Empirical and analytical analyses of laboratory granular flows to investigate rock avalanche propagation

Laboratory experiments which consist of releasing dry rigid non-cohesive grains or small bricks on an unconfined chute have been designed to investigate rock avalanche propagation mechanisms and to identify parameters influencing their deposit characteristics. Factors such as volume, fall height, basal friction angle, material used, structure of the material before release, i.e. bricks randomly poured into the reservoir before failure or piled orderly one on top of the other, and type of slope break, i.e. curved or sharp angular, are considered and their influence on apparent friction angle, travel angle of the centre of mass, deposit length and runout is analysed. Results highlight the influence of the structure of the material before release and of the type of transition at the toe of the slope on the mobility of granular avalanches. The more angular and sharp is the slope break, the more shearing (friction) and collisions will develop within the sliding mass as it changes its flow direction, the larger will be the energy dissipation and the shorter will be the travel distance. Shorter runout is also observed when bricks are randomly poured into the reservoir before release compared to when they are piled one on top of the other. In the first case, more energy is dissipated all along the flow through friction and collisions within the mass. Back analysis with a sled block model of experiments with a curved slope break underlines the importance of accounting centripetal acceleration in the modelling of the distance travelled by the centre of mass of a granular mass. This type of model though is not able to assess the spreading of the mass and its total runout because it does not take into account the internal deformation and the transfer of momentum within the mass which, as highlighted by the experimental results, play an important role in the mobility of rock avalanche

The propagation and emplacement mechanisms of the Tenteniguada volcanic debris avalanche (Gran Canaria):Field evidence for brittle fault-accommodated spreading

The Tenteniguada volcanic debris avalanche deposit is located on the east of the island of Gran Canaria, Spain. Its internal structure is composed of a complex assemblage of extensional features and shearing structures including normal faults, horst and graben, brittle/ductile boudinage and clastic dike injections. Examination of these features in the field and evaluation of their distribution have allowed the generation of a new conceptual model for the transport and emplacement of this debris avalanche, and potentially others. In the majority of the deposit, the degree of disaggregation is low, with large portions of the original edifice preserved, although displaced by brittle deformation. Greater disaggregation is observed deeper and in the more distal section of the deposit. The findings suggest that the propagation of the volcanic debris avalanche was most likely facilitated by the normal fault-accommodated spreading and extension of the mass, with the majority of stress focused in fault zones. The greater disaggregation exhibited in the deeper and the more distal part of the deposit is likely to be due to greater stress accommodation from fault convergence and momentum transfer respectively. The abundance of competent lava lithologies and scarcity of weaker material that could be easily disaggregated is the most likely reason Tenteniguada did not fully evolve from a slide to a granular flow, and therefore generated a deposit which bears resemblance to non-volcanic blockslide deposits. Therefore, lithological properties are potentially a vital factor for the propagation mechanisms, distribution of stress and consequently the evolution of a debris avalanche from the initial collapse to its emplacement. The present study highlights the importance of dedicated field examinations of sedimentological, morphological, and structural features for providing constraints for models of debris avalanche propagation mechanisms and the factors dictating them.</p

Distributed stress fluidisation: Insights into the propagation mechanisms of the Abona volcanic debris avalanche (Tenerife) through a novel method for indurated deposit sedimentological analysis

