What Causes Floods?
Flooding occurs in known floodplains when prolonged rainfall over several days, intense rainfall over a short period of time, or an ice or debris jam causes a river or stream to overflow and flood the surrounding area. Melting snow can combine with rain in the winter and early spring; severe thunderstorms can bring heavy rain in the spring and summer; or tropical cyclones can bring intense rainfall to the coastal and inland states in the summer and fall.
Flash floods 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, and flash floods can catch people unprepared. You will not always have a warning that these deadly, sudden floods are coming. So if you live in areas prone to flash floods, plan now to protect your family and property.
As land is converted from fields or woodlands to roads and parking lots, it loses its ability to absorb rainfall. Urbanization increases 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.
Several factors contribute to flooding. 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 important roles. Most flash flooding is caused by slow-moving thunderstorms, thunderstorms repeatedly moving over the same area, or heavy rains from hurricanes and tropical storms. Floods, on the other hand, can be slow- or fast-rising, but generally develop over a period of hours or days.
Learn about flooding and flash flooding in your area by contacting the local emergency management office, National Weather Service (NWS) office, your American Red Cross chapter, or your planning and zoning department. If you are at risk, take steps to reduce damage and the risk of injury or loss to your family.
Know the difference between WATCHES and WARNINGS.
A National Weather Service (NWS) WATCH is a message indicating that conditions favor the occurrence of a certain type of hazardous weather. For example, a severe thunderstorm watch means that a severe thunderstorm is expected in the next six hours or so within an area approximately 120 to 150 miles wide and 300 to 400 miles long (36,000 to 60,000 square miles). The NWS Storm Prediction Center issues such watches. Local NWS forecast offices issue other watches (flash flood, winter weather, etc.) 12 to 36 hours in advance of a possible hazardous-weather or flooding event. Each local forecast office usually covers a state or a portion of a state.
An NWS WARNING indicates that a hazardous event is occurring or is imminent in about 30 minutes to an hour. Local NWS forecast offices issue warnings on a county-by-county basis.
Many more WATCHES are issued than WARNINGS. A WATCH is the first sign a flood may occur, and when one is issued, you should be aware of potential flood hazards.
Be aware of flood hazards. Floods can roll boulders, tear out trees, destroy buildings and bridges, and scour out new channels. Flood waters can reach heights of 10 to 20 feet and often carry a deadly cargo of debris. Flood-producing rains can also trigger catastrophic debris slides.
Regardless of how a flood or flash flood occurs, the rule for being safe is simple: head for higher ground and stay away from flood waters. Even a shallow depth of fast-moving flood water produces more force than most people imagine. The most dangerous thing you can do is to try walking, swimming, or driving through flood waters. Two feet of water will carry away most automobiles.
Plan for a Flood
Develop a Family Disaster Plan. Develop flood- specific planning. Learn about your area’s flood risk and elevation above flood stage.
Contact your local Red Cross chapter, emergency management office, local National Weather Service office, or planning and zoning department about your area’s flood risk.
Knowing the elevation of your property in relation to nearby streams and dams will let you know if forecasted flood levels will affect your home.
If you are at risk from floods:
Talk to your insurance agent. Homeowners’ policies do not cover flooding. Ask about the National Flood Insurance Program (NFIP).
Use a NOAA Weather Radio with a tone-alert feature, or a portable, battery-powered radio (or television) for updated emergency information.
Develop an evacuation plan. (See “Evacuation” in the “Family Disaster Plan” section.) Everyone in your family should know where to go if they have to leave. Trying to make plans at the last minute can be upsetting and create confusion.
Discuss floods with your family. Everyone should know what to do in case all family members are not together. Discussing floods ahead of time helps reduce fear and anxiety and lets everyone know how to respond.
What to Tell Children
If you come upon flood waters, stop, turn around, and go another way. Climb to higher ground. If it is moving swiftly, even water six inches deep can knock you off your feet. Many people are swept away wading through flood waters, resulting in injury or death.
Stay away from flooded areas. Even if it seems safe, flood waters may still be rising.
Never try to walk, swim, drive, or play in flood water. You may not be able to see on the surface how fast flood water is moving or see holes and submerged debris.
If you are in a vehicle and become surrounded by water, if you can get out safely, do so immediately and move to higher ground. Vehicles can be swept away in two feet of water.
Watch out for snakes in areas that were flooded. Flood waters flush snakes from their homes.
Stay away from creek and stream banks in flooded and recently flooded areas. The soaked banks often become unstable due to heavy rainfall and can suddenly give way, tossing you into rapidly moving water.
Never play around high water, storm drains, ditches, ravines, or culverts. It is very easy to be swept away by fast moving water.
Throw away all food that has come into contact with flood waters. Contaminated flood water contains bacteria and germs. Eating foods exposed to flood waters can make you very sick.
How to Protect Your Property
Keep insurance policies, documents, and other valuables in a safe-deposit box. You may need quick, easy access to these documents. Keep them in a safe place less likely to be damaged during a flood.
Avoid building in a floodplain unless you elevate and reinforce your home. Some communities do not permit building in known floodplains. If there are no restrictions, and you are building in a floodplain, take precautions, making it less likely your home will be damaged during a flood.
Raise your furnace, water heater, and electric panel to higher floors or the attic if they are in areas of your home that may be flooded. Raising this equipment will prevent damage. An undamaged water heater may be your best source of fresh water after a flood.
Install check valves in building sewer traps to prevent flood water from backing up into the drains of your home. As a last resort, when floods threaten, use large corks or stoppers to plug showers, tubs, or basins.
Construct barriers such as levees, berms, and flood walls to stop flood water from entering the building. Permission to construct such barriers may be required by local building codes. Check local building codes and ordinances for safety requirements.
Seal walls in basements with waterproofing compounds to avoid seepage through cracks.
Consult with a construction professional for further information if these and other damage reduction measures can be taken. Check local building codes and ordinances for safety requirements.
Contact your local emergency management office for more information on mitigation options to further reduce potential flood damage. Your local emergency management office may be able to provide additional resources and information on ways to reduce potential damage.