What does the term '100-year flood' actually mean?
Scientifically, the term 100-year flood is a misinterpretation of terminology
A motorist drives a car through a flooded expressway in Brooklyn, New York early on September 2, 2021
(Ed Jones / AFP / Getty Images)
Many of us have heard the term "100-year flood" when describing the frequency of significant flooding in a given area.
But if we experience a 100-year flood, does that mean it won't happen again for a century?
Not exactly.
Scientifically, the term '100-year flood' is a misinterpretation of terminology that leads to a misconception, according to the United States Geological Survey.
Forecasters measure the size of flooding by using the stage or height above a reference point, and the most common term we hear is "above flood stage." The flood stage is the level at which water will begin to spill out of the banks of a river or other body of water.
Suppose you hear a meteorologist say a local river crested at 25 feet, which resulted in a 100-year flood. In that case, that means the river reached a peak height of 25 feet and caused significant flooding that occurs statistically once every 100 years.
Or there's a 1 percent chance of the same event or more significant happening in any given year.
A 100-year flood doesn't occur frequently. It's rare. According to the Federal Emergency Management Agency, most flooding consists of lesser frequency events such as 1, 5 or 10-year floods.
Take a look at the map above.
That data shows the areas that received historic rainfall amounts in September from the remnants of what was Hurricane Ida.
The areas that are shaded in yellow experienced a 10-year event. That means there is a 10 percent chance those areas could experience the same amount of historic rainfall during any given year. The dark green areas experienced a more than 1000-year event. There's a 0.1 percent chance those areas could receive that amount of rain in such a short period of time in a year.
So whether is rainfall amounts or flooding, just remember that hearing about a "100-year" event doesn't mean it only happens once every 100 years. It's based on the probability that the event will be equaled or exceeded in any given year.