We present the late Holocene eruption history of the poorly known Doña Juana volcanic complex, in SW Colombia, which last erupted in the twentieth century. This represents a case study for potentially active volcanism in the rural Northern Andes, where tropical climate conditions and a fragmented social memory blur the record of dormant volcanoes. We reconstructed the volcanic stratigraphy of the central-summit vent area by integrating new mapping at 1:5000 scale with radiocarbon ages, sedimentology analysis, and historical chronicles. Our results revealed cyclic transitions from lava-dome growth phases and collapse to explosive Vulcanian and possibly subplinian phases. Pyroclastic density currents were generated by dome collapse producing block-and-ash flows or by pyroclastic fountain/ column collapse and were rapidly channelized into the deeply incised fluvial valleys around the volcano summit. The pyroclastic density currents were ~4–10 × 10⁶ m³ in volume and deposited under granular flow– or fluid escape–dominated depositional regimes at high clast concentrations. In places, more dilute upper portions reached a wider areal distribution that affected the inhabited areas on high depositional terraces. The coefficient of friction (ΔH/L) is higher for block-and-ash flows and dense lava–bearing fountain/low-column-collapse pyroclastic density currents compared to pumice-bearing, column-collapse pyroclastic density currents. Associated mass-wasting processes included syneruptive and intereruptive debris flows, with the last one documented in 1936 CE.

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Granular flows are a complex process, involving a wide range of grain sizes, materials, varied viscous fluids, among others. For this reason, the simulation of granular flows requires a certain level of simplification, allowing the isolated study of its governing variables and extending the global observations to field events. Here, we present the planar setup as an alternative for studying simplified processes associated to granular flows. The planar setup consists of two windows separated by a thin gap and enclosing a granular assembly. We present two examples where the planar setup is adapted for the study of the competing action of segregation and disaggregation in a fractured grain under shear flow, and for the study of the stability scenarios of a flow impacting a permeable obstacle. The close visualization of the kinematics at the particle scale provides an ideal opportunity for describing the mechanisms behind the grain disaggregation or controlling the obstacle stability. Both examples highlight the advantages of the planar setup for the study of granular flows.

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The pile response is of great importance to the effectiveness of a row of piles used as a landslide mitigation method. However, the current classifications of the pile responses and failure modes do not consider the effects of soil displacement and the relative pile rigidity. An attempt is made in this paper to improve this using a non-dimensional model based on simplified ground consisted of two layers of cohesive soils. A hyperbolic p–y model is adopted to consider non-linear soil reactions. The calculations of three case histories show that the proposed model predicts the pile responses well. The dependencies for obtaining a pile response with maximum pile resistance have been identified and the influencing factors have been examined. The efficacy of the proposed approach has been demonstrated by the satisfactory result that it achieves in reproducing the more complex three-dimensional finite-element analyses. The application of the non-dimensional solutions indicates that it can increase the efficiency of the preliminary pile design.@en

In the SW flank of the Doña Juana Volcanic Complex, Colombia, the dynamic geomorphic system responds to the complex interaction between volcanic, climatic, and tectonic driving forces, where the recent landscape (last ~20 years) is being shaped as a function of denudational processes. Despite the rapid rates of landforms development, the geomorphology of this area is poorly documented. To overcome the lack of information we mapped the area using a GeoSAR DEM and an Unmanned Aerial Vehicle based DEM. This paper presents two maps, a 1:25,000 scale map and a 1:5,000 detailed map of landforms along the Humadal Creek. Detailed categorization of landforms (at 1:5,000) allowed us to identify geomorphic processes in the village of Las Mesas and rural areas that triggered hazards for the communities. The morphologic evolution interpretation of this volcanic tropical area serves as a tool for future geohazard assessment in inhabited areas with important information gaps.@en

The granular column collapse is a simplified version of granular flows such as landslides, avalanches, and other industrial processes mobilized in air or within a fluid. In this configuration, the particles collapse in an accelerating phase, reaching a state of constant spreading velocity until they decelerate and stop. Granular flows commonly involve particles of different sizes, a property termed polydispersity. Understanding the role of polydispersity remains a challenging task that is often analyzed with nearly monodisperse systems and demanding a series of simplifications when coupled with a fluid in a numerical model. Here, we study the effect of particle-size polydispersity in dry and immersed granular columns, using a finite element method-discrete element method model for fluid-particle interactions. We show that the velocity of the column collapse and runout distance decrease with an increase in the level of polydispersity in immersed conditions, and remain nearly independent of the level of polydispersity in dry conditions. Moreover, we find that the runout scales with the spreading front kinetic energy, weighted by the ratio between the particles' density and the density difference between particles and fluid. This scaling helps in identifying the governing processes in polydisperse granular columns, unifying the runout description of both dry and immersed collapses, and indicating that the column initial packing fraction is the governing parameter.@en

