1-in-100-year return period flood 

A large flood that has a one per cent chance of occurring in any given year. It can also be called a 1% flood or a 100-year flood, and is often written as 1:100 year flood. Although called a “1 in 100 year flood” there will not necessarily be one every 100 years. It is even possible to have more than one 1 in 100 year flood in the same year. “”