NH3.1

The collision between the Indian and the Eurasian Plates make crustal deformation and develop many faults of the Qinghai-Tibet Plateau. Debris flows affected by tectonic activities occur frequently and are various types on the edge of plateau. It is essential to scientifically categorize the debris flow gullies on active fault to understand their mechanisms, prevent and mitigate debris flow disasters. The tectonic landforms are the foundation for debris flows occurrence. Topographical measurements and statistical analyses of seven basins on the edge of the Qinghai-Tibet Plateau were carried out (Yarlung Zangbo River, Nu River, Indus River, Gaizi River, Bailong River, Xiaojiang River and Daheba River), in which typical debris flow gullies were concentrated. The results showed that debris flows were mainly distributed in the most active tectonic uplift zone of seven basins. The debris flow gullies were classified into three types by means of nonmetric multidimensional scaling. Type I was formed by rainstorms in exposed bedrock areas, Type II was formed by glaciers in exposed bedrock areas, and Type III was formed by rainstorms in depositional basins. Based on entropy method and fuzzy mathematics, the susceptibility of debris flow on seven watersheds was analyzed. Type I had good sediment connectivity due to rainstorms and main-river incision, and was easy to form small and middle-scale debris flow. Type II was easy to form high-frequency, middle and large-scale debris flows caused by abundant moraine deposit and good sediment transport under the glacier erosion. Type III was prone to form high-frequency and small-scale debris flows triggered by rainfall and loose depositional materials.

How to cite: Liang, X., Ge, Y., Xu, M., and Lyu, L.: Topographic analysis of debris flow gullies affected by tectonic activities on the edge of Qinghai-Tibet Plateau, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6857, https://doi.org/10.5194/egusphere-egu22-6857, 2022.

The Sichuan-Tibet Highway spans the Qinghai-Tibet Plateau and the Sichuan Basin. Due to its special geological and geographical environment of steep, cold, high earthquake intensity and high ground stress, it is one of the most typical areas characterized by most serious natural disasters in China. In particular, frequently occurred debris flow disasters seriously affect the distribution of highway lines, the stability of subgrade slopes, road traffic safety, etc. In order to better serve the early warning, forecasting and disaster prevention and mitigation works in disaster-prone areas, it is necessary to carry out risk assessment. Comparatively, the southern traffic line of Sichuan-Tibet Highway was more convenient with more relating researches. At present, little attention has been paid to the northern line of Sichuan-Tibet Highway. However, the northern line passed through Dege, Sichuan and Changdu, Tibet, which is of great value to the traffic and life of the local Han and Tibetan people. At the same time, the northern line passed through Ganzi-Luhuo earthquake zone, and a large section of the line was distributed in parallel along Xianshuihe fault zone, so the risk of debris flow disaster cannot be avoided, and the research significance of the northern line of Sichuan-Tibet Highway was evident. Therefore, in this paper, focus on the debris flow along the northern Sichuan-Tibet highway, combined with field investigation and GIS technology, the characteristics and pregnant environment of debris flow along the highway were analyzed, and the risk assessment of debris flow was carried out by the method of evidence weight. Based on the idea of "discretization", highway vulnerability assessment was carried out for highway structures and moving disaster-bearing bodies. Based on above researches, the debris flow risk zoning along the northern line of Sichuan-Tibet highway was completed. The results shown that: (1) There were 235 debris flows along the northern line of Sichuan-Tibet Highway, of which 136 were hidden danger spots and 101 were disaster spots, which are distributed in Daofu-Luhuo, Dege-Jiangda and Qamdo Karuo. (2) The hazards of debris flow on the northern line of Sichuan-Tibet Highway mainly include blocking culverts, impacting bridges and burying roads. Among the existing 136 hidden danger points of debris flows, 44% of which directly affect culverts, 39% of which were bridges, and 17% were hidden danger points or damaging roadbed/roads. (3) The risk zone of debris flow in the northern Sichuan-Tibet highway indicated that the middle and high-risk road sections taking part of 63.30%, more than half of which were mainly distributed in Jiangda County, dege county and Luhuo-daofu county, which were basically in consistent with the distribution of major debris flow disaster points in the study area and verified the reliability of the evaluation results in this paper. The risk zoning map obtained from this research provided references for risk avoidance, disaster prevention and mitigation of debris flow along the northern Sichuan-Tibet highway.

How to cite: Sun, Y., Ge, Y., Chen, X., and Guo, X.: Characteristics and Risk Assessment of Debris Flow Disasters along the Northern Sichuan-Tibet Highway, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6876, https://doi.org/10.5194/egusphere-egu22-6876, 2022.

