Types Of Debris Flows Research Articles

Submarine landslides on the Eastern South Korea Plateau (ESKP)—Do pumice-rich tephra layers control slope stability?

Tephra layers embedded in marine sediments can be significant in controlling submarine landslide dynamics and seafloor morphology, but their preconditioning effects on slope failure remain uncertain. Here, we study the morphology and preconditioning factors of submarine landslides from a volcanically active region, the Eastern South Korea Plateau (ESKP) using a recently acquired multibeam echosounder (MBES), high-resolution sub-bottom chirp profiler, and piston core data, together with previous geophysical data. At least 50 translational landslides were identified on the southern margin of the ESKP in an area of ∼470 km2, with a relatively small volume of remobilized sediment (∼10 km3). The landslide headscarps are arcuate, up to 400 m in height, and lie at water depths of 980 to 2300 m. Landslides on the upper ESKP margin are more disintegrative (debris flow type) than those on the lower margin which consists largely of blocky type failures with small runouts. Landslide deposits identified in the cores consist of debris flows and slides/slumps while background deposits contain hemipelagic mud interbedded with ca. 10-cm-thick coarse-grained, poorly-sorted pumiceous tephra. Seismic and core integration indicate that the glide planes of the observed slides correspond to the pumice-rich tephra layers. These tephras are predominantly composed of fresh volcanic glass devoid of clay, indicating that the composition of the tephra did not control the formation of weak layers or slide planes. We infer therefore that the weakness of the glide planes resulted from high hydraulic (pore) pressure at the interface between some of the porous tephra layers and their overlying, relatively impermeable, hemipelagic mud. In addition to burial compaction and build-up of pore pressure in the tephra layers, fluid overpressure could have been facilitated by earthquakes causing granular convection within, and compaction of, the poorly sorted tephra. The landslide predominantly formed > ca. 84 ka ago, which suggests time-restricted triggering associated with a temporary increase in seismicity and/or sediments reaching their overpressure threshold during burial compaction. Our study suggests that regional occurrences of discrete layers of porous tephra deposits within finer grained hemipelagic sediments can potentially control the generation of landslides in subaqueous environments affected by explosive volcanism.

An integrated hydrodynamic model for runoff-generated debris flows with novel formulation of bed erosion and deposition

Debris flow is one of the most common geohazards in mountainous regions, posing significant threats to people, property and infrastructure. Among different types of debris flows, runoff-generated debris flows are attributed to rain storms, which provide abundant runoff that entrain large quantities of bed material, resulting in the formation of a solid-liquid current known as a debris flow. One of the keys to effectively simulating runoff-generated debris flows is modelling the erosion-deposition process. The commonly used approach for formulating erosion and deposition, although constrained by physics, suffers from a singularity in the presence of vanishing velocity, which poses a major challenge for practical applications. It is also argued that the deposition rate cannot be represented by simply reversing the sign of the erosion rate. To address these two issues, we have developed a depth-averaged debris flow model with a novel method of calculating the erosion-deposition rate. We have demonstrated that the singularity is due to the non-linear erosion-deposition term but quickly disappears while the flow converges to the equilibrium that is defined by the classic Takahashi's formula. To resolve the non-linearity and avoid the singularity, an implicit method within a Godunov-type finite volume framework has been proposed. An additional parameter is introduced to differentiate the erosion rate from the deposition rate. The model is validated against several test cases, including a real-world debris flow event. Satisfactory results are obtained, demonstrating the model's simulation capability and potential for wider applications such as risk assessment and impact-based early warning.

Discussion on the Relationship between Debris Flow Provenance Particle Characteristics, Gully Slope, and Debris Flow Types along the Karakoram Highway

