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The safety of tailings impoundments has long been a concern of academic researchers globally. The tailings dam breaks are extremely likely to induce secondary disasters such as mudflows, landslides and water and soil pollution, thus further aggravating the risk of accidents. On the basis of practical engineering, this study performed a dam-break physical model experiment and theoretical analysis to examine tailings dam overtopping and dam body collapse processes. It also discussed the leaked tailings flow evolution characteristics after dam failure, based on important parameters such as the dam-breach variation rules after dam break, flow velocity of the leaked tailings flow, particle deposition characteristics and submerged depth. Our research discovered that dam-breach development was mainly constituted by water current erosion-induced longitudinal downcutting and dam-breach slope instability-caused horizontal expansion. A large sand inrush amount resulted in a high downstream flow rate and large sediment depth after dam failure. In addition, when the sampling site was > 100 cm (model scale) away from the dam site, the discharged tailings flow size grading phenomenon was obvious during dam breach, and the particle size gradually decreased with increasing distance between the sampling characteristic sites and dam site.

The events of landslides induced by the combined effect of earthquakes and rainfall are more dangerous than those induced by earthquake or rainfall. The Yongguang loess landslide induced by the Minxian–Zhangxian Ms 6.6 earthquake in 2013 is a typical example of a landslide induced by the combined effect of an earthquake and rainfall. A few scholars have carried out relevant research on this landslide, but the initiation mechanism of this kind of landslide has not been elucidated. Therefore, the initiation mechanism of loess landslide under the combined effect of earthquakes and rainfall is studied through indoor geotechnical tests, shaking table slope model tests and numerical simulations. The indoor geotechnical tests show that the mechanical properties of Q3 loess would change significantly (the mechanical strength would decrease sharply) when soaked in water and that its strength would be further lowered under dynamic action. The slope model test results show that the infiltration depth at the slope top is deeper than that of the slope surface after continuous heavy artificial rainfall and that the natural frequency of the shoulder of the front edge of the slope top is closer to the frequency of the loading wave than it is in other parts of the slope; so the amplification effect of the seismic ground motion at the shoulder of the front edge of the slope top is more significant and the soil damage is more serious under seismic dynamic action. With the rapid increase in the residual deformation and pore water pressure of the soil mass, the loess properties change substantially, that is, loess liquefaction occurs when the pore water pressure and residual deformation reach a certain threshold. Under the action of the subsequent seismic ground motion, the liquefied loess mass of the front edge of the slope top rushes out along the sliding bed and slides down the slope surface in the form of a loess mudflow; that is, a mudflow-like loess landslide is induced. A simplified mechanical model is used in the numerical simulation test and can reproduce the triggering process of this kind of loess landslide. In this research, the initiation mechanism of mudflow-like loess landslide induced by the combined effect of earthquakes and rainfall is revealed by comprehensively considering a variety of test results, and a sliding distance calculation formula is proposed, providing a foundation for the prevention and control of this kind of landslide disaster.

Multi-source mudslides are common in the Daunia Apennines, i.e., the Apulian sector of the Southern Apennines of Italy, and play an important role in the historical evolution of the landscape, with significant impact on the social and economic activities of the area. They also represent the most relevant geological hazard along the front of the Apennine Chain and expose to a considerable risk the local population and infrastructures. The outer sector of the Daunia Apennines is part of the foreland thrust belt system, made up of fissured fine-grained materials, belonging to the Daunia tectonic unit formations. Concerning landslide typology, the most frequent slope movements are represented by flows and composite mudslides, the latter typically starting as slumps evolving from flows. This paper deals with the production of landslide activity maps of three case studies of composite mudslides from the front of the Daunia Apennines. The maps were produced integrating traditional interpretation of multi-year aerial photograph coverage and field surveys with the available digital elevation models to analyze surface morphology. Each landslide activity map corresponds to multi-temporal landslide inventories compiled for the last 15–20 years. This paper outlined that in the last 20 years the front of the Daunia Apennines underwent a significant reactivation of slope failure phenomena after years of quiescence, or of limited activity. The three case studies clearly confirm that: (i) the active portions of the mudslides are located inside or near preexisting slope failures, and (ii) the spatial distribution of instability phenomena is strictly dependent upon the presence of older, larger and dormant, phenomena. Landslide activity maps are an important tool for the evaluation of landslide susceptibility and hazard in the study area, and for quantitative geomorphological analyses it aimed at understanding the long-term geomorphological evolution of this portion of the Southern Apennines. Moreover, we argue that high-quality multi-temporal inventories could have positive effects on all derivative products and analyses, including erosion studies and landscape modeling, susceptibility and hazard assessments, and risk evaluations as well.