Introduction: Volcanic debris avalanches mobilise large volumes and achieve long runouts with high destructive potential. However, the propagation processes that generate them are not currently explained by theoretical or numerical models, which are unable to represent deposit observations. Evaluation of the dynamics represented in deposits is therefore vital for constraining su ch models. The Abona volcanic debris avalanche deposit is located on the southern flank of the island of Tenerife, Spain. The deposit exhibits universal microfracturing and cataclasis. Fluidal features such as fluidal mixing of lithological units and diffuse boundaries, and mixed matrix are observed throughout the deposit.Methods: Field description including sedimentology and facies identification and the evaluation of their distribution have allowed the generation of a new conceptual model for the propagation dynamics of this volcanic debris avalanche, and potentially others with similar properties. The deposit is indurated making the detailed study of its sedimentology difficult, especially clast-size analysis. A novel method utilising structure from motion photogrammetry and photographic sampling was employed.Results: The universal cataclasis of the material and fluidal features suggest that the lack of a major competent material component allowed the mass to fragment and enabled fluidised granular flow behaviour. It is proposed that shear was periodically distributed throughout the body of the avalanche in chaotic temporary shear networks rearranging according to the instantaneous distribution of the mass. Stress and agitation were not temporally or spatially homogenous during propagation. This is also reflected in the unsystematic erosion of the substrate according to the variable basal shear accommodation.Discussion: It is proposed that lithological properties are potentially a determining factor for the propagation mechanisms, stress distribution, and consequently the evolution of a volcanic debris avalanche from the initial collapse to its emplacement. This study highlights the importance of dedicated field examinations of sedimentological, morphological, and structural features for providing constraints for models of volcanic debris avalanche dynamics and the factors dictating them. The novel methodology proposed has the potential of broadening the number of events that can be studied and enhancing the understanding of these complex and hazardous phenomena

Grain size distribution and sedimentology in volcanic mass-wasting flows: implications for propagation and mobility

The sedimentological characteristics of mass-wasting flow deposits are important for assessing the differences between phenomena and their propagation and emplacement mechanisms. In the present study, nine volcanic debris avalanche deposits and eight lahar deposits are considered, from the literature. Their sedimentology is expressed in the descriptive statistics: median grain size, sand, gravel and finer particle fractions, skewness and sorting. Analysis of the data confirms that lahars and debris avalanches diverge in their grain size distribution and in their evolution during propagation. Water saturation in lahars is the main factor enabling debulking, a mechanism that is not recorded in the data derived from debris avalanches deposits. On the contrary, evidence of comminution of particles due to particle-particle interactions is observed in debris avalanches, and not in lahars. These findings support previous studies suggesting that although water content in debris avalanches plays a role in propagation, the effects of inertial collision of solid fragments are more important than fluid effects, confirming that particle-particle interactions are the main factor influencing the mobility of non-saturated mass wasting flows

Evaluating the use of smart sensors in ground-based monitoring of landslide movement with laboratory experiments

Boulders and cobbles embedded on the body of landslides are carried downstream under the action of gravity, and the study of their transport can give important insight on their dynamics and hence the related hazard. The study examines the reliability of smart sensors to track movements of a cobble and discern between intensity and mode of movement in laboratory experiments. A tag equipped with accelerometer, gyroscope, and magnetometer sensors was installed inside a cobble. The experiments consisted of letting the cobble fall on an inclined plane. By tilting the inclined plane at different angles, different modes of movement such as rolling, bouncing, or sliding were generated. Sliding was generated by embedding the cobble within a thin layer of sand. The position of the cobble travelling down the slope was derived from camera videos. Raw sensor data allowed detection of movement and separation of two modes of movement, namely rolling, and sliding. Additionally, reliable values for the position, velocity, and acceleration were determined by feeding a Kalman filter with smart sensor measurements and camera-based positions. Furthermore, by testing LoRaWAN wireless transmission through sand, the study showed that the signal strength tended to decrease for thicker sand layers. These findings confirm the potential to use these sensors to improve early warning systems and further studies are in progress to assess practicalities of their use in field settings

Smart sensors to detect movements of cobbles and large woody debris dams. Insights from lab experiments.