Debris flow, the most extensive and most severe geological hazard along the Karakoram Highway, frequently blocks the Karakoram Highway. Based on the methods of field measurement, indoor statistical analysis and theoretical research, this paper discusses the relationship between the four types of debris flow along the Karakoram Highway. The four types are the rain type, the rain glacier type, the glacier ice lake break type and the freeze–thaw type, and their particle characteristics and gully slope are also considered in the discussion. The results are as follows: (1) The provenance particle size of debris flow is controlled by the type of debris flow. Generally, the provenance average particle equivalent diameter of the debris flow induced by the glacier ice lake type is relatively small, followed by the freeze–thaw type and glacier ice lake break type, and the equivalent diameter of the debris flow induced by the rain type is relatively large; (2) The gully slope coefficient of the debris flow C along the Karakoram Highway is greater than 1, and it increases with the increase in gully slope α, that is, the larger C is, the steeper the gully slope will be; (3) The gully slope coefficient C and the average particle equivalent diameter D of the four types of debris flow are distributed in the ellipse with them as the axis. This ellipse quantitatively describes the relationship between the gully slope of the four types of debris flow and the corresponding provenance particle characteristics. This paper analyzes the formation and causes of debris flow along the Karakoram Highway. It accurately understands the scientific connotation of debris flow formation in the surface matrix layer and improves the diversity, stability, and sustainability of the ecosystem. The paper also proposes ideas and suggestions for promoting the ecological protection and restoration of the Karakoram Highway. Therefore, the research has a certain theoretical significance and practical application value for the appropriate selection and rational design of the debris flow prevention projects along the China–Pakistan Highway.

Prediction model of outburst debris flow triggered by landslide dam failure considering the erosion process

The formation of outburst debris flow is different from other types of debris flow as it is formed by the gradual erosion of the riverbed by the unsteady and rapid outburst flow triggered by landslide dam failure. However, a prediction model for outburst debris flow that comprehensively considers the erosion effect has not yet been established. In this paper, the shallow water equation is used to describe the motion of outburst flow. This study proposes a method for constructing a virtual unit and resetting the terrain variables to reproduce the wet and dry boundary. An erosion model considering the effect of hindered erosion is introduced. By combining the shallow water equations, a debris flow prediction model that considers the entire erosion process of outburst flow is thus proposed. The established model is verified by numerical experiments on the evolution of dam-break flow on a dry fixed bed and on a moveable bed; additionally, it is verified by physical experiments on the formation of outburst debris flow. Combined with the calculation results of erosion models with and without a hindered erosion effect, the erosion model without the hindered erosion effect overestimates the flow concentration in this paper. The simulation results are closer to the actual values when considering the effect of hindered erosion in the analysis of outburst debris flow formation. The proposed model in this paper can be used as a tool to predict the occurrence of outburst debris flow.

Keynote lecture. A general two phase depth integrated model considering pore water pressure and dewatering

Debris flows can be considered a type of landslide with large velocities and long run-out distances. There are many types of debris flows, depending on the properties of the solid and fluid components of the mixture. The triggering and propagation of debris flows can be studied using a single 3D mathematical model. The computational cost can be very high because of their length, and depth-integrated models provide a good combination of accuracy and cost. Both types of models can be combined in the analysis, using 3D models for initiation and at singular points where more accuracy is wanted. As in a chain where the strength is never higher than that of the weaker link, we have to ensure that all the models are accurate enough in a joint model. This paper deals with a new depth-integrated model which can take into account the changes caused by dewatering in a debris flow.

A new two-layer two-phase depth-integrated SPH model implementing dewatering: Application to debris flows

Debris flows can be considered a type of landslide with large velocities and long run-out distances. There are many types of debris flows, depending on the properties of the solid and fluid components of the mixture. The triggering and propagation of debris flows can be studied using a single 3D mathematical model. The computational cost can be very high because of their length, and depth-integrated models provide a good combination of accuracy and cost. Both types of models can be combined in the analysis, using 3D models for initiation and at singular points where more accuracy is wanted. As in a chain where the strength is never higher than that of the weaker link, we have to ensure that all the models are accurate enough in a joint model. This paper deals with a new depth-integrated model which can take into account the changes caused by dewatering in a debris flow. An important limitation of existing two-phase models allowing different velocities of solid and water particles is that when water abandons the mixture, porosity decreases and tends to zero. Here, a two-layer model is introduced, including an unsaturated upper layer on top of a saturated layer.

