NH3.2

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/HS9
Convener: Marcel Hürlimann | Co-conveners: Velio CovielloECSECS, Xiaojun Guo, Sara Savi
vPICO presentations
| Fri, 30 Apr, 09:00–15:00 (CEST)

Session assets

Session materials

vPICO presentations: Fri, 30 Apr

Chairpersons: Marcel Hürlimann, Xiaojun Guo
Part 1: Numerical modelling (9:00 - 10:30)
09:00–09:05
09:05–09:15
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EGU21-15175
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solicited
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Highlight
Matteo Berti and Alessandro Simoni

Rainfall is the most significant factor for debris flows triggering. Water is needed to saturate the soil, initiate the sediment motion (regardless of the mobilization mechanism) and transform the solid debris into a fluid mass that can move rapidly downslope. This water is commonly provided by rainfall or rainfall and snowmelt. Consequently, most warning systems rely on the use of rainfall thresholds to predict debris flow occurrence. Debris flows thresholds are usually empirically-derived from the rainfall records that caused past debris flows in a certain area, using a combination of selected precipitation measurements (such as event rainfall P, duration D, or average intensity I) that describe critical rainfall conditions. Recent years have also seen a growing interest in the use of coupled hydrological and slope stability models to derive physically-based thresholds for shallow landslide initiation.

In both cases, rainfall thresholds are affected by significant uncertainty. Sources of uncertainty include: measurement errors; spatial variability of the rainfall field; incomplete or uncertain debris flow inventory; subjective definition of the “rainfall event”; use of subjective criteria to define the critical conditions; uncertainty in model parameters (for physically-based approaches). Rainfall measurement is widely recognized as a main source of uncertainty due to the extreme time-space variability that characterize intense rainfall events in mountain areas. However, significant errors can also arise by inaccurate information reported in landslide inventories on the timing of debris flows, or by the criterion used to define triggering intensities.

This study analyzes the common sources of uncertainty associated to rainfall thresholds for debris flow occurrence and discusses different methods to quantify them. First, we give an overview of the various approaches used in the literature to measure the uncertainty caused by random errors or procedural defects. These approaches are then applied to debris flows using real data collected in the Dolomites (Northen Alps, Itay), in order to estimate the variabilty of each single factor (precipitation, triggering timing, triggering intensity..). Individual uncertainties are then combined to obtain the overall uncertain of the rainfall threshold, which can be calculated using the classical method of “summation in quadrature” or a more effective approach based on Monte Carlo simulations. The uncertainty budget allows to identify the biggest contributors to the final variability and it is also useful to understand if this variability can be reduced to make our thresholds more precise.

 

How to cite: Berti, M. and Simoni, A.: Uncertainty in debris flow rainfall thresholds, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-15175, https://doi.org/10.5194/egusphere-egu21-15175, 2021.

09:15–09:17
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EGU21-14333
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ECS
Nina Marlovits, Martin Mergili, and Thomas Glade

History has shown that cascading landslides, such as the debris avalanches from Huascarán in 1962 and 1970, the Kolka-Karmadon rock-ice-avalanche in 2002, or the rock avalanche-debris flow event of Bondo in 2017, can be very destructive due to their high energies, velocities and volumes. They can lead to large numbers of fatalities, huge material damage, and disruption of critical infrastructure.

Cascading landslides are a specific class of multi-hazard events in which one type of motion transforms into another or an initial, primary movement triggers a secondary process. High-mountain areas are particularly prone to this type of landslides due to their dynamic, rapidly changing environments and their high relief. For example, an initial rock fall can reach snow or ice masses and transform into a rock-snow- or rock-ice-avalanche, or into a debris flow. Physically-based numerical modelling is often used for the attempt to predict such events as a basis for the design of risk management strategies such as early warning systems. However, we identify at least two specific types of challenges making accurate and reliable predictions highly difficult:

  • (a) The dynamic behaviour of such process chains, especially in the transition phase, is not yet fully understood. Existing models are either developed for (i) fall or (ii) flow processes. Whereas substantial progress has been made in previous years in the integrated simulation of flow-type movements, no software which fully and directly considers the transformation of fall to flow processes is known to the authors. Therefore, it is not yet possible to simulate fall-flow sequences of cascading landslide events with one single tool. Model chains have to be used instead, which have a limited capacity for appropriately representing the transition phase between the two types of processes.
  • (b) Limited knowledge on the initial conditions and input parameters represents another severe limitation. Model input relies on available information on previous events and on certain characteristics of the (possible) release and impact area. Obviously, the quality of the data set is significantly influencing the model results. Whereas the scientific community is far away from exact predictions of landslide impact, an important objective should consist in better constraining the definition of possible scenarios to be considered for hazard and risk management.

For the reasons highlighted, it remains highly challenging to adequately predict the impact areas, energies, and travel times of cascading landslides in space and time. Nevertheless, stakeholders require such predictions for decisions on sustainable hazard and risk management strategies. Therefore, the aims of this study are (i) to evaluate possibilities to appropriately combine models for fall and flow processes and (ii) to examine data acquisition methods for the model input. Furthermore, (iii) appropriate strategies to present and to communicate simulation results need to be discussed.

How to cite: Marlovits, N., Mergili, M., and Glade, T.: Challenges in the predictive simulation of cascading landslide processes , EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14333, https://doi.org/10.5194/egusphere-egu21-14333, 2021.

09:17–09:19
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EGU21-10498
Hock Kiet Wong, Ching-Yuan Ma, Chi-Jyun Ko, and Yih-Chin Tai

The movement of a debris flow is channelized by the mountain topography. It slows down and begins to deposit, forming the so-called debris-flow fan, when the slope is gentle. Since the flow body is composed of solid grains with interstitial fluid, the solid fraction may vary and plays a crucial role in the deposition process. In the present study, an entrainment-deposit law together with the two-phase model for grain-fluid flows (Tai et al., 2019) is proposed for describing the development of a debris flow fan. The model equations are derived in a terrain-following coordinate system, in which the coordinates are in coincidence with the topographic surface and the deposition/erosion is treated as the sub-topography. Numerical validation is performed against flume experiments (Tsunetaka et al., 2019), where the sediment-water mixture is released from a channel and merging into a gentle inclined flat plain via a steady water inflow. In this study, we shall illustrate the impacts of the sediment concentration on the evolution of the debris-flow fan, such as the location, distribution, geometry of debris-flow fan as well as the flow paths. 

How to cite: Wong, H. K., Ma, C.-Y., Ko, C.-J., and Tai, Y.-C.: An investigation of two-phase grain-fluid model on the development process of debris flow fan, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10498, https://doi.org/10.5194/egusphere-egu21-10498, 2021.

09:19–09:21
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EGU21-2224
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ECS
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Highlight
Luca Crescenzo, Gaetano Pecoraro, Michele Calvello, and Richard Guthrie

Debris flows and debris avalanches are rapid to extremely rapid landslides that tend to travel considerable distances from their source areas. Interaction between debris flows and elements at risk along their travel path may result in potentially significant destructive consequences. One of the critical challenges to overcome with respect to debris flow risk is, therefore, the credible prediction of their size, travel path, runout distance, and depths of erosion and deposition. To these purposes, at slope or catchment scale, sophisticated physically-based models, appropriately considering several factors and phenomena controlling the slope failure mechanisms, may be used. These models, however, are computationally costly and time consuming, and that significantly hinders their applicability at regional scale. Indeed, at regional scale, debris flows hazard assessment is usually carried out by means of qualitative approaches relying on field surveys, geomorphological knowledge, geometric features, and expert judgement.

In this study, a quantitative modelling approach based on cellular automata methods, wherein individual cells move across a digital elevation model (DEM) landscape following behavioral rules defined probabilistically, is proposed and tested. The adopted model, called LABS, is able to estimate erosion and deposition soil volumes along a debris flow path by deploying at the source areas autonomous subroutines, called agents, over a 5 m spatial resolution DEM, which provides the basic information to each agent in each time-step. Rules for scour and deposition are based on mass balance considerations and independent probability distributions defined as a function of slope DEM-derived values and a series of model input parameters. The probabilistic rules defined in the model are based on data gathered for debris flows and debris avalanches that mainly occurred in western Canada. This study mainly addresses the applicability and the reliability of this modelling approach to areas in southern Italy, in Campania region, historically affected by debris flows in pyroclastic soils. To this aim, information on inventoried debris flows is used in different study areas to evaluate the effect on the predictions of the model input parameter values, as well as of different native DEM resolutions.

How to cite: Crescenzo, L., Pecoraro, G., Calvello, M., and Guthrie, R.: A probabilistic model for assessing debris flow propagation at regional scale: a case study in Campania region, Italy, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-2224, https://doi.org/10.5194/egusphere-egu21-2224, 2021.

09:21–09:23
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EGU21-13776
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ECS
Qiang Zou, Cong Li, Bin Zhou, Zhenru Hu, and Hu Jiang

The failure mechanism of building structure is important for quantitatively assessing vulnerability of elements at risk, which is a critical step in risk assessment of debris flow. Scholars have recently made great processes in the researches on debris flow hazard effects and vulnerability of elements at risk. Statistical analysis methods have widely used to analyze field survey data and build vulnerability functions. Based on numerical simulation and model experiment, structural dynamic response process was analyzed to evaluate structure vulnerability. However, due to the lack of quantitative relationship between the debris flow hazard-forming mechanism and the dynamic response of building structure, it is essential to analyze the dynamic response characteristics and process of building structure subject to debris flow, which would play an important guiding role in disaster prevention and disaster mitigation.

Through hazard field investigation, the failure modes of rammed earth building caused by debris flow were summarized as burying, scouring and impact. Figure 1 shows the debris flow hazard in Jiende Gully, Liangshan. In addition, by using the finite element analysis method, the structure model of rammed earth building was established to simulate to the impact process of debris flow on the structure. During the dynamic failure process of rammed earth building shown in Figure 2, the failure types of building wall impacted by the debris flow mainly presented at crushed failure of the impact point, tensile failure of the inside wall and shear failure of the corner. Then debris flow destroyed the gable wall, rushed into the room, and broke the doorway, which resulted in damage of the longitudinal wall. Moreover, the response characteristics and failure mechanism of rammed earth buildings under the impact of debris flow further show that the integrity of rammed earth building is poor and the development of cracks cuts off the propagation path of stress, which effectively protects other walls. The transform-shape locations of the rammed earth building including were initially destroyed at the points of the wall foundation, corners of wall and the points impacted by big rocks of debris flow. Therefore, the reinforced measures on the locations where stress suddenly changes, such as wall foundations and wall corners should be paid more attention to protect rammed structure of buildings.

How to cite: Zou, Q., Li, C., Zhou, B., Hu, Z., and Jiang, H.: Dynamic process and failure mechanism of rammed building structure subject to debris flow, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13776, https://doi.org/10.5194/egusphere-egu21-13776, 2021.

09:23–09:25
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EGU21-15407
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ECS
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Highlight
Filippo Mauro, Alessandro Leonardi, and Marina Pirulli

Debris flows are amongst the most hazardous landslide phenomena (Jakob & Hungr, 2005). They are mixtures of flowing water and granular materials, which range in size from microscopic soil particles to massive rock boulders. Due to their unpredictability and rapidity, they pose severe hazard on infrastructure, structures, and human lives. To dissipate the destructive kinetic energy of debris flows and induce deposition of the coarsest fraction of the flow, mitigation systems often include the use of filter barriers. Filter barriers are built both in steel and reinforced concrete, and their openings should be designed according to a reference grain diameter. This key parameter is often chosen arbitrarily due to the difficulties in considering the full grain size distribution of the deposit. Sufficiently small outlets, however, leads to premature clogging of the barriers, blocking further outflow (Ashour et al., 2017). This can result in excessive maintenance costs.

