Why are there so many 100-year floods?

It seems every time it rains heavily, a journalist (selling news clicks) loves to use this headline. But it's misleading. And I'll tell you why. Firstly, the storm "size" is expressed as a probability of the rainfall exceeding: - a certain amount

Why are there so many 1-in-100 Year Floods ?



Factors that Determine a 1-in-100-Year Flood (some of them)


Diagram 1 - Issue 16


  • How the rainfall changes during the storm(storm shape and intensity) 

  • Size of water catchment 

  • Shape of the land(steep or hollow) 

  • Position of your house in the catchment

  • Condition of stormwater pipes and PRFs

  • Land coverage(trees, buildings, highways) 


With so many independent factors, each square metre of your city has a unique 1-in-100-year flood flow - which means in any significant rain event, there is a good chance somewhere in your city experiences a 1-100 year flood or even a 1-1000 year flood! 


For example, you could make a grid of about 100 points in the Sydney area. The 1-hour, 1% AEP (100 year) value has 1% chance of occurring at each of these points in a particular year, which means that there is a good chance (63% assuming full independence of points) of a 1- hour, 1% AEP event occurring somewhere in the general Sydney area in each calendar year. 



Climate modeling that predicts storm frequency and intensity are likely to increase, but be careful not to let journalists trick you into believing your house will flood every second year.

Ian Warren