The interactions of pyroclastic density currents with obstacles : a large-scale experimental study : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Earth Sciences at Massey University, Manawatū, Palmerston North, New Zealand

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Pyroclastic Density Currents (PDCs) are hot multiphase flows of volcanic particles and gas that are frequently produced during explosive volcanic eruptions. These fast-moving currents show variable runout distances that range from a few kilometres to more than 100 kilometres from their sources. Through their high velocities, large contents of respirable fine ash, temperatures and dynamic pressures, PDCs constitute extreme suffocation, burn and damage hazards for people living around volcanoes. An important additional source of hazard arises from the ability of PDCs to surmount significant topographic obstacles, such as hills and ridges. Because of their ferocity, to date there are no direct observations and measurements of PDCs interacting with obstacles and current knowledge comes from characterizing PDC deposits and PDC damage features across terrain after eruptions. The interaction of dense granular flows and dilute aqueous and gaseous particle-laden gravity currents with simple topographic barriers has been studied in laboratory experiments. These represent dense, non-turbulent or dilute, fully turbulent end-members of PDC behaviour. How the behaviour of dense and dilute end-member flows interacting with obstacles differs from those of real-world PDCs, which encompass a complete multiphase spectrum from dilute to dense transport regimes, remains unknown. This PhD research aims at bridging this gap in knowledge through synthesizing the behaviour of interaction of PDCs with ridge-shaped obstacles in large-scale experiments. The experiments were designed and conducted at the eruption simulator facility PELE (Pyroclastic flow Eruption Large-scale Experiment) to visualise the processes of PDCs interacting with obstacles, measure the changes in the flow velocity and density structure induced by this interaction, characterise the effects of the interaction on downstream flow behaviour, and study the variations in deposition across obstacles. Three hill-shaped obstacles were designed for these experiments, with the same shape and aspect ratio, but varying sizes compared to the size of the experimental PDC. The experiments reveal strong changes in the vertical velocity and density structures of the currents immediately before and after obstacles and strong losses in flow momentum. This is associated with flow compression and acceleration along the stoss side of the ridge, flow detachment with boundary layer separation behind the ridge crest, and formation of a turbulent wake underneath the detached flow before re-attachment. The amount of flow acceleration, the size of the turbulent wake and the flow re-attachment distance increase with obstacle size. High-speed video footage of the interaction shows evidence for a typical sequence of transient behaviour that could be linked to the time-variant velocity and density structure of the head, proximal body, distal body and a tail regions of the experimental PDCs. Four phases of interactions are noted in all three experiments: (1) during head passage - flow acceleration and compression on the stoss side, followed by detachment after the crest, generation of the wake behind the hills and re-attachment downstream; (2) during passage of the proximal body - the development of an alternatively thickening and collapsing turbulent jet structure along the stoss side that forms the base of the detached flow, and which separates and shields the wake from the detached flow above; (3) during passage of the remaining body - an increase in flow density leads to the blocking of a dense underflow forming thick deposits on the stoss side and to the advection of particles from the lower flow region into the detached flow above the wake; (4) during waning flow and passage of the gravity current tail - the velocity field rotates and the angle of attack of the flow approaches the inclination of the lee side of the obstacle. In this situation, the size of the turbulent wake decreases and eventually flow detachment ceases. The compression and acceleration of material on the stoss side of obstacles allow particles at the base of the flow to be conveyed upward back into the detached flow. The higher the obstacle, the stronger the acceleration and the larger the proportion of the flow that is advected. Ballistic trajectory models, which have been used to predict flow paths of dense and dilute analogues flows across simple obstacles, do not describe well the wake measured in experiments and under-estimate its dimensions. As evidenced by vortex shedding and high detachment angles in a flow with high Reynolds number, PDC-obstacle interactions are instead controlled by boundary layer separation in a turbulent flow. Therefore, they are linked to the drag coefficient of the hill and the drag force exerted by the obstacle onto the flow. A study of the wake dimensions revealed that a lift force assists in maintaining the wake aloft and in countering gravity. The ratio between the drag and lift forces controls the wake dimensions. An empirical scaling relationship between the re-attachment distance of the flow and the height of the obstacle was derived and tested against natural data of preserved tree patches behind hills. The experimental measurements also showed that loss in flow momentum due to obstacle drag is associated with complete blocking of the basal granular-fluid underflow and partial blocking of the upper dilute turbulent part of the experimental PDCs. Data from the three experimental runs, in combination with measurements from experiments with no obstacles, allowed extrapolation of the minimum ridge size that leads to complete flow blocking. This relationship agrees well with results from previous laboratory experiments on dilute gas-particle gravity currents and could find application in volcanic hazard estimates. The increasing loss in flow momentum with increasing obstacle size is associated with a strong reduction in the bulk flow density. Thus, experimental PDCs propagating over larger obstacles show a lower density contrast with the ambient air, and therefore a lower driving force, than currents propagating over smaller obstacles. Despite this, the final runout distances are remarkably similar in experiments with different obstacle sizes. This finding is explained by two different processes. First, flow compression and acceleration on the stoss side of obstacles leads to the acceleration of internal gravity waves. The gravity waves move faster than the surrounding flow, intrude and provide momentum into the PDC head. Initially slower currents behind large obstacles thus accelerate periodically and ‘catch up’ with less compressed and less accelerating currents downstream of smaller obstacles. Second, particles that sedimented below the level of the obstacle crest before the obstacle become advected with the detached flow into flow regions above the height of the crest. Larger obstacles, which induce stronger flow acceleration, advect particles higher into the detached flow than smaller obstacles. The duration and downstream length over which the advected particles re-sediment, deposit and eventually become inactive to drive the flow as excess density therefore increases with obstacle size. With increasing obstacle height, this second process generates increasingly hotter flows with slightly coarser and thicker deposits downstream of obstacles. The experimental results and relationships derived in this research add critical complexity to the understanding of PDCs interacting with topographic obstacles and resulting downstream hazards. The reported compressibility effects in the experimental PDCs are currently not captured in PDC flow and hazard models. The local flow acceleration against gravity on the obstacle stoss side warrants caution for the application of kinetic to potential energy conversion models that are used to estimate bulk velocities of PDCs. These findings encourage further experimental and numerical experiments to investigate, for instance, the more complex situations of three-dimensional obstacles, systematic test of different obstacle geometries and series of obstacles in PDC pathways to help development of predictive PDC flow and hazard models.
Figure 2.4 is re-used with the publisher's permission.
Explosive volcanic eruptions, Volcanic ash, tuff, etc., Density currents, Fluid dynamics, Mathematical models