Debris Flow Impact Research Articles

Experimental and Numerical Study of the Performance of Self-Centering Frame Structures Subjected to Debris-Flow Impacts

AbstractTo investigate the performance of self-centering RC frame structures subjected to debris-flow impacts, a series of debris-flow impact tests of a traditional RC frame structure (TF) and a se...

Dynamic Response Study of Impulsive Force of Debris Flow Evaluation and Flexible Retaining Structure Based on SPH-DEM-FEM Coupling

Flexible retaining structure is demonstrated to be an effective measure for debris flow prevention in mountainous areas, which can effectively separate water and stone, reduce particle mass, and dissipate kinetic energy. In order to explore the impulsive force and dynamic response of flexible retaining structure impacted by solid-liquid two-phase debris flow, a complex dynamic interaction model of particle-fluid-structure has been established by employing the SPH-DEM-FEM coupling numerical analysis method. In the present study, the process, impulse force, and dynamic response of a flexible retaining structure subjected to debris flow under different slopes were investigated, respectively. The numerical results are compared with those of the calculation formula of the peak impulsive force of the semiempirical debris flow. Meanwhile, the effectiveness of the coupling numerical simulation is verified. The simulation results show that the coupled SPH-DEM-FEM numerical analysis method can visualise the impact of the solid-liquid two-phase debris flow on the flexible retaining structure, reproducing the impact, retaining, water-stone separation, climbing, silting, and deposition process of debris flow. The dynamic time-history curve of the coupling numerical analysis method for the flexible retaining structure is consistent with the results of the existing literature. The debris flow evaluation results of flexible retaining structure impacted by solid-liquid two-phase debris flow are in an order of magnitude with the empirical formula results. Moreover, the results are reliable. The obtained results have a certain reference value for the study on the impulsive force and dynamic response of the flexible retaining structure impacted by solid-liquid two-phase debris flow and the engineering design of the flexible retaining structure.

The effects of upstream flexible barrier on the debris flow entrainment and impact dynamics on a terminal barrier

The destructive nature of debris flows is mainly caused by flow bulking from entrainment of an erodible channel bed. To arrest these flows, multiple flexible barriers are commonly installed along the predicted flow path. Despite the importance of an erodible bed, its effects are generally ignored when designing barriers. In this study, three unique experiments were carried out in a 28 m long flume to investigate the impact of a debris flow on both single and dual flexible barriers installed in a channel with a 6 m long erodible soil bed. Initial debris volumes of 2.5 and 6 m3 were modelled. For the test setting adopted, a small upstream flexible barrier before the erodible bed separates the flow into several surges via overflow. The smaller surges reduce bed entrainment by 70% and impact force on the terminal barrier by 94% compared to the case without an upstream flexible barrier. However, debris overflowing the deformed flexible upstream barrier induces a centrifugal force that results in a dynamic pressure coefficient that is up to 2.2 times higher than those recommended in guidelines. This suggests that although compact upstream flexible barriers can be effective for controlling bed entrainment, they should be carefully designed to withstand higher impact forces.

Debris-flow hazard assessment at the archaeological UNESCO world heritage site of Villa Romana del Casale (Sicily, Italy)

Archaeological sites are extremely vulnerable to the impacts of weather-related events, which may lead to irreparable damages to cultural heritage. Here an assessment of the debris-flow hazard for the UNESCO site of Roman Villa del Casale (Italy) is carried out, through a combination of historical analyses, field surveys, geomorphological and hydrological investigations and two-dimensional hydraulic numerical modelling, all performed at river catchment scale. Historical analyses reveal that the site has been hit by several landslides in the far and recent past. This is presently confirmed by the high level of exposure to the impact of rain-triggered debris-flow events, due to the position of the Villa at a closure section of the related river basin and to the hydro-geomorphological characteristics of the basin itself. By applying the proposed approach, a scenario analysis is carried out. Results allow one to highlight the dynamics of the impact of debris flows, thanks to space and time-dependent maps about deposition areas, water depth and speed values, and to identify the most vulnerable archaeological elements within the study site. The numerical simulations are also used to test the efficiency of the existing hydraulic defense systems and to support the implementation of an early warning system for the site protection. Here, we also synthetize the design of the architecture of the wireless monitoring network, the sensor technology adopted to develop an effective real time environmental monitoring system and management platform, to construct a Wireless Sensor Network (WSN) - early warning and reporting system, which can be applied as a prevention measure.

