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.
Beach survey
Undertaking a beach profile survey to analyse changing sand volumes along the New Zealand coast. Surveys in the 1970s used a jet boat to echo-sound the offshore profile and an instrument onshore to measure the boat’s position. Willem is in the bow of the boat, monitoring the echo sounder.
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!
Explosion-induced tsunamis
Modelling tsunami formation after volcanic eruptions and explosions. This water-tank explosion experiment was set up to investigate how the eruption of Krakatau, Indonesia, in 1883 gave rise to tsunamis.
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.
Erosion of sand dunes
These sand dunes in the Bay of Plenty have eroded in response to north-easterly storms. The remnants of an ancient podocarp/broadleaf forest are being exposed at the base of the dunes.
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?
Marram grass and spinifex
Marram grass (top) and spinifex (bottom) are both used to restore coastal sand dunes. The two plants have different effects on the shape of the beach over time and the flow of water in the freshwater table in the dunes.
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.
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.
The level 3 Connected article Building for the future looks at how a scientist uses scientific modelling and is working with others to build a safer future for a Samoan village.
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.”
Related content
Explore other research in:
Studying storm surge and coastal hazards – NIWA scientist Dr Rob Bell studies the hazards of storm surge waves when they combine with large tides. Numerical modelling, measurements and historical accounts have led to the development of a simple red-alert tide warning system.
Predicting ocean waves – Dr Richard Gorman, a NIWA scientist, predicts wind-forced wave conditions in the ocean using computer modelling. These forecasts are very useful for being aware of hazards in the sea and at the coasts.
Activity ideas
Read the Connected article The tsunami that washed time away and try the learning activities in the teacher support materials.
Try this activity with your students, in which they use a shallow tray of water to demonstrate wave generation and behaviour.
This activity looks at change – natural and human induced. It’s ideal to support critical thinking and use of evidence and reasoning.
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.
Read more about toheroa and how their harvest is controlled.
