Gregory Smith, Rebecca Williams, Pete Rowley , Dan Parsons

Pyroclastic Density Currents (PDCs) are hazardous flows of hot gas and volcanic particles which have a diverse range of flow behaviours and depositional mechanisms. Here we use defluidising granular currents, analogous to dense PDCs, to examine the region known in the volcanological literature as the flow-boundary zone. This consists of the lower part of the current and upper part of its aggrading deposit, and its behaviour is thought to control the characteristics of the deposit, independent of bulk current conditions. In these experiments we define the top of the flow-boundary zone as equal to the top of the exponential tail of the velocity profile through the current. Using part of the viscous law of the wall to acquire estimates of shear velocity and shear stress it is shown how variations in parameters in the flow-boundary zone control deposition. In waning currents, the flow-boundary zone transitions from thin, high-shear granular-flow dominated, to thick, low-shear fluid-escape dominated, during the deposition of a sequence of steepening bedforms. This process results in inverse grading at the base of the deposit as initial high shear allows effective vertical particle segregation. Attention is also drawn to how the near-wall viscous sublayer of turbulent fluid flows is analogous to the flow-boundary zone in granular currents. This work demonstrates that the deposits of defluidising granular currents are controlled by the characteristics of the flow-boundary zone, as well as factors such as the current’s response to topography.