In a context of climate change, sediment availability and occurrence of heavy rainfall episodes are tending to increase, resulting in a likely rise in frequency and magnitude of debris flows. For effective hazard management, predicting the velocity, depth, and run-out of these flows in realistic settings, while considering all relevant processes, is crucial. In particular, as debris flows may exhibit viscoplastic characteristics, a better understanding of the interplay between inertia, rheology and topographical features is necessary. We report on well-controlled laboratory experiments of viscoplastic surges flowing down a model topography. A volume of viscoplastic fluid (Carbopol) is released onto four 3D-printed topographies featuring different patterns of mounds and ridge. Using Moiré projection, we monitor flow depth with a temporal resolution of 250 Hz, exploring a wide range of configurations involving varying volumes and fluid rheological properties. This set-up enables us to investigate the front velocity, the deviation and accumulation of fluid induced by the obstacles and the shape of the deposit. Two distinct flow regimes are observed. Initially, a rapid regime develops with a high front velocity, and the fluid spreads in both transversal and longitudinal directions. This regime is driven mainly by inertial forces. Subsequently, the front velocity drops drastically and the fluid flows mainly along the slope. In this second regime, controlled by rheological effects, the flow is strongly influenced by the topography with various mechanisms depending on the case (deceleration, accumulation, channelization, etc.). These experimental results are then systematically compared with depth-averaged numerical simulations based on shallow-water hypothesis. We compare the outcomes of two models implementing different representations of the complex rheology. The experimental data serves as a benchmark to assess the predicting capabilities of the models and evaluate the resulting uncertainties. These findings offer new insights into the physical processes driving debris flows, and they will ultimately contribute to the enhancement of simulation tools used in hazard management.

How to cite: Ginot, C., Chambon, G., Wallon, M., Vigneaux, P., and Philippe, P.: Experimental study of viscoplastic surges down complex topographies and comparisons with numerical simulations., EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-9580, https://doi.org/10.5194/egusphere-egu24-9580, 2024.

Landslides and debris flows are massive geophysical processes that could occur in subaerial or submerged conditions. They involve granular materials in a wide range of Grain Size Distributions (GSD). In such processes, granular materials are subjected to large deformations, reaching a state where strains accumulate at constant shear stress. This state is known in the geotechnical community as the residual or critical state. The influence of the GSD on the residual state has been a matter of discussion between conflicting experimental and numerical observations. In this work, we confirm, at a grain scale and in dry conditions, that the residual shear strength is independent of the GSD. Moreover, we experimentally validate this result on dry granular flows, comparing the influence from monodisperse materials (materials with one grain size) to well-graded materials (materials with multiple grain sizes) on the mobility of a granular column. In this configuration, a granular column is let to collapse by self-weight and spread horizontally. The column mobility can be interpreted as a macro representation of the material's effective shear strength. Furthermore, we explore the effect of the GSD in immersed columns, finding a strong dependence of the GSD on the flow dynamics arising from the evolution of basal pore pressure P. At the flow initiation, negative P changes beneath the column produce a temporary increase in the column strength. This positive change lasts longer and with a larger amplitude in granular flows with well graded materials than in monodisperse ones. Then, during the column horizontal spreading, positive changes of P provoke a decrease in shear strength. For column collapses of graded materials, the excess of P lasts longer, allowing the collapses to reach farther distances compared with collapses of monodisperse materials. Finally, considering the relevance of mobility in granular flows, we propose a mobility model that scales the final runout with the collapse kinetic energy. This model works for both dry and immersed flows with different GSDs and has been validated for results from different authors, methodologies, and grain characteristics. Our results offer a novel perspective on the influence of GSD on the complex relationships between solid and fluid phases in granular flows, highlighting features that can be extended to massive natural processes.

How to cite: Polanía, O., Azéma, E., Renouf, M., Estrada, N., and Cabrera, M.: Grain size distributions, from a single grain size to well-graded distributions, on dry and immersed flows, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-9068, https://doi.org/10.5194/egusphere-egu24-9068, 2024.

While there are field observations and studies on erosion of debris flows over loose sediment, bedrock erosion by debris flows has not yet been studied comprehensively. Nevertheless, traces of erosion in the bedrock channel after debris-flow events, such as scars, ridges, grooves or individual impacts, indicate a non-negligible entrainment of material as a result of process-related impact and shear forces. In the Alps, such erosion phenomena are often found in the upper steep and inaccessible parts of the catchment areas and are thus difficult to analyse.

In this study we therefore investigate the potential erosion capacity of debris flows of different rheological characteristics on immobile channel beds with a small-scale physical model. We try to understand how bedrock strength influences erosion and if shear or impact forces dominate bedrock erosion by debris flows. To this end, erosion rates in terms of volume and the forces causing erosion are examined in over 100 laboratory experiments. We also compare and scale these rates to natural bedrock erosion caused by debris flows.

