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NH3.5

Debris flows are among the most dangerous natural hazards that threaten people and infrastructures in both mountainous and volcanic areas. The study of the initiation and of the dynamics of debris flows, along with the characterization of the associated erosion/deposition processes, is of paramount importance for hazard assessment, land-use planning and design of mitigation measures, including early warning systems.
A growing number of scientists with diverse backgrounds are studying debris flows and lahars. The difficulties in measuring parameters related to their initiation and propagation have progressively prompted research into a wide variety of laboratory experiments and monitoring studies. However, there is a need of improving the quality of instrumental observations that would provide knowledge for more accurate hazards maps and modeling. Nowadays, the combination of distributed sensor networks and remote sensing techniques represents a unique opportunity to gather direct observations of debris flows to better constrain their physical properties.
Scientists working in the field of debris flows are invited to present their recent advancements. In addition, contributions from practitioners and decision makers are also welcome. Topics of the session include: field studies and documentation, mechanics of debris-flow initiation and propagation, laboratory experiments, modeling, monitoring, hazard and risk assessment and mapping, early warning, and alarm systems.

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Co-organized by GM3/HS13
Convener: Marcel Hürlimann | Co-conveners: Velio CovielloECSECS, Xiaojun Guo, Roland Kaitna
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| Tue, 05 May, 14:00–18:00 (CEST)

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Chat time: Tuesday, 5 May 2020, 14:00–15:45

Chairperson: Velio - Xiaojun (Marcel - Roland)
D1975 |
EGU2020-5546
| Highlight
Lorenzo Marchi, Massimo Arattano, Marco Cavalli, Federico Cazorzi, Stefano Crema, and Sara Cucchiaro

Debris-flow research requires experimental data that are difficult to collect because of the intrinsic characteristics of these processes. Both post-event field observations and monitoring in instrumented channels are suitable to collect debris-flow field data, even if with different resolutions and purposes. Monitoring in instrumented channels enables recording data that cannot be gathered by means of post-event surveys in ungauged channels. Extending the monitoring activities over multidecadal time intervals increases the significance of collected data because longer time series permit recognizing changes in debris-flow response as a consequence of changes in controlling factors, such as climate, land use, and the implementation of control works.

This paper presents debris-flows data recorded in the Moscardo Torrent (eastern Italian Alps) between 1990 and 2019. As far as we know, the Moscardo Torrent basin was the first catchment equipped with permanent instrumentation for debris-flow monitoring in Europe. The monitoring activities in the Moscardo Torrent began in 1989-1990 and still keep on, although with some gaps due to the implementation of control works in the instrumented channel (1998-2000) and the obsolescence of the instrumentation between 2007 and 2010.

Thirty debris flows were observed between 1990 and 2019; 26 of them were monitored by sensors installed on the channel (at two measuring stations for most events), while four debris flows were documented by means of post-event observations. Monitored data consist of debris-flow hydrographs, measured by means of ultrasonic sensors, and rainfall. Debris flows in the Moscardo Torrent occur from early June to the end of September, with higher frequency in the first part of summer.

This contribution presents data on triggering rainfall, flow velocity, peak discharge and volume for the monitored hydrographs. The relatively large number of debris-flow events recorded in the Moscardo Torrent has permitted to recognize the main characteristics of the debris-flow hydrographs. We used the data related to duration and the maximum depth of the debris-flow surges to define triangular hydrographs related to different event severity. Simplified triangular hydrographs show the distinctive features of debris flows (short total event duration and very short time to peak) and can help defining realistic inputs to debris-flow propagation models. A more detailed representation of hydrographs shape was achieved by averaging the recorded hydrographs of debris-flow surges. This analysis was performed on the debris flows recorded between 2002 and 2019: data for 12 surges for each of the two flow measuring stations were available. Dimensionless hydrographs were generated normalizing the flow depth by its maximum value and the time by the total surge duration. Flow peaks were aligned to preserve the sharp shape that is a distinctive feature of debris-flow hydrographs. Finally, the ordinates were averaged, and mean debris-flow hydrographs were obtained.

