Instead, it refers to a phenomenon that has a 1 in 100 chance of occurring each year.
The probability of no 100-year flood happening in Paris over the next 100 years is 36.6%. The probability of at least one 100-year flood taking place over the next century is therefore 63.4%.
The Seine has exceeded 6 metres in height 36 times since the 17th century.
3 water level rises have taken the river over 8 metres and can be classed as 100-year floods: 1658, 1740 and 1910. The 19th century is therefore the century that saw the greatest number of floods, but not a 100-year flood!