In our small-scale laboratory investigation, polyurethane foam boards act as bedrock surrogates. This material has already been used as a bedrock simulant in studies into fluvial bedrock erosion. The boards were installed in the lower 2.5 m of the flume channel, which has a total length of 5.6 m. Different board strengths ­­– indicating different erosion susceptibility – were tested with three different debris-flow mixtures. The slope of the channel was kept constant at 34°. After each debris flow, the change in bed elevation was measured with a laser scanner to determine erosion rates at submillimeter accuracy. Laser distance sensors, pore water pressure sensors, a load cell and a geophone were used to quantify debris-flow dynamics and different erosion forces, including shear and impact.

Our results show an exponential increase in erosion with a decreasing tensile strength of the bedrock simulant. While impact and shear forces both influence erosion rates, the decisive erosion force component appears to depend on the proportion of gravel and clay in the debris-flow mixture. The results of this study serve to deepen our understanding of the debris-flow process and expand our knowledge of erosion processes in the upper reaches of debris-flow catchments. In addition, our results can be used to tune existing models for longer-term landscape evolution.

How to cite: Friedl, C., Roelofs, L., Hansen-Löve, J., Scheidl, C., and de Haas, T.: Hitting rock bottom - Experimental study of bedrock erosion by debris flows, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-7750, https://doi.org/10.5194/egusphere-egu24-7750, 2024.

Landslide runout analyses are conducted to predict the motion and distal reach of potential future landslides to inform landside hazard zonation, risk management, and the design of the optimal location and height of mitigation strategies such as barrier systems. A key uncertainty in these analyses relates to the erodibility and entrainment of sediment which may unexpectedly increase the volume of the landslide and affect travel distance. In this study we explore the case of a landslide overriding and entraining loose saturated valley floor sediments; in particular, whether such a scenario may cause the overridden sediments to liquefy, and if so, the extent to which that liquefaction affects the depth of erosion of the bed material and mobility of the slide.

The presence of a loose saturated layer of soil at the base of a slope inherently introduces a soil region potentially prone to static liquefaction. This scenario fulfills the criteria required for instability: a) loose contractive soil, which happens to be co-located in an area that is both b) fully saturated, and c) within reach of a shear trigger (i.e., being overridden by the landslide). This scenario was reproduced in the Queen’s landslide flume, using a horizontal liquefiable bed of saturated fine sand 2 m in width, 7 m in length, and 0.3 m in height located at the bottom of the inclined portion of the flume. Landslides of up to 1,200 kg of granular material were then released from the top of a 6.5 m long slope inclined at 30 degrees to impact the bed at speeds of up to 6 m/s. Behaviour of the sand bed upon impact was captured using ultrahigh speed imaging of the landslide and bed profiles, a Blickfeld LiDAR sensor positioned opposite to the landslide to capture point cloud scans of the slide, and a linear array of porewater pressure sensors within the sand bed. Preliminary results of experiments attempting to liquefy the valley floor sediments are presented as we explore the requisite conditions for different rates of entrainment, and in extreme cases, liquefaction, of the sand bed, as well as the effect of liquefaction and entrainment on the speed and volume of the slide.  

How to cite: Waring, A. and Take, A.: Exploring the entrainment of liquefied bed material in landslides, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-13638, https://doi.org/10.5194/egusphere-egu24-13638, 2024.

Debris flows are extremely fast landslides whose complex dynamics are still not fully understood, primarily due to challenges in acquiring direct field measurements. In modern monitoring stations, cameras represent cost-effective data sources, providing essential information for characterising the documented events.

Digital Particle Image Velocimetry (DPIV) algorithms have been extensively employed in the literature to reconstruct velocity fields in laboratory physical models under controlled conditions. However, the resolution of field camera footage is typically suboptimal due to weather and lighting conditions, as well as non-zenithal recording geometry, hindering a straightforward application of DPIV. Landslide flume experiments, conducted in collaboration with the Civil and Structural Engineering Department of the University of Sheffield and the British Geological Survey office in Keyworth, revealed that also suboptimal quality footage can be effectively utilised provided appropriate orthorectification algorithms are applied to eliminate the original image distortions.

In this study, the methodology established through the laboratory flume experiments was applied to analyse a real debris flow event in an active catchment in the Camonica Valley (Lombardia, Italian Alps) between the municipalities of Ono San Pietro and Cerveno. The Blè Stream catchment, with a drainage area of approximately 3.5 km², a maximum elevation of 2,527 m a.s.l.,  and a main channel length of about 2.9 km, experienced a debris flow event on October 22, 2022. This was documented by several monitoring stations equipped with cameras and a flow-depth radar sensor along the main channel track.

The frame-by-frame orthorectified surface velocity field of the recorded debris flow was obtained through a DPIV analysis, employing two open-source tools in Matlab sequentially: PIVlab (Thielicke & Stamhuis 2014) and RIVeR (Patalano et al. 2017). The discharge at a specific instant along a reference section was computed as the product of the reconstructed flow velocity distribution and the area of the section defined by its topography, known from pre- and post-event LiDAR and drone surveys, and the measured flow level. Throughout this phase, careful consideration was given to assessing the primary sources of uncertainty arising from the continuously changing section geometry and the measured surface velocity, which typically overestimates the actual depth-averaged velocity, with a divergence depending on flow rheology. Calculating the discharge for each frame along the reference section ultimately yielded the hydrograph of the documented debris flow event, along with an estimate of the involved volume of material.

