Flooding

Cost


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 - 2004. During this same period, an annual average of 93 people a year lost their lives due to flooding.


Flooding

Occurrence


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 due to 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:
  • Degree of vegetation and/or cleared/impervious surfaces
  • Precipitation
  • Recent soil moisture condition
  • River basin topography
  • Weather patterns

Categories of Flooding

Flash

 
Flash floods can be caused by low 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

 
General flooding is typically a long term event that may last for several days. General flooding can be further divided into three categories.

Riverine

 
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

 
Coastal flooding is typically a result of storm surge, wind driven waves, and heavy rainfall produced by hurricanes or other coastal storms.

Urban

 
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.

High Risk Areas

 
A high risk area is a special flood hazard areas (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 a 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 a letter D on flood maps. 

Periodic Flooding

Occurrence

 
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

 
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 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. 

Flood Safety

  1. Before
  2. During
  3. After
  4. Flood Insurance

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!

Additional Resources

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):