Granular suspension avalanches. II. Plastic regime

We present flume experiments showing plastic behavior for perfectly density-matched suspensions of non-Brownian particles within a Newtonian fluid. In contrast with most earlier experimental investigations (carried out using coaxial cylinder rheometers), we obtained our rheological information by studying thin films of suspension flowing down an inclined flume. Using particles with the same refractive index as the interstitial fluid made it possible to measure the velocity field far from the wall using a laser-optical system. At long times, a stick-slip regime occurred as soon as the fluid pressure dropped sufficiently for the particle pressure to become compressive. Our explanation was that the drop in fluid pressure combined with the surface tension caused the flow to come to rest by significantly increasing flow resistance. However, the reason why the fluid pressure diffused through the pores during the stick phases escaped our understanding of suspension rheology

Effondrement granulaire immergé : rôle de la fraction volumique solide initiale

Nous présentons l'étude expérimentale de l'effondrement d'une colonne de grains dans un liquide visqueux. Contrairement au cas sec, le rapport d'aspect initial du tas n'est plus le seul paramètre contrôlant la morphologie du dépôt ni de la dynamique de l'avalanche. Dans le régime visqueux, cette dernière est contrôlée par la fraction volumique solide initiale de la colonne. Nous avons identifié deux régimes principaux selon que le compactage initial est dense ou lâche et les enregistrements de pression sous le tas suggèrent que la dilatance est un mécanisme physique majeur

Dam Break of Newtonian Fluids and Granular Suspensions:Internal Dynamics Measurements

The objective of this thesis was to increase our understanding of two-phase geophysical flows (e.g. debris flows) by providing velocity profiles in idealized laboratory avalanches. To that end, we developed a new experimental platform made up of an inclined flume coupled to an imaging system to measure velocity in granular suspension. The inclined flume was 3.5 m long and 10 cm wide and could be inclined from 0 to 35°. A reservoir with the capacity for 10 l of fluid was located in the upper part of the flume and closed with a pneumatic controlled gate. Velocity profiles were obtained using Particle Image Velocimetry (PIV) and index-of-refraction matching of the solid and liquid phases. We used transparent PMMA beads with mean diameters of 200 µm and the interstitial fluid was composed of a mixture of three fluids. The interstitial fluid was adapted in order to match the refraction index and the density of the solid phase. Using pulsed laser and a high speed camera we were able to measure velocity profiles at frequencies up to 1000 Hz with very good precision. Two additional cameras tracked the front position along the flume with a frequency of 30 Hz and a spatial resolution of 1 mm. Prior to acquiring data on the granular suspension, we tested our system on Newtonian fluids. Eight flow configurations were selected with different fluids (glycerol and triton X100), different slopes and different released masses. Velocity profiles were found to be parabolic far from the front as well as very close to the contact line. However, near the front, quantitative theoretical predictions given by lubrication theory diverged from experimental results. Velocities were significantly overestimated (∼ 400%) by the theory at low Reynolds numbers (Re 8). Very good agreement with theory far from the front indicated that the accuracy of the setup was good (reliable calibration procedure and image processing methods). Experiments on granular suspensions revealed a variety of behaviors depending on the particle concentration, the slope and the mass released. At solid fractions up to 45%, suspensions behaved as homogeneous viscous fluids. For the duration of the experiment, it was not possible to detect any inhomogeneity due to migration or sedimentation. In the range of shear rate tested and with the precision allowed by the setup no shear thickening or shear thinning was observed since velocity profiles remained perfectly Newtonian. For slightly more concentrated suspensions (up to 55%), we found that the flow dynamics at the bulk scale could still be described using a viscous theory. However, at the local scale, migration gave rise to concentration inhomogeneities producing a blunted velocity profile. The shape of the blunted profile was well described by the Mills and Snabre migration model coupled to a Krieger-Dougherty effective viscosity. However, magnitudes of the velocities were largely overestimated, most probably because we fitted the effective viscosity at higher shear rates. Above 55%, small released masses with high solid fractions stopped after a finite time and separation between fluid and solid phases occurred. The solid frame stayed at rest while the fluid seeped through the granular media eroding the front. For larger released masses, we observed successions of different regimes: After an inertial regime and a pseudo-viscous regime, the flow slowed down, corresponding to a new regime in which the shearing was localized in a thin layer at bottom and there was no shearing of the front. At the same time, we observed that the free surface deformed and became wavy. Fractures developed on the top of the flow and, if they grew sufficiently, modified the local velocity field substantially. Finally, at longer time (≥ 4 min) an intermittent motion (stick-slip) was observed with phases during which the suspension was flowing in a quasi-steady regime and phases during which the suspension was at a halt

