Propagation and hindered settling of laboratory ash flows

Overview & Synopsis

The fluidal behavior of pyroclastic flows is commonly attributed to high gas pore pressures and associated fluidization effects. We carried out experiments on flows of fluidized volcanic ash at 170°C, which is hot enough to reduce cohesive effects of moisture. The flows were generated in a 3‐m‐long, horizontal lock‐exchange flume. The ash was fluidized and expanded uniformly in the flume reservoir by up to 43% above loose packing and was then released. Each flow defluidized progressively down the flume until motion ceased. Initial expansion E and initial height h0 were varied independently of one another. The flows traveled in a laminar manner. Flow fronts exhibited three main phases of transport: (1) a brief initial phase of gravitational slumping, (2) a dominant, approximately constant velocity phase, and (3) a brief stopping phase. Phase 2 frontal velocities scaled with equation image, like other types of dam‐break flow. Deposition from initially expanded flows took place by progressive sediment aggradation at a rate that was independent of distance and varied only with E. Despite rates of shear up to 80 s−1, aggradation rates were identical to those determined independently, at the same value of E, in quasi‐static collapse tests. Sedimentation caused the flows to thin progressively during transit until they ran out of volume. The dynamics were governed to a first order by two dimensionless parameters: (1) the initial aspect ratio h0/x0 in the lock reservoir and (2) the ratio tsett/tgrav of two timescales: a particle settling time tsett and a gravitational acceleration time tgrav.