Oceans on the Rise

This article first appeared in the Winter 2015 edition of Fire Australia magazine. By Nathan Maddock.

Researchers are investigating where extreme water levels could impact Australia’s coast and what can be done to mitigate the risk.

In Fremantle Harbour last August a container ship broke away from its moorings and collided with the bridge on the Perth to Fremantle train line, closing the bridge for two weeks. Unknown to most casual observers, this part of the West Australian coast had been hit by a meteorological tsunami, or meteotsunami for short. The strong currents of the meteotsunami broke the moorings and the increased water height allowed the ship to travel over the sandbar separating the harbour from the Swan River.

Along Perth’s waterfront, meteotsunamis, in combination with high tides and storm surges, have led to flooding of the Swan River. June 2012 saw the highest water level recorded in 115 years, which resulted in the closing of the Kwinana Freeway.

“Most people do not know that a meteotsunami has occurred,” said Professor Charitha Pattiaratchi, an oceanographer at the University of Western Australia.

“They think it is just a high tide or a king tide. But it is the stacking up of the events that causes problems.

“The timing is really important. If a meteotsunami occurs at low tide there is negligible impact, whilst in contrast, if it happens close to the high tide level, it will lead to high water levels. This is what happened at the Perth waterfront in June 2012.”

Around 85% of Australia’s population live within 50 km of the coast, alongside a significant proportion of key assets and infrastructure. Yet these coastlines, and the communities that call them home, are exposed to a range of natural hazards that can cause severe damage. Research underway by the Bushfire and Natural Hazards CRC will improve our knowledge of such hazards.

Professor Pattiaratchi is leading the CRC study, with fellow UWA researchers Drs Sarath Wijeratne, Ivan Haigh, Matt Eliot, Ivica Janekovic and Yasha Hetzel rounding out the project team.

Professor Pattiaratchi said that extreme water levels can result from the combination of different physical processes including tides, storm surges, tsunamis (both seismic and meteorological) as well as seasonal and inter-annual mean sea level variations.

Extending our understanding of meteotsunamis and their risk around the country is a key focus of the study.

“Meteotsunamis are relatively common phenomena, but not well understood,” Professor Pattiaratchi said.

“We could consider Western Australia a global hotspot for meteotsunamis—25 were recorded in 2014.”

Other areas where they are particularly common are the Mediterranean, Japan, China and Korea.

“We have seen them often in Western Australia, maybe two or three a month in certain places,” Professor Pattiaratchi said.

But what is a meteotsunami and how does it differ from a tsunami caused by an earthquake?

“In Western Australia, and across most of Australia, meteotsunamis are caused by the passage of thunderstorms, or very fast travelling frontal (weather) systems,” Professor Pattiaratchi said.

These weather systems result in a change to the atmospheric pressure, and if the propagation of the pressure disturbance matches the speed generated by a wave, then a meteotsunami could occur.

“From an oceanography point of view, a one hectopascal change in atmospheric pressure is equal to a one centimetre change in water level.

“With a meteotsunami, we are talking about a two or three hectopascal change, creating a water level change of about 50 cm. The effects are 20 to 30 times higher than what you would expect naturally. This is due to a resonance condition and that is what makes the water level higher.”

But not every thunderstorm will cause a meteotsunami.

“If the passage of the storm or weather system is too slow, or too fast, then you will not have that resonance. It is not every thunderstorm. But it is more likely if a thunderstorm is travelling parallel to the coast where a range of water depths are experienced,” Professor Pattiaratchi said.

Many people would not be aware of meteotsunamis because compared to a seismic tsunami, often associated with very large, destructive waves, meteotsunamis do not necessarily cause loss of life.

“Meteotsunamis are not that large—in Australia the maximum height is around one metre,” noted Professor Pattiaratchi.

But that does not mean there is not a risk. When the tidal range is factored in, one metre can become very significant.

“We know Western Australia is at risk, but we want to understand what other areas of the country are at risk too. This study is about extending our knowledge around the entire coastline to gain a better understanding of their occurrence and, therefore, risk.”

Predicting extreme water levels

The project is seeking to document not only the risk posed by meteotsunamis, but also the risk associated with a combination of different processes such as tides and storm surges generated by tropical and extratropical cyclones. A computer model will enable the research team to map the coastline around the country, highlighting where extreme water levels could occur.

This will enable an accurate assessment of potential impacts on the coastline, leading to better emergency planning and management.

A key aspect of the research is not just about finding the areas that might have problems, but being able to tell when these extreme water levels could occur.

“We will analyse the identified problem areas to find out under what specific conditions problems occur,” Professor Pattiaratchi said.

“This will allow coastal engineers, emergency managers and planners to be better prepared, with accurate estimates of extreme water levels.”

This outcome is precisely what CRC end user Dr Martine Woolf believes is needed.

“We are an incredibly coastal nation, but there is not really a good handle on the likelihood of extreme water heights occurring,” said Dr Woolf, Section Leader for Hazards and Risk Application and Infrastructure at Geoscience Australia.

“With storm surges and meteotsunamis, we have a poor understanding of what the likely hazard is.”

Localised studies have been undertaken at various locations, but it is the national aspect of this project that appeals most to Dr Woolf.

“It is vital that the whole country is modelled using a consistent approach,” she said.

“Professor Pattiaratchi and his team are trying to come up with data on what is ultimately a very localised hazard, but in such a way that you can compare it across the country, from different types of events, from meteotsunamis to tropical cyclones, to extra-tropical east coast lows.

“It will be a unique national dataset and benchmark point across the nation, identifying where the issues are, even if they have not yet been experienced at that location.”

Ultimately, these outcomes will strengthen the resilience of coastal communities and infrastructure.