Pyroclastic Flows: the impact of the valleys

Pyroclastic flows are gravity currents carrying particulate matter whose deposits are the key source of information to evaluate the volcanic hazards. Study of such currents by analogue flume experiments and numerical simulations is usually carried out in flat bottom geometries. The valleys along which such currents generally flow are thus ignored. Here we show the significant impact of the valleys by studying particulate gravity currents at high Reynolds number flowing along a V-shaped valley. Lock-exchange experiments and a box model were both used. The currents were initiated by releasing a fixed volume of fluid consisting of pure water mixed with particles from a lock at one end of the 5 m long tank. The particle volumetric concentrations were typically 3%. The presence of the V-shaped valley brings significant differences to the motion and deposit of these currents compared to those flowing along a flat bottom tank: 1) the front of the currents has a parabolic shape, 2) the width of the currents narrows with time, 3) the currents travel faster, 4) the runout lengths are higher, and 5) the mass deposited is much larger in the central part of the valley than it is on the flanks. However, bi-dispersed currents of similar grain sizes and initial effective gravity show similar material deposit density in either of the two geometries, and runout lengths vary in the same way with the grain size distribution. The speed, width and mass deposited by these currents are described with remarkable accuracy by a box model using a generalization of the equation for sedimentation from a turbulent medium due to Martin and Nokes. The box model only fails to match the mass deposited of some bi-dispersed currents suggesting that particle interactions with the turbulence or in the viscous sublayer might need to be considered.