Debris-flow mobilization from shallow landsliding is widespread, can be deadly, and is the focus of warning systems worldwide. One challenge of forecasting is that some shallow landslides transform into rapid debris flows, whereas others do not. Although early warning systems can depend on forecasting this poorly understood transition, high-resolution hydrologic and deformation data from debris-flow mobilization events are rare. As part of the APERIF project (Ochiai et al., Landslides, 2004), we performed a field-scale artificial rainfall (sprinkling) experiment on a planar 33º natural hillslope near Mt. Kaba-san, Japan. Using 100-Hz sampling, we recorded synchronous subsurface pore-pressure response and ground-surface motion throughout the transition from slide (~1 m thick) to flow, including slow precursory deformation, abrupt rapid failure, and subsequent debris-flow motion down a small channel. The experiment began with ~6 hours of high-intensity sprinkling (at 78 mm/hr), which triggered motion of the slide in response to rising positive pore-water pressures. Continued sprinkling led to persistent slow motion of the slide for ~1 hour, until acceleration and abrupt failure. Our data revealed that during the several seconds of rapid failure, pore pressures dropped (indicative of soil dilation) then oscillated greatly in response to deformation, thereby enhancing liquefaction and flow.

An abrupt transition from slow to fast motion in dilative soils can present a mechanical conundrum. Although loose, contractive soils may collapse and liquefy, many hillslope soils are dense and dilate in shear, thereby impeding motion. We explored slide behavior using a 1D model (Iverson, 2005) that fully couples slide motion and pore pressure with evolving shear-zone dilatancy, and utilized measured and estimated parameters from our hillslope experiment. Simulations demonstrated that dilative soils impede motion, as observed initially in our experiment. However, dilatant systems can evolve dynamically through persistent landslide motion driven by prolonged rainfall. When motion-inhibiting dilatant effects are exhausted, our analysis showed rapid acceleration during a swift drop and subsequent increase in pore pressures (within a few sec), as was also observed in the field experiment. This behavior provides a mechanism to mobilize debris flows from shallow landsides in dense hillslope soils. Our results suggest that although high-resolution monitoring might detect precursory motion, forecasting liquefaction and debris-flow genesis is still dependent on soil properties and transient hydrologic conditions.

How to cite: Reid, M. and Ochiai, H.: Abrupt debris-flow mobilization in a hillslope experiment, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-4021, https://doi.org/10.5194/egusphere-egu23-4021, 2023.

Laboratory landslide flume tests provide valuable insights into the mechanics of multi-phase granular flows within highly controlled settings. Past studies have revealed the complex fluid-particle interactions associated with saturated granular flows result in greater mobility than their dry counterparts, being notably faster, further reaching, and experiencing enhanced spreading. The ability to reliably measure the pore pressures at the base of these flows in the laboratory is critical for developing, evaluating, and validating constitutive relationships linking the effects of pore pressure to the mechanisms causing increased mobility. Unfortunately, experience has shown that two identical sensors installed in the base of a landslide flume can yield wildly different responses to the same multi-phase landslide. In this session, we explore an answer to the question “Why is it so difficult to reliably measure the pore pressure at the base of a fast landslide in a laboratory flume test?” using evidence accumulated from ten years of flume testing using the Queen’s University Landslide Flume. In particular, we explore the hypothesis that surface roughness around pore pressure sensor filter elements can influence sensor readings. A unique experimental strategy of simplifying the flow into a single fluid phase is used to validate sensor readings, prior to application in multi-phase flows. Dam-break releases of 600 kg of water at the top of the inclined flume slope are used as a parametric study to provide evidence to support the hypothesis that surface roughness significantly impacts the pore pressure recorded in high velocity flows. These results are then contrasted to observations of releases of multi-phase flows to derive best practices for the reliable measurement of pore pressure in landslide flume tests.

How to cite: Fawley, A., Taylor-Noonan, A., Tauskela, L., Treflik-Body, E., and Take, W. A.: Why is it so difficult to reliably measure the pore pressure at the base of a fast landslide in a laboratory flume test?, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-11999, https://doi.org/10.5194/egusphere-egu23-11999, 2023.

