̽��ֱ�� of Cambridge - avalanche

What makes a sand dune sing?

sc604 — Fri, 04 Nov 2016 08:50:28 +0000

For Marco Polo, the desert could be a spooky place, filled with evil spirits. Writing in the 13th century, he described the famous singing sands, which “at times fill the air with the sounds of all kinds of musical instruments, and also of drums and the clash of arms.” But the low, loud rumbles coming from the dunes were not the work of spirits. They were the work of physics.

As grains of sand slide down the side of certain dunes, they create vibrations that can be heard for miles around. ̽��ֱ��sand avalanches trigger the dune’s natural resonance, but only when conditions are just right. It can’t be too humid, and the grains of sand need to be just the right size and contain silica. Only then will an avalanche cause the dunes to start singing.

An avalanche, whether it’s made of sand or snow, is an example of a granular flow, when solid particles flow like liquids, colliding, bouncing around, interacting, separating and coming back together again. Granular flow processes can be found everywhere from the world’s highest mountains to your morning bowl of cereal.

Dr Nathalie Vriend, a Royal Society Dorothy Hodgkin Research Fellow in the Department of Applied Mathematics and Theoretical Physics, is a specialist in granular flows. Her PhD research at the California Institute of Technology unravelled some of the physics at work in the same singing sands that mystified Marco Polo. At Cambridge, her research focuses both on sand dunes and on avalanches, and how to quantify their behaviour, which can have practical applications in industries including pharmaceuticals, oil and gas. Vriend’s work relies as much upon laboratory experiments and fieldwork as it does on mathematical models.

“An avalanche can behave as a solid, liquid or gas, depending on various factors, which is what makes them so difficult to model mathematically,” says Vriend. “For me, modelling their behaviour starts with observation, which I then incorporate into a model – it’s nature where I get my inspiration from. That, and curiosity – I see something and I want to try to explain it.

“Since there are particles which collide and interact in a granular flow, there is a certain degree of randomness to the process, so how do you incorporate that into a model? You try to translate what you’re seeing into a physical description, and then you perform numerical or theoretical simulations to see if the behaviour you get from the models is the same as you observe in nature.”

Despite their somewhat chaotic nature, avalanches and other types of granular flows share some distinct patterns. Owing to a phenomenon known as segregation, larger particles tend to rise to the top in an avalanche, whereas smaller particles sink to the bottom, falling into the gaps between the larger particles. A similar phenomenon can be seen in your breakfast cereal: the smaller, tastier bits always seem to end up at the bottom of the bowl. Larger grains are also pushed to the side and the front, forcing the flow of the avalanche into channels.

Similar processes are at work in sand dunes. As wind blows across a dune, there is segregation of the individual grains of sand, as well as small avalanches taking place on the granular scale. But for a sand dune that is 40 m high, there are also processes taking place on the macro scale. ̽��ֱ��entire dune itself can move and race across the desert floor. Small dunes migrate faster than large dunes, as if playing a “catch me if you can game”.

A cross section of a typical sand dune would reveal a slope on one side of a ridge, and a sharp drop on the other. As the wind blows across the dune, it pushes grains of sand up the slope, where they gather in a heap. When the heap gets too big, it becomes unstable and tumbles over the other side, causing an avalanche, eventually coming to a stop. This process happens again and again, causing layers to form within the dune. “A sand dune may look like a monolithic mass of sand, but there are multiple layers and structures within it,” says Vriend.

How does this understanding of the anatomy and movement of a sand dune translate into practical applications? Understanding granular flows can be useful in the pharmaceutical industry, where two different active ingredients may need to be mixed properly before a pill is made. Granular flows are also highly relevant to the oil and gas exploration process, and with this in mind Vriend is working with Schlumberger, the oilfield services company.

Sand dunes are major sources of noise in seismic surveys for oil and gas in deserts, which are conducted to probe the location and size of underground oil and gas reserves. ̽��ֱ��surveys use an acoustic pulse from a source and carefully placed receivers at different points to listen to the signal that is received, which can then be used to calculate what is hidden underground. ̽��ֱ��problem encountered by surveyors is that the sand dunes are composed of loose sand and therefore have a much lower wave velocity than the rocky desert floor, and as a result they act as traps of wave energy: the energy keeps reverberating and creates a source of noise in the post-processing of the seismic surveys. As part of a secondment at Schlumberger, one of Vriend’s PhD students is performing numerical simulations to understand the origin and features of this noise.

Another industrial problem that Vriend’s group is currently working on is the phenomenon of ‘honking’ grain silos. As grains are let out of the bottom of a silo, the friction of the pellets on the walls of the silo makes a distinctive ‘honking’ sound. Annoying for the neighbours perhaps, but hardly dangerous. However, when the vibrations get loud enough, it can cause a resonance within the silo, leading to structural failure or collapse. Vriend’s students are attempting to understand what affects the way that silos honk, which could someday be used to minimise noise, or even to prevent collapse.

