Synthesizing Field and Experimental Observations to Investigate the Behavior of Pyroclastic Density Currents

One of the major hazards associated with volcanic eruptions are pyroclastic density currents (PDCs), which are fast-moving volcanic avalanches consisting of ash, boulders, and gas. Because of their unpredictability, studying PDCs in real time is dangerous and difficult. Therefore, we investigate the deposits produced by PDCs and use granular flow experiments to simulate PDCs in the laboratory. The experimental results allow us to understand sediment transport and erosional processes at small scales, and then we can extrapolate those results to natural PDCs. By better understanding what controls PDC behavior, we hope to ultimately improve risk assessment for these dangerous flows

Influence of particle density on flow behavior and deposit architecture of concentrated pyroclastic density currents over a break in slope: Insights from laboratory experiments

Geological granular flows are highly complex, gravity-driven phenomena whose different behaviors depend on the mechanical properties, density and granulometric distributions of the constituent materials. Years of research have produced significant advances in understanding transport and deposition processes in granular flows. However, the role and effects of clast densities and density contrast in a granular flow are still not fully understood. In this paper we show the effect that pumice has on dry granular flows; specifically on flow velocity and longitudinal segregation of the deposits. Our work confirms, by experimental results, field observations on pumice/lithic segregation and longer pumice runout. We report results of velocity decay and deposit architecture for a granular flow passing over a break in slope (from 38° to 4° inclination). The 30 experimental runs were carried out in a five-meter long laboratory flume equipped with a series of sensors that include laser gates and high-speed cameras (400 fps). We used two polydisperse mixtures of dacitic lithics and rhyolitic pumice in varying amounts, with Weibull and Gaussian particle size distributions. The pumice/lithic ratio changes the flow response passing over a break in slope. This effect is particularly evident starting from 10% of pumice volume into the flow mixture, independently of its granulometric distribution. Runout relates to mass following a power law, with an exponent close 0.2. The experiments confirm that pumice segregation affects polydispersed mixtures, similarly to what has been observed in real field deposits, where density decoupling produces lithic-enriched proximal areas and pumice-enriched distal areas. The results obtained prove that the presence of low-density materials in a dense granular flow has a strong influence on its behavior

Reconstrucci\uf3n del evento eruptivo asociado al emplazamiento del flujo pirocl\ue1stico El Refugio hace 13 ka, volc\ue1n Nevado de Toluca (M\ue9xico)

The Nevado de Toluca is a quiescent volcano located in the central sector of the Trans-Mexican Volcanic Belt, 80 km southwest of Mexico City. The activity began ca. 2.6 Ma ago, with andesitic to dacitic lava flows and domes that lasted until 1.15 Ma. During the last 42 ka, the volcano has been characterized by different eruptive styles, including fi ve dome collapses dated at 37, 32, 28, 26, and 13 ka and five plinian eruptions at 42 ka, 36 ka, 21.7 ka, 12.1 ka and 10.5 ka. The 13 ka dome destruction is the youngest event of this type, and originated a 0.11 km3 block-andash flow deposit on the northeastern sector of the volcano, here named El Refugio flow. The deposit consists of two facies: channel-like, up to 10 m thick, monolithologic, that is composed of up to fi ve units, with decimetric dacitic clasts set in a sandy matrix; and a lateral facies that consists of a gray, sandy horizon,up to 4 m thick. A 30 cm-thick surge layer lies down at the base of the sequence. The main component is a dacitic lava, with variable degree of vesciculation, with mineral association of Pl-Hbl-Opx. Stratigraphic and petrographic features indicate that the dome was quickly extruded on the summit of the volcano, and its collapse was accompanied by an explosive component. The magmatic process that probably triggered the eruption was an overheating of the magma chamber that induced a self-mixing mechanism yielding to an overpressurization of the system. Finally, the identifi cation of an explosive component associated with dome destruction events at Nevado de Toluca volcano clearly indicate the high risk that a future event with such characteristics can represent for populated areas around the volcano

The 17 July 1999 Block-And-Ash Flow (BAF) at Colima Volcano: new insights on volcanic granular flows from textural analysis

On July 17 1999, a strong explosion occurred at Colima Volcano (Mexico) that produced a 10 km high eruptive column. The partial column collapse originated a block-and-ash flow (BAF) that flowed to the south, along the San Antonio and Montegrande ravines, travelling 3.3 km from the volcano summit. The flow filled the ravines with a volume estimated at 7.9 x 10(5) m(3). The erosion of these deposits occurred between 1999 and 2002 (time of sampling), providing excellent longitudinal outcrops that allowed their detailed textural study. The study was carried out by means of quantitative textural analysis: (1) Rosiwal intersections, for carrying out vertical granulometric profiles; (2) total grain-size analysis, from -11 to +9 phi; and (3) Fourier and fractal analysis of the particle morphology. Grain size and morphometric parameters obtained with these methods were used to identify vertical and longitudinal variation patterns in the BAF deposit. The grain size variations allowed to infer the main particle segregation mechanisms that acted during transport and deposition of the studied BAFs. The two methods used for studying the particle shape morphologies yielded results with different accuracy and reliability. In particular, fractal analyses have been found to be the most effective in describing the particle support mechanisms that acted during transport and deposition of the studied BAFs. The results highlight the importance of the information obtained by means of these techniques, and provide new insights in transportation and deposition mechanisms of BAFs

