It is wrong to think that a “100-year flood” happens only once every 100 years. The phrase “100-year flood” describes the estimated probability of a flood event happening in any given year.
A 100-year event has a 1 percent chance (or 1-in-100) of occurring in any given year.
While not likely, 200-year flooding events can occur within a month of each other.
Scientists collect data on how frequently different sizes of floods occur and the time between these floods.
They use the data to calculate the probability that a flood of a particular size will be equaled or exceeded during any year.
The term “100-year flood” is a statistical designation of an unlikely event.
Statistically, a 100-year flood has roughly a 63 percent chance of occurring in any 100-year period, not a 100 percent chance of occurring.
Climate can be defined as the average state of the atmosphere for a given place over a specified interval of time.
Extreme events, such as 100-year events, are part of a region’s climate. These extreme values are important for assessing the risk of unusual events and are used in determining flood insurance rates.
Recently, the front range of Colorado experienced a flooding event. Flooding is part of this region’s climate.
For example, Boulder, Colo., lies outside a canyon where rapid runoff from precipitation in the mountains can lead to flash flooding.
A flash flood is a sudden local flood characterized by a great volume of water and a short duration.
The area had major floods in 1919 and 1995. In 2013, between September 10-15, 17 inches of rain fell over Boulder causing major flooding that destroyed many bridges in the area. Many people had to be airlifted to safety.