**In their recent JPhysD Topical Review, author Renaud Delannay and colleagues review a range of experimental and numerical modeling techniques for geophysical granular and particle laden flows. Such flows consist of solid-fluid mixtures driven by gravity and propagate in an ambient fluid over generally complex topographies. Professor Delannay explains why investigating these phenomena is so important:**

The study of natural granular flows on Earth and on other planets is an important subject both in terms of understanding, modeling and prediction, in relation to the processes of erosion and deposition, but also for societal issues (on Earth) in the context of natural hazards. Knowledge obtained from laboratory experiments is the basis for addressing natural flows, which are often very complex (consisting of grains with a wide variety of sizes, unsteady, transporting fluids, etc.). The mechanisms of triggering these flows are also interesting (volcanism, earthquakes or others), as well as the subsequent events (tsunamis, modification of deposits, etc.).

The study of subsequent events is of particular importance here; since the geologist sees most of the time only the final state (direct observations of flows are rare, difficult or dangerous). He thus has to solve an inverse problem consisting in inferring the dynamics of the flow from the characteristics of the deposit. It is a very difficult exercise, especially since the processes involved in these geophysical flows are very complex and many parameters come into play. The classical simplification approach involves identifying the important processes and parameters in order to neglect those which can be disregarded. One way of extracting elementary mechanisms is to perform lab experiments under controlled conditions. If we formulate simple questions about the natural flows, we can hope to draw simple but pertinent information. The knowledge of the elementary mechanisms then helps the physicist to develop theoretical modeling. Lab experiments together with numerical simulations allow proposing interpretation of the field data. One of our motivations is to give an overview of the studies carried out in this direction.

Among the important issues discussed here are: the role of the interstitial fluid, the description of the erosion processes, flows with unexpected high mobility – long run out avalanches -and the role of the side walls. In particular, we highlight the remaining issues, such as the up-scaling. It is not known at this time whether it is possible to move from small scale experiments in the laboratory to large-scale natural flows. The mechanisms may be very different in nature.

The specificity of our approach is to stand at the interface between physics and geology. We have the will to analyze the experiments in a geophysical context, which physicists do not usually do. But, we provide analysis that go beyond qualitative comparisons and adopt a quantitative approach as a physicist.

The Topical Review is available now on IOPscience.

**About the authors**

**Renaud Delannay: **His current research focuses on rapid granular flows with emphasis on the effect of boundary conditions. He also investigates the initiation of the flows: avalanches and avalanche precursors as well as heat and mass transfers in static granular piles. His work combines theoretical approach with laboratory experiments.

**Alexandre Valance**: His current research focuses on granular flows, two-phase particle/fluid flows and fluid-mediated particle transport in geophysical flows (e.g., wind-blown sand, aquatic sediment transport), combining laboratory experiments, theoretical approaches and numerical simulations. He is also interested in morphogenesis in the context of aeolian and marine sand dunes.

**Olivier Roche**: His research focuses on volcanic process, with special emphasis on explosive eruptions, edifice destabilization and gravitational flows such as pyroclastic density currents and debris avalanches. He investigates these phenomena and similar geophysical events through laboratory experiments, combined with field and theoretical approaches. His recent works have addressed the erosional mechanisms of gas-particle flows and their significance for the emplacement dynamics of pyroclastic flows.

**Anne Mangeney**: She is interested in the physical processes at work in natural flows with complex rheology (landslides, avalanches, glaciers, etc.) on Earth and on other planets. Her work combines theoretical and numerical approaches as well as laboratory and field experiments. Recently she has focused on the interactions of these processes with the solid earth and the ocean by studying the seismic and water waves generated by landslides and glaciers. She also enjoys working on hazard assessment and the study of volcanic activity through gravitational flow monitoring.

**Patrick Richard** : His current research focuses on investigations of the packing, mechanics and rheology of granular materials. He studies these systems with the aim of better understanding the underlying mechanisms that govern their behaviour. Optimizing processes involving granular materials also motivates his research. His work combines laboratory experiments and theoretical or numerical approaches. In particular, the computer modelling of discrete particles by means Discrete Element Methods (DEM) is one of his areas of expertise. As an example, he recently used DEM to simulate the aggregate crushing process.

This work is licensed under a Creative Commons Attribution 3.0 Unported License.

Figure 1 taken from R Delannay et al 2017 J. Phys. D: Appl. Phys. **50** 053001, © IOP Publishing, All Rights Reserved.

Figure 2 taken from Iverson R M et al 2010 Geophys. Res. 115 F03005. Copyright 2010. This material is reproduced with permission of John Wiley & Sons, Inc.

Author image © Renaud Delannay.

Categories: Journal of Physics D: Applied Physics