Influence of fluctuating supply on the emplacement dynamics of channelized lava flows

The evolution of lava flows emplaced on Mount Etna (Italy) in September 2004 is examined in detail through the analysis ofmorphometricmeasurements of flow units. The growth of the main channelized flow is consistent with a layering of lava blankets, which maintains the initial geometry of the channel (although levees are widened and raised), and is here explicitly related to the repeated overflow of lava pulses. A simple analytical model is introduced describing the evolution of the flow level in a channelized flow unit fed by a fluctuating supply. The model, named FLOWPULSE, shows that a fluctuation in the velocity of lava extrusion at the vent triggers the formation of pulses, which become increasingly high the farther they are from the vent, and are invariably destined to overflow within a given distance. The FLOWPULSE simulations are in accordance with the observed morphology, characterized by a very flat initial profile followed by a massive increase in flow unit cross-section area between 600 and 700 m downflow. The modeled emplacement dynamics provides also an explanation for the observed substantial “loss” of the original flowing mass with increasing distance from the vent

Reconstructing eruptive source parameters from tephra deposit: a numerical study of medium-sized explosive eruptions at Etna volcano

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Uncertainty quantification and sensitivity analysis of volcanic columns models: results from the integral model PLUME-MoM

The behavior of plumes associated with explosive volcanic eruptions is complex and dependent on eruptive source parameters (e.g. exit velocity, gas fraction, temperature and grain-size distribution). It is also well known that the atmospheric environment interacts with volcanic plumes produced by explosive eruptions in a number of ways. The wind field can bend the plume but also affect atmospheric air entrainment into the column, enhancing its buoyancy and in some cases, preventing column collapse. In recent years, several numerical simulation tools and observational systems have investigated the action of eruption parameters and wind field on volcanic column height and column trajectory, revealing an important influence of these variables on plume behavior. In this study, we assess these dependencies using the integral model PLUME-MoM, whereby the continuous polydispersity of pyroclastic particles is described using a quadrature-based moment method, an innovative approach in volcanology well-suited for the description of the multiphase nature of magmatic mixtures. Application of formalized uncertainty quantification and sensitivity analysis techniques enables statistical exploration of the model, providing information on the extent to which uncertainty in the input or model parameters propagates to model output uncertainty. In particular, in the framework of the IAVCEI Commission on tephra hazard modeling inter-comparison study, PLUME-MoM is used to investigate the parameters exerting a major control on plume height, applying it to a weak plume scenario based on 26 January 2011 Shinmoe-dake eruptive conditions and a strong plume scenario based on the climatic phase of the 15 June 1991 Pinatubo eruption

Insights into the formation and dynamics of coignimbrite plumes from one-dimensional models

Coignimbrite plumes provide a common and effective mechanism by which large volumes of fine-grained ash are injected into the atmosphere. Nevertheless, controls on formation of these plumes as a function of eruptive conditions are still poorly constrained. Herein, two 1-D axysymmetric steady state models were coupled, the first describing the parent pyroclastic density current and the second describing plume rise. Global sensitivity analysis is applied to investigate controls on coignimbrite plume formation and describe coignimbrite source and the maximum plume height attained. For a range of initial mass flow rates between 108 and 1010 kg/s, modeled liftoff distance (the distance at which neutral buoyancy is attained), assuming radial supercritical flow, is controlled by the initial flow radius, gas mass fraction, flow thickness, and temperature. The predicted decrease in median grain size between flow initiation and plume liftoff is negligible. Calculated initial plume vertical velocities, assuming uniform liftoff velocity over the pyroclastic density current invasion area, are much greater (several tens of m/s) than those previously used in modeling coignimbrite plumes (1 m/s). Such velocities are inconsistent with the fine grain size of particles lofted into coignimbrite plumes, highlighting an unavailability of large clasts, possibly due to particle segregation within the flow, prior to plume formation. Source radius and initial vertical velocity have the largest effect on maximum plume height, closely followed by initial temperature. Modeled plume heights are between 25 and 47 km, comparable with Plinian eruption columns, highlighting the potential of such events for distributing fine-grained ash over significant areas

IMEX_SfloW2D 1.0: a depth-averaged numerical flow model for pyroclastic avalanches