This work focuses on the clogging mechanism of three different kinds of filter barriers: nets, slit dams, and slot dams. The aim is to evaluate the influence of grainsize dispersity into the clogging/non-clogging transition. Starting from simpler monodisperse granular material, we determine via DEM simulations the particle diameter D that induces clogging in the openings, as a function of the opening size S. Thus, for monodisperse grains, a set of threshold values for S/D can be detected: on one side of the threshold the particles are too small to clog the opening, on the other side they are too large to allow free passage of the material.

However, natural debris deposits are far from uniform. To analyse the role of grainsize dispersity, bidisperse specimens are created mixing grains with two different diameters: a small diameter and a large diameter. By varying the composition of large and small particles, a transition is observed between clogging and free-flow, in analogy with what obtained in the simulation with monodisperse grains. The comparison of results obtained with bidisperse and monodisperse samples indicates that an analogy in terms of trends and thresholds exists, as long as an equivalent diameter D* is introduced for bidisperse mixtures (Marchelli, 2018). This parameter is therefore suggested as the reference diameter to be adopted in the barrier design.

How to cite: Mauro, F., Leonardi, A., and Pirulli, M.: Clogging mechanisms of filter barriers against debris-flow hazard, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-15407, https://doi.org/10.5194/egusphere-egu21-15407, 2021.

09:25–09:27
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EGU21-14914
Chaojun Ouyang

Massflow is based on the depth-integrated continuum and solved by second-order MacCormack-TVD finite difference method. Shared code and friendly GUI are provided for researchers and engineers. It adopted CPU and GPU accellerated algorithm to improve the efficiency. Now around 1000 people adopted Massflow to do their own research. Based the framework, we have done several insightful simulations of real landslides and debris flows. Meanwhile, we are developing a solution for catchment-based rainfall- flood-debris flow prediction. We will introduce the basic of the software, the mechanism and related model to modeling the real hazards, and the framework and finished work of forecasting of catchment flood or debris flow. 

How to cite: Ouyang, C.: Massflow—A software for dynamic modeling and risk evaluation of earth-surfaced flow, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14914, https://doi.org/10.5194/egusphere-egu21-14914, 2021.

09:27–09:29
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EGU21-13938
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ECS
Hilbert Villafane Gomez, Juan C. Torres Lázaro, Adriana Caballero Bedriñana, Harrinson W. Jara Infantes, Enver L. Melgarejo Romero, Julia E. Araujo Reyes, Christian Yarleque, Stephan Harrison, Ryan Wilson, Joanne L. Wood, and Neil F. Glasser

The Cordillera Blanca is undergoing rapid deglaciation due to climatic warming, especially since the late 20th century. This process has resulted in the formation of new glacial lakes and an increase in the volume of existing lakes, some of which pose a risk in the form of Glacier Lake Outburst Floods (GLOF); such as Parón lake in the Cordillera Blanca, which represents a significant hazard to the Caraz city and smaller populations located in the Llullán-Parón sub-basin. Here, we model a potential dam breach and GLOF generation scenario at Parón lake using a novel numerical modelling procedure that, amongst other factors, considers the geological structure of the natural dam. Overall, this procedure includes four distinct phases: (1) estimation of the potential for ice avalanche impact on Parón lake sourced from surrounding glacial cirques; (2) modelling of subsequent impulsive wave generation and propagation; (3) analysis of the hydraulic parameters of a possible breach of the natural dam, considering the non-erodible material within empirical estimations of the hydrograph where the composition of the dam is interpreted based on surface geological mapping and drill sampling carried out in the area; and (4) simulation of a potential GLOF using the FLO-2D model with input data from the previous phases. Modelling results indicate that Parón lake is most at risk from ice avalanches that originate from the adjacent Hatunraju glacier and that such events have the potential to generate impulse waves that could initiate erosion and a subsequent breach of the natural dam. Considering a worst-case ice avalanche scenario, our results indicate the potential generation of a GLOF with average peaks flow of 25,264.22 m3/s. This GLOF event would reach the urban area of the  Caraz city in around 36 - 42 minutes with now rates and flood heights fluctuating between 11.2 m/s to 22.4 m/s and 9.9 m to 19.7 m, respectively.

How to cite: Villafane Gomez, H., Torres Lázaro, J. C., Caballero Bedriñana, A., Jara Infantes, H. W., Melgarejo Romero, E. L., Araujo Reyes, J. E., Yarleque, C., Harrison, S., Wilson, R., Wood, J. L., and Glasser, N. F.: Modelling the impact of a GLOF scenario at Parón lake, Cordillera Blanca, Perú, using a novel multi-phase topographical and geological procedure, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13938, https://doi.org/10.5194/egusphere-egu21-13938, 2021.

09:29–09:31
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EGU21-13139
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ECS
Federico Gómez Cardona, Edier Aristizábal Giraldo, Maria Isabel Arango, and Martin Mergili

Debris-flow processes are highly destructive phenomena that endanger life and infrastructure located in mountainous areas. The Colombian Andes are especially susceptible to this type of processes. Disaster databases include 1,387 channelized debris flow, debris flood, and flash flood records between 1921 and 2020, causing 3,332 deaths and affecting 1,152,613 people. These statistics show the importance of carrying out a regional debris flow hazard assessment to prioritize resources and actions to reduce risk.

One of the main challenges when evaluating debris-flow processes hazard is their multi-hazard nature: they are understood as part of a concatenated phenomenon at catchment scale, including cascading effects of landslides, flash floods, debris floods and channelised debris flows. In this study, a multi-hazard approach was implemented to assess debris-flow processes susceptibility and hazard on both regional and local scale, combining statistical and physically based models in combination with geomorphological observations.

The study area is located in the central Colombia Andes, with an extension of 63,612 km2 where 3,039 catchments were analysed for their debris flow-processes susceptibility, using machine learning methods based on morphometric parameters. This analysis was joined with a physically-based slope stability model to estimate potential sediment volumes that might be supplied by intense rainstorms. By combining susceptibility, slope stability, and soil type at the catchment scale, it was possible to understand the magnitude of the potential of different debris-flow processes. Susceptibility analysis allowed to differentiate the catchments into alluvial and torrential and their magnitude level was categorized based on the volume of unstable soil to find hazard and then, used to select critical catchments for a more detailed scale.

A detailed hazard analysis was carried out for those selected areas through hydrological and hydraulic software, along with fluid-dynamic mass routing models. These methodologies were used with a sub- metric resolution and provide detailed information such as flow height, speed, and pressure to categorize more accurate hazard levels, always framed on the torrential geomorphology units.

Traditional hydraulic and hydrological models were insufficient to provide accurate heights and extents of debris-flow processes since they do not consider their multi-hazard nature nor the volume of sediments from landslides and channel erosion that are added to the flow. As a result, the extent of the flow was smaller than the observed morphological features. The fluid dynamic model r.avaflow considers the rheologic change and fitted better to the type of events. The model was used to simulate different sediment concentrations and flow types. The model results were complemented with the different torrential units mapped through fieldwork. This way, it was possible to establish the events’ maximum potential extent linked to their return periods.

This multi-hazard and multi-scale methodology is a useful tool for stakeholders to prioritize and improve urban planning. It grants a perspective from regional to local scale, can be adapted to fit into specific environments and contexts.

How to cite: Gómez Cardona, F., Aristizábal Giraldo, E., Arango, M. I., and Mergili, M.: Regional and detailed multi-hazard assessment of debris-flow processes in the Colombian Andes, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13139, https://doi.org/10.5194/egusphere-egu21-13139, 2021.

09:31–09:33
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EGU21-4501
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ECS
Andrea Pasqua, Alessandro Leonardi, and Marina Pirulli

Debris flows are landslide phenomena which occur worldwide, posing a major threat to mountain settlements. They consist of flowing fine and coarse sediment saturated with water, which propagate mainly in channelized paths. Because of their high velocity and unpredictability, the evacuation of local populations is often impossible. Losses of human lives and economical damages can be avoided if a correct risk mitigation procedure is adopted. Hence, mitigation structures, such as filter barriers or flexible barriers are often installed in high-risk areas. The primary goal of these structures is to reduce the flow energy and to retain the coarsest boulders. Their design process, which is still frequently based only on empirical or simplified models, would greatly benefit from the support of a reliable numerical model.

 

In this framework, continuum-based Depth-Averaged Models (DAMs) have been the dominant numerical tool since the 90s. DAMs can simulate events propagating over a wide area while keeping the computational time low, even on complex topographies (Pirulli, 2010). Nevertheless, the averaging process applied to velocity and pressure causes a loss of information, which is critical when the flow impact against structures is evaluated. A full 3D model would allow for a more accurate resolution of fluid-structure interaction (Leonardi et al., 2016). However, debris flows may propagate up to kilometres, and a complete 3D analysis would therefore require exceedingly long computational times.

 

To bypass the shortcomings mentioned above, this work aims to couple DAMs to a 3D model based on the Lattice Boltzmann Method (LBM). Thus, the domain is split into two parts. First, DAMs describes the flow evolution from its initialization to the transport phase. In this portion of the domain, no structures are present. When the flow approaches a structure, DAMs is coupled to a 3D model. To verify the coupling procedure accuracy, the model is benchmarked on the laboratory tests conducted by Moriguchi et al. (2009). These laboratory tests targeted the flow of dry sand on a steep chute, evaluating the flow impact on a barrier. Preliminary results suggest that the coupled model reproduces the laboratory results reasonably well.

 

Keywords: debris flow, coupled numerical modelling, depth-averaged method, 3D Lattice-Boltzmann Method

 

REFERENCES

Leonardi, A., Wittel, F. K., Mendoza, M., Vetter, R., & Herrmann, H. J. (2016). Particle-Fluid-Structure Interaction for Debris Flow Impact on Flexible Barriers. Computer-Aided Civil and Infrastructure Engineering, 31(5), 323–333.

Moriguchi, S., Borja, R. I., Yashima, A., & Sawada, K. (2009). Estimating the impact force generated by granular flow on a rigid obstruction. Acta Geotechnica, 4(1), 57–71.

Pirulli, M. (2010). On the use of the calibration-based approach for debris-flow forward-analyses. Natural Hazards and Earth System Science, 10(5), 1009–1019.

How to cite: Pasqua, A., Leonardi, A., and Pirulli, M.: Coupling Depth-Averaged and 3D models for debris flow: a multi-domain strategy , EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-4501, https://doi.org/10.5194/egusphere-egu21-4501, 2021.