Additional insights to EC7 from the application of reliability-based design methods: the case of debris flow protection structures

Debris flows are dangerous natural processes that cause extensive damages to infrastructures and urbanized areas and can lead to loss of human lives. Their unpredictability, their extremely high motion and their magnitude are the main causes of these harms. Mitigation measures are fundamental for reducing the associated risk and protecting infrastructures in mountainous areas. Their design is still an open issue: there are many formulations to evaluating impact pressure. Moreover, the uncertainties in the determination of flow characteristics (velocity and thickness) are significantly high and difficult to quantify. In the European Union, the design of any type of structures involved in rock mechanics field must comply with EN-1997 Geotechnical Design (CEN 2004) (EC7). For debris flow countermeasures, EC7 requirements are very difficult to apply in practice since partial safety factors are not provided for these phenomena. However, the basic philosophy of reliability-based design (RBD), as defined in EN1990 (CEN 2002) may be a suitable and complementary approach to provide geotechnical structures with a uniform probability of failure. Reliability Based Design (RBD) can provide additional insights to EC7 design and can be applied when partial factors have still to be proposed (by EC7) to cover uncertainties of less common parameters, as in case of debris flow countermeasures. This paper presents an analysis of the advantages and limitations on the applicability of RBD approach to debris flow countermeasures, by using the first-order reliability method (FORM). In particular, data availability, the possibilities for analysing data in a statistical framework and the choice of performance function are the main limitation of the method, which force to make assumptions regarding statistical distribution of the considered parameters. A sensitivity analyses, comparing different equations, commonly used for debris flow impact pressure estimation, were performed for quantifying the effect of the selected performance function on the RBD results.

Exploring new methods to analyse spatial impact distributions on debris‐flow fans using data from south‐western British Columbia

AbstractPredicting the spatial impact of debris flows on fans is challenging due to complex runout behaviour. Debris flow mobility is highly variable and flows can sporadically avulse the channel. For hazard and risk assessments, practitioners typically base the probability of spatial impact or avulsion on their experience and expert judgement. To support decision‐making with empirical observations, we studied spatial impact distributions on 30 active debris‐flow fans in south‐western British Columbia, Canada. We mapped 146 debris‐flow impact areas over an average observation period of 74 years using orthorectified airphotos, satellite imagery, topographic base maps, LiDAR data, orthophotos, and field observations. We devised a graphical method to convert our geospatial mapping into spatial impact heat maps normalized by fan boundaries, enabling comparison of runout distributions across different fans. About 90% of the mapped debris flows reached beyond the mid‐points of fans, while less than 10% avulsed more than half‐way across the fan relative to the previous flow path. Most avulsions initiated at distances of 20% to 40% of the maximum fan length from the fan apex and upstream of the fan intersection point. Large volume events tend to be more mobile in the down‐fan direction, but the relation between volume and cross‐fan runout (e.g., avulsions) is more complex. Differences in spatial impact distributions can be explained, in part, by the degree of fan incision and whether a fan is truncated at its toe by a river or lake. There were no significant differences in spatial impact distributions based on the geology of the source area, sediment supply condition, or hydrogeomorphic process classification.

Assessment and Analysis of a Rainfall–Time-Lagging Water-Related Disaster in Mountainous Areas

Both global climate change and human activities are continuously impacting the abruptness and frequency of water-related natural disasters such as flash floods, debris flows, and landslides in mountainous areas, greatly threatening the safety of lives and properties. A recent rainfall-induced debris flow event happened on July 6, 2020 in the Chenghuangmiao Gully, in Sichuan Province, China, resulting in severe damage to buildings at the outlet. An integrated analysis of the consequence and triggering mechanism of this debris flow event was conducted with hydrologic information, topographic details, vegetation regimes, and drone aerial imagery. The result shows that the entire runout of the debris flow differs from that of common ones (debris flow and rainfall were highly related and synchronized), which happened 4 h after the stop of the rainfall. The hysteretic feature increases the difficulty of the prediction and warning of the debris flow due to lack of a responsible triggering mechanism. The hillslope surface is well covered by vegetation, hindering regular observation and cleaning up of long-term deposited wood and sediment debris. This effect increases the crypticity and abruptness of potential debris flows. With field evidence and analysis, it is speculated that long-term accumulative processes of dead wood sand sediment deposition formed a small-scale debris dam, and the continuous water release from the watershed led to dam breaching, subsequently triggering the initiation of the debris flow. Multiple steps distributing along the gully of an average slope of 15.65° contributed to the amplification of the debris flow once the breach of the upstream wood and sediment dam occurred. Along the gully, small-scale landslide scars can be observed, possibly amplifying the scale of the debris flow and disaster impact. This debris event gives a lesson of necessary demands of predicting and managing the risks of a low-frequency debris flow non-synchronized with rainfall events.

