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.

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.

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.

    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.