More than 2330 landslides/mudflows in loess were triggered by the 2013 Minxian earthquake (MS 6.6) in its epicentral area in Minxian County, Gansu, China. A comprehensive investigation, including in-situ and laboratory tests, was conducted for the largest landslide/mudflow in Yongguang village in order to gain a better understanding of the mechanism and rainfall impact on the landslides/mudflows. A series of triaxial laboratory tests showed that the strength of loess is significantly reduced with increased water content. The static and dynamic parameters of loess, obtained from in-situ and laboratory tests, were used to analyze ground-motion site response. The field observations and site-response analyses suggested that peak ground accelerations generated by the Minxian earthquake were in the range of 0.2–0.4 g in the epicentral area. The static and dynamic parameters of loess were also used to analyze dynamic slope stability and liquefaction potential. The results showed that loess slopes would not fail or liquefy under dry conditions and ground motions with PGA of 0.2–0.4 g, whereas loess slopes would fail or liquefy under saturated conditions. Although difficult to quantify, the water content of the loess was significantly increased, even to the point of being partially saturated, by abnormally high rainfall prior to the earthquake. Thus, the abnormal rainfall before the earthquake was a contributing factor for the massive loess landslides/mudflows in the epicentral area.

Debris flows are one of the most dangerous and common hydrological phenomena in mountainous regions. They are extremely various in their type and character, but they are always mountain flows consisting of a mixture of water and loose-fragmental debris. The problem of calculation and forecasting the mudflows still remains intractable. There are several reasons for that: Firstly, the representatives of the whole spectrum of the Earth Sciences (Hydrology, Geology, Geomorphology, Geography, Mechanics, Rheology) deal with this problem from their point of view. Secondly, systematic monitoring of passing debris flows are currently held only in several countries only (USA, Canada, Austria, Switzerland, Japan, China), because they require significant funding. Thirdly, the calculation methods, having been accepted for the present time, give certain errors. In this article, the results of the artificially triggered debris flow experiments conducted in 1972–1976 in the Chemolgan river basin, organized by the Kazakh Research Hydrometeorological Institute are described. These were the first full-scale experiments with the detailed recording of the numerous debris flows characteristics ever conducted. The movie is attached as supplementary material to the Editorial of the Special Issue. The information about the used measurement equipment, the obtained characteristics of debris flows, the debris flow classification accepted as a result of the experiments is given. Conducting such experiments in nature allowed us to assess various aspects of the formation of these natural phenomena and made it possible to build the mathematical models of the debris flow processes.

On August 7, 2010, a giant mudflow occurred in Zhouqu County, in China’s Gansu Province. It killed at least 1,467 people and destroyed many buildings. The extensive damage resulted partially from the impact of the swift mudflow, and partially from subsequent flooding caused by damming of the Bailong River, creating a lake. The mudflow had both internal and external causes. We discuss these in detail from the perspective of their relationship to human activities, with the goal of helping to provide advance warning of such disasters in the future.