Multi-scale multiphase modelling of granular flows

Geophysical hazards usually involve multiphase flow of dense granular solids and water. Understanding the mechanics of granular flow is of particular importance in predicting the run-out behaviour of debris flows. The dynamics of a homogeneous granular flow involve three distinct scales: the microscopic scale, the meso-scale, and the macroscopic scale. Conventionally, granular flows are modelled as a continuum because they exhibit many collective phenomena. Recent studies, however, suggest that a continuum law may be unable to capture the effect of inhomogeneities at the grain scale level, such as orientation of force chains, which are micro-structural effects. Discrete element methods (DEM) are capable of simulating these micro-structural effects, however they are computationally expensive. In the present study, a multi-scale approach is adopted, using both DEM and continuum techniques, to better understand the rheology of granular flows and the limitations of continuum models. The collapse of a granular column on a horizontal surface is a simple case of granular flow; however, a proper model that describes the flow dynamics is still lacking. In the present study, the generalised interpolation material point method (GIMPM), a hybrid Eulerian – Lagrangian approach, is implemented with the Mohr-Coloumb failure criterion to describe the continuum behaviour of granular flows. The granular column collapse is also simulated using DEM to understand the micro-mechanics of the flow. The limitations of MPM in modelling the flow dynamics are studied by inspecting the energy dissipation mechanisms. The lack of collisional dissipation in the Mohr-Coloumb model results in longer run-out distances for granular flows in dilute regimes (where the mean pressure is low). However, the model is able to capture the rheology of dense granular flows, such as the run-out evolution of slopes subjected to lateral excitation, where the inertial number I < 0.1. The initiation and propagation of submarine flows depend mainly on the slope, density, and quantity of the material destabilised. Certain macroscopic models are able to capture simple mechanical behaviours, however the complex physical mechanisms that occur at the grain scale, such as hydrodynamic instabilities and formation of clusters, have largely been ignored. In order to describe the mechanism of submarine granular flows, it is important to consider both the dynamics of the solid phase and the role of the ambient fluid. In the present study, a two-dimensional coupled Lattice Boltzmann LBM – DEM technique is developed to understand the micro-scale rheology of granular flows in fluid. Parametric analyses are performed to assess the influence of initial configuration, permeability, and slope of the inclined plane on the flow. The effect of hydrodynamic forces on the run-out evolution is analysed by comparing the energy dissipation and flow evolution between dry and immersed conditions

Tracing back the source of contamination

From the time a contaminant is detected in an observation well, the question of where and when the contaminant was introduced in the aquifer needs an answer. Many techniques have been proposed to answer this question, but virtually all of them assume that the aquifer and its dynamics are perfectly known. This work discusses a new approach for the simultaneous identification of the contaminant source location and the spatial variability of hydraulic conductivity in an aquifer which has been validated on synthetic and laboratory experiments and which is in the process of being validated on a real aquifer

Proceedings Of The 18th Annual Meeting Of The Asia Oceania Geosciences Society (Aogs 2021)

The 18th Annual Meeting of the Asia Oceania Geosciences Society (AOGS 2021) was held from 1st to 6th August 2021. This proceedings volume includes selected extended abstracts from a challenging array of presentations at this conference. The AOGS Annual Meeting is a leading venue for professional interaction among researchers and practitioners, covering diverse disciplines of geosciences

