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Understanding Geophysical Granular Flows: from Small-scale Experiments to Full-scale Simulations

Final Report Summary - GEOGRAF (Understanding Geophysical Granular Flows: from Small-scale Experiments to Full-scale Simulations.)

The GEOGRAF Project (Understanding Geophysical Granular Flows: from Small-scale Experiments to Full-scale Simulations) investigated the high mobility of natural dry granular flows, which constitute an important natural hazard. Explaining this behaviour has remained a long-lasting scientific challenge, and the crushed rock, soils and debris forming the core of the flowing material show an ability to travel and spread extensively. Particles of different sizes have a natural tendency to segregate in granular flows, with finer material migrating towards the base, thus providing a mechanism to reduce friction between the flow and the underlying surface and increase flow mobility. The shape of the underlying surface (or ‘topography’) can also potentially enhance mobility. The GEOGRAF project used laboratory experiments and numerical simulations to investigate these effects, and provided research training in laboratory experimental methods to a researcher with expertise in numerical simulations.

The mechanisms of particle segregation in granular flow were investigated using predominantly numerical simulation, and a small number of experiments where techniques used in medical imaging were used to track segregating particles. A new description of the segregation process in terms of lift and drag forces acting on the larger particles was developed, and the role of transient stresses from particle contacts and related velocity fluctuations was quantified. These results lead to a physical characterisation of segregation time scales. The energy cost of the segregation process and the inclusion of larger boulders in flows were shown to slow down the flow, implying that segregation can enhance mobility only through ‘structuring’ processes, such as self-channelisation of unconfined flows down slopes. Self-channelisation occurs when larger particles preferentially segregate to the edges of the flow and arrest, building a channel that confines the remainder of the flow and increases its runout along the channel.

The role of the large-scale topography and small-scale roughness of the surface below spreading granular materials was studied in the laboratory using experiments where an initially static column of spherical grains collapsed and then flowed in a channel and over an unconfined surface. For a given material, the different topography shape strongly influenced the effective material properties. Numerical simulations showed that the roughness of the underlying surface induced large variations in the flow behaviour, without affecting directly the flow measurable dissipative properties. An important result of this research was the recognition that the effective friction between the granular flow and the underlying surface provided a dominant control on the flow dynamics, being more influential than friction between the constituent particles or any reduction of this due to particle segregation.

Research undertaken in the GEOGRAF project provides a detailed physical characterisation of the roles of segregation and topography in controlling the mobility of hazardous geophysical granular flows such as dry landslides and rockslides. The project results show that the mobility of these flows is primarily controlled by the underlying surface properties (topography and roughness), and differences in constituent material sizes provides a weaker influence on flow mobility. However, the importance of size segregation for self-channelization within flows or for confining flows on flatter topography has been recognised, and these understanding these effects in detail constitutes an emerging research agenda. The dominance of the basal friction condition over internal particle friction provides important new physical justification for the application of simplified numerical models to large-scale natural hazard assessment, and increases confidence that the assumptions about the flow properties made in these models are appropriate.

This project has provided important contributions to knowledge that have direct socio-economic impact, primarily in their application to natural hazard assessment and mitigation, but also to the efficiencies of industrial processes involving transport of granular material. The results support the use of forward physics-based modelling as a key tool in prediction of dry landslide and rockslide inundation and dynamics, because the dominance of the basal friction means that the approximations made in these models are highly likely to be reasonable for full-scale natural flows. In addition, the widespread application of these tools is also reasonable, since the influence of segregation from the wide range of natural material sizes on flow mobility is secondary. Future hazard assessment must better characterise and account for the role of the surface topography in model input as this provides a key influence on the flow mobility, so future research should be directed towards improved resolution of field measurements of surface topography. The characterisation of the mechanism and timescale of granular segregation and the associated potential reduction in granular friction could be exploited to reduce the energy costs associated with the transport of industrial granular materials. The research results are directly relevant to agencies tasked with hazard assessment for dry geological mass flows including landslides, and industrial organisations involved in transport and processing of granular materials.

The GEOGRAF project has provided research training to enable a researcher with expertise in numerical modelling to develop new skills in laboratory experimentation, including experimental design, measurement and analysis, and geophysical hazards and their assessment. New research questions have been identified during the project that will form the basis of future collaboration between the project participants. The research results have been published in six international journal and peer-reviewed conference articles, and been presented at three international meetings including the European Geophysical Union General Assembly. The project objectives and results to date are available on the project website