Submarine slides are one of the most complex geotechnical problems causing serious damages to the seabed environment. Damages may lead to imminent destruction of offshore facilities such as platform and subsea pipeline. Mud sediment was the most susceptible of subsea material involved in the event. The completeness of field investigation is very limited; therefore, laboratory experiment is implemented in view of modeling the dynamics movement at the time of submarine slides occurs. The current experiment was conducted by generating the mudflow in water ambient over an inclined base to observe the flow-front structures of the mudflow. As facility to investigate the drag force exerted by mudflow, additional pipe model was positioned at certain flow distance. Mud models were prepared from mixtures of 10–35 % by weight of kaolin and water. Mudflow observations were limited to 3.5-m run-out distance with an exemption for 35 % KCC since this percentage stopped flow at 2.3-m flow distance. The role analysis of destructive flow was developed by the determination of drag force coefficient exerted by mudflow incorporating the Reynolds number. Velocity analysis indicates that all flows were defined as initial flow since they have only one phase of slumping phase. With respect to head-flow height and densimetric Froude number, the general form of velocity propagation formula provided the satisfactorily results in tracing mudflow velocity observation of laboratory experiment. Furthermore, the higher percentage of KCC exerted the higher drag force than the lower one.

The debris flow, which was triggered in the Wenjia Gully on August 13, 2010, is an extreme example of mass movement events, which occurred after the Wenchuan earthquake of May 12, 2008. This Earthquake triggered in the Wenjia Gully the second largest co-seismic landslide, which can be classified as a rockslide-debris avalanche. A lot of loose sediments was deposited in the basin. In the main so called Deposition Area II of this landslide, with a volume of 30 × 106 m3, flash floods can easily trigger debris flows because of the steep bottom slope and the relative small grain sizes of the sediments. The largest debris flow of August 13, 2010 destroyed the most downstream dam in the catchment during a heavy rain storm. The debris flow with a peak discharge of 1,530 m3/s and a total volume of 3.1 × 106 m3 caused the death of 7 persons, 5 persons were missing, 39 persons were injured and 479 houses buried. After three rainy seasons, only 16 % of the landslide-debris deposition was taken away by 5 large-scale debris flow events. Since the threshold for rainfall triggered debris flows in the Wenjia Gully and other catchments drastically decreased after the Wenchuan Earthquake, new catastrophic events are expected in the future during the rainy season.

The 2008 Wenchuan earthquake induced a large number of landslides, and a vast amount of loose landslide materials deposited on steep hill slopes or in channels. Such loose materials can become sources of deadly debris flows once triggered by storms. On 13 August 2010, a storm swept Yingxiu and its vicinity, triggering a catastrophic debris flow with a volume of 1.17 million m3 in Xiaojiagou Ravine. The debris flow buried 1,100 m of road, blocked a river and formed a debris flow barrier lake. A detailed field study was conducted to understand the initiation mechanisms and runout characteristics of this debris flow. Two types of debris flows are identified, namely hill-slope debris flow and channelized debris flow. The hill-slope debris flow was triggered in the forms of firehose effect, rilling and landsliding, whereas the channelized debris flow was triggered in the form of channel-bed failure. This debris flow was a water–rock flow since most particles were gravel, cobble or larger rocks and the fraction of silt and clay was less than 2%. Grain contact friction, pore-pressure effects and inertial grain collision were the three most important physical interactions within the debris flow. Such interactions yielded a smaller runout distance (593 m) compared with those of mud–rock flows of similar size.

During the three flood seasons following the Wenchuan earthquake in 2008, two catastrophic groups of debris flow events occurred in the earthquake-affected area: the 2008-9-24 debris flow events, which had a serious impact on rebuilding; and the 2010-8-13/14 debris flow events, which destroyed much of the progress made in rebuilding. The Wenjia gully is a typical post-earthquake debris flow gully and at least five debris flows have occurred there. As far as the 2010-8-13 debris flow is concerned, the deposits of the Wenjia gully debris flow reached a volume of 3.1 × 106 m3 in volume and hundreds of newly built houses were buried. This study took the Wenjia gully debris flow as an example and discussed the formation and characteristics of post-earthquake debris flow on the basis of field investigations and a remote sensing interpretation. The conclusions drawn from the investigation and analysis were as follows: (1) Post-earthquake debris flows were a joint result of both the earthquake and heavy rainfall. (2) Gully incision and loose material provision are key processes in the initiation and occurrence of debris flows and a cycle can be presented as the following process: runoff—erosion—collapse—engulfment—debris flow—further erosion—further collapse—further engulfment—debris flow enlargement. (3) The amount of rainfall that triggered debris flows from the Wenjia gully was significantly less than the average daily rainfall, while the intraday rainfall threshold decreased by at least 23.3%. (4) The occurrence mechanism of Wenjia gully debris flow was an erosion type and there was a positive relationship between debris flow magnitude and rainfall, which fitted an exponential model. (5) There were five representative characteristics of Wenjia gully debris flow: the long duration of the occurring process; the long distance of deposition chain conversion during the process of damage; magnification in the scale of debris flow; and the high frequency of debris flow events.

