People who live on the eastern coasts of Australia are experiencing an uncommon meteorological phenomenon. In some areas, record-breaking rainfall, as well as extremely long-lasting and heavy rain in other regions, caused significant flooding.
In various places in the world, this is described as one in 30 or fifty as well as a one per 100 incident. What exactly is this referring to?
What is a one in 100 years?
First, let’s dispel the common misconception regarding what a one-in-a-million 100-year event is. It doesn’t mean that the event will only occur once every 100 years or that it will never occur again for the next 100 years.
For meteorologists who study meteorology, the one-in-100-year event is one in the size of which it can be greater or equal typically every 100 years. That means over 1,000 years, to see that the one in 100 year event to be equal or exceeded 10 times. However, a few of those ten occasions could occur within a couple of years of one another, and none will be repeated for a long period of time.
Read more: Explainer: was the Sydney storm ‘once-in-a-century’?
Ideally, we would avoid using the phrase “one in 100 year event” because of this common misunderstanding, but the term is so widespread now it is hard to change. Another way to think about what a one in 100 year event means is that there is a 1% chance of an event of at least that size in any given year. (This is known as an “annual exceedance probability”.)
What is the probability of one in 100 years instances?
Many people are shocked by the fact that one in 100 things happen more often than one would think. While a probability of 1% could seem extremely unusual and unlikely it ,actually happens to be more frequent than you believe. There are two main reasons for this.
For a specific place (such like where you live) the one in 100 year event is typically occurring once every 100 years. But, all across Australia it is expected for to see the one-in-100 year event to occur in someplace much more frequently than every 100 years!
Many residents in NSW are being evacuated from their homes because of the flooding. Jason O’Brien / AAP
Similar to, you might have a one-in-a million chance to win the lottery, however the probability of winning will win the lottery is definitely greater.
A second, while a single hundred-year flood may have a 1 percent chance of occurring in any particular year (hence it’s known as a “1% flood”) “1% flood”) However, the likelihood is greater when you consider longer periods. If, for instance, your house is built to be able to withstand a flood of 1, that means over 70 years, there’s about a 50% chance that the home could be inundated at some point in this period! It’s not the most favorable odds.
What do we know about the frequency of floods?
Events like these 1% annual events with a probability of exceeding are usually called “flood planning levels” or “design events” because they are frequently used in various urban engineering design and planning applications. However, this assumes that we can determine what exactly the 1% event actually is that sounds more straightforward than what it actually is.
In the beginning, We used historical data to calculate the 100-year event. However, Australia is home to only 100 years of reliable weather data and records that are even less of the flow of rivers in a majority of places. We are confident that this record of 100 years does not include the biggest possible event that could occur in terms of drought, rainfall, flood, and so on. We have evidence from indirect paleoclimate data that point to larger-scale historical events.
Read more: Sydney storm: are extreme rains and flash floods increasing?
So, a 1% event is by no means a “worst case” scenario, and some of the evidence from paleoclimate datasuggests the climate has been very different in the deep past.
Second, estimating the single in 100 years by using historical data assumes that the underlying conditions don’t change. However, in many regions of the world, we are aware that the streamflow and rainfall are changing, increasing the risk of flooding.