References:

Patalano A, García C, Rodriguez A, 2017. Rectification of Image Velocity Results (RIVeR): A simple and user-friendly toolbox for large scale water surface Particle Image Velocimetry (PIV) and Particle Tracking Velocimetry (PTV). Computers and Geosciences. 109. 323-330. 10.1016/j.cageo.2017.07.009.

Thielicke W, Stamhuis EJ, 2014. PIVlab – towards user-friendly, affordable and accurate digital Particle Image Velocimetry in MATLAB. J. Open Res. Softw. 2 http://dx.doi.org/10.5334/jors.bl. 

How to cite: Zuccarini, A., Ioriatti, E., Redaelli, M., Albertelli, L., Reguzzoni, M., Reguzzoni, E., Nedumpallile Vasu, N., Banks, V., Bowman, E., Leonardi, A., and Berti, M.: Estimating the hydrograph of a debris flow event through low-cost field camera monitoring and Digital Particle Image Velocimetry, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-10832, https://doi.org/10.5194/egusphere-egu24-10832, 2024.

The experimental-based μ(I) rheology is now prevalent to describe the movement of gravitational mass flows. Though the μ(I) rheology has been successfully applied to the modelling of historical debris flows and rock avalanches, its physical implication is not fully understood. In this study, we re-formulate the μ(I) rheology as a Voellmy-type relationship, which is composed of a Coulomb friction term and a turbulent term. We find that different from the classic Voellmy rheology (ξ is a constant), the turbulent coefficient ξ in the μ(I) rheology is heavily dependent on the avalanche height and velocity, indicating the shear-thinning features. However, as μ(I) rheology is a pure function of velocity (for a constant height), the friction exhibits no change during the acceleration and deceleration stage. With this purpose, we introduce a newly proposed μ(R) rheology that relates the friction to the production and decay of fluctuation energy (granular temperature) R. Using one-dimensional block models, we show the equivalence of I and R, and elucidate why similar results of μ(I) and μ(R) rheologies are easily obtained. Ultimately, this comparative analysis offers valuable insights into improving geophysical flow models, enhancing our understanding of flow behavior's dependence on various factors and leading to more accurate assessments and mitigation of geophysical hazards.

How to cite: Zhuang, Y., McArdell, B., and Bartelt, P.: Comparative Analysis of Geophysical Flow Models: Voellmy, μ(I), and μ(R) Rheologies , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-14924, https://doi.org/10.5194/egusphere-egu24-14924, 2024.

Earthflows are landslide processes characterized by the viscous movement of predominantly fine-grained and often water-saturated material down a slope or gully. They occur at a broad range of velocities, but generally do not display extremely rapid movement (such as debris flows, snow avalanches, or rock avalanches). Examples include the Gschliefgraben earthflow in the Austrian Alps and the Chirlești earthflow in the Romanian Carpathians. Although earthflows are common mass movement processes, they have not received the same attention as extremely rapid flows when it comes the development of dynamic simulation models. Here, we present a novel mechanical model and dynamical solution technique for earthflows. We develop a strategy of balancing the flux, viscous, and other forces. Our model essentially employs the flux-controller, viscosity-controller, and the deformation-controller. Within a single unified frame, we can now simulate a broad range of earthflows for different viscous, plastic, or visco-plastic behaviors and any degree of mechanically controlled deformation over a wide spectrum of time scales. We demonstrate the performance of the new earthflow model and its applicability with the advanced open-source computational mass flow simulation tool r.avaflow. Simulated earthflow deformation and motion are very smooth, typical of a hugely viscous material, as it is anticipated for earthflows as commonly observed for real-world events. As expected, the motion and deformation are exceptionally sensitive to the changes in the viscosity of the earthflow.

How to cite: Pudasaini, S. P. and Mergili, M.: A dynamic earthflow model, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-11956, https://doi.org/10.5194/egusphere-egu24-11956, 2024.

Debris flows are characterized by rapid movement of a mixture of water, mud, and unsorted debris along natural channels due to gravity. With propagation speeds exceeding 5 m/s and lack of premonitory signals, evacuating local populations is challenging, mitigation measures like barriers become necessary . The current difficulties in designing barriers regard the simplified methods employed that overlook event variability, hindering optimal structural design.

In this study, continuum numerical models, specifically depth-averaged (DA) and three-dimensional (3D) models, are employed to investigate debris flows. DA models depth-average Navier-Stokes equations, reducing the number of variables, allowing for efficient analyses of entire mountain valleys in a short timeframe. However, due to depth-averaging, essential details of vertical momentum transfer are missing, crucial for studying flow-structure interaction (FSI). By contrast, 3D models faithfully replicate FSI but are computationally demanding, making their application challenging for valley-scale flow propagation studies.

