Although it is widely believed that a 100-year flood will occur every one hundred years, the terminology adopted to define flood levels is really the root cause of this misunderstanding. To characterize flood levels and their chance of being equaled or exceeded each year, engineers and scientists use the term "estimated exceedance probability" to identify flood risk in areas susceptible to flooding.

The term “100-year” flood is used to describe a flood event that has a 1 in 100, or 1% chance (0.01 probability), of being equaled or exceeded each year.  Like the very fine print on a lottery ticket, this chance of flooding is also written as 1:100.

Hence, a 50-year flood (0.02 exceedance probability) has a 1:50, or 1 in 50, or 2% chance of occurring; a 200-year flood (0.005 exceedance probability) has a 1:200, or 1 in 200, or 0.5% chance of occurring; and a 500-year flood (0.002 exceedance probability) has a 1:500; or 1 in 500, or 0.2% chance of being equaled or exceeded each year.

The occurrence of a large flood does not mean that similar flooding won’t occur again in the near future.  If the necessary weather conditions are present (persistent or heavy rainfall, snowmelt, etc.), flooding may occur again.  Consider:

• The chance of a 100-year flood occurring is about the same as being injured in a car accident.
• During the span of a 30-year mortgage
  • A house in the 100-year floodplain is 27 times more likely to experience a flood than a residential fire.
  • There is better than a 1 in 4 chance that a home in the 100-year floodplain will be flooded.

With large floods occurring more often than previously estimated, it is important to be aware of your risk of flooding and to take appropriate action(s) to reduce future flood risk.  The Corps is working with state and local governments and the Federal Emergency Management Agency (FEMA) to develop the way forward.  It is too early to determine what impacts the results of this study will have on existing Corps flood-risk-management projects and reservoir operations, or on FEMA flood hazard mapping and levee accreditation.