Weathering pits as a geochronometer of environmental changes in the State of Pernambuco, Northeastern Brazil

This research was intended to interpret, by means of several empirical bases, the quaternary climate variabilities in the area of Fazenda Nova. The premise that guides the investigation is that the sedimentary fills of the weathering pits favors the understanding of the landscape and geomorphological changes in the area, by means of the investigation of a depositional environment that is still closely linked to the current landscape. Due to that, the analysis of the depositional events and climate variabilities was done based on a morphostratigraphic approach, dating by Optically Simulated Luminescence (LOE), O18 isotope of carbonatic crust and phytolith analysis. The results showed that the deposits that fill the weathering pits were derived from gravitational sedimentation of the debris flow type, under torrential conditions, evidencing that the genesis of these records was associated with cycles of pedogenesis/morphogenesis under several combinations of semiaridity at work in the area.

Grain Size Curve Characteristics of 2nd Member of Sangonghe Formation in Qianshao Area and Its Indicative Significance of Hydrodynamic Environment

There are mainly two sedimentary types of the 2nd member of Jurassic Sangonghe Formation (J1s2) in the Qianshao area: shallow-water delta and sandy debris flow. In order to accurately understand the characteristics and further identify the difference of sedimentary microfacies, the characteristics of the grain size curve are analyzed. The results show that the cumulative probability curve of sandstone particle size in the research area mainly includes two basic types: tractive current type and sandy debris flow type. The tractive current type is mainly developed in the shallow-water delta front. Among them, the inner front subaqueous distributary channel microfacies develop two patterns: tri-segment pattern and one bouncing segment-one suspension segment-one complex transitional zone pattern. The transitional section is widely developed. There are two patterns in the subaqueous distributary channel microfacies of the outer front (bi-segment pattern and one bouncing segment-one suspension segment-one transitional zone pattern) and two patterns in the estuary bar microfacies (bi-bouncing segment-one suspension segment-one transitional zone pattern and one bouncing segment-one suspension segment-one transitional zone pattern). The main tractive current characteristics are higher bouncing component content and slope and lower suspended component content. The sand debris flow type mainly develops two patterns: one bouncing segment-one suspension segment-one transitional zone pattern and one bouncing segment-one suspension segment-one complex transitional zone pattern with lower jump component content and slope and higher suspended component content.

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.

Effect of debris-flow sediment grain-size distribution on fan morphology

Abstract. Knowledge of how debris flows result in the fan-shaped morphology around a channel outlet is crucial for mitigation of debris-flow-related disasters and investigation of previous sediment transport from the upper channel. Therefore, using a flume connected to a deposition area (inundation plane), this study conducted fan-morphology experiments to assess the effects of differences in grain-size distribution within debris flows on changes in fan morphology. Two types of debris-flow material, i.e., monogranular particles comprising monodispersed sediment particles and multigranular particles comprising polydispersed sediment particles, were used to generate monogranular and multigranular experimental debris flows, respectively. By adjusting the average grain size coincident between the monogranular and multigranular flows, we generated two types of debris flow with similar debris mixture hydrographs but different grain-size distributions in the flume. Although the flow depths were mostly similar between the monogranular and multigranular flows before the start of the debris-flow runout at the deposition area, the runout distances of the front of the multigranular flows were shorter than those of the monogranular flows. The difference in runout distance was responsible for the variations in the extent to and location in which the debris flows changed their direction of descent, resulting in the different shapes and morphologies of the fans in response to grain-size distribution. Although the direction of descent of the flows changed repeatedly, the extent of morphological symmetry of the debris-flow fans increased at a similar time during fan formation irrespective of the grain-size distribution. In contrast to this similarity in the rate of change in fan symmetry, the shift of the multigranular flow directions eventually increased the extent of asymmetry in fan morphology and expanded the scale of deviations in fan morphology between experimental test runs. Therefore, wide-ranging grain-size distributions within debris flows likely result in complex fan morphology with a high degree of asymmetry.

