Strong ground motions from major earthquakes trigger tens of thousands of landslides in mountain landscapes initiating a sediment cascade that ultimately elevates sediment and carbon fluxes in rivers. The magnitude and duration of the fluvial response to earthquake-induced landsliding is relevant for quantifying post-earthquake hazard, landscape evolution and carbon cycling but remains poorly constrained in many mountain settings because post-earthquake sediment cascades are rarely captured by instrumental data series. The sedimentary record may provide a valuable archive of the landscape response to earthquakes in the absence of instrumental data but requires the signature of post-earthquake sediment cascades to be reliably identified and quantified. Here we use sedimentary archives of lakes adjacent to the Southern Alps, New Zealand to reconstruct earthquake-induced erosion in response to great earthquakes on the range bounding Alpine Fault; the timing, location and magnitude of which have been well constrained by independent paleoseismic data. High-resolution chronology combined with volumetric reconstructions of lacustrine sedimentary fills based on a dense network of sediment cores from two lakes fed by range front catchments allow sediment and carbon fluxes to be quantified over millennial timescales. The volumetric reconstructions show earthquake-induced landsliding increased suspended sediment and organic carbon (OC) transfers from the mountain belt by more than an order of magnitude immediately after each earthquake. While elevated fluxes persisted for decades, the majority of sediment and OC was exported within the first five to ten years after each large earthquake. In total, the last four M_w>8 earthquakes on the Alpine Fault have driven sediment and OCtransfers that equate to ~40% of the total flux over the last millennium. Further, biomarkers encoded in the OC allow the location and depth of earthquake-triggered landslides to be reconstructed. The Southern Alps case study demonstrates that post-earthquake sediment cascades in mountain catchments are reliably recorded in the sedimentary record. These records provide unprecedented insights into post-earthquake hazard in settings where major earthquakes have not occurred during the period of instrumental observation.

How to cite: Howarth, J., Fitzsimons, S., Hilton, R., Moody, A., Croissant, T., Wang, J., Densmore, A., and McClymont, E.: Reconstructing post-earthquake sediment cascades in mountain landscapes from the sedimentary record, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-7123, https://doi.org/10.5194/egusphere-egu21-7123, 2021.

 On 29th of November of 1987, a large ice-rock avalanche occurred in a permafrost area of the central Andes of Chile. This event has been considered one of the most destructive events in that area in the last decades. The ice-rock avalanche initiated at an elevation of 4350 m, above the Estero Parraguirre. Due to the large amounts of ice and snow and the high potential energy, this avalanche developed into a debris flow propagating down the valley, reaching a travel distance of approx. 57 km after 2 hours. On the way, many people lost their lives, and two hydroelectric power plants were destroyed. The avalanche was likely triggered by warm temperature anomalies and snow build-up at high elevation linked to the concurrent and strong El Nino event in 1987.

In this study, we use old topographic maps and aerial photographs, acquired just a few days after the event, and satellite imagery to constrain the trigger volume and to accurately compute the general mass displacement. A physically-based multi-phase mass flow model is employed to retrace the dynamics and characteristics of this debris-flow event. Previous studies suggested a trigger volume of about 6 x 10⁶ m³. After entrainment along the flow path, the debris flow reached a total volume of 15 x 10⁶ m³. First results of our study suggest that the trigger volume was significantly larger than previously thought. The next step is to shed light on possible entrainment scenarios, which will be constrained by and assessed against the observed elevation changes/mass displacement.

The reconstruction of this event is crucial to better assess future events and thus to develop successful mitigation strategies.

How to cite: Bruckner, T., Farías-Barahona, D., Fürst, J., Mergili, M., Sepulveda, S., Pena, H., Casassa, G., and Braun, M.: Reconstruction constraints on the Estero Parraguirre ice-rock avalanche in 1987, Central Andes of Chile: New insights from remote sensing and numerical modeling.  , EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13019, https://doi.org/10.5194/egusphere-egu21-13019, 2021.