Global warming has led to environmental changes in the alpine Himalayan mountains, with significant glacier retreat, an increase in the area and number of glacial lakes, and an increase in the frequency and scale of glacial lake outburst debris flows, causing significant damage to people and facilities in downstream. In this study, we analyzed the spatial heterogeneity and variations of disaster-forming environments on the north and south areas of the Himalayan, identified the distribution patterns of glacial lake outburst debris flows, and predicted debris flows’ changing trends in the Himalayas. The results demonstrate that the distribution and variations of glaciers and glacial lakes in the Himalayan region have apparent spatial heterogeneity. The Central and South Himalayas are where glaciers and glacial lakes undergo the most dramatic changes. Glacial lakes are widely distributed in the Central Himalayas and southern slopes, with an increase in area and number from 1990-2015. New glacial lakes at higher elevations and alterations in moraine lakes dominate glacial lake variations across the region. Since the 20th century, there have been 249 outbursts of 113 glacial lakes in the Himalayan, Karakorum, and Southeast Tibetan regions, with the majority of outbreaks occurring in the Central and Eastern Himalayas along steep sections of main rivers. In the period 1901-2019, the inflection point for glacial lake outburst hazard is 1966⁺³⁷/_-31 years (median and 95% HDI), and the frequency of glacial lake outbursts proliferates before the inflection point and slowly increases after the break-point; the annual mean temperature changes have opposite trends before and after the inflection point, reflecting the lag effect of glacial lake outbursts on temperature changes. In addition, the measured data were calibrated and down-scaled the future simulated climate prediction data to reveal the spatial and temporal trends of glacial lake outburst debris flow disaster risk under the influence of future climate-causing factors. The annual mean temperature and precipitation in the Himalayas generally exhibited an upward trend in the 21st century, with higher increment speeds of warming and humidification on the northern slopes; Increasing very high and high glacial lake outburst debris flow hazard zones are a consequence of climate change, with a more concentrated distribution in the centre and northwest of the Himalayan Mountains.

How to cite: Zou, Q., Zhou, B., Chen, S., Zhou, W., Jiang, H., and Yao, H.: Glacial lake outburst debris flows in the Himalayas in response to climate change, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-10744, https://doi.org/10.5194/egusphere-egu23-10744, 2023.

On the southeastern Tibetan Plateau, which is an area widely covered by alpine glaciers, two types of debris flow generally occur: glacier-related debris flows (GDFs) and rainfall-related debris flows (RDFs). It is widely accepted that topographic conditions influence debris flow activities; however, few studies have examined the differences between such influence on GDFs and RDFs. This study investigated the GDFs and RDFs in the periglacial area of the Parlung Tsangpo Basin, and calculated 12 geomorphic indexes to reveal the topographic features associated with these two types of debris flow. It was found that lower values in the drainage area, main channel length, and relative elevation occurred in RDFs compared to the GDFs, whereas higher values in the channel gradient, relief ratio, and effective basin area appeared in RDFs. The discrepancy is mainly related to the different topographic and geomorphic shaping of modern glaciers. According to its geomorphological characteristics, the Parlung Tsangpo Basin can be divided into three sections: the upper V-shaped canyon section, middle wide valley section, and lower steep canyon section. The scale and frequency of debris flows in the upstream canyon region are substantially lower than those of debris flows in the downstream canyon region. Moreover, the frequency and scale of RDFs are substantially different to those of GDFs, primarily because of the different geomorphic evolutionary stages of debris flows gullies in different regions.

How to cite: Wang, J. and Zhang, G.: Study on the influence of  the geomorphic  on debris flow activity in the paraglacial zone of the Southeast Tibetan Plateau, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-10888, https://doi.org/10.5194/egusphere-egu23-10888, 2023.

In mountain regions, debris flows are responsible for major damage to infrastructure and many casualties every year. Early Warning Systems (EWSs) based on sensor networks installed along the debris-flow channel have been implemented in some catchments around the world, including the Alps. Detecting the early phase of debris flows would allow expanding the lead time of an EWS compared to the monitoring of channelized flows upstream a vulnerable site. In this study, monitoring data gathered from 2019 to 2022 in the headwaters of the Gadria catchment, eastern Italian Alps, are analyzed. One active channel located at 2200 m a.s.l. was instrumented with a time-lapse video camera, a tipping-bucket rain gauge, and a 4.5-Hz vertical geophone. The dataset includes 5 debris-flow events that propagated from the monitored channel to the basin outlet and a large number of signals produced by other seismic sources (e.g., rockfalls, earthquakes, animals, wind). The peak amplitude, the duration and the frequency content of the seismic signals were analyzed with the support of video images to identify the different seismic sources. Results show that different seismic sources produce signals with different characteristics and that it is possible to discriminate the most intense channel processes by analyzing seismic data only. Adopting an approach similar to the rainfall thresholds, debris-flow and runoff events have been bounded by means of a power relationship between peak amplitude and signal duration. The next step of the research would be the development of an algorithm able to automatically classify the seismic sources and identify intense channel processes that can generate debris flows. A similar approach will be applied to the Blé catchment (Val Camonica, central Italian Alps) to study the triggering mechanisms and dynamics of debris flows and analyse whether the proposed approach is valid in other locations. The analysis of seismic data will be combined with the identification of triggering rainfall thresholds and the analysis of infrasound signals to develop reliable EWSs for debris flows.