Spatio-Temporal Distribution Characteristics of Debris Flow Impact Pressure and Impact Velocity on the Check Dam Upstream Surface

Spatio-Temporal Distribution Characteristics of Debris Flow Impact Pressure and Impact Velocity on the Check Dam Upstream Surface

Impact dynamics of debris flow against rigid obstacle in laboratory experiments

Debris-flow impact against rigid obstacle involves instant redirection of flow velocity and elevation of flow depth, and could serve as a stringent approach for verification of the two-phase flow models. In this study, kinetic and dynamic behavior of debris-flow impact with varying flow regime has been analyzed on the basis of laboratory tests with distributed measurement. Debris flows interact with obstacle in the modes of vertical jet (deflected vertically) and momentum jump (reflected upstream). The experimental results are further adopted to verify the predictability of vertical jet model and momentum jump model. Our findings highlight that, due to the elevated confining stress in the impact area, the liquefaction ratio increases, resulting in a more “fluid-like” state of debris flow. The two analytical models could distinguish the runup height around the threshold Froude number 3–4. However, the impact loads under both modes could be very close, because the effective impact in the vertical jet mode only concentrates in the lower portion of runup height. The potential threat of flip-through impact (shock impact by a steep or concave wave front) should also be considered, as its impulse load could be much higher than the predicted load by vertical jet and momentum jump models.

Discrete modelling of debris flows for evaluating impacts on structures

Lahars (volcanic debris flows) are natural phenomena that can generate severe damage and wreak havoc in densely populated urban areas. The evaluation of the forces and pressures generated by these mass flows on constructions (e.g., buildings, bridges and other infrastructure) is crucial for civil protection, assessment of physical vulnerability and risk management. The current tools developed to model the spread of flows at large scale in densely populated urban areas remain inaccurate in the evaluation of mechanical efforts. Here, we developed a discrete numerical model for evaluating debris-flow (DF) impact forces at the local scale of one structure (pillar or column) like a building, a bridge and other infrastructure. In this model, the large-sized solid particles that damage infrastructures and edifices are explicitly modelled using Distinct Element Method (DEM). We considered the fluid and fine-grained solid particles not only in the frame of the pressure exerted on structures, but also through their effects on the movement of particles, i.e. buoyancy and drag. The fluid velocity field and the fluid free surface obtained from Computational Fluid Dynamics (CFD) calculation based on Navier–Stokes equations are imported in the DEM simulation. This model is able to reproduce a range of magnitudes of DFs in terms of volumes, velocities and flow heights. Finally, the model provides insights on impact forces generated by particles on structures and on hydrostatic and/or dynamic pressure due to the combined effect of fluid and solid phases. The model provides results consistent with existing empirical models.

Influence of particle size on the buffering efficiency of soil cushion layer against rockfall impact

Soil cushion layers are commonly paved in front of protection structures to avoid the direct impact of rockfall and debris flow. This study is devoted to investigating the influence of particles size on the buffering efficiency of a soil cushion layer by experimental and numerical tests. A series of experimental tests are performed by releasing a spherical granite rock block from different heights to impact onto a soil cushion layer consisting of uniformly graded particles. The influence of particle grading on the buffering efficiency is then analyzed in a set of parametric studies. The impact force of rock block and the interacting force at the interface between the soil layer and the protection structure are monitored and evaluated. The discrete element method is utilized to explore the cratering process of rock block, together with the force chains and energy evolutions in the soil cushion layer. The results indicate that soil particles size can influence significantly on the impact force, especially for high-velocity impacts. This effect can be explained by the number and the stability of force chains formed in the granular packing. These findings can provide some insights to designing effective soil cushion layers for protection structures.

Impact of Debris Flows on Filter Barriers: Analysis Based on Site Monitoring Data

ABSTRACT Debris flows are one of the most complex and devastating natural phenomena, and they affect mountainous areas throughout the world. Structural measures are currently adopted to mitigate the related hazard in urbanized areas. However, their design requires an estimate of the impact force, which is an open issue. The numerous formulae proposed in the literature require the assignment of empirical coefficients and an evaluation of the kinematic characteristics of the incoming flow. Both are generally not known a priori. In this article, we present the Grand Valey torrent site (Italian Alps). A monitoring system made up of strain gauges was installed on a filter barrier at the site, allowing the evaluation of impact forces. The system provides pivotal information for calibrating impact formulae. Two debris flows occurred during the monitoring period. We present the interpretation of videos, impact measurements, and the results of numerical analyses. The combined analysis allows a back calculation of the events in terms of forces, flow depth, and velocity. Thus, we investigate the applicability of the impact formulae suggested in the literature and of the recommended empirical coefficients. The results highlight that hydrostatic effects dominated the impact during the first event, while hydrodynamic effects prevailed in the second one.