Rock-ice avalanches correspond to three-phase mixtures composed of a liquid and of particles of rock and ice. The presence of ice inside the mixture plays a key role in the mobility of rock-ice avalanches, since the heat produced by basal friction and particle collisions induces its transformation into water. Due to this continuous supply of liquid to the mixture, rock-ice avalanches can threaten populations living in cold mountainous areas. Thus, for a good hazard assessment and management, there is the need to construct mathematical models able to predict the flow of rock-ice avalanches. In the literature, there exist only few models that deal with this type of mass flows (Pudasaini & Krautblatter 2014, Bartelt et al. 2018, Sansone et al. 2021). As proposed in Sansone et al. (2021), a framework of different simplified rock-ice avalanche models can be derived by starting from a complete three-phase approach and by imposing two specific assumptions, namely the isokinetic and incompressibility hypotheses. In this way, five classes of simplified approaches can be detected, and these mathematical models are characterized by different levels of approximations of the physics of rock-ice avalanches.

In this work, we provide some numerical solutions for the depth-integrated one-dimensional versions of all the simplified mathematical models detected in Sansone et al. (2021). These numerical solutions are constructed using three different numerical schemes that distinguish themselves from the way the numerical fluxes are evaluated. While one of the three chosen numerical methods evaluates the numerical fluxes without considering the eigenstructure of the systems of equations, the other two schemes take partially or entirely account of the eigenstructure of the equation systems. Due to the possible loss of hyperbolicity detectable in some simplified models, we consider as test cases the problems of the small perturbations of the flow depth and of the concentrations.

The first result of the analysis computed corresponds to the comparison between the numerical solutions derived from the three numerical schemes for each class of models. In this way, the responses of the different numerical methods to each equation system can be investigated. The second result consists in comparing numerically the different classes of simplified models detected by Sansone et al. (2021), thus allowing us to quantify the effects of the assumptions of each class of models on the flow dynamics.

References:

Bartelt P., Christen M., Bühler Y., Buser O. (2018), Thermomechanical modelling of rock avalanches with debris, ice and snow entrainment. In 9th European Conference on Numerical Methods in Geotechnical Engineering (NUMGE), University of Porto, Porto, PORTUGAL.

Pudasaini S., Krautblatter M. (2014), A two-phase mechanical model for rock-ice avalanches. Journal of Geophysical Research: Earth Surface 119 (10), 2272-2290.

Sansone S., Rosatti G., Zugliani D. (2021), A mathematical framework for modelling rock-ice avalanches. Paper under review.

How to cite: Sansone, S., Rosatti, G., and Zugliani, D.: Numerical comparison between simplified mathematical models for rock-ice avalanches , EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-7090, https://doi.org/10.5194/egusphere-egu21-7090, 2021.

Large rock slope failures play a pivotal role in long-term landscape evolution and are a major concern in land use planning and hazard aspects. While the failure phase and the time immediately prior to failure are increasingly well studied, the nature of the preparation phase remains enigmatic. This knowledge gap is to a large degree related to challenges in collecting appropriate data in such high mountain terrain. Classic monitoring techniques provide detailed data but mostly of point character and only reflecting the surface expression of processes within the rock mass. Thus, the integral behaviour of a peak, at the surface and at depth remains elusive.

Here, we present results from a continuous multi-sensor seismic analysis of the Hochvogel summit, a 2592 m high Alpine peak, which is deemed to fail in the near future, as a 5 m wide and 40 m long crack is progressively opening and mobilising up to 260,000 cubic metres of rock. The seismic network consisted of up to seven sensors, installed during July--October 2018 (with 43 days of data loss). We develop and discuss proxy time series indicative of cyclic and progressive changes of the summit.

Modal analysis, horizontal-to-vertical spectral ratio data and end-member modelling analysis reveal diurnal cycles of increasing and decreasing coupling stiffness of the fragmented rock volume, due to thermal forcing. Relative seismic wave velocity changes mimic this pattern but also reveal the release of stress within the rock mass. At longer time scales, there is a superimposed pattern of stress evolution, which increases for five to seven days and suddenly drops within a few days, also expressed in an increased emission of short seismic pulses indicative of rock cracking. Our data provide essential first order information on an early stage of a large-scale slope instability, which evolves towards a catastrophic failure.

How to cite: Dietze, M., Krautblatter, M., Leinauer, J., Illien, L., and Hovius, N.: The melody of a failing peak – seismic constraints on rock damaging and stick-slip motion at the Hochvogel (DE/AT Alps), EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-2602, https://doi.org/10.5194/egusphere-egu21-2602, 2021.

