r.avaflow represents an innovative open-source computational tool for routing rapid mass flows, avalanches, or process chains from a defined release area down an arbitrary topography to a deposition ...area. In contrast to most existing computational tools, r.avaflow (i) employs a two-phase, interacting solid and fluid mixture model (Pudasaini, 2012); (ii) is suitable for modelling more or less complex process chains and interactions; (iii) explicitly considers both entrainment and stopping with deposition, i.e. the change of the basal topography; (iv) allows for the definition of multiple release masses, and/or hydrographs; and (v) serves with built-in functionalities for validation, parameter optimization, and sensitivity analysis. r.avaflow is freely available as a raster module of the GRASS GIS software, employing the programming languages Python and C along with the statistical software R. We exemplify the functionalities of r.avaflow by means of two sets of computational experiments: (1) generic process chains consisting in bulk mass and hydrograph release into a reservoir with entrainment of the dam and impact downstream; (2) the prehistoric Acheron rock avalanche, New Zealand. The simulation results are generally plausible for (1) and, after the optimization of two key parameters, reasonably in line with the corresponding observations for (2). However, we identify some potential to enhance the analytic and numerical concepts. Further, thorough parameter studies will be necessary in order to make r.avaflow fit for reliable forward simulations of possible future mass flow events.
•We proposed a novel, process-based, mechanical erosion/deposition model for two-phase mass flows•Mechanically consistent erosion-rate models are derived that incorporate momentum/rheology changes ...across flow-bed interface•Inclusion of mass/momentum production is essential for physically correct & mathematically consistent description of erosion•Reduced friction is equivalent to momentum production, we solved long-standing dilemma - erosion enhances mass flow mobility•New model adequately describes complex erosion phenomena in landslide, avalanche and debris flow
Erosion, entrainment and deposition are complex and dominant, but yet poorly understood, mechanical processes in geophysical mass flows. Here, we propose a novel and process-based two-phase erosion-deposition model capable of adequately describing these complex phenomena commonly observed in landslides, avalanches, debris flows and bedload transports. The model is based on the jump in the momentum flux including changes of material and flow properties along the flow-bed interface and enhances an existing general two-phase mass flow model (“Pudasaini S.P., 2012, A general two-phase debris flow model, Journal of Geophysical Research, 117, F03010, doi:10.1029/2011JF002186”). A two-phase variably saturated erodible basal morphology is introduced which allows for the evolution of erosion-deposition-depths, incorporating the inherent physical process including momentum and rheological changes of the flowing mixture. By rigorous derivation, we show that appropriate incorporation of the mass and momentum productions or losses in conservative model formulation is essential for the physically correct and mathematically consistent description of erosion-entrainment-deposition processes. We show that mechanically deposition is the reversed process of erosion. We derive mechanically consistent closures for coefficients emerging in the erosion-rate models. We prove that effectively reduced friction in erosion is equivalent to the momentum production. With this, we solve the long standing dilemma of mass mobility, and show that erosion enhances the mass flow mobility. The novel enhanced real two-phase model reveals some major aspects of the mechanics associated with erosion, entrainment and deposition. The model appropriately captures the emergence and propagation of complex frontal surge dynamics associated with the frontal-drag with erosion.
•We propose a new multi-directional separation-flux mechanism leading to strong phase separation in debris flows.•This addresses a long-standing challenge of understanding physics of phase separation ...between solid and fluid.•Separation-flux includes dominant physical/mechanical aspects of mixture flow; relative velocity induced by effective forces is key in triggering phase separation.•Separation-flux describes dynamically evolving phase separation in multi-phase, geometrically 3D debris flow.•This results in solid-rich, mechanically strong frontal surge & lateral levees followed by a weaker tail consisting of viscous fluid.
