Investigation of variable aeration of monodisperse mixtures: implications for pyroclastic density currents

Smith, Gregory; Smith, Gregory M.; Williams, Rebecca; Rowley, Pete; Parsons, Daniel R.

Abstract

The high mobility of dense pyroclastic density currents (PDCs) is commonly attributed to high gas pore pressures. 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. In this study, the effect of spatially heterogeneous gas pore pressures in experimental PDCs was investigated. Sustained, unsteady granular currents were released into a flume channel where the injection of gas through the channel base was controlled to create spatial variations in aeration. Maximum current front velocity results from 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 current results in greater runout distances than currents which are closer to fluidisation (i.e. 0.9 Umf_st) near to source, then de-aerating distally. Additionally, although all aerated currents are sensitive to channel base slope angle, the runout distance of those currents where aeration is sustained throughout their lengths increases by up to 54% with an increase of slope from 2° to 4°. Deposit morphologies a primarily controlled by the spatial differences in aeration, where there is a large decrease in aeration the current forms a thick depositional wedge. Sustained gas-aerated granular currents are observed to be spontaneously unsteady, with internal sediment waves travelling at different velocities.

• Journal Article Type: Article
• Publication Date: 2018-08
• Print ISSN: 0258-8900
• Electronic ISSN: 1432-0819
• Publisher: Springer Verlag
• Peer Reviewed: Peer Reviewed
• Volume: 80
• Issue: 8
• Article Number: 67
• APA6 Citation: Smith, G. M., Williams, R., Rowley, P., & Parsons, D. R. (2018). Investigation of variable aeration of monodisperse mixtures: implications for pyroclastic density currents. Bulletin of volcanology, 80(8), https://doi.org/10.1007/s00445-018-1241-1
• DOI: https://doi.org/10.1007/s00445-018-1241-1
• Keywords: Pyroclastic density current; Aerated currents; Flume; Fluidisation; Pore pressure; Slope angle
• Publisher URL: https://link.springer.com/article/10.1007%2Fs00445-018-1241-1
• Copyright Statement: © The Author(s) 2018; ; This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made.