Real Debris Flow Research Articles

Discrete element modeling of rock-filled gabions under successive boulder impacts

A discrete element model consisting of irregular crushable filling particles and a flexible wire mesh is established. The numerical model is validated against the static net punching test and the dynamic pendulum impact test to ensure that the large irreversible plastic deformations and the impact forces during successive impacts can be captured. The impact force reduces at small friction coefficients, which is associated with the significant particle rearrangement during the impact process. The important role of friction is further confirmed by the energy evolution, showing that friction is the dominant mechanism for energy dissipation, instead of the more intuitionistic collision or particle crushing. Besides, the increase of impact force with the number of impacts becomes more significant at a higher impact energy due to the faster rate of momentum exchange. A bounce-back behavior of the boulder is observed when the impact energy is low, which may attenuate the impact force like the reflection waves in real debris flow events where multiple boulders are present. Our results highlight the combined effects of contact properties and impact energy, which are valuable in the design of rigid barriers shielded by rock-filled gabions for hazard mitigation in engineering practice.

Characterization of a debris flow event using an affordable monitoring system

This study presents monitoring data of a debris flow event in the Central Italian Alps. The debris flow occurred on August 16, 2021 in the Blè basin (Val Camonica valley, Lombardia Region) and was recorded by a monitoring station installed just few weeks before. The monitoring system was deployed to document the hydrologic response of the catchment to rainfall, and was designed to be lightweight, relatively cheap, and easy to deploy in the field. To this purpose, we combined video cameras with geophysical sensors (geophones and infrasound) and optimized the power supply system. The data recorded during the event allowed to identify the triggering rainfall, document the flow behaviour, and estimate surface flow velocity and flow rate using Particle Image Velocimetry algorithms. Moreover, the seismic signal generated by the debris flow revealed a peculiar frequency spectrum compared to regular streamflow. These results show that even a relatively simple monitoring system may provide valuable data on real debris flow events.

A dilatant two-phase debris flow model with erosion validated by full-scale field data from the Illgraben test station

We propose a dilatant, two-layer debris flow model validated by full scale density/saturation measurements obtained from the Swiss Illgraben test site. Like many existing models we suppose the debris flow consists of a matrix of solid particles (rocks, boulders) that is surrounded by muddy fluid. However, we split the muddy fluid into two fractions. One part, the inter-granular fluid, is bonded to the solid matrix and fills the void space between the solid particles. The combination of solid material and inter-granular fluid forms the first layer of the debris flow. The second part of the muddy fluid is not bonded to the solid matrix and can move independently from the first layer. This free fluid forms the second layer of the debris flow. During flow the rocky particulate material is sheared which induces dilatant motions that change the solid/fluid concentration of the first layer and then his density. As suggested by real data of Illgraben, the rheology used is not constant and uniform but a function of the flow composition/ density. The model is then compared to real debris flow data of Illgraben and tested on a real event in Ritigraben for which erosion data are available.

Realistic visualization of debris flow type landslides through virtual reality

Immersive media technologies, such as virtual and augmented reality, have recently enabled a more holistic way to comprehend natural hazards. In this work, we aim at visualizing the temporal and spatial evolution of a debris flow in a virtual reality environment. We develop a framework to integrate the output results obtained from a debris flow numerical model into virtual reality. To guide the framework, a real debris flow event, which happened in Hunnedalen (Norway) in 2016 and blocked a road network, is considered as a case study. The debris flow is back-calculated using a depth-averaged numerical model and the simulation results are imported into a dedicated game engine to construct a digital model of the debris flow event. The debris flow is visualized using a Head-Mounted Display. We therefore discuss a wide range of potential applications of virtual reality to manage and grasp landslide phenomena: training for rescue operations; improving decision-making; studying early warning systems, and educating communities affected by natural hazards. We finally provide a quantitative evaluation of the hazard perception for a road user. We show that the debris flow movement is perceived at variable delayed times from the triggering of the landslide, depending on the position along the road where the debris flow is observed. Evaluating the realistic perception time of the natural hazard may be fundamental to designing more effective road networks, signs, and mitigation measures.

