In the Wenchuan area in SW China, an abundance of loose co-seismic landslide debris was present on the slopes after the Wenchuan earthquake, which in later years served as source material for ...rainfall-induced debris flows or shallow landslides. Slopes composed of Cambrian sandstones and siltstones intercalated with slates appeared to be most susceptible to co-seismic landsliding. A total of 20 debris flows are described in this paper; all were triggered by heavy rainfall on 13th of August 2010. Field reconnaissance and measurements, supported by aerial photo interpretation, were conducted to identify the locations and morphological characteristics of the debris flow gullies in order to obtain information about surface area and volume of landslides and the debris flows. The debris flows in the study area were initiated by two processes: a) run-off erosion on co-seismic landslide material, and concentrated erosion of landslide debris in steep channels; b) new landslides that transform into debris flows. The volume of debris flow deposits on individual fans varies by many orders of magnitude. The smallest deposit has a volume of from 5760 to 3.1
million
m
3. A comparison of the measured volumes, deposited on the fan with the volumes of debris stored in the catchment shows the huge potential for future debris flow activity. Whilst there is a weakly significant positive correlation between these two volumes, no significant statistical correlation could be established between volumes of debris flow deposits and other morphometric parameters of the catchment.
A catastrophic debris flow catchment (the Wenjia catchment) was selected as an extreme case to show in detail the mechanism of debris flow formation as a result of intensive erosion in loose material, which was deposited by a rock avalanche during the 2008 Earthquake event. Analyses of the meteorological conditions that triggered these debris flows show one day antecedent precipitation varying between 67.7 and 137.6
mm, with a mean rainfall intensity of about 7.3 to 22.5
mm/h. A rainfall event with peak intensity of 38.7
mm/h triggered the largest debris flow event. Rainfall data related to five debris flow events in the Wenjia torrent was used to establish a primary rainfall intensity–duration relationship for the triggering of debris flows, which was compared to other rainfall duration thresholds from other parts of the world. In the discussion emphasis is laid on the need to unravel the process mechanisms which initiated the debris flows for a better understanding and assessment of meteorological thresholds.
► The Wenchuan earthquake area is more susceptible to rainfall initiated debris flows. ► The sediments for the debris flows were supplied by the co-seismic landslides. ► Two process mechanisms play a role in the increased activity of debris flows. ► Debris flows transport huge amounts of sediment downstream generating debris-dams.
Although numerous studies focus on the mechanical behaviour and instability of tailings dam materials, a framework that adequately describes all the processes involved in their failure and ...fluidization is still needed. Therefore, we used a series of instrumented flume tests, with pore pressure transducers, micro-seismic accelerometers, internal displacement transducers and laser scanning, to investigate the mechanisms of tailings-dam failure and fluidization. The artificial rainfall induced water infiltration was adopted to induce the failure. The test results showed that continuous rainfall expanded macro-voids and led to particle rearrangement and local collapse, producing a gradual buildup of pore pressure. Furthermore, the increasing porosity enhanced seepage forces and thus reduced the shear strength of the tailing's materials so that the dam models were close to liquefaction. However, a dam model with a higher hydraulic conductivity would not be totally damaged, even if failure and fluidization occurred. The amount of antecedent rainfall likely significantly contributed to dam failure because it raised the moisture content of the tailing's materials, which promoted internal erosion; thus, the rainfall might have been sufficient to generate local fluidization. Rainfall decreased the peak pore pressures required to initiate failure and shortened the time to failure but hardly influenced the fluidization movement distance.
•We use a series of instrumented flume tests to investigate the mechanisms of tailings-dam failure and fluidization.•We find that the relative density of tailing material can largely influence the failure pattern of the tailing slope.
