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Modelling tsunami waves

Computer models developed by a Monash researcher are helping to unravel some of the mysteries about how devastating tsunamis are produced.

By Tim Thwaites

A Monash mathematician has developed computer models that can determine the areas of coastline most vulnerable to tsunamis.

Professor Joe Monaghan has applied a technique he used to study the formation of stars to generate computer simulations of the likely impact of tsunamis, such as the one which recently hit the northern coast of Papua New Guinea.

"We can use these models to identify which parts along a coast are in danger, and which areas are safe. We can now say: 'This town or village is at extreme risk.'"

Range of applications

The work has application to chemical engineering processes, such as the operation of furnaces, and to predicting the environmental impact of piping waste materials out to sea. "We are already talking to mining companies," Professor Monaghan said.

The trigger for Professor Monaghan's productive line of research actually occurred about 3000 years ago - the sudden destruction of the advanced Minoan civilisation on Crete, southeast of mainland Greece. One explanation for the disappearance of the Minoans is that Crete was hit by a massive tsunami set off by the volcanic explosion of the island of Santorini (Thera), about 100 kilometres to the north. After reading about this speculation, Professor Monaghan recognised that he could use his astrophysical modelling technique known as 'smooth particle hydrodynamics' to calculate how such a tsunami might behave when it hit the northern coast of Crete.

While plenty of work has been undertaken on how tsunamis move through the ocean, Professor Monaghan soon realised that little was known about how the giant waves were produced and what happened when they encountered a coastline. So he began to develop his computer models, using a series of simple experiments in a wave tank to provide basic information about how tsunamis were initiated and what form they took.

Tsunamis are generated by massive earth movements, such as rapid flows of ash from an erupting volcano into the sea, large undersea earthquakes, and huge avalanches from the flanks of islands involving many cubic kilometres of material. These events can produce waves, kilometres wide from front to back, which can travel thousands of kilometres across the deep ocean at speeds of more than 350 kilometres an hour. In the open ocean, the crests of these waves may be a few metres high, but when such a wave reaches shallow water, the mass of onrushing water can pile up into a crest higher than a three-storey building.

To simulate the initiation and impact of such an event, Professor Monaghan used a tank containing a coloured solution of dense saltwater. At its side was stationed another, smaller tank containing fluid of a different colour and of a different density. The water from the side tank was then allowed to flow down a ramp into the main tank, simulating the flow of ash down the side of a volcano into the sea. The whole experiment was videoed.

If the fluid in the side tank was less dense than that in the main tank, when the two met, the incoming fluid would tend to flow along the top, creating a surface current. If the fluid in the side tank was denser, then it plunged into the saltwater and created a more conventional tsunami-style wave. Professor Monaghan analysed such videos to produce descriptions of wave formation and movement for his models.

Collaborative research

It soon became clear that such work dovetailed with the interests of several researchers in Monash's Department of Earth Sciences, most notably Dr Ray Cas, who has been working on flows from a volcano on the island of Kos to the east of Santorini. And the kind of movement generated in the initiation of tsunamis has much in common with events such as the discharge of floodwater into the sea, and fluid flows in furnaces.

The upshot of the research has been the coming together at Monash of a multidisciplinary group of researchers interested in modelling the key features of large-scale earth movements, volcanoes, earthquakes, landslides, tsunamis and the like. They now work in a new laboratory known as Epsilon - the Earth Process Simulation Laboratory - which has been fitted out with wave tanks and several high-speed computers using money from the Special Monash University Research Fund and from the Faculty of Science.

Professor Monaghan's work has been used to direct field research on Crete looking for evidence of a tsunami 3000 years ago. So far such evidence has been hard to find, because the climate of Crete, with its severe winter storms, discourages the build-up of sediments where the information would be preserved. But just a few months ago, Professor Monaghan directed an international team which probed some surprisingly deep sediments discovered in a swamp on the north coast of Crete. And they were excited to find the first signs of ancient marine inundation.

Research into tsunamis and volcanoes are among the activities of the new Earth Process Simulation Laboratory. The laboratory is an inter-faculty facility dedicated to the integrated physical and numerical simulation of such catastrophic Earth processes. For further information about courses and projects, contact Professor Joe Monaghan (Mathematics and Statistics) or Professor Ray Cas (Earth Sciences) on (03) 9905 4431.