ALBERT

Description: Multiphase flow models represent valuable tools for the study of the complex, non-equilibrium dynamics of pyroclastic density currents. Particle sedimentation, flow stratification and rheological changes, depending on the flow regime, interaction with topographic obstacles, turbulent air entrainment, buoyancy reversal, and other complex features of pyroclastic currents can be simulated in two and three dimensions, by exploiting efficient numerical solvers and the improved computational capability of modern supercomputers. However, numerical simulations of polydisperse gas-particle mixtures are quite computationally expensive, so that their use in hazard assessment studies (where there is the need of evaluating the probability of hazardous actions over hundreds of possible scenarios) is still challenging. To this aim, a simplified integral (box) model can be used, under the appropriate hypotheses, to describe the kinematics of pyroclastic density currents over a flat topography, their scaling properties and their depositional features. In this work, multiphase flow simulations are used to evaluate integral model approximations, to calibrate its free parameters and to assess the influence of the input data on the results. Two-dimensional numerical simulations describe the generation and decoupling of a dense, basal layer (formed by progressive particle sedimentation) from the dilute transport system. In the Boussinesq regime (i.e., for solid mass fractions below about 0.1), the current Froude number (i.e., the ratio between the current inertia and buoyancy) does not strongly depend on initial conditions and it is consistent to that measured in laboratory experiments (i.e., between 1.05 and 1.2). For higher density ratios (solid mass fraction in the range 0.1–0.9) but still in a relatively dilute regime (particle volume fraction lower than 0.01), numerical simulations demonstrate that the box model is still applicable, but the Froude number depends on the reduced gravity. When the box model is opportunely calibrated with the numerical simulation results, the prediction of the flow runout is fairly accurate and the model predicts a rapid, non-linear decay of the flow kinetic energy (or dynamic pressure) with the distance from the source. The capability of PDC to overcome topographic obstacles can thus be analysed in the framework of the energy-conoid approach, in which the predicted kinetic energy of the flow front is compared with the potential energy jump associated with the elevated topography to derive a condition for blocking. Model results show that, although preferable to the energy-cone, the energy-conoid approach still has some serious limitations, mostly associated with the behaviour of the flow head. Implications of these outcomes are discussed in the context of probabilistic hazard assessment studies, in which a calibrated box model can be used as a fast pyroclastic density current emulator for Monte Carlo simulations.

Description: Pyroclastic density currents (PDCs) represent one of the most dangerous volcanic hazards for people living in proximity of explosive volcanoes. The zonation of areas potentially affected by this threat is therefore of paramount importance and is the first step needed to set up appropriate mitigation measures. Campi Flegrei (CF) caldera represents a high-risk volcano with a remarkable PDC hazard due to the frequent occurrence of this phenomenon in its eruptive history. Despite the fact that CF caldera has been the object of many studies in recent decades, the mapping of PDC hazard there remains particularly challenging due to the remarkable variability of potential vent locations and eruption scales, and the complex dynamics of PDC propagation over the caldera topography. In this study we have produced, through the application of a doubly stochastic model, quantitative background (also called long-term or baseline) probabilistic maps of PDC invasion able to incorporate some of the main sources of epistemic uncertainty that influence the models for aleatoric (physical) variability. The new method developed combines the spatial probability distribution of vent opening locations, the density distribution of PDC invasion areas, and a simplified PDC model able to describe the main effect of topography on flow propagation. Our results indicate that the entire caldera has the potential to be affected (with a mean probability of flow invasion higher than about 5%) and the central-eastern area of the caldera (i.e. Agnano-Astroni-Solfatara) has invasion probabilities above about 30% (with local peaks of mean probability of about 50% in Agnano). Significant mean probabilities (up to values of about 10%) are also computed in some areas outside the caldera borders. Our findings are quite robust against different assumptions about several of the main physical and numerical parameters adopted in the study. In addition to mean values of probability of PDC invasion, the study provides the estimates of the credible uncertainty ranges associated with such probabilities in relation to some key sources of epistemic uncertainty. From our analysis, uncertainty spreads on invasion probabilities inside the caldera typically range between ±15 and ±35% of the local mean value, with an average of about ±25%; wider uncertainties are found outside the caldera, with an average above ±50% and a significantly larger range of variability from place to place.