An increase in population pressure and severe storms under climate change have greatly impacted landslide and flood hazards globally. At the same time, recent advances in Wireless Sensor Network (WSN) and Internet of Things (IoT) technologies, microelectronics and machine learning offer new opportunities to effectively monitor stability of boulder and woody debris on landslides and in flood-prone rivers. In this framework, smart sensors embedded in elements within the landslide body and the river catchment can be potentially used for monitoring purposes and for developing early warning systems. This is because they are small, light-weight, and able to collect different environmental data with low battery consumption and communicate to a server through a wireless connection. However, their reliability still needs to be evaluated. As data from field sites could be fragmented, laboratory experiments are essential to validate sensor data and see their potential in a controlled environment. In the present study, dedicated laboratory experiments were designed to assess the ability of a tag equipped with an accelerometer, a gyroscope, and a magnetometer to detect movements in two different settings. In the first experimental campaign, the tag was installed inside a cobble of 10.0 cm diameter within a borehole of 4.0 cm diameter. The experiments consisted in letting the cobble fall on an experimental table composed of an inclined plane of 1.5 m, followed by a horizontal one of 2.0 m. The inclined plane can be tilted at different angles (18˚- 55˚) and different types of movement have been generated by letting the cobble roll, bounce, or slide. Sliding was generated by embedding the cobble within a layer of sand. The position of the cobble travelling down the slope was derived from camera videos by a tracking algorithm developed within the study. In the second experimental campaign, a simplified analogue model of a woody debris dam was built from a single hollowed dowel with a length of 40 cm and a diameter of 3.8 cm. The sensor tag is installed in the woody dowel within a 2.5 cm longitudinal borehole. Two metal rigs are mounted at both sides of the woody dowel to allow different modes of movement. Specifically, the woody dowel is allowed to move either horizontally or vertically within a range of 20-30 mm, whereas it is always free to complete full rotations. The woody dowel is mounted on a frame within a 20 m long and 0.6 m wide flume. In these two experimental settings, combining data from the accelerometer, gyroscope and magnetometer it was possible to detect movements and differentiate between different type of motions both in a woody dowel and in the cobble under different initial conditions. Data were analysed to understand which type of information could be retrieved. This gives important insights for the assessment of the feasibility and effectiveness of the use of smart sensors in the detection of movements in woody logs within dams and boulders embedded in landslides, thus providing indications for the development of early warning systems using this innovative technology

The Influence of Particle Concentration on the Formation of Settling-Driven Gravitational Instabilities at the Base of Volcanic Clouds

Settling-driven gravitational instabilities observed at the base of volcanic ash clouds have the potential to play a substantial role in volcanic ash sedimentation. They originate from a narrow, gravitationally unstable region called a Particle Boundary Layer (PBL) that forms at the lower cloud-atmosphere interface and generates downward-moving ash fingers that enhance the ash sedimentation rate. We use scaled laboratory experiments in combination with particle imaging and Planar Laser Induced Fluorescence (PLIF) techniques to investigate the effect of particle concentration on PBL and finger formation. Results show that, as particles settle across an initial density interface and are incorporated within the dense underlying fluid, the PBL grows below the interface as a narrow region of small excess density. This detaches upon reaching a critical thickness, that scales with (ν2/g′)1/3, where ν is the kinematic viscosity and g′ is the reduced gravity of the PBL, leading to the formation of fingers. During this process, the fluid above and below the interface remains poorly mixed, with only small quantities of the upper fluid phase being injected through fingers. In addition, our measurements confirm previous findings over a wider set of initial conditions that show that both the number of fingers and their velocity increase with particle concentration. We also quantify how the vertical particle mass flux below the particle suspension evolves with time and with the particle concentration. Finally, we identify a dimensionless number that depends on the measurable cloud mass-loading and thickness, which can be used to assess the potential for settling-driven gravitational instabilities to form. Our results suggest that fingers from volcanic clouds characterised by high ash concentrations not only are more likely to develop, but they are also expected to form more quickly and propagate at higher velocities than fingers associated with ash-poor clouds.</jats:p