How to cite: Ioriatti, E., Coviello, V., Comiti, F., Macconi, P., Reguzzoni, M., and Berti, M.: Detection of debris-flow initiation with seismic techniques for early-warning purposes, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-568, https://doi.org/10.5194/egusphere-egu23-568, 2023.

The surface velocity of debris flows is constantly subject to strong temporal and spatial fluctuations. These are amplified by the pulse-like occurrence of surges throughout the event and by the high variance of the solids fraction. However, continuous information on velocities of multiple consecutive surges within a single debris-flow event with high temporal resolution is rare. In this study, we test a pulse-Doppler (PD) radar over a total torrent length of 180 m. The PD radar utilizes pulsed transmission and provides spatially resolved cells, called range gates, that extend over a width of 20 meters. Doppler velocity spectra composed of velocity classes and echo intensities are obtained at 4 Hz for each range gate. From these, we derived continuous velocity-time data sets. We present PD radar data for three debris flows that occurred on 05.06, 30.06, and 08.09.2022 at the Illgraben creek, Switzerland. The radar data were validated at the first event with a velocity data set obtained from a LiDAR scanner installed at the same location. This novel method collects high-resolution 3D point clouds at 10 Hz. This data was used to derive a high-resolution velocity vector field for one of the events. We isolate a ~ 2x2 m box in the middle of the channel and compare the LiDAR derived velocities at this location to those measured by the PD radar. Our comparison shows a strong correlation between the two data sets, with a coefficient of determination of 0.85. In addition, we note a minor consistent offset in the two velocity data sets of 0.5 m/s. We attribute this to the nature of the different measurement methods and conclude that the two methods may be sensitive to different features of the surface of the flow. However, our results show the high effectiveness and reliability of both methods in debris-flow monitoring. We anticipate that further analysis of the data sets will provide new insights into the geophysical principles of debris flows.

How to cite: Schöffl, T., Aaron, J., Kaitna, R., and Hübl, J.: Application of pulse-Doppler radar and 3D LiDAR for high-resolution velocity measurements of three debris flows at Illgraben (Switzerland), EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-17212, https://doi.org/10.5194/egusphere-egu23-17212, 2023.

Debris flows are a highly hazardous landslide type, and their impact forces, peak discharges and runout distances are dependent on the flow velocity. Knowledge of flow velocities is therefore often required for hazard planning and mitigation, as well as for validating numerical models. One commonly used method for post-hoc estimation of debris flow velocities uses the mudlines left behind by a passing flow as it travels through a bend. The surface inclination derived from these mudlines can be used to estimate velocity based on the forced vortex equation, originally developed for clear water flows and later adapted to debris flows using a correction factor k to back-calculate the flow velocity^1,2:

where R_c is the radius of curvature of the bend, g* is the bed-normal component of acceleration due to gravity, B is the flow width, and Δh is the difference in elevation of the flow surface between the inner and outer bend.

This approach involves some uncertainties, however, such as how best to define the radius of curvature, the influence of roll waves and splashing on the post-event mudlines used to measure the surface inclination, as well as the meaning and appropriate value of the correction factor k. In this study, we first derive a database of superelevation velocity estimates based on pre- and post event UAV data for seven events from the years 2019 to 2021 in the monitored Illgraben torrent in Switzerland. Analysis of this database firstly indicates that the placement of cross-sections for surface inclination measurements is more important than how the radius of curvature is defined due to the large influence of local topography on mudlines. Secondly, the data indicates that the correction factor k increases nonlinearly with decreasing Froude numbers, as has been previously suggested^2,3. The correction factors were back-calculated using eq. 1 and reference velocities from geophone detections of the front arrival, and seemed to range between approximately 1 and 7. We next present a first comparison of these data to surface inclination and radius of curvature values derived from high-resolution 3D LiDAR scanners for one event in the summer of 2022. We use this unique dataset to directly derive the radius of curvature (based on surface velocity vectors) and surface inclination of the flow, as well as the appropriate correction factor.  We compare these values to those derived by the above-mentioned commonly used method based on bend topography and post-event mudlines to assess the efficacy of these methods. This preliminary study thus provides a validation of the superelevation approach and will provide a basis for more in-depth research on this topic.

