Study Aims to Better Understand Iceberg-Tsunami Dynamics

Iceberg tsunamis can be dramatic and violent events. A recent paper used large-scale experiments to better understand tsunamis generated by iceberg calving. The team of scientists set up a large tank and used heavy blocks to create waves under controlled conditions. The different iterations of the experiments revealed some of the differences that can be found when icebergs fall into water or rise to the surface in various ways. 

The findings were published at the 38th International Association for Hydro-Environmental Engineering and Research World Congress (IAHR 2019) in Panama City. The researchers sought to better understand the different features of iceberg-tsunamis that result when icebergs of different sizes calve. They aimed to expand their research by comparing the new findings to the features of tsunamis caused by landslides. The team hopes that their work will serve to create benchmark test cases that future research can benefit from.   

Lead author Valentin Heller, a professor of environmental fluid mechanics at the University of Nottingham, highlighted the work’s immediate and future impacts. “The research enables the efficient systematic prediction of iceberg-tsunamis for a wide range of calving mechanisms for the first time,” Heller told GlacierHub. “In the longer term, this is likely to impact the design of coastal infrastructure and disaster risk assessment in areas where iceberg-tsunamis occur.”

The process through which blocks of ice break off the terminus (end) or margins (sides) of glaciers, ice shelves, or ice sheets and fall into a body of water, typically an ocean, is called iceberg calving. Calving events range from rarer instances in which very large chunks of ice break off, like in the video above, to more frequent events with much smaller pieces of ice separating, like in the video below. Calving events can cause iceberg-tsunamis, examples of which can be seen in both videos.

Though glacier melt is increasing worldwide due to the climate emergency, Heller said an increase in ice loss will not automatically bring about an increase in number or strength of iceberg-tsunamis. This is because other melting mechanisms are playing a role as well. “Ice mass loss is primarily driven by two main components; (i) melting of ice and runoff in the form of water from the ice sheet surface and (ii) discharge through glaciers terminating in the sea in the form of iceberg calving.” He continued, saying that “an acceleration of ice mass loss through (ii) does not necessarily result in larger iceberg-tsunamis.” 

Iceberg-tsunamis are dangerous to coastal communities, tourists, and the fishing and shipping industries. Greenland has been the site of multiple significant iceberg-tsunamis; one tsunami at the Eqip Sermia glacier in 2013 produced waves so substantial a tourist boat landing was destroyed. The inhabitants of the village Innaarsuit, located in Greenland, were on high alert in 2018 when a 330-foot tall iceberg drifted into the waters near their homes, bringing with it the threat of flooding

The research team conducted 66 unique, large-scale experiments in a 50 by 50 meter basin with heavy blocks of up to 187 kilograms each with different variations of iceberg volume, geometry, kinematics, and initial position relative to the water surface. They looked at five iceberg calving mechanisms; capsizing, gravity-dominated fall, buoyancy-dominated fall, gravity-dominated overturning, and buoyancy-dominated overturning. The researchers wrote that “gravity-dominated icebergs essentially fall into the water body whereas buoyancy-dominated icebergs essentially rise to the water surface,” distinguishing the two categories. 

The pool used by the researchers during the experiments. In the image, a gravity-dominated experiment is being conducted. 
Source: Figure 2/ Large-scale experiments of tsunamis generated by iceberg calving

The researchers looked at nine parameters influencing iceberg-tsunamis that could impact wave heights and their decay. The parameters monitored were released energy, water depth, iceberg velocity, iceberg thickness, iceberg width, iceberg volume, iceberg density, water density, and gravitational acceleration. 

The data showed that tsunami heights caused by gravity-dominated fall and gravity-dominated overturning are approximately an order of magnitude larger than those generated by capsizing, buoyancy-dominated fall, and buoyancy-dominated overturning. In other words, icebergs that fall into the water from above are much more hazardous than icebergs released underwater. Heller told GlacierHub that the researchers were surprised about this large difference because it had not been quantified before.

Diving deeper into the researchers’ analysis reveals that the wave magnitudes generated by the gravity-dominated overturning mechanism created the largest tsunamis, the gravity-dominated fall mechanism created the second largest tsunamis, and the three other mechanisms had waves that were up to a factor of 27 smaller. In other words, the two processes that result from icebergs essentially falling into the water created much larger tsunamis than the mechanisms where icebergs rise to the water surface.

A further difference between the two largest wave producers and the three smaller is that for the gravity-dominated mechanisms the largest wave amplitude was observed earlier in the wave train. For the three processes that resulted in smaller waves, the largest wave amplitude was found in the middle of the wave train. 