The study proposes a novel approach by coupling DA and 3D models to comprehensively investigate a flow propagating in a mountain valley and impinging against barriers. This approach combines the efficiency of DA models with the precision of 3D models, without neglecting upstream flow evolution. The DA model is employed when the flow is far from barriers, and a coupling section is placed upstream of a barrier, with the DA results as input for the 3D model to study the FSI.

The DA-3D coupled model is validated through replicating a laboratory experiment and a real-world event, with the same rheological law in DA and 3D frameworks for consistency.

A laboratory experiment with glass beads in a flume was replicated. Using µ(I) rheology, the study initially compared 3D results with the original experiment. Subsequently, the DA-3D model, employing µ(I) rheology, replicated the experiment. Striking similarity were found in DA-3D results when compared with 3D and experimental results. Additionally, forces on the barrier were compared between the 3D and DA-3D models, affirming results consistency and effectiveness of the DA-3D model.

In the site-scale investigation of an event occurred in St. Vincent (Aosta Valley, Italy), the study focused on a filter barrier installed to mitigate the risk associate to debris flows in the area. Field data are available because  the barrier was monitored to evaluate forces.  In this scenario, the DA-3D model was utilised, avoiding the the µ(I) rheology due to calibration challenges and because it was formulated for dry flows. Instead, the study opted for Voellmy rheology, extensively used in DA frameworks and specifically adapted for the 3D framework. The examination of debris flow impact on the barrier involved a comparison of field data with numerical values. The findings highlighted the realistic representation of FSI and forces by the DA-3D model at the site scale, emphasizing its potential for the comprehensive study of debris flows.

Acknowledgments

This study was carried out within the RETURN Extended Partnership and received funding from the

142 European Union Next-GenerationEU (National Recovery and Resilience Plan – NRRP, Mission 4,

143 Component 2, Investment 1.3 – D.D. 1243 2/8/2022, PE0000005) – SPOKE VS 2.

How to cite: Pasqua, A., Leonardi, A., and Pirulli, M.: The impact of debris flow on mitigation structures: a novel depth-averaged and three-dimensional coupled model for the flow dynamic simulation , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-20635, https://doi.org/10.5194/egusphere-egu24-20635, 2024.

Corresponding author: Shun Wang       E-mail: shun.wang@whu.edu.cn

Abstract: Overtopping failure of landslide dams is a complex process that involves strong soil-water coupling and structural failure. Physically based numerical models are needed for breach mechanism as well as failure process and flood prediction. In this study, we establish an SPH-DEM dam-break model that considers the combined effect of seepage and overflow. The key feature of the proposed high-fidelity dam-break model is that both solid and fluid phases are solved simultaneously in two different sets of Lagrangian particles using their own governing equations. In the numerical framework, the water phase is modeled as weakly-compressible Newtonian fluid using the SPH method, and the soil phase is modeled using the DEM method. The interactions between these two phases including drag force, buoyancy and adhesion. The capillary force generated by the meniscus between two soil particles is solved to characterize the saturated and unsaturated processes of soil. The model is validated by three benchmarks including the simulations of seepage through an earth dam, a small-scale dam-break test and the whole progress of dam profile erosion. In a small-scale dam break test, the calculation error of overtopping peak flow is 3.5%. Simulation results predicted by the SPH-DEM dam-break model show good agreements with the finite element method and experimental results. Furthermore, the high-fidelity dam-break model is able to simulate many other soil-water coupling processes, such as reservoir water infiltration, dam slope erosion and collapse, breach development, and dam failure. In the future, the proposed SPH-DEM soil-water coupling framework could be applied to the modeling of geohazard chain triggered by rainfall-induced landslides, blocking river and dam outburst flood.

Keywords: landslide dam; overtopping modeling; SPH-DEM; soil-water coupling

How to cite: Su, Z., Wang, S., and Li, D.: High-fidelity modeling of landslide dam overtopping failure using SPH-DEM method , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-9914, https://doi.org/10.5194/egusphere-egu24-9914, 2024.

Lahars are a common and potentially long-lived hazard in river basins affected by explosive volcanic eruptions. They result from the hydrological disturbance of surrounding landscapes, driven by the destruction of vegetation and deposition of pyroclastic material. These modifications typically result in heightened rainfall runoff responses, via reduced interception and infiltration, leading to increased water and sediment flux, manifesting as lahars. With time, landscapes recover via removal of sediment, establishment and stabilisation of channels, and the redevelopment of vegetation. This recovery subsequently reduces the runoff response to rainfall and in turn limits the potential magnitude and frequency of lahars. Numerical modelling is an important approach for assessing the hazard posed by lahars. Most modelling approaches related to lahars consider the remobilisation susceptibility of pyroclastic deposits under particular conditions, or the runout/inundation potential of individual or probabilistic ensembles of flows. To date, very limited research has sought to address the longer-term (years to decade) evolution of lahar activity in affected catchments as they respond to and recover from disturbance. Here we present and discuss SedCas_Volcano, a simple model designed to simulate the longer-term evolution of lahar incidence in a catchment on the island of Montserrat that has been repeatedly disturbed by episodic volcanic activity since 1995. Using this simple and computationally inexpensive numerical framework, we account for variability in sediment supply, vegetation cover, and rainfall. Here we will discuss the merits of this model and identify possible next steps for continued model development.