¹ Hungr, O., Morgan, G.C. and Kellerhals, R., 1984. Quantitative analysis of debris torrent hazards for design of remedial measures. Canadian Geotechnical Journal, 21(4), pp.663-677.

² Scheidl, C., McArdell, B., Nagl, G. and Rickenmann, D., 2019. Debris flow behavior in super-and subcritical conditions. Association of Environmental and Engineering Geologists; special publication 28.

³ Scheidl, C., McArdell, B.W. and Rickenmann, D., 2015. Debris-flow velocities and superelevation in a curved laboratory channel. Canadian Geotechnical Journal, 52(3), pp.305-317.

How to cite: Åberg, A., Aaron, J., Hirschberg, J., de Haas, T., McArdell, B., and Kirchner, J.: LiDAR-based investigation of debris flow superelevation and velocity, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-12134, https://doi.org/10.5194/egusphere-egu23-12134, 2023.

Debris flows are water-laden masses of soil and rock, which are common geological hazards in mountainous regions worldwide. They can grow greatly in size and hazardous potential by eroding bed and bank materials. However, erosion mechanisms are poorly understood because debris flows are complex hybrids between a fluid flow and a moving mass of colliding particles, bed erodibility varies between events, and field measurements are hard to obtain. Here, we combine detailed flow measurements, rainfall data, and high-resolution UAV measurements of channel-bed erosion and deposition for 13 debris flows in the Illgraben (CH), to identify the key controls on debris-flow erosion and deposition. We show that flow conditions and bed wetness jointly control debris-flow erosion. Flow conditions that describe the cumulative forces exerted at the bed over the full event (flow volume, cumulative shear stress, and seismic energy) have the strongest correlations with measured erosion and deposition. However, we also find statistically significant correlations between erosion and deposition and frontal flow properties, including frontal velocity, flow depth, shear stress, and peak discharge. Antecedent rainfall over a period of 2-3 hours prior to the debris-flow events strongly correlates to erosion and deposition, while the correlation decreases in strength and diminishes towards shorter and longer time periods of antecedent moisture. Shear forces and particle-impact forces are strongly correlated and act in conjunction in the erosion process. A shear-stress approach accounting for bed erodibility may therefore be applicable for modelling and predicting debris-flow erosion.

How to cite: de Haas, T., McArdell, B., Nijland, W., Åberg, A., Hirschberg, J., and Huguenin, P.: Flow and Bed Conditions jointly control Debris-Flow Erosion and Bulking, Illgraben (CH), EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-1454, https://doi.org/10.5194/egusphere-egu23-1454, 2023.

In order to calibrate and validate debris flow models, high precision in situ measurements are essential. However, it is quite difficult to acquire detailed information about debris flows, as they only rarely occur during exceptional high precipitation intensities. In July 2022, a series of such high-intensity short-duration precipitation events triggered several debris flows within the area of the Stubai Alps/Austria, which caused severe damage. On the 20^th and 23^rd of July 2022, two of these convective events initiated multiple debris flows on the slopes of the Horlachtal, a side valley of the Oetztal.

These events have been registered by measurements of three different meteorological stations and four different discharge gauges distributed over the study area. In addition, INCA (Integrated Nowcasting through Comprehensive Analysis) rainfall data provided by ZAMG (Austrian Central Institute for Meteorology and Geodynamics) allow insights in the spatial and temporal characteristics of the rainfall patterns. Furthermore, two airborne LiDAR (Light Detection and Ranging) campaigns of the Chair of Physical Geography at the University of Eichstätt-Ingolstadt covering the whole Horlachtal (about 55 km²) provide detailed pre and post event topographical data.

