Flooding is a coast-to-coast threat in the United States and its territories in all months of the year. It is the most frequent and costly hazard with approximately 75% of all presidential disaster declarations resulting from natural events where flooding was the major component.
In most years it causes more damage in the United States than any other severe weather-related event with an average of $5.3 billion spent a year from 1975 to 2004. During this same period, an annual average of 93 people a year lost their lives due to flooding.
Flooding typically occurs when prolonged rain falls over several days, when intense rain falls over a short period of time or when an ice or debris jam causes a river or stream to overflow onto the surrounding area.
The most common cause of flooding is rain and/or snowmelt that accumulates faster than soils can absorb it or rivers can carry it away. Flooding can also be caused by the failures of water control structures such as dams and levees.
The severity of flood events is typically determined by a combination of:
Categories of Flooding
Flash floods can be caused by slow-moving thunderstorms and dam or levee failures. These types of events are rapid and usually occur within six hours of the immediate cause and result in a very rapid rise of water over low-lying areas.
Steep, hilly, or mountainous terrain are at greater risk of flash flooding as are urban areas due to the large expanses of concrete and asphalt surface that do not allow for adequate water absorption.
General flooding is typically a long-term event that may last for several days. General flooding can be further divided into three categories.
Riverine flooding occurs when river levels rise and overflow their banks or the edges of their main channels. River flooding can be caused by heavy rainfall, dam failures, rapid snowmelt, and ice jams.
Coastal flooding is typically a result of storm surges, wind-driven waves, and heavy rainfall produced by hurricanes or other coastal storms.
Urban flooding results because urbanization increases the magnitude and frequency of floods by increasing impermeable surfaces, increasing the speed of drainage collection, reducing the carrying capacity of the land, and occasionally overwhelming sewer systems.
Flood Prone Risk
The National Flood Insurance Program has developed a mapping system to identify flood-prone areas in an effort to help communities understand their risk. Three main categories of risk areas are defined.
A high-risk area is a special flood hazard area (SFHA) where there is a 1 in 4 chance of flooding during a 30-year period. They are typically indicated on flood map zones with the letter A or V.
Moderate to Low-Risk Areas
A moderate to low-risk area is a non-special flood hazard area (NSFA) where the risk of flooding may be low but cannot be completely removed. They will typically be identified on flood map zones with the letter B, C, or X (may or may not be shaded).
Undetermined Risk Areas
Undetermined risk areas are areas where no flood hazard analysis has been conducted but a flood risk may still exist. These areas will be labeled with the letter D on flood maps.
Periodic flooding of lands adjacent to non-tidal rivers or streams (areas known as a floodplain) is a natural and inevitable occurrence and can be expected to take place within regular recurrence intervals. The recurrence interval of a flood is defined as the average time, in years, expected between a flood event of a particular magnitude and an equal or larger flooding.
Floodplains are defined by the frequency of a flood that is large enough to cover it. For example, a reference to a 100-year floodplain means that a 100-year flood will cover that specified area.
To help better define the floodplain the National Flood Insurance Program (NFIP) developed a common standard of baseline probability. The "base" flood is the 1% (1 out of 100) annual chance of a flood occurring during any given year. The base flood is thus referred to, informally, as the 100-year flood.
Often misunderstood, the term 100-year flood does not mean that a flood will only occur once in a 100-year span but instead means that on any given year an area has a 1% chance of a flood occurring. Thus an area could see a 100-year flood twice in the same year, two years in a row, or not at all over a 200-year span.
Before a Flood
You should know that anywhere it rains, it can flood. Floods can even be in areas with a low risk of flooding. Just because you haven't had a flood in the past doesn't mean you won't in the future. Flood risk is based on a lot of factors including rainfall, landscape, flood-control measures, river-flow and tidal-surge data, flood history and changes due to new construction and development.
To prepare for a flood:
- Build an emergency kit.
- Make a family communications plan.
- Do not build in a floodplain unless you raise it up and support your home.