We present a transdisciplinary perspective for multiple actors interested in disaster-risk-reduction strategies in inhabited volcanoes, based on the novel approach of complexity and collaborative research. Our reflection is based on an ongoing research process that puts into dialogue scientific knowledge and local knowledge from Andean campesino communities inhabiting the Doña Juana Volcano-Páramo in the Latin American tropics. These local communities have experienced historically complex social, economic, and environmental conflicts in their territories; currently, most of them live in the buffer zones of the National Natural Park Doña Juana-Cascabel, which is part of Colombia’s national system of natural protected areas. We address inhabited Andean volcanoes as complex systems, which combine multi-temporal and spatial processes that allow for the emergence of nonlinear and heterogeneous social and ecological interdependencies. By transgressing disciplinary boundaries, we see an opportunity for building horizontal dialogues with communities inhabiting active volcanoes, understanding resilience, and eventually developing situated and collaborative disaster-risk-reduction strategies. The resulting vision is proposed as a fundamental methodology in settings such as south-western Colombia, marked by convoluted social dwelling patterns, and unequal biosocial and socioeconomic histories. Detached from positivistic and victimising perspectives, we seek to build knowledge on the antecedents, inheritance, persistence, and preservation of systems, ultimately enabling response-abilities for decision-making. The motivations, knowledge, and self-organisation capacities of the volcano inhabitants bring about new possibilities of doing locally while thinking globally. The transdisciplinary research approach allows us to situate our exchanges with the local community, and envision future collaborative paths towards promoting community-oriented forms of appropriation and transformation of a volcanic lifeworld.

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Abstract: Granular flows are typically studied in laboratory flumes based on common similarity scaling, which create stress fields that only roughly approximate field conditions. The geotechnical centrifuge produces stress conditions that are closer to those observed in the field, but steady conditions can be hardly achieved. Moreover, secondary effects induced by the apparent Coriolis acceleration, which can either dilate or compress the flow, often obscure scaling. This work aims at studying a set of numerical experiments where the effects of the Coriolis acceleration are measured and analyzed. Three flow states are observed: dense, dilute, and unstable. It is found that flows generated under the influence of dilative Coriolis accelerations are likely to become unstable. Nevertheless, a steady dense flow can still be obtained if a large centrifuge is used. A parametric group is proposed to predict the insurgence of instabilities; this parameter can guide experimental designs and could help to avoid damage to the experimental apparatus and model instrumentation.

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The granular column collapse experiment is an important benchmark case for the physical and numerical study of transitional mass flows. Unlike columns of dry granular materials, the presence of a relatively incompressible fluid, such as water, in the voids of saturated columns complicates the shear behavior of the column by becoming a function of the coupled shear and volumetric behavior of the grain–fluid system. Dilative or contractive behavior at the pore level will cause a decrease or increase, respectively, in the pore fluid pressure. These changes in effective stress, in turn, will define stability or instability and length of runout. Here we use the new opportunity provided by transparent soil to observe air entry within saturated columns to explore the hypothesis that the entry pressure provides the maximum contribution of capillary pressure at incipient failure, thereby providing a quantitative control on the stability of dilative granular columns. Furthermore, the mobility of densely packed saturated columns subject to collapse was significantly influenced by air entry. An analytical model, based on this assumption of limiting capillary pressure, is able to describe the stability of the experimental columns as well as the larger dataset from the literature, reframing the previous empirical stability threshold using limit equilibrium and soil material parameters. Our results demonstrate the importance of stress-dilatancy and air-entry phenomena on the rapid shear behavior of saturated granular materials.@en

By means of the Discrete Element Method, we investigate the relationship between the power-law exponent of the grain size distribution (GSD) and the mobility of granular transitional flows. This is done with the aim of testing a recently posed hypothesis, suggesting that high power-law exponents are related to the large mobility observed in natural mass flows (Lai et al., 2017, https://doi.org/10.1002/2017gl075689), as a consequence of a lubrication mechanism in which small grains act as rotational bearings reducing the shear strength of the flowing mass. We show that both the mobility of the flow and the shear strength of the material are independent of the GSD power-law exponent, as long as the system-to-grain size ratio of the studied system is sufficiently large, as it is expected in natural granular flows. This confirms that the power-law exponent, or the proportion of small and big grains, are not plausible origins of the large mobility of granular avalanches.