Numerical investigation of debris flows using a two-phase continuum model incorporating a visco-inertial rheology

The motion of debris flows is controlled by the interaction of their fluid and solid components. In this work, a general two-phase model framework (Pudasaini, 2012) is adopted which captures the coupled effects of the individual phase dynamics to the overall mobility. While solving the model equations, the fluid phase is treated as a viscous Newtonian liquid while the solid phase is considered to be a granular material obeying a recently developed visco-inertial constitutive rheology. Solid and fluid components in the mixture are coupled through the interaction forces, namely buoyancy, drag, and virtual mass. The model is calibrated against results from instrumented flume experiments and from field measurements of saturated, channelized debris flows. The numerical model captures the enhanced mobility of debris flows due to the presence of interstitial fluid and provides better predictions of the flow dynamics relative to those obtained from single-phase frameworks. Better modeling agreement is obtained for granular-fluid flows with relatively high fluid content. The model is then used to simulate real debris flow events, including a case that occurred in the proximity of the Sichuan-Tibet Railway, where good agreement with reported field measurements is obtained. This modeling framework is expected to improve mitigation strategies for debris flows hazards.

Variable hydrograph inputs for a numerical debris-flow runout model

Abstract. Debris flows affect people and infrastructure around the world, and as a result, many numerical models and modelling approaches have been developed to simulate their impacts. Observations from instrumented debris-flow channels show that variability in inflow depth, velocity, and discharge in real debris flows is much higher than what is typically used in numerical simulations. However, the effect of this natural variability on numerical model outputs is not well known. In this study, we examine the effects of using complex inflow time series within a single-phase runout model utilizing a Voellmy flow-resistance model. The interactions between model topography and flow resistance were studied first using a simple triangular hydrograph, which showed that simulated discharges change because of local slopes and Voellmy parameters. Next, more complex inflows were tested using time series based on 24 real debris-flow hydrographs initiated from three locations. We described a simple method to scale inflow hydrographs by defining a target event volume and maximum allowable peak discharge. The results showed a large variation in simulated flow depths and velocities arising from the variable inflow. The effects of variable-inflow conditions were demonstrated in simulations of two case histories of real debris flows, where the variation in inflow leads to significant variations in the simulation outputs. The real debris-flow hydrographs were used to provide an indication of the range of impacts that may result from the natural variability in inflow conditions. These results demonstrate that variation in inflow conditions can lead to reasonable estimates of the potential variation in impacts.

An optimal selection method for debris flow scene symbols considering public cognition differences

Disaster scene symbols can reduce memory and cognitive burdens and improve the transmission efficiency of debris flow information. There is no unified standard for disaster scene symbols, although many academic institutions and emergency departments have thoroughly studied debris flow disasters. Instead, many disaster scene symbols of different styles interfere with the public's understanding of disaster information. Here, an optimal selection method for debris flow scene symbols considering public cognition differences is proposed. First, public knowledge is incorporated into the fuzzy analytic hierarchy process (FAHP) model for debris flow scene symbols selection. Second, debris flows are visualized in 3D with a virtual geographic environment (VGE). Finally, a real debris flow is selected for experimental analysis. The proposed method can support the optimal selection of debris flow scene symbols considering public cognition differences, and a 3D scene constructed with the selected scene symbols can improve the transmission efficiency of disaster information and provide support for public-oriented debris flow information services.

Automatic debris flow detection using geophones

Ground vibration recorded using geophones can provide information on debris flows. This information can be used for debris flow detection and warning. However, real debris flow signals change not only from site to site but also during different times at the same location. Thus, a constant threshold calibrated with historic data at one location may not accurately detect debris flow at the same location, let alone at other locations. Therefore, a debris flow detection method requires a criterion that can transform current environmental conditions into a detection criterion. The characteristic of a debris flow tremor was discussed and transformed into two mathematical indices. We used accumulation energy and the rate of energy change in a specific spectrum domain as the representative indicators. To distinguish debris flows from noise, we assumed debris flow energy to be at least an order of magnitude larger than noise before debris flow arrival. To avoid a short-term spike event confusing the detection of debris flow, accumulation energy at an interval longer than 2 s was used. This method was tested on three debris flow events with complete records at the Ai-Yu-Zi Creek, Taiwan. The detection of arrival time for debris flows using geophone data had a delay of less than 10 s compared with the detection of arrival time through video recording.

