National Weather Service United States Department of Commerce

Severe Weather Awareness - Flash Floods and River Floods

The Number ONE weather-related killer in the United States


Flooding kills  almost twice as many people each year as tornadoes and hurricanes combined!

Unlike hurricanes,
floods are a threat in every state.

Weather related fatalities by year
 

Flooding can take many forms including river floods, coastal floods, urban floods, and flash floods.

River flooding from Hurricane Floyd
Ed Hayden
Goldsboro News- Argus

Coastal flooding from offshore winter storm
Tom Herde
Boston Globe

Urban flooding from heavy rains
David Vann
The Sentinel-Record
River flooding...like coastal flooding...are long term events and may last a week or more. River flooding at Modoc
Flash flooding and urban flooding are short-term events ... usually occurring with 6 hours of the beginning of heavy rain. Man wading through chest deep flood waters
Photo by: Brian Strickland News-Argus
Goldsboro , N.C.

How do flash floods occur? The two key elements are:

  • rainfall intensity

  • duration

Topography, soil conditions, and ground cover
also play an important role.

Radar display showing thunderstorms in a line (training) over the Pee Dee region
Flash floods occur within a few minutes or
hours of excessive rain or a dam or levee
failure. Flash floods can roll boulders, tear out
trees, destroy buildings and bridges, and scour
out new channels. Rapidly rising water can
reach heights of 30 feet or more.
A car stranded in flood waters - water up to the windows
Photo by: David Vann
The Sentinel-Record
Hot Spring, AR

 Most flash flooding is caused by slow-moving thunderstorms, thunderstorms repeatedly moving over the same area (called "training"), or heavy rains from hurricanes and tropical storms.

 

River flood waters

Urban Floods are the result of land being converted from fields and woodlands into roads and parking lots. 

As land is paved, it loses its ability to absorb rainfall. Urbanization increases runoff 2 to 6 times over what would occur on natural terrain. 

During periods of urban flooding, streets can become swift moving rivers and basements can become death traps as they fill with water.

Street flooding - evacuation using small boats

Photo by: Kaye Farmer
News-Argus
Goldsboro, N.C.

Nearly half of all flash flood deaths are auto
related. Even 6 inches of fast moving flood
water can knock you off your feet and at a
depth of 2 feet will float your car.
Never drive into flood waters!
Water weighs 62.4 pounds per cubic foot and is typically flowing downstream at 6 to 12 miles an hour. Water to the bottom of a sedan
When a vehicle stalls in the water, the water's momentum is transferred to the car. For each foot the water rises, 500 pounds of lateral force are applied to the car. Water is now midway up the sedan door
The biggest factor is bouyancy. For each foot the water rises up the side of the car, the car displaces 1,500 pounds of water. In effect, the car weighs 1,500 pounds less for each foot the water rises. The combination of bouyancy and lateral force of the flood waters begins to move the car
Two feet of water will carry away most cars - 1,000 pounds of lateral (or side force) plus 3,000 pounds of buoyancy! In only 24 inches of flood water the sedan begins to float away
Clues to potential flash flooding:
  • Distant thunder - runoff from a faraway thunderstorm could be heading your way
  • Look out for water rising
    rapidly
  • In your car look out for flooding at highway dips, bridges, and low areas.
Heavy rainfall from a thunderstorms
Before the flood...
  • Know your flood risk and elevation
    above flood stage
    • Do your local streams or rivers
      flood easily? If so, be prepared
      to move to a place of safety.
  • Know your evacuation routes.
  • Keep your automobile fueled; if
    electric power is cut off, gas stations
    may not be able to operate pumps for
    several days
  • Store drinking water in clean bathtubs
    and in various other containers. Water
    service may be interrupted or water
    contaminated.
  • Keep a stock of food that requires
    little cooking and no refrigeration.
  • Keep first aid supplies on hand.
    Keep a NOAA Weather Radio, a
    battery-powered radio, emergency
    cooking equipment, and flashlights in
    working order.
  • Install check valves in building sewer
    traps to prevent flood water from
    backing up into the drains of your
    home.
In flood areas, be prepared before flooding occurs!

A safety kit should be a part of every home

Assemble a disaster supplies kit containing: first-aid kit, canned food and can opener, bottled water, rubber boots, rubber gloves, NOAA Weather Radio, battery-powered radio, flashlight, and extra batteries.

River Flood Watch
  • River levels may reach or rise above flood stage
  • Be ready to move to a safe area if necessary
  • Stay tuned to NOAA Weather Radio or local radio and TV for possible warning
River Flood Warning
  • Rivers will rise above flood stage
  • Take all precautions necessary to protect your life and property
  • Move to a safe area if you are in the flood area
  • Stay tuned to NOAA Weather Radio or
    local radio and TV
Flash Flood Watch
  • Heavy rains are possible
  • Be ready to move to higher ground immediately if flooding occurs
  • Stay tuned to NOAA Weather Radio or
    local radio and TV for possible warning
Flash Flood Warning
  • Flash flooding has been reported or is
    imminent
  • Move to higher ground immediately
  • Never drive through floodwater on roadways
  • Stay tuned to NOAA Weather Radio or local radio and TV

The rule for being safe in a flooding situation is simple:


Head for higher ground and stay away from flood waters!

Stay tuned to NOAA WEather Radio or local tv and radio

Flood and Flash Flood warnings are sent to
local radio and television stations and are
broadcast over your local NOAA Weather Radio serving the warning area. These warnings are also relayed to local emergency management and public safety officials who can activate local warning systems to alert communities to the danger.

   
   

Contact us if you'd like a free copy of Flash Floods and Floods...the Awesome Power - a preparedness brochure produced by the National Weather Service, Federal Emergency Management Agency, and the American Red Cross.

Related Web Sites on Flooding:


Credits:

Thanks to the Goldsboro NC News-Argus for allowing us to use pictures from Hurricane Floyd flooding