How to cite: Christie, J. and Bennett, G.: SedCas_Volcano: a novel approach to modelling decadal evolution of lahar hazard in response to episodic volcanic eruptions, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-1051, https://doi.org/10.5194/egusphere-egu24-1051, 2024.

As a natural hazard in mountainous terrain, debris flows cause considerable disruptions, human casualties and economic damage in many regions world-wide. However, the spatially localized nature of debris flows together with the lack of data at sufficient temporal and spatial resolutions make the triggering processes difficult to describe. As a result, debris flows are problematic to predict. Effective regional and local early warning systems, built on both process-based or statistical models, have therefore so far remained elusive. Even more, common statistical models, such as  precipitation-intensity threshold models, rely on precipitation. As debris flows are essentially in-channel processes, precipitation is an indirect predictor and proxy for in-channel processes. As such it is not surprising that precipitation has limited predictive power. In spite of recent progress, general and detailed descriptions of in-channel processes that control debris flow triggering only start to emerge. Most generally, sediment supply and channel flow magnitudes can be considered major direct controls on debris flow occurence. As both are difficult to observe, they have so far not been systematically exploited and quantitatively described for their role as debris flow triggers.  

Based on 20-year records of hydro-climatic data, several dozens of well documented debris flow events in three contrasting head-water catchments in the Central Alps and a semi-distributed, process-based hydrological model, the objectives of our analysis are to (1) quantify the critical channel runoff magnitudes that have triggered past debris flows and to establish whether characteristic magnitudes can be found as a function of topography, soils, geology and other factors, (2) identify the relevance of snow melt vs. rainfall for the generation of debris flow triggering critical channel runoff, and (3) to test whether modelled critical channel runoff has higher power to predict debris flows than standard precipitation-intensity models.

Overall, we have found that indeed, relatively well-defined minimum critical channel flows as lower limits above which debris flows occur feature each of the three study catchments. It was also found that the general magnitudes are highly site specific. In spite of that, no obvious relation between the average critical flow magnitudes and landscape characteristics, such as local terrain or channel slopes, vegetation cover, soil type or geology at the three sites could be identified. In general, it was found that flow peaks, generated by short-duration, high-intensity rainfall events, mostly during summer, dominated the debris flow trigger dynamics at the study sites. In addition, several instances when debris flows were triggered by flow peaks of similar magnitudes but generated by high-intensity snow melt in combination with rain-on-snow were observed, highlighting the importance of quantifying liquid water input dynamics instead of bulk precipitation as system input that causally leads to the occurrence of debris flows. Intrinsically accounting not only for this distinction but also additional effects by evaporation, modelled channel flow magnitudes were found to be better predictors of debris flows, with respect to both, higher rates of true positives (correctly predicted debris flows) and lower rates of false positives (predicted but not occurred in reality), than traditional precipitation thresholds. 

How to cite: Hrachowitz, M., Sklar, L., and Kaitna, R.: Critical channel runoff as direct trigger of debris flows in mountainous terrain., EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-7378, https://doi.org/10.5194/egusphere-egu24-7378, 2024.

Debris flows pose significant natural hazards in mountainous regions globally, with the potential to cause substantial damage to villages and infrastructure in lowland areas. To effectively mitigate these hazards, it is essential to identify catchments prone to delivering debris flows to fan areas. Topographic metrics such as fan slope and catchment Melton number are useful at a regional scale but in recently deglaciated alpine landscapes do not explain high variability in debris flow frequency among neighboring catchments. Here we assess debris flow susceptibility by quantifying the spatial extent of sediment cover across upstream catchment areas and the connectivity of potential debris flow source areas to fans at catchment outlets.

The Pitztal Valley in the Austrian Alps serves as the study site, benefiting from elevation data (digital terrain model) and historical debris flow event data since the 1950s for 28 catchments, along with sediment cover maps for 18 catchments. To assess the likelihood that debris flows originating within sediment covered areas could reach the fan, we calculated the minimum mean slope angle along every flow path to the fan apex. Source areas with minimum angles lower than a dynamic friction angle are assumed to be disconnected from the fan apex because potential debris flows would come to rest and form coarse deposits upstream of the fan. This concept is utilized to calculate the fractional connectivity for sediment areas in each catchment as a function of dynamic friction angle.  Rather than assume a single friction angle, we compare catchments based on the angle corresponding to a connectivity rate of 50% for the sediment covered areas (referred to as the 50% connectivity angle).

We find a highly significant positive exponential correlation between the 50% connectivity angle and the historical debris flow frequency (in units of debris flow events per square kilometer of catchment area) using data from the 18 catchments with sediment cover maps. Validation is performed in 10 additional catchments where we identified sediment covered areas using a new algorithm that can distinguish bare bedrock from sediment deposits from the local topographic roughness. The observed debris flow event frequencies align closely with or fall within the confidence bounds predicted by the 50% connectivity angles, confirming that the combined evaluation of catchment connectivity and sediment availability successfully explains debris flow frequency in this landscape.

