High speed granular debris flows: new paradigms and interactions in geomechanics

Lead Research Organisation: University of Sheffield
Department Name: Civil and Structural Engineering

Abstract

Debris flows (often called mudflows) are a type of rapid landslide where soil, rocks and water flow together at high speed downslope. These natural hazards travel for long distances and pose risks to lives and infrastructure that they encounter in their flow paths. During a debris flow event, solid particle sizes and fluid segregate so that large particle are concentrated in the front, leading to high impact forces. This renders their behaviour both dangerous and physically complex so that there are several mathematical and numerical granular flow theories competing to explain their motion.

This research seeks to improve our understanding of debris flows and the impact stresses they can cause to obstacles they encounter, by conducting a series of novel laboratory scale tests in 2D and in 3D on flows confined within model debris flow channels. In the 2D model experiments, the soil and rock will be replaced by acrylic particles, and a photoelastic method used to enable the forces in the individual particles to be determined as the material flows downslope without considering fluid interactions. The results then will be compared against current theories of granular flow in 2D in order to determine which theories best match the results in terms of particle motion and stresses induced in the particles. In the 3D experiments, the soil and rock will be replaced by glass particles and the water by an optically matched fluid which will render the mixture virtually transparent. A laser plane passing through the system then will create a visual slice of the flow at its centre, so that the 2D slice of particles will appear as dark against a bright fluid background during high speed motion. Capturing this behaviour via high speed photography will enable comparison with granular flow theories in 3D in terms of particle motion in the presence of fluid and away from sidewall boundary effects.

In the second series of tests in 3D, an obstacle, representing a structural barrier to the flow or infrastructure, will be placed in the path of a model glass-and-fluid debris flow so that its interaction with the obstacle can be examined. As well as using more conventional techniques, the use of a novel method, holographic interferometry will be trialed to determine how the deformation and stresses in the structure itself are related to particle impacts from the model debris flow. This method can enable the detection of very small deformations over a whole object so is ideally suited to examining the complex interactions of multiple particle impacts with obstacles of differing shape and size.

The overall outcomes will be a several datasets that can be used to better model debris flows numerically, which uniquely includes both the influence of a debris flow on a structure and a structure on a debris flow. It is hoped that the results may lead to better design of barriers to debris flows, enhancing and protecting the natural and built environment and hence leading to greater protection of human life.

Planned Impact

This research and its outcomes will have impact on a number of diverse groups and in several ways. The initial (first test series) outcomes will be a 2D and 3D dataset that can be compared with granular mechanics theories of flow in better detail (by generating force-chain behaviour in 2D, and particle interactions and flow velocities via digital image correlation in 3D) than has previously been developed. This dataset will be published in academic journals that are read by members of both the granular physics and engineering communities, thereby fostering a multidisciplinary understanding between geotechnical / hydraulic engineering and physics.

The second test series outcomes will have a much wider impact beyond academia, being of particular value to model developers in the hazards mitigation community. This goal of this group, working in academia, private companies and in government organisations, is better predictive modelling of flow behaviour towards the reduction of risk posed by debris flows, floods and landslides. The soil-structure interaction effects are particularly important as engineered barriers are usually designed by private companies and implemented (in general) according to local government edict to protect vulnerable populations and lifelines. Such barriers must meet the needs of the local community in question and data from these physical model experiments will assist in determining both debris flow behaviour upon meeting a rigid structural barrier and the best design of such barriers. This will have both an economic and societal impact in enhancing protection of our natural and built environment and hence, of quality of life.

The second series of outcomes also will generate impacts directly in geotechnical and multiphase research, being the result of the development of a novel research approach to experimental geotechnical engineering. This novel approach, holographic interferometry, while used in other disciplines such as medical research and in non-destructive testing, will be extended further for use in the multiphase problem posed by fluidized soil. If such an approach should prove successful, it could pave the way for further outcomes in research on geotechnical soil-structures interaction and has the potential to feed into other multiphase problems (e.g. industrial processing of granulated media and biomedical research).

Finally, in terms of public outreach, natural hazards occupy a special fascination with the public as does novel physics. This research proposal combines both within the context of an engineering and societal problem - namely, how to cope with an increasing global population living on more marginal land at the same time as more extreme climatic events lead to greater risks from debris flows and related hazards. The research project and its outcomes would be a unique opportunity to showcase to the public the interconnected reality of science, engineering and the natural world.

Publications

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Gollin D. (2015) Methods for the physical measurement of collisional particle flows in IOP Conference Series: Earth and Environmental Science

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Gollin Devis (2015) Granular temperature field of monodisperse granular flows in EGU General Assembly Conference Abstracts

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Gollin Devis (2016) Internal characteristics of refractive-index matched debris flows in EGU General Assembly Conference Abstracts

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Sanvitale Nicoletta (2016) Erosion and basal forces in granular flow experiments in EGU General Assembly Conference Abstracts

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Sanvitale Nicoletta (2017) Small scale modelling of dynamic impact of debris flows in EGU General Assembly Conference Abstracts

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Turnbull Barbara (2015) Debris flows: Experiments and modelling in Comptes Rendus Physique