09:33–09:35
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EGU21-6979
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ECS
jinbo Tang, Peng Cui, and Jiangang Chen

Mudflow behaves normally the flowing properties of viscoplastic or pseudo-plastic stemming from flocculent network structures formed by fine particles within the mud. In order to obtain the dynamical characteristics of the dam-break of mudflow, a numerical model has been developed in the present study. The numerical model solved the Navier-Stokes equation with the Herschel-Bulkley model, which exhibits a plastic properties of fluid with shear-thinning. The two-step projection method are employed to solve the velocity field in present numerical model, and the Bi-CGSTAB technique are implemented to solve the pressure Poisson equation. The volume of fluid (VOF) method is used to track the broken the free surface. In this study, the numerical simulation of dam-break with Herschel-Bulkley fluid are implemented, the numerical results agree well with the other numerical results. Furthermore, when the shear-thinning index is equals to unity, the Herschel-Bulkley model becomes Bingham model. In this study, laboratory experiments of dam-break of slurry in the flume have been conducted to record data with time of the surface height of mudflow and pressure in the bottom of flume. The same cases with laboratory experiments are implemented in our numerical model, the numerical results match with the laboratory experiments. Finally, as a demonstration, the impact of mudflow on the structures   are simulated and discussed.

How to cite: Tang, J., Cui, P., and Chen, J.: Numerical simulation of dam-break mudflow based on the Herschel-Bulkley model, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-6979, https://doi.org/10.5194/egusphere-egu21-6979, 2021.

09:35–09:37
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EGU21-5819
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ECS
Johnnatan Palacio, Edier Aristizábal, Martin Mergili, and Oscar Echeverri

Colombia is located in a tropical environment with mostly warm and humid climatic conditions and complex mountainous terrain, where landslides triggered by intense rainfall are very common. Therefore, determining the occurrence and propagation of these events is of great interest in risk management and territorial planning programs.

Landslide propagation is difficult to predict due to uncertainties of rheological properties as well as initiation and dynamics of rock or sediment mobilization.  In Colombia, methodologies and models for landslide propagation have been less addressed than those corresponding to the occurrence, even though the consequences on people and infrastructure are generally are strongly related to travel distances and impact areas. Most propagation models are based on empirical methods to establish the travel distance of the sliding material, employing geometric approximations or geomorphological interpretation. In the last decades, physically based dynamic landslide propagation models have been proposed. These models use digital elevation models in combination with flow parameters. Their application represents a complex task because of the difficulty in constraining – depending on the model – sometimes the large number of relevant flow parameters.

In this study, we apply two open source models that work as extensions to the GRASS GIS software: (i) the r.slope.stability model for slope stability assessment using a limit equilibrium model for different sliding surface geometries, together with a probabilistic analysis applied to a range of geotechnical parameters (cohesion, internal friction); and (ii) r.avaflow for landslide propagation, which employs a multi-phase model considering solids and fluids. The models are implemented in the catchment area known as La Arenosa (9.9 km2), located in the municipality of San Carlos (Antioquia, Colombia). On September 21, 1990, an event of rainfall of short duration and high intensity precipitated on La Arenosa catchment. approx. 200 mm of precipitation fell within the study area in less than 3 hours, triggering approx. 700 landslides many of which have converted into hillslope debris flows. The zones categorized with a high probability of failure through r.slope.stability are defined as source areas and propagated down with r.avaflow. The results are evaluated against the landslide inventory in order to evaluate the potential of the proposed model combination for predictive simulations.

How to cite: Palacio, J., Aristizábal, E., Mergili, M., and Echeverri, O.: Shallow landslide occurrence and propagation in tropical mountainous terrain with open source models. A case study in the Colombian Andes., EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-5819, https://doi.org/10.5194/egusphere-egu21-5819, 2021.

09:37–09:39
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EGU21-2752
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ECS
Marc Peruzzetto, Clara Levy, Yannick Thiery, Gilles Grandjean, Anne Mangeney, Anne-Marie Lejeune, Aude Nachbaur, Yoann Legendre, Benoit Vittecoq, Jean-Marie Saurel, Valérie Clouard, Thomas Dewez, Fabrice R. Fontaine, Martin Mergili, Sophie Lagarde, Jean-Christophe Komorowski, Anne Le Friant, and Arnaud Lemarchand

This work focuses on the use of thin-layer models for simulating fast gravitational flows for hazard assessment. Such simulations are sometimes difficult to carry out because of the uncertainty on initial conditions and on simulation parameters. In this study, we aggregate various field data to constrain realistic initial conditions and to calibrate the model parameters. By using the SHALTOP numerical code, we choose a simple and empirical rheology to model the flow (no more than two parameters), but we model more finely the geometrical interactions between the flow and the topography. We can thus model both a rock avalanche, and the subsequent remobilization of the deposits as a high discharge debris flow.

Using the Prêcheur river catchment (Martinique, Lesser Antilles) as a case study, we focus on extreme events with a high potential to impact populations and infrastructures. We use geological and geomorphological data, topographic surveys, seismic recordings and granulometric analysis to define realistic simulation scenarios and determine the main characteristics of documented events. The latter are then reproduced to calibrate rheological parameters. With a single rheological parameter and the Coulomb rheology, we thus model the emplacement and main dynamic characteristics of a recent rock avalanche, as well as the travel duration and flooded area of a documented high discharge debris flow. Then, in a forward prediction simulation, we model a possible 1.9x106 m3 rock avalanche, and the instantaneous remobilization of the resulting deposits as a high-discharge debris flow. We show that successive collapses allow to better reproduce the dynamics of the rock avalanche, but do not change the geometry of the final deposits, and thus do not influence the initial conditions of the subsequent debris flow simulation. A progressive remobilization of the materials slows down the debris flow and limits overflow, in comparison to instantaneous release. However, we show that high discharge debris flows, such as the one considered for model calibration, are better reproduced with an instantaneous initiation. The range of travel times measured for other significant debris flows in the Pr\^echeur river is consistent with our simulation results, with various rheological parameters and the Coulomb or Voellmy rheology.

How to cite: Peruzzetto, M., Levy, C., Thiery, Y., Grandjean, G., Mangeney, A., Lejeune, A.-M., Nachbaur, A., Legendre, Y., Vittecoq, B., Saurel, J.-M., Clouard, V., Dewez, T., Fontaine, F. R., Mergili, M., Lagarde, S., Komorowski, J.-C., Le Friant, A., and Lemarchand, A.: Simplified simulation of rock avalanches and subsequent debris flows with a single thin-layer model. Application to the Prêcheur river (Martinique, Lesser Antilles), EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-2752, https://doi.org/10.5194/egusphere-egu21-2752, 2021.

09:39–10:30
Break
Chairpersons: Velio Coviello, Sara Savi
Part 2A: Monitoring
11:00–11:05
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EGU21-1226
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ECS
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solicited
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Highlight
Giacomo Belli, Emanuele Marchetti, Fabian Walter, Brian McArdell, Małgorzata Chmiel, and Michaela Wenner

Debris flows are episodic gravitational currents, consisting of mixtures of water and debris in varying proportions. They occur in steep mountain torrents with volumes commonly exceeding thousands of m3. Given their unpredictability and their capability to transport large boulders, debris flows rank among the most dangerous natural hazards in mountain environments. Nevertheless, moderate flow velocities (typically < 10 m/s) make early warning in principle possible if the flows are detected early upon formation.

Infrasound studies of debris flows increased significantly in the last decade, focusing mostly on event detectability and application for early-warning. The use of infrasound arrays and the combined use of collocated seismic and infrasound sensors have turned out to be efficient systems for reliable detection of debris flows in near-real time.

Despite these advances, open questions remain about the possibility to infer debris-flow source characteristics and event magnitude from recorded infrasonic signals. This requires theoretical and/or empirical source models describing elastic energy radiation in the atmosphere, in the form of infrasound, and relating it with fluid dynamic processes within a debris flow. Infrasound radiated by debris-flows is believed to be generated by standing waves that develop at the free surface of the flow, but details of the involved dynamic processes are not fully understood.

Here, we present the analysis of infrasonic signals recorded with a small aperture array during the 2017-2020 debris-flow seasons in the Illgraben catchment (Switzerland, Canton Valais), including more than 20 events of variable sizes. In order to better understand infrasound source mechanisms and to investigate the fluid dynamics processes involved in the infrasonic energy generation, debris-flow infrasound signals are quantitatively compared with independent hydraulic information of the flow (velocity, maximum flow depth and flow density). Finally, we discuss the use of extrapolated empirical relationships between infrasound signal features and flow characteristics for debris-flow monitoring and risk management.

 

How to cite: Belli, G., Marchetti, E., Walter, F., McArdell, B., Chmiel, M., and Wenner, M.: Investigating infrasound sources within Illgraben debris-flows, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1226, https://doi.org/10.5194/egusphere-egu21-1226, 2021.

11:05–11:07
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EGU21-10499
Takahiro Itoh, Takahiko Nagayama, Satoru Matsuda, and Takahisa Mizuyama

The monitoring method for direct debris flow measurements using loadcells and so on, that were preliminary developed by WSL in Switzerland (McArdell et al., 2007), was firstly installed in Sakura-jima Island in Japan, where volcanic activity was severe, and many debris flows took place due to deposition of falling ash after eruptions. Debris Flow measurements with Loadcells and Pressure sensors (DFLP) system was installed referring to the method by WSL, and debris flow characteristics such as specific weight and volumetric sediment concentration have been obtained (e.g., Osaka et al., 2014).

 In Japan, as well as in Sakura-jima island, attempts for debris flow monitoring were also carried out at KamiKamihori Creek since 1970s (e.g., Okuda et al., 1980), and there were a lot of debris flow events due to heavy rainfall. KamiKamihori Creek is at western side of Mt. Yake, where volcanic activity was severe at those time. The DFLP system was modified and installed there in November in 2014, because there were a lot of sediment deposition and debris flows took place though volcanic activity has been inactive. Present research could report the following results.  

(1) Multiple debris floe over five surges were monitored using DFLP system installed in 2014 during 15 minutes in debris flow events on August 29th, 2019. Rainfall intensity for 10 minutes was 12 mm and accumulated depth was 56 mm just before those events. Antecedent time before those events was 4.5 hours.

(2) The DFLP system measured multiple debris flow surges in events on August 29th, 2019, and sediment concentration was calculated temporary and continuously. Time-averaged sediment concentration and relative mass density are calculated as 0.470 and 1.73, respectively, under flow discharge obtained by images analysis of CCTV video camera. Equilibrium sediment concentration of coarse sediment particles is estimated 0.160 for bed slope of 0.141 (8 degrees) and calculated value using the DFLP system is over than the equilibrium value because of mud phase due to fine sediment particles.

 

References

McArdell B.W., Bartelt P., Kowalski J. (2007). Field observations of basal forces and fluid pore pressure in a debris flow, Geophysical Research Letters, Vo. 34, L07406.

Okuda, S., Suwa, H., Okunishi, K., Yokoyama, K., and Nakano, M. (1980). Observation of the motion of debris flow and its geomorphological effects, Zeitschrift fur Geomorphology, Suppl.-Bd.35, pp. 142–163.

Osaka T., Utsunomiya R., Tagata S., Itoh T., Mizuyama T. (2014). Debris Flow Monitoring using Load Cells in Sakurajima Island, Proceedings of the Interpraevent 2014 in the Pacific Rim (edited by Fujita, M. et al.), Nov. 25-28, Nara, Japan, 2014, O-14.pdf in DVD.

How to cite: Itoh, T., Nagayama, T., Matsuda, S., and Mizuyama, T.: Multiple debris flow surges monitored directly by DFLP system at Kamikamihori Creek, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10499, https://doi.org/10.5194/egusphere-egu21-10499, 2021.