Understanding the physics of phase separation between solid and fluid phases as a mixture mass moves down-slope is a long-standing challenge. Here, we propose an extension of a two phase mass flow model (“Pudasaini (2012), A general two-phase debris flow model, Journal of Geophysical Research, 117, F03010, doi:10.1029/2011JF002186”) by including a new mechanism, called separation-flux, that leads to strong phase separation in avalanche and debris flows while balancing the enhanced solid flux with the reduced fluid flux. The relative velocity between the phases is key in triggering the phase separation mechanism, which is induced by the effective forces that appear in the system. The novel separation-flux can be written as a product of the separation-rate, solid and fluid volume fractions, and the flow depth which amplify the separation-flux. Its magnitude is further controlled by the separation-rate-intensities which are functions of volume fractions and the density ratio. The separation-fluxes are multi-directional. One of the most important characteristics of the separation-flux is that phase separation ceases as soon as one of the components in the mixture vanishes. As the solid density approaches the fluid density, the phase separation intensity is reduced. Furthermore, as the drag increases, the phase separation decreases. The separation velocity emerges from the separation-flux as a function of the relative phase velocity, volume fraction of solid or fluid, and the respective separation-rate-intensity. The separation-rate takes into account different dominant physical and mechanical aspects of the mixture flow, such as the hydraulic pressure gradients, topography induced pressure gradients, the gradients of the volume fractions of solid and fluid phases, flow depths, grain size, densities, friction, viscosities, and buoyancy. The separation-flux mechanism is capable of describing the dynamically evolving phase separation and levee formation in a multi-phase, geometrically three-dimensional debris flow. These are often observed phenomena in natural debris flows and industrial processes that involve the transportation of particulate solid-fluid mixture material. Due to the inherent separation mechanism, as the mass moves down-slope, more and more solid particles are transported to the front and the sides, resulting in solid-rich and mechanically strong frontal surge head, and lateral levees followed by a weaker tail largely consisting of viscous fluid. The primary frontal solid-rich surge head followed by secondary fluid-rich surges is the consequence of phase separation. Such typical and dominant phase separation phenomena are revealed for two-phase debris flow simulations. Finally, changes in flow composition, that are explicitly considered by the new modelling approach, result in significant changes of impact pressure estimates. These are highly important in hazard assessment and mitigation planning and highlight the application potential of the new approach.
The Cordillera Blanca in Peru has been the scene of rapid deglaciation
for many decades. One of numerous lakes formed in the front of the
retreating glaciers is the moraine-dammed Lake Palcacocha, ...which drained
suddenly due to an unknown cause in 1941. The resulting Glacial Lake
Outburst Flood (GLOF) led to dam failure and complete drainage of Lake
Jircacocha downstream, and to major destruction and thousands of fatalities
in the city of Huaráz at a distance of 23 km. We chose an integrated
approach to revisit the 1941 event in terms of topographic reconstruction
and numerical back-calculation with the GIS-based open-source mass
flow/process chain simulation framework r.avaflow, which builds on an
enhanced version of the Pudasaini (2012) two-phase flow model. Thereby we
consider four scenarios: (A) and (AX) breach of the moraine dam of Lake
Palcacocha due to retrogressive erosion, assuming two different fluid
characteristics; (B) failure of the moraine dam caused by the impact of a
landslide on the lake; and (C) geomechanical failure and collapse of the
moraine dam. The simulations largely yield empirically adequate results with
physically plausible parameters, taking the documentation of the 1941 event
and previous calculations of future scenarios as reference. Most simulation
scenarios indicate travel times between 36 and 70 min to reach
Huaráz, accompanied with peak discharges above 10 000 m3 s−1. The results of the scenarios indicate that the most likely initiation mechanism would be retrogressive erosion, possibly
triggered by a minor impact wave and/or facilitated by a weak stability
condition of the moraine dam. However, the involvement of Lake Jircacocha
disguises part of the signal of process initiation farther downstream.
Predictive simulations of possible future events have to be based on a
larger set of back-calculated GLOF process chains, taking into account the
expected parameter uncertainties and appropriate strategies to deal with
critical threshold effects.
Mass flow simulations are considered important tools for hazard analysis. For the simulation of single process mass flows such as debris flows, robust tools and reasonable parameter range estimates ...are available. However, this is much less the case for more complex mass flows, e.g. involving process chains and flow transformation. We explore the challenges of simulating complex flow-dominated landslides by back-calculating the Huascarán events of 1962 and 1970 with r.avaflow, a two-phase mass flow model (Pudasaini, 2012) in a GIS-based open source simulation framework. Both events started as rock-ice falls on the western slope of the north summit of Nevado Huascarán (Cordillera Blanca, Peru) and entrained large volumes of glacial till at lower elevation, resulting in highly mobile debris avalanches. Whereas the 1962 event badly affected the village of Ranrahirca when spreading over a debris cone, the 1970 event overtopped a ridge and led to the complete destruction of the town of Yungay. Well documented in the literature, these events provide an opportunity as a natural laboratory for testing innovative mass flow simulation tools and their features. In a first step, we consider (i) the 1962 event and (ii) the 1970 event separately, for each of them optimizing the key input parameters in terms of empirical adequacy. In a second step, we apply the optimized parameter set for (i) to the 1970 event and the parameter set derived for (ii) to the 1962 event. In a third step, we explore the sensitivity of the model outcomes to selected key parameters (basal friction angle and entrainment coefficient). The results (a) demonstrate the general ability of r.avaflow to reproduce the spatio-temporal evolution of flow heights and velocities as well as travel times and volumes of these complex mass flow events reasonably well; and (b) highlight the challenges and uncertainties involved in predictive simulations with parameter sets obtained from back-calculations. We suggest a strategy to appropriately deal with uncertain outcomes by superimposing the results of multiple simulations.