The impact of mountain disasters on human society continues to increase under the background of climate change and social economy development, especially for the developing countries or regions with relatively backward social and economic development level and fragile natural ecological environment. China is one of the countries suffered most serious mountain disasters in the world. In particular, after Wenchuan earthquake in 2008, the frequency and scale of secondary mountain disasters caused by heavy rainfall and the earthquake increased significantly, which seriously threatens the life and property safety and post-disaster reconstruction in earthquake-hit areas. Therefore, some events with mass deaths and injuries occurred. For example, on July 10, 2013, the massive landslide in Sanxi Village, Zhongxing Town, Dujiangyan City, Sichuan Province caused 166 deaths or missing. On June 24, 2017, the high mountain collapse in Xinmu Village, Dixi Town, Maoxian County, Sichuan Province buried 62 farm houses, caused 10 deaths, 73 missing and 3 injures. What’s more, mountain disasters also caused mass deaths and injuries in some areas less affected by Wenchuan earthquake. On June 28, 2012, the large debris flow occurred in Aizi Gully, Ningnan County, Sichuan Province, China was the annually most serious debris flow in construction site in China, resulting in 40 deaths or missing. On June 28, 2020, debris flow caused 17 deaths or missing in Caogu Township, Mianning County, Liangshan Prefecture, China. Lots of disaster cases show that disaster awareness and emergency capacity are the base of scientific emergency avoidance，which  is one of the important ways to reduce the casualties of mountain disasters in high-risk areas. Through the analysis of disaster cases, the experience and lessons of mountain disasters in western China were summarized and the measures to avoid mass deaths and injuries in the process of mountain disaster emergency avoidance were explored. So this research aims to  provide a scientific basis for the reduction of casualties in mountain disasters in similar areas.

How to cite: Chen, R., Tan, R., and Zhang, J.: How to avoid mass deaths in the emergency avoidance process of mountain disasters: Lessons from the mountainous areas of western China, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-7878, https://doi.org/10.5194/egusphere-egu22-7878, 2022.

Large earthquakes trigger landslides and collapses, which not only increase the loose solid materials, but also change the topography in the catchments. The debris flow activities in response to earthquake are widespread concerned, but most of the researches focus on the material conditions and the flow properties. In this research, we investigated the temporal variations of debris flow activities in a typical catchment in the Wenchuan Earthquake area, by considering the index of sediment connectivity (IC), which reflects the efficiency of sediment delivery in the catchment. The IC values in different tributaries and different period were calculated to indicate the spatial distribution and temporal variation. The results show that the high IC values distributed in the tributaries on the right hand of the mainstream in the catchment. The IC values decreased significantly after the earthquake, indicating the sediment transfer ability decreased continuously. Meanwhile, the debris flow history and loose solid material amounts were investigated via field surveys. The debris flows activities show a close consistency with the variations of debris flow source amounts and the IC values in the catchment. This research presents a new method of assessment the characteristics of sediment transfer of debris flows affected by the earthquake, and also provides a new insight to assess the debris flow actives for its close relationship with distribution of loose solid materials and sediment connectivity. 

How to cite: Li, Y., Hu, K., Guo, X., and Hu, X.: Assessment of debris flows activity in response to earthquake using an index of sediment connectivity, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-1860, https://doi.org/10.5194/egusphere-egu22-1860, 2022.

On June 17, 2020, a large debris flow occurred in the Meilong catchment following a short-duration, high-intensity rainstorm. The debris flow was initiated by two shallow landsides upstream of the catchment and had a volume of approximately 7.7×10⁵ m³. It blocked the river and then induced flooding, which caused a great loss to the local residents. Through a combination of field observation, image interpretation and laboratory experiments, the initiation mechanism, erosion depth along the main channel and deposition area of this debris flow were comprehensively analyzed. A sequentially integrated numerical model considering the vegetation interception, infiltration and runoff process was developed. Considering the spatial variations in the climatic, hydrological and geotechnical parameters, the whole process of debris flow initiation, motion, entrainment and deposition were simulated. The computational outcomes matched well with the field observation results. A combination of the proposed integrated model and spatially varying parameters can be used to effectively describe the debris flow characteristics in the initiation and propagation stages and provide significant insights into physical processes involved in such hazards.

How to cite: An, H.-C., Ouyang, C.-J., and Wang, F.-L.: Integrated numerical modeling of a large debris flow in the Meilong catchment, China, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6045, https://doi.org/10.5194/egusphere-egu22-6045, 2022.

The shallow landslide-generated debris flow on hillside catchments plays a critical role in the change of landscape features caused by natural hazards. When these debris flows occur in dams or reservoirs, they reduce the efficiency of facilities, and when they occur in residential areas, they cause many casualties and property damage. To minimize such damages, some methods can be performed through 1) installation of the warning system and 2) construction of check dam. However, in the case of rainfall-induced debris flow, preparation through a warning system is challenging because debris flows very rapidly. Therefore, to reduce the damage caused by debris flow events, the check dam needs to be installed, and for an efficient installment, a study on numerical modeling needs to figure out. Therefore, in this study, the Deb2D numerical model was used to analyze the mitigation effect through the check dam. This model is a two-dimensional debris flow simulation software based on quadtree-grid. The debris flow was simulated by Voellmy rheology, and the erosion, entrainment, and deposition processes that must be considered for the analysis of debris flow were simulated through the algorithm suggested in our recent study. The Raemian apartment and Galram-ri debris flow events were analyzed which occurred at Mt. Umyeon in 2011 and Gangwon-do in the Republic of Korea. In addition, a check dam was hypothetical by changing the distance from the collapse zone. The efficient location can be suggested through the simulation results.

