NH3.1

EDI
Debris flows: advances on mechanics, controlling factors, monitoring, modelling and risk management

Debris flows are among the most dangerous natural hazards that threaten people and infrastructures in both mountainous and volcanic areas. The study of the initiation and dynamics of debris flows, along with the characterization of the associated erosion/deposition processes, is of paramount importance for hazard assessment, land-use planning and design of mitigation measures, including early warning systems. In addition, the impacts of climate change on debris-flow activity must be considered and carefully analysed, as the number of mountain areas prone to these events may increase in future.
A growing number of scientists with diverse backgrounds are studying debris flows and lahars. The difficulties in measuring parameters related to their initiation and propagation have progressively prompted research into a wide variety of laboratory experiments and monitoring studies. However, there is a need of improving the quality of instrumental observations that would provide knowledge for more accurate hazards maps and modeling. Nowadays, the combination of distributed sensor networks and remote sensing techniques represents a unique opportunity to gather direct observations of debris flows to better constrain their physical properties.
Scientists working in the field of debris flows are invited to present their recent advancements. In addition, contributions from practitioners and decision makers are also welcome. Topics of the session include: field studies and documentation, mechanics of debris-flow initiation and propagation, laboratory experiments, modeling, monitoring, impacts of climate change on debris-flow activity, hazard and risk assessment and mapping, early warning, and alarm systems.

Co-organized by GM3/HS13
Convener: Marcel Hürlimann | Co-conveners: Velio Coviello, Xiaojun GuoECSECS, Roland Kaitna, Sara SaviECSECS
Presentations
| Thu, 26 May, 08:30–11:50 (CEST), 13:20–16:40 (CEST)
 
Room M2

Session assets

Session summary

Presentations: Thu, 26 May | Room M2

Chairpersons: Marcel Hürlimann, Xiaojun Guo
08:30–08:35
Part1: Assessment (Asia)
08:35–08:42
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EGU22-6857
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ECS
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Virtual presentation
Xinyue Liang, Yonggang Ge, Mengzhen Xu, and Liqun Lyu

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.

08:42–08:49
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EGU22-6876
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ECS
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Virtual presentation
Yuqing Sun, Yonggang Ge, Xingzhang Chen, and Xiaojun Guo

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.

08:49–08:56
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EGU22-7878
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ECS
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Highlight
Rong Chen, Rongzhi Tan, and Jianqiang Zhang

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.

08:56–09:03
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EGU22-3418
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Highlight
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Virtual presentation
Soil mass domination in debris-flow disasters and strategy for hazard mitigation
(withdrawn)
Ningsheng Chen, Shufeng Tian, Guisheng Hu, Mingfeng Deng, and Taixin Peng
09:03–09:10
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EGU22-1860
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ECS
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Virtual presentation
Yanji Li, Kaiheng Hu, Xiaojun Guo, and Xudong Hu

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.

09:10–09:17
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EGU22-4675
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ECS
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Virtual presentation
Xudong Hu and Miao Ding

Numerical modelling is the physically-based method for in-depth analyzing the process from landslide to debris flow. Particle flow analysis method (PFC) has the advantage of dealing with such processes, like debris flow formation, propagation, and deposition. Hence, our study to analyze the dynamic characteristics of a landslide-generated debris-flow, taking the Shaziba landslide-debris-flow as example, which occurred in Enshi City on June 8, 2020, under complex landslide material composition, Combined the field survey, unmanned aerial vehicle (UAV) aerial photography, and laboratory direct shear tests, the velocity, displacement and the characteristics of the landslide-generated debris-flow were simulated. The results indicated that the initial stage of the landslide starts with an overall motion acceleration with a time around 733 s. The maximum velocity of the landslide body is 17.5 m/s, and the maximum displacement is 1500 m with a total volume of 9.31×105 m3. The simulation results are closer to the actual landslide volume (1.0×106 m3) and the form of the dam in Qingjiang. The study reveals the mechanism of dam formation, which could be served as useful information for natural hazards management to prevent the river from being blocked by landslides or debris flows.

How to cite: Hu, X. and Ding, M.: Modeling the propagation and run-out from gravel-silty clay landslide to debris flow in Shaziba, southwest Hubei Province, China, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-4675, https://doi.org/10.5194/egusphere-egu22-4675, 2022.

09:17–09:24
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EGU22-6045
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ECS
Hui-Cong An, Chao-Jun Ouyang, and Fu-Lei Wang

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.

09:24–09:31
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EGU22-6729
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ECS
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Virtual presentation
Seungjun Lee, Hyunuk An, and Minseok Kim

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.

09:31–09:38
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EGU22-2174
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ECS
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Virtual presentation
Kahlil Fredrick Cui, Gongdan Zhou, and Lu Jing

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.

