Synthesizing large‐scale pyroclastic flows: Experimental design, scaling, and first results from PELE

Abstract

AbstractPyroclastic flow eruption large‐scale experiment (PELE) is a large‐scale facility for experimental studies of pyroclastic density currents (PDCs). It is used to generate high‐energy currents involving 500–6500 m3 natural volcanic material and air that achieve velocities of 7–30 m s−1, flow thicknesses of 2–4.5 m, and runouts of >35 m. The experimental PDCs are synthesized by a controlled “eruption column collapse” of ash‐lapilli suspensions onto an instrumented channel. The first set of experiments are documented here and used to elucidate the main flow regimes that influence PDC dynamic structure. Four phases are identified: (1) mixture acceleration during eruption column collapse, (2) column‐slope impact, (3) PDC generation, and (4) ash cloud diffusion. The currents produced are fully turbulent flows and scale well to natural PDCs including small to large scales of turbulent transport. PELE is capable of generating short, pulsed, and sustained currents over periods of several tens of seconds, and dilute surge‐like PDCs through to highly concentrated pyroclastic flow‐like currents. The surge‐like variants develop a basal <0.05 m thick regime of saltating/rolling particles and shifting sand waves, capped by a 2.5–4.5 m thick, turbulent suspension that grades upward to lower particle concentrations. Resulting deposits include stratified dunes, wavy and planar laminated beds, and thin ash cloud fall layers. Concentrated currents segregate into a dense basal underflow of <0.6 m thickness that remains aerated. This is capped by an upper ash cloud surge (1.5–3 m thick) with 100 to 10−4 vol % particles. Their deposits include stratified, massive, normally and reversely graded beds, lobate fronts, and laterally extensive veneer facies beyond channel margins.