11:07–11:09
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EGU21-15399
Philipp Aigner, Erik Kuschel, Christian Zangerl, Johannes Hübl, Markus Hrachowitz, Leonard Sklar, and Roland Kaitna

Debris flows (DFs) pose a severe risk to Alpine communities and infrastructure. The Lattenbach catchment (basin area 5,3 km², relief 2134 m) in Tyrol, Austria, is an example for an active DF-site with several DFs occurring per year. To improve our understanding of the DF-process cascade in this catchment, we raise the questions: where does the sediment originate, are hillslope processes the drivers for DF-activity, and how is the relationship of rainfall amount and DF-magnitude?

We employ an approach that makes use of the data richness of this study site: High resolution ALS and TLS terrain models and aerial photographs are considered to locate significant elevation differences. Furthermore, we performed an in-detail UAV-based surveying campaign of the active channel reaches for the 2019 and 2020 DF-season. Additionally, we use datasets captured by a DF monitoring station (discharge, volume, timing, precipitation) at the catchment outlet to assess triggering rainfall as well as DF-frequency and magnitudes.

We find that in the last fifteen years up to three events occurred annually. A single location, where all DFs originate from, is not detectable, indicating a variety of sediment sources is relevant for DF-initiation, including bank failures and channel incision, partly driven by deep-seated landslides that continuously feed the channel with sediment. Between the years 2005 and 2018 the DF-volumes recorded at the catchment outlet varied between about 5.000 m³ (small) and 46.000 m³ (large). A first analysis suggests that there is a prevailing “background noise“ pattern of relatively small DF-events that happen regularly during every DF-season. We hypothesize that rare, very large events represent a tipping point in the catchment system, which leads to a period of increased large-scale DF-activity over following seasons.

How to cite: Aigner, P., Kuschel, E., Zangerl, C., Hübl, J., Hrachowitz, M., Sklar, L., and Kaitna, R.: Multi-sensor approach towards understanding debris-flow activity in the Lattenbach catchment, Austria, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-15399, https://doi.org/10.5194/egusphere-egu21-15399, 2021.

11:09–11:11
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EGU21-13348
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ECS
Ivonne Martínez Valdés, Víctor Hugo Márquez Ramírez, Lucia Capra, Velio Coviello, and Raúl Arámbula Mendoza

Rainfall-induced lahars are one of the most common phenomena in tropical volcanoes. Volcán de Colima (VdC) is the most active volcano in Mexico regarding intra-eruptive lahar generation. Lahars represent one of the main hazards for local communities located within a radius of 15 km from the summit. During the rainy season, from May to October dozens of lahars occur in the different ravines draining the VdC. Since 2007, lahar monitoring is performed for both research and civil purposes. Rain gauges, seismic sensors, cameras, and infrasound sensors are part of the current monitoring system deployed at Montegrande ravine (MR) which is located in the southern flank of the volcano. Here we present the data collected during the 2018 monitoring season that are composed of seventeen flow events, six of which feature the most complete dataset ever collected at MR. Data are recorded with multiple stations including broad-band seismic sensors (120 s), geophones (4.5 Hz), short-period seismometers (1 Hz) and a video camera installed along a 1.5 km channel reach. Three types of lahars have been classified based on the join-analysis of seismic signals and video images of these latter six events: dry front, diluted and multi-front. These classes are related to the solid-liquid composition and dynamics of the flows, and to the rainfall amount possibly triggering the processes. A linear discriminant analysis (LDA) is proposed to classify the rest of the events using seismic and rainfall records. Preliminary results show how the flow velocity and the presence of coarse fronts, inferred by means of cross-correlation method and inspection of the video images respectively, are the first factors controlling the characteristics of the seismic signals. This work also demonstrate how seismic techniques represent a valuable tool to describe the remarkable variability of flow dynamics along the travel path.

 

How to cite: Martínez Valdés, I., Márquez Ramírez, V. H., Capra, L., Coviello, V., and Arámbula Mendoza, R.: Seismic classification of rainfall-induced lahars at Volcán de Colima, México, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13348, https://doi.org/10.5194/egusphere-egu21-13348, 2021.

11:11–11:13
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EGU21-4968
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ECS
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Highlight
Oliver Francis, Hui Tang, Carlo Gregoretti, Matteo Berti, Martino Bernard, and Alessandro Simoni

Runoff-generated debris flows are a significant hazard in steep mountain ranges across the world. During intense rainfall storms, runoff can rapidly form in small steep basins and mobilise large volumes of sediment triggering debris flows which can damage infrastructure and endanger lives. A common method for forecasting debris flows is deriving empirical rainfall intensity–duration (ID) thresholds from previously recorded debris flow events in a given area. However, the storms which trigger debris flows usually are short and intense with high spatial variation making an accurate recording of the conditions responsible for initiation difficult.

In this study, we investigate the impact of the spatial variability of rainfall on debris flow initiation in small, steep, and debris flow prone catchments in the eastern Italian Alps (Dolomites) using the SWEHR (Shallow Water Equation Hairsine-Rose) numerical model. The modelled catchments are monitored by multiple rain gages which we use to quantify the uncertainty of the rainfall ID thresholds due to the spatial variation of rainfall by comparing empirical and numerically modelled thresholds. We also compare simulated triggering discharges for debris flows with available field observations in the study area. This study will help to improve the quality of hazard forecasting of debris flows in mountainous regions

How to cite: Francis, O., Tang, H., Gregoretti, C., Berti, M., Bernard, M., and Simoni, A.: Rainfall Spatial variability, rainfall intensity–duration (ID) thresholds, and the initiation of debris flows in the eastern Italian Alps, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-4968, https://doi.org/10.5194/egusphere-egu21-4968, 2021.

11:13–11:15
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EGU21-16257
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ECS
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Highlight
Verena Stammberger, Benjamin Jacobs, and Michael Krautblatter

High-intensity precipitation events and the resulting extreme discharges in mountain torrents are immensely dangerous and destructive hazards that can put lives in danger and cause expensive damages to infrastructure. There is a high probability that further changes in climate will favour the genesis and therefore increase the frequency of such extreme events. Nevertheless, there is a pronounced desire to experience breathtaking mountainous landscapes, especially when easy accessible. An example is the Höllental gorge (between 1032 and 1062 m a.s.l., Wetterstein mountains, Germany), a key touristic attraction in the region with up to 100k visitors per year. Especially for such highly frequented places, the knowledge and comprehension of possible risks from hydrological and geomorphic hazards is crucial. With this in mind, we are reconstructing and discussing possible modelling approaches of a recent event of a hyperconcentrated flow through the gorge.

In June 2020 a local extreme precipitation event between 50 and 60 mm/h caused a rapid accumulation of the surface runoff due to the steep slopes of the Höllental (inclination of ø 110%). Secondary sediment storages were mobilized and transported to the main channel where a hyperconcentrated flow developed at the beginning of the gorge. Depending on the percentage of transported sediment in the flow, temporary transitions to a debris flow were possible. Throughout the ravine, massive forces reshaped the rock walls and the channel bed by particle erosion, shearing and relocation of boulders up to 20 m3.

In this study we present a comparison of two terrestrial laser scan campaigns, the first two weeks prior to the event and the second just five days after. We were able to accurately calculate the morphological changes along the sides of the channel and obtained a unique data set for bedrock erosion rates due to the impact of a hyperconcentrated flow. We mapped the flow height throughout the whole gorge by identifying the visible transition of undisturbed to roughened rock surfaces. DEM difference calculation upstream allows to determine the erosion and deposition heights as well as the corresponding volumes. Additionally, electrical resistivity tomographies reveal the thickness of (still) available sediment upstream.

Here we discuss possible numerical and analytical modelling approaches and analyse preliminary results. We aim at coupling the observed erosion rates to calculated velocities of a model that integrates the complex topography as well as the rheological parameters of the flow. A calibration of the model will be achieved with the mapped flow height in the gorge. Due to the complexity of the gorge, a frequently used numerical simulation as well as a analytical open-channel flow model will be analyzed and compared.

This study presents a unique dataset of effective erosion rates with records collected pre- and post-event. The results contribute to strongly improve the understanding of the flow dynamics in hyperconcentrated flows and give unparalleled information about erosion processes in narrow bedrock channels.

How to cite: Stammberger, V., Jacobs, B., and Krautblatter, M.: TLS-recorded massive bedrock erosion of a hyperconcentrated flow in an Alpine Gorge: approaches to modelling the event (Höllentalklamm, Germany), EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-16257, https://doi.org/10.5194/egusphere-egu21-16257, 2021.

11:15–11:17
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EGU21-11815
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ECS
Raffaele Spielmann, Jordan Aaron, and Brian McArdell

Debris flows are extremely rapid, flow-like landslides that can impact people and infrastructure far from their source. Reducing debris flow hazard requires an understanding of the mechanisms which govern debris flow behavior, such as grain size segregation, entrainment, deposition and liquefaction, as well as accurate numerical models, which must be validated based on laboratory and field data.  Thus, knowledge of debris flow mechanisms, as well as forecasts of debris flow behavior, require accurate measurement of a number of quantities that vary through time, which include slope inclinations, front velocity, flow depth and flow volume.  These parameters are difficult to measure in moving granular flows, however newly available sensors have the potential to accurately capture them. These sensors include high resolution LiDAR and depth cameras, which are potentially useful for the field and laboratory scale, respectively.  In the present work, we implement a processing pipeline to attempt to estimate these four parameters, and associated errors, using two such sensors; a LiDAR sensor (Ouster OS1-64 Gen 1) which is planned for a field installation and a depth camera (Intel RealSense Depth Camera D415) which is useful for laboratory scale experiments. 

We performed a series of laboratory experiments, where different dry sediment mixtures were released down an inclined plane, and both sensors were used to collect time-lapse point clouds of the moving material.  These point clouds were recorded at a rate of 10 Hz and 30 Hz, for the Ouster OS1-64 sensor and Intel RealSense Depth Camera D415 respectively.  Our processing pipeline involves aligning the point cloud frames, isolating the moving material based on reflectivity or infrared thresholds, and then measuring the four parameters detailed above in each frame.  Both sensors are able to measure slope angle, velocity and volume, however the measurement of the free surface gradient was subject to more error.  The experimentally determined noise levels for the sensors were different, with  3 cm for the LiDAR scanner, and 0.5 cm for the depth camera.  Additionally, the more accurate depth camera enabled tracking of larger particles on the flow surface.  Interestingly, we measure a consistent volume dilation of the flowing debris, with a volume increase of about 39% over 0.1 s.

Thus, both sensors will be useful tools for understanding fundamental debris flow mechanisms.  Future work will focus on defining a relationship between flow velocity and material contraction/dilation, and will further refine the processing algorithm to more accurately assess these four parameters.  Additionally, we will present an overview of the set-up and installation of an OS1-64 sensor in the Illgraben catchment, Europe's most active debris flow catchment.

How to cite: Spielmann, R., Aaron, J., and McArdell, B.: Time-lapse point clouds of moving granular flows: Preliminary insights and future outlook, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-11815, https://doi.org/10.5194/egusphere-egu21-11815, 2021.