A laboratory experience on the effect of grains concentration and coarse sediment on the rheology of natural debris-flows

Not only solid volumetric concentration, but also coarse particle content play a relevant role on the rheology of soil mixtures involved in mud, debris, hyper-concentrated and earth flows. This paper is devoted to investigating the influence of bulk solid volume concentration and of coarse fraction on the rheological behavior of granular slurries. Laboratory activity is carried out involving different soils from the source area of real debris flows (from the Campania region in Italy). Experimental results demonstrate that the flowing behavior is the same as yield stress fluids with a static yield stress value larger than the dynamic one. A generalized Herschel–Bulkley model is considered, accounting for a consistent index which is a function of solid volume concentration. Experimental evidence demonstrates that the viscous characteristics (i.e., yield stresses and bulk viscosity) are very sensitive to coarse grain content, and the flow characteristics are deeply affected by small variations of both solid content and particle size. In effect, the tested materials show increasing yield stress (both static and dynamic) with solid concentration and decreasing yield stress for an increment of coarser solid fraction. To the contrary, assuming a constant fine grain relative content, the higher the coarse grain fraction is, the lower the yield stress results. Moreover, the typical transition from solid-like to fluid-like behavior, and vice versa, is strongly influenced by solid concentration and grain size distribution.

A hands-on approach to estimate debris flow velocity for rational mitigation of debris hazard

Rational estimation of debris flow velocity is one of the key issues in debris hazard mitigation. Among the various procedures, back-calculation of debris flow velocity is one of the most frequently used approaches. Back-calculation includes determination of super-elevations and channel properties, and velocities are calculated using the forced vortex equation. Super-elevation and bend radius are biasing parameters in the back-calculation scheme. Therefore, an iterative approach based on simplified assumptions is developed to avoid the direct and subjective determination of bend radius. Besides, as only the highest mud prints are visible after the disaster, this misreads the actual super-elevation. To seek better ways to fix this anomaly, a series of three-dimensional numerical curved flume tests using smoothed particle hydrodynamics are carried out. Estimated velocities from highest flow marks underestimate the actual velocities near the source, while they converge on the actual velocities as the distance to source increases. A best-fit line is then proposed to adjust the mud-marks-derived velocities to the real velocities. The assumption taken during development of the iterative approach for determining bend radii is also justified from the numerical simulations. The law of similarity allows application of these findings to real debris flow disasters. Thus, two real debris flow disasters in Japan are examined for validation of the developed procedure and adjusted velocities are proven to be consistent with verbal evidences and previous analyses.

A two‐phase SPH model for debris flow propagation

SummaryThis paper presents a model which can be used for fast landslides where coupling between solid and pore fluid plays a fundamental role. The proposed model is able to describe debris flows where the difference of velocities between solid grains and fluid is important. The approach is based on the mathematical model proposed by Zienkiewicz and Shiomi, which is similar to those of Pitman and Le and Pudasaini. The novelty of the present work is the numerical technique used, the smoothed particle hydrodynamics (SPH). We propose to use a double set of nodes for soil and water phases, the interaction between them being described by a suitable drag law. The paper presents both mathematical and numerical models, describing the main assumptions and their limitations. Then, the model is applied to (1) a simple case where shocks and expansion waves appear, (2) a dam break problem on a horizontal plane with a frictional soil phase, and (3) a debris flow which happened in Hong Kong. The main conclusions that can be drawn from the applications are: Debris flows having 2 phases with important relative mobility present a rich structure of shocks and rarefaction waves, which has to be properly modeled. Otherwise, the model will have numerical damping or dispersion. Dambreak exercises provide interesting information in simple and controlled situations. We can see how both phases move relative to each other. Real debris flows can be simulated with the proposed model, obtaining reasonable results.

Modelling of a Debris Flow Event in the Enna Area for Hazard Assessment

In the paper a modelling of a real debris flow in the Enna area in the south of Italy is described. Starting from the study of the geological framework and the historical background for landslides of the Enna district, the research has focused on the causes triggering the landslides. In order to study the performance of debris flow, the real case of 1st-2nd February 2014 which affected Enna city has been modeled. The event caused damage to private buildings and above all the interruption of the main infrastructure connecting Enna city at the motorway, due to the material on the road. The modelling of the real debris flow using a mono-phase model (FLO-2D) was carried out in order to investigate the global dynamic of the event. The study allows to acquire a better knowledge of the hydraulic parameters that can be used in other modelling events for areas with a similar soil composition in order to assess the most appropriate mitigation works, reducing damage to structures and infrastructures.

