Saltation threshold for pyroclasts at various bedslopes: Wind tunnel measurements

Pyroclastic density currents represent one of the most destructive hazards associated with explosive volcanism. This destructive nature does not only urge the need for but also prevents the obtainment of in situ measurements of their physical characteristics. The resulting deposits offer, however, evidence of the physics of their sedimentation phase. Deposits of dilute pyroclastic density currents frequently exhibit repeated cycles of deposition and erosion, yielding insights into the turbulent shearing along the ground. The utilization of such field observations can be greatly enhanced by the calibration of physical properties of such flows under well-constrained laboratory conditions. Here, wind tunnel measurements were performed using pyroclastic particles. The saltation threshold and surface roughness length were calculated for wind above a pyroclastic bed. The results serve as an aid in linking field observations to quantitative values of turbulent shear at the base of a flow. Scoria and pumice particles were investigated as a function of grain size (1 ϕ fractions between 0.125 and 4 mm), as well as the influence of bedslope (− 20° to + 25° in 10° steps). The results point to the dominant control of density, grain size and, contrary to previous assumptions, differ moderately from results obtained for round beads. Properly utilized, the dataset enables the establishment of a link between the grain size of natural deposits and the shearing extant during their emplacement. Depending on the type of sedimentary structure observed in the field, the saltation threshold can be used as a minimal or a maximal shearing limit during emplacement of dilute pyroclastic density current deposits. Stoss-aggrading laminations likely involve the saltation threshold as an upper limit, whereas for truncation events it must have been overcome. The effect of particle concentration within the flow, a critical parameter for pyroclastic density currents and the extent of validity of the data, are discussed.