 
Description (1) Debris flows consist of particles of many sizes (from boulders to silt sized) and shapes. Spherical particles may be used to idealise debris flow material for simple experiments, however, their use as an idealisation requires several particle sizes to be used within a single experiment (i.e. challenging the notion from some numerical models that one or two particle sizes along with a fluid phase to represent the rest of the flow is sufficient). Without several particle sizes and suitable basal roughness, particles can behave in an unrealistically crystalline manner, lining up rather than colliding / shearing past each other as they flow downslope. This means that the mechanics of mixing and segregation of particle sizes are not otherwise captured and produces a gross oversimplification of the reality.
(2) The use of holographic interferometry to examine impacts on structures may be possible in future work but considering the refraction of light through transparent granular materials at high speed, it was found to be too difficult at this stage of experimentation. Instead we have focused on establishing a link between overall structural load (which can be visually apportioned by use of the laser images along with appropriate load cells) and impacts from particles; this information can be used to help to design debris flow resistive barriers by linking the particle impact time with that of the load.
(3) We have developed methods to use both PIV (Particle Image Velocimetry) to track ensemble behaviour of the flow and PTV (Particle Tracking Velocimetry) to track particular particles within the flow. Both methods have been evaluated for their ability to measure granular temperature (related to collisional transfer of stress and specific requirements for some numerical models) and while PTV was found to be better for flows of single sized particles (i.e. highly idealised flows), PIV was found to have merit due to the possibility of applying it over a larger range of particle sizes, even though the results were less reliable due to the averaging. Determining the most appropriate PIV algorithm is the subject of further research.
Exploitation Route Debris flow hazard models and barrier interactions sorely need some physical modelling validation. This research can provide this validation - to help with the development of codes of practice in hazard mitigation. Numerical modelling of debris flows is a holy grail in landslides hazard mitigation, however there is very little data that provides the localised insights needed for sophisticated codes. This research will enable such codes to be better validated by giving both visual insight and measurements at margins - particularly codes that use a particulate approach (e.g. discrete element method or molecular dynamics). Also the flow of granular materials is not just associated with high speed landslides, but with a number of other fields, including powder technology (hence, pharmaceuticals, chemicals and manufacturing) in which similar physical problems (e.g. particle size segregation during mixing; rheologies of fluid-particulate systems) and numerical codes are used.
Sectors Aerospace, Defence and Marine,Chemicals,Construction,Environment,Manufacturing, including Industrial Biotechology,Pharmaceuticals and Medical Biotechnology,Transport

 
Description Leverhulme Trust International Network
Amount £125,000 (GBP)
Funding ID IN-2016-041 
Organisation The Leverhulme Trust 
Department Leverhulme International Network
Sector Charity/Non Profit
Country United Kingdom
Start 02/2017 
End 01/2020
 
Description Collaboration with Queen's University, Canada 
Organisation Queen's University
Country Canada 
Sector Academic/University 
PI Contribution Co-investigating debris flow mechanics through experimental work
Collaborator Contribution Co-investigating debris flow mechanics through experimental work
Impact Geotechnical links
Start Year 2017
 
Description UK Fluids Network Special Interest Group on Granular Flows in the Environment and Industry 
Organisation Medical Research Council (MRC)
Department MRC Laboratory of Molecular Biology (LMB)
Country United Kingdom 
Sector Public 
PI Contribution Two workshops have been held so far. The first was in Cambridge (March 2017); the second in Sheffield (organised by the PI) in September 2017 in which industrial partners to the UKFN on Granular flows in the environment and industry were invited to give talks.
Collaborator Contribution PI from Cambridge organised the first meeting; the PI from Edinburgh will be organising the 3rd.
Impact Discussions on research collaboration.
Start Year 2016
 
Description UK Fluids Network Special Interest Group on Granular Flows in the Environment and Industry 
Organisation University of Edinburgh
Department Centre for Population Health Sciences
Country United Kingdom 
Sector Academic/University 
PI Contribution Two workshops have been held so far. The first was in Cambridge (March 2017); the second in Sheffield (organised by the PI) in September 2017 in which industrial partners to the UKFN on Granular flows in the environment and industry were invited to give talks.
Collaborator Contribution PI from Cambridge organised the first meeting; the PI from Edinburgh will be organising the 3rd.
Impact Discussions on research collaboration.
Start Year 2016
 
Description GeoFlow Workshop and 4 week long Seminar (Dresden, Germany) 
Form Of Engagement Activity Participation in an activity, workshop or similar
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Postgraduate students
Results and Impact Attended 2 week-long segments of a workshop / seminar in which scientists, engineers, mathematicians and their postgraduate students discussed all aspects of geophysical flows - from maths of cloud formation and climate change through to flooding and sediment transport, though to catastrophic engineering failures.
Year(s) Of Engagement Activity 2016,2017
URL https://www.mpipks-dresden.mpg.de/events/
 
Description Pint of Science talk (Sheffield) 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach Regional
Primary Audience Public/other audiences
Results and Impact Delivered a "Pint of Science" lecture to the general public: "Looking inside a debris flow" at The Sheffield Tap as part of a trio of talks on natural hazards. There was a lively discussion afterwards along with a lot of curiosity in playing with the demonstration items. There appeared to be a lot of interest from younger members of the audience.
Year(s) Of Engagement Activity 2017
URL https://pintofscience.co.uk/