The role of gravitational instabilities in deposition of volcanic ash

Volcanic ash is a significant hazard for areas close to volcanoes and for aviation. Gravitational instabilities forming at the bottom of spreading volcanic clouds have been observed in many explosive eruptions. Here we present the first quantitative description of the dynamics of such instabilities, and correlate this with the characteristics of the fall deposit from observations of the 4 May 2010 Eyjafjallajökull (Iceland) eruption. Gravitational instabilities initially took the form of downward-propagating fingers that formed continuously at the base of the cloud, and appeared to be advected passively at the crosswind speed. Measurements of finger propagation are consistent with initial conditions inferred from previous studies of ash cloud dynamics. Dedicated laboratory analogue experiments confirmed that finger downward propagation significantly exceeded the settling speed of individual particles, demonstrating that gravitational instabilities provide a possible mechanism for enhanced sedimentation of fine ash. Our observations challenge the view that aggregation is the primary explanation of proximal fine ash sedimentation, and give direct support for the role of gravitational instabilities in providing regions of high particle concentration that can promote aggregation

Settling-driven gravitational instabilities associated with volcanic clouds: new insights from experimental investigations

Downward propagating instabilities are often observed at the bottom of volcanic plumes and clouds. These instabilities generate fingers that enhance the sedimentation of fine ash. Despite their potential influence on tephra dispersal and deposition, their dynamics is not entirely understood, undermining the accuracy of volcanic ash transport and dispersal models. Here, we present new laboratory experiments that investigate the effects of particle size, composition and concentration on finger generation and dynamics. The experimental set-up consists of a Plexiglas tank equipped with a removable plastic sheet that separates two different layers. The lower layer is a solution of water and sugar, initially denser than the upper layer, which consists of water and particles. Particles in the experiments include glass beads as well as andesitic, rhyolitic and basaltic volcanic ash. During the experiments, we removed the horizontal plastic sheet separating the two fluids. Particles were illuminated with a laser and filmed with a HD camera; particle image velocimetry (PIV) is used to analyse finger dynamics. Results show that both the number and the downward advance speed of fingers increase with particle concentration in the upper layer, while finger speed increases with particle size but is independent of particle composition. An increase in particle concentration and turbulence is estimated to take place inside the fingers, which could promote aggregation in subaerial fallout events. Finally, finger number, finger speed and particle concentration were observed to decrease with time after the formation of fingers. A similar pattern could occur in volcanic clouds when the mass supply from the eruptive vent is reduced. Observed evolution of the experiments through time also indicates that there must be a threshold of fine ash concentration and mass eruption rate below which fingers do not form; this is also confirmed by field observations.Published395V. Dinamica dei processi eruttivi e post-eruttiviJCR Journa

Il ruolo della Corte di giustizia nella definizione della politica economica e monetaria europea

Il ruolo giocato dalla Corte di giustizia nel campo della politica economica e della politica monetaria è stato importante quanto sottostimato. Le due competenze dell’Unione sono profondamente differenti - ciascuna dotata di significative peculiarità - e tuttavia strettamente interconnesse. Definire i confini incerti tra politica economica e politica monetaria è stato il ruolo più importante svolto dalla Corte in questo campo. Questo ha comportato una precisazione dei ruoli delle istituzioni chiamate ad agire - Commissione, Consiglio, Banca centrale - così come dei limiti all’indipendenza dell’istituto di emissione, ma anche della ripartizione di competenze tra Stati e Unione. Dal 2012, un’ulteriore funzione è stata esercitata dalla Corte: la verifica della legittimità dell’intervento straordinario della BCE nell’economia per gestire le crisi. L’assenza nei trattati europei di disposizioni specifiche che contemplassero un ruolo della Banca come prestatore di ultima istanza o nella gestione delle crisi economiche e finanziarie spiega bene le contestazioni dell’ultimo decennio, così come la necessità di pronunce autorevoli da parte della Suprema Corte europea. Queste si collocano a buon diritto nel solco della giurisprudenza in tema di rule of law e di garanzia del rispetto dei principi generali nell’ordinamento europeo