•Two-phase models are needed for the simulation of complex mass flows.•r.avaflow successfully back-calculates complex mass flows with optimized parameters.•Predictive simulations based on optimized parameter sets remain a challenge.•Hazard mapping needs scenario analyses or likelihood measures (parameter ranges).
Flow behavior and mobility of gravitational mass flows such as rapid‐moving landslides, ice‐rock avalanches, or snow avalanches strongly depend on the material temperature. Flow temperature ...dependence is particularly pronounced for materials with high homologous temperatures, such as snow or ice under natural conditions. The interplay between mechanisms driving the temperature evolution in flowing geomaterials remain largely unknown. Here we present laboratory experiments in a rotating drum, measuring the temperature evolution of steadily flowing snow at ambient temperatures below freezing. After initial heating the flow reaches thermal equilibrium. To describe the thermal energy balance, we derive an analytical model, taking into account frictional energy dissipation and heat exchange with the ambient medium. The model accurately captures the measured temperature evolution and predicts the observed thermal equilibrium, where ambient cooling compensates frictional heating. It allows to determine heat transfer coefficients and total shear stresses of the flowing material based on measured temperatures.
Plain Language Summary
Snow avalanches, landslides, or ice‐rock avalanches produce heat as they flow down the mountain. We quantify this heat production in laboratory experiments with flowing snow. Alongside we discovered a new phenomenon, namely, the thermal equilibrium in gravitational mass flows. We reproduce this phenomenon experimentally and derive an analytical model to predict the temperature evolution of the flowing geomaterial.Exciting aspects of the model are its simplicity and the possibility to determine material properties, such as heat transfer coefficients and total shear stresses, in an entirely new way.This type of simple model and experiment helps to uncover the physics behind flow type transitions and opens a new way to investigate the frictional behavior for various kinds of mass flows. The thermal equilibrium appears due to the natural compensation of frictional energy dissipation by ambient cooling. For a wide range of gravitational mass flows and in particular snow avalanches, the temperature evolution dictates the resulting mobility and run out. Therefore, this research is an important piece in the puzzle to develop methods to predict the destructive potential of natural hazards.
Key Points
The temperature evolution of a flowing geomaterial was analytically modeled and measured in laboratory drum experiments
We found thermal equilibrium as a result of frictional energy dissipation compensated by ambient cooling
Material properties of the gravitational mass flow were determined from temperature measurements
Debris flows affect vulnerable areas by unloading their destructive impact and depositing sediments in debris-flow lobes in the runout area. The debris flow deposition morphology characteristically ...displays a complex process and is of particular interest for hazard mapping, economic activities planning, construction of civil structures, and settlements in vulnerable areas. This laboratory study is based on large-scale stony debris flow experiments, focusing on the runout distance, mobility, and inundated area. The experiments were conducted under different initial and boundary conditions like particle size, solid volume fraction, and basal roughness. Our experimental results reveal that runout distance and lobe shape change dramatically under different solid volume fractions and are highly dependent on basal roughness. On the contrary, the particle size has a marginal effect on deposit morphology. The newly introduced dimensionless runout capacity allows the evaluation of the momentum transfer of the debris flows to the end of the flume into a runout distance. The experimental results show that the runout capacity is inversely proportional to the Froude number of the stony debris flow. We deduce a relationship for the runout distance from this concept, which reveals that flow velocity contributed to the runout distance on a par with the flow depth. A weak velocity can be counterbalanced by an increased front depth to achieve a similar runout distance. Furthermore, we evaluate flow mobility, deposited volume, as well as the inundated area, and compare our results with other experimental and field data. The presented data match existing hypermobility models in-between small-scale laboratory and field-scale events. Consequently, this study complements missing data to the previous dataset of runout distance, elevation difference, mobility, and inundated area, contributing to better understand debris flow dynamics.