A combined evaluation of the different data sets allows us to characterise the debris flow events in the study area in great detail. Topographical analyses show that a total number of 156 debris flows were triggered with accumulation volumes up to 40.000 m³. These volumes can be related to the individual catchment areas in combination with precipitation intensities. Furthermore, the spatial distribution of the triggered debris flows show a concentration to a certain region within the study area, which relates to the spatial patterns of the precipitation events.

How to cite: Rom, J., Haas, F., Hofmeister, F., Heckmann, T., Altmann, M., Fleischer, F., Pfeiffer, M., and Becht, M.: Quantitative evaluation of the debris flow events in July 2022 in Horlachtal/Austria by combining different data sets, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-6251, https://doi.org/10.5194/egusphere-egu23-6251, 2023.

Debris flows initiate by a critical combination of abundant sediment, steep inclination, and water. The latter is mostly provided by rainfall that can lead to landslides at the hillslope or along the channel and/or erosion and bulking of sediment due to increased runoff. Location of sediment sources and channel recharge are related to short- and long-term geomorphological processes within the watershed. Up to now, there are only a few studies investigating sediment dynamics in high alpine watersheds that are regularly affected by debris flows. In this contribution we report of our ongoing efforts to monitor sediment dynamics and debris-flow activity in three very different watersheds in the Austrian Alps. We use a combination of remote sensing and in-channel monitoring techniques including UAV, air-borne and terrestrial laser scanning before and after debris-flow events. We find that debris-flows frequency and volumes are strongly related to movement rates of landslides present in the watershed. At high movement rates, most of the channel refill occurs within the time scale of hours. In the absence of active landslides, debris-flow activity is limited by rainfall-triggered embankment failures along the channel and continuous transfer of hillslope sediment into the channel. In the steepest and smallest monitored watershed, active landslides and continuous surface erosion from landslide scars leads to a high frequency of debris flows of all magnitudes, even in the absence of rainfall. Our study shall provide the basis for a more complete modeling framework for a better prediction of debris flows now and in a future climate. 

How to cite: Kaitna, R., Aigner, P., Bernardi, T., Wagner, P., Kuschel, E., Zangerl, C., Hrachowitz, M., and Sklar, L.: Sediment dynamics related to the triggering of debris flows in different alpine watersheds, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-8751, https://doi.org/10.5194/egusphere-egu23-8751, 2023.

Active volcanic environments, due to their rapid processes of growth and deformation, can be considered a typical setting for the formation of both subaerial and subaqueous gravitational instabilities. Occasionally, such conditions - coupled with heavy rainfalls or earthquakes - can trigger sudden and rapid mass movements such as mud-debris flows and/or debris avalanches, representing serious hazards for human settlements.

The active volcanic island of Ischia is localized within the Gulf of Naples and its complex volcanism began prior to 350ka and continued, with centuries to millennia of quiescence, until the last eruption occurred in the historical period (AD 1302). 56ka a caldera-forming eruption occurred, followed by a resurgence process that has affected the caldera floor and generated a net uplift of about 900m since 33ka. Thus, the main morphostructural feature is the Mt. Epomeo resurgent block.

From a geological point of view, the island is composed of volcanic rocks, epiclastic deposits, and terrigenous sediments, demonstrating an alternation of constructive and destructive phases. In this context, the continuous occurrence of landslides on the island, even causing death to humans, is historically documented. In the last decades, the risk has been greatly exacerbated by the high level of human exposure due to not properly planned urban development.

On November 26, 2022, as a consequence of heavy rainfall, diffuse landslide events occurred along the island, the main of which affected a small catchment basin in the vicinity of an urban settlement within the Casamicciola Municipality. The area affected by the landslide events is located along the northern slope of Mt. Epomeo. On the summit, Mt. Epomeo is characterized by the strongly weathered Green Tuff Formation. The generated debris flow affected about 30 buildings causing 12 casualties, 5 injured, 230 people displaced, and severe damages to roads and properties. It should be pointed out that the maximum occurred rainfall - recorded by the local weather station - for durations from 1 to 24 hours are all higher than the corresponding maximum values recorded in the years 2007÷2021.

The present work shows the first outcomes, in terms of landslide types, volumes, extent, etc., from a preliminary multidisciplinary investigation carried out immediately following the event by using desk research, on-field survey, geomorphological mapping, remote sensing, and numerical modeling.