- Raise up the furnace, water heater and electric panel in your home if you live in a high flood risk area.
- Think about putting in "check valves" to prevent flood water from backing up into the drains of your home.
- If you can, build barriers to stop floodwater from entering the building. Seal walls in basements with waterproofing compounds.
Know the terms:
- Flood watch – rainfall is heavy enough to cause rivers to overflow their banks. Flooding is possible.
- Flood warning – flooding is occurring or very likely to happen in an affected river, lake or tidewater area. If told to leave, do so immediately.
- Flash flood watch – flash flooding in specified areas is possible. Be alert! You may need to take quick action.
- Flash flood warning – flash flooding is occurring or is likely to happen along certain streams and select areas. Get to a safe place immediately!
During a Flood
If a flood is likely in your area:
- Listen to the radio or television to learn what to do.
- Know that flash flooding can occur. If there is any possibility of a flash flood, move quickly to higher ground. Do not wait to be told to move.
- Know that streams, drainage channels, canyons and other areas can flood quickly. Flash floods can happen in these areas with or without typical warnings.
If you must leave:
- Secure your home. If you have time, bring in outdoor furniture. Move important items to an upper floor.
- Turn off water, gas and power at the main switches or valves if told to do so. Disconnect electrical appliances. Do not touch electrical equipment if you are wet or standing in water.
- Do not walk through moving water. Six inches of moving water can make you fall. If you have to walk in water, walk where the water is not moving. Use a stick to check the firmness of the ground in front of you.
- Do not drive into flooded areas. If floodwaters rise around your car, leave the car and move to higher ground if you can do so safely. You and the vehicle can be swept away quickly.
- Do not camp or park your vehicle along streams, rivers or creeks, particularly during threatening weather.
After The Flood
- Return home only when authorities say it is safe.
- Be aware of areas where floodwaters have receded and watch out for debris. Floodwaters often erode roads and walkways.
- Do not attempt to drive through areas that are still flooded.
- Stay away from floodwaters. Water may be dirty with oil, gasoline or raw sewage.
- Fixed broken septic tanks, cesspools, pits and leaching systems as soon as you can. Damaged sewer systems are serious health problems.
- Listen for news reports to learn whether the community’s water supply is safe to drink.
- Clean and cleanse everything that got wet. Mud left from floodwaters can contain sewage and chemicals.
- Avoid standing water as it may be electrically charged from underground or downed power lines.
- Photograph damage to your property for insurance purposes.
- Check references if you hire cleanup or repair contractors. Be sure they are trained to do the job. Be wary of people who drive through neighborhoods offering help in cleaning up or fixing your home.
Be aware that most insurance policies do not include flood damage. Think about buying a separate flood insurance policy. Go to www.ncfloodmaps.com to find out if you are in a flood zone. Or contact the Onslow County Floodplain Manager at:
Flood losses are not normally covered under renter and homeowner’s insurance policies. You can get separate flood insurance in most areas through insurance agents. Policies are available whether the building is in or out of a known flood-prone area. There is a 30-day waiting period before flood insurance can begin.
FEMA manages the National Flood Insurance Program (NFIP). This program makes federally-backed flood insurance available in areas that agree to adopt and apply floodplain management rules to lower future flood damage.
What you can do:
- Find out if your home or business is at risk for flood. Learn about what a flood can do to you and your family.
- Talk to your insurance provider about your policy and decide if you need more coverage.
- Flood insurance is available in most areas through insurance agents.
- Contact the NFIP. It can help give a way for property owners to financially protect themselves if more coverage is needed. The NFIP offers flood insurance to homeowners, renters and business owners if their area joins the NFIP. To find out more about the NFIP visit www.FloodSmart.gov.
Want to learn more about Flooding and how to protect yourself and your home? Visit these sites:
Want to learn more about the National Flood Insurance Program and Floodplain protection measures? Check out these FEMA/NFIP brochures (Note: Some of these are LARGE documents best opened on a computer):