Queensland tsunami modelling shows how coastal communities will be impacted
Three hours — that's the minimum amount of time Queenslanders would have to prepare in the event a destructive tsunami was headed towards Australia's east coast.
Millions of people would need to be warned and evacuations along the coast completed before low lying communities were swamped.
By the time the first wave hit Sandy Cape on the northern tip of Fraser Island, it would only take another hour to reach the mainland, but the first wave might not be the biggest.
It's one of the worst-case scenarios that has been modelled by Queensland's Environment Department to make sure emergency services, local councils and other state agencies are equipped to respond.
Communities most at risk
Scientists have been working for seven years assessing what would happen in a one-in-750 year, 3,000-year or 10,000-year event — including the potential speed, height and inundation levels at various tides.
The modelling was based on tsunamis generated at three likely locations — the New Hebrides (Vanuatu region), South America and the Kermadec-Tonga region.
Computer-generated tests found tsunamis generated in the New Hebrides region would reach Queensland's coast in the shortest amount of time but waves generated in the Kermadec-Tonga zone would likely be the most extreme.
The arrival time of these waves could be anywhere between three to 17 hours.
In the event of a tsunami, ocean buoys off the coast managed by the Bureau of Meteorology and Geoscience Australia would send instant alerts to authorities.
Researchers found the Great Barrier Reef would provide some protection to the north coast by dissipating a tsunami as it travels through shallow shoals and that large islands would protect some areas.
But more exposed regions like the Gold Coast and the stretch of coast from Agnes Water to Bundaberg could see greater wave heights and inundation.
Department of Environment hydraulic engineer, Paul Boswood, said researches surprisingly found the most populated communities in and around Brisbane would largely be protected by Moreton and Stradbroke islands.
"We knew that swell waves don't really come into the bay because of the shallow shoals at the entrances," he said.
"We didn't know whether or not tsunamis would behave in a similar way but what we found was tsunamis lose a lot of energy as they come over those entrances ... they are about one fifth the size as they are on the ocean side."
"The lesson learned is that the impact within Moreton Bay is much less than might have been expected than not having done the study."
The long-running project was funded by the Commonwealth's Natural Disaster Mitigation Program initiated in the wake of the 2004 Indian Ocean tsunami and 2011 Japan disasters.
Mr Boswood said researchers conducted tests across 23 different scenarios.
"We picked scenarios that are very rare and unlikely to occur but would expect to produce a result that we could get some conclusions out of," he said.
"And we looked at different stages of tide because of course the tide plays a part as well. When you're looking at tsunamis of only 1 metre high and you've got tides of 2 metres, it has an impact.
"We used a static tide level at Mean Sea Level, Highest Astronomical Tide and we also introduced Sea Level Rise into it as well — so when you do all the permutations of those, we did about 23 runs."
Researchers found there was an amplified impact on coastal communities if sea levels were to rise with climate change.
The first four reports focussed on the Sunshine Coast, Hervey Bay and Brisbane regions, but researches hope to conduct further assessments into the Gold Coast region.
Spiro Spiliopoulos from Geoscience Australia said the three subduction zones used in the study were classified as the most likely to produce tsunamis that could threaten the Queensland coast.
"A subduction zone is where the plates on the surface of the Earth collide and one of the plates will then be thrust down into the mantle," he said.
"It's along these boundaries where you get the largest earthquakes and it's because of the friction.
"The New Hebrides zone and the Kermadec-Tonga zone are the two closest subduction zones to Queensland — they're very active zones.
"They produce large earthquakes but we don't have historical evidence of earthquakes above say 8.5 (magnitude) there so we then chose the South American subduction zone because even though it's quite some distance away ... we do have evidence of very, very large earthquakes there."
He said a one-in-10,000-year tsunami event could only be generated by an extremely large and rare earthquake.
"The largest earthquakes would have the longest return periods — so a one-in-10,000-year earthquake might be a magnitude 9," he said.
Regions across the state with a higher tsunami hazard risk in descending order:
- Gold Coast
- Ocean side of Bribie, Moreton and Stradbroke islands
- Sunshine Coast
- Fraser Island
- Flying Fish Point
- Capricorn coast
- Agnes Water
- Hervey Bay
Good and bad news for Brisbane and Moreton Bay
The hazard is significantly greater on the ocean side of the islands that protect Moreton Bay, producing a significant marine and land hazard with maximum currents up to 8 metres per second and waves of up to 10 metres.
Tsunamis generated from the New Hebrides have the shortest arrival time of just over four hours while waves from South America would arrive much later at more than 18 hours.
Once the leading wave reaches Cape Moreton, the additional time it takes to reach locations within the bay range between one and one-and-a-half hours and two-and-a-half hours to reach Indooroopilly within the Brisbane River.
Maximum water levels within the Brisbane River ranged between 0.4m and 0.6m with the waves slowing at about St Lucia.
There is a potential for shorter steep waves to travel on top of the underlying tsunami in areas where the tsunami wave steepened significantly (particularly close to the coast at Redcliffe and over the entrance sandbanks).
Sea level rise associated with climate change could exacerbate inundation in and around the bay in the event of a tsunami.
The good news is that infrastructure at the Port of Brisbane is situated at elevations that did not pose an inundation risk, according to the modelling.
The results showed there was also no significant risk to infrastructure at Brisbane Airport although there was potential for inundation of the north-eastern runway.
The modelling showed that ocean currents would also pose a marine hazard.
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