Keywords: Debris flow; Numerical model; Check dam; Mitigation effect

Acknowledgments

This subject is supported by the Korea Ministry of Environment as “The SS projects; 2019002830001”

How to cite: Lee, S., An, H., and Kim, M.: Analysis of debris flow according to the location of the check dam: suggesting the optimal location by numerical simulation, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6729, https://doi.org/10.5194/egusphere-egu22-6729, 2022.

During the course of a debris flow’s motion, large particles, such as rocks and boulders, rise to the free-surface while the finer sand and silt-sized particles settle to the base. This inverse-grading process influences the development of coarse-grained heads and levees in debris flows that consequently enhance the flow mobility. Size segregation is well-studied in dry granular flows wherein it is found to be highly efficient and results in sharply separated layers of differently sized particles. Segregation diminishes in the presence of pore fluids (i.e. water or muddy slurry) and in some cases is no longer evident, although the mechanisms behind this inhibitive effect is poorly understood. In order to accurately capture size segregation in debris flows, and its impacts on the flow dynamics, it is important to understand how different types of pore fluids influence the segregation process. In this research, we systematically investigate the effects of various interstitial fluids, characterized by their density and viscosity, on the rate of particle size segregation through coupled granular-fluid simulations. Debris flows are simulated as sheared granular mixtures composed of spheres having two distinct particle sizes, immersed in ambient fluids. Solid and fluid interactions are modelled through drag and buoyant forces. Fluid effects are also evaluated across different shear rates, confining pressures, mean diameters, and gravity. It is found that the segregation slows down as the fluid viscosity is increased, but is unaffected by it below certain threshold values. In the low viscosity limit, segregation is affected only by the relative density between the particles and the fluid, and by flow inertial conditions. Analysis of stresses acting on a segregating particles reveals that the decrease of segregation rates with the viscosity is due to the increase of fluid drag forces which effectively weaken the contact stress gradients and velocity fluctuations responsible for driving the large particles upward. An empirical scaling formula is developed which accounts for the effects of fluid viscosity and the relative density on size segregation immersed in different fluids.

How to cite: Cui, K. F., Zhou, G., and Jing, L.: Particle size segregation in debris flows: insights from simulations of immersed sheared granular flows, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-2174, https://doi.org/10.5194/egusphere-egu22-2174, 2022.

The collisions of a particle against other particles or walls in the ambient fluid are one of the key processes in debris flow. Understanding the kinematics of this process, especially the role of particle rotation, is of great significance. We conducted a series of experiments studying the kinematics of a free-falling sphere colliding with a flat wall in the ambient fluids. Seven water-glycerol mixtures of different viscosities and densities are used. The kinematic behavior of the sphere is measured using both MEMS and optical techniques. The relationships between the coefficient of restitution (CR), contact time, and the Stokes number (St) are obtained. We found that when the St is greater than the upper critical value (448), the coefficient of restitution is stable at around 0.63. With the decrease of St, the CR drops rapidly before it approaches 0 when St is less than the lower critical value. The rotation process leads to wider distribution of CR. These results implicit the particle-particle collision might be significantly different when the viscosity of the liquid phase in debris flow varies and the particle scale kinematics of the particle phase is not trivial.

How to cite: Jiao, J., Zhou, Y., An, Y., and Liu, Q.: Experimental measurement of kinematic behavior of particle collisions in ambient liquid, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-11100, https://doi.org/10.5194/egusphere-egu22-11100, 2022.

    In recent years, shallow landslides and debris flow usually have occurred successively in areas with good vegetation coverage, causing casualties and economic losses. After the occurrence of shallow landslides, the failure mass accumulated in the channel, providing the material source for debris flow. And the quantity of the failure mass determines the scale of debris flow. Therefore, it is an important basis for debris flow disaster management in vegetated mountainous areas to deeply understand the influence of vegetation on the hydro-mechanical properties of debris flow sources. This study takes the shallow landslides that occurred in Mengdong village, China in 2018 as the objects, analysis the changes in soil hydro-mechanical properties influenced by tree roots through field investigation and laboratory tests, and discusses the failure mechanism of the shallow landslides. The field investigation results indicate that the vertical root distribution can be expressed as an exponentially decayed polynomial model, that is, with the increase of depth, the distribution of tree roots increased first and then decreased. Furthermore, the maximum root area density is 0.266 mm²/cm² at 20-40cm depth, and 80% of the roots are distributed in the soil above the slip surface. Laboratory test results show that the root-soil density above the slip surface was lower which was 1.04 g cm^-3, and the maximum porosity of the root-soil is 61.23%. In addition, the saturated permeability of the root-soil above the slip surface is 10-17 times that of the soil below. The shear strength of the root-soil above the slip surface is lower than that below it under saturated conditions. The difference in root distribution and the resulting changes in the hydro-mechanical properties of soil may increase the risk of slope failure and the probability of debris flow after heavy rainfall. This research could be used as a reference for debris flow source analysis and hazard management.

