Research maps real risk of major tsunami in Sydney

A tsunami triggered by a major earthquake in New Zealand's south could reach Sydney in two hours and cause harbour currents to reach speeds up to eight metres a second, according to new modelling to be published on Tuesday.

Any tsunami that reaches Sydney is unlikely to have the walls of water seen in Hollywood films, but a large once-in-every 5000 year event would cause whirlpools in the Spit, flood Manly's Corso, damage jetties, boats and beaches, and trigger dangerous and powerful currents across the harbour, a new paper to be published in Nature's Scientific Reports on Tuesday concludes.

For the first time, coastal researchers from the University of Newcastle and the Bureau of Meteorology have modelled the effects of tsunami inundation in Sydney, using 3D mapping to show what would happen from the heads to inland at Parramatta.

Clontarf in 1960 after a tsunami triggered in Chile.

University of Newcastle coastal geoscientist, Dr Hannah Power, said a Sydneysider would probably experience a tsunami in their lifetime. Since 1805, about 55 tsunamis have been detected in Australia. Of those with a known source, most started in Chile, New Zealand, Indonesia and the Solomon Islands, the Australian Bureau of Meteorology says.

But given Australia's distance from tectonic plates that cause many tsunamis, such as those in New Zealand and Chile, the impact on Sydney would most likely be small compared to other, more vulnerable places around the world.

Clontarf in 2006 during normal sediment conditions.

A major tsunami reaching Sydney would very likely result in water rising tens of centimetres in tens of minutes with very long periods - even 20 minutes - between two crests of waves. "It would be very strange," Dr Power said.

The water level could likely rise and fall for as long as 24 hours.

Sydney current speeds with largest modelled event. Credit: University of Newcastle

The 1960 Chilean tsunami - caused by a magnitude-9.2 earthquake - damaged boats in harbour on NSW's coast from Evans Head, Newcastle, Sydney and Eden. The Fort Denison tide gauge in Sydney Harbour recorded wave heights up to 84cm.

The 2004 Indian Ocean tsunami caused flooding in Western Australia and more than 30 swimmers had to be rescued. Boats were damaged in marinas from WA to as far away as Tasmania.

Very often the Australian public didn't take tsunami warnings seriously, and flocked to the beaches to see the large waves rather than recognising even a minor event could make swimming, boating and visiting the beach dangerous.

The Hobart Mercury reported "chaos" in Sydney in 1960, caused by the Chilean tsunami.

NSW had been affected by serious tsunamis in the past, and the tsunami risk needed to be taken seriously, the lead author and PhD researcher Kaya Wilson said.

“When you broach the idea of Australia being susceptible to tsunami, you’re usually met with a
dichotomy – either total disbelief that we’re at any risk, or panic as to what the threat means to
the individual,” Mr Wilson said.

According to the American science institute, the National Oceanic and Atmospheric Administration, 73 per cent of tsunamis are generated by earthquakes. The remaining 27 per cent are triggered by underwater volcanos, landslides, and a combination of events including those with unknown causes.

A big tsunami - the kind that would cause whirlpools and significant coastal flooding, for instance – is forecast to occur at worst on average once in every 5000 years. Events the same size as the 1960 Chilean tsunami were more likely, on average about every 50 to 100 years. These smaller events would result in waters levels rising up and down and rapid current speeds.