̽��ֱ��phenomenon behind honking silos on a busy farm is similar to that which causes massive desert sand dunes to sing, although one could be perceived as an annoyance while the other is considered captivating. For Vriend, however, it’s the real-world observations and the opportunity to spend time in nature that motivate her.

She explains: “What I love about my research, whether it’s looking at silos or avalanches, is that you can observe it, see it, feel it, touch it.”

Inset images: Avalanche Zinal, credit: dahu1. Slip face GPR, credit Matthew Arran.

When solids flow like liquids they can make sand dunes sing, and they can also result in a potentially deadly avalanche. Cambridge researchers are studying the physics behind both of these phenomena, which could have applications in industries such as pharmaceuticals, oil and gas.

A sand dune may look like a monolithic mass of sand, but there are multiple layers and structures within it

Nathalie Vriend

̽��ֱ��District

Sand dune

̽��ֱ��text in this work is licensed under a Creative Commons Attribution 4.0 International License. For image use please see separate credits above.

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Perspectives on the Nepal earthquake

jeh98 — Tue, 28 Apr 2015 14:50:40 +0000

As many agencies are now reporting, the death toll associated with the 7.9 magnitude earthquake that hit Nepal on Saturday is likely to rise considerably over the coming days and weeks. On Tuesday it stands at over 4,000 but the Nepalese Prime Minister, Sushil Koirala, announced that it might reach 10,000. ̽��ֱ��UN declared that 8 million people have been affected, with 1.4 million people urgently needing aid.

̽��ֱ��full scale of the damage will become clear as contact is made with remote settlements away from the capital, which are now largely cut off from communication and supply. In Kathmandu and other urban centres, the greatest cause of injury and death was collapsing buildings.

But in more isolated, mountainous regions, further problems arose from the shaking ground triggering a range of natural hazards. One such region is the Langtang Valley, 60km north of Kathmandu, where we have been doing research for the last two years.

A recent analysis shows the entire valley would have been particularly susceptible to landslides following the earthquake due to its proximity to the epicentre and the topography of the mountain slopes there.

We are exceptionally fortunate not to have been in the area when the earthquake struck. We were in Kathmandu for an International Glaciology Society Symposium in early March.

One of us (Ian Willis) stayed on to do glaciological fieldwork with two other scientists from Cambridge (Dr Hamish Pritchard and PhD student Mike McCarthy) towards the top of the Langtang Valley, returning very recently.

In fact Hamish Pritchard is still in Kathmandu, safe and now helping the UN effort.

̽��ֱ��other of us (Evan Miles) was due to fly to Nepal on Sunday and walk to the head of the Langtang Valley this week, but of course his trip was cancelled.

For the past two years, we have been working there with science colleagues from Switzerland, Netherlands and Nepal and aided by a professional Nepali team of guides, porters and cooks.

̽��ֱ��overall aim of the research project has been to better understand the climate of the region, and to investigate how the changing climate is affecting the glaciers and the discharge of water in the streams.

This is of huge societal importance, as the people of the valley rely on ground and stream water for their livelihoods – drinking, washing and irrigating crops.

In addition, a small hydro-electric plant was due to be built later this year at the uppermost village in the valley, Kyanjin Gompa, but this will presumably now be put on hold.

Our specific work focuses on improving knowledge about the glaciers of the region. And it is while undertaking our research that we have come to appreciate many of the natural hazards that occur in the area.

Many of the glaciers in Nepal and elsewhere across High Mountain Asia are covered by debris, which may inhibit the rate of ice melting underneath.

̽��ֱ��debris gets onto the glaciers through rockfalls, debris avalanches and mudflows. These are continuous processes, but would have been orders of magnitude more severe during the recent earthquake than anything we ever saw.

Many of the glaciers across the Himalaya and surrounding mountains are nourished by snow avalanches.

Again, these occur regularly (we have both been caught in snow avalanches sweeping down the glacier we work on) but the energy they contain is typically dissipated by the time they reach the valley bottoms.

As the recent footage from the Everest region shows, however, snow avalanches can be particularly large and devastating when triggered by an earthquake.

Video of a snow avalanche that swept down the Lirung Glacier on 20th March 2015. This one was harmless by the time it reached the village of Kyanjin Gompa. Bigger snow avalanches triggered by the earthquake would have been much more destructive. Video by Ian Willis.

Finally, many glaciers in the region are associated with lakes – these form on the glacier surface where they are dammed by ice, or in front of the glacier where they are blocked by moraines (large ridges of sediment ‘bulldozed’ by a formerly more extensive glacier).

̽��ֱ��rapid draining of such lakes provides another hazard, causing floods or mudflows to downstream regions. Again, the flooding and mudflows associated with lake dams rupturing is likely to have had a significant impact during the recent earthquake.