A coupled SPH-DEM-FEM model for fluid-particle-structure interaction and a case study of Wenjia gully debris flow impact estimation

Debris flows are rapid gravity-driven unsteady flows of highly concentrated mixtures of water and solid particle material, destroying numerous mountain building structures and traffic facilities. The investigation of debris flows is thus of significance to hazard prevention and mitigation. This paper aims to provide a numerical model capable of reproducing debris flow impact estimation by accounting for the complicated fluid-particle-structure interaction (FPSI) using a coupled smoothed particles hydrodynamics (SPH), discrete element method (DEM), and finite element method (FEM) approach. The fluid phase is represented by SPH. The solid phase consists of physical particle(s) and is represented by the DEM, and the deformable structure is represented by the FEM. The interaction forces among the fluid, solid particles, and structure are computed using the penalty function method. The proposed model is capable of simultaneously simulating a fluid-particle interaction (FPI), particle-structure interaction (PSI), and fluid-structure interaction (FSI), with good agreement between the complicated hybrid numerical and experimental results. Finally, a Wenjia gully debris flow is carried out to demonstrate the capability of the coupled model in simulating the FPSI as an application of debris flow impact simulations. When compared with the actual situation, the propagation of the debris flow and destruction of the structures were predicted accurately. Additionally, the Wenjia gully debris flow was treated, and the treatment measure was analyzed from the impact evolution (e.g., interception by dams, impact force, and destruction of dams). The developed method will contribute to a better understanding of the FPSI and is a promising tool for hazard analysis and mitigation.

Reliability-based formulation of building vulnerability to debris flow impacts

Physical building vulnerability to debris flows is defined as the potential damage degree of buildings for a given debris flow intensity. In this paper, the physical characteristics of both debris flow intensity and building response are considered. Uncertainties in building capacity and debris flow intensity are explicitly quantified to evaluate the damage probability of a typical reinforced concrete building subjected to debris flow impact. Four damage states with clear failure mechanisms are defined using multi-source information from field observations, numerical simulation, and expert experience. Two series of fragility models are proposed based on practical debris flow impact pressure models. Several debris flow intensity measures are investigated. A better indication can be provided using the intensity measure that represents a specific failure mechanism; e.g., impact force (hv2) for force-dominated failures or overturning moment (h2v2) for moment-dominated failures, where h and v are debris flow depth and velocity, respectively. The corresponding fragility surfaces best express potential building damage. The intensity thresholds in the proposed fragility curves are consistent with those in empirical vulnerability curves. The methodology presented in this paper promotes vulnerability assessment using physics-based modeling, leading to a more reliable evaluation of building damage caused by debris flows.

Experimental study on the performance of CFRP-strengthened masonry structures under debris flow impacts

This paper takes common masonry structures built in mountainous areas prone to debris flow as a research object and proposes carbon fiber-reinforced polymer (CFRP) to improve the resistance of the structures under debris flow impacts. According to a series of impact loading tests on an unreinforced model (hereinafter referred to as UM) and a CFRP-strengthened model (hereinafter referred to as CM), dynamic responses and failure mechanisms of these two structures under debris flow impacts are analyzed. To simulate the impact of large boulders in debris flows, steel balls with different diameters are placed on a loading device, and they roll down along the track from different heights to input homologous impact energy to the scaled models. The test results reveal that the maximum peak acceleration of the CM is only 51.3% of that of the UM, and the peak displacement of the CM is 50% of that of the UM, while the residual displacement is 65% of that of the UM (under the loading case G6 in this paper). When the models reach the serviceability limit state (defined in this paper), the absorbed impact energy of the CM is 7.35 times that of the UM. The test results show that CFRP is an effective reinforcement material to significantly improve structural bearing capacity and reduce dynamic response of structures under debris flow impacts. This study will be helpful for building construction and reinforcement in mountainous areas prone to debris flow impact.