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Debris flows involve the high-speed downslope motion of rocks, soil, and water. Their high flow velocity and high potential for impact loading make them one of the most hazardous types of gravitational mass flows. This study focused on the roles of particle size grading and degree of fluid saturation on impact behavior of fluidsaturated granular flows on a model rigid barrier in a small-scale flume. The use of a transparent debris-flow model and plane laser-induced fluorescence allowed the motion of particles and fluid within the medium to be examined and tracked using image processing. In this study, experiments were conducted on flows consisting of two uniform and one well-graded particle size gradings at three different fluid contents. The evolution of the velocity profiles, impact load, bed normal pressure, and fluid pore pressure for the different flows were measured and analyzed in order to gain a quantitative comparison of their behavior before, during, and after impact.

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Compaction is probably the industrial process more intensively used in road construction. The current compaction practice is based on the useful methodology proposed by Proctor (1933). This methodology intends to reproduce field compaction in the laboratory by applying a controlled mechanical energy to the soil and relating the dry density with the water content. However, recent developments in soil compaction analysis in the laboratory and field seek a better understanding of the link between the mechanical stresses applied to the soil and the strains it undergoes. This requirement needs analyzing the stresses applied to the soil by the compaction machine, the distribution of stresses within the soil layer undergoing compaction, and the behavior of the partially saturated soil. This article explores the feasibility of using a series of simplified analytical equations to model compaction within a mechanical framework. The proposed model focuses on the compaction produced by a cylinder applying a static load. The methodology combines the Hertz contact stress theory with the Fröhlich stress distribution and the Barcelona Basic Model for partially saturated soils. The results of the proposed analytical model successfully agree with the experimental observations obtained using a geotechnical centrifuge model and confirming its potential use for compaction control and monitoring.

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The granular column collapse is a simplified system of the complex dynamics observed in gravity-driven natural mass-movements (i.e., landslides, debris flows, rock avalanches) and industrial applications (i.e., pharmaceutics, concrete, and food industry). In this system, a granular column is built with an initial height and initial width and then is allowed to collapse by self-weight onto a horizontal plane, while observing the variation in runout as a function of its initial geometry. Despite its wide use in the study of mass-movements mobility, either dry or with a liquid, little is known on the internal physics during collapse and its variation when immersed in an ambient fluid. This work presents a planar setup that allows the study of fully and partially immersed granular columns, with little disturbance at release [1]. The use of a planar configuration allows the monitoring of the moving mass and its deformation patterns, providing a unique insight into the particle-fluid interactions at release and during collapse that were not possible before. These observations are of great importance for the understanding of particle-fluid interactions at a mesoscale and can shed light into larger processes like a submarine and subaerial landslides. This work addresses these interactions by varying the geometry and measuring the mobility in dry and immersed conditions. The associated deformation patterns are observed both at the column-scale and at the particle-scale, reflecting in the velocity scaling of a deformable and moving granular mass and the occasional ejection of particles at its surface. We observed that the area of the released portion decreases during collapse and converges toward an equivalent portion of surface particles with little influence by the initial column geometry. These observations validate the planar setup for the study of granular columns, provides a novel interpretation in the momentum transfer in particle-fluid systems, and sets a validation case for future numerical simulations.@en

Landslide generated tsunamis are a primary natural hazard to coastal communities and infrastructure, but the present state of knowledge is limited to the specific details of momentum transfer from the landslide to the water body for either subaerial or fully submerged conditions. Here we report a series of novel experiments and an analytical model that bridges this gap, by exploring the collapse of a granular column for a wide range of water depths. We show that the maximum seaward wave amplitude is governed by a single dimensionless ratio, the relative depth of submergence. Based on the experimental observations, we propose a continuous function that quantifies the maximum wave amplitude by considering the momentum flux from the initially vertical granular column to the initially still fluid. Predictions made using the momentum function are in good agreement with observations of the present study and with other experimental studies of granular column collapse at larger scales. The analytical model allows the prediction of maximum wave amplitude over the full range of submergence conditions from subaerial to partially submerged and fully submerged collapses, with potential applications for tsunami hazard assessment.@en