For a quantitative assessment of debris flow risk, it is essential to consider not only the hazardous process itself but also to perform an analysis of its consequences. This should include the ...estimation of the expected monetary losses as the product of the hazard with a given magnitude and the vulnerability of the elements exposed. A quantifiable integrated approach of both hazard and vulnerability is becoming a required practice in risk reduction management. This study aims at developing physical vulnerability curves for debris flows through the use of a dynamic run-out model. Dynamic run-out models for debris flows are able to calculate physical outputs (extension, depths, velocities, impact pressures) and to determine the zones where the elements at risk could suffer an impact. These results can then be applied to consequence analyses and risk calculations. On 13 July 2008, after more than two days of intense rainfall, several debris and mud flows were released in the central part of the Valtellina Valley (Lombardy Region, Northern Italy). One of the largest debris flows events occurred in a village called Selvetta. The debris flow event was reconstructed after extensive field work and interviews with local inhabitants and civil protection teams. The Selvetta event was modelled with the FLO-2D program, an Eulerian formulation with a finite differences numerical scheme that requires the specification of an input hydrograph. The internal stresses are isotropic and the basal shear stresses are calculated using a quadratic model. The behaviour and run-out of the flow was reconstructed. The significance of calculated values of the flow depth, velocity, and pressure were investigated in terms of the resulting damage to the affected buildings. The physical damage was quantified for each affected structure within the context of physical vulnerability, which was calculated as the ratio between the monetary loss and the reconstruction value. Three different empirical vulnerability curves were obtained, which are functions of debris flow depth, impact pressure, and kinematic viscosity, respectively. A quantitative approach to estimate the vulnerability of an exposed element to a debris flow which can be independent of the temporal occurrence of the hazard event is presented.
The quantification of risk has gained importance in many disciplines, including landslide studies. The literature on landslide risk assessment illustrates the developments which have taken place in ...the last decade and that quantitative risk assessment is feasible for geotechnical engineering on a site investigation scale and the evaluation of linear features (e.g., pipelines, roads). However, the generation of quantitative risk zonation maps for regulatory and development planning by local authorities still seems a step too far, especially at medium scales (1:10,000-1:50,000). This paper reviews the problem of attempting to quantify landslide risk over larger areas, discussing a number of difficulties related to the generation of landslide inventory maps including information on date, type and volume of the landslide, the determination of its spatial and temporal probability, the modelling of runout and the assessment of landslide vulnerability. An overview of recent developments in the different approaches to landslide hazard and risk zonation at medium scales is given. The paper concludes with a number of new advances and challenges for the future, such as the use of very detailed topographic data, the generation of event-based landslide inventory maps, the use of these maps in spatial-temporal probabilistic modelling and the use of land use and climatic change scenarios in deterministic modelling.
Debris flows are one of the most hazardous types of landslides in mountain regions. In the upper part of the Zêzere valley (Serra da Estrela, Portugal) several debris flows events occurred in the ...last 200 years, some of them causing loss of lives and material damages. In this work, a methodology for pedestrian evacuation modelling, in a debris flow hazard scenario, was implemented. A dynamic run-out model, developed in previous studies, was used to evaluate the debris flows velocities, thickness of the deposits and extent of the mobilized material. The buildings potentially affected by the impact of debris flows were identified and the potentially exposed population was estimated by applying a dasymetric distribution. The results lead to the conclusion that, in the study area, the elderly are those who are most exposed to debris flows. Furthermore, the time lapse between the debris flows initiation and the arrival at the buildings at risk was estimated, allowing to account for the overall number of buildings where the evacuation time takes longer than the debris flows arrival. Additionally, the safe areas within the study area were identified, and several safe public buildings with the capacity to gather a large number of persons were selected. Considering that the study area is located in a mountain region, characterized by steep slopes, the evacuation modelling was performed based on an anisotropic approach, in order to consider the influence of slope direction on travel costs. At the end, three pedestrian evacuation travel time scenarios, based on different walking speeds to accommodate residents with different ages in safer places, were compared and the results mapped. The implemented methodology is not local dependent, which allows its reproduction elsewhere.