Large landslide complexes in flysch are among the largest landslides on earth. These landslides often feature a rotational landslide at the head, the weathering and downslope transport of which produces one or more earthflows that terminate in a bulging toe at a valley bottom. These landslide complexes typically undergo ductile movements, on the order of mm/year to cm/year, and thus loss of life risk is typically low. However, the earthflow portions of these complexes can surge, which can result in significant infrastructure damage. Thus, understanding annual landslide displacements, the partitioning of strain within the landslide body, as well as subsurface groundwater recharge are crucial factors for understanding and managing these landslide complexes.

In the present work we present and analyze a uniquely detailed dataset collected for the Triesenberg Landslide, a landslide complex in Flysch located in Liechtenstein. This dataset contains accurate measurements of surface displacements that occurred between 1978 and 2012, InSAR displacement time series from 2011 to 2020, periodic measurements (once or twice a year) of over 30 inclinometers since 1995, continuous and periodic pore pressure measurements at a number of locations since 2001 as well as climatic data from nearby climate stations. We combine the surface and subsurface displacement measurements to understand how strain is partitioned in the landslide, as well as seasonal and annual landslide displacement rates. We then combine pore-pressure measurements and climatic data to investigate groundwater recharge mechanisms, as well as the water balance of our study area. The analysis of the InSAR data, as well as its comparison to previous displacement measurements, reveals annual displacement rates up to 4.5 cm/year. Additionally, the inclinometer data shows that the depth to the rupture surface varies throughout the landslide body, and is measured as deep as 70 m in some locations. Surprisingly, very few internal shear planes were noted within the earthflow portion of the landslide. We find that recharge into the landslide body is complex, and that the water mass balance is potentially influenced by the adjacent Valuna valley. By combining these analyses, we are able to gain preliminary insights into the behavior of the Triesenberg landslide, which has important implications for understanding this landslide as well as many other landslide complexes in flysch.

How to cite: Maissen, J., Löw, S., and Aaron, J.: Annual displacements, strain partitioning and pore pressure variation in the Triesenberg Earthflow, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8463, https://doi.org/10.5194/egusphere-egu21-8463, 2021.

Multiple hazards (e.g. floods, landslides, earthquakes, glacial and landslide lake outburst floods) are threatening people, their goods and infrastructures in the high mountains of Nepal Himalaya. Floods and landslides are mainly driven by monsoonal precipitation. However, human impact often increases natural risks, like in the Kali Gandaki (KG) valley, the deepest valley (>5500 m) on earth, where the new two-lane road construction (since 2017) has caused many undercut and instable slopes.

In the light of previous events, we intend to assess the cascading multi-hazard events of 2020 in three tributary catchments of KG.

We adopted a pluri-disciplinary approach: interpretation of Sentinel-2 satellite images (March and November 2020), analysis of precipitation (stations of Lete and Tatopani, GPM satellite precipitation measurements), hydrologic and seismic data (Beni), geomorphological mapping, hydrological modelling in HEC-RAS, and field visits in July and November 2020, including interviews with locals.

On 20 July 2020 major hyper-concentrated flood events and landslides occurred in the Rupse, Thaplyang and Kahiku catchments (between Tatopani and Lete) destroying parts of the KG road, road bridges and a hotel (Rupse site). We focus on the Rupse River entering the KG valley at Rupse waterfall (height 108 m; kyanitic gneisses) then flowing down to the KG road and to KG River 200 m below. The major flood event lasted two hours and reached a max. flood level of 35 m at the edge of the waterfall. Upstream of the waterfall, four landslides (each about 250m wide, 200 m high) were triggered. Due to cloud coverage satellite scenes are missing to unravel whether the landslides caused the damming of the river and a landslide lake outburst flood or if the landslides were mainly triggered by the flood and increased sediment input to it.

Floods from these tributary catchments caused a major KG flood especially south of the Rupse catchment, which led to severe erosion and sedimentation in the channel; i.e. destruction of a pole of the national electricity grid, reactivation of the Kham Bhitta deep-seated landslide, destruction of the KG road (the construction of which probably contributed to this reactivation).
Seismic data from Beni, approximately 27 km downstream of the affected catchments, provide constraints on the timing and relative magnitude of the flood in the KG. The data show that a short duration high magnitude flood with a very rapid rise and recession passed through Beni on the afternoon of 20 July. In addition, station data of Lete and Tatopani shows that yearly rainfall totals of 1839.5 and 2140.2 mm, respectively, were the highest since 1970. March and April were already very wet, followed by extremely monthly rainfall totals of 499.7 mm and 551.5 mm at Lete and Tatopani, respectively.