Large-volume debris flow events are defined when the volume of solid materials exceeds 1 million m3. Traditional engineering measures, such as check dams, diversion channels, and flumes, are effective for normal debris flow control but are not sufficient to control large-volume debris flows. Experiments were conducted with an artificial step-pool system on the new Wenjiagou Gully to mitigate large-volume debris flows. The old Wenjiagou Gully was buried by 81.6 million m3 of loose solid material created by a landslide that was triggered by the Wenchuan earthquake on May 12, 2008. The new gully was formed during the scouring process caused by debris flows in 2008. Large-volume debris flows were initiated by rainstorm flood with high kinetic energy. The artificial step-pool system was constructed with huge and big boulders on the new Wenjiagou Gully in 2009. The step-pool system dissipated flow energy in steps and hydraulic jumps. Analysis proved that the step-pool system dissipated two-third of the kinetic energy of flow; thus, the critical discharge for triggering debris flow increased threefold. Due to the step-pool system maximized the flow resistance and protected the bed sediment and banks from erosion, the rainstorm floods in 2009 did not trigger debris flows. In 2010, the step-pool system was replaced with 20 check dams. Huge boulders were broken into small pieces of diameter less than 0.5 m and were used as building materials for the 20 dams. Without the protection of the step-pool system, a rainstorm flood scoured the base of the dams and caused failures for all of the 20 check dams in August 2010. The flow incised the gully bed by 50 m. The loose bank materials slid into the flow mixed with water and formed a large-volume debris flow with a volume of 4.5 million m3. Many houses were buried by the debris flow, and 12 people were killed. Comparison of the two strategies proved that energy dissipation structures are necessary for controlling large-volume debris flows. Check dams, if they are stable, may reduce the potential of bank failures and control debris flows. The step-pool system dissipates flow energy and control gully bed incision and bank failure. A combination of check dams and step-pool systems may be the most effective for mitigating debris flows.

Soil properties of major landslides that occurred recently on the mid-altitude slopes of Mount Elgon, eastern Uganda were analysed. A mudflow, located at the Kitati protected forest site, and two deep debris flows on the Nametsi and Buwabwala deforested steep slopes (36°–58°) were surveyed. In order to test the hypothesis that ‘soils at the landslide sites are particularly ‘problem soils’ and thus prone to landslides’, the following analyses were undertaken: particle size distribution, Atterberg limits, shear strength and factor of safety (Fs). Soils at the Kitati and Buwabwala sites exhibited expansive potential, owing to clay contents well above 20%. A clay content exceeding 32% was identified at the Nametsi debris flow site implying an extremely high expansive potential of the soil. High liquid limits (LLs) at Kitati (59%) and Buwabwala (53%) meant that the soils qualified as vertisols susceptible to landslides. High plasticity indices (PIs) (averaging 33%) also confirmed the vertic nature of soils at the Nametsi debris flow site. Whereas the value of F s 1). Despite this finding, the stable sites could be described as only conditionally stable because of the interplay of various physical, pedological and anthropogenic factors. The results point to the fact that soils at the landslide sites are inherently ‘problem soils’ where slope failure can occur even without human intervention. Therefore, the hypothesis that soils at three landslide sites are inherently ‘problem soils’ and prone to landslides, is accepted.