Keywords: Root-soil system; Landslide-induced debris flow; Geohazard chain; Hydro-mechanical properties

How to cite: Qin, M., Guo, J., Jiang, Y., and Zhang, G.: Effects of vegetation root on hydro-mechanical properties of debris flow source, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6743, https://doi.org/10.5194/egusphere-egu22-6743, 2022.

Debris flow is a mixture of water and granular materials of wide-ranged grain size, which carries huge quantity of sediment. Generally, the flow is implicitly assumed a fluid of water plus solid, ignoring the when and how the mixing is going on. However, as far as the forming processes are concerned, the solid phase (granular sediments) do not always move in step with the flush water. In most cases, material supplies are scattering and discontinuous from the source areas and streambed sediment does not initiates as whole but separately in certain time intervals, while water flow is continuous from upper to downstream channels. The separation of sediment and water in debris flow developing is vividly encoded in the successive surges as ubiquitously observed in the world, especially in the Jiangjia Gully (JJG) in southwest China. Fig.1 shows the time series of water and the carried sediment of two events, indicating the out-of-synch between water and sediment.

Using the data of debris flows in JJG, we attempt to disclose the sediment-water separation effects on the developed surge properties, which is expected to be heuristic for understanding the forming and developing mechanisms of debris flows from sources to the mainstream. Specifically, we consider the following issues as exhibited by the surge sequences.

1) The temporal variability of water and sediment flow series, including the fluctuation, autocorrelation, power spectrum, Hurst exponent;

2) The statistical features of the two series, especially the probability distribution of the quantity (discharge or total volume) and the physical implication of the distribution parameters;

It is found that both the water and sediment bear high autocorrelation and Hurst index, while the sediment sources are randomly supplied. Furthermore, the series satisfies a unified distribution in form of P(x) = Kx^-μexp(x/x_c), with x being the discharge and volume of sediment and water.   The parameters μ and x_c vary with the events (e.g., Fig.2 for the distribution of magnitude).

These findings are expected to shine a light on how the non-synch processes of water and sediment influence the developing of debris flow and the peak discharge, and this also poses a question in dynamics, which should incorporate the random and discontinuous sediment entrance in the evolution of flow.

Fig.1   Water and sediment flow discharge series of debris flow surges (E990716 and E990816)

Fig.2   Probability distribution of water and sediment quantity

How to cite: Yao, Y., Li, Y., and Zhang, J.: The nonsynchronous processes in debris flow developing, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3410, https://doi.org/10.5194/egusphere-egu22-3410, 2022.

Abstract: Debris flow is one of the most destructive geomorphological events in mountainous watersheds, which usually appears in form of successive surge waves as observed all over the world. In particular, debris flows in the Jiangjia Gully (JJG) in southwest China have displayed a great variety of surge phenomena; each debris flow event contains tens or hundreds of separate surges originating from different sources. Therefore, the surge sequence of an event must encode the information of debris flow developing. The UAV (unmanned aerial vehicle) photos provide an overview of debris-flow sources, showing the different potential of debris flow; and surge sequences present various patterns responding to the rainfall events. Then the variety of rainfalls and material sources determine the diversity of surge sequence. Using time series analysis to the surge discharge sequences, we calculate the Hurst exponent, the autocorrelation function, and the power spectrum exponent, and find that all the sequences commonly share the property of long-term memory and these parameters are correlated in exponential form, with values depending on rainfall patterns. Moreover, all events show a gross trend of discharge decay, despite the local rainfall process, which implies the intrinsic nature of the surge sequence as a systematic behavior of watershed. It is expected that these findings are heuristic for establishing mechanisms of debris flow initiation and evolution in a watershed.

How to cite: Zhang, J., Li, Y., Guo, X., Yang, T., Liu, D., and Yu, B.: Temporal characteristics of debris flow surges, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3302, https://doi.org/10.5194/egusphere-egu22-3302, 2022.

Slope failures are important material supplies for debris flows, and field observations have indicated that failures are random and discontinuous. However, few studies focus on the nature of failures in succession. This study reports groups of field experiments of soil failures under artificial rainfall on slopes in two debris flow valleys, the Jiangjia Gully (JJG) in Yunnan Province, and the Niujuan Gully (NJG) in Sichuan Province, in southwest China (Fig.1).

Fig.1 Experimental sites of the study (upper, NJG; lower, JJG)

It is found that failures occur separately and intermittently on slopes; a slope process is composed of a failure sequence (Fig.2), which presents similar properties under different rainfalls and slope conditions: 1) the sequence is primarily random, with weak autocorrelation and small correlation to time progress and less dependence on rainfall; 2) the time interval between failures satisfies the exponential distribution, and the average interval decreasing with rainfall intensity, implying the frequency increases with rainfall intensity; 3) the magnitude of failure fluctuates up to three orders, from several to hundreds of volume unit (10^-3m³); and the distribution follows the power law, with total amount increasing with rainfall intensity.

