According to the National Weather Service, flash floods are the No. 1 cause of weather-related deaths in the United States. Be aware of the awesome power of floodwater, and take steps to educate yourself about flooding dangers now.

What causes flash floods?

Unlike standard river floods, which take time to develop and are well-monitored by experts, dangerous flash floods can happen suddenly. Flash floods can occur within six hours of a rain event, or after a dam or levee failure, or following a sudden release of water held by an ice or debris jam.

Here in Illinois, flash flooding is typically caused by heavy, intense rain from severe thunderstorms. If it has been raining hard for several hours, or steadily raining for several days, be alert to the possibility of a flood.

Flash floods can hit urban and suburban areas. Indeed, as more and more land is converted from fields or woodlands to roads and parking lots, the land loses its natural ability to absorb rainfall. Urbanization increases stormwater runoff two to six times over what would occur on natural terrain.

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

Preparing for flash floods

Flash floods can catch people unprepared. Due to the sudden nature of flash flooding, you may not hear a warning from emergency officials if your area has been affected. Nonetheless,

• Listen to NOAA Weather Radio, commercial radio, and television for the latest flash flood/flood watches, warnings and advisories. See National Weather Service advisories here.
• Know the difference between a flash flood watch and warning:
• A WATCH means a flash flood is possible. If a flash flood watch is issued in your area, be sure to fill your car's gas tank so you can be prepared to leave immediately in case an evacuation notice is issued.
• A WARNING means flash flooding is already occurring or will occur soon. If a flash flood warning is issued, move to higher ground away from rivers, creeks and storm drains.
• Install check valves in building sewer traps to prevent flood water from backing up into the drains of your home.

What to do in an area of flash flooding

Floodwaters are dangerous and powerful. Be extremely cautious when confronted with floodwaters, and know their awesome power.

In the car

A depth of 2 feet will float your car. How can this happen? It’s physics. Water weighs 62.4 lbs. per cubic foot and typically flows downstream at 6 to 12 miles an hour. When a vehicle stalls in water, the water's momentum is transferred to the car. For each foot the water rises, 500 pounds of lateral force is applied to the car. And for each foot the water rises up the side of the car, the car displaces 1,500 lbs. of water.

• Look out for flooding at highway dips, bridges, and low areas. If you come upon flood waters, don’t drive through them; the road could be washed out underneath.
• Do not attempt to drive through road barricades. They are there for your safety. About 80 percent of people who die in floods die in their vehicles. If you come upon a barricade warning of high water, seek an alternate route.
• If your car stalls in rapidly rising waters, abandon it immediately and climb to higher ground.

Out of the car

Even 6 inches of fast-moving flood water can knock you off your feet. Many people are swept away wading through flood waters.

• Never try to walk or swim through swift water. If you come upon flood waters, stop, turn around and go another way.
• Never play around high water, storm drains, ditches, ravines, or culverts. It is very easy to be swept away by fast-moving water.

General flooding

In the Midwest, flooding occurs in known floodplains when prolonged or intense rainfall causes a river or stream to overflow and saturate the surrounding area. Most flooding happens in spring, summer and fall, as severe thunderstorms bring heavy rain. However, in late winter, melting snow can combine with rain and cause a river or stream to spill its banks.

Topography, soil conditions, and ground cover influence how severe any flood event is. Floods can be slow- or fast-rising, but generally develop over a period of hours or days.

Preparing for floods

If you live in areas prone to flooding, plan now to protect your family and property.

• Consider keeping insurance policies, documents, and other valuables in a safe-deposit box.
• Check to see whether your homeowner’s insurance policy covers flood damage.
• Raise your furnace, water heater, and electric panel if they are in areas of your home that may be flooded.
• Seal walls in basements with waterproofing compounds to avoid seepage through cracks.

Know the difference between a flood watch and warning:

• A flood WATCH means a flood is possible in your area. If a flood watch is issued in your area, be sure to fill your car's gas tank, in case an evacuation notice is issued. Move your furniture and valuables to higher floors of your home.
• A flood WARNING means flooding is already occurring or will occur soon in your area. Listen to local radio and TV stations for information and advice. If told to evacuate, do so as soon as possible.

Sources: Red Cross, National Weather Service