Why can flooding in northern river cities such as Lismore and Byron Bay occur in clusters?

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The residents of Lismore are currently experiencing their second major flood in a month.

On February 28, the devastating first flood peaked at 14.4 metres, more than two meters higher than the previous record of 12.27 meters in 1954, and well above the city’s 10-metre high embankment wall, built in 2005. Four people died, with 2,000 homes destroyed or unlivable of the 19,000 in the city.

Even as the residents of Lismore and Northern Rivers struggle to recover from the initial flood, the floods come again. On March 29, more heavy rain began to fall on the soggy watershed that fed the Wilsons River.

Once again, the traumatized community of Lismore had to evacuate, with flood forecasts of 10.6 meters. The flood reached only 9.7 meters. But then another 279mm of rain fell between 9 a.m. Tuesday and 6 a.m. Wednesday, March 30, with a flood forecast of 11 metres. Again residents had to evacuate.

Is this unprecedented? Although the height of the first flood is a new record, there have been successive floods before. Brisbane suffered three of the largest floods recorded in quick succession then, in 1893. Floods can come in clusters.

As the world warms, the atmosphere can hold more water. That means we are more likely to experience major floods. It’s time to consider moving flood-prone cities.

We have seen clusters of floods before

Lismore is no stranger to flooding. The town was built at the meeting point of Wilsons River and Leycester Creek. Lismore’s deep bowl shape has earned the town the nickname “The Wok”. As many as 30,000 people live on low-lying land at risk of flooding.

The last major flood in Lismore occurred in 2017 and reached a height of 11.59 meters. Just five years later, these devastating floods came. That’s an unusually small gap, and many locals were shocked by the small pause between major floods. Then the second struck, with almost no interruption between the floods.

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Residents of Lismore start cleaning up floods

Although unusual, clusters of flooding have occurred before. In a scenario eerily familiar to the residents of Lismore, Brisbane suffered three floods in a month. In addition, two of these were the largest ever.

On February 5, 1893, Brisbane experienced a flood of 8.35 meters. It was the second-highest recorded flood since 1841. Bridges, railroads, businesses and entire streets of houses were destroyed. Roads became canals as the city flooded. Houses and shops were covered in stinking mud. There was very little time to recover as the city was flooded again on February 11, albeit only up to 2.4 meters high.

Just four days later, as recovery began again, a major new flood swept through the city. Newspapers reported that damage from this 8.09-meter flood was much less than the first, as much of the potential damage had already been done. This was not entirely true, as many buildings had been significantly weakened from the first flood. The sodden land and roads were more prone to landslides and collapses.

These floods have killed 35 people and left hundreds of people homeless and unemployed. Successive floods took a psychological toll, with newspapers reporting people feeling despair and misery. Many in Brisbane in 1893 could identify with the feeling Lismore Mayor Steve Krieg recently described as “flood fatigue”.

Why does Australia suffer from flood clusters?

Australia has highly variable rainfall. This is due to the well-known El Niño-Southern Oscillation, a natural climate phenomenon that can make Australia drier or wetter. In El Niño years, rainfall is significantly reduced, making us more vulnerable to drought. But in La Niña years, wet weather sets in, making flooding more likely.

Both 1893 and 2022 are La Niña years. We see the result of wet summers saturating our watersheds and soils, making them less able to absorb heavy rainfall and more prone to flooding.

In La Niña years, major floods are more common in clusters with dry spells in between. We can see this clearly in the history of Lismore.

Between 1887 and 1893, the city experienced three major floods ranging from 10.43 to 12.46 meters.

Between 1962 and 1965, the city withstood three more floods of more than 10 meters.

And in 1967, Lismore flooded five times between March and June, with floods ranging from 5.09 to 10.27 meters.

A police car submerged in water on a street in Lismore
Flood clusters in Lismore are unusual, but they are not unprecedented: in 1967, Lismore flooded five times between March and June.PAA: Jason O’Brien

Although La Niña years often come in pairs, the Bureau of Meteorology has warned that it is possible to have three consecutive La Niña years, as we saw in 1954-57, 1973-1976, and 1998-2001. All of these caused flooding in Lismore. Fortunately, at this stage, it’s considered unlikely that our two years of La Niña will stretch to three.

So what can we learn?

As the world warms, floods are becoming increasingly difficult to predict. While dams and levees can reduce flood damage, they work best during minor and moderate flooding. In catastrophic events such as the February flood, there is little they can do.

While La Niña years and soggy watersheds make flooding more likely, they don’t guarantee flooding.

Scientists have repeatedly warned us that climate change can both dry out soils and intensify rainfall, depending on the area. That means that smaller floods may be less frequent, but floods that do occur are likely to be more frequent and intense.

As I write this, flood records are tumbling again in some parts of northern NSW. The floods at Ballina, Bellingen and Lennox Head are meters above previous heights.

While it may not lead to another catastrophic flood in Lismore, it won’t be the city’s last flood. In fact, the record may be surpassed. It’s time to talk about moving the city away from the floodplain.

Margaret Cook is a history professor at the University of the Sunshine Coast. This piece first appeared on The Conversation.

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