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Item Effects of multiphase turbulence on the flow and hazard behavior of dilute pyroclastic density currents : a thesis submitted in partial fulfilment for the degree of Doctor of Philosophy in Earth Science at Massey University, Palmerston North, New Zealand(Massey University, 2024-06-16) Uhle, Daniel HolgerPyroclastic surges (also dilute pyroclastic density currents or dilute PDCs) are amongst the most hazardous volcanic phenomena associated with explosive volcanic eruptions and hydrothermal explosions. These fast-moving, turbulent, polydisperse multiphase flows of hot volcanic particles and gas occur frequently and have severe impacts on life and infrastructure. This is attributed to a compounding of hazard effects: large flow-internal dynamic pressures of tens to hundreds of kilopascals destroy reinforced buildings and forests; temperatures of up to several hundreds of degrees Celsius pose severe burn hazards; and readily respirable hot fine ash particles suspended inside dilute PDCs cause rapid asphyxiation. Direct measurements inside pyroclastic density currents are largely absent, and previous research has used a combination of detailed field studies on PDC deposits, laboratory experiments on analog density currents, numerical modeling, and theoretical work to interrogate the internal flow structure, gas-particle transport, sedimentation and destructiveness of dilute PDCs. Despite major scientific advances over the last two decades, significant fundamental gaps in understanding the turbulent multiphase flow behavior of dilute PDCs endure, preventing the development of robust volcanic hazard models that can be deployed confidently. Critical unknowns remain regarding: (i) how turbulence is generated in dilute PDCs; (ii) how multiphase processes modify the flow and turbulence structure of dilute PDCs; and (iii) if and how turbulent gas-particle feedback mechanisms affect their destructiveness. To address these gaps in understanding, this PhD research involved high-resolution measurements of velocity, dynamic pressure, particle concentration, and temperature inside large-scale experimental dilute PDCs. It is shown that dilute PDCs are characterized by a wide turbulence spectrum of damage-causing dynamic pressure. This spectrum is strongly skewed towards large dynamic pressures with peak pressures that exceed bulk flow values, routinely used for hazard assessments, by one order of magnitude. To prevent severe underestimation of the damage potential of dilute PDCs, the experimentally determined ratio of turbulence-enforced pressure maxima and routinely estimated bulk pressures should be used as a safety factor in hazard assessments. High-resolution measurements of dynamic pressure and Eulerian-Lagrangian multiphase simulations reveal that these pressure maxima are attributed to the clustering of particles with critical particle Stokes numbers (𝑆𝑡=𝒪(1)) at the margins of coherent turbulence structures. The characteristic length scale and frequency of coherent structures modified in this way are controlled by the availability of the largest particles with critical Stokes number. Through this, spatiotemporal variations in peak pressures are governed by the mass loading and subsequent sedimentation of these clustered particles. In addition to the ‘continuum phase’ loading pressure, the measurements also revealed that the direct impact of clustered margins and high Stokes number particles decoupling from margins with structures generate instantaneous impacts. These piercing-like impact pressures exceed bulk pressure values by two orders of magnitude. Particle impact pressures can cause severe injuries and damage structures. They can be identified as pockmarks on buildings and trees after eruptions. This new type of PDC hazard and the magnitude of pressure impacts need to be accounted for in hazard assessments. Systematic measurements of the evolving experimental pyroclastic surges along the flow runout demonstrate that time-averaged vertical profiles of all flow velocity components and flow density obey self-similar distributions. Variations of the roughness of the lower flow boundary, geometrically scaling ash- to boulder-sized natural substrates, showed the self-similar distributions are independent of the roughness. Mathematical relationships developed from the self-similar velocity and density distribution reveal the self-similar vertical distribution of mean dynamic pressure. This empirical model can inform multi-layer PDC models and estimate the height and values of peak time-averaged dynamic pressure for dilute PDCs of arbitrary scale. Turbulence fluctuations around the mean were investigated through Reynolds decomposition. The large-scale turbulence structure and the dominant source of turbulence generation are shown to be controlled by free shear with the outer flow boundary, while strong density gradients at the basal high-shear flow boundary dampen turbulence generation. The large-scale, shear-induced coherent turbulence structures can be tracked along the runout and were found to be