Lastly, the results are compared to models employing previously published metrics of connectivity and debris flow susceptibility, providing insights into the contribution and efficacy of this new approach.

How to cite: Chen, T.-Y. K., Sklar, L., Pilkington, B., Dickson, E., and Kaitna, R.: Assessing Debris Flow Susceptibility in Deglaciated Alpine Catchments: A Novel Approach Integrating Flow-path Connectivity and Sediment Availability, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-6819, https://doi.org/10.5194/egusphere-egu24-6819, 2024.

In the Dolomites area, steep rocky cliffs are incised by several chutes that concentrate runoff and deliver it to the scree at their base. This interaction prompts erosive actions, creating channels for debris flows. After intense, short-lasting rainfall, the resulting runoff carries a significant amount of loose debris, forming a solid-liquid surge. This surging flow entrains boulders, gravel, and sand along its path, transforming into an increasingly substantial stony debris flow. To explore how headwater rocky catchments respond hydrologically and trigger stony debris flows, we utilize data gathered from three monitoring stations placed in distinct debris-flow catchments. These stations, located in the debris-flow initiation area of the basins, capture videos and flow-stage data, enabling us to observe the timing and type of the incoming flows. Over 15 years of monitoring, numerous instances of runoff and mass-transport phenomena have been documented. This comprehensive dataset is precious for analyzing the hydrological behavior of small, steep headwater basins and investigating stony debris flow initiation. An existing hydrological model has been partially reformulated, and its updated version was calibrated using the hydrographs measured via a sharp-crested weir. Testing this updated model against observations from two larger debris-flow catchments affirms its capability to replicate the initial phases of a debris flow, particularly when the sediment concentration is quickly increasing. Moreover, combining simulated runoff volume with the entrained sediment volume in the Rovina di Cancia catchment, estimated through DEM of Differences for the debris-flow events that occurred from 2009 to 2022, provides values for solid concentration suitable for predicting sediment volumes carried by debris flows.

How to cite: Bernard, M., Barbini, M., Berti, M., Boreggio, M., Schiavo, M., Simoni, A., Vesco Lopez, S., and Gregoretti, C.: Prediction of runoff-generated debris flows using hydrological modeling, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-3161, https://doi.org/10.5194/egusphere-egu24-3161, 2024.

2^nd October 2020, the Atlantic storm « Alex » triggered an exceptional Mediterranean flood event in the south-eastern region of France, affecting the coastal Alpine valleys of the Tinée, Vésubie and Roya rivers. The event is considered exceptional due to the unprecedented rainfall recorded within a 24-hour period, surpassing 650 mm at Mesche dam and the elevated liquid peak flows around 1 350 m³/s at Breil-sur-Roya. Geomorphological changes consequence of the flood event includes river-related effects such as debris flows, landslides and partial destruction of the forest cover on the slopes, together with the destruction of bridges, roads, and houses.

Our study focuses on the Viévola sub-watershed upstream of the Roya river, made up of 4 confined torrential sub-valleys: Dente, Morte, Para, Scabrie and Rabay valleys. Theses generated numerous of debris flows feeding the Viévola alluvial fan. Our observations show that these valleys were responsible for massive input of sediment which led to widening of the active channel of Roya river,bank erosion and embankments during the Alex flood event.

The major areas of sediment accumulation were quantified in order to determine the balance between the 'initial' volumes eroded such as upstream gullies sediments and landslide deposits and the 'deposited' volumes (Viévola alluvial fan). The sediment volumes were quantified by Digital Elevation Model of Differences (DoD) from aerial imagery available before and after the flood.

Pre-flood topographical data is of low quality while post-flood topography are better thanks to LiDAR datasets acquired in october 2020 and june 2021. A statistical study of tree heights on tributary slopes, using aerial and infrared images, allowed us to subtract them from the pre-flood digital surface model, creating a tree-free pseudo topographic surface.

Our results show that the Viévola sub-watershed produced approximately 304,000 m³ of sediment within a 24-hour period, with a margin of error of +/- 40,000m³ (an average error of 13%). The gullies at the head of the Dente and Rabay valleys produced 125,000 m³ of sediment, with an error margin of +/- 21,000 m³. These initial inputs from these gullies caused significant bank erosion in the Dente valley, resulting in the release of over 140,000 m³ of sediments, with an error margin of +/- 5,000 m³. This erosion led to a considerable widening of the Dente torrent channel, which expanded from an average width of 6.1m before the flood to 18.6m after the event. Based on field studies, we found that the cause of the bank erosion is linked to the debris flows that occurred in the Dente valley.

Comparatively, the Morte, Para and Scabrie valleys primarily contributed sediment from landslides, totalling around 21,000 m³, with an error margin of +/- 6.300m³. On average, the width of the torrents in these valleys doubled, with a factor of 2.1. Moreover, an alluvial cone formed at the Vievola holiday resort, resulting in a deposited volume of approximately 90,000 m³, with an error margin of +/- 20,000 m³. Consequently, an estimated 214,000m³ of sediment was exported to the Roya River.