Numerical simulation for runout process of debris flow using depth-averaged material point method

Debris flows, i.e., combined flows of watery mud, chunks of boulders, trees, etc., can be extremely destructive to life and property and can exhibit complicated mechanical features. These complicated features of heterogeneous debris-flow surges can be rationally modeled by considering the rheological behaviors of solid, liquid, and interaction-force phases. Herein, we present the simulation results obtained using a numerical method for a runout process of debris flows based on the depth-averaged equation of motion implemented with the material point method. This method enabled us to describe the debris-flow behaviors in a more adequate manner than previous methods, by simulating debris flows using a depth-averaged model. The simulation results were verified by comparing them with the results of flume tests on dry and wet sand flows. The test results were calibrated to identify the key parameters for simulating the runout process of soil flows. In addition, a real debris-flow event was simulated, and important features of the simulated debris flow, i.e., the geographical dimensions of the depositional zones, conformed well to the observed dimensions of the real debris flow.

A new data assimilation procedure to develop a debris flow run-out model

Parameter calibration is one of the most problematic phases of numerical modeling since the choice of parameters affects the model’s reliability as far as the physical problems being studied are concerned. In some cases, laboratory tests or physical models evaluating model parameters cannot be completed and other strategies must be adopted; numerical models reproducing debris flow propagation are one of these. Since scale problems affect the reproduction of real debris flows in the laboratory or specific tests used to determine rheological parameters, calibration is usually carried out by comparing in a subjective way only a few parameters, such as the heights of soil deposits calculated for some sections of the debris flows or the distance traveled by the debris flows using the values detected in situ after an event has occurred. Since no automatic or objective procedure has as yet been produced, this paper presents a numerical procedure based on the application of a statistical algorithm, which makes it possible to define, without ambiguities, the best parameter set. The procedure has been applied to a study case for which digital elevation models of both before and after an important event exist, implicating that a good database for applying the method was available. Its application has uncovered insights to better understand debris flows and related phenomena.

A comparative assessment of two different debris flow propagation approaches – blind simulations on a real debris flow event

Abstract. A detailed comparison between the performances of two different approaches to debris flow modelling was carried out. In particular, the results of a mono-phase Bingham model (FLO-2D) and that of a two-phase model (TRENT-2D) obtained from a blind test were compared. As a benchmark test the catastrophic event of 1 October 2009 which struck Sicily causing several fatalities and damage was chosen. The predicted temporal evolution of several parameters of the debris flow (such as flow depth and propagation velocity) was analysed in order to investigate the advantages and disadvantages of the two models in reproducing the global dynamics of the event. An analysis between the models' results with survey data have been carried out, not only for the determination of statistical indicators of prediction accuracy, but also for the application of the Receiver Operator Characteristic (ROC) approach. Provided that the proper rheological parameters and boundary conditions are assigned, both models seem capable of reproducing the inundation areas in a reasonably accurate way. However, the main differences in the application rely on the choice of such rheological parameters. Indeed, within the more user-friendly FLO-2D model the tuning of the parameters must be done empirically, with no evidence of the physics of the phenomena. On the other hand, for the TRENT-2D the parameters are physically based and can be estimated from the properties of the solid material, thus reproducing more reliable results. A second important difference between the two models is that in the first method the debris flow is treated as a homogeneous flow, in which the total mass is kept constant from its initiation in the upper part of the basin to the deposition in a debris fan. In contrast, the second approach is suited to reproduce the erosion and deposition processes and the displaced mass can be directly related to the rainfall event. Application of both models in a highly urbanized area reveals the limitation of numerical simulation which is inadequate in describing some disturbances of the flows that occurred during the alluvial event (e.g. the cars, the volume of debris within buildings etc.) which have a crucial influence on the evaluation of the maximum and final flow depths.

A Particle Floating Model for the Initiation of Debris Flows

Riverbeds in Taiwan are full of granular materials such as boulder, cobble, gravel, sand, and silt. Under the action of floods, particles with their sizes from little to large will be floating upward and then flowing with floods gradually. Mechanical models for critical conditions of debris flows taken by some other researchers in Taiwan are mainly based on slope stability criteria. However real debris flows are generally observed to be initiated when granular soils start floating upward. Under such circumstances, a more proper mechanical model was proposed by the authors. It was proved that such a model can provide proper solutions as compared with those produced from tests.

