United States Search and Rescue Task Force
FLOODS AND FLASH FLOODS
Flash floods occur within a few minutes or hours of excessive rainfall, a dam or levee failure, or a sudden release of water held by an ice jam. 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. Furthermore, flash flood-producing rains can also trigger catastrophic mud slides. You will not always have a warning that these deadly, sudden floods are coming. Most flood deaths are due to FLASH FLOODS.
Most flash flooding is caused by slow-moving thunderstorms, thunderstorms repeatedly moving over the same area, or heavy rains from hurricanes and tropical storms.
Types of Flooding
- River Flooding
Flooding along rivers is a natural and inevitable part of life. Some floods occur seasonally when winter or spring rains, coupled with melting snow, fill river basins with too much water too quickly. Torrential rains from decaying hurricanes or tropical systems can also produce river flooding.
- Coastal Flooding
Winds generated from tropical storms and hurricanes or intense offshore low pressure systems can drive ocean water inland and cause significant flooding. Escape routes can be blocked off and blocked by high water. Coastal flooding can also be produced by sea waves called tsunamis, sometimes referred to as tidal waves. These waves are produced by earthquakes or volcanic activity.
- Urban Flooding
As land is converted from fields or woodlands to roads or parking lots, 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, while basements can become death traps as they fill with water.
- Flash Flooding
Several factors contribute to flash flooding. The two key elements are rainfall intensity and duration. Intensity is the rate of rainfall, and duration is how long the rain lasts. Topography, soil conditions, and ground cover also play an important role.
Learn flood warning signs and your community alert signals.
Request information on preparing for floods and flash floods.
If you live in a frequently flooded area, stockpile emergency building materials.
These include plywood, plastic sheeting, lumber nails, hammer and saw, pry bar,shovels, and sandbags.
Have check valves installed in building sewer traps to prevent flood watersfrom backing up in sewer drains. As a last resort, use large corks or stoppers to plug showers, tubs, or basins.
Plan and practice an evacuation route.
This plan should include information on the safest routes to shelters. Individuals living in flash flood areas should have several alternative routes.
Have disaster supplies on hand.
- Flashlights and extra batteries
- Portable, battery-operated radio and extra batteries
- First aid kit and manual
- Emergency food and water
- Nonelectric can opener
- Essential medicines
- Cash and credit cards
- Sturdy shoes
Develop an emergency communication plan. In case family members are separated from one another during floods or flashfloods (a real possibility during the day when adults are at work and children are at school), have a plan for getting back together.
Ask an out-of-state relative or friend to serve as the “family contact.” After a disaster, it's often easier to call long distance. Make sure everyone in the family knows the name, address, and phone number of the contact person.
Make sure that all family members know how to respond after a flood or flash flood.
Teach all family members how and when to turn off gas, electricity, and water.
Teach children how and when to call 9-1-1, police, fire department, and which radio station to tune to for emergency information.
Learn about the National Flood Insurance Program. Ask your insurance agent about flood insurance. Homeowners policies do not cover flood damage.
DURING A FLOOD WATCH
- Listen to a battery operated radio for the latest storm information.
- Fill bathtubs, sinks, and jugs with clean water in case water becomes contaminated.
- Bring outdoor belongings, such as patio furniture, indoors.
- Move valuable household possessions to the upper floors or to safe ground if time permits.
- If you are instructed to do so by local authorities, turn off all utilities at the main switch and close the main gas valve.
- Be prepared to evacuate.
DURING A FLOOD
- Turn on battery-operated radio or television to get the latest emergency information.
- Get your preassembled emergency supplies.
- If told to leave, do so immediately.
- Climb to high ground and stay there.
- Avoid walking through any floodwaters. If it is moving swiftly, even water 6inches deep can sweep you off your feet.
If In A Car:
- If you come to a flooded area, turn around and go another way.
- If your car stalls, abandon it immediately and climb to higher ground. Many deaths have resulted from attempts to move stalled vehicles.
DURING AN EVACUATION
- If advised to evacuate, do so immediately.
- Evacuation is much simpler and safer before flood waters become too deep for ordinary vehicles to drive through.
- Listen to a batter-operated radio for evacuation instructions.
- Follow recommended evacuation routes–shortcuts may be blocked.
- Leave early enough to avoid being marooned by flooded roads.
Flood dangers do not end when the water begins to recede. Listen to a radio or television and don't return home until authorities indicate it is safe to do so.
Remember to help your neighbors who may require special assistance–infants, elderly people, and people with disabilities.
Inspect foundations for cracks or other damage.
Stay out of buildings if flood waters remain around the building.
When entering buildings, use extreme caution.
- Wear sturdy shoes and use battery-powered lanterns or flashlights when examining buildings.
- Examine walls, floors, doors, and windows to make sure that the building is not in danger of collapsing.
- Watch out for animals, especially poisonous snakes, that may have come into your home with the flood waters. Use a stick to poke through debris.
- Watch for loose plaster and ceilings that could fall.
- Take pictures of the damage–both to the house and its contents for insurance claims.
Look for fire hazards.