Volcanic habitats host a dynamic environment for sudden and long-lasting relationships between nature and culture, becoming an archetypal case for the study of resilient communities. In these habitats, the study of the occurring phenomena is often addressed independently and in disciplinary isolation, focusing on the uncertainty and contingency of geohazards, the abrupt and recurrent resetting of biophysical conditions due to natural disturbances, or the intrinsic repercussions on the anthropogenic memory. Under this perspective, mass-movements within a volcanic habitat can be addressed as a complex system built over various generations of interacting and interdependent human societies, ecological systems, climate and geological processes. Understanding this multivariable and multi-scalar coexistence becomes central in how mass-movements are perceived. In this work, we propose a transdisciplinary approach for the formulation and design of alternative strategies in the mitigation of mass-movements hazards, by responsibly collaborating between geoscientists, social scientists, and local actors. Mass-movement mitigation strategies rarely take into account the cultural relationship of the inhabitants with their territories and the complexity of the local knowledge and capabilities of the communities to resolve their condition [2]. This limits the effectiveness in the response capacity and resilience of communities and ecosystems to extreme events [2]. Through this research, we aim at finding ways to democratize knowledge, and change academic practices within a geoethical context, recognizing and valuing the local perspectives. In this work, we study an area within the Doña Juana-Cascabel volcanic-complex, located in SW Colombia, and focus on the processes in the vicinity to the Humadal stream and neighbouring communities. This stream is recognized as the main preoccupation of the inhabitants with the recent occurrence of mass-movements in its basin. We address this issue through a team consisting of key local social actors and researchers in anthropology, archaeology, biology, design, engineering, geology, pedagogy, and pedology. We collaborate within a Historical Ecology framework, aiming to the empowerment of sociological resilience-based decision making [3]. This work started with the site recognition, mapping the geological, biological, and social settings. In parallel, we listened and valued the local knowledge about physical geography, ecosystems, and mass-movements in an active volcanic habitat, and merge it with the scientific knowledge. Moreover, this local knowledge enlighted key aspects on the interaction between the inhabitants and the State’s agencies and governmental processes, which underlay the dynamics of any reliable policy and sustainibile process. In this particular site, we identified the organizational capacity to work on reforestation, road maintenance, and weaving as fundamental capabilities for connecting with the design, potential implementation, and sustainability of a set of potential mitigation strategies. With this case study, we invite the multiple actors involved in disaster risk reduction to find common languages beyond disciplinary boundiaries aiming to horizontalize knowledge with the local actors in risk. Through this excercise, we avoid the victimization of the communities, reduce power relationships, and empower resilience.@en

The use of the geotechnical centrifuge to obtain scaled physical models is a useful tool in geomechanics. When dealing with granular flows, however, the traditional scaling principles are challenged by the complex rheology of the material and by the non-trivial effects of the Coriolis apparent acceleration. In a laboratory centrifuge, obtaining a clear understanding of these effects is further complicated by the technical difficulties in obtaining flows in steady conditions. In this work, the scaling principles for granular flows are studied using a numerical model based on the discrete-element method. In this way it is possible to obtain a steady flow in a rotating reference frame, and to explore the variation of macroscopic properties by changing the scaling factor and the distance from the rotation centre. The outcome is compared with the prediction obtained with a continuum theory for frictional flows. Results show that granular flows scale consistently only when the Coriolis acceleration is negligible, and are severely altered otherwise. The augmented acceleration field is also responsible for an alteration of the flow state, driving the system towards the inertia-driven collisional regime.@en

In this paper, we study the flow of angular grains in a split-bottom Couette cell. Grains departing from a spherical shape result in collective flow fields that form a heap on the free surface. Here we extend on previous observations in split-bottom cells, exploring a wider range of flows within the inertial regime and finding a richness collection of behaviours. Surface height profiles and velocity profiles are accurately measured with digital image analysis. These measurements allow the characterization of the flow regimes within the cell and the heap morphology. We show that the known flow regimes in split-bottom geometries, like the universal and wall-collapsed regimes, can also be observed in moderately high inertial flows, extending the range for studying universal shear banding. The heap morphology is amplified by the flow inertia, with a partial collapse when the cell comes to a halt. Moreover, at high angular velocities, flows under low confinement will spread radially outwards, while flows under high confinement will develop localized particle ejections. Our results complement the observation of free-surface deformations of flows of nonspherical grains. These observations suggest a need for considering deformable free surface boundary conditions in the simulation of angular grains during shear, with repercussions in the characterization and prediction of natural mass flows.@en