How to cite: Kerverdo, R., Lafuerza, S., Gorini, C., Rabaute, A., Granjeon, D., and Fouache, E.: Budgeting sediment volumes mobilised during Storm Alex in the upper part of the Roya valley, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-17806, https://doi.org/10.5194/egusphere-egu24-17806, 2024.

To accurately assess landslide susceptibility and model debris flow paths, acquisition of high-resolution elevation data is essential. These data enable precise topographical analysis, considering factors like slope and curvature. But most of the time such high-resolution data will have inaccuracies due to vegetation, especially in tropical region. This study proposes a comprehensive approach that integrates field data and Structure from Motion (SfM) technology to rectify such DEM inaccuracies caused by dense vegetation. The goal is to enhance the accuracy of landslide simulations and volumetric analysis for effective post-disaster management. The Western Ghats, running parallel to India's western coast, has experienced a surge in rainfall-induced landslides, resulting in significant casualties in recent years. Notably, the Pettimudi village landslide in 2020 caused 70 deaths. The unique geomorphological features of the Western Ghats, such as concave curvature, colluvium deposits, and evidence of paleo landslides, contribute to the area's susceptibility. The study emphasizes the need for detailed assessment and mitigation strategies. The proposed method aims to improve the post-event high-resolution DEM accuracy by integrating field-collected elevation values and utilizing Agisoft Metashape software with the SfM algorithm. The rectification process involves combining elevation differences measured during a field study with photogrammetric elevation data. The field-collected elevation differences are crucial for rectifying these points, and enhancing the Digital Elevation Model (DEM) accuracy. The inaccessible source region is improved using a SfM-created DEM from drone footage, resulting in a more accurate post-event DEM. Correcting the 1 m DEM using an SfM-generated DEM proves challenging but significantly improves detail and accuracy, especially over the landslide source area. The impact of the rectification on accuracy is assessed by comparing volumes calculated from the initial DEM and the newly corrected DEM. The difference in volumes of debris depletion and accumulation, computed using initial and corrected DEMs, highlights variations, with depleted volumes being significantly larger due to the extensive depth increase over the landslide source region. These volumes are then utilized in Rapid Mass Movement Simulation (RAMMS) to validate the rectification process and enhance landslide impact predictions. Debris flow simulations in RAMMS, utilizing the rectified 1 m DEM, show specific outcomes at the landslide source region and toe of the landslide. The study emphasizes the importance of integrating SfM technology with field data to improve DEM accuracy, acknowledging the significance of additional field data for further refinement. The potential adoption of highly precise aerial imagery from Unmanned Aerial Vehicle (UAV) surveys is suggested to further enhance the SfM DEM. Debris flow modeling with RAMMS serves as a vital step in validating the accuracy and reliability of the rectified elevation model.

Keywords: Landslides, Western Ghats, Pettimudi, Digital elevation model (DEM), Structure-from-Motion (SfM) technology, Rapid Mass Movement Simulation (RAMMS)

How to cite: Vk, K. P., Ambujendran, R., Nedumpallile-Vasu, N., J Banks, V., Arnhardt, C., and Ks, S.: Integrated UAV and Field Data Analysis for High-Resolution DEM Enhancement, Rectification, and Debris Flow Modeling, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-13686, https://doi.org/10.5194/egusphere-egu24-13686, 2024.

Hydrogeomorphic hazards are natural hazards that involve the mobilization, transport, and deposition of mixtures of water and debris or sediment and can take the form of floods, debris floods and debris flows. Such hazards occur in a continuum with varying size and concentration of entrained sediment and debris. Within this continuum, debris floods occur when large volumes of water in a creek or river entrain the gravel, cobbles, and boulders on the channel bed; also known as “full bed mobilization”. Debris floods have transient behaviour over the duration of the event with pulses of sediment laden (i.e., boulders and woody debris) flow and more diluted (i.e., water-like) flows. There is limited guidance in available literature on the hydraulic modelling of debris floods, in particular, Type 2 debris floods (i.e., diluted debris flows). There are several fluid dynamics models that could be used, and several rheologies that can be used to parametrize the flows, however, the complexity of real debris flood behaviour generally needs to be simplified to an equivalent fluid rheology in practice.

Following heavy, prolonged rainfall in southwestern British Columbia, Canada, in mid-November 2021, several road and railway crossings were damaged by hydrogeomorphic hazards and erosion. These events highlight the need for the design and construction of bridge crossings able to withstand hydrogeomorphic hazards for transportation network resiliency. The modelling work described in this study was in support of the design of an armoured channel for a site that was impacted by a debris flood in November 2021.