Effects of water infiltration into an unsaturated streambed on debris flow development

Debris flows gain much of their mass by entraining material as they descend a steep channel, growing many times before the deposition begins on gentler terrain downstream. Understanding such development processes and mechanisms is essential for predicting the scale of a debris flow arriving downstream. Notably, some field observations revealed that bed sediment was unsaturated before and during debris flow descent over them, implying the infiltration of the debris flow interstitial water into the streambed. In this study, small-scale flume experiments were conducted to simulate debris flow development processes over an unsaturated bed to investigate the relationship between water infiltration into the streambed and debris flow development. Subsequently, we established a model that can represent an unsaturated debris flow, which has pores containing air in the upper flow layer. This model successfully quantified the scale effect of our experiments. Our experimental and theoretical results show that water infiltration into a streambed can change the type of debris flow from saturated to unsaturated and influence the velocity and depth of unsaturated debris flow. An unsaturated debris flow repeatedly accelerates and decelerates owing to the water infiltration into a streambed and local gradient change of a channel. Therefore, considering water infiltration into a streambed is essential for predicting the velocity and depth of unsaturated debris flow.

Alteration Characteristics and Precious Metal Availability in Gunung Gembes & Surroundings, Jeruk Village, Pacitan Regency, East Java Province

The research area is located in the area of Mount Gembes, Jeruk Village in Pacitan (East Java). This study aims to determine the characteristic and temperature in the formation of alteration which refers to petrography with the help of samples taken in the study area. The petrographic observations carried out were thin section observations which were then analyzed for the mineral content contained in the sample rock. After knowing the minerals contained in the sample rock, it is then entered into the mineral formation temperature diagram (White & Hedenquist, 1995). The method used in this study includes primary data in the form of geological observations such as lithology, geological structure geomorphology, environmental geology and alteration observations and combined with secondary data. Alteration observations in this study were carried out descriptively (megascopic) and subjective (microscopic). The geomorphic unit that works is of volcanic origin, with high hilly sub-units of porphyry dacite intrusion (volcanic neck). Geological structures that develop in the study area are faults, namely the left horizontal fault of Jeruk 1, the left horizontal fault of Jeruk 2 and the interpretation of the oranges right horizontal fault. Geological resources or sources found in the research area are mineral potential resources that are economical, and negative sources are in the form of mass movement of debris flow type. The Zonation and the type of alteration that developed in the research area are the clay alteration zone and silicification. The clay zone is formed at a temperature >100⁰C-220⁰C with a pH of 3-5 and a silicification zone is formed at a temperature>100⁰C-300⁰C with a pH <2 which is directly controlled by the faults found in the study site.

Postwildfire Soil‐Hydraulic Recovery and the Persistence of Debris Flow Hazards

AbstractDeadly and destructive debris flows often follow wildfire, but understanding of changes in the hazard potential with time since fire is poor. We develop a simulation‐based framework to quantify changes in the hydrologic triggering conditions for debris flows as postwildfire infiltration properties evolve through time. Our approach produces time‐varying rainfall intensity‐duration thresholds for runoff‐ and infiltration‐generated debris flows with physics‐based hydrologic simulations that are parameterized with widely available hydroclimatic, vegetation reflectance, and soil texture data. When we apply our thresholding protocol to a test case in the San Gabriel Mountains (California, USA), the results are consistent with existing regional empirical thresholds and rainstorms that caused runoff‐ and infiltration‐generated debris flows soon after and three years following a wildfire, respectively. We find that the hydrologic triggering mechanisms for the two observed debris flow types are coupled with the effects of fire on the soil saturated hydraulic conductivity. Specifically, the rainfall intensity needed to generate debris flows via runoff increases with time following wildfire while the rainfall duration needed to produce debris flows via subsurface pore‐water pressures decreases. We also find that variations in soil moisture, rainfall climatology, median grain size, and root reinforcement could impact the median annual probability of postwildfire debris flows. We conclude that a simulation‐based method for calculating rainfall thresholds is a tractable approach to improve situational awareness of debris flow hazard in the years following wildfire. Further development of our framework will be important to quantify postwildfire hazard levels in variable climates, vegetation types, and fire regimes.