superimposed by smaller turbulence structures. In Fourier spectra of dynamic pressure, flow velocity, and temperature, these sub-structures are observed as discrete frequency bands that correspond to the coarse modes of the spatiotemporally evolving flow grain-size distributions. This can be associated with the preferential clustering of particles at the peripheries of the sub-structures. Following the decoupling of particle clusters, the rapid sedimentation of particle clusters occurs periodically at the characteristic frequency of the turbulence sub-structures. This mechanism of preferential clustering, decoupling and rapid sedimentation of particles with critical particle Stokes numbers is an important mechanism of turbulent sedimentation to explain the spatiotemporally evolving flow grain-size distribution of pyroclastic surges.Item 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(Massey University, 2023) Corna, LucasPyroclastic 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.Item Understanding magmatic processes and their timescales beneath the Tongariro Volcanic Centre through microanalytical investigations of the tephra record : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Earth Sciences at Massey University, (Manawatū Campus), Palmerston North, New Zealand(Massey University, 2020) Lormand, CharlineThe Tongariro Volcanic Centre (TgVC) is a complex volcanic system located at the southern end of the Taupo Volcanic Zone in New Zealand, and has produced historical explosive eruptions of different eruptive styles. Its three ski fields and its iconic Tongariro Alpine Crossing attract more than 130,000 visitors annually. The last eruption occurred in 2012 on the northern flank of Tongariro, at the Te Maari vent. Due to the lack of precursory activity, this eruption could have turned into a tragedy if it had happened during day time. Previous studies have focused on the TgVC phenocrysts, which do not provide insights into shallow magmatic processes, essential to mitigate the resulting volcanic hazards. To understand magma ascent processes and their associated timescales, the textures and compositions of the micrometre-sized crystal cargo (i.e. microlites and micro-phenocrysts) carried during explosive eruptions are investigated, along with their conditions of crystallisation [i.e. P-T-X(H₂O)], which are constrained using hygrothermobarometry and MELTS modelling. Glass shards from five tephra formations spanning from c. 12 ka BP to 1996 AD, associated with explosive eruptions ranging from Strombolian to Plinian in style, are studied here. High resolution images and chemical maps of the tephras and the crystals are acquired using scanning electron microscopy (SEM) and secondary ion mass spectrometry. The variety of disequilibrium textures and compositions found in the micro-phenocrysts (< 100 μm) indicates multiple events of magma mixing, magma recharge, pressure fluctuations, and suggests an antecrystic origin. Crystal size distribution (CSD) of 60,000 microlites (< 30 μm) of plagioclase and pyroxene are generated from back-scattered-electron (BSE) images using a semi-automatic method developed here to undertake this study, employing the Weka Trainable Segmentation plugin to ImageJ. Combined with a well-constrained growth rate, crystallisation times are derived and indicate that microlites crystallised 2 to 4 days before the eruption, regardless of the eruption style. Microlite crystallisation occurred between mid-crustal depths and the surface (average of c. 4 km), at unusually high temperature for arc magmas of intermediate composition (average of 1076 °C), and at low water contents (average of 0.4 wt%). Considering the inferred depths and the crystallisation times of 2 to 4 days, ascent rates of only up to 9 cm s⁻¹ prior to shallow water exsolution are calculated. Vent exit velocities are not exceeding 27 m s⁻¹ after complete water exsolution, too slow to feed explosive eruptions characterised by supersonic exit velocities. This research proposes a new conceptual model for the magmatic plumbing system beneath TgVC, where the microlitic crystal cargos result from multiple intrusions of aphyric melts through dykes, which most of the time stall and evolve at depth as deep as the mid-crust. Eventually, a magma injection percolates through previous intrusions and entrains crystals of differing textures and histories. Dykes feeding volcanism funnel into a narrow cylinder towards the surface, allowing acceleration and triggering explosive eruptions. Therefore, the conduit geometry at TgVC is a key controlling factor on the explosivity, with narrower conduits resulting in more explosive eruptions, suggesting that volatile-poor magmas can still trigger explosive eruptions. This study supports that vertical foliation of the igneous upper crust is consistent with dyking and thus may be more common than typically acknowledged.Item Inside pyroclastic surges - a characterisation of the flow behaviour, hazard impact mechanisms and sedimentation processes through large-scale experiments : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Earth Science