11:17–11:19
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EGU21-13846
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ECS
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Highlight
Elijah Orland, Dalia Kirschbaum, and Thomas Stanley

As the risk of wildfires increases worldwide, burned steeplands are vulnerable to the secondary hazard of widespread sediment mobilization through debris flows. Following an initial burn, sediment and soil previously restrained by vegetation are no longer consolidated, allowing for easy mobilization into channels and along steep hillslopes through runoff.  Sufficiently powerful rainfall incorporates entrained material into turbulent flows and serves as the primary trigger for debris flow initiation. There is thus an ongoing need to establish the relationship between rainfall and debris flow initiation based on a variety of spatiotemporal preconditions. Previous work establishes regional and local thresholds to constrain the effect of rainfall in recently burned areas, but no empirical or numerical solution has worldwide application. Building from regionally-based efforts in the U.S., this work considers how remote sensing data can be applied to better approximate the post-fire debris flow hazards worldwide using freely available global datasets and software. Our work assesses the utility of remote sensing resources for analyzing burn characteristics, topography, rainfall intensity/duration, and, thus, debris flow initiation. Early results show that global observations are sufficient to delineate background rainfall rates from storms likely to cause debris flows across a variety of burn severity and topographic conditions. However, the dearth of publicly-available post-fire debris flow inventories globally limit the ability to test how the model framework performs within different climatologic and morphologic areas. This work will present preliminary analysis over the Western United States and demonstrate the feasibility of a global, near-real time model to provide situational awareness of potential hazards within recently burned areas worldwide. Future work will also consider how global or regional precipitation forecasts may increase the lead time for improved early warning of these hazards.

How to cite: Orland, E., Kirschbaum, D., and Stanley, T.: Towards real-time global assessment of post-fire debris flow hazards with remotely sensed data, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13846, https://doi.org/10.5194/egusphere-egu21-13846, 2021.

11:19–11:21
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EGU21-16520
Maxime Morel, Guillaume Piton, Caroline Le Bouteiller, Alexandre Mas, and Guillaume Evin

In mountain areas, the quantification of sediment yield is essential in the diagnosis of a torrential watershed. The objective of this study is to present a prediction method based on multivariate statistical models calibrated from an original data set covering nearly 130 torrential basins in the Northern French Alps. Data on sediment yield and occurrence of torrential events were collected on these catchments thanks to registries from sediment retention basins (average monitoring period of 20 years) and historical archives of the catchment basin managers. On these catchments, several morphological and hydro-meteorological characteristics were calculated (e.g. geological and sediment connectivity indices, the rate of connected eroding areas in the catchment, the Melton index, the slope of the fan, etc.) in order to relate them to sediment production and the frequency of occurrence of torrential events. These models allow the estimation of quantiles of the sediment yield in small torrent catchments. These models could be useful to evaluate sediment yield and the occurrence of torrential events on catchment not equipped with sedimentation structures.

How to cite: Morel, M., Piton, G., Le Bouteiller, C., Mas, A., and Evin, G.: Relating sediment supply to the morphological and hydro-meteorological characteristics of torrent catchments, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-16520, https://doi.org/10.5194/egusphere-egu21-16520, 2021.

Part 2B: Experiments
11:21–11:26
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EGU21-10777
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ECS
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solicited
Lukas Reider, Anna-Lisa Fuchs, Lisa Dankwerth, Susanna Wernhart, Roland Kaitna, Georg Nagl, Dirk Proske, and Christian Scheidl

For the design of mitigation measures knowledge of debris-flow impact forces, usually estimated based on hydrostatic, hydrodynamic, or combined approaches, is essential. As these approaches are based on Newtonian fluids, they must be adjusted by empirical correction factors to account for the solid-fluid nature of debris flows. The values for the correction factors shown in the literature vary over a wide range and several studies showed a clear dependence with the Froude regime of debris flows.

To better understand the correction factors and to be able to calculate them using parameters that describe the flow behaviour a total of 32 experiments were conducted in the course of the project “Debris flow impact forces on bridge super structures (DEFSUP)”, funded by the Austrian Science Fund (FWF). Two different material compositions, different water contents as well as a total impact and a bypassing of the measuring block were tested.

The experimental setup designed within the project consists of a 4 m long semi-circular channel with a diameter of 300 mm and an inclination of 20°. The material is released from a rectangular reservoir in a dam-break scenario and accelerated with zero roughness on a length of 1.2 m and transferred to the semi-circle profile. The subsequently introduced roughness with a grain diameter of 1-2 mm generates a stationary phenomenological debris flow until it hits the measuring setup. With a starting volume of 50 kg, flow heights between 8 and 12 cm and velocities from 0.8 to 2.2 m/s were achieved according to the material composition and different water content. With these different mixtures a Froude-range from 0.6 to 3.6 was covered. In addition, normal stresses and pore water pressures were measured at the exact same point.

A detailed analysis of the measured impact forces together with the above mentioned measured parameters showed that the hydrodynamic correction factor is a constant mainly corresponding to the liquification ratio of the debris-flow mixture. Hence, the hydrodynamic correction factor can be regarded as a drag coefficient and seems to depend mainly on the internal friction of the flowing medium. At low Froude numbers measured impact forces exceed even a full momentum transfer if the mean bulk density is used for the calculation. This indicates that the impact forces can no longer be described by the hydrodynamic approach alone. For this reason, an additional pressure term based on a hydrostatic approach is considered in the combined concept. This additional pressure term depends on the dynamics of flow (Froude number) and can be modelled via a dynamic earth pressure coefficient.

The findings from these experiments contribute to a better prediction of debris-flows impact forces in terms of their material composition and flow behaviour.

How to cite: Reider, L., Fuchs, A.-L., Dankwerth, L., Wernhart, S., Kaitna, R., Nagl, G., Proske, D., and Scheidl, C.: A combined model approach for debris-flow impact forces, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10777, https://doi.org/10.5194/egusphere-egu21-10777, 2021.

11:26–11:28
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EGU21-14877
Chan-Young Yune and Beom-Jun Kim

A debris flow with a high speed along valleys has been reported to cause serious damages to urban area or infrastructure. To prevent debris flow disaster, countermeasures for flow-impeding structures are installed on the flow path of debris flows. Recently, an installation of cylindrical baffles which are open-type countermeasures has increased because of a low construction cost, filtering out rocks, and an increased hydraulic continuity. However, a comprehensive design guideline for specification and arrangement on cylindrical baffles has not yet been suggested. Moreover, the design of baffle installation is mainly based on empirical approaches as the influence of baffle array on debris mobility is not well understood. In this study, to investigate the effect of cylindrical baffles on the flow characteristics of debris flow, a series of small-scale flume tests were performed according to the varying baffle height and row numbers of installed baffles. High-speed cameras and digital camera to record the flow interaction with baffles were installed at the top and side of the channel. To reproduce the viscosity of debris flows caused by fine-grained soil in the flume, glycerin was mixed with debris materials (sand and gravel). After the test, the velocity and energy dissipation according to various baffle arrays were estimated. Test results showed that the installation of baffles reduced the frontal velocity of debris flows. Furthermore, taller baffles also increased the effect of the energy dissipation in debris flows, but additional rows of the baffle did not have a major effect on the energy dissipation. Thus, increasing the height of baffle led to an increased efficiency of energy dissipation of debris flows.

How to cite: Yune, C.-Y. and Kim, B.-J.: Flume investigation of cylindrical baffles for dissipation of debris flow energy, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14877, https://doi.org/10.5194/egusphere-egu21-14877, 2021.

11:28–11:30
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EGU21-9144
Jiangang Chen, Xi'an Wang, and Huayong Chen

A series of check dams were constructed for debris-flow hazard mitigation in China. Based on the results of field investigation, check dam has a significant impact on the geomorphology of debris flow gully, especially the upstream and downstream of a check dam. According to the relationship between the sediment deposition thickness and the check dam height, the running status of a check dam can be divided into three states: without sediment deposition, half of the storage capacity with sediment deposition, and full of sediment deposition. With the accumulation of sediment transport, the running state of a check dam gradually changed and the sediment-trapping effect of check dams has gradually weakened, leading to the loss of part of the disaster mitigation effect, increasing the risk of downstream infrastructure and human security. Therefore, experiments with multi-surges of debris flows were carried out to study the geomorphic and sediment-trapping effectiveness of check dams. The results showed that with the increase of the sediment amount with multi-surges, the deposition slope in the downstream dam approached or even exceeded that of upstream dam. For one surge, deposition morphology has slightly difference in the cascade dams. At last, a method for calculating the reduction coefficient of deposition slope considering the check dam height and sediment amount with multi-surges is proposed.

How to cite: Chen, J., Wang, X., and Chen, H.: Sediment-trapping effectiveness of check dams with multiple debris-flow surges: Experimental study, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9144, https://doi.org/10.5194/egusphere-egu21-9144, 2021.

11:30–11:32
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EGU21-2565
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ECS
Tjalling de Haas, Amanda Aaberg, Fabian Walter, and Zhen Zhang

Debris flows are gravity-driven mass movements that are common natural hazards in mountain regions worldwide. Previous work has shown that measurements of ground vibrations are capable of detecting the timing, speed, and location of landslides and debris flows. A remaining question is whether or not additional flow properties, such as grain-size distribution, flow depth, and impact stress can be inferred reliably from seismic data. Here, we experimentally explore the relation of seismic vibrations and normal-force fluctuations with debris-flow composition and dynamics. We show that seismic vibrations and normal-force fluctuations induced by debris flows are strongly correlated, and that both are strongly affected by debris-flow composition. We find that the effects of the large-particle distribution on seismic vibrations and normal-force fluctuations are substantially more pronounced than the effects of water fraction, clay fraction, and flow volume, especially when normalized by flow depth. We further show that for flows with similar coarse-particle distributions seismic vibrations and normal-force fluctuations can be reasonably-well related to flow depth, even if total flow volume, water fraction, and the size distribution of fines varies. Our experimental results shed light on how changes in large-particle, clay, and water fractions affect the seismic and force-fluctuation signatures of debris flows, and provide important guidelines for their interpretation.

How to cite: de Haas, T., Aaberg, A., Walter, F., and Zhang, Z.: Deciphering the seismic and normal-force fluctuation signatures of debris flows: an experimental assessment of the effects of flow composition and dynamics, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-2565, https://doi.org/10.5194/egusphere-egu21-2565, 2021.

11:32–11:34
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EGU21-14075
Yong Li

Debris flow is composed of solid grains of different sizes. the characteristics of grain size distribution reflect the movement mode and dynamic conditions of the fluid, and have different effects on the movement of debris flow. Due to the high variability of debris flow materials, the granular interaction is bound to affect the fluid properties. The grain size distribution (GSD) of debris flow satisfies the formula: P(D)=CDexp(-D/Dc), where, GSD parameters μ and Dc can comprehensively reflect the change of grain composition. with μ reflecting the structure and variation characteristics of fine grains, and Dc reflecting the range of grain size. Field surveys in various regions indicate that the GSD parameters are distinct in materials of flow, source, and deposition. The GSD parameters of source soil and deposition soil are random and discrete, while the GSD parameters of fluid samples show obvious negative power function form: Dc= aμb (Figure 1). This shows that the grain composition of debris flow contains some dynamic information. In this paper, we use natural soil materials in a typical debris flow valley to conduct a series dynamically mixing and rotating experiments to simulate the flow evolution, and explore the change of grains under the action of dynamics and the effect of grain adjustment on the mobility of debris flow. The results show that the GSD shows a significant regularity after dynamic rotation. The specific performance is that μ and Dc change from the initial random discrete state to negative power correlation (Figure 2), and the appearance of this correlation corresponds to the best mobility of debris flow. At the same time, the Malvern laser grain size analyzer was used to analyze the specific surface area of fine grains (<0.20 mm) in the dynamic rotation experiment. The results show that with the increase of dynamic time, the specific surface area increases according to power law, and when the time reaches about 100 minutes, the growth slows down, and the specific surface area changes little. The experimental results are helpful for a deep understanding of the dynamics of debris flow.