Comparison between FLO-2D and Debris-2D on the application of assessment of granular debris flow hazards with case study

Numerical simulation has been widely applied to the assessment of debris flow hazards. In East Asia and especially Taiwan , the most widely used numerical programs are FLO-2D and Debris-2D . Although these two programs are applied to the same engineering tasks, they are different in many aspects. These two programs were compared according to their fundamental theories, input and output data, computational algorithms and results. Using both programs, the simulations of a real debris flow with abundant granular material induced by landslides at Xinfa village in southern Taiwan are performed for comparison. The simulation results show that Debris-2D gives better assessment in hazard area delineating and flow depth predicting. Therefore, Debris-2D is better for simulation of granular debris flows.

Numerical modelling of debris surges based on shallow-water and homogeneous material approximations

The objective of this contribution is to analyse the formation of debris waves in natural channels. Numerical simulations are carried out with a ID code based on shallow-water equations and on the weighted averaged flux method. The numerical code represents the incised channel geometry with a power-law relation between local width and flow depth and accounts for all source terms in the momentum equation. The debris mixture is treated as a homogeneous fluid over a fixed bottom, whose rheological behaviour alternatively follows Herschel-BulkJey, Bingham or generalised visco-plastic models. The code is applied to real debris flow events that consisted of a single wave and multiple surges, in particular in the Illgraben catchment (Switzerland) and in the Cortina d’Ampezzo area (Dolomites). Numerical results are presented and compared with available flow depth registrations. A statistical analysis of debris waves shows that a good representation of wave statistics can be obtained with a proper calibration of rheological parameters. Wave propagation in time along the channel is investigated to verify if overtaking phenomena like in water roll waves do develop. Finally, an original interpretation for the formation of finite amplitude waves in a transient unstable flow is provided, based both on field data and numerical simulations.

Numerical modelling of debris surges based on shallow-water and homogeneous material approximations

The objective of this contribution is to analyse the formation of debris waves in natural channels. Numerical simulations are carried out with a 1D code based on shallow-water equations and on the weighted averaged flux method. The numerical code represents the incised channel geometry with a power-law relation between local width and flow depth and accounts for all source terms in the momentum equation. The debris mixture is treated as a homogeneous fluid over a fixed bottom, whose rheological behaviour alternatively follows Herschel-Bulkley, Bingham or generalised visco-plastic models. The code is applied to real debris flow events that consisted of a single wave and multiple surges, in particular in the Illgraben catchment (Switzerland) and in the Cortina d'Ampezzo area (Dolomites). Numerical results are presented and compared with available flow depth registrations. A statistical analysis of debris waves shows that a good representation of wave statistics can be obtained with a proper calibration of r...

Simulating debris flows through a hexagonal cellular automata model: SCIDDICA S<sub>3–hex</sub>

Abstract. Cellular Automata (CA) represent a formal frame for dynamical systems, which evolve on the base of local interactions. Some types of landslide, such as debris flows, match well this requirement. The latest hexagonal release (S3–hex) of the deterministic model SCIDDICA, specifically developed for simulating debris flows, is described. For CA simulation purposes, landslides can be viewed as a dynamical system, subdivided into elementary parts, whose state evolves exclusively as a consequence of local interactions within a spatial and temporal discretum. Space is the world of the CA, here constituted by hexagonal cells. The attributes of each cell ("substates") describe physical characteristics. For computational reasons, the natural phenomenon is "decomposed" into a number of elementary processes, whose proper composition makes up the "transition function" of the CA. By simultaneously applying this function to all the cells, the evolution of the phenomenon can be simulated in terms of modifications of the substates. SCIDDICA S3–hex exhibits a great flexibility in modelling debris flows. With respect to the previous releases of the model, the mechanism of progressive erosion of the soil cover has been added to the transition function. Considered substates are: altitude; thickness and energy of landslide debris; depth of erodable soil cover; debris outflows. Considered elementary processes are: mobilisation triggering and effect (T1), debris outflows (I1), update of landslide debris thickness and energy (I2), and energy loss (T2). Simulations of real debris flows, occurred in Campania (Southern Italy) in May 1998 (Sarno) and December 1999 (San Martino V.C. and Cervinara), have been performed for model calibration purposes; some examples of analysis are briefly described. Possible applications of the method are: risk mapping, also based on a statistical approach; evaluating the effects of mitigation actions (e.g. stream deviations, topographic alterations, channelling, embankments, bridges, etc.) on flow development.