In the United States, maps of floodplains often indicate the 100-year floodplain. This is used by the insurance industry to assess risk and by city planners to determine where building should and should not be located.
A 100-year flood is often mistakenly thought to be a flood that happens once every 100 years. In this activity, students explore that this is not the case. A 100-year flood is a flood that has a probability of 1 in 100 (1%) of occurring in any given year. With a 1% probability each year, it is possible for floods to happen two years in a row.