09:38–09:45
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EGU22-12035
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ECS
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Virtual presentation
Zhitian Qiao, Wei Shen, Matteo Berti, and Tonglu Li

Numerical models have become a useful tool for predicting the potential risk caused by debris flows. Although a variety of numerical models have been proposed for the runout simulation of debris flows, the differences and performances of these models are unknown. To this end, in this paper, two typical depth-averaged models have been selected to analyze the debris-flow event that occurred in the Cancia basin on July 23rd, 2015. The simulations with and without entrainment are conducted to analyze the influence of entrainment on the runout behavior of the debris flow. The simulated results are compared and discussed in detail. In the scenario without entrainment, a part of the debris mass deviates from the main path during propagation, while the debris mass propagates along the channel if entrainment is considered. This conclusion illustrates that entrainment cannot be ignored in this case. Additionally, the comparison between measured and simulated results shows that both models perform generally well in the terms of simulating the erosion-deposition distribution, but the DAN3D model will present a greater lateral spreading and a thinner depositional thickness than Shen’s model.

How to cite: Qiao, Z., Shen, W., Berti, M., and Li, T.: Modeling the run-out behavior of the July 23rd, 2015 Cancia debris-flow event using two numerical models, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-12035, https://doi.org/10.5194/egusphere-egu22-12035, 2022.

09:45–10:00
Coffee break
Chairpersons: Roland Kaitna, Xiaojun Guo
Part 2: Experiments - monitoring (Asia)
10:20–10:27
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EGU22-11123
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ECS
Wangxin Yu, Su Yang, Xiaoliang Wang, and Qing-quan Liu

Granular flow impacting structures is an important problem in the research of providing scientific basis for disaster prediction and mitigation, so it is of great significance to deepen the understanding of the interaction law. We studied the spread and deposit behaviors of fast granular flow impacting an array of semi-ellipsoid obstacles with different parameters such as the height, distribution density and deflection angle. It is found that the flow and deposit state of granular matter are controlled by the obstacle array through both dissipation and deflection effect. We quantified the deposit behavior by two dimensionless indices, one pre-existing index called runout efficiency, and a new proposed index termed as deflection efficiency. This work would provide help in designing protective obstacle arrays by exploring the relationship between regulation effect and parameters of the obstacle array.

How to cite: Yu, W., Yang, S., Wang, X., and Liu, Q.: Deflection effect in the interaction between granular flow and semi-ellipsoid obstacle array, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-11123, https://doi.org/10.5194/egusphere-egu22-11123, 2022.

10:27–10:34
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EGU22-4690
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ECS
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Virtual presentation
Experimental study on failure mechanism of debris flow dam
(withdrawn)
Hechun Ruan, Huayong Chen, Xiao Li, and Yunhan Yu
10:34–10:41
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EGU22-4597
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ECS
Jianqi Zhuang

Designing the experiment on debris flow formation process at artificial rainfall at the conditions of the loose material unusually rich for studying the debris flow formation process. The main results showed: (1) the fine particles moving first for the initiation of debris flow, and then mixed with the large particle and runoff increased sharply, along with channel block-outburst phenomenon; the debris flow phenomena gradually disappeared with the fine particles migration off and the channel rough serious. (2) the slop failure and moving at the rainstorm, the failure material deposited in channel and formed the dams which effect the erosion and deposits of the channel with moving down to downstream. (3) the erosion sediment was main from middle and lower channel, then from the upstream and slope; the debris flow fan materials was main from the downstream channel, then from the upstream and slope. (4) the pore-water pressure and water content, which not only effected by rainfall, but also effected by fine particles content and soil structure, changed obviously and varied in different time and different sites with fluctuation. (5) the fine particles played an important role in the process of debris flow initiation and it’s accumulation and displacement effected the evolution of the basin topography and the formation of debris flow. In the debris flow forecast, the fine particles of soil content should be considered duo to its critical water content and pore-water pressure quite different in different content of fine particles of debris flow initiation.

How to cite: Zhuang, J.: The Experiment Study on Debris Flow Formation Process Based on REE, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-4597, https://doi.org/10.5194/egusphere-egu22-4597, 2022.

10:41–10:48
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EGU22-11100
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ECS
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Virtual presentation
Jiajun Jiao, Yiyang Zhou, Yi An, and Qing-quan Liu

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.

10:48–10:55
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EGU22-6743
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Highlight
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Virtual presentation
Mingyue Qin, Jian Guo, Yao Jiang, and Guotao Zhang

    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.

10:55–11:02
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EGU22-3410
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ECS
Yingjie Yao, Yong Li, and Jun Zhang

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/xc), with x being the discharge and volume of sediment and water.   The parameters μ and xc 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.

11:02–11:09
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EGU22-3302
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ECS
Jun Zhang, Yong Li, Xiaojun Guo, Taiqiang Yang, Daochuan Liu, and Bin Yu

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.

11:09–11:16
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EGU22-3386
Yong Li

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).