Rock slope instability in alpine geomorphic systems, Switzerland

Faced with the hazard potential and geomorphic importance of rock slopes adjusting to glacier retreat and current climate warming, the motivation of this dissertation is to increase our systemic and process understanding of rock slope instability in alpine geomorphic systems. It is hypothesised that a deeper understanding of rock slope instability can be achieved by thinking and working across scales and accounting for the emergence of non-linear, complex rock slope systems. For this reason, a novel hierarchical methodological approach has been developed. The methodology integrates multivariate modelling and geomorphic field mapping at the valley-scale, rockwall-scale geotechnical, geomorphological and sedimentological field surveys in the Turtmann Valley and Swiss National Park as well as numerical frost cracking modelling and laboratory weathering simulations at the intact rock scale. By means of this multi-method and, most importantly, multiscale systems approach, some progress was made towards current research debates about (i) the key controls of rock slope instability in areas affected by glacier retreat, (ii) associated paraglacial and short-term rockfall activity and (iii) their geomorphic consequences for alpine sediment cascade systems.Felsinstabilitäten in alpinen geomorphologischen Systemen, Schweiz Die Instabilität von Felswänden ist ein komplexes Phänomen das in Zeit und Magnitude variiert. Vor allem in Hochgebirgsregionen sind Felsinstabilitäten von großer Relevanz für die langzeitliche Relieferosion und Landschaftsentwicklung, sowie für die Sedimentproduktion und Effizienz von alpinen Sedimentflüssen. Die damit verbundene Disposition von Sturzereignissen stellt zudem ein ernstzunehmendes Naturgefahrenpotenzial für Mensch und Infrastruktur dar. Untersuchungen zeigen weltweit, und speziell für die Schweizer Alpen, eine Zunahme von Felsinstabilitäten unterschiedlicher Magnituden in den letzten Jahrzehnten. Das komplexe Zusammenspiel von topoklimatischen, kryosphärischen und felsmechanischen Kontrollfaktoren, insbesondere in von Gletscherrückzug betroffenen alpinen Tälern, ist jedoch noch unzureichend verstanden. Folglich stehen nur begrenzt Informationen über die kurz- und langzeitlichen Konsequenzen von Felsinstabilitäten bezüglich Magnituden, Intensitäten und Frequenzen von Sturzprozessen in alpinen Kaskadensystem zur Verfügung. Angesichts dieser Wissenslücken hat diese Doktorarbeit zum Ziel unser System- und Prozessverständnis von alpinen Felsinstabilitäten auf unterschiedlichen Zeit- und Raumskalen zu vertiefen. Ein neuer multiskaliger methodologischer Ansatz wird entwickelt, welcher erlaubt die Skalenabhängigkeit und Emergenz von Felssystemen zu adressieren. Die Arbeit umfasst fünf empirische Studien auf unterschiedlichen räumlichen und zeitlichen Skalen mit Untersuchungsgebieten im Turtmanntal (Schweizer Waliser Alpen) und Schweizer National Park. Auf der größten und längsten Skale untersucht diese Arbeit Hauptkontrollfaktoren für die raumzeitliche Aktivität von Felsinstabilitäten in alpinen Tälern seit dem letzten Glazialen Maximum. Zum ersten Mal in der Sturzprozessforschung wird ein Random Forest Klassifikationsalgorithmus angewandt und durch die Kombination mit einem Hauptkomponenten-basierten, logistischen Regressionsmodell weiter entwickelt. Die Modellkombination zeigt auf, dass Permafrostdegradation im Laufe des Gletscherrückzugs einer der wichtigsten Kontrollfaktoren für die Instabilität entgletscherter Felswände darstellt. Mit Hilfe eines ergodischen Ansatzes werden drei Szenarien paraglazialer Felsanpassung entwickelt, welches nichtlineare tektonische und strukturelle Konditionierungen von Permafrostwänden berücksichtigt. Die Arbeit liefert zudem quantitative und qualitative Beweise für die geomorphologische Signifikanz von Felsinstabilitäten für Sedimentkaskaden in alpinen Einzugsgebieten. Die Kombination aus einem GIS-basierten Konnektivitätsmodell und einer detaillierten geomorphologischen Feldkartierung ermöglicht es Sedimentflüsse von instabilen Felswänden zum fluvialen System zu identifiziert und im Hinblick auf ihre Effizienz zu bewerten. Die feld- und modellierungsbasierten Beobachtungen zeigen eine Dominanz von Sturzprozesse kleiner bis mittlerer Magnitude. Allerdings wird deutlich, dass aktuell ein Drittel des gespeicherten Sturzmaterials aufgrund der glazialen Talmorphometrie vom Hauptkaskadensystem entkoppelt ist. Auf der Skale individueller Felswände analysiert diese Arbeit die Ursache-Wirkung Beziehung zwischen Felsverwitterung, Felsinstabilität sowie Materialspeicherung auf Schutthalden in drei vergletscherten Hängetälern. Ein neuer holistischer Ansatz wird vorgestellt, welcher abduktive Schutthaldenuntersuchungen mit deduktiven geotechnischen Kartierungen an Felswänden, einem zweijährigen Felstemperaturmonitoring und numerischer Frostverwitterungsmodellierung integriert. Dieser integrative Ansatz zeigt auf, dass die Komplexität aus Kluftabstand, der vorgegebenen Kinematik aus Haupttrennflächen sowie der tiefenvariierenden Intensität saisonaler Eissegregation wesentlich das jährlich-dekadische Frequenz-Magnituden Spektrum von Sturzprozessen steuert sowie, in Kombination mit Permafrostdegradation, die langzeitliche Variabilität von Sedimentproduktion und Formeigenschaften von Schutthalden kontrolliert. Auf der Skale des intakten Fels widmet sich diese Arbeit der Frage nach der individuellen und synergetischen Verwitterungseffizienz hochfrequenter thermaler Zyklen und täglicher Eiskristallisation in Glimmerschiefer geringer Porosität. Ein neuartiges zweiphasiges Laborexperiment liefert Evidenzen für mikroskalige, strukturabhängige Felsermüdung in Folge wiederholter Frostzyklen, insbesondere in Felsproben, welche zuvor einer Phase thermalen Stresses ausgesetzt waren. Die Langzeitmessungen zeigen sowohl positive als auch negative Feedbackeffekte im Laufe verändernder mechanischer Felseigenschaften. Diese Beobachtungen haben Implikationen für aktuelle Forschungsdebatte über die Rolle subkrtitischer Verwitterungsmechanismen für oberflächennahe Felsinstabilitäten. Diese Arbeit hebt hervor, dass geomorphologische Forschung dringend mehr Aufmerksamkeit auf die Quellgebiete in alpinen Systemen, also Felswände und ihre inhärenten Systemeigenschaften, richten muss. Zudem zeigen die Befunde dieser Arbeit auf, dass die Instabilität von Felswänden eine Skalenfrage ist. Jede räumliche und zeitliche Skale ist mit unterschiedlichen Kausalzusammenhängen und Erklärungen verbunden hinsichtlich Hauptkontrollfaktoren, raumzeitlicher Sturzprozessaktivität und geomorphologischen Effekten für Sedimentkaskaden. Um diese Skalenabhängigkeit und Nichtlinearität von Felssystemen zu adressieren, liefert diese Arbeit verschiedene praktische und philosophische Lösungsansätze für zukünftige Forschung