at Massey University, Palmerston North, New Zealand(Massey University, 2020) Brosch, ErmannoDilute pyroclastic density currents (or pyroclastic surges) are frequently occurring and highly dangerous volcanic flows generated during explosive volcanic eruptions. These fast and ground-hugging flows of hot volcanic particles and gas swiftly sweep across landscapes to cause significant risk to life and infrastructure at many volcanoes globally. Understanding the flow dynamics of pyroclastic surges, and developing a quantitative understanding of the mechanisms behind their hazard impacts, are thus important requisites for the development and testing of robust hazard mitigation strategies. Despite strong progress through field, theoretical, laboratory and numerical approaches during the past 50 years, the understanding of the flow and hazard behaviour of pyroclastic surges is still highly fragmentary. An important reason behind current gaps in knowledge stems from the hostile nature of these currents, which, to date, has prevented any direct observations and internal measurements. This leaves many theories behind flow and hazard models untested and un-validated. Over the past ten years, large-scale experiments have provided a novel approach to generate the missing ‘view’ inside pyroclastic density currents. While recent experiments have improved the understanding of the internal flow behaviour of dense pyroclastic density currents, comparable large-scale experimental studies for pyroclastic surges remain outstanding. This thesis describes the first systematic series of large-scale experiments that aimed to obtaining detailed measurements of the dangerous interior conditions of pyroclastic surges. Conducted at the international eruption simulator facility PELE (the Pyroclastic flow Eruption Large-scale Experiment), this study aimed to provide answers to the following three research questions: What are the flow-internal processes that cause the extreme destruction potential of pyroclastic surges? What is the detailed internal structure of pyroclastic surges? What are the particle-transport and sedimentation processes occurring in the basal region of dilute PDCs? Through the development of new measurement techniques and the refinement of existing set-up approaches, a systematic series of experiments were completed, which provided comprehensive datasets of dilute PDC analogues. The main results and implications of this research are as follows. Direct measurements of the internal velocity and density structure inside experimental flows show that turbulence is an important driver of the destructiveness of pyroclastic density currents. The effects of turbulence manifest themselves through three cumulative mechanisms. First, most of the flow energy becomes focussed into large eddies whose turbulent excursions generate destruction-causing dynamic pressures that exceed traditionally estimated mean values manifold. Second, self-developed pulsing inside flows, associated with the propagation of large coherent turbulence structures, leads to high dynamic pressures, propagating and perpetuating downstream. Third, the characteristic frequencies associated with large eddy motion are able to excite resonance in large buildings. The characterisation of gas-particle interactions inside the dynamically and kinematically scaled experimental pyroclastic surges revealed that commonly assumed near-homogeneous coupling between particles and gas is the exception, while strong to intermediate feedback loops between gas and solid phases are omnipresent throughout most of the evolving flow. This leads to interesting mesoscale turbulence effects, which are here shown to modify and control the evolving vertical flow stratification, the spatiotemporally varying deposition mechanisms, and the generation of turbulence in addition to the typically assumed shear and buoyancy processes. The spatially variable velocity of the leading front of the experimental surges is here demonstrated to behave differently to aqueous particle-laden gravity currents. The propagation of flow-internal pulses inside the highly turbulent gravity currents is a key mechanism in determining the flow runout and consequent hazard characteristics. The deposits of the experimental pyroclastic surges are here shown to have strong similarities to real-world deposits. Simultaneous measurements of the evolving structure of the lower flow boundary and the accreting deposit add complexity to our current qualitative view of the sediment transport and deposition mechanisms inside pyroclastic surges. The occurrence of mesoscale turbulence modifies particle supply into the lower flow boundary and, through the rapid passage of large-eddy passages in this region, gives rise to dynamic changes between a range of bedload transport processes and deposition rates. A correlation of the contribution of the different regions of the experimental surge to the spatially and temporally evolving deposit provides new insights in to how real-world deposits originate and how best to sample them to characterise the behaviour of past PDC-forming