Our field research is on hold at present while we wait to hear the fate of the people of the Langtang Valley and other remote regions of Nepal. But initial reports from Langtang sound very bleak. Eye witness accounts state “From where we were, there was nothing you could see. All the villages were gone,” and “the whole valley has been destroyed”.

Helicopter-based photographs seem to confirm that Langtang village has been wiped out by a large landslide. We are busy scouring satellite data to identify zones of the worst impact, but Nepal has been shrouded in heavy clouds and rain since the earthquake inhibiting our efforts.

We are holding our breath awaiting a clear picture of Langtang Valley. We are hoping for the best but fearing the worst for the Nepali families that reside there.

DEC Nepal Earthquake Appeal

Inset images:

Looking downvalley to Langtang village in May 2014. Reports suggest that this entire village has been buried by a debris avalanche during the earthquake. Credit: Ian Willis.

Preliminary landslide susceptibility map created by Dr Tom Robinson ( ̽��ֱ�� of Canterbury). Susceptibility ranges from 0 to 1 with higher numbers indicating a greater chance of landslides occurring. Earthquake epicentre shown with a star. Langtang Valley is circled.

Evan Miles on the extremely debris-covered Lirung Glacier in 2014. Credit: Eduardo Soteras.

Lake on the surface of Lirung Glacier. ̽��ֱ��rapid drainage of such lakes may cause flooding downstream and may have contributed to devastating mudflows during the earthquake. Credit: Evan Miles.

Ian Willis with the owners of the Shangri-La Guest House, Langtang. L-R: Saylie, Tsering Dolma Lama, Karma, Ian, Nima, Samden Dindu. Photo taken May 2014. This family and many others are in our thoughts.

As the death toll continues to rise in Nepal, Senior Lecturer Dr Ian Willis, and PhD student Evan Miles, from the Scott Polar Research Institute contemplate the fate of people in a remote part of the country, where they have been doing research for the past two years.

We are holding our breath awaiting a clear picture of Langtang Valley. We are hoping for the best but fearing the worst for the Nepali families that reside there.

Ian Willis

Evan Miles. Homepage banner image credit: Bhuwan Maharjan

Lake on the surface of Lirung Glacier. ̽��ֱ��rapid drainage of such lakes may cause flooding downstream and may have contributed to devastating mudflows during the earthquake.

̽��ֱ��text in this work is licensed under a Creative Commons Attribution 4.0 International License. For image use please see separate credits above.

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̽��ֱ��mathematics of avalanches

ns480 — Sat, 01 Sep 2007 13:47:44 +0000

Dr Jim McElwaine, in the Department of Applied Mathematics and Theoretical Physics, is interested in developing such models. He has recently returned from a three-month trip to the Swiss Federal Institute for Snow and Avalanche Research in Davos, where he has been validating his models using real avalanche situations.

Historically, the disciplines of forecasting and hazard zoning have been used in an effort to reduce the deaths and damage caused by avalanches. Both rely heavily on the historical record and the wealth of data collected over hundreds of years.

Forecasting estimates the chance of an avalanche occurring on a particular day in a particular region and is carried out by collecting weather data and information about the snowpack, and comparing it with the frequency of avalanches on similar days in the past. As avalanches tend to fall on the same track year on year but with different sizes, hazard zoning estimates how far an avalanche will travel and what damage it will cause. In the most dangerous ‘red’ zones, which are highly vulnerable to avalanches, no building is permitted.

Although these methods have proved very successful in the Alps, there have been rare but disastrous occasions where they have gone wrong. In 1999, the worst avalanche winter in the Alps for 20 years, an ‘Alpine Tsunami’ travelling at over 100 mph entered the safe ‘green’ zone of the Austrian village of Galtür, killing 31 people. ‘These methods are becoming increasingly uncertain for the Alps as the climate changes and past statistics stop being useful,’ explains Dr McElwaine, ‘and, in countries like Turkey and Iran, where fewer data are available, bad accidents frequently devastate villages.’

Where data are scant, or where the past record can no longer be relied upon, predictive models are necessary. To do this, Dr McElwaine has designed and installed sensors in the Swiss test site Vallée de la Sionne. Powder snow avalanches are triggered by dropping explosive charges from a helicopter. ̽��ֱ��speed of the air in front of, and inside, the very largest avalanches are then measured and compared with mathematical models. ‘ ̽��ֱ��data from these experiments confirmed our model,’ says Dr McElwaine, ‘and showed how natural avalanches can indeed be related to laboratory experiments. ̽��ֱ��aim now is to use these models for designing defensive avalanche structures such as dams or snow sheds.’

This research was supported by the Royal Society. For more information, please contact Dr Jim McElwaine (jnm11@amtp.cam.ac.uk).

Each year more than a million avalanches fall worldwide, killing around a hundred people in the Alps alone. Can mathematical models be used to predict and prevent these disasters?

̽��ֱ��aim now is to use these models for designing defensive avalanche structures such as dams or snow sheds

Dr McElwaine

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