Assessing the 2020 events demonstrates how important localized events in relatively small areas are to understand cascading multi-hazard processes in Himalayan mountain regions. In addition, such hydro-geomorphic functioning and related hazards should be carefully considered when planning road design and bridge sites together with landslide and water level monitoring, for a better traffic maintenance and safety.

How to cite: Bell, R., Gurung, N., Andermann, C., Fort, M., Arnaud-Fassetta, G., and Cook, K. L.: A catastrophic multi-hazard event in 2020 in Kali Gandaki valley, Nepal Himalaya, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-15530, https://doi.org/10.5194/egusphere-egu21-15530, 2021.

Smaller systems have thresholds lower than those of larger ones. Therefore, the response of a small change can be easily observed in small catchments where a larger system might not respond.

In this study, we investigated sub-catchments of a small 4^th order Pranmati River Catchment, located in the North-west Himalayas; it is the part of the Ganga River System. We selected eight sub-catchments of 1^st to 3^rd order with area not more than 16 km² and analyzed their response to high-intensity rainfall. We calculated drainage density, length of overland flow, infiltration number, and constant of channel maintenance to analyze their behavior in terms of infiltration and surface runoff. The results show that two of the sub-catchments show tendency of low infiltration and higher surface runoff compared to the other sub-catchments. To validate our results, we compared them with the observations and available data of a highly localized high-intensity precipitation event that occurred in July 2018 within the catchment. During this event, there was focused rainfall in one of the sub-catchments that initiated a flash flood. The flood propagated from that sub-catchment along the trunk channel while all other sub-catchments suffered negligible impact. A significant increase in channel width has been observed along the path of the flood. We ran simulations of storm events in HEC-RAS for various rainfall patterns within the given time interval to replicate the event.

The hydrological simulation of basin-wide uniform rainfall with a Gaussian temporal distribution shows high overland sheet flow in these basins whereas the rest of the basin showed channel flow and low surface runoff. One of these two catchments was the initiation point of the flood event. The results indicate the high sensitivity of the basins and their contrasting responses under similar forces. Minor differences in the values of geomorphic parameters which are inconsequential in the case of large catchments become significant for smaller catchments. It also highlights the degree of spatial heterogeneity of rainfall and the inconsistency of the presently available precipitation datasets.

How to cite: Sarkar, A. and Singh, V.: Assessing the sensitivity of small catchments to an extreme event in the North-west Himalayas, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-16118, https://doi.org/10.5194/egusphere-egu21-16118, 2021.

Meteorological events characterized by extreme rainfall intensity have recently struck the hilly and mountainous territory of the northern Apennines (Italy) as well as many other geographic areas of the world. These extreme rainfall events trigger fast flows of debris along the slopes, stream channels, landslides, and floods, which damage many man-made structures such as roads, houses, water-pipes, etc. There is thus a strong practical interest in predicting the frequency and intensity of these effects for emergency management and to reduce the vulnerability of the territory.

In 2015 an intense rainfall event hit the Valleys of the Trebbia, Nure, and Aveto watercourses in the emilian-ligurian Apennines. In about 6 h a mesoscale convective system deployed a stunning amount of precipitation of 340 mm, with an extreme hourly rainfall intensity of >100 mm/h. During this event, several types of widespread effects on the ground developed i.e., fast flows of debris along the slopes and stream channels (a total number of 305 occurrences), shallow landslides (342) and overbank flooding occurred. Instrumental as well as geological and historical data clearly suggest that extreme rainfall events are increasing in the northern Apennines, in good agreement with the international literature. Through the optimal combination of rainfall data and radar volumes, in this work we present a detailed rainfall analysis, which will serve as a basis to create a quantitative correlation with debris flows over elementary hydrological units. The meteorological analysis of the storm led us to consider the 3 h accumulation rain field as the most relevant for flood triggering. This time interval is short enough to describe the intensity peak of macro precipitating structures, and at the same time it is long enough to allow the development of the debris and stream-flow processes described. The very good match between the 3 h peak intensity and the distribution of high-discharge and hillslope-debris flow support the hypothesis. The 3 h interval further emphasizes the meteorological event with respect to its overall duration of 6 h.