The results of the study will be useful to both scientists and policy-makers. Heller told GlacierHub that the “results [will] help scientists looking into wave runup at shorelines and wave impact on infrastructures, such as coastal buildings, by providing the necessary offshore wave parameters to support their work.” He elaborated, saying that predicting the heights of iceberg-tsunamis “helps to make decisions on how close to a glacier front ships can safely navigate or if evacuations are necessary, as in the case of the village Innaarsuit on Greenland.” 

“Iceberg-tsunamis is a relatively new field of research and people are just starting to realize the significance of such waves for coastal infrastructure, tourists and coastal communities,” Heller said. As the body of research grows, we will have a better understanding of how iceberg-tsunamis function. Once more information is available, impacted communities will be better able to prepare for such events.

Not All Iceberg-Generated Tsunamis Are Alike. Here’s How They Differ

Many people are familiar with ocean tsunamis caused by earthquakes, such as the devastating Japan 2011 tsunami, but fewer know they can also be caused by iceberg calving. As glaciers and ice sheets undergo intensified melting, we can expect to see more frequent tsunamis triggered by icebergs dropping off the face of the world’s glaciers. These events threaten the lives of people in nearby coastal settlements, whether residents or tourists, and infrastructure as well.

In a recent study published in the journal Scientific Reports, lead researcher Valentin Heller and colleagues investigate the potential for five different calving mechanisms in producing tsunami waves. They knew that iceberg calving, also known as glacier calving, accounted for most of the mass loss from the Antarctic Ice Sheet and about a third for the Greenland Ice Sheet between 2009-2012. Their research could not only contribute to science but have practical effects. Identifying the impacts of  different calving scenarios is beneficial for implementing disaster management strategies and strengthening disaster resilience in coastal regions.

Scientists observed that iceberg calving events in polar regions interact differently with the surrounding waters through distinct calving mechanisms. They investigated five types of calving events: capsizing, gravity-dominated fall, buoyancy-dominated fall, gravity-dominated overturning, and buoyancy-dominated overturning.

To test the tsunami energy potential of each type of calving event, large-scale experiments were conducted in a 50 by 50 meter wave basin at Deltares in Delft, Netherlands. Sixty-six experiments were conducted , at depths of 1 or 0.75 meters. The researchers used PPH blocks, a thermoplastic material with similar density to ice, as a proxy for icebergs.

The researchers implemented various methods of control to simulate the five types of calving events. To represent capsizing, for example, the researchers  fed a wooden rod through the centers of the blocks in order to control the rotation. They simulated buoyancy-dominated fall by pulling the blocks underwater with rope and stabilizing them with a steel beam from above.

Falling and overturning icebergs, and sketches of calving mechanisms (Source: Heller et al.)

They then quantified the maximum heights and energies of the iceberg-tsunamis and found the relative energy releases of the iceberg calvings. They then  analyzed and compared the results with the predictive methods of landslide-tsunamis. By doing this, researchers aimed to transfer knowledge from a well-established research field to the relatively new field of iceberg-tsunamis.

The team found large differences in tsunami height between the mechanisms. The two gravity-dominated mechanisms were found to be better predicted by landslide-tsunami models than the others. These results are significant in understanding the relative impact and prediction capabilities of specific calving events, which is vital to disaster management. Yet the results will be of most use for cases of gravity-dominated calving events. More research will need to be done to better analyze the other calving mechanisms.

One thing not considered in the comparison was the movement of icebergs along coastal locations such as harbours. Researchers noted that even significantly smaller iceberg-tsunamis from capsizing can cause large destruction. They team also scrutinized the existing landslide-tsunami models for failing to capture the physics of the capsizing and buoyancy-driven mechanisms of A, C, and E, which are important iceberg events.

Calving at the Hubbard glacier in eastern Alaska (Source: Navin Rajagopalan/Flickr)

Lead author Valentin Heller, who’s an assistant professor of hydraulics at the University of Nottingham, said the experiments showed that icebergs falling into water were about 10 times larger than those breaking off underwater and moving to the surface, as well as capsizing icebergs. He said the researchers were surprised that this large difference has never been quantified before.

“The overall aim of the study is to be able to predict the tsunami magnitude in function of the size of the iceberg, its initial position relative to the water surface, and on how it interacts with the surrounding water,” Heller said. “This helps to predict the iceberg-tsunami height at any location in front of the glacier front to provide guidelines for tourist boats on how close they can safely approach a glacier front.”  

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