that are favorable for flash flooding in and close to the watch area, but the occurrence is neither certain or imminent.
  • FLASH FLOOD WARNING: Issued to inform the public, emergency management, and other cooperating agencies that flash flooding is in progress, imminent, or highly likely.

Flash Flooding Risks


Significant street flooding can pose risks to both pedestrians and drivers.

  • Avoid walking through moving water.
    • Water may be contaminated by oil, gasoline, or raw sewage.
    • Water may be electrically charged from underground or downed power lines. Stay away from downed power lines, and report them to the power company.
  • If you have to walk in water, walk where the water is not moving or use a stick to check the firmness of the ground in front of you.
  • Do not drive into flooded areas.
    • Six inches of water will reach the bottom of most passenger cars, causing loss of control and possible stalling.
    • A foot of water will float many vehicles.
    • Two feet of rushing water can carry away most vehicles, including sport utility vehicles (SUVs) and pick-ups.
  • Stay out of any building if it is surrounded by floodwaters.


One of the causes of flooding is surcharged sewers/sewer back-ups. Sewage is carried in different pipes than those for drinking or washing water. Unless otherwise notified, it is safe to drink tap water in an area with flooding; however, floodwater from SBUs can pose serious health risks.
Watch a simulated SBU
See an illustration of an SBU

Reduce Your Risk

The New York City Department of Environmental Protection (DEP) illustrates how the City's sewer system manages rainwater, and offers residents and business owners methods to reduce flooding:

  • Depressed driveway protection: If your property has a driveway that slopes below street level, the City recommends you contact a contractor to help prevent flooding into your basement. Obtain two or three quotes from different contractors before beginning the work, as the job can vary widely in price depending on various field conditions.
  • Green spaces, trees, and plants absorb rain water and prevent it from collecting and pooling on concrete surfaces. When possible, plant vegetation and avoid paving over green space. If you identify a good area for a tree in your neighborhood, visit, to request a tree from the New York City Department of Parks and Recreation.
  • For additional resources, visit DEP's website.

Cleaning Up After the Flood

  • Dry all areas and items quickly and thoroughly.
  • Dryclean or wash and dry all clothing and other home items.Clean floors, furniture, and other surfaces with detergent and water.
  • Stay out of deep water. Extensive flooding damage may require clean-up and restoration by professionals.
For more cleanup tips, including tips on handling sewage and mold growth, visit the NYC Department of Health and Mental Hygiene online.

Preparedness for Homeowners and Building Owners
Get information on how to prepare buildings for weather emergencies, natural disasters, and power outages

More Resources
Emergency Preparedness and Response: Floods (Centers for Disease Control and Prevention)
Find out about past flash flooding events in New York City (National Climatic Data Center)
Learn about flood insurance
Ready New York: Flooding
Are you Ready for a Flood or a Flash Flood? (Red Cross)

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