How to cite: Li, Y.: Dynamic evolution of debris flow grain composition, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14075, https://doi.org/10.5194/egusphere-egu21-14075, 2021.

11:34–11:36
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EGU21-14116
Taiqiang Yang

Debris flow is characterized by the multi-disperse grain composition and intergranular collision and friction, but the granular effects on rheology are often reduced to the volumetric concentration of solid (Cv), almost ignoring the specific grain size distribution (GSD). In this study, small debris flows occurring in a tributary of Jiangjia Gully were taken as the material sources for rheology experiments. From the real flows we selected slurries with different Cv and maximum grain sizes (Dm) for rheological tests under shearing rate up to 40 (s-1), which is usually the real rate for debris flows in natural conditions. The results indicate that the flows follow the Herschel-Bulkley (HB) rheology, with randomly changing consistency coefficient and relatively constant exponent of 0.45 on average. Only at high shear rate will the flow exhibit Bingham behavior. The HB rheology also reveals shear thinning behavior in surge phenomena observed in the field. Shear-thinning behavior is revealed by the viscosity-shear rate relationship: ηa=pγq, with the exponent (thinning index) dependent on shear rate. This greatly concerns the surge phenomena observed in field. Moreover, both the yield stress and the effective viscosity are found to be perfectly related to the scaling GSD parameters in power-law and exponential form, with nearly constant exponents independent of the shear rate(Figure 1). The rheology properties can be calculated from their relationships to GSD parameters (μ, Dc), which in turn can be used to infer the HB rheology for the concerned flows and then build the dynamical equations(Figure 2). This implies the presence of some interlock between the fine and coarse grains. Finally the rheology model (general in HB form) can be completely determined by the GSD parameters. This study has for the first time proposed quantitative formulas for rheology incorporating GSD parameters, which is helpful for more accurate dynamic analysis of debris flow.

How to cite: Yang, T.: Granular effect on debris flow rheology, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14116, https://doi.org/10.5194/egusphere-egu21-14116, 2021.

11:36–11:38
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EGU21-12139
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ECS
Yanju Fu, Ziming Liu, and Yao Jiang

       Glacial tills are ubiquitous in periglacial mountains and can be destabilized as the main source materials of glacial debris flows due to atmospheric warming. In general, these surface hillslope materials are internally mixed with debris, ice, fluids etc., where the constituent fluids may experience prolonged freeze-thaw cycles. Although many studies including laboratory tests, field investigations and numerical simulations have been conducted to examine the formation mechanism relating to glacial debris flows in a variety of circumstances, largely unknown mechanisms impel destabilization of loose, frozen, or non-frozen glacial tills on steep slopes. In the present study, a series of simple direct-shear tests were performed to further investigate the shear behavior and strength properties of glacial tills subjected to short-term thawing. The samples with differing water contents and dry densities were firstly frozen under the same period but sheared with varying thawing intervals. The results directly show that (1) the stress-strain curves of all tested samples depict strain-softening characteristic to some extents, but the difference between peak and critical resistance decreases with increase of thawing intervals; (2) the dry density can enhance the shear resistance but the initiation water content may result in the decrease of shear resistance for the relative denser samples; (3) the shear strength profiles manifest that the internal friction angle increases but the cohesion decreases with increase of thawing intervals. These laboratory results suggest that the frozen water content can have measurable effect on the strength properties of glacial tills in shear, and the phase transition process from ice to water may affect the water distribution as a consequence of thawing interval. It should be mentioned that the results preliminarily provide fundamental information regarding shear strength properties of glacial tills by considering short-term thawing effect, and further study will be needed to examine the shear behavior of glacial tills under other potential factors.

How to cite: Fu, Y., Liu, Z., and Jiang, Y.: Effect of short-term thawing on the mechanical properties of frozen glacial tills, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12139, https://doi.org/10.5194/egusphere-egu21-12139, 2021.

11:38–12:30
Break
Chairpersons: Marcel Hürlimann, Velio Coviello
Part 3: Case studies
13:30–13:40
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EGU21-13952
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solicited
Anna Serra-Llobet, John Radke, Mathias Kondolf, and Sarah Lindbergh

On January 9, 2018 a series of debris flows killed 23 people and caused over a $1 billion in economic losses in Montecito, Santa Barbara County. The debris flows followed a classic pattern in mountainous areas of southern California: A large wildfire (the 2017 Thomas Fire) burned the headwaters of streams draining the Transverse Ranges southward to the Pacific, creating hydrophobic soil conditions that prevented infiltration of water, resulting in larger runoff during rains. A cell of intense precipitation over Montecito triggered debris flows, affecting areas along the stream channels. 

The 2018 Montecito debris flows raise compelling questions about the role of scientific information in decision making generally, and specifically how hazardous areas along rivers and streams are mapped, how land use is regulated in these zones, and how best to respond in emergency situations. 

This presentation analyzes the evacuation planning process during the emergency management (making emphasis on the maps used by public officials), the recovery planning strategies that the local government adopted after the event, and the evolution of houses in flood hazard areas since the beginning of the 20th century, to highlight the importance of exposure as a key element to reduce risk.

How to cite: Serra-Llobet, A., Radke, J., Kondolf, M., and Lindbergh, S.: Planning for floods after fires: Lessons from the 2018 Montecito Debris Flow (California), EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13952, https://doi.org/10.5194/egusphere-egu21-13952, 2021.

13:40–13:42
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EGU21-7007
Li Wei and Kaiheng Hu

Sichuan Province in southwest China is highly susceptible to debris flow disasters and suffers much damage to buildings and loss of human lives in concentrated rural settlements each year. By combining geographic information system (GIS) and Deep Encoding Network (DE-Net) methods, we proposed an automatic identification method for buildings highly susceptible to debris flows with large-scale digital elevation data and high-resolution remote sensing imagery based on a vulnerability matrix containing different threshold values of the horizontal distance (HD) and vertical distance (VD) between buildings and channels. A case study in Puge County, Sichuan Province, demonstrated the high identification potential of the method for buildings susceptible to debris flows in large areas with only scarce information available. Meanwhile, We chose a high-risk village in Puge County to study debris flow risk to buildings and residents. Different types of days and diurnal periods were considered in the analysis of societal risk to residents. The results indicated that societal risk to residents on holidays is always higher than that on weekdays, and societal risk at night is also much higher than that in the daytime. The identification results of buildings vulnerability provide valuable information regarding high-risk residential areas to governments and facilitate targeted measure design at the initial planning stage, and the proposed method of societal risk provides a basis for decision-making in the planning of mitigation countermeasures in a specific settlement.

How to cite: Wei, L. and Hu, K.: Quantitative analysis of the debris flow risk to concentrated rural settlement in southwest Sichuan, China, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-7007, https://doi.org/10.5194/egusphere-egu21-7007, 2021.

13:42–13:44
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EGU21-373
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Highlight
Juan Daniel Rios-Arboleda

This research expands the original analysis of Baker and Costa (1987) including data from Europe and South America with the objective to understand if there are emerging latitudinal patterns. In addition, the threshold proposed by Zimmermann et al. (1997) it is evaluated with the data from tropical zones finding that this is a good predictor.

Mainly, recent Debris Flow occurred in South America are analyzed with the aim of identifying the best risk management strategies and their replicability for developing countries, particularly, the cases that have occurred in Colombia and Venezuela in the last 30 years are analyzed in order to compare management strategies and understand which are the most vulnerable areas to this phenomenon.

It is concluded that large-scale and multinational projects such as SED ALP are required in South America to better characterize events that have left multiple fatalities (sometimes hundreds of people) and better understand how to manage the risk on densely populated areas.

Finally, the use of amateur videos is proposed to characterize these events in nations with limited budgets for projects such as SED ALP, methodology that will be described extensively in later works.

How to cite: Rios-Arboleda, J. D.: Reanalysis of Flash Floods and Debris Flows on two continents and their management strategies, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-373, https://doi.org/10.5194/egusphere-egu21-373, 2021.

13:44–13:46
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EGU21-11841
Shin-Ping Lee, Yuan-Jung Tsai, Yun-Chung Tsang, Ching-Ya Tsai, Shang-Ming Wang, Kui-Lin Fu, Zheng-Xiu Tsai, and Wei-Di Chen

Under climate change impact, the frequency of extreme hydrological events increases. The occurrence of extreme rainfall events may lead to large-scale flooding or sediment disasters resulting in serious property damage and casualties. Large-scale sediment disasters include large-scale landslides and debris flows which are the main types of disasters causing casualties. In Taiwan, during Typhoon Morakot in 2009, the long duration and high-intensity rainfall led to a large-scale sediment disaster resulting in heavy casualties. A disaster with certain magnitude and complexity cannot be coped with a single disaster management approach. In this study, a risk assessment method considering climate change impacts proposed by the Intergovernmental Panel on Climate Change (IPCC) was adopted. By analyzing hazard, exposure, and vulnerability indicators of large-scale sediment disasters in Xinfa catchment of Kaohsiung City, Taiwan, a disaster risk adaptation strategy was proposed based on the impact of disaster factors.

Two scenarios were applied for the catchment sediment hazards risk assessments including 50-year recurrence period (high frequency and low impact) and extreme scenario (low frequency and high impact). Multiple factors for hazard (impact area of landslides and debris flows), exposure (lifeline roads and land use intensity), and vulnerability (disaster prevention and relief resources and settlement population characteristics) assessments were considered. The correlation factor selection and weighting analysis was calibrated by the 2009 Typhoon Morakot event. All disaster-recorded locations were above moderate risk indicating that the risk assessment method was reasonable. A risk map for Xinfa catchment was completed based on the validated risk assessment model to identify the high-risk settlements. After analyzing the spatial characteristics and disaster risk impact factors of high-risk settlements, both software and hardware disaster prevention measures and adaptation strategies were suggested. According to the analyzed results, although the hardware measures were effective in reducing sediment hazards generally, under extreme hydrologic events, those measures could be ineffective due to limited protection capacity of the engineering facilities. Hence, reducing exposure and vulnerability is essential to deal with the impact of extreme events.

Keywords: Large-scale sediment disasters, Risk assessment, Adaptation strategies

How to cite: Lee, S.-P., Tsai, Y.-J., Tsang, Y.-C., Tsai, C.-Y., Wang, S.-M., Fu, K.-L., Tsai, Z.-X., and Chen, W.-D.: Risk Assessment and Disaster Prevention and Mitigation Strategies for Large-Scale Sediment Disasters, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-11841, https://doi.org/10.5194/egusphere-egu21-11841, 2021.

13:46–13:48
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EGU21-14201
Yingjie Yao

The intermittent surge is the basic manifestation of viscous debris flow, which emerges universally over the world, especially exemplified by those in Jiangjia Gully (JJG), a valley famous for its high frequency and variety of debris flow surges. It has been found that the surges originate from various sources in the watershed, thus identifying the source areas plays a fundamental role in studying the mechanism and process of surge developing. Advancement of GIS provides an apparent convenience in geospatial analysis of the watershed, which is used as a dominate tool in this paper.