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•The deposition for landslide-induced stony debris flows was studied in the laboratory.•Runout distance and lobe shape change crucially under solid volume fractions.•The runout capacity as a new dimensionless number is introduced.•A velocity can be counterbalanced by a front depth to achieve similar runout distance.•The Pudasaini and Miller hypermobility model matches the experimental data very well.
Landslides, debris avalanches and flows are common events in mountainous regions, causing tremendous damages to people and infrastructures. Their dynamics are substantially affected and altered by ...obstacles such as trees, big boulders and civil structures on their way. Appropriately designed and optimally installed obstacles, including braking mounds, catching or deflecting dams, in the flow path can drastically change the flow dynamics by deflecting, re-directing or stopping the debris mass. Such structures can substantially reduce the kinetic energy of the flow and associated risks. So, a proper understanding of the flow-obstacle-interaction is required to construct adequate defense structures. Here, we simulate a two-phase debris flow as a mixture of solid particles and viscous fluid down an inclined surface with tetrahedral obstacles of different dimensions, numbers and orientations. This is achieved by employing a physically-based general quasi-three dimensional two-phase mass flow model (Pudasaini, 2012) consisting of a set of non-linear and coupled partial differential equations representing mass and momentum conservations for both the solid- and fluid-phases. Simulations on mass flows are performed with a high-resolution and efficient numerical scheme that is capable of capturing rapid and detailed dynamics, including the strongly re-directed flow with multiple stream lines, mass arrest, strong shock waves and debris-vacuum generation and flow pattern formations, as the rapidly cascading mass suddenly encounters the obstacles. The estimated impact pressure is useful for designing the defense structures. The solid and fluid phases show fundamentally different interactions with obstacles, flow spreading and dispersions, and run-out dynamics and deposition. The observations are in line with natural debris flows and experiments. Our understanding of the complex interactions of real two-phase mass flows with multiple obstacles helps us to plan defense structures and constitute advanced and physics-based engineering solutions for the prevention and mitigation of risks caused by different gravitational mass flows.
•Simulated complex interactions of two-phase debris flow with tetrahedral obstacles of different dimension, number, orientation.•Solid and fluid phases show fundamentally different interactions with obstacles, spreading, run-out and depositions.•Different obstacle configurations produce different redirections, mass arrest, phase-separation, inundation/mitigated areas.•We present new/better method of impact pressure estimation that is useful for designing defense structures.•Simulations constitute advanced, physics-based engineering solutions for planing defense structures and mitigating risks.
The run out and destructive potential of gravitational multi-phase flows is largely determined by the mixture composition, the material properties of the solid particles and the fluid. One instrument ...to expand the understanding of the governing processes of flow is laboratory experiments. In this study, we concentrate experimentally on landslide-induced stony debris flows as a particular type of flow-like mass movement. We aim to observe different natural flow types for varying initial and boundary conditions. In a laboratory flume, 12.0 m long, 1.3 m wide and 0.3 m deep, we initiate stony debris flows and measured flow variables such as flow depth, mass, bulk density, front velocity and front shape for varying particle size, solid volume fraction and basal roughness. Our experimental results reveal that flow type and evolution changes significantly for different solid volume fractions, as well as for different basal roughness. The particle size had a noticeable effect on flow velocity and front shape. The smooth surface facilitated rapid, shallow, and turbulent flows. In contrast, experiments with rough beds showed relatively lower velocities and dense flow behaviour. Although the flow parameters covered only a small spectrum of the naturally possible parameter space, flow phenomena such as phase-separation, longitudinal sorting, steep front surges, or front overtopping were observed. We group our observed flows regarding the flow properties and classify them into common flow types: (1) debris flood (hyperconcentrated flow), (2) debris flow, and (3) non-liquefied debris flow. To compare our results with natural events and other experimental results, we analyse the data with several dimensionless numbers. The flows were generally dominated by grain collision on the smooth surface. Naturally, frictional forces gain more influence on the rough surface but did not overrule collisional forces. Viscous forces played only a minor role in our experiments, due to the lack of highly viscous fluid. Overall, we infer, that our well-controlled experiments mimicked natural stony debris flow and give new profound insights into the causal relationship of how the initial and boundary conditions affect the flow evolution.
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•Landslide‐induced stony debris flows were initiated in a 6.7 m long laboratory flume.•Flow type and evolution change significantly for different solid volume fractions.•Particle size had a noticeable effect on flow velocity and front shape.•Experimental flows could be assigned to three natural flow types.•The flows were generally dominated by grain collision.
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