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•Identification of buildings at risk and safe areas in a debris flow hazard scenario•Estimation of the potentially exposed population•Identification of the elderly as the most exposed to debris flows•Pedestrian evacuation modelling and travel times estimation•Simulation of different walking speeds
Landslides are one of the commonly natural disasters triggered by rainfalls, earthquakes, and human activities, which cause fatalities, damage to properties, and economic losses in the world. Early ...warning method may predict landslide occurrence and reduce the risk. In this paper, we propose a self-developed site-specific landslide early warning system (LEWS), which has been progressively designed and accomplished over the past decade in Southwest China. The warning model for the prediction of slope instability is focused on multiple thresholds, including deformation rate, rate increment and tangential angle related to surface displacement measurement. The debris-flow initiation is determined by critical rainfall thresholds integrated with the topographic and geological conditions at a catchment scale. The recent Xingyi rockslide, Baige debris slide and debris flows in Zoumaling catchment are selected to explain the specific process of landslide early warning in a real-time operating system. The performance of the presented warning system is evaluated by comparison with the inverse-velocity model (INV) and gradient model (SLO) in the prediction of slope failure time. The Receiver Operator Characteristic (ROC) analysis is used to prove its feasibility and reliability in the debris-flow initiation prediction. The presented warning model and system can be applied to other regions in landslide early warning and to be useful to mitigate landslide losses and damages.
•Provide a successfully real-time landslide early warning system.•Warning model and warning system can be applied in an emergency response to mitigate landslide losses.•Comparing with the SLO, INV model and using ROC analysis to evaluate the performance of the warning system.
The initiation of debris flows is commonly attributed either to fluidization as a result of rainfall-induced landslides or to gully erosion induced by concentrated runoffs. A series of flume tests ...have been performed to show how the initial soil moisture influences the initiation of debris flows. At the start of each experiment, surface runoff was generated over loose granular deposits, triggering debris flows. These experimental debris flows enacted different scenarios according to the small variations among the initial soil moistures. In the loose granular deposits with initial soil moistures ranging from 1 to 5 %, most runoff water could infiltrate and trigger a landslide, which accelerated within 1 s to speed over 1 ms
−1
and then transformed into a debris flow. In the same soil deposits with initial moistures >5 or <1 %, the debris flow was initiated by slow gully erosion with episodic events of damming and breaching due to small-scale landslides occurring on the side-slopes of the erosion valley. The slope failures were not triggered by positive pore pressure but by a decrease in suction due to the wetting of the soil. This suction decrease in initially unsaturated slopes explains why the transformation of these slope failures into debris flows are due not only to an increase of pore pressure leading to soil liquefaction, which is one of the expected triggering mechanisms, but also to a loss of the cohesive strength of the soil.
The frequency of huge debris flows greatly increased in the epicenter area of the Wenchuan earthquake. Field investigation revealed that runoff during rainstorm played a major role in generating ...debris flows on the loose deposits, left by coseismic debris avalanches. However, the mechanisms of these runoff-generated debris flows are not well understood due to the complexity of the initiation processes. To better understand the initiation mechanisms, we simulated and monitored the initiation process in laboratory flume test, with the help of a 3D laser scanner. We found that run-off incision caused an accumulation of material down slope. This failed as shallow slides when saturated, transforming the process into debris in a second stage. After this initial phase, the debris flow volume increased rapidly by a chain of subsequent cascading processes starting with collapses of the side walls, damming and breaching, leading to a rapid widening of the erosion channel. In terms of erosion amount, the subsequent mechanisms were much more important than the initial one. The damming and breaching were found to be the main reasons for the huge magnitude of the debris flows in the post-earthquake area. It was also found that the tested material was susceptible to excess pore pressure and liquefaction in undrained triaxial, which may be a reason for the fluidization in the flume tests.
In the Wenchuan area in the southwest of China, a huge amount of loose co-seismic landslide material was deposited on slopes during the Wenchuan earthquake of May 2008. These loose deposits formed ...the source material for rainfall-induced debris flows or shallow landslides in the years after the earthquake. On August 13, 2010, about 20 large debris flows were triggered by heavy rainfall in the area around the epicenter of the Wenchuan earthquake. Field reconnaissance revealed that the initiation of these post-earthquake debris flows was closely related to severe erosion of the loose deposits.