We aim at providing an objective basis for future predictions, starting from the recognition of the forcing meteorological events, allowed us to clearly identify high-intensity-precipitation thresholds triggering flood in small mountain catchments.

Keywords: floods; catchment; threshold; extreme rainfall events; northern Apennines

How to cite: segadelli, S. and grazzini, F.: Predicting extreme precipitation effects on the geomorphology of small mountain catchments, northern Apennines (Italy), EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-6058, https://doi.org/10.5194/egusphere-egu21-6058, 2021.

Geophysical mass flows often break down into large amplitude wave pulses and/or spontaneously form channels with static levees in the arrest zone, enhancing overall run-out. This talk reviews recent depth-averaged models that are able to capture the formation of:- (i) rollwaves, (ii) erosion-deposition waves (which exchange mass with an erodible substrate) and (iii) channel and levee formation, within a single framework. The key is the inclusion of frictional hysteresis, which allows static and moving zones to coexist, as well as depth-averaged viscous terms that incorporate further details of the granular rheology. As well as being able to compute time-dependent spatially evolving solutions numerically, the resulting model allows steady-state solutions to be constructed for the height, width and depth-averaged velocity profile across a leveed channel, which are in good quantitative agreement with small scale analogue experiments using monodisperse dry sand. Colour change experiments are used to show that erosion-deposition waves really do propagate downslope as a wave, rather than a coherent body of grains, and that the presence of the erodible substrate gives them surprising mobility over very long distances. Photos and videos of the similar effects at field scale will be shown to emphasize the importance of these ideas for a wide range of geophysical mass flows. There are, however, still many open challenges in how to generalize these results to multiphase mixtures with broad grain size distributions.

How to cite: Gray, N.: Spontaneous formation of waves, channels and levees in geophysical mass flows, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-6558, https://doi.org/10.5194/egusphere-egu21-6558, 2021.

Natural catastrophes like landslides are often caused by the nucleation and propagation of fractures in heterogeneous materials. Landslides are typically initiated by heavy raining events when water penetrates the pores and reduces the cohesion of soils leading to instability and cracking. When it happens on a steep slope, the moving mass could break up into pieces and the landslide gives rise to a debris flow composed of rapidly traveling fragments of soil and rocks. Such devastating catastrophes endanger the infrastructure and take thousands of lives every year.

In order to understand the emergence of landslides and debris flows we investigated the collapse of a granular column under the action of gravity by means of discrete element simulations. In the model, a cylindrical sample of soil is represented as a random packing of spherical particles. Cohesion is introduced by connecting the particles with non-linear spring elements along the edges of Delaunay triangles determined in the initial configuration of the particles. The constitutive law of springs captures the elastic behavior of particle contacts at small deformations, the plasticity beyond a yield threshold, and the gradual softening and final breaking at large separation distances. A very important feature of the interaction is that particle contacts can be healed, i.e. if two particles approach each other within a capture distance, a new cohesive contact is established between them. Computer simulations were performed varying the strength of cohesion in a broad range.

Our calculations revealed that at high cohesion the granular column sinks in, i.e. its height gradually decreases while it undergoes restructuring and flattening, however, in the final state the system keeps its integrity. When the cohesion is sufficiently week, the process of collapse cannot stop: the system breaks up into a large number of fragments which run out at a high speed. The two phases of high and low cohesion represent the mass movement and the debris flow states of real landslides, respectively. We demonstrate that the transition occurs at a critical cohesion showing analogies to continuous phase transitions.

How to cite: Szuszik, C. and Kun, F.: Collapse of a weakly cohesive granular column, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12977, https://doi.org/10.5194/egusphere-egu21-12977, 2021.