eruptions. Parts of the experimental datasets obtained during this research form the first international benchmark case to test, validate and compare the current range of numerical dilute PDC flow and hazard models.Item Modelling and analysis of leaching of copper from volcanic ash soil : a thesis presented in partial fulfilment of the requirments [i.e. requirements] for the degree of Master of Technology in Environmental Engineering(Massey University, 1999) He, DaliSoil contaminated by heavy metal ions has become a global problem. Besides legislation to restrict the input of heavy metals, remediation of contaminated soil is also essential. The most common means of remediation is leaching. There have been many studies published in this field, some of which relate to development of mathematical models. Volcanic ash soil is common in New Zealand. Developing a model to predict the process of leaching heavy metal from volcanic ash soil is important for New Zealand. No model was found for predicting the process of leaching heavy metal from volcanic ash soil. Heavy metal soil contamination can not be remedied by microorganisms, so the heavy metals will inevitably accumulate in soils over time. Once heavy metals have accumulated in soil to exceed a threshold, they will be released and then be taken up by plants, entering the food chain or moving into the groundwater system. Therefore, it is necessary to leach the heavy metals from the contaminated soil. The batch stirred process is a fast and convenient method, and it is easily used in the field. The main purpose of this study is to develop a model that can predict the bulk liquid concentration of heavy metal in the stirred vessel. In the present study, the internal model is pore diffusion model. The explicit method is used to translate a partial differential equation to a finite difference equation. The results from thermodynamic and kinetic experiments agree with the model. With the exception of the equilibrium parameters for Freundlich isotherm derived by experiment, all other parameters were obtained from literature on volcanic ash soil. Therefore, the model can be used for leaching of other heavy metals from volcanic soil under similar conditions. The leaching of heavy metals from volcanic soil is shown to be an internal diffusion controlled process, so increasing the agitating speed in a stirred reactor is of no use for improving the mass transfer. Decreasing the size of volcanic soil aggregates by breaking them clearly increases the rate of the leaching process. The equilibrium relationships of the adsorption process and the desorption process are different for the system, and there is a hysteresis.Item A mineralogical and textural study of the central North Island tephra, Okareka ash and its overlying tephric loess deposits : a thesis presented in partial fulfilment of the requirements for the degree of Master of Science in Soil Science at Massey University(Massey University, 1982) Benny, Lynette AIn Central North Island, New Zealand, Post-Okareka tephric loess rests upon Okareka Ash (c.17,000 years B.P.). Tephric loess accumulation occurred under semiarid conditions which coincided with glacial advances in southern areas of New Zealand. Morphological and grain-size evidence indicates the tephric loess has been derived from a localised source, most probably that of Okareka Ash material, reworked and redeposited by aeolian processes. Optical and electron optical evidence reveals that Okareka Ash particles are angular and relatively unweathered, whereas tephric loess grains are subangular and more weathered. The sand and clay mineralogy of the tephra and tephric loess are similar. Sand fractions contain mainly rhyolitic volcanic glass, quartz, plagioclase feldspar, biotite, hypersthene, hornblende, titanomagnetite and traces of cristobalite, tridymite and augite, whereas clay fractions contain halloysite, allophane, imogolite and gibbsite in varying amounts. Grain-size analysis reveals Okareka Ash deposits show decreasing mean grain-size with increasing distance from source, are poorly-sorted, fine-skewed, and lepto/platykurtic. In contrast to tephra, tephric loess samples exhibit a narrow mean grain-size range, and are better sorted, but show similar skewness and kurtosis values to ash. Grain-size results also indicate that due to minimal weathering of Okareka Ash and Post-Okareka loess, the distinction between the two deposits is less well-defined than data from similar deposits reported by Fisher (1966). Furthermore, where ash deposits are thin, in distal areas from source, and under certain environmental conditions, textural and morphological characteristics of the tephra are similar to those of the tephric loess. Nevertheless, grain-size parameters may be used to differentiate airfall tephra and tephric loess deposits, although this differentiation is enhanced by post-depositional weathering. The contrasting clay mineralogies of tephra and tephric loess samples from sections of similar topography, altitude, drainage and rainfall, illustrates the problems of field sampling in weathering studies.