Two-phase SPH numerical study of pore-water pressure effect on debris flows mobility: Yu Tung debris flow

A debris flow is one of the most dangerous types of geophysical mass flows moving downward mountain slopes and consist of a mixture of soil and rocks with a significant quantity of interstitial fluid. They can travel at extremely rapid velocities affecting long distances due to the significant influence of pore-water pressures on the propagation stage. In this paper, a novel two-phase depth-integrated SPH numerical model is applied to simulate a debris flow in which the effects of bed entrainment and pore-water pressure evolution, essential components to performed debris flows risk assessment, are included. The model was applied to simulate the dynamics of Yu Tung debris flow which is an appropriate benchmark case to examine the applicability of the developed model, as bed entrainment and pore-water pressure evolution were key aspects that affected the moving mass dynamics. The developed two-phase depth-integrated SPH-FD model is applied to assess the structural countermeasure of the bottom drainage screen, used to reduce the impact of debris flows. The reasonable results obtained from the analysis indicate that the model is capable to properly reproduce the propagation of the debris flow and more importantly to correctly perform the time-space evolution of pore water pressures during the whole deformation process from initiation through propagation, over an impermeable and permeable bottom boundary, up to deposition.

Debris flow impact on a flexible barrier: laboratory flume experiments and force-based mechanical model validation

Nowadays, hydrogeological instability is a very critical issue because of its impact on population, infrastructures and economic and productive sectors. This makes essential to use advanced and reliable defense technologies, which are able to guarantee high performance and reliability. Flexible and permeable structures are ideal for stopping natural phenomena like debris flow, thanks to their high deformation capacity and their water permeability. This study presents some results obtained from laboratory tests related to the impact of a simulated debris flow against a scaled physical model of the barrier. A granular flow composed of aggregates of known size particle was released in a channel with variable inclination and known length and height. Using this experimental setup, several tests were carried out by varying the inclination of the channel and the geometry of the barrier. The results obtained were correlated with data deriving from a simplified analytical model; the comparison between the measured and calculated values made possible to assess the good prediction capability of the model, providing a simple and innovative tool to design and assess the safety of deformable protection barriers.

Development of fragility curves for road embankments exposed to perpendicular debris flows

Debris flows cause recurrent interruptions and permanent damages in rural road networks, representing significant economic losses. Embankments are road assets frequently exposed to debris flows, especially in mountainous areas. The potential risk of infrastructure exposed to debris flows can be assessed in terms of the probability of expected damage based on fragility curves. The aim of the study is to develop fragility curves for road embankments exposed to debris flows representative to the expected physical damage and capacity loss. Two conceptual models are proposed to describe the impact and erosion of debris flows that run perpendicularly to road embankments. Probabilistic models are then developed in terms of limit state functions and the simulation of potential scenarios. The resulting models are finally fit to log-normal distributions and compared to historical data. Headcut erosion has higher probabilities of occurrence compared to sliding failure caused by the impact of debris flow. Lower road embankments present higher probabilities of failure to the impact of debris flows, especially for dense flows. Whereas, longer interaction times of less dense flows increase the probability of headcut erosion. Two-lane roads may present 50% more probabilities of headcut erosion compared to multilane roads for similar debris flows intensity.

Influence of the particle shape on the impact force of lahar on an obstacle

Lahars represent natural phenomena that can generate severe damage in densely populated urban areas. The evaluation of pressures generated by these mass flows on constructions (buildings, infrastructure…) is crucial for civil protection and assessment of physical vulnerability. The existing tools to model the spread of flows at large scale in densely populated urban areas remain inaccurate in the estimation of mechanical efforts. A discrete numerical model is developed for evaluating debris flow (DF) impact pressures at the local scale of one structure. The large-sized solid particles are modelled explicitly using Distinct Element Method (DEM) and the fine-grained solid particles are integrated in a fluid phase which generates two effects on the movement of particles, i.e. buoyancy and drag. Fluid velocity field and the fluid free surface are obtained from Computational Fluid Dynamics (CFD) code then imported in the DEM simulation in a one way coupling scheme. In this paper, the influence of particle shape on the impact forces generated on the obstacle is investigated: spherical particles and polygonal rigid blocks (r-blocks) are considered. The shape of the particle influences the contact surface and therefore the impact pressure. With an angular shape and several facets like r-blocks, the impact pressure on an obstacle is more important for a flow with the same characteristics.

Impact resistance analysis of grille dam based on fluid structure interaction

Aiming at the impact failure of debris flow grille dam, considering the interaction of boulder-debris flow slurry- grille dam based on SPH-FEM, this article analyzed the variation laws of velocity, impact force and support reaction before and after debris flow slurry and boulders passing through grille dam. The results show that: SPH-FEM coupling method can truly reappear the impact of debris flow on the grille dam; the velocity of debris flow slurry and boulder are reduced by nearly 60% after passing through the dam, and the effect is remarkable; debris flow slurry and boulder have secondary impact on the grille dam. In the first impact, the greater the radius of the boulder, the greater the impact force; in the second impact, the impact force has nothing to do with the radius of the boulder.