Granular Avalanches Past Obstacles and Erosion and Deposition Processes in Shallow Granular Flows

Abstract

Geophysical mass flows, such as debris flows, pyroclastic flows and snow avalanches, can pose a significant hazard to people and infrastructure in mountainous areas. A better understanding of the underlying physical processes of these granular phenomena is essential for developing and improving effective mitigation strategies. This thesis comprises three studies which investigate the interaction of static and moving material in shallow granular flows. Throughout this thesis, a combination of small-scale experiments and numerical modelling is used with a depth-averaged modelling approach that encompasses the hysteretic behaviour of grains through a non-monotonic basal friction law and lateral viscous dissipation through a depth-averaged viscous term. The first research chapter of the thesis considers the features and behaviour of a granular flow impacting a blunt obstacle on a rough inclined plane. For a rapid (supercritical) inflow, material is diverted around the obstacle by a combination of a static dead zone and a bow shock upstream of the obstacle. For slower (subcritical) inflows, material is diverted without the presence of the bow shock. In both cases, a grain-free (vacuum) region is created downslope of the obstacle. For flows on rough beds, this vacuum region is bounded by levees of static material that increase its length. This is the opposite to what is observed for flows on smooth beds, where the fastest flow is situated next to the vacuum region. The second chapter focuses on an entrainment problem for shallow granular flows on an erodible bed and describes a ripple-like instability, which is observed localised to the wavefront. Understanding how a geophysical mass flow entrains material is essential in order to describe the dynamics at the front of the avalanche and the resulting growth and decay of the flow. This study shows that the ripple instability, which retreats from the front prior to being diffused into the bulk flow, is a distinct waveform from roll waves, which propagate forward. The study also indicates that there are properties of an eroding front that are not yet fully captured by simple models in which the material is instantaneously activated through the full depth. The last chapter examines the effects of basal topography on the descriptions of the hysteretic behaviour of granular materials. Many granular flow models assume a simplification that the avalanche flows over a uniform plane. However, a real-world basal topography is likely more complex. This study contributes to our understanding of transitioning regimes and shows that the model adapted for flows over complex topography can recover the locally predicted deposit depths over the transition and, thus, show behaviour converges to local effects. These discussions contribute to our understanding of a wide variety of complex granular flows which can occur on varying topography, interact with obstacles and entrain material influencing the dynamics of the flow.

Date of Award	1 Aug 2022
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Nico Gray (Supervisor) & Christopher Johnson (Supervisor)