Fig 2 Failure sequences under different rainfall intensities on the experimental slopes

We propose that these properties are ascribed to the spatial heterogeneity of soil, which can be described by two parameters, m and D_c, of the grain size distribution (GSD). The point-to-point variation of (m, D_c) leads to dramatic changes in the distribution of strength, infiltration, and pore water pressure generation, and finally results in the variety of failures across the slope.

Correspondingly, the discontinuous failures translate into separate debris flow surges in the tributaries, thereby providing a scenario for surge formation in the mainstream flow of the valley. It is suggested that surges in the mainstream channel result from cascading development of tributary surges, and that the spatiotemporal characteristics observed in mainstream surges are rooted in the sources of slope failures.

How to cite: Li, Y.: Spatiotemporal characteristics of discontinuous slope failures, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3386, https://doi.org/10.5194/egusphere-egu22-3386, 2022.

Viscose debris flows always move in the manner of intermittent surges and show obvious fluctuation. In the traditional design of debris flow control engineering, the impact of single surge has on the check dams was the only factor to be taken into consideration. Whereas stability variation law of the check dams under the impact of intermittent surges was always neglected. On the basis of debris flow observation material from the Jiangjia Gully (JJG), we initially analysis the fluctuating and decaying characteristics of intermittent surges. Results indicate that intermittent surges exhibit obvious decaying characteristics and finally decay in a power-law form, showing a strong no-linear behavior. Next, based on loading combination and stability analysis of check dams, we deduced the expression of the stability coefficient when intermittent surges impact on the check dams in empty and half reservoir conditions. Meanwhile, stability variation law of the check dams in the different conditions were compared. Results indicated that when intermittent surges impact on check dams, anti-sliding stability coefficient (K_c) and anti-overturning stability coefficient (K_y) decrease with the increase of surges, and the former 3^th~5^th surges experienced the largest decaying rate. On the other hand, the deeper deposits in the reservoir corresponds to the smaller stability coefficient under the impact of the same intermittent surges. Finally, the relationship between flow depth and stability coefficients is in the form of an envelope curve, inferring that the variation of flow depth restraint the stability coefficient of check dams.

How to cite: Liu, D., Xiang, B., Shao, J., He, Y., and Liang, M.: Stability analysis of check dam impacted by intermittent surge, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-2482, https://doi.org/10.5194/egusphere-egu22-2482, 2022.

Mount Gamalama is a stratovolcano forming Ternate Island in Indonesia. Collapse of the volcanoes flank has the potential to generate large tsunamis, potentially mega-tsunamis. This active volcanic island has a history of tsunami generation in 1608, 1840, and 1871. However, the generation mechanism of these tsunamis is unknown. Numerical simulation was used to understand the level of instability of the volcano flanks and the travel time and velocity of of the potential landslides and ensuing tsunamis on nearby coastlines. We also determined the factors that affect the size of the tsunami generated. An open-source finite-element code, Fluidity, was used to simulate the tsunami generation and propagation. A three-material model is considered: a viscous subaerial slide material, water, and air to capture the complex physics and interaction of the landslide and water. The results show that the subaerial mass failure takes around 2 to 6 minutes to enter the sea and can generate an initial wave of heights ranging from 35 m to 110 m. A volcanic flank collapse on Mount Gamalama would therefore have serious implications for the coastal population in neighbouring islands and submarine infrastructures like underwater cables.

How to cite: Saaduddin, S., Neuberg, J., Thomas, M., and Hill, J.: Numerical modelling of the potential for landslide-induced tsunamis, Mount Gamalama, Indonesia, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-11688, https://doi.org/10.5194/egusphere-egu22-11688, 2022.

Debris flows are the dominant process delivering sediment from hillslopes into channels following the 2008 Wenchuan earthquake. Post-earthquake debris flows continue to pose a significant hazard to the recovering local communities. In 2019, a period of intense rainfall triggered several extremely large debris flows. The flows bulked to volumes in excess of 100 000 m³,  much larger than their initiation volumes, and transited catchments to be deposited in the Min Jiang river. The scale of these flows highlights our limited understanding of why and where large debris flows deposit. Previous studies have shown that topography (notably bed slope and channel width), flow composition (grain size), and flow characteristics (velocity and depth) can all control debris flow runout. Yet, there is limited understanding of how these interrelate. For example, whether abrupt changes in topography, such as increased channel width, lead to the deposition of certain grain size fractions and subsequently encourage further deposition. Alternatively, whether changes in bed slope affect flow velocity and this results in the entrainment of specific grain size fractions by the flow. An understanding of these relationships will help to better constrain where and how post-earthquake debris flows are more likely to deposit.

In this study, we determine how debris flow characteristics (velocity and depth) and the grain size distribution (GSD) deposited by the debris flow evolve with changes in topography and distance from the initial debris flow source. To achieve this, we simulated two post-earthquake debris flow events in the Liusha and Luoquan catchments, China, using the 2D dynamic debris flow model, Massflow. GSDs were collected by sampling and sieving pits located equidistantly along the centre of each 2019 debris flow deposit. Bed topography data was recorded both in the field and using a 30 m resolution DEM. We compared changes in the flow characteristics and GSDs deposited for each debris flow with the data for bed topography to explore how controls on debris flow runout interrelate. Preliminary findings for the Luoquan debris flow suggest a relationship between negative changes in curvature and the deposition of fine-grained material. This work will help to better understand controls on debris flow runout, subsequently aiding future studies of post-earthquake debris flow hazard prediction.

