Ice avalanche enters the lab
Jan 4, 2012 3 comments
A new experiment in the UK has been created to simulate ice avalanches, a hazard that can follow the collapse of a glacier or the eruption of an ice-capped volcano. Despite the obvious risk these events pose to nearby people and communities, they are still a relatively unexplored geological phenomenon. Early results from the experiment suggest that melting at the surface of individual ice particles helps to explain how these flows can travel so fluidly down a slope. Ultimately, the research could lead to more accurate systems for predicting the onset and characteristics of ice avalanches in regions of the world prone to the hazard.
Current engineering models of ice flows tend to be based on data collected in specific geographic regions, where the approach is focused on creating a model that corresponds with field measurements. In Europe, for example, a lot of research has been based in alpine areas, particularly in Switzerland, France and Norway. These models provide town planners and other local authorities in these regions with information such as the likely extent and speed of ice flow in areas susceptible to avalanches.
But the limitation with this approach is that the models do not necessarily apply to other parts of the world, where defences against avalanches are often less developed. "The current need in Europe is to help predict these types of avalanche in the Carpathian mountain range across Eastern Europe and there are limited data available in these regions," says Barbara Turnbull, a researcher at the University of Nottingham in the UK. "Furthermore, aspects of the underlying physics of these very complex flows are not understood, leading in some cases to flow behaviour that cannot be explained with traditional theories."
Avalanche in a drum
Turnbull has sought to develop a more generalized approach to studying ice flow in avalanches. She has designed an experiment to examine the behaviour of flowing ice particles within controlled conditions in a laboratory at the University of Cambridge in the UK. The set-up consists of a rotating drum filled with ice spheres, which she monitors using high-speed video in order to examine the interactions between individual spheres of ice.
In a series of tests, ice spheres with diameters of roughly 5 mm were created by dripping water droplets slowly into a bath of liquid nitrogen. These spheres were then transferred to a Perspex drum, which has a diameter of 350 mm and a width of 20 mm, and the drum was rotated at a rate of 3.75 s per revolution. Over the course of 45 min, the drum was filmed every 2 min at a rate of 250–500 frames per second. Turnbull then repeated the experiment at –4 °C, –2 °C, –1 °C, and 0 °C.
At all four temperatures, Turnbull discovered that melting at the interface of particles significantly increased the overall speed of the ice flow, and this process subsequently led to more melting and faster flow. Reporting her findings in Physical Review Letters, Turnbull believes that the feedback system created by these surface interactions can in part explain the large distances and speeds that ice avalanches can reach.
"This work will allow the development of a new type of avalanche model based on the key physical processes," Turnbull told physicsworld.com. "Most people are focused on developing a model that works – a model that can be validated against field measurements," she says. Turnbull believes that her approach is more fundamental, and for that reason it could lead to a more generalized understanding of ice avalanches that is not tied to particular areas of the world.
Importance of experiment
Demian Schneider, a glacial-hazards researcher at the University of Zurich in Switzerland, is impressed by the experimental approach taken by Turnbull. "The basis of preventing future avalanche disasters is on the one hand practical knowledge including empirical data. And on the other hand, a high level of understanding of the physical process, which can be achieved by experiment."
Schneider does note, however, that the controlled experimental approach taken by Turnbull has both pluses and minuses when compared with field studies. One advantage he cites is that because Turnbull's laboratory avalanche is effectively endless, it provides the researcher with time to study changes in flow behaviour, including the melting of ice and changes in grain size as particles fragment. On the downside, however, Schneider says that the rotating drum is very different from a real-world avalanche, not least because the ice flows over a perfectly curved bed, which would not exist in nature.
In the short term, Turnbull intends to carry out further testing at the Cambridge laboratory. But within the next five years she hopes to begin calibrating her findings against measurements in the field by developing a network of observation stations in areas prone to ice avalanches, including Caucasus, Switzerland, Norway and Canada.
Turnbull acknowledges that from a global perspective the current risk posed by avalanches is low compared with that of other geophysical flows such as volcanic eruptions. But she believes that the risk is changing quickly as climatic patterns appear to be altering and certain areas may be particularly affected. "Ice-capped volcanoes close to habitation, such as Mount Rainier [US] and Cotopaxi [Ecuador] and lower-altitude permafrost areas are the key locations of vulnerability," she says.
A developing hazard
An increasing risk posed by avalanches is also predicted by Stephan Herminghaus, a researcher of complex flow at the Max Planck Institute for Dynamics and Self-Organization in Germany. "I think it is sufficiently clear from the IPCC [The Intergovernmental Panel on Climate Change] reports that there are many places on Earth that will be subject to increased avalanche hazards," he says. Herminghaus predicts that all mountainous regions where soil and rock has been stabilized in part by permafrost conditions could soon change.
Christian Huggel, a researcher at the University of Zurich who specializes in high-mountain and glacial hazards, goes further and suggests that certain regions are already experiencing an increased risk. "We have relatively robust evidence of an increase in high-mountain rock fall and avalanches in the European Alps over the past decades," he says. Huggel stresses, however, that it is not yet clear whether we can say we are experiencing a global increase.
In addition to its environmental application, this new research could also lead to an improved understanding of ice flow in other areas of science. Turnbull suggests that astronomers could use the experiment to build more detailed histories of fragments retrieved from comets, which are essentially a mix of ice and rock. She says it could also inform industry, for example helping to improve the efficiency of production lines where frozen food is transported in close contact around a factory.
About the author
James Dacey is a reporter for physicsworld.com