We are pleased to welcome Nemi Walding for a HotStuff seminar on: The PDC Flow Units Problem - Deposit heterogeneity from varying cohesive behaviour and sediment flux.

Abstract:

Pyroclastic Density Currents (PDC) are high-temperature, rapidly moving flows that can form extensive ignimbrite deposits. As a PDC propagates, entrainment from both internal and external environments can decrease temperatures and introduce water vapour (e.g., exsolving juvenile magma, external hydrological factors, combusting plant matter, water-laden sediment). As a PDC flows away from source the comminution of grains will lead to fragmentation and subsequent higher ash content. These factors are expected to affect cohesive and frictional behaviours within the flow, and the resulting deposits.

Fluidisation within PDCs plays a substantial role in their high mobility and is accepted as an outcome of excess pore pressure from exsolution and entrainment. Defluidising material may alter the profile of a deposit by remobilising grains through gas escape structures (i.e., elutriation pipes) and can cause secondary eruptions in a deposit. The ability for gas escape to reorganize the deposit will be affected by the mechanical properties of the deposit, which will include cohesion. Experiments investigating the cohesive behaviour of analogue and ignimbrite material have been undertaken to explore how static packs of sediment respond to gas escapes under a range of conditions. Material properties including angle of repose, bulk and tapped density and fluidisation behaviour have been recorded under varying moisture content conditions to better understand the static and dynamic behaviours of these materials. 

Results show just small amounts of moisture (0.25 – 0.50%) greatly affect the behaviour of analogue and volcanic material. Increasing moisture content results in higher angle of repose and minimum fluidisation velocity values. As materials becomes fluidised, cohesional variations within the deposit affect bubble and channel formations and can create vertical pressure profiles. These results begin to explore the impact of capillary cohesion and its implications for PDC dynamics, deposit architecture and validity of different analogue materials in experimental modelling.