Debris-flow data collected in the Moscardo Torrent dataset could contribute to further analysis, including the comparison of triggering rainfall and flow variables with those recorded in other basins instrumented for debris-flows monitoring under different climate and geolithological conditions.

How to cite: Marchi, L., Arattano, M., Cavalli, M., Cazorzi, F., Crema, S., and Cucchiaro, S.: Debris-flow data collected in the Moscardo Torrent (eastern Italian Alps) between 1990 and 2019, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5546, https://doi.org/10.5194/egusphere-egu2020-5546, 2020

D1976 |
EGU2020-1614
Tjalling de Haas, Wiebe Nijland, and Brian McArdell

Debris flows can grow greatly in size and hazardous potential by eroding bed material, but effective hazard assessment and mitigation is currently hampered by limited understanding of erosion and deposition dynamics. We have collected high-resolution pre- and post-flow topography with drone-based photogrammetry in the Illgraben channel in the Swiss Alps. We present erosion and deposition patterns as a result of six debris flows and intensive subcatchment activity over a 3.3 km long unconsolidated reach with check dams, and interpret these erosion and deposition patterns with in-situ flow measurements. We show that the spatio-temporal patterns of erosion and deposition in natural debris-flow torrents are highly variable and dynamic. We identify a memory effect where erosion is strong at locations of strong deposition during previous flows and vice versa. Large sediment inputs from subcatchments initially result in new channel erosion through the subcatchments deposits and at the same time upstream deposition as a result of backwater effects. It is generally believed that erosion increases with debris-flow magnitude, but we show that there is a limit to debris-flow bulking set by channel geometry. Large flows that overtop their channel deposit large amount of sediment in levees and on overbanks, leading to net deposition despite strong thalweg erosion, and thus a decrease in flow volume. These findings provide key guidelines for flow volume forecasting, emphasizing the importance of memory effects and the need to resolve both erosion and deposition for accurate flow volume estimation.

How to cite: de Haas, T., Nijland, W., and McArdell, B.: Spatio-temporal patterns of debris-flow erosion and deposition in the Illgraben torrent, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1614, https://doi.org/10.5194/egusphere-egu2020-1614, 2019

D1977 |
EGU2020-21300
| Highlight
Carolin Kiefer, Michael Krautblatter, Christoph Mayr, Patrick Oswald, and Michael Strasser

Debris flows represent a widespread geomorphological hazard in mountainous regions. Understanding the long-term dynamics of debris flow activity in view of climate change is crucial for the prevention and mitigation of future events. The activity of debris flows is evidently linked to the magnitude of rainstorms. Dietrich & Krautblatter (2017) found an increase in debris flow volumes after 1980 by a factor of 2 compared to the period 1947-1980 and by a factor of 3 compared to the mean Lateglacial/Holocene debris flow volumes by investigating aerial photos of the surroundings of lake Plansee (Reutte, Austria) and estimating debris flow cone volumes with geophysical methods.

In this study, the terrestrial observations of increasing debris flow volumes were compared with the subaquatic deposits from the deepest basin of the lake. The debris flow volume within a three-month period on a large debris cone was monitored by Terrestrial Laserscanning (TLS) and the debris flow activity over the last 3 600 years was reconstructed using sediment cores. Four short cores of up to 145 cm depth were recovered in a transect from the shallow subaquatic debris cone area to the deepest basin of the lake. The grain size, density, Magnetic Susceptibility as well as the d13-C, d15N- and C/N-ratios of the sediment were analyzed.

The Terrestrial Laserscans revealed a sediment delivery ratio of 30% for the steep debris cone bordering the lake. In the four correlated short cores, 52 debris flow events were differentiated within the last 3 600 years of sedimentation. The proportion of event layers in the cores ranges between 34% and 57% of the total section thickness. The sedimentation rates from a dated core confirm the increase of debris flow activity that was observed with terrestrial methods by Dietrich & Krautblatter (2017). The sedimentation rates show an 11-fold increase after 1930 compared to the rates before 1930 and a 5-fold to 