EVOLUTION OF THE SUBCONTINENTAL LITHOSPHERE DURING MESOZOIC TETHYAN RIFTING: CONSTRAINTS FROM THE EXTERNAL LIGURIAN MANTLE SECTION (NORTHERN APENNINE, ITALY)

Our study is focussed on mantle bodies from the External Ligurian ophiolites, within the Monte Gavi and Monte Sant'Agostino areas. Here, two distinct pyroxenite-bearing mantle sections were recognized, mainly based on their plagioclase-facies evolution. The Monte Gavi mantle section is nearly undeformed and records reactive melt infiltration under plagioclase-facies conditions. This process involved both peridotites (clinopyroxene-poor lherzolites) and enclosed spinel pyroxenite layers, and occurred at 0.7–0.8 GPa. In the Monte Gavi peridotites and pyroxenites, the spinel-facies clinopyroxene was replaced by Ca-rich plagioclase and new orthopyroxene, typically associated with secondary clinopyroxene. The reactive melt migration caused increase of TiO2 contents in relict clinopyroxene and spinel, with the latter also recording a Cr2O3 increase. In the Monte Gavi peridotites and pyroxenites, geothermometers based on slowly diffusing elements (REE and Y) record high temperature conditions (1200-1250 °C) related to the melt infiltration event, followed by subsolidus cooling until ca. 900°C. The Monte Sant'Agostino mantle section is characterized by widespread ductile shearing with no evidence of melt infiltration. The deformation recorded by the Monte Sant'Agostino peridotites (clinopyroxene-rich lherzolites) occurred at 750–800 °C and 0.3–0.6 GPa, leading to protomylonitic to ultramylonitic textures with extreme grain size reduction (10–50 μm). Compared to the peridotites, the enclosed pyroxenite layers gave higher temperature-pressure estimates for the plagioclase-facies re-equilibration (870–930 °C and 0.8–0.9 GPa). We propose that the earlier plagioclase crystallization in the pyroxenites enhanced strain localization and formation of mylonite shear zones in the entire mantle section. We subdivide the subcontinental mantle section from the External Ligurian ophiolites into three distinct domains, developed in response to the rifting evolution that ultimately formed a Middle Jurassic ocean-continent transition: (1) a spinel tectonite domain, characterized by subsolidus static formation of plagioclase, i.e. the Suvero mantle section (Hidas et al., 2020), (2) a plagioclase mylonite domain experiencing melt-absent deformation and (3) a nearly undeformed domain that underwent reactive melt infiltration under plagioclase-facies conditions, exemplified by the the Monte Sant'Agostino and the Monte Gavi mantle sections, respectively. We relate mantle domains (1) and (2) to a rifting-driven uplift in the late Triassic accommodated by large-scale shear zones consisting of anhydrous plagioclase mylonites. Hidas K., Borghini G., Tommasi A., Zanetti A. &amp; Rampone E. 2021. Interplay between melt infiltration and deformation in the deep lithospheric mantle (External Liguride ophiolite, North Italy). Lithos 380-381, 105855