Parts of NSW’s mid-north coast are experiencing a one-in-500-year flood, according to Natural Hazards Research Australia, with water levels expected to rise further as hundreds of millimetres of rain continue to pour.
But just four years ago the region experienced severe flooding, which the state’s premier at the time, Gladys Berejiklian, described as a one-in-100 year flood.
When a flood like this occurs, it doesn’t rule out another flood happening, and in some cases, floods can occur within a week of each other, Andrew King, an associate professor in climate science at The University of Melbourne, told SBS News.
He said defining floods in such a way can be confusing, but the figure is based on probability and not certainty.
What does a one-in-500-year flood mean?
Essentially, a one-in-500-year flood is one that’s extreme and highly unlikely, based on long-term data for the area.
“If you have a record of the past river flow conditions or river high conditions, you can make estimates then of what would be a really extreme event,” King said.
He said the data for many of the areas flooding this week only goes back around 100 years which may not be enough time to accurately predict the water-level variations and rainfall patterns of an area.
Another way to think about a one-in-500-year flood is that it has a one-in-500 probability of occurring, or 0.2 per cent likelihood, he said.
“The term is misleading in a way because I think people think when they hear one in 500 year event, we won’t have another one of these events for 500 years, but that’s not how it works at all because you can have two or three really extreme events in a very short space of time and then long periods without any extreme events,” King said.
He said after bad flooding, further flooding becomes more likely in the following weeks because the ground is already saturated.
On 28 February 2022, the NSW northern rivers city of Lismore recorded something between a one-in-100,000-year flood and a one-in-500-year flood level, as the river catchment rose to 14.4m.
But just four days later, the river height dropped to somewhere between a one-in-10-year flood and a one-in-100-year flood level.
These probabilities were based on the Lismore City Council’s definition of the highest expected water level — 16m above sea level — which is the “probable maximum flood” level for a one-in-100,000-year flood.
What factors can affect the probability of an extreme flood?
According to the Bureau of Meteorology (BoM), the chance of an above-average rain event increases when measuring over a larger area.
The BoM says above-average floods are caused by heavy rain, but factors like the duration of the rainfall and how dry the area is — not just the volume of rain — also impact flooding.
Roger Stone, professor emeritus in climate science at the University of Southern Queensland, said the flooding in NSW was foreseeable based on rain forecasts.
“It was predictable, at least in terms of much heightened risk,” he said.
“We should expect such instances at this stage of the decay of a weak La Niña event in the Pacific.
“The NSW coast forecast for total rain for April to June this year showed a 90 per cent chance of well above normal rain.”
Are severe flooding events occurring more often?
King said it’s possible we have been underestimating the risk of severe flooding because the long-term data only showed a particularly dry period. But he also noted it’s equally possible that multiple rare events are simply occurring by chance.
He said there’s evidence that shorter, more intense storms are occurring more frequently and causing more flash flooding.
King said in general we can expect to see changes in rainfall patterns that might make extreme flood events more common, “especially ones related to extreme rainfall that occurs on short timescales within hours”.
“We expect that to become a bit more common as the climate changes and the world warms in most parts of Australia, we’d expect to see an increase in those types of flood events for multi-day extreme rain events that cause large-scale river flooding,” he said.
Experiencing multiple severe floods
Some Taree residents have shared the “extremely stressful” impact of multiple floods through social media posts.
“Unfortunately, my parents’’ home, our home, has faced the brunt of this impact with the river swallowing up our street,” a resident wrote on Facebook.
“We’re yet to see what the real impact will be and the damage we are dealing with. It has been extremely stressful as this is the second time we’ve been hit, but the worst. It took years to finally get our place up to scratch from the last flood, only to have this happen again.”
Another wrote: “The flood of 2021 was just a training exercise for the current #midnorthcoastfloods, which have now broken every flood level record in history and have seen Martin Bridge [the big main bridge] under water for the first time. Currently, there is no accessible road in or out of Taree, with more torrential rain and wind to come, overnight and all day tomorrow.”
Many in the area are without power, and some who have lost their homes have nowhere to go.
“We lost mobile reception for 24 hours and currently have no power. We have access to parts of Taree, but that may change as more rain and water from further up the catchment continues to flow down on Thursday. We have everything we need to be isolated for more time,” another user wrote.
“This is a devastating flood for our community, with many houses and businesses going under for a second time since the last major flood in 2021, and places that have never been impacted by flood being hit this time.”
