NH3.11

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.

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 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.

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.

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.

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.

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.