Description: Published

Description: Napoli

Description: 6V. Pericolosità vulcanica e contributi alla stima del rischio

Description: Campi Flegrei is an example of active and densely populated caldera with a very high volcanic risk associated with the occurrence of Pyroclastic Density Currents (PDCs) produced by explosive events of variable scale and vent location. The mapping of PDC hazard in such caldera setting is particularly challenging due to the complex dynamics of the flow, the large uncertainty of future vent location and the complex topography affecting the flow propagation. Nevertheless, probabilistic mapping of PDC invasion, able to account for the intrinsic uncertainties affecting the system, is needed for hazard assessment. In this study, we show the results of new field work and statistical analysis of past eruptive activity aimed at producing long-term probabilistic maps of vent opening at Campi Flegrei. The field work was focused on the structural and morphological nature of the caldera and particularly on the reconstruction of the location of past eruptive vents as well as of main faults and fissures formed in the last 15 kyrs of activity. One objective of the analysis was to incorporate into the vent opening maps the main uncertainties affecting the system. This was done by adopting appropriate density distributions of the probability of vent opening of the different areas of the caldera and by relying on expert judgement. Then, we used these maps to produce a variety of probabilistic PDC hazard maps of the Campi Flegrei area based on different invasion models and accounting for the uncertainty in vent opening and event size. Invasion models were based on simple correlations derived from field reconstruction of past events, one-dimensional models based on a linear decay of the flow energy (e.g. energy line), and simple energy decay models tuned on transient and 2D numerical simulations of the flow dynamics.

Description: Unpublished

Description: Pisa

Description: 6V. Pericolosità vulcanica e contributi alla stima del rischio

Description: Mapping of pyroclastic density currents (PDCs) hazard in caldera settings is particularly challenging due to the large uncertainty on future vent location and eruption scale as well as the complex dynamics of the flow over the irregular caldera topography. Nevertheless, probabilistic mapping of PDC invasion, able to account for the intrinsic uncertainties affecting the system, is needed for hazard assessment, particularly for highly populated regions. Campi Flegrei (CF) is a vivid example of active and densely populated caldera with a very high risk associated with the occurrence of PDCs produced by explosive events. In this presentation we show the results of new field work and mathematical modelling of past eruptive activity aimed at producing long-term probabilistic vent opening and PDC invasion maps at CF. Field work was focused on the structural and morphological features of the caldera and particularly on the reconstruction of the location of past eruptive vents as well as of the distribution of the main faults and fractures formed in the last 15 kyr of activity. One specific objective of the analysis was to incorporate into the vent opening maps the main uncertainties affecting the system by adopting appropriate density distributions and by relying on expert judgement. We then used these maps to produce, by adopting a Monte Carlo approach, a variety of probabilistic PDC hazard maps of the CF area based on different invasion models and accounting for the uncertainty on vent location and event scale. In particular we developed and adopted a simplified invasion flow model based on the so-called box model approximation and tuned on transient and 2D numerical simulations of the flow dynamics. The new model allowed to describe the exponential decay of the flow energy as well as to account for first-order topographic effects. The developed methodology appears able to provide relatively quick and robust probabilistic assessments of PDC hazard in caldera settings and could be potentially extended to other calderas worldwide.

Description: Published

Description: San Francisco (CA)

Description: 6V. Pericolosità vulcanica e contributi alla stima del rischio

Description: Campi Flegrei is an example of active and densely urbanized caldera with a very high risk associated with the occurrence of pyroclastic density currents (PDCs) produced by explosive events of variable scale and vent location. The mapping of PDC hazard in such a caldera setting is particularly challenging not only due to the complex dynamics of the flow but also due to the large uncertainty on future vent location and the complex topography affecting the flow propagation. Nevertheless, probabilistic mapping of PDC invasion, able to account for the intrinsic uncertainties affecting the system, is needed for hazard assessment. In this study we present a variety of probabilistic PDC hazard maps of the Campi Flegrei area based on different invasion models and accounting for the uncertainty in vent opening and event size. Invasion models were based on simple empirical correlations derived by field reconstruction of past events, simplified one-dimensional models based on a linear decay of the flow energy (e.g. energy line), and correlations derived from 2D and transient numerical simulations of the flow dynamics. Field data referred mostly to the third epoch of activity of the volcano (i.e. last 5 kyr) although the analysis was extended to the last 15 kyr. In addition to the uncertainty affecting the vent location the probability invasion maps illustrate some of the uncertainties and features affecting the invasion models adopted. Results show that, consistently with field evidences, the central-eastern part of the caldera (i.e. Agnano-Astroni) is the area most exposed to flow invasion whereas values up to about 5-10% are estimated in some limited areas outside the caldera (e.g. Posillipo Hill). This latter result appears consistent with the outcomes of 3D transient simulations of large-scale PDCs when vent location was assumed in the eastern area of the caldera.