Flume tests were carried out to study the initiation mechanism of these post-earthquake debris flows and the related influencing factors. The flume was instrumented with ten combination sets of suction and pore-pressure sensors. These sensors were accompanied by TDR probes to measure the soil water content. The flume has a length of 2.5m and a width of 1.5m.
A series of 26 tests was conducted to study the influence of slope gradient and discharge on the initiation mechanism and scale of the debris flows. The amount of debris-flow discharge was obtained by collecting the washed out deposits every 20s.
The experimental results showed that the initiation mechanism of debris flows for gentle slopes and steep slopes was different. On steeper slope, incision by run-off water was very rapid initiating directly the start of a debris flow. The debris flow volume increased rapidly by a chain of subsequent cascading processes starting with collapses of the side walls, damming and breaching leading to a rapid widening of the erosion channel. At gentler slopes less intensive run-off incision caused an accumulation of material down slope, which, after saturation, failed as shallow slides transform in a second stage into debris flows. It was demonstrated that the slope was the dominant factor in controlling the scale of the debris flows while the effect of discharge on erosion and the size of the debris flows was less clear.
The frequency of snowmelt‐induced soil slope instabilities is increasing in some seasonally cold regions because of climate change. Reliable hazard assessment and risk mitigation of snowmelt‐induced ...landslides require physically‐based prediction models. However, existing models either apply only at the slope scale or assume precipitation as the sole landslide trigger. In doing so, they neglect the complexity and coupled nature of the thermo‐hydro‐mechanical processes leading to slope instability in seasonally cold regions (such as snow accumulation and melting, infiltration and surface runoff, soil saturation, pore water pressure buildup and dissipation). Here, we present a spatially distributed and sequentially coupled numerical model to simulate snowmelt‐induced slope instabilities at the catchment scale. The model accounts for temperature‐dependent changes in the soil hydraulic behavior related to changes in water state by means of a routine implemented in a geographic information system. We verified the performance of the model using a case study of spring snowmelt‐induced soil slope failures that occurred after the 2004 Mid‐Niigata earthquake in Japan. Considering limitations and simplifications, the model was able to predict the triggering condition, magnitude, and spatial distribution of the snowmelt‐induced landslides with a satisfactory degree of accuracy. We believe that the robustness and simplicity of our numerical approach make it suitable for implementation in early warning systems.
Plain Language Summary
Climate change is responsible for an increasing number of natural disasters in seasonally cold regions, where heavier snowfalls and sudden snowmelts can cause destructive landslides on hill slopes. These landslides are not easy to study because the mechanisms that trigger them are different from those of shallow landslides occurring in response to rainfall. Moreover, advanced models that incorporate the complex relations among rain, snow, temperature, soil freezing, melting, and slope stability at a detailed scale are still cumbersome and very demanding in terms of computational power. For these reasons, evaluating the risk to people and assets in seasonally cold regions is not straightforward, and existing models tend to underestimate it. In this paper, we propose a novel modeling strategy to study the stability of soil slopes under the influence of both snowmelt and rainfall at a regional scale. First, we explain the governing mechanisms of snowmelt‐induced landslides; then, we present our strategy along with detailed explanations of the equations we use. We test our model by predicting a known case study (the snowmelt‐induced landslides of 2005 in Niigata, Japan) and obtain a reasonably good result. This simple strategy could be applied in an early warning system for snowmelt‐induced landslides.
Key Points
Developed a novel spatially distributed numerical model for snowmelt‐induced shallow landslides
Simplified account of below‐freezing temperature on hydro‐mechanical‐behavior for catchment‐scale slope stability assessment
Model performance evaluated on snowmelt‐induced post‐seismic landslides occurred in Niigata, Japan, in 2005
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