A rock slide on top of the clayey–silty–sandy–pebbly masses in the Gschliefgraben (Upper Austria province, Lake Traunsee) having occurred in 2006, together with the humid autumn of 2007, triggered a mudslide comprising a volume up to 4 million m³ and moving with a maximum displacement velocity of 5 m/day during the winter of 2007–2008. The possible damage was estimated up to 60 million € due to the possible destruction of houses and of a road to a settlement with intense tourism. The movement front ran ahead in the creek bed. Therefore, it was assumed that water played an important role. Inclinometer measurements showed that a less permeable layer was sliding on a thin, more permeable layer. During the last centuries, mudslides had already pushed farms into the lake, as reported by chronicles. Thus, the inhabitants of 46 houses had to be evacuated for safety reasons. They could return to their homes after displacement velocities had decreased. It was decided (a) to prevent soaking of water into the uppermost, less permeable layer by transversal drainages, (b) to lower the pore water pressures by longitudinal trenches filled with blocky material, (c) to pump water out of the more permeable layer by well drillings upslope of the houses in order to create a stable block below the houses and (d) to remove material thrust over the stable blocks in order to avoid damage to the houses. These mitigation measures costing 11.5 million € led to a deceleration of the process to displacement velocities of some cm/year up to now. The houses and the road were not damaged.

Every year, and in many countries worldwide, wildfires cause significant damage and economic losses due to both the direct effects of the fires and the subsequent accelerated runoff, erosion, and debris flow. Wildfires can have profound effects on the hydrologic response of watersheds by changing the infiltration characteristics and erodibility of the soil, which leads to decreased rainfall infiltration, significantly increased overland flow and runoff in channels, and movement of soil. Debris-flow activity is among the most destructive consequences of these changes, often causing extensive damage to human infrastructure. Data from the Mediterranean area and Western United States of America help identify the primary processes that result in debris flows in recently burned areas. Two primary processes for the initiation of fire-related debris flows have been so far identified: (1) runoff-dominated erosion by surface overland flow; and (2) infiltration-triggered failure and mobilization of a discrete landslide mass. The first process is frequently documented immediately post-fire and leads to the generation of debris flows through progressive bulking of storm runoff with sediment eroded from the hillslopes and channels. As sediment is incorporated into water, runoff can convert to debris flow. The conversion to debris flow may be observed at a position within a drainage network that appears to be controlled by threshold values of upslope contributing area and its gradient. At these locations, sufficient eroded material has been incorporated, relative to the volume of contributing surface runoff, to generate debris flows. Debris flows have also been generated from burned basins in response to increased runoff by water cascading over a steep, bedrock cliff, and incorporating material from readily erodible colluvium or channel bed. Post-fire debris flows have also been generated by infiltration-triggered landslide failures which then mobilize into debris flows. However, only 12% of documented cases exhibited this process. When they do occur, the landslide failures range in thickness from a few tens of centimeters to more than 6 m, and generally involve the soil and colluvium mantled hillslopes. Surficial landslide failures in burned areas most frequently occur in response to prolonged periods of storm rainfall, or prolonged rainfall in combination with rapid snowmelt or rain-on-snow events.

An investigation is made of the influence of fire-caused deforestation on precipitation-triggered, shallow landslide susceptibilities in southern California, using a scalable and extensible geo-fluid model (SEGMENT), that accounts for soil mechanics, root distribution, and relevant hydrologic processes. SEGMENT is applicable to variable regional topography, soil thickness profiles, and vegetation coverage. In this study, for southern California following the 2007 wildfires, three experiments were performed with rainfall recurrence periods chosen to be 2, 10, and 25 years. These intervals correspond to 24-h storm rainfall totals of 17, 25.4, and 33 mm. The model generated landslide stability maps that identified three areas of high instability. These unstable regions are located in the San Fernando Valley, the San Gabriel Mountains, and the Santa Monica Mountains. In each case, the vegetation cover had been severely burned during the preceding 2007 wildfires. The model results showed that burning from wildfires is a major destabilizing factor for southern California. Burn sites are more prone to landslides than vegetated slopes because the soil more readily exceeds its critical moisture content. Severe droughts in a future warming climate are expected to increase the likelihood of more frequent and intense wild fires. Higher temperatures combined with decreased total rainfalls facilitate more intense landslides, including devastating mudslides, following heavy precipitation. Finally, the model is designed to assist in developing timely mitigation measures for post-fire, storm-triggered landslides.