- Broken or leaking gas lines
- Flooded electrical circuits
- Submerged furnaces or electrical appliances
- Flammable or explosive materials coming from upstream
Throw away food–including canned goods–that has come in contact with flood waters.
Pump out flooded basements gradually (about one-third of the water per day) to avoid structural damage.
Service damaged septic tanks, cesspools, pits, and leaching systems as soon as possible. Damaged sewage systems are health hazards.
Check for gas leaks–If you smell gas or hear blowing or hissing noise, open a window and quickly leave the building. Turn off the gas at the outside main valve if you can and call the gas company from a neighbor's home. If you turn off the gas for any reason, it must be turned back on by a professional.
Look for electrical system damage–If you see sparks or broken or frayed wires, or if you smell hot insulation, turn off the electricity at the main fuse box or circuit breaker. If you have to step in water to get to the fuse box or circuit breaker, call an electrician for advice.
Check for sewage and water lines damage–If you suspect sewage lines are damaged avoid using the toilets and call a plumber. If water pipes are damaged, contact the water company and avoid the water from the tap. You can obtain safe water by melting ice cubes.
Impacts to Automobiles
Nearly half of all flash flood fatalities are auto related!!!
Never drive your automobile into moving water, especially if you cannot tell how deep the water is.
|Water weighs 62.4 lbs. per cubic foot and typically flows downstream at 6 to 12 miles per hour.||When a vehicle stalls in the water, the water's momentum is transferred to the car. For each foot the water rises, 500 lbs. of lateral force is applied to the automobile.|
|But the biggest factor is bouyancy. For each foot the water rises up the side of the car, the car displaces 1500 lbs. of water. In effect, the automobile weighs 1500 lbs. less for each foot the water rises.||Two feet of water will carry away most automobiles!|
FLOODING – THE HIDDEN DANGER
Nearly half of all flood related deaths occur in vehicles. Most of these deaths take place when people drive into flooded highway dips of low drainage areas. A low water crossing is where a road, without a bridge, dips across a normally dry creek bed or drainage area. Motorists who attempt to cross these flooded low water crossings are putting themselves, their vehicles and any other occupants of their vehicles at deadly risk.
Fluid Dynamics –
Water weighs about 62.4 pounds 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 care. For each foot the water rises, 500 pounds of lateral force are applied to the car. For each foot the water rises up the side of the car, the car displaces 1,500 pounds less for each foot the water rises. Therefore, most cars will float in just two feet of water.
The Human Element –
Most vehicles will become buoyant in two feet of water or less. People who have previously driven successfully through a flooded low water crossing often do not recognize that an increase of an inch or so in the water level may be all it takes to tip the balance of buoyancy against them. Few people including public safety and rescue personnel, appreciate the power of flowing water. Fewer people realize how fast water can rise in a small stream to flood a low water crossing area.
More than half of all low water crossing vehicular related deaths occur at night. Under conditions of low visibility the vulnerability of the driver and passengers to the hidden danger is greatly magnified. High volumes of moving water play havoc on bridges, road beds, and other structures. What may appear as a normal road, may in fact, be a death trap.
Flash Flood Warnings –
A flash flood watch means flash flooding is possible within the flash flood watch area. When a flash flood watch is posted, persons in the watch area should take precautionary measures. Keep informed and be ready for quick action if flash flooding is observed or a warning is issued. A flash flood warning means flash flooding is imminent and to take action immediately. Motorists and pedestrians in or passing through the advisory area should:
- Keep abreast of road conditions through the news media.
Avoid crossing flooded roads, and allow extra time to reach your destination.
Avoid steep terrain where mudslides can occur.
Be aware that heavy rainfall can reduce visibility to zero.
Be aware that road beds may have been scoured or even washed away during flooding creating unsafe driving conditions.
Be aware that driving too fast through low water will cause the vehicle to hydroplane and lose contact with the road surface.
Dangerous Advertising –
With this information in mind, it can be shocking to see current television advertising for sports utility vehicles. Their advertising campaign demonstrates these vehicles driving through low water crossing areas which promotes the idea that their vehicles can handle just about anything mother nature throws at them. This can be misleading and dangerous as it presents a false sense of security.
The general public as well as public safety and rescue personnel must remember the following critical points:
- Cars and emergency vehicles can easily stall in the water and be carried away in any current.
Most vehicles will be swept away by less than two feet of running water.
Do not try to cross a flooded road or stream in your vehicle.
Heed all flood and flash flood warnings issued by the National Weather Service.
Do not drive around barricades and low water crossings.
Be especially vigilant at night or when traveling on unfamiliar roads.
Do not cross flowing water.
Observe any water level. Indicators at low water crossings, remembering that six inches of water may be enough to cause you to lose control of your vehicle.
Appreciate how fast water can rise.
Be aware that beneath the water’s surface, road beds may have been washed away.
If you choose to abandon your vehicle, respect the force of the water. Six inches of fast moving water will knock you off of your feet.
Thanks to Gerald Dworkin, NOAA and the U.S. Department of Commerce for the contents of the above article.
Water Rescue in Progress
Severe Flooding From Hurricane Floyd in 1999