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Modelling tsunamis and protecting the coast

Dr Willem de Lange is a researcher and senior lecturer in the Department of Earth and Ocean Sciences at the University of Waikato. He studies many aspects of tsunamis and sits on the Tsunami Expert Panel.

Through his research, Willem aims to understand how tsunamis form from volcanic eruptions and in-water explosions. He is involved in the Firewaves Project, an international collaboration that aims to identify the range of tsunami-related hazards and predict the frequency and magnitude of tsunamis.

He is also interested in how coastal vegetation can minimise damage caused by waves, and he studies how different kinds of beach plants affect sand dunes.

Modelling tsunamis from volcanic eruptions

Volcanic eruptions can sometimes generate tsunamis, but scientists are still trying to understand exactly how this happens. Willem and his team develop models to predict which aspects of volcanic eruption are the cause of tsunamis.

Willem is particularly interested in how the steepness of the volcano sides affects the size of the tsunami that is generated. This research involves heating up material to 600°C and chucking it into the water. This creates an explosion and an ensuing tsunami.

For his research on volcano-induced tsunamis, Willem wanted to use explosives in a wave tank. Unfortunately for him, he was not allowed to use explosives in his research here in New Zealand for safety reasons. The research was eventually conducted by a graduate student of Willem’s in Indonesia – but was canned when the laboratory roof blew off!

During the 1940s, the idea of creating a tsunami bomb from explosions was tested in top-secret experiments in New Zealand and also further afield at Bikini Atoll (using nuclear explosions), but it was found that the bigger explosion did not give bigger waves. There was a limit to the energy that could be imparted to the tsunami wave. In contrast, steep pyroclastic flow leads to big tsunami waves being generated. Willem’s research into this effect involved computer modelling.

Restoring coastal vegetation 

Another focus of Willem’s research is restoration of coastal areas with native vegetation. Coastal planting can protect against coastal erosion. Also, tsunamis do not penetrate as far inland when there is more coastal vegetation.

Coastal vegetation can also provide direct protection. In the Indonesian tsunami in October 2010, it was reported that some people were saved by climbing trees to get beyond the reach of the surge and waiting for the waves to pass.

The best thing for New Zealand coasts in terms of hazard would be to have coastal trees – the forests – restored, but restoring the dunes is much better than not having them there at all.

Dunes are often restored by planting marram grass or spinifex. Both of these plants are good at growing in sandy environments and can prevent erosion by holding the sand in place. They have important differences though – the type of vegetation affects the steepness of the beach, which, in turn, affects the types of waves at the coasts.

The type of vegetation also affects the shape of the beach – with marram grass, the sand blows inland and the dunes get higher, but with spinifex, the beach gets wider and increases the buffer between houses and the sea.

Restoring the dunes – a surprising outcome

Replanting coastal vegetation can have unexpected consequences. In the 1950s, the population of the shellfish toheroa dropped markedly in Northland (as well as elsewhere in New Zealand) because they were over-harvested. Even after harvesting was strictly controlled, toheroa numbers stayed low. Dunes at toheroa-containing beaches had been restored and planted with marram grass instead of spinifex – could this have been one reason why toheroa weren’t thriving?

Willem and his team did tests on Matakana Island, where marram and spinifex native dunes had been restored right next to each other (about 10m apart). Willem’s team found that the type of vegetation planted had an effect on the freshwater table in the dunes, which the toheroa depended on. The spinifex-planted dunes allowed a freer flow of fresh water than dunes planted with marram grass, so replanting the dunes with marram grass might have made it more difficult for toheroa to thrive.

This research really highlighted the importance of waves and dunes in ecology. Willem says, “It all sort of inter-relates, and it’s [about] trying to tease out the linkages between the ecology and the [coastal] processes.”

Nature of Science

In a good scientific experiment, as few variables as possible are tested at once. In Willem’s Matakana Island tests, his team could draw strong conclusions about the effect of planting on the water table because marram- and spinifex-planted dunes were in such close proximity. This eliminated the variables of climate, wind conditions and surf conditions.

Useful links

Read this New Zealand Herald article about New Zealand’s role in Project Seal, which investigated the possibility of developing a ‘tsunami bomb’ during World War II.
www.nzherald.co.nz/nz/news/article.cfm?c_id=1&objectid=14727external link

Read more about toheroa and how their harvest is controlled.
www.mahingakai.org.nz/resources/what-are-kai/external link

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