The collapse of granular columns, which sheds light on the kinematics, dynamics, and deposition morphology of mass-driven flows, is crucial for understanding complex flows in both natural and engineering systems, such as debris flows and landslides. However, our research shows that a strong size effect and cross-section shape influence exist in this test. Thus, it is essential to better understand these effects. In this study, we explore the influence of both relative column sizes and cross-section shapes on the run-out behavior of collapsed granular columns and analyze their influence on the deposition morphology with the discrete element method (DEM) with Voronoi-based spheropolyhedron particles. We link the size effect that occurs in granular column collapse problems to the finite-size scaling functions and investigate the characteristic correlation length associated with the granular column collapses. The collapsing behavior of granular columns with different cross-section shapes is also studied, and we find that particles tend to accumulate in the direction normal to the edge of the cross-section instead of the vertex of it. The differences in the run-out behavior in different directions when the cross-section is no longer a circle can also be explained by the finite-size analysis we have performed in this study. We believe that such a study is crucial for us to better understand how granular material flows, how it deposits, and how to consider the size effect in the rheology of granular flows.

How to cite: Man, T., Huppert, H., Li, L., and Galindo-Torres, S.: Finite-size analysis and the influence of the cross-section shape of the granular column collapses, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1647, https://doi.org/10.5194/egusphere-egu21-1647, 2021.

Dry granular fronts and watery tails often develop in debris flows, but their formation mechanisms are still poorly understood. Dry bouldery debris flow fronts are often attributed to particle-size segregation, but idealized experimental mixtures of fluid and mono-disperse grains also exhibit the formation of dry fronts. This motivates the development of a new depth-averaged model that treats grain-water mixtures as a buoyancy and Darcy drag coupled multiphase medium. This system is able to describe the temporal and spatial evolution of the grain and water depths as well as the associated grain and water depth-averaged velocities. It considers the layered development of the flow and incorporates a shear velocity profile into the model, instead of the standard plug flow assumption that is employed by almost all debris-flow models. By revisiting Davies' moving bed flume experiments, it is shown that, in the under-saturated region, shear results in the surface layer of dry grains moving faster than the bulk and they are preferentially transported to the flow front to develop a dry snout. Conversely, in the over-saturated region, the flow thickness is sufficiently small that the water friction is stronger than the friction acting on the grains. As a result, the surface grains can move faster than the water and leave it behind. This novel theory provides a rational framework that describes the complete longitudinal profile of debris flows from the dry granular front to the pure watery tail without the need to consider particle-size segregation.

How to cite: Meng, X., Johnson, C., and Gray, N.: Formation of dry granular fronts and watery tails indebris flows, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-5337, https://doi.org/10.5194/egusphere-egu21-5337, 2021.

Stress anisotropy affects the motion of gravitational mass flows, including debris flows, rock and snow avalanches. Though widely used in analytical models and numerical simulation tools, direct measurements of stress anisotropy in debris flows are not yet available. The present study aims to investigate the ratio of longitudinal to normal pressure exerted by two natural debris flows impacting a monitoring structure in the Gadria creek, IT. The fin-shaped structure in the middle of the channel is equipped with a force plate upstream of the barrier and load cells on the vertical wall of the barrier, continuously recording forces in flow and bed-normal direction. Additionally, the flow height and basal pore fluid pressure were measured. Here we present data from surges of two debris-flow events with peak flow heights of 2.5 m and velocities up to 4 m/s. The ratio of pore fluid pressure to normal stress (often termed liquefaction ratio) reached values up to 0.8. We find an anisotropic stress state during most of the flow event, with stress ratios ranging between 0.1 and 3.5. Video recordings reveal complex deposition and re-mobilization patterns in front of the barrier during surges and highlight the unsteady nature of debris flows. We find a correlation of the stress ratio with flow depth. There is a weak correlation between stress ratio and liquefaction ratio during the falling limb of the surge hydrographs.  Our monitoring data confirm the assumption of stress anisotropy in natural debris flows and support the earth-pressure concept used for gravitational mass flows.

How to cite: Nagl, G., Hübl, J., and Kaitna, R.: In-situ measurements of stress anisotropy in natural debris flows , EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-4378, https://doi.org/10.5194/egusphere-egu21-4378, 2021.

