Effects of topography on pyroclastic density current runout and formation of coignimbrites

Laboratory experiments of dilute mixtures of warm talc powder in air simulate dilute pyroclastic density currents (PDCs) and show the effects of topography on current runout, buoyancy reversal, and liftoff into buoyant plumes. The densimetric and thermal Richardson, Froude, Stokes, and settling numbers for our experiments match those of natural PDCs. The laboratory currents are fully turbulent, although the experiments have lower Reynolds numbers than PDCs. In sum, our experiments are dynamically similar to natural currents. Comparisons of currents traversing flat topography or encountering barriers show that runout distance is not significantly reduced for currents that traverse barriers with height <1.5 times the current thickness, but currents do not pass taller barriers. Buoyancy reversals occur in most currents, resulting in liftoff and generation of a buoyant plume. Liftoff occurs near the maximum runout distance for currents traveling over flat topography, but is focused near or above barriers for currents that encounter barriers. Notably, plume formation above barriers can result in reversal of flow direction downstream of the obstruction as portions of the current flow back and feed the rising plume.