Propagation of aerated pyroclastic density current analogues : flow behaviour and the formation of bedforms and deposits

Smith, Gregory Michael

Abstract

Pyroclastic Density Currents (PDCs) are deadly volcanic phenomena which pose an active risk to millions of people. Particularly dangerous due to their great unpredictability, despite decades of study the internal physics of PDCs are still poorly understood, especially in dense currents. Much of our understanding relies on the interpretation of PDC deposits, but there is a lack of quantitative links between internal processes and deposit characteristics. Analogue modelling of PDCs attempts to bridge this gap. Recent modelling has emphasised the importance of high gas pore pressures within dense PDCs, which allows them to behave as a fluid and so travel great distances. However, the heterogeneity of pore pressure in PDCs has not yet been replicated.
A series of flume experiments are presented using a novel apparatus to investigate heterogeneous pore pressures within granular currents, analogous to dense PDCs. Experiments show that flow behaviour is affected by variable pore pressures, which also control the morphology of the deposit, with thick wedges of sediment rapidly aggrading where the current undergoes a large drop in pore pressure.
These deposits are further investigated by using coloured particles to visualise internal surfaces. Numerous bedforms are identified, despite most conventional models suggesting that bedforms are indicative of deposition from dilute currents. Their stoss-aggrading nature results in very steeply-dipping upstream beds, which are usually interpreted as recording the transition from supercritical to subcritical flow, although in these experiments they form by topographic blocking.
Particle Image Velocimetry allows the high-resolution characterisation of the granular currents and the identification of the flow-boundary zone through analysis of velocity profiles. Velocity and shear conditions are observed to have some control on the deposit characteristics, in conjunction with other factors such as topography.
The experimental bedforms are validated by detailed comparison with field examples, which shows that they share similar geometries and scaling parameters. Therefore, interpretations made from observing the analogue currents can apply to PDCs. Greater understanding of how PDC behaviour is recorded in their deposits has important ramifications for hazard assessment.

Citation

Smith, G. M. (2020). Propagation of aerated pyroclastic density current analogues : flow behaviour and the formation of bedforms and deposits. (Thesis). University of Hull. Retrieved from https://hull-repository.worktribe.com/output/4223270

• Thesis Type: Thesis
• Deposit Date: Jul 8, 2021
• Publicly Available Date: Feb 23, 2023
• Keywords: Geography; Geology
• Public URL: https://hull-repository.worktribe.com/output/4223270
• Additional Information: Department of Geography, Geology and Environment, The University of Hull
• Award Date: Sep 1, 2020