Pyroclastic density currents: state of the art and perspectives

Pyroclastic density currents (PDCs) are mixtures of twocomponents, namely solid particles and fluid (gas) phase. Theymacroscopically behave as dense,multiphase gravity currents (flowing pyroclastic mixtures of particles and gas) immersed in a less dense, almost isotropic fluid (the atmosphere). As for other natural phenomena, their study needs a multidisciplinary approach consisting of direct observations, analysis of the associated deposits, replication through laboratory experiments, and numerical simulations. This review deals with the description of the current state of the art of PDC physics, and combines analysis of data from various methodologies. All of the above-mentioned approaches have provided significant contributions to advancing the state of the art; in particular, laboratory experiments and numerical simulations deserve a special mention here for their tumultuous growth in recent years. A paragraph of the review is dedicated to the puzzling behaviour of large-scale ignimbrites,which are (fortunately) too rare to be directly observed; they cannot be easily reproduced through laboratory experiments, or investigated by means of numerical simulations. The final part is dedicated to a summary of the whole discussion, and to a comment on some perspectives for future developments of PDC studies

Investigating the effect of polydispersity on the dynamics of multiphase flows using computational fluid dynamics tools

Granular flows consist of discrete macroscopic particles. If they are non-cohesive, their status is determined by the interaction of particle-particle frictional forces, external boundaries and gravity. In particular, the understanding of the transport mechanisms of granular materials is of paramount importance for the characterization of volcanic granular flows and for hazard assessments associated with these flows. In order to investigate dynamics of these kinds of flows, we replicated large-scale experiments with multiphase computational fluid dynamic (CFD) simulations using the Two-Fluid Model approach, with an emphasis on the polydispersity effect on the flow behaviour. The CFD simulations were run using the software MFIX. The present work consists of: 1) investigations on the drag force relationships implemented in MFIX; 2) applications of MFIX to replicate largescale experiments on volcanic dry granular flows sliding on an inclined channel; 3) comparisons between experimental and simulated data with particular emphasis on the velocity of the granular flow front. Simulations on polydisperse granular flows demonstrated the simulated flows capability to replicate segregation dynamics active in real granular flows, and the polydispersity effects on velocities and shapes of granular flows. The nonuniformity of solid phases highly affects the dynamic of the whole flow and results in a better agreement between simulated and experimental flow velocities than the simplest monodisperse particles systems. In particular, the greater the number of the solid phases, the lower the velocity of granular flows and the mean square error, which decreases by ca. 50%

Flow Connectivity in Active Volcanic Areas: Use of Index of Connectivity in the Assessment of Lateral Flow Contribution to Main Streams

Connectivity analysis is an important geomorphological and hydrologic tool that can be used to identify spaces that are prone to removal of primary sediments which are eventually assimilated into granular flows and related epiclastic processes. Studies of connectivity have been made in various areas, but to date, not in active volcanic areas, where such studies can be very useful due to the constant presence of loose volcanic material easily removed by epiclastic processes. Mobilization of loose pyroclastic sediments can trigger phenomena such as lahars, which are among the most dangerous in nature. In this study, the index of connectivity (IC) (Borselli et al. 2008) was calculated by obtaining a weight coefficient (W) combining two elements: the coefficient C related to the universal soil loss equation and the revised universal soil loss equation (USLE-RUSLE) proposed by Borselli et al. (2008) for areas with vegetation cover or crops, and the roughness index (RI) developed by Cavalli et al. (2013), which characterizes bare soil areas. Combining both methods, we propose a new joint index of connectivity (ICJ) that does not overestimate the degree of connectivity in bare areas, while areas with vegetation cover are characterized based on their wellrecognized hydrologic impedance properties. This methodology may enable better characterization in highly dynamic environments such as active volcanic areas. We also propose a new lateral hydrological efficiency index (LHEI) that increases the ability to identify watersheds that supply major amounts of sediment to main streams in ravines. The application of this methodology in the active volcanic area of Volcán de Colima, the most active volcano in Mexico, is of great importance, because of the constant supply of new pyroclastic material from the top of the edifice, the high dynamicity of geomorphological processes, and the widespread presence of bare soil areas consisting of loose materials easily sourced, or assimilated, into epiclastic processes