How to cite: Romeo, S., Bonasera, M., Chiessi, V., D'Angiò, D., Fraccica, A., Olivetta, L., Perrotti, M., Roma, M., Trigila, A., Vitale, V., and Amanti, M.: Investigation and preliminary assessment of the Casamicciola landslide in the island of Ischia (Italy) on November 26, 2022, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-6690, https://doi.org/10.5194/egusphere-egu23-6690, 2023.

Climate change is forecasted to result in more frequent and intense storms, which in turn are likely to cause more flash floods and other hazardous processes in steep hilly and mountainous catchments. These flash floods are driven by complex and rapid overland flow responses to intense rainfall across these catchments. Where loose slope or valley-based deposits are available, flood water may mobilise these materials and transform into dynamic high-velocity, high-density debris flows that can pose significant threats to people, property, and infrastructure considerable distances away from the areas where these deposits are mobilised, exacerbating the already devastating situation caused by flooding. Hydro-dynamic models solving the full shallow water equations (SWEs) have shown great potential to reliably simulate the dynamics of overland flows and flash floods at catchment scales. However, simulating the transition from flash flood into debris flow is still technically challenging because of the difficulty of simulating erosion and deposition processes robustly. A reason is that the commonly used method for calculating erosion and deposition rate may suffer from singularity in the presence of vanishing velocity, which poses a major challenge for practical applications. In this work, we have developed a novel integrated hydrodynamic model for simulating flash floods and debris flows. Overland flows, change of debris concentration and bed elevation change are simulated simultaneously to model the transition between flash flood and debris flow. The overland flow processes are simulated by solving the full SWEs using a Godunov-type finite volume method. A novel method for calculating erosion and deposition rates is incorporated into the SWEs-based model to simulate the change of debris concentration and bed elevation change. The new method can maintain numerical stability and accuracy even in the presence of vanishing velocity. Therefore, the new model can effectively simulate the full process of rainfall-runoff-flooding turning into debris flows. Satisfactory simulation results have been obtained for both laboratory-scale and real-world test cases. The new model has the potential to be applied for flash flood/debris flow risk assessment and early warning.

How to cite: Xia, X., Jarsve, K., Dijkstra, T., Liang, Q., and Meng, X.: An integrated hydrodynamic model for flash flood and debris flow simulations, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-5869, https://doi.org/10.5194/egusphere-egu23-5869, 2023.

Climate change-induced geohazards pose significant threat to the sustainability and serviceability of built environment. Among such disasters debris flows are prominent in hilly areas and pose threat to life and property all over the world. Debris flows are coupled geo-hydro-mechanical phenomena with high flow velocity and long runout distance, resulting in a large impact force on the associated built environment. For effective hazard mitigation it is crucial to investigate the dynamic impact of debris flows on structures. It has been established that barriers in the way of debris flow helped to reduce the energy of the flow, leading to a lesser impact on the downstream end. As such many studies have focused on installing barriers at varied locations and of various sizes. However, there is still need for innovative research on how to increase the performance of these barriers. In this study an investigation to evaluate the impact on a structure on downstream due to debris flows is carried out. Besides, the implications of introducing a barrier structure with passages on upstream end is also studied. Smoothed Particle Hydrodynamics (SPH), a mesh-free Lagrangian method, is employed to capture the motion of debris flow and its impact on a rigid structure. For this study the authors have considered the Rishiganga river valley of Uttarakhand state in India, where a recent event of debris flow on February 07, 2021, caused large destruction to important facilities including the hydroelectric power plant. Located in the southern part of the Himalayas, this region is geo-morphologically sensitive and seismically active, making it susceptible to frequent events of landslides, debris flows and other mass movements. Three dimensional (3D) analyses are carried out for three different cases: case1, with no barrier structure on the upstream, case 2, where a barrier structure with one large passage has been placed and case 3, where a barrier structure with two passages has been placed on the upstream. Based on the outcomes, it is inferred that the presence of a rigid structure at the upstream end reduces the impact on the downstream structure considerably. The impact is found to be highest for case 1, followed by cases 2 and 3, with impact values which are only 35% and 30% of case 1, respectively. Similar trend is found in the velocity gradient at a location between the barrier and the main structure. After the introduction of barrier structure, there is a decrease of approximately 10% in maximum velocity for case 2 and a drastic decrease of approximately 90% for case 3 as compared to case 1, showing consistency with the impact values. It is established, that the introduction of passages decreases the impact considerably owing to the decrease in velocity as well as the volume of debris reaching the main structure, because of some accumulation behind the barrier. Moreover, increasing the number of passages, while keeping the passage area constant, causes the flow to become more streamlined, hence making the flow more uniform, which leads to a further reduction in impact forces.