Debris flows in the Lushan earthquake area: formation characteristics, rainfall conditions, and evolutionary tendency

Debris flows often occur in the mountainous watersheds of earthquake-affected areas, and in the Lushan earthquake area of southwestern China, they have become a significant hazard. In this study, the influencing factors and spatial distribution of debris flows were analyzed through a review of their occurrence history. Debris flows are mainly distributed in the northwestern part of the study area, which hosts the greatest density of active faults. The debris flows are generally formed by the ‘progressive bulking’ effect in channels, and deep incision, lateral erosion, and blockage breaking are common processes that amplify the magnitude of such debris flows. Rainfall thresholds for different types of debris flow were proposed to explain the spatial differences between debris-flow regions, and the temporal variations of those thresholds highlighted how the rainfall conditions required for the occurrence of debris flows have changed. Natural vegetation recovery, reduction in the availability of solid material, and artificial debris-flow control projects play important roles in raising the threshold of the rainfall conditions required for triggering debris flows.

Morphometric analysis of debris flows basins in the Eastern Rhodopes (Bulgaria) using geospatial technologies

Debris flows are considered as a rapid movement of highly water-saturated colluvial and proluvial deposits. Besides the intensive rainfall and snowmelt, there are several other factors—prerequisites to debris flows: morphometric characteristics, rocks composition, active tectonics and deforestation. The current research is directed to the analysis of morphometric parameters of debris flow areas located in Eastern Rhodopes (Bulgaria). Two types of debris flows are analysed—gully-induced debris flows and stream torrent section with debris flood occurrence. The aim is to determine morphometric indicators to be used for discrimination of debris flow types and further susceptibility assessment. The following parameters are calculated: basin area and length, basin relief, relief ratio, Melton index, slope, topographic wetness index, stream power index and curvature. The analysis shows that slopes in a range 30–45° have most important role, among the other topographic factors, for debris flow initiation. Melton index is a meaningful indicator for discrimination of debris flow types. It is greater than 1 at gully-induced debris flows and much lower at the debris flood section (0.14–0.16). The analysis of curvature rasters, generated as a second derivative of digital elevation model (DEM), shows that the considered gullies are subject of intensive erosion and the process is still active. The research is elaborated in GIS environment on the basis of DEM and field investigations. Terrestrial laser scanning (TLS) of the low part of the basins and debris flow fan is done, and detailed terrain models are developed for the purpose of morphometric analyses.

Intermittent viscous debris flow formation in Jiangjia Gully from the perspectives of hydrological processes and material supply

Debris flows are among the most destructive natural processes to affect areas of mountainous terrain. Debris flow formation involves watershed-scale processes of hydrology and material supply. This study explored both processes based on long-term monitoring data from the Jiangjia Gully in southwestern China. Based on hydrological simulations, we found that the debris flows formed as “normal” or “abnormal” hydrological processes. The former, with normal time lag τ and normal peak discharge Qmax, formed as a typical hydrological process in which slope failure material was mixed instantaneously with channel runoff. Conversely, the latter, with longer time lag τ and/or abnormal discharge Qmax, formed through blockage and outburst associated with landslides. We found that the different types of debris flow required similar rainfall conditions and that the threshold could be expressed as I = 6.25 D−0.73 (0.5 ≤ D ≤ 14.5 h). Furthermore, we established that rainfall pattern influences debris flow occurrence, that is, such events are more likely during short-duration rainfall. The findings of this study highlight the significance of soil supply to the process of debris flow formation via random disturbance of the normal hydrological process.

A Qualitative Study of the Critical Conditions for the Initiation of Mine Waste Debris Flows

Mine waste debris flows are a type of man-made debris flow that commonly lead to major disasters. In this study, the Xiaotong Gully, which is located in the Xiaoqinling gold mining area in China and contains a typical mine waste debris flow gully, was selected as the study area. Since a debris flow can be classified as either a geotechnical debris flow or hydraulic debris flow based on its initiation mode, we conducted 46 experimental model tests to explore the initiation conditions of these two different types of debris flows. According to our tests, the initiation conditions of hydraulic debris flows were mainly affected by the flume gradient, the water content of the mine waste, the inflow discharge, the water supply modes, and the clay particle content. A larger flume gradient and higher mine waste water content were more conducive to initiating a hydraulic debris flow. However, the influence of the water supply mode on the initiation of a hydraulic debris flow was complex (influenced by factors such as water content of mine waste, runoff discharge rate and rainfall intensity). The critical runoff of a hydraulic debris flow, which starts with a parabolic relationship to the clay particle content of the mine waste, decreased with increasing clay particle content and then increased. There was a minimum critical runoff when the clay content of the mine waste was 30%. The initiation conditions of a geotechnical debris flow were mainly affected by the flume gradient, the water content, and the clay particle content. The critical gradient of a geotechnical debris flow decreased with increasing water content and had a parabolic relationship to the clay particle content. In tests 31–46 of this study, the second and third critical slopes both decreased and then increased with increasing clay particle content. These preliminary research results provide a scientific reference for subsequent research on the prevention and mitigation of mine waste debris flows.