During the last 50 years, an average of 30 persons per year was killed by avalanches in Austria. About one-third of all avalanche fatalities occurred as a result of so-called ‘catastrophic avalanches’. ‘Catastrophic avalanches’ are spontaneously released avalanches that affect villages and cause damage to property (buildings, roads and other infrastructure). The biggest avalanche events in Austria were in 1950/1951 (135 fatalities), in 1953/1954 (143 fatalities) and in February 1999, when 38 persons were killed in Galtur and Valzur. This article deals with an analysis of nine major avalanche cycles in the last 55 years. An avalanche cycle in this article is defined as 50 recorded avalanches of at least size 3 in two days and/or 5 persons killed in villages within two days. The basis of this study are the well-documented records from Fliri (1998), who analysed natural disasters in the western part of Austria and the Trentino, including floods, mudflows, earthquakes and avalanches. The meteorological data were taken from two relevant observation sites in the northern part of the Austrian Alps, from two sites in an intermediate and continental region, respectively and from one site in the southern part of the Austrian Alps. Atmospheric patterns were analysed by using weather charts for the relevant periods. Both the meteorological data and the weather charts were provided by the Central Institute for Meteorology and Geodynamics (ZAMG). It was found that there was a major cycle every 6 years (on average). Two-thirds of all investigated cycles were characterised by a continuous increase of snow depth over a period of at least three days. In only three periods (1975, 1986, 1988), daily extreme values could be observed. More than 40% of all the cycles occurred in January. In two-thirds, a north-westerly oriented frontal zone was responsible for the formation of a major cycle. The remaining cycles were released by low-pressure areas over Central Europe and the Mediterranean Sea, respectively.

Debris flows are more frequent in central Taiwan, because of its mountainous geography. For example, many debris flows were induced by Typhoon Herb in 1996. The Chi-Chi earthquake with a magnitude of 7.3, which took place in 1999 in central Taiwan, induced many landslides in this region. Some landslides turned into debris flows when Typhoon Toraji struck Taiwan in 2001. This study investigates the characteristics of the gullies where debris flows have occurred for a comparison. Aerial photos of these regions dated in 1997 (before the earthquake) and 2001 (after the earthquake) are used to identify the occurrence of gully-type debris flows. A Geographic Information System (GIS) is applied to acquire hydrological and geomorphic characteristics: stream gradient, stream length, catchment gradient, catchment area, form factor, and geology unit of these gullies. These characteristics in different study regions are presented in a statistical approach. The study of how strong ground motion affects the debris flows occurrence is conducted. The characteristics of the debris flow gullies triggered by typhoons before and after the Chi-Chi earthquake are quantitatively compared. The analysis results show that a significant transformation in the characteristics was induced by the Chi-Chi earthquake. In general, the transformation points out a lower hydrological and geomorphic threshold to trigger debris flows after the Chi-Chi earthquake. The susceptibility of rock units to strong ground motion is also examined. The analysis of debris flow density and accumulated rainfall in regions of different ground motion also reveal that the rainfall threshold decreases after the Chi-Chi earthquake.

Debris and mudflows are some of the main geological hazards in the mountain foothills of the Chilean capital city of Santiago. There, the risk of flows triggered in the basins of ravines that drain the range into the city increases with time due to the city growth. A multivariate statistical study based on the logistic regression method is presented. The model provides equations that allow the computation of combined meteorological triggering factors associated with a probability of rain-induced flow occurrence. Daily rainfall, accumulated rainfall and the snowfall level, traditionally considered as the relevant factors, are analysed for a 25-year period. The results show a strong relevance of the rainfall on the day of the flow event over the other factors. However, the relatively low probabilities returned for some real flow events suggest that the model does not capture all the significant variables and the problem is more complex than as it has been traditionally assumed, and further investigations are needed to develop predictive models of flow triggering.