How to cite: Mubarak, N. and Kumar, R.: Mitigating the Impact of Debris Flows on the Built Environment: A Case Study of Southern Himalayas, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-2407, https://doi.org/10.5194/egusphere-egu23-2407, 2023.

Vibrating boundaries are widely encountered, for example, between soil and bedrock during earthquake shaking. We understand that vibration of such boundaries can lead to instabilities in granular media with many applications to geological hazards, such as liquefaction and landslides, and during geological engineering applications. Although numerous studies have been dedicated to revealing the behavior of granular flows under various flowing regimes, the significance of vibrating boundaries remains an open problem. To fill this gap, we introduce a vibrating base boundary into the collapse of a granular column with a numerical scheme. To understand the role of fluids, we contrast the behavior of granular flows under dry and fluid-saturated conditions. From the simulations, the development of anisotropy in spatial inter-grain contact force distribution is studied. The fluid-saturated condition is achieved via a two-way coupled CFD-DEM method. From these simulations, a scaling law of granular flow is derived for vibrating boundaries. We illustrate for the first time the energy evolution of the granular system with vibrating boundaries. This work demonstrates the role of vibration in increasing the runout distance and the maximum kinetic energy of granular flows, this suggests a link between the mesoscale inter-grain responses and macro-scale dynamics of granular geological hazards triggered by earthquakes. Additionally, the spatial distribution of inter-grain contact forces is presented under dry and fluid-saturated conditions to indicate the anisotropic development inside the granular assembly.

How to cite: Wang, Y., Mao, W., and Rafn Heimisson, E.: On the effect of vibration on dry/fluid-saturated granular flows: Implications for geological hazards induced by earthquakes, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-8143, https://doi.org/10.5194/egusphere-egu23-8143, 2023.

Flowslides, which occur mainly in shallow granular soils resting on bedrock, are among the most dangerous natural hazards to humans and utilities in both mountainous and volcanic areas. These phenomena are strongly controlled by the stratigraphic and topographic characteristics of the slope and the groundwater regime, which are commonly recognized as predisposing factors for the occurrence of flow-like landslides. Therefore, the study of the spatial variability of the local geological setting and hydrogeological conditions in partially saturated slopes is of fundamental importance for the prediction of flowslide events. In this framework, we propose a procedure based on 3D time-lapse electrical resistivity tomography imaging of the slope, integrated with geotechnical numerical modelling of hydraulic phenomena affecting the land cover, to analyse the effects of the stratigraphic variability in terms of geometry, continuity, and thickness of the soil horizons on the groundwater regime over time. The goal of the proposed approach is to set up an effective tool for predicting debris-flow landslides occurrence at the slope scale, thereby increasing the predictive capacity of early warning systems. The proposed multidisciplinary study was applied to the test site on Mt. Faito, in the northernmost part of the Lattari Mountains (Naples, Southern Italy), where loose pyroclastic deposits from the explosive eruptions of the nearby Somma-Vesuvius volcano cover a karst-fractured carbonate bedrock with a natural slope more than 30° steep. Specifically, seasonally repeated 3D electrical resistivity tomography surveys, suitably complemented with geological and geotechnical investigations, were carried out to determine the electro-stratigraphic and geological setting of the pyroclastic cover, the local morphology and physical conditions of the underlying carbonate bedrock, and the saturation degree distribution on a seasonal time scale. The latter was estimated through the resistivity vs. water content characteristic curves of the different soil horizons, which were obtained from laboratory measurements on specimens sampled in the survey area. The maps of the water content distribution within the pyroclastic cover, determined by the repeated field resistivity surveys, were validated by comparison with those obtained from 2D geotechnical numerical modelling aimed at simulating hydraulic phenomena affecting the soil cover. As main findings, the integrated approach showed that i)  the buried bedrock morphology heavily influences the pore water distribution in the soil cover and ii) ashy material fills the upper karst portion of the bedrock, providing a hydraulic connection of the water flow infiltrating from the topsoil downward.

How to cite: Salone, R., Pirone, M., De Paola, C., Forte, G., Santo, A., Urciuoli, G., and Di Maio, R.: Integration of geophysical and geotechnical modelling to define hydrogeological conditions for unsaturated soils prone to shallow flowslides, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-8083, https://doi.org/10.5194/egusphere-egu23-8083, 2023.