The development of a failure plane in the subsurface can result in movement of large volumes of solids. Depending on the type of rock and initiation mechanism, this material can partially contain cohesive structure. Such semi-structured landslides or rock avalanches feature alternate dynamics compared to debris flows and other types of granular movements. The way in which this structure influences the impact on structures, and thus the resulting hazard, remains largely unknown. Recently, a two-phase semi-structured generalized mass movement model was developed. This model implements a full stress-strain relationship for the moving mixture. In this work, this model is applied to a set of hypothetical and real mitigation measures and structures. The effect of various types of checkdams and blocking pillars is investigated. The resulting impacts are converted to impact pressures and potential damage fractions. Finally, the model results are compared with traditional unstructured debris flow and landslide runout models. Results indicate the strong increase of structural damage with increased cohesive structure. The model furthermore predicts complex interactions between the semi-structured mass movements and mitigation measures. Moving aggregates can break, after which individual rocks or particles might continue in diverging trajectories. Depending on the physical parameters of the material, fragmentation can also occur before impact, after which movement is similar to granular flows. 

How to cite: van den Bout, B., van Westen, C., Jetten, V., and Dhakal, O.: Semi-structured mass movement interactions with flow-blocking mitigation, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-4642, https://doi.org/10.5194/egusphere-egu21-4642, 2021.

Gravitational mass movements pose a threat to the population of numerous mountainous regions around the globe. Climate change affects these processes and their related hazards by influencing their triggering, flow and deposition mechanisms, overall increasing the number of natural catastrophes. Numerical modelling is an essential tool for the analysis and the management of such hazards: it allows the quantitative description of the runout and pressure of rapid mass movements and may contribute to better understand the effects of climate change on their size, frequency, and dynamics. Several depth-averaged models are already operational and commonly applied by practitioners and scientists. Yet, a unified model able to simulate multi-phase cascading events, including their initiation, propagation, entrainment and finally impact on structures is still missing. Hence, more detailed models are  required to advance our understanding of the physics behind gravitational mass movements and ultimately to contribute improving hazard assessment and risk management.

Here, we present some preliminary results of the development of a hybrid Eulerian-Lagrangian Material Point Method (MPM) with finite strain elasto-plasticity to simulate in a unified manner: i) permafrost instabilities and failure initiation; ii) rock and ice avalanche dynamics; iii) solid-fluid interaction and phase transition from rock avalanches to debris-flows. In order to simulate the mechanical behaviour of rock and ice, we propose a Drucker-Prager softening constitutive law accounting for cohesion, internal and residual friction. We calibrate this constitutive law on the basis of state of the art laboratory experiments. The model is applied to synthetic slope geometries to evaluate their stability and investigate subsequent rock fragmentation processes. At a larger scale, dynamics simulations are compared against observations of full-scale process chains. In particular, we implement the two real-scale cases of the rock-avalanche from the Piz Cengalo (CH) and ice- and snow-avalanche from the Grandes Jorasses (IT). The 3D implementation of the model allows to accurately reproduce the initial conditions of an event and complex phenomena such as reported ballistic trajectories non adherent to the ground. Secondary releases due to the mass flow (such as snow or glacier-ice entertainment) and phase changes can be simulated realistically. We test the potential of the model in a broad range of settings and highlight the major gaps to be filled in the near future.

How to cite: Cicoira, A., Blatny, L., Li, X., Troilo, F., Kenner, R., and Gaume, J.: A Material Point Method for Alpine Mass Movements, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-5258, https://doi.org/10.5194/egusphere-egu21-5258, 2021.

Erosion and entrainment are critical processes in gravity-driven mass flows like snow avalanches, as they can significantly change the flow mass and momentum and thus affect the flow dynamics. In snow avalanches, snow cover can be considerably eroded but only partially entrained into the flow. Differentiating erosion and entrainment gives more accurate prediction of the increased flow mass and offers information on eroded snow cover remaining on the slope, but is challenging in practice. This study investigates snow avalanche erosion and entrainment with the material point method, focusing on exploring various erosion mechanisms, differences in erosion and entrainment, and their possible influences on runout distance. By using different mechanical properties for the flowing snow, distinct erosion patterns are observed and the corresponding temporal evolutions of entrainment, erosion, and deposition in the erodible bed are examined. Erosion and entrainment require an appropriate combination of snow friction and cohesion of the bed. If cohesion and/or friction are too low, the bed will naturally be unstable. On the other hand, highly cohesive and frictional bed will prevent erosion. For intermediate values, erosion and entrainment can be notable, and the amount of eroded snow shows a clear negative correlation with snow friction and cohesion while the entrained snow does not demonstrate a strong tendency. Furthermore, the release and erodible bed lengths are varied to study their effect on erosion and entrainment propensity. It is found that the increase in the lengths of the release zone and erodible bed leads to more erosion and entrainment as expected, but not necessarily to a longer runout distance. In our simulations, the release and erodible bed lengths are positively and negatively correlated with the runout distance, respectively. This implies that the runout distance can have opposite trends with erosion and entrainment, which might be closely related to the energy change of the simulated avalanches from the outlet of the erodible bed to the final deposit. Our results shed more light into the erosion and entrainment mechanisms and may contribute to improve related parametrizations in large-scale avalanche dynamics models.