Description: Published

Description: Kagoshima (Japan)

Description: 6V. Pericolosità vulcanica e contributi alla stima del rischio

Description: Campi Flegrei (CF) is an example of an active, densely populated, caldera with very high risks associated with the occurrence of explosive eruptions. In particular, mapping of pyroclastic density currents (PDCs) hazard is challenging due to the large uncertainty on future vent location and eruption scale as well as the complex dynamics of flows over caldera topography. In this presentation we show how volcanological datasets of different type, mathematical modelling and expert elicitation techniques have been used to produce base-rate probabilistic vent opening and PDC inundation maps. The analysis particularly focused on the reconstruction of the location of past eruptive vents and it allowed the incorporation of additional volcanological datasets, such as the distribution of faults and surface fractures assumed to be representative of areas of crustal weaknesses in the caldera. One key objective was to directly incorporate some of the main sources of epistemic uncertainty relating to an understanding of the volcanic system. We used a formal and structured expert elicitation procedure to quantify uncertainties for the main parameters and evaluate the outcomes through different expert weighting models. A set of probabilistic PDC inundation hazard maps were then produced by the Monte Carlo approach based on a simplified inundation model and incorporating uncertainties on future vent location and event scale.

Description: Published

Description: Roma

Description: 6V. Pericolosità vulcanica e contributi alla stima del rischio

Description: Multiphase flow models represent valuable tools for the study of the complex, non-equilibrium dynamics of pyroclastic density currents. Particle sedimentation, flow stratification and rheological changes, depending on the flow regime, interaction with topographic obstacles, turbulent air entrainment, buoyancy reversal, and other complex features of pyroclastic currents can be simulated in two and three dimensions, by exploiting efficient numerical solvers and the improved computational capability of modern supercomputers. However, numerical simulations of polydisperse gas-particle mixtures are quite computationally expensive, so that their use in hazard assessment studies (where there is the need of evaluating the probability of hazardous actions over hundreds of possible scenarios) is still challenging. To this aim, a simplified integral (box) model can be used, under the appropriate hypotheses, to describe the kinematics of pyroclastic density currents over a flat topography, their scaling properties and their depositional features. In this work, multiphase flow simulations are used to evaluate integral model approximations, to calibrate its free parameters and to assess the influence of the input data on the results. Two-dimensional numerical simulations describe the generation and decoupling of a dense, basal layer (formed by progressive particle sedimentation) from the dilute transport system. In the Boussinesq regime (i.e., for solid mass fractions below about 0.1), the current Froude number (i.e., the ratio between the current inertia and buoyancy) does not strongly depend on initial conditions and it is consistent to that measured in laboratory experiments (i.e., between 1.05 and 1.2). For higher density ratios (solid mass fraction in the range 0.1–0.9) but still in a relatively dilute regime (particle volume fraction lower than 0.01), numerical simulations demonstrate that the box model is still applicable, but the Froude number depends on the reduced gravity. When the box model is opportunely calibrated with the numerical simulation results, the prediction of the flow runout is fairly accurate and the model predicts a rapid, non-linear decay of the flow kinetic energy (or dynamic pressure) with the distance from the source. The capability of PDC to overcome topographic obstacles can thus be analysed in the framework of the energy-conoid approach, in which the predicted kinetic energy of the flow front is compared with the potential energy jump associated with the elevated topography to derive a condition for blocking. Model results show that, although preferable to the energy-cone, the energy-conoid approach still has some serious limitations, mostly associated with the behaviour of the flow head. Implications of these outcomes are discussed in the context of probabilistic hazard assessment studies, in which a calibrated box model can be used as a fast pyroclastic density current emulator for Monte Carlo simulations.

Description: Published

Description: 257-272

Description: 5V. Dinamica dei processi eruttivi e post-eruttivi

Description: JCR Journal