FLATModel (Medina et al. 2008) was developed as a modeling tool for debris flow (DF) events. It includes several specific DF features, among them, a simple erosion mechanism was proposed and implemented. The previous validation of this model capability was based on event volume comparison. Now, a more detailed analysis has been performed, and several constraints in its applicability emerge. Model results for erosion present an important dependence on numerical parameters configuration, hence, not only the physical parameters calibration is required. Accuracy in erosion modeling requires proper numeric setup resulting in high computational effort, compromising the model performance. New specific algorithm or erosion computation approach is required.
A part from these numerical aspects, we present some results obtained in the Rebaixader torrent (Central Pyrenees), where a debris-flow monitoring system is installed and annual UAV-surveys are performed.

Medina, V., Hürlimann, M., Bateman, A. (2008) FLATModel: 2D finite volume code for debris-flow modelling. Application to different events occurred in the Northeastern part of the Iberian Peninsula. Landslides, 5, 127-142. https://doi.org/10.1007/s10346-007-0102-3

How to cite: Medina, V., Hürlimann, M., and Molano, L.: Drawbacks in modeling debris flow erosion using the shallow water equations and the finite volume method. Examples from FLATModel and the Rebaixader torrent, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-16507, https://doi.org/10.5194/egusphere-egu23-16507, 2023.

Extreme climate events (e.g., extreme rainfall and glacier melting) in alpine areas can result in significant runoff or mass movements (e.g., flash floods and debris flows) that pose substantial threats to human life and infrastructure downstream. However, due to sparse measurements, understanding the sediment transport mechanisms that control these processes still needs to be completed. Scaling laws and dimensionless numbers provide valuable insights into complex physical systems and processes. Several dimensionless numbers (e.g., Einstein number and Savage number) have been proposed to investigate the relative importance of different sediment transport-related stresses for different systems or processes. In this study, we propose a new data-driven approach that embeds the principle of dimensional analysis in an unsupervised machine learning scheme to discover the best combination of dimensionless numbers that can describe sediment transport mechanisms in torrent processes. We reduce high-dimensional parameter spaces (12 dimensions) to descriptions involving only a few (about 3-4) physically interpretable dimensionless numbers. Using a unique field dataset, we demonstrate this idea to investigate the transition in different transport mechanisms. The result is generally applicable criteria that can improve existing classification models and aid in developing appropriate hazard assessments in mountainous regions based on scarce hydrologic measurements. 

How to cite: Tang, H.: Data-driven discovery of dimensionless numbers for extreme flow, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-5870, https://doi.org/10.5194/egusphere-egu23-5870, 2023.

Typhoon debris flows are recurrent phenomena with a high capacity to cause significant economic and life loss in the coastal areas. Accurately predicting the movement process and determining the potential zones and risk assessment are crucial to design mitigation strategies and to reduce societal and economic losses. In this study, the Wangzhuangwu (WZW) gully was chosen as the study object, which once broke out a debris flow induced by the Typhoon Likima on 10 August 2019. First, a detailed field investigation and interpretation of remote sensing imagery were carried out to study the trigger mechanism and quantify the characteristics of the debris flow. Second, the movement and deposition process of the 2019 WZW debris flow were reconstructed based on the Soil Conservation Service-curve number (SCS-CN) approach and a two-dimensional finite model (FLO-2D PRO model). The debris flow inundation and evolutionary trajectory were shown to be reasonably comparable with historical debris flows. Then, the potential hazard zones of debris flows with different recurrence intervals were determined based on the validated rheological parameters. Here we established a two-factors model that couples maximum flow depth with momentum to classify the hazard zones. Finally, we calculated the vulnerability distribution and economic risk of the buildings with different recurrence intervals based on a quantitative risk formula. This study provides a complete and efficient mean to determine the values of debris flow parameters and to implement a hazard and risk assessment based on numerical simulation. This proposed approach efficiently generated a debris flow risk distribution map that can be used for effective disaster prevention in the debris flow-prone areas.

How to cite: Wang, T., Yin, K., Li, Y., Du, J., Gui, L., and Guo, Z.: Reconstruction and risk predication of a typhoon-triggered debris flow via numerical simulation: A case study of Zhejiang Province, SE China, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-5593, https://doi.org/10.5194/egusphere-egu23-5593, 2023.