How to cite: Harvey, E., Hales, T., Hobley, D., Horton, A., Liu, J., and Fan, X.: Controls on the deposition of extremely large post-earthquake debris flows in Wenchuan, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-5246, https://doi.org/10.5194/egusphere-egu22-5246, 2022.

Debris flows/floods are natural hazards occurring in steep mountain catchments. Debris material mainly derives from processes of channel/channel head, bed erosion, bank destabilization or shallow landslides. More rarely landslide deposits within the channel could be sources of debris. Some studies pointed out the potential increment in debris flow magnitude because the flow may increase its volume and peak discharge after the impacts against an in-channel deposit. The objective of this investigation is to estimate the potential consequences of a debris flow impacting a landslide deposit located in the channel bed.

The project has been developed analysing the rio Rudan catchment (Belluno province, North-eastern Italy), characterized by a frequent occurrence of debris flows in the last decades. In the rio Rudan a wide shallow landslide, highly connected to the transport channel reach,   occurred on the 15^th December 2020 and deposited the majority of the volume within the channel. The landslide was capable to generate only a low magnitude debris flow (of the order of 10’000 m³). Most of the released material (40’000 m³) remained in the channel close to the slope failure zone. In order to analyse the effects of following different types of debris-flows encountering the deposit, different scenarios have been simulated considering the landslide deposit as an entrainable layer. We created five triangular shaped input debris flow hydrographs characterized by different peak discharge (20, 40, 60, 80 and 100 m³ s^-1) and a flow hydrograph representing a debris flood (peak of 20 m³s^-1). Simulations have been performed using the r.avaflow model (version 2.4) for which we employed the two-phase routing model together with the empirical erosion model.

Results of the simulations showed that the magnitude of possible future debris flow events was reduced due to the presence of the landslide deposit. In particular, the peak discharges of the simulated output debris flow hydrograph was reduced of 60-70% compared to the input hydrograph. Even if the coefficient of erosion was set to high values, the quantity of entrained material was low and, surprisingly, most of the solid component of the simulated debris flows deposited in the upper part of the landslide deposit due to the decrease in slope. Most of the erosion process occurred in the lower part of the deposit for the increase in slope. Conversely, in the numerical simulation of the longer-duration debris flood event (or even characterized by multiple peak discharge), the landslide deposit has proved to furnish a constant input of debris material, magnifying the total volume of the event but not the peak discharge. Looking at the results of the simulated case study, we can conclude that the big landslide deposit within the Rudan channel could have a mitigation effect in reducing the peak discharge of future debris flow events considering those debris flows with an important (return periods of 20-30 years) but not extreme magnitude. This highlights the importance of a dedicated modelling in companion cases to avoid excessive costs for interventions and to correctly assess residual risks in case of non-interventions.

How to cite: Baggio, T., Bettella, F., and D'Agostino, V.: In-channel landslide deposits and future debris flows, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-7533, https://doi.org/10.5194/egusphere-egu22-7533, 2022.

Debris flows are fast, hazardous and massively erosive mass movements that can cause severe danger to infrastructure and have been responsible for a significant number of casualties in the last decades. The European and German Alps face an increasing frequency and magnitude of hazardous debris-flows due to more frequent rainstorms in a warming climate. While the erodibility of the channel bed is a major contributor to the magnitude of debris-flows and the effective erosion often represents more than 80% of the final volume (Dietrich and Krautblatter, 2019) which, it is not or not sufficiently implemented in present debris-flow models.

Here, we present a concept of a simple predictive erosive debris-flow model calibrated with two erosive debris-flow events in the German Alps in June 2015. Both torrent channels were recorded with terrestrial laser scans and compared with an airborne laser scan performed in 2007. The detected geomorphic change was subdivided by same-length segments and correlated with modelled flow velocities at the cross-sections between the segments. The flow velocity at the cross sections was calculated by individual RAMMS Debris Flow simulations for every segment, each including the cumulated erosion volume of the sections upstream as well as the initial volume estimated from a rainfall-runoff calculation. As a result, we obtain a linear relationship between flow velocity and mean erosion depth, which can be used in a predictive debris-flow model to iteratively calculate the entrainment in every channel segment.

By analysing further geological and topographical debris-flow settings, we aim to create an inventory of different catchment characteristics and calibrate the model to various dimensions and properties. This would enable enhanced magnitude predictions of anticipated erosive debris-flows in comparable catchments by a fully forward-modelling approach.

Reference:

Dietrich, A. and Krautblatter M. (2019): Deciphering controls for debris-flow erosion derived from a LiDAR-recorded extreme event and a calibrated numerical model (Roßbichelbach, Germany). Earth Surface Processes and Landforms 44: 1346-1361, doi: https://doi.org/10.1002/esp.4578.

