Investigation of variable aeration of monodisperse mixtures: implications for Pyroclastic Density Currents
High gas pore pressures are known to be important in dense pyroclastic density currents (PDCs), causing the flows to be highly mobile. However, the influence of spatial and temporal variations in pore pressure within PDCs has yet to be investigated. Theory suggests that variability in the fluidisation and aeration of a current will have a significant control on PDC flow and deposition. Here, the effect of spatially heterogeneous gas pore pressures in experimental PDCs was investigated.
... estigated. Sustained, unsteady granular flows were released into a flume channel where the injection of gas through the channel base was controlled to create spatial variations in aeration. Maximum flow front velocity is achieved by high degrees of aeration proximal to the source, rather than lower sustained aeration along the whole flume channel. However, moderate aeration (i.e. ~0.5 minimum static fluidisation velocity (Umf_st)) sustained throughout the propagation length of a flow results in greater runout distances than flows closer to fluidisation (i.e. 0.9 Umf_st) near to source then de-aerating distally. Additionally, although all aerated flows are sensitive to channel base slope angle, the runout distance of those flows where aeration is sustained throughout the length of the flow increase by up to 54% with an increase of slope from 2° to 4°. Deposit morphologies are primarily controlled by the spatial differences in aeration; where there is large decrease in aeration the flow forms a thick depositional wedge. Sustained gas-aerated granular currents are observed to be spontaneously unsteady, with internal sediment waves travelling at different velocities.