Nico Gray is an expert on the flow of avalanches over complex natural terrain. This is of importance for understanding the dynamics of hazardous geophysical flows (e.g. snow avalanches, debris-flows, pyroclastic flows and lahars) for planning and risk assessment in alpine, mountainous and volcanic regions. Recent work has focussed on how avalanches flow pastdeflecting dams and obstacles, that are used to slow the flow, or steer it into less harmful areas. Nico uses a novel combination of theory, numerical computation, small scale laboratory experiment and field observations to investigate how obstacles generate shock waves, expansion fans and zero thickness regions.
Avalanches are part of a much wider class of gravity-driven granular free-surface flows that frequently occur in industrial processes as well as in foodstuffs in our kitchens! Indeed, small avalanches are responsible for the transport of our cereal grains from the box into a bowl at breakfast!
Over recent years Nico has become particularly interested in particle-size segregation that takes place within the flowing avalanche and the patterns that are formed as the grains are subsequently deposited. An example of a Catherine wheel pattern formed in a partially filled rotating drum is shown on the right. Most of the grains are in slow solid body rotation, but there is a rapid thin avalanche just near the surface, in which the large (white) grains rise to the top and small (dark) grains fall towards the base. Dependent on whether the grains are brought to rest by the passage of a shock wave, or, by basal deposition from the avalanche, different patterns are formed. Nico is developing simple theoriesfor the segregation process and investigating the complex patterns that are generated in heaps and rotating containers.
In some situations the segregation can have a direct feedback on the bulk velocity of the avalanche. Nico Gray and Peter Kokelaar (Liverpool University) have a joint NERC project to investigate the dynamic feedbacks that give rise to the spontaneous formation of coarse grained lateral levees, that channelize the flow and enhance the run-out of hazardous geophysical grain flows.
Further details of Nico's research including pictures and movies of some of the flows can be found at his personal webpage and in his publication list.