In this study the JJG is divided into 97 tributaries (sub-watershed) and the hypsometric analysis is performed for each, from which derive the height of inflection points and the gravitational potential energy, coupled with the fitted parameters of specific power function. Then the morphology parameters, including slope, roundness, vegetation and soil, are revealed in tributaries. Besides, spatial autocorrelation among tributaries is quantified both globally and locally through Moran’s I and Getis-Ord Gi*, so that the HI spatial distributions are quantified and visualized. In particular, hot spots are conspicuously visible and highlight the geologic meaning of the HI when exploratory spatial data analysis is applied to the data distributions through local indices of spatial autocorrelation.

The results show that H-curves approximately present as S-shaped, and the integral values (HI) range from 0.18 to 0.69 and show positive relationship with both gravitational potential energy and the height of the inflection points. By the HI value, the tributaries are identified as in 5 phases of evolution. The younger tributaries (HI>0.49) make up the majority, which are expected to be the main possible sources for debris flows. Additionally, the slope distribution of tributaries all conform to the extreme distribution while the curves for the upstream, where the HI of tributaries generally manifest higher coupled with larger roundness, tends to skew to the right.

Finally the correlation between possible sources are explored through geospatial analysis. The spatial association in JJG provides an explanation of the debris flow source areas. Global spatial autocorrelation manifests significantly clustered (Moran’s I shows 0.449, passing the significance test) while tributaries with high HI value concentrate mainly in the Menqian Valley. Moreover, the drainage form of Menqian Valley represents a large possibility of debris flow source area with the respect of that being in Duozhao Valley.

Keywords: debris flow source area; hypsometric analysis; topographical characteristics; spatial autocorrelation; evolutionary phases

How to cite: Yao, Y.: Spatial heterogeneity of debris-flow watershed, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14201, https://doi.org/10.5194/egusphere-egu21-14201, 2021.

13:48–13:50
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EGU21-14564
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ECS
Veronica Zoratti, Elisa Arnone, Giuseppe Formetta, Silvia Bosa, and Marco Petti

The Northeastern Italy and the therein Friuli Venezia Giulia (FVG) region are frequently hit by heavy and prolonged precipitations, which cause frequent debris flow and diffused shallow landslides. In this study we focus on a mountain sub-basin of the Fella river watershed, the Uque at Ugovizza, located in the northeastern Julian Alps of the FVG, where a disruptive rainfall-triggered debris flow occurred in 2003.

The work aims at pursuing two main targets: (i) implementing a rainfall-runoff and hydro-morphodynamical framework for the analysis of debris flow initiated by intense heavy precipitation; ii) exploiting, for the first time, the flexibility of the GEOframe-NewAge semi-distributed hydrological model simulating high temporal resolution simulations (5-minutes) rainfall-runoff events.

The GEOframe-NewAge is an open-source component-based modeling framework, which simulates the entire hydrological cycle of the study area, including the snow melting, the soil water storage and the runoff production and routing in the river network; the model is suitable for the rainfall-runoff event scale simulations in Alpine environment with scarce measurements.

Specifically, we describe the results of the calibration and validation procedures applied to four selected intense events occurred in the period 2009-2019. Meteorological data at 5 minutes-step are used to rainfall-runoff modeling, whereas streamflow at 30 minutes is used for the model calibration and validation. Preliminary results show that the models is able to capture the temporal and spatial dynamic of extremes short events, providing satisfying Nash and Sutcliffe coefficient values.

How to cite: Zoratti, V., Arnone, E., Formetta, G., Bosa, S., and Petti, M.: Calibrating a semi-distributed hydrological model on Fella river basin (Italian Alps), EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14564, https://doi.org/10.5194/egusphere-egu21-14564, 2021.

13:50–13:52
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EGU21-13750
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ECS
Jian Guo

In mountainous areas, large-scale landslides usually cause serious disasters. A large number of studies have found that complex terrain may affect the landslides dynamic, which may be one of the significant factors in catastrophic events. However, the mechanism is rarely explored. On July 23, 2019, a large-scale landslide occurred in Jichang town, Shuicheng County, Liupanshui City, Guizhou Province in China. The landslide, which moved along two gullies, resulted in strong punching-shear, induced scarping on vegetation and large destruction of houses and finally formed a deposit with a volume of 2×106 m3. This research aims to understand the effect of topography on landslide kinematics. To achieve this aim, a detailed field investigation was first carried out with an unmanned aerial vehicle (UAV) aerial photography survey, resident interviews, and field sampling. The rainfall analysis indicate the effective rainfall within seven days before landslides was 70.14 mm which exceeded the rainfall threshold of 54.3 mm in this region, which finally triggered the landslide. Traditional soil mechanics tests were then performed to identify the soil properties of the source material. Combined with numerical simulation using the nonlinear shallow water equation, the whole process of landslides was divided into four stages: instability stage, acceleration stage, transformation stage and impact and accumulation stage. The simulations results show the landslide block slid with a low velocity of 8 m/s for about 100 m. Then, Froude number of landslide increase from 2 to 3 when passing the high and steep terrain, indicating that landslide change to inertial dominated with potential same Froude behavior of classic debris flow. The rupture mass slid with the peak velocity of 23 m/s and diverged in two gullies and ran out for about 600 m. The maximum velocity is 23 m/s in east gully while only 15 m/s in west gully. Compared with deep and incised valleys in west, shallow and straight valley in east decrease the deposit depth, further increase the velocity of landslide material with increased runout distance. This research may provide a fast flow path of back analyzing geo-hazards on complex terrain and serve as a basis for future research on long runout landslides. 

How to cite: Guo, J.: Catastrophic landslide affected by topography: a case study in Guizhou, China, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13750, https://doi.org/10.5194/egusphere-egu21-13750, 2021.

13:52–13:54
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EGU21-9557
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ECS
Xuemei Liu, Yong Li, Pengcheng Su, Taiqiang Yang, and Jun Zhang

Abstract: Susceptibility assessment of landslides over a large area depends on the basic spatial unit of mapping, each unit is assumed to have unique assessment value, so the division of mapping unit is directly related to the evaluation rate, grid cell or slope unit are usually be used in many researches. Grid cell divide the study region into regular squares of predefined size, each cell is assigned a value of influence factor. Slope unit based on hydrology divides the region by ridge and valley lines, which is more related to geological environment and it is hard to identify the subbasin boundary. Both units are used in this study for the assessment of small shallow and clustered landslides in vegetated slopes in Malipo, southwest China. Google earth map on February 7, 2019 was used to interpret the landslides. ArcGIS 10.2 software was used to produce landslide inventory map and obtained 1435 landslides in the study area; most frequent landslide areas are in the range of 62m2 to 900m2. Field survey was carried out to verify uncertain factors and measure moisture soil content. Soil moisture content (SMC) map was obtained by Kriging Interpolation methods based on the field measured soil moisture content of 48 sample points. Information value (IV) model was used to generate landslide susceptibility assessment map and improved information value (IIV) model was used to determine whether the mapping unit with or without landslide. Seven factors, including slope angle, slope aspect, elevation, normalized difference vegetation Index (NDVI), Soil Moisture Content (SMC), distance to river and road were used as landslide influence factors. The Area under curve (AUC) values of the slope unit IIV, IV and grid cell were 0.814, 0.802 and 0.702 respectively for success rate. For prediction rate, the AUC values of the slope unit and grid cell were 0.803(IIV), 0.790(IV) and 0.699 respectively. Slope unit is more suitable than grid cell for assessing susceptibility of Small, Shallow and Cluster Landslide (Fig.1). Improved information value model can increase the accuracy of susceptibility assessment model for this characteristic landslide.

Keywords: Landslide susceptibility assessment; Slope unit; Grid cell; Information value

                                                 (a)     (b)                                                                                    

                                                              Figure 1 Landslide susceptibility maps (a)Slope unit-based and (b)Grid cell-based

How to cite: Liu, X., Li, Y., Su, P., Yang, T., and Zhang, J.: Susceptibility Assessment of Small, Shallow and Clustered Landslide in Malipo, southwest China, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9557, https://doi.org/10.5194/egusphere-egu21-9557, 2021.

13:54–13:56
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EGU21-3229
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ECS
Viktoriia Kurovskaia, Sergey Chernomorets, Tatyana Vinogradova, and Inna Krylenko

Debris flow is one of the most hazardous events that occur in all mountain regions.  Direct debris flow damage includes loss of human life, destruction of houses and facilities, damage to roads, rail lines and pipelines, vehicle accidents, and many other losses that are difficult to quantify. In July 2015, in the valley of the Barsemdara River (Gorno-Badakhshan Autonomous Region, Tajikistan), plenty of debris flows were observed. As a result, residential areas, social facilities, and infrastructure in Barsem village and neighboring settlements were destroyed and flooded. Besides, debris flow deposits blocked the Gunt River with the subsequent formation of a dammed lake with a maximum volume of 4.0 million m3
The aim of this study was to obtain hydrographs of debris flow waves in the source and detailed zoning of the Barsemdara river valley. For the debris flow source, we applied transport-shift model. Equations of this model were developed by Yu.B. Vinogradov basing on Chemolgan experiments of artificial debris flows descending. Previously, the model characteristics were compared with the observational data of the Chemolgan experiments, and the results were found to be satisfactory [Vinogradova, Vinogradov, 2017]. Based on the equations, a computer program was created in the programming language Python. Besides, we improved the model by adding flow velocity calculations, and eventually it became possible to obtain hydrographs. To investigate quantitative characteristics of the debris flow in the river valley we implied a two-dimensional (2D) model called FLO-2D PRO. It is based on the numerical methods for solving the system of Saint-Venant equations. Besides, in this model, it is assumed that debris flows move like a Bingham fluid (viscoplastic fluid) [O'Brien et al., 1993]. The input information for modeling was digital elevation model (DEM) and previously obtained hydrographs. The output information included flow depth, velocity distribution and hazard level of the territory. The results of the study will be reported.

1.    Vinogradova T.A., Vinogradov A.Y. The Experimental Debris Flows in the Chemolgan River Basin // Natural Hazards. – 2017. – V. 88. – P. 189-198.
2.    O'Brien J. S., Julien P.Y., Fullerton W.T. Two-dimensional water flood and mudflow simulation //Journal of hydraulic engineering. – 1993. – V. 119, No 2. – P. 244-261.

How to cite: Kurovskaia, V., Chernomorets, S., Vinogradova, T., and Krylenko, I.: Modeling debris flow triggered by snow melting in the Barsemdara river valley, Tajikistan, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3229, https://doi.org/10.5194/egusphere-egu21-3229, 2021.

13:56–13:58
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EGU21-10280
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ECS
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Highlight
Tobias Schöffl, Richard Koschuch, Philipp Jocham, and Johannes Hübl

After a heavy rainfall event on August 31st, 2019, a debris flow at the Dawinbach in the municipality of Strengen (Tyrol, Austria) caused a blockage of the culvert below the provincial road B-316 and deposition in the residential area. The debris deposition raised up to 2 to 3 meters on the road and led to property damage to real estate. The total volume of the debris flow was approximately 15 000 cubic meters.

In order to control a further debris flow of this magnitude, the Austrian Service of Torrent and Avalanche Control started to construct mitigation measures. They include a channel relocation in order to significantly increase the channel crosssection. Hence the construction company STRABAG is also relocating the provincial road bridge.

Since the risk for this road section and for the workers on site is particularly high during the construction period, a combined monitoring and early warning concept was developed and implemented by the BOKU, Vienna and the company IBTP Koschuch.