How to cite: Stammberger, V., Dietrich, A., and Krautblatter, M.: Towards a simple predictive erosive debris-flow model calibrated with contrasting environmental settings, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-9131, https://doi.org/10.5194/egusphere-egu22-9131, 2022.

Debris flows are a common hazard in Alpine headwater catchments during intense convective rainstorms. These debris flows are commonly triggered by runoff entraining previously deposited sediment within the catchment. A debris flow will be initiated if rainfall exceeds the given rainfall intensity threshold. We usually define the rainfall intensity threshold as a function of storm duration (rainfall intensity-duration threshold). Above this empirically recorded threshold, the resulting surface runoff can mobilise sediment from the hillslopes and within the channel network. Thresholds are usually defined empirically for a given geographic region via monitoring of debris flow occurrence and the triggering rainfall intensity. However, direct field observations and rainfall data are sparse and noisy, and it is impossible to define rainfall thresholds when historical data are unavailable. An alternative methodology to derive rainfall ID thresholds is to use simplified physics-based model simulations. In this case, a greater understanding of the controlling factors for debris-flow activities could enable better threshold estimation in unmonitored catchments.

Here we present the initial simulation results of three different monitored catchments in the Dolomite mountains of Northeast Italy. These catchments are dominated by steep dolomite bedrock walls, which can provide large volumes of surface runoff to the catchment during rainfall. To simulate the response to rainfall in these catchments, we use the SWEHR (Shallow Water Equation & Harsine Rose) debris flow model, which we calibrate using a combination of field data and a correlation maximising framework. By focusing on the runoff response to rainfall in the catchments, we identified several key factors in the calibration of the model. The timing and magnitude of the runoff is controlled by the hydrological characteristics of the bedrock, the roughness of the catchment, the availability of sediment in the catchment, and the characteristics of the rainfall. By running multiple rainfall simulations for the catchments, we show how these factors impact rainfall ID thresholds

How to cite: Francis, O. and Tang, H.: The initiation of runoff-generated debris flow in steep carbonate catchments, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-7428, https://doi.org/10.5194/egusphere-egu22-7428, 2022.

Volcanic debris avalanches occur when volcanic edifices collapse and flow as landslides. They are preserved in the geological record as volcanic debris avalanche deposits (VDADs). Analysis of these deposits can provide insight into the flow characteristics of the avalanche and its possible triggers.

Here we provide preliminary textural data on the shear zone layer at the base of a small-volume VDAD on Ascension Island, South Atlantic. The deposit has a volume of ~4 x 10⁶ m³, covers 2 km² and originated from the partial collapse of the northern flank of the 300ka Green Mountain scoria cone, which sits at 550 metres above sea level. The avalanche flowed 2 km down a ~10° slope, before stopping at in a small basin against a lava dome at 190 m above sea level.

Over most of its length the VDAD overlies an in-situ Green Mountain scoria fall deposit that was dispersed north during the eruption. The base of the deposit is marked by a fine-grained, ~2 cm-thick shear zone with slickensides. The shear zone is distinguishable in hand specimen from the rest of the deposit by being finer grained and indurated. The bulk of the VDAD is composed of semi-coherent, metre scale blocks of scoria with a poorly sorted volcaniclastic matrix composed of a hetereolithic clast population including randomly orientated clasts of basaltic scoria, pumice and lavas. The toe of the deposit is fractured and flame structures are abundant.

Preliminary Back-scattered Scanning Electron Microscope imaging of the shear zone reveal that porosity and pore interconnectivity decrease markedly towards the centre of the shear zone, and clasts become finer-grained, better sorted and more rounded. Experiments will be conducted on samples of Green Mountain Scoria using Rotary Shear Equipment to place constraints on slip rates and shear parameters. Ultimately, we hope to understand potential triggers of the failure and explore the hazards and potential for similar events on the island in the future.

How to cite: James, H.: Volcanic Debris Avalanche and accompanying shear zone slip surface formed by a perched scoria cone collapse on Ascension Island, South Atlantic, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-4185, https://doi.org/10.5194/egusphere-egu22-4185, 2022.

Arctic landscapes are among the most vulnerable on Earth to climate change, largely due to the degradation and thawing of permafrost. In steeper bedrock-dominated terrains, slope instability from warming permafrost leads to larger and more frequent rockfall and frost cracking events, which in turn increases the production and delivery of sediment to hillslopes and channel networks by debris flow and fluvial processes. However, there is a fundamental lack of data on past and current rates of sediment production and transport in Arctic watersheds. Without an understanding of these phenomena, it is impossible to predict the transient responses, rates, and directions of periglacial processes in response to future climate change. To begin to address this knowledge gap, we conducted a field-based study of the Black Mountain catchment in the Aklavik Range (Northwest Territories, Canada). This site was chosen due to its position within a zone of continuous permafrost and the presence of an alluvial fan at the base of the catchment, providing a closed system.