The proposed crossing is a steep and complex channel geometry, with channel slopes between 8 and 35%. Estimates of, flow depths, velocities, and shear stresses, were required for design. To capture the full effects of the steep and complex geometry of the proposed channel, debris floods were modelled in both two-and three-dimension using HEC-RAS and FLOW-3D, respectively. To provide conservative but realistic design values, multiple debris flood scenarios were modelled with the intention of capturing the range of transient behaviour expected over the duration of a debris flood and evaluate the uncertainties in the model parameterization. The debris flood model parameterization included a high and low mobility Bingham rheological parameters set (e.g. viscosity and yield stress) and modelling the flood as laminar or turbulent flow. The higher mobility turbulent flows are more representative of a flood condition that has a lower sediment concentration during the later stages of a debris flood event, while the lower mobility laminar cases are expected to be more representative of surge fronts with a higher sediment concentration.

Different debris flood cases provided critical design values for different parameters. Generally, the low mobility laminar flow was the most conservative for flow depth. Modelled velocity and shear stress were not only dependent on the debris flood case, but varied within the channel sections between the two and three-dimensional results. Design values were proposed using a percentile of the amalgamated results of all debris flood cases modelled to capture the variation of the modelled cases.

How to cite: Cheff, I., Taylor, J., Mitchell, A., Horita, K., Shepherd, D., Rintoul, S., and Millar, R.: Evaluating uncertainty in debris flood modelling for the design of a steep built channel, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-20781, https://doi.org/10.5194/egusphere-egu24-20781, 2024.

Debris flow phenomena are difficult to predict because their occurrence depends on multiple factors (e.g. availability of sediment, volume, and rainfall intensity, as well as slope conditions before an intense rainfall event, etc.). When they occur, debris flows can significantly modify the topographic surface of conical-shaped fans and cones via erosion and deposition phenomena.
This contribution aims, for the first time, to use a Monte Carlo approach to simulate the most probable avulsion paths of debris flows and define those conveying a certain drainage area with a minimum occurrence probability. We rely upon various digital terrain models (DEM) available for the Fiames area (Cortina d'Ampezzo, BL), covering approximately 1.6 km² , and the average local elevation and the standard deviation of each cell of the domain are evaluated. We then generate N=2000 Monte Carlo realizations of possible topographic (equiprobable) surfaces, according to the geostatistical procedure known as Sequential Gaussian Simulations (SGSIMs). On each topographic surface obtained, the optimal drainage network is extracted using algorithms that guide the flow, by gravity, along the directions of maximum slope, as commonly used in hydrology. It is therefore possible to obtain as many drainage networks as there are simulated topographies. The ensemble of drainage networks (networks) is used to obtain the most probable network, extracted from the average topographic surface among the simulated ones.
Furthermore, we set a threshold on the drainage area variable, thus it is possible to calculate the probability of having, in a domain's location, a flow conveyance (per unit of area) higher than the threshold one. Finally, it is possible to (at least preliminarily) evaluate the probability of existing infrastructure vulnerability by appraising the probability that they are located along possible drainage routes or not. The presented approach moves the analysis of avulsion paths within the probability space. It can be validated by verifying the correspondence between a part of the probable (synthetic) pathways with those that historically occurred. Furthermore, it allows us to define routes and (synthetic) triggering points different from the historical ones.

How to cite: Schiavo, M., Bernard, M., Barbini, M., Boreggio, M., Lopez, S., and Gregoretti, C.: Probabilistic delineation of debris-flow routing pathways in cones and fans within a Monte Carlo framework, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-3898, https://doi.org/10.5194/egusphere-egu24-3898, 2024.

On October 5, 2021, a landslide-debris flow disaster chain occurred suddenly in Hanping village, Shaanxi Province, China. This catastrophic disaster chain damaged 7 houses, 41.9 hectares of arable land and 3 roads and resulted in 1 death. Based on a detailed field investigation of the disaster site, we analyzed the dynamic evolution of the disaster chain by using experimental analysis, unmanned aerial vehicle (UAV) photogrammetry, satellite remote sensing interpretation and the SBAS-InSAR technique and then preliminarily revealed the movement process and causal mechanism of the disaster chain. The results suggested that the first landslide initiated in the upper part of Canger cliff, which is the result of the combined effects of slope structure, earthquake damage, engineering disturbance, and rainfall infiltration. Among them, extreme rainfall events are the primary factors that induce landslides. Before the landslide, InSAR results showed that deformations had already appeared in the source area, and the deformation rate had a strong correlation with precipitation. Then, the potential-to-kinetic transformation effect and air cushion effects generated by the landslide movement in the narrow and steep section of Canger cliff led to the disintegration of the sliding body. With replenished surface runoff, the clastic flow gradually transformed into debris flow. Moreover, due to the dam-breaching effects at bayonets and bends and the entrainment effect of the high-density debris flow along the gully, the scale of debris flow increases gradually, resulting in catastrophic damage during the movement. The findings of this study provide a significant reference and guidance for understanding the chain-generation mechanism of landslide-debris flow disaster chains, as well as informing disaster prevention and mitigation strategies.

How to cite: Zhan, J. and Yao, Z.: Characteristics and mechanism of a catastrophic landslide-debris flow disaster chain triggered by extreme rainfall in Shaanxi, China, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-9638, https://doi.org/10.5194/egusphere-egu24-9638, 2024.