The monitoring site consisting of a pulse compression radar and a pull rope system was installed 800m upstream from the fan. The combination of the two sensors now results in three major advantages.

  • At sensor level, the system operates redundantly.
  • A more reliable differentiation between increased discharge or debris flow is given.
  • In the event of a false alarm, the system provides easier diagnosis and assignment of the fault.

Two events of increased runoff occurred during the deployment period. Both were successfully detected by the pulse compression radar. Here, the first event was used for threshold validation of the radar unit. Thus, an alarm could already be sent out automatically for the second one. The road is controlled by an integrated light signal system consisting of three traffic lights. A siren near the construction site can warn workers of an impending event by means of an acoustic signal. The reaction time after the alarm has been triggered is between 75 and 150 seconds, depending on the speed of the debris flow. The responsible authorities are informed by sending an SMS chain, which includes details about the type of process and the type of the activated triggering system.

How to cite: Schöffl, T., Koschuch, R., Jocham, P., and Hübl, J.: Application of a combined and automated monitoring and early warning system for debris flows at the Dawinbach, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10280, https://doi.org/10.5194/egusphere-egu21-10280, 2021.

13:58–14:00
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EGU21-3680
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ECS
Emanuel Castillo Cardona and Edier Aristizábal

Debris flow fans are commonly occupied by urban and rural settlements in mountainous regions such as in the northern Colombian Andes. Those fans are originated by violent surges of high sediment concentration that are then mobilized downstream by strong currents during torrential events highly destructive. Then, characterization and understanding of the dynamics that give rise to fans in tropical and mountainous regions such as Andean zone is a fundamental tool for land use planning. This research focuses on cartography of fans and catchments using digital elevation models in the central and western mountain range of the northern part of the Andean mountain belt. The methodology considered: morphometric measurements of the catchments and fans, lithological aspects of the catchments, type of catchments (torrential or no torrential). Then the correlation between morphometric parameters of fans and catchments is carried out, including relationships with qualitative variables by multivariate statistical analysis and machine learning techniques to find patterns between quantitative and qualitative variables. The results indicate that slope of the fans has a high correlation with Melton index of the catchments and with the slope of the main stream of the catchments. About the qualitative classification of the catchments in torrential and no torrential, it is observed that there are good discriminations for slope of the fan, volume of the deposits(fans), the relationship between the relief of the catchments and other variables. On the other hand, the lithology of the catchments does not have strong influences on the morphometry of the fans.

How to cite: Castillo Cardona, E. and Aristizábal, E.: Morphometrical analysis of Debris flow fans and torrential catchments in mountainous terrain in the northern Colombian Andes by machine learning techniques., EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3680, https://doi.org/10.5194/egusphere-egu21-3680, 2021.

14:00–14:02
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EGU21-14988
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ECS
Toshiyuki Horiguchi, Hiroshi Kokuryo, and Nobutaka Ishikawa

The human settlement tend to be close to a mountainous area all over the world. The prevention concept is necessary to mitigate or prevent the origin occurrence of debris flow in zero-order basin. The zero-order basin is a mountain stream that is unclear valley topography, and don’t flow always running water. Although it is designated as a debris flow prone zone, the area of the basin is small, and the arrangement of sabo dam is difficult, and there are a lot of mountain streams which are judged that the facility function is hard to be fully demonstrated for the actual phenomenon of the sediment. The prevention and/or mitigation of measurement facilities is required. This study presents a full design of a protection barrier against debris flow, including woody debris in small, inactive, and usually dry catchments in small-scale torrents or zero order basin. The concept of performance-based design is proposed for a protection barrier as exemplifying slit barrier in small torrent. In addition, the safety performance for two types of a slit barrier is verified to protect debris flow under the situation of three difference small torrents.

How to cite: Horiguchi, T., Kokuryo, H., and Ishikawa, N.: Prevention of Debris Flow from Inactive, Usually Dry Catchment in zero-order basin, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14988, https://doi.org/10.5194/egusphere-egu21-14988, 2021.

14:02–14:04
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EGU21-14294
Rahul Devrani and Rohit Kumar

Debris flow events are recognized as one of the most prominent mechanisms for landscape evolution in the Himalayan river basins. Triggered by cloud bursts, glacial and landslide lake outburst floods; debris flows can erode, transport and deposit vast amount of sediments with profound landscape changes. The Himalayan river basins frequently experience such debris flow events during the monsoon. However, only a few morphological and hydrological studies are available for such events. Hence, we studied a high-magnitude, low-frequency debris flow event in the Asiganga River basin (a headwater tributary of the Ganga River) on 3rd August 2012.

In the present study, we (i) computed landscape change during the event and (ii) calculated the paleohydrology of the event. The pre and post geomorphic mapping is carried out using satellite imageries (Google Earth), field data, and published literature to analyze landscape modification/change. The paleohydrology of the event is calculated using dimensions of 440 mobilized stream boulders at 11 locations in the Asiganga River basin. Our results suggest that the Asiganaga River’s reaches encountered sediment deposition and erosion on a massive scale; especially in the lower terrace levels. Channel shifting and widening was also a dominating geomorphic response, and it occurred in different magnitude along the course of the Asiganga River. A significant alteration trend is observed in sediment bars, especially in the reaches, which were exceedingly influenced by morphological and hydraulic parameters. The peak discharge is calculated using D95, D90, D85, and D80 of the mobilized stream boulders. Overall, the calculated highest peak discharge is around 4500 m3s-1. Interestingly, the peak discharge from D90 yielded the value of 2661 m3s-1 , and it corresponds with the peak discharge (i.e., 2665 m3s-1 ) measured using an instrument based previous study. 

In the Himalayan River basins, documentation of such debris flow events is crucial. Such studies will provide a unique database to study river sensitivity towards future debris flow events.

How to cite: Devrani, R. and Kumar, R.: Geomorphic response and paleohydrology of a debris flow event in the upper Ganga River basin, NW Himalaya, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14294, https://doi.org/10.5194/egusphere-egu21-14294, 2021.

14:04–14:06
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EGU21-14583
Baoliang Wang, Yong Li, and Xiaojun Guo

Spontaneous initiation of debris flow surges from sedimentary deposits

Wang Baoliang1 , Li Yong2, Guo Xiaojun2

1 Yellow River Engineering Consulting Co., Ltd., Zhengzhou, 450004, China

2 Key Laboratory of Mountain Hazards and Surface Process / Institute of Mountain Hazards and  Environment, Chinese Academy of Sciences, Chengdu, 610041, China

 

We report here that a small gully of sedimentary deposit has spontaneously produced a sequence of debris flow surges in great variety of appearances. The event occurred in a tributary gully of 0.16km2, without slope failures and rainstorms; the only triggering force was the groundwater at small discharge (0.5L/s). Individual surges originated separately from the sediment in different manners and appeared in a variety of flow regimes and material compositions(Fig.1). We’ve taken a detailed scrutiny on the whole processes, i.e., the surge sequence spontaneously occurring in the stream (Fig.2), and suggested a mechanism for the surge scenario. It is proposed that the sediment is heterogeneously composed of blocks of granular aggregates, featured by the grain size distribution (GSD). With different GSD parameters, sediment blocks have different critical condition of liquefaction or fluidization, which determines the manner of surge initiation (Fig.3). Fine grains are easily to run out with infiltration to form slurry and lubricate the substrate sediment layer, facilitating the mobility of succeeding surges; while coarse grains collapse as Coulomb failure and turn into high concentrated surges. And variation of the substrate granular structure causes on and off of the surges. In summary, it is the randomness of GSD and block of sediment that lead to the variety of surge initiation; and the initiation and motion of tributary surges provides a vivid scenario for intermittent surges in the mainstream channel.

 

How to cite: Wang, B., Li, Y., and Guo, X.: Spontaneous initiation of debris flow surges from sedimentary deposits, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14583, https://doi.org/10.5194/egusphere-egu21-14583, 2021.

14:06–14:08
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EGU21-13933
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ECS
Mingyu Lei

The influence of forest fire on soil hydrology and mechanical effects promotes the occurrence of post-fire debris flow. However, the variation of soil properties with time has not been much studied. In order to determine the temporal variation characteristics of soil hydrology and mechanical effects after forest fires under different fire severities, and link them to the initiation of post-fire debris flows, a two-year continuous testing of samples from the source area of post-fire debris flows in Muli County, China, were investigated. Saturated direct shear test, saturated permeability experiment and scanning electron microscope experiments of undisturbed soil were carried out. The results showed that the burning of organic matter occurred, which reduced soil permeability sharply. In addition, high temperatures destroyed the structure of soil, causing the internal collapse of aggregates and reducing soil shear strength. While the herbaceous plants can quickly regenerate after fire, improving the hydrological and mechanical properties of soil, and reducing the frequency of post-fire debris flow. These results can evaluate the changes of soil physical and mechanical properties in short term after fire, and also explain the formation mechanism of post-fire debris flow. 

How to cite: Lei, M.: Temporal Variation Characteristics of Soil After Fire in A Post-fire Debris Flow Source Area, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13933, https://doi.org/10.5194/egusphere-egu21-13933, 2021.

14:08–14:10
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EGU21-13937
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ECS
Dongxu Yang and Hai Huang

Located in the north of the Great Bend of Yarlung Zangbo River, the Purlung Zangbo basin is the largest distribution area of marine glaciers in China. Here is one of the most serious sections for geological disasters in the Sichuan-Tibet transportation corridor, which poses a serious threat to the planning and construction of the Sichuan-Tibet Railway and Expressway and other important infrastructures. In order to assess the risk of geological disasters along the proposed Sichuan-Tibet Railway, more than 80 typical moraine landslides, glacier collapsed debris flows, and glacial lake breakouts in the basin were selected through remote sensing interpretation and field verification, which were carried out ground investigation and geotechnical properties tests in detail. The main types, material sources and key influencing factors of the moraine geological hazards are analyzed by in-situ tests of large-scale straight shear, double ring penetration, matrix suction, dynamic penetration and indoor sample testing methods such as consolidation compression, triaxial shear, and OSL dating, combined with regional temperature, rainfall, multi-stage remote sensing images and geological mapping data. Finally, the starting mechanism and disaster scale of loose moraine are preliminarily simulated by combining the water and thermal product index method and FLOW 3D numerical analysis software. The results show that the physical and mechanical properties of moraine have good statistical relations with its water content, void ratio and clay content. The natural density and compression modulus are negatively linear related to the void ratio, while the vertical permeability coefficient and the free expansion ratio show a quadratic relationship with clay content, and also the shear strength and compressive strength are binary relationship with void ratio and moisture content. The permeability, consolidation degree and free surface characteristics are the key parameters affecting the stability of moraine slope. Moraine located in three depositional positions (the front of modern glacier tongue, the middle reaches of glacial valley and the bank slope of main river ) has different starting modes, which are "shoveled-scraped and migration type", "erosion and blocking-burst type", and "unloaded and permeated type". When it intersects with the main traffic lines, different countermeasures and application should be adopted: avoidance by bridges or tunnels, slope reinforcement, and cutting or subgrade. 

How to cite: Yang, D. and Huang, H.: Development Mechanism of Moraine Debris Flows in Parlung Zangbo Basin, Southeast Tibet, China, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13937, https://doi.org/10.5194/egusphere-egu21-13937, 2021.

14:10–15:00