In the summer of 2019, after a summer storm event, we observed several debris flows that initiated from ice-filled gullies, as well as fluvial sediment transport from snowmelt. We documented flow and sediment transport conditions on the fan, yielding modern-day fluvial transport rates of 0.2–2 m³/hr for water runoff rates of 0.01–0.2 mm/hr. However, less-frequent mass flow events can rapidly deposit large amounts of sediment. For example, we estimate that a mass flow event that occurred in 2016 delivered ~1.5*10⁵ m³ of sediment to the fan—equivalent to ~8–85 years of continuous fluvial sediment transport. Based on our surficial and sedimentological mapping, the fan has likely been forming under a periglacial climate over the last ~13,000 years from a combination of mass flow and fluvial processes. Most of the fan (~67%) was deposited fluvially, but the upper, steeper portion of the fan was deposited by coarse granular debris flows. We hypothesize that accelerated warming has increased sediment supply due to frost cracking, leading to aggradation, increased debris flow activity, and upper fan steepening.

How to cite: Palucis, M., Marshall, J., and Strauss, J.: Sediment production and transport processes in an arctic watershed undergoing climate change , EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-10812, https://doi.org/10.5194/egusphere-egu22-10812, 2022.

Understanding erosion and entrainment of material by debris flows is essential for modelling debris-flow volume growth and prediction of hazard potential. Recent advances have highlighted two driving forces behind debris flow erosion; impact and shear forces. How erosion and these forces depend on debris-flow composition and interact remains unclear. We experimentally investigated the effects of debris-flow composition and volume on erosion processes in a small-scale flume with a loosely packed bed. We quantified the effects of gravel, clay and solid fraction in the debris flow on bed erosion. Erosion increased linearly with gravel fraction and volume, and decreased with increasing solid fraction. Erosion was maximal around a volumetric clay fraction of 0.075 (fraction of the total solid volume). Under varying gravel fractions and flow volumes erosion was positively related to both impact and shear forces, while these forces themselves correlate. Results further show that the internal dynamics driving the debris flows, quantified by Bagnold and Savage numbers, correlate to erosional processes and quantity. Impact forces became increasingly important for bed erosion with increasing grain size. The experiments with varying clay and solid fractions showed that the abundance and viscosity of the interstitial fluid affect debris-flow dynamics, erosional mechanisms and erosion magnitude. High viscosity of the interstitial fluid inhibits the mobility of the debris flow, the movement of the individual grains, the transfer of momentum to the bed by impacts, and therefore inhibits erosion. High solid content possibly decreases the pore pressures in the debris flow and the transport capacity, inhibiting erosion, despite high shear stresses and impact forces. Our results show that bed erosion quantities and mechanisms may vary between debris flows with contrasting composition, and stress that entrainment models and volume-growth predictions may be substantially improved by including compositional effects.

How to cite: de Haas, T., Roelofs, L., and Colucci, P.: Unraveling debris-flow erosion: experimentally assessing the effects of debris-flow composition on erosion, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-678, https://doi.org/10.5194/egusphere-egu22-678, 2022.

Mountainous areas tend to have a high density of bridges due to their topography and mobility requirements. Furthermore, such areas are often characterized by frequent debris-flow activity, which in turn can endanger the structural integrity of bridges. The influence of debris flows on bridge piers has already been analyzed in the past, but mechanisms and consequences of debris-flow impact on bridge superstructures remain unclear.

We hypothesize that in addition to horizontal forces, frictional shear-forces and uplift forces may play a considerable role in bridge failure caused by debris-flow impacts. We also conjecture that the type of the bridge superstructure, specifically the bridge profile has an influence on the occurring forces.

In order to obtain a deeper understanding of impact forces on bridge superstructures, we aim to measure and quantify the forces exerted on different bridge profiles during debris-flow impact based on small scale experiments. We will investigate debris-flow impact on five different bridge profiles in the course of the project “Debris-flow impact forces on bridge superstructures (DEFSUP)”, funded by the Austrian Science Fund (FWF).

The laboratory setup consists of a 4 m long semi-circular channel with a diameter of 0.3 m and an inclination of 20°. The cement miniature bridge in the scale of 1:30 is mounted on a metal frame and is installed at the end of the flume. The debris-flow material corresponds to a granular debris flow, the mass is fixed at 50 kg for each experiment. The flume itself has been optimized in preliminary studies and ensures high reproducibility of stationary debris flows with predictably sufficient flow-heights for the impact on the miniature bridge. Each profile is subjected to at least three impacts. The impact forces on the bridge profile are measured with 3-axis-force sensors at both abutments of the bridge. Thereby it is possible to determine horizontal impact forces as well as uplift and shear forces. Additionally, flow heights, pore water pressure and normal stresses are gauged.

The results of the study are intended to contribute to recommendations for the structural design of bridges in vulnerable areas. This aims not only to protect human lives and to increase the safety of structures, but also to provide financial relief in the future, since there is evidence that the areas prone to debris-flow events are likely to increase as a consequence of climate change.

How to cite: Friedl, C., Scheidl, C., Wernhart, S., and Proske, D.: Investigation of debris-flow impact forces on bridge superstructures – laboratory experiments on the influence of bridge profiles, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-1154, https://doi.org/10.5194/egusphere-egu22-1154, 2022.