Abstract. Pyroclastic avalanches are a type of granular flow generated at active volcanoes by different mechanisms, including the collapse of steep pyroclastic deposits (e.g., scoria and ash cones), fountaining during moderately explosive eruptions, and crumbling and gravitational collapse of lava domes. They represent end-members of gravity-driven pyroclastic flows characterized by relatively small volumes (less than about 1 Mm3) and relatively thin (1–10 m) layers at high particle concentration (10–50 vol %), manifesting strong topographic control. The simulation of their dynamics and mapping of their hazards pose several different problems to researchers and practitioners, mostly due to the complex and still poorly understood rheology of the polydisperse granular mixture and to the interaction with the complex natural three-dimensional topography, which often causes rapid rheological changes. In this paper, we present IMEX_SfloW2D, a depth-averaged flow model describing the granular mixture as a single-phase granular fluid. The model is formulated in absolute Cartesian coordinates (whereby the fluid flow equations are integrated along the direction of gravity) and can be solved over a topography described by a digital elevation model. The numerical discretization and solution algorithms are formulated to allow for a robust description of wet–dry conditions (thus allowing us to accurately track the front propagation) and an implicit solution to the nonlinear friction terms. Owing to these features, the model is able to reproduce steady solutions, such as the triggering and stopping phases of the flow, without the need for empirical conditions. Benchmark cases are discussed to verify the numerical code implementation and to demonstrate the main features of the new model. A preliminary application to the simulation of the 11 February pyroclastic avalanche at the Etna volcano (Italy) is finally presented. In the present formulation, a simple semi-empirical friction model (Voellmy–Salm rheology) is implemented. However, the modular structure of the code facilitates the implementation of more specific and calibrated rheological models for pyroclastic avalanches

Grain size distribution uncertainty quantification in volcanic ash dispersal and deposition from weak plumes

We present the results of uncertainty quantification and sensitivity analysis applied to volcanic ash dispersal from weak plumes with focus on the uncertainties associated to the original grain size distribution of the mixture. The Lagrangian particle model Lagrangian Particles Advection Code is used to simulate the transport of inertial particles under the action of realistic atmospheric conditions. The particle motion equations are derived by expressing the particle acceleration as the sum of forces acting along its trajectory, with the drag force calculated as a function of particle diameter, density, shape, and Reynolds number. Simulations are representative of a weak plume event of Mount Etna (Italy) and aimed at quantifying the effect on the dispersal process of the uncertainty in the mean and standard deviation of a lognormal function describing the initial grain size distribution and in particle sphericity. In order to analyze the sensitivity of particle dispersal to these uncertain variables with a reasonable number of simulations, response surfaces in the parameter space are built by using the generalized polynomial chaos expansion technique. The mean diameter and standard deviation of particle size distribution, and their probability density functions, at various distances from the source, both airborne and on ground, are quantified. Results highlight that uncertainty ranges in these quantities are drastically reduced with distance from source, making them largely dependent just on the location. Moreover, at a given distance from source, the distribution is mostly controlled by particle sphericity, particularly on the ground, whereas in air also mean diameter and sorting play a main role

IMEX_SfloW2D v2: a depth-averaged numerical flow model for volcanic gas–particle flows over complex topographies and water

We present developments to the physical model and the open-source numerical code IMEX_SfloW2D (de' Michieli Vitturi et al., 2019). These developments consist of a generalization of the depth-averaged (shallow-water) fluid equations to describe a polydisperse fluid–solid mixture, including terms for sedimentation and entrainment, transport equations for solid particles of different sizes, transport equations for different components of the carrier phase, and an equation for temperature/energy. Of relevance for the simulation of volcanic mass flows, vaporization and entrainment of water are implemented in the new model. The model can be easily adapted to simulate a wide range of volcanic mass flows (pyroclastic avalanches, lahars, pyroclastic surges), and here we present its application to transient dilute pyroclastic density currents (PDCs). The numerical algorithm and the code have been improved to allow for simulation of sub- to supercritical regimes and to simplify the setting of initial and boundary conditions. The code is open-source. The results of synthetic numerical benchmarks demonstrate the robustness of the numerical code in simulating transcritical flows interacting with the topography. Moreover, they highlight the importance of simulating transient in comparison to steady-state flows and flows in 2D versus 1D. Finally, we demonstrate the model capabilities to simulate a complex natural case involving the propagation of PDCs over the sea surface and across topographic obstacles, through application to Krakatau volcano, showing the relevance, at a large scale, of non-linear fluid dynamic features, such as hydraulic jumps and von Kármán vortices, to flow conditions such as velocity and runout

Evolution of Conduit Geometry and Eruptive Parameters During Effusive Events

The dynamics of effusive events is controlled by the interplay between conduit geometry and source conditions. Dyke-like geometries have been traditionally assumed for describing conduits during effusive eruptions, but their depth-dependent and temporal modifications are largely unknown. We present a novel model which describes the evolution of conduit geometry during effusive eruptions by using a quasi steady state approach based on a 1-D conduit model and appropriate criteria for describing fluid shear stress and elastic deformation. This approach provides time-dependent trends for effusion rate, conduit geometry, exit velocity, and gas flow. Fluid shear stress leads to upward widening conduits, whereas elastic deformation becomes relevant only during final phases of effusive eruptions. Simulations can reproduce different trends of effusion rate, showing the effect of magma source conditions and country rock properties on the eruptive dynamics. This model can be potentially applied for data inversion in order to study specific case studies

Book of Abstracts

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