Formation of levees, troughs and elevated channels by avalanches on erodible slopes

Significance 

Comprehensive measurements of snow avalanches have indicated that the principle mechanism for entrainment and growth of an avalanche is by frontal ploughing into a layer of fresh snow. Amount of available materials as well as the structure of the snow pack are the primary factors that have been identified to affect avalanche flow dynamics. Entrainment has been found to occur at, or very close to the flow front, and basal erosion by the flowing body being significantly less.

The entrainment process is responsible for avalanche build up, therefore becoming more destructive as compared to the initial release. Snow may be deposited at the rear, base and sides, in a similar fashion as the eroding materials. Therefore, there is a subtle balance that dictates if there will be an overall avalanche growth or decay. Deposition is also responsible for critical qualitative changes in flow dynamics, for instance the formation of static levees at avalanche flanks leaving a trail in the deposit. The avalanche flow could also rapidly stop when the inclination changes and the avalanche decays away by mass deposition.

In a move to investigate the effects of erosion and subsequent deposition experimentally, University of Manchester research team, Andrew Edwards, Sylvain Viroulet and Nico Gray in collaboration with Peter Kokelaar at University of Liverpool performed small-scale analogue experiments on a rough inclined plane designed with static erodible carborundum grains layer. Their research work is published in Journal of Fluid Mechanics.

The research team prepared the static layer by slowly closing down a flow from a hopper at the summit of the slope. This left behind a uniform layer at a selected slope inclination. Owing to the hysteresis of the rough bed friction law, this layer could be inclined to higher angle on condition that the thickness did not exceed the initial height, which could be considered as the maximum depth that could be held static on the rough bed. The authors then triggered an avalanche on top of the static layer by releasing a selected volume of the carborundum grains.

The researchers observed three behaviors depending on the slope inclination as well as the static layer depth. For initial deposit depth greater than Hstop, they observed that the avalanche rapidly grew in size by entraining more grains progressively at the sides and front, while depositing just a few particles at the tail and base. This left behind a trough eroded to a depth below the initial deposit surface and whose maximal areal extent had a triangular shape. However, the release on a shallower slope with Hstop deposit thickness led to net deposition. In this case, the avalanche left behind a levee-flanked channel whose floor lay above the level of the initial deposits and narrowed downstream.”

Therefore, it is possible to produce avalanches having an ideal balance between deposition and net erosion. Granular flow problems entailing erosion and deposition are challenging due to the fact that there is no accepted method of modelling the phase change between moving and static particles. However, it was shown in their study that by integrating Forterre’s and Pouliquen extended friction law with the depth-averaged rheology of Edwards and Gray, it was possible to come up with a 2-Dimensional shallow-water-like avalanche model capturing quantitatively all the experimentally observed behavior.

The developed model will have important practical applications for modeling the initiation, growth and decay of snow avalanches for assessing hazards and risk mitigation.

Formation of levees, troughs and elevated channels by avalanches on erodible slopes.. Advances in Engineering

About the author

Andrew N. Edwards received a 1st class (hons) MMath degree in 2010 and a PhD in 2014 from the University of Manchester. Since then he was worked as a postdoctoral research associate at the School of Mathematics and Manchester Centre for Nonlinear Dynamics, apart from a period at the Department of Applied Mathematics and Theoretical Physics, University of Cambridge in 2015. His research interests are focused around shallow granular flows and segregation effects, in particular those flows that propagate whilst exhibiting erosion and deposition of material from a substrate layer.

About the author

Nico Gray is Professor of Applied Mathematics at The University of Manchester and is an expert on granular avalanches and the particle segregation that occurs within them. He holds a BSc in Mathematics from Manchester, a PhD on “sea ice dynamics” from the University of Cambridge and a Habilitation in “continuum mechanics and geophysical dynamics” from the Technical University of Darmstadt. A key feature of Nico’s research is that he performs small scale experiments that provide a strong motivation for his theoretical and computational work.

Over recent years he has also collaborated extensively with geologists working on hazardous geophysical flows, such as debris-flows, rockfalls and pyroclastic flows. This has included field work, as well as novel large-scale experiments at the United States Geological Survey (USGS) debris-flow flume in Oregon. Nico holds a prestigious Royal Society Wolfson Research Merit Award as well as EPSRC Established Career Fellowship. Both these awards are focused at applying the significant theoretical breakthroughs that he has made in understanding the rheology of granular flows and how they segregate to important industrial unit operations, such as chute flows, silos, conveyor belts and rotating drums.

About the author

Peter Kokelaar received BSc 1st Class and PhD in Geology from the University of Wales and MSc in Geochemistry from the University of Oxford. As a mountaineer and volcanologist his main discoveries from world-wide fieldwork and laboratory experiments concern physical processes of avalanches and pyroclastic granular flows. He founded the International Commission for study of Volcanic Sediments, led field excursions at home and abroad, and dived on the submarine flanks of the island volcanoes of Surtsey (Iceland), Stromboli (Italy) and White Island (New Zealand). Most recently he discovered previously unknown phenomena in avalanches on the Moon. He was awarded the Murchison Medal of The Geological Society (London) and recently retired as George Herdman Professor of Geology at University of Liverpool.

About the author

Sylvain Viroulet received his MSc in 2010 from the Université de Bretagne Occidentale (France) and his PhD in Fluid Mechanics in 2013 from the Université of Aix-Marseille (France). He worked as a post-doctoral research associate at The School of Mathematics and Manchester Centre for Nonlinear Dynamics (Manchester, UK) from 2013 to 2016. Since 2017, he works as a post-doctoral research associate at the Institut de Physique du Globe de Paris (IPGP, France). His main research topics are the generation and propagation of tsunamis waves generated by landslides and the study of instabilities developing in dry granular flows with application to geophysical events.

Reference

A. N. Edwards, S. Viroulet, B. P. Kokelaar and J. M. N. T. Gray. Formation of levees, troughs and elevated channels by avalanches on erodible slopes. Journal of Fluid Mechanics, Volume 823 (2017), pages 278–315.

 

Go To Journal of Fluid Mechanics 

Check Also

Recovery potential of flotation tailings