How to cite: Li, X., Sovilla, B., Ligneau, C., Jiang, C., and Gaume, J.: Investigating erosion and entrainment of snow avalanches using the material point method, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-6232, https://doi.org/10.5194/egusphere-egu21-6232, 2021.

We experimentally investigated the internal dynamics of stationary mono- and bidisperse granular avalanches in an inclined conveyor belt flume. We used the refractive index matching technique to visualize and obtain information from within the granular bulk. In combination with particle tracking velocimetry and coarse-graining techniques, we were able to calculate continuum particle distributions and velocity fields. The experimental avalanches had distinct flow regions: (i) a convective-bulged front, (ii) a compact-layered tail, and (iii) a breaking size segregation wave structure, serving as a transition between the former two. To describe the dynamics of these regions, we computed local strain rates in the form of its tensor invariants. The invariants varied notably between regions; while the largest values and non-linear distributions were found at the front, linear distributions were observed in the tail. In general, and although that slip was considerable at the base of the flow, time-averaged velocity profiles were found to be well captured by a Bagnold model. Based on recent developments in particle-size segregation theory, we calculated the segregation flux within the bidisperse avalanches. In those experiments, we found that segregation flux was higher at the front than at the back, a fact that was confirmed by the observed recirculation of large particles at the front. All our experimental data show a strong link between rheology and segregation, a result that will provide grounding for new developments in segregation theory.

How to cite: Trewhela, T. and Ancey, C.: Internal dynamics of steady and nearly uniform granular avalanches, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12535, https://doi.org/10.5194/egusphere-egu21-12535, 2021.

Guillaume Meyrat, Brian McArdell, Ksenyia Ivanova, Perry Bartelt

WSL Institute for Forest, Snow and Landscape Research, 8903 Birmensdorf, Switzerland

Keywords: Debris flows, multi-phase models, dilatancy, shear stress, density distribution

To implement an accurate numerical tool to simulate debris flow hazard is a longstanding goal of natural hazard research and engineering. In Switzerland the application of numerical debris flow models has, however, been hampered by many practical and theoretical difficulties. One practical problem is to define realistic initial conditions for hazard scenarios that involve both the rocky (granular solid) and muddy (fluid) material. Still another practical problem is to model debris flow growth by entrainment [1]. These problems are compounded by theoretical uncertainties regarding the rheological behavior of multi-phase flows. Recent analysis of debris flow measurements at the Swiss Illgraben test-site [2] (shear and normal stresses, debris flow height) show that the shear force, and therefore the entire debris flow behavior, is largely influenced by the debris flow composition, i.e. the amount of solid particle and muddy fluid at any specific location within the debris flow body (front, tail, etc.). The debris flow composition is, in turn, determined by the initial and entrainment conditions for a specific event. As a consequence, we have concluded that the very first step to construct a robust numerical model is to accurately predict the space and time evolution of the solid/fluid flow composition for any set of initial and boundary conditions. To this aim, we have developed a two-phase dilatant debris flow model [3, 4, 5] that is based on the idea that the dispersion of solid material in fluid phase can change over time. The model is thus able to predict different flow compositions (rocky fronts, watery tails), using shallow-water type mass, momentum and energy conservation equations. This helps to predict when the solid phase deposits, and when muddy fluid washes and channel outbreaks in the runout zone can occur. The parameters controlling the evolution of debris flow density and saturation have been derived by direct comparison to the full-scale measurements performed at the Illgraben test site.

References

How to cite: Meyrat, G.: A Dilatant, Two-Phase Debris Flow Model for Hazard Mitigation, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10815, https://doi.org/10.5194/egusphere-egu21-10815, 2021.