Submerged mass flows are commonly considered a natural geohazard that interacts with terrestrial and submarine infrastructure and communities and are linked to the generation of tsunamis. Previous studies have investigated submerged mass flows by means of the benchmark case of a granular column collapse. This study explores the mobility and collapse dynamics of submerged granular columns within a planar configuration as a function of the column aspect ratio a. The use of a planar configuration allows the monitoring of the moving mass and its deformation patterns and provides a novel insight into the particle-fluid interactions at release and during collapse not possible before. Special attention was devoted to the column saturation and release mechanism. The column mobility is observed to be directly controlled by a and is linked to a clear distinction between triangular and trapezoidal deposits. The column collapse is found to be described by two characteristic times, marking transition points through the collapse stages. The associated deformation patterns reflect both at a column scale and at localized particle groups during collapse, reflecting in the velocity scaling of a deformable and moving granular mass and the occasional ejection of particles at its surface. We observed that the area of the released portion decreases during collapse and converge toward an equivalent portion of surface particles with little influence by a. The experimental results provide a novel insight into the study of submerged mass flows and set a reference behavior for future numerical validations.@en

On the night of March 31, 2017, a series of mass movements took place in the Mocoa Basin and devastated the northwestern part of its urban area. The mass movements were triggered by a 4-day rainfall accumulation and high rainfall intensities on the night of the event. After the trigger, about 420 mass movements transported material as landslides, debris flows, and hyperconcentrated flows along the Taruquita and Taruca creeks and Sangoyaco River on the northern side, and as landslides and mudflows along the Mulato River on its southern side, resulting in major damages to both its population and infrastructure. Previous reports by Cheng et al. (2018) and García-Delgado et al. (2019) described the event from a hydrological and geological perspective, connecting them with the damage caused by the event, but leaving aside an analysis of the series of mass movements and its characteristics. This work presents an extended review of these mass movements and studies them via a statistical analysis, providing a detailed review of the event’s characteristics and their relationship with the resultant damage. These characteristics are compared with the existing hazard maps and they provide valuable information and awareness on hazard management for communities settled in the northern Andes.@en

The column collapse experiment is a simplified version of the complex granular flows observed in both natural and industrial contexts. Due to its simple setup and rich behavior, the column collapse has been studied experimentally and numerically by several authors. The purpose of the analyses presented in this paper is to verify whether some of the results presented in these publications are affected by grain-size effects. In order to do so, we simulate two-dimensional granular columns by means of a discrete-element method, i.e., contact dynamics. Specifically, we study the influence of the grain size, as compared to the system size, on the resultant deposit geometry and collapse duration. We show that (i) some of the previously published results may be affected by grain-size effects, (ii) in order to avoid these effects, the system-size to grain-size ratio must be larger than 75 for short columns and larger than 50 for tall columns, and (iii) the quantities that are the most affected by grain-size effects are the column mobility and the collapse duration. Our findings serve as a tool for comparing results obtained by different researchers and draw guidelines on the number of grains that must be used in order to avoid grain-size effects.@en

The granular column collapse is a simplified system of the complex dynamics observed in gravity-driven natural mass flows and granular media manufacture systems. The granular column collapse has become a benchmark case for the study of granular materials under transitional flow regimes. In the current paper, we study the granular collapse of ceramic beads in a fully two-dimensional experimental set-up with the inclusion of a novel gate mechanism, avoiding any disturbance on the granular media at release. The system is backlighted by a high-intensity LED panel, allowing a clear distinction between beads and their surroundings. The collapse runout and kinematics are studied by means of digital image analysis, combining an edge-detection technique with the particle image velocimetry method. The column mobility is studied as a function of the initial aspect ratio a=H0/R0, with H0 and R0 as the initial column height and width, respectively. Results are in agreement with previous experimental studies, recognizing the influence of a on the final column deposit and validating the experimental set-up for further studies.@en