Landslides Mapped using Satellite Data in the Western Ghats of India After Excess Rainfall During August 2018

Excess rainfall during August 2018 triggered numerous landslides in the Western Ghats region of India covering the states of Kerala, Karnataka and Tamil Nadu. These landslides caused widespread damage to property, loss of life and adversely affected various land resources. In this article, we present an inventory of landslide prepared from the analysis of multitemporal high-resolution images acquired before and after the rainfall event from Resourcesat-2, WorldView-2, GF-2, SPOT-6 and 7, Pleiades-1, Kompsat-3 and Sentinel-2 Earth observation satellites. A total of 6970 landslides with a cumulative area of 22.6 sq. km were mapped for this rainfall event. Majority of landslides have occurred in Kerala (5191), followed by Karnataka (993) and Tamil Nadu (606). Landslides are mostly debris slide and debris flow type with entrainment along the channels. Results show that landslides (83.2%) are triggered by very high rainfall. Also, very high rainfall has resulted in 14.9% of landslides even though slopes are moderate, mainly in the Kodagu district of Karnataka.

Initiation and Runout of Post‐Seismic Debris Flows: Insights From the 2015 Gorkha Earthquake

AbstractPost‐seismic debris flows are an important hazard following large earthquakes, propagating destruction downstream from hillslopes where coseismic landslides occur and extending damage for years after shaking stops. Data sets of post‐seismic debris flows are necessary to predict initiation and runout characteristics but are presently scarce. We used satellite imagery supplemented by field observations to compile an inventory of >1,000 debris flows associated with the 2015 Gorkha Earthquake in Nepal. We identified two distinct debris flow types: (1) Material from a coseismic landslide was remobilized in a steep channel during a later monsoon; and (2) a new post‐seismic hillslope failure occurred in saturated conditions and became fluidized and channelized. Runout distance was constrained by channel confluences and may be related to confluence geometry. Unstable landslide debris was largely flushed from steep channels during the first monsoon following the earthquake, and the rate of new hillslope failures tailed off over a few years.

Debris Flow Activity in Trans-Ili Alatau in the 20th — Early 21st Centuries

We examine the debris flow activity in Trans-Ili Alatau since 1900 till the present. Characteristics of the debris flow formation conditions and data on the largest debris flows are presented. A study is made of the interannual and intra-annual variability of debris flow activity. The debris flow activity was characterized by the number of debris flows in a year, and by the total volume of debris transported. For the period from 1900 to 2017 we recorded 481 debris flows. It was found that the genetic types of debris flows are dominated by rain-induced and glacial flows (87 % of the total number of debris flows). The other genetic types (snow-caused, seismogenic and anthropogenic) account for 13%. Most of the debris flows have a rain genesis (71% of the total number of rain-induced and glacial debris flows). Glacial debris flows account for 79% of the volume of debris transported. The volumes of the largest debris flows, both glacial and rain-induced, exceed one million cubic meters. They make up 3% of the total number of debris flows but they transported 70 % of debris. Small debris flows with a volume of less than 10 thousand cubic meters, in terms of their number, constitute 74%; however, they account for a mere 3% of the volume of debris flow deposits. Observations indicate that the earliest debris flows occurred in the second ten-day period of March, and the latest occurred in the first ten-day period of September. The distribution of the number of debris flows according to the dates of their occurrence clearly shows two maxima: one maximum in the second ten-day period of June, and the other maximum in the second ten-day period of July. The June maximum is due to rain precipitation, and the July maximum is associated with glacial debris flows. From 1950 to 2017 there were only 5 (7%) years without any debris flows. Four of them corresponded to the 2010s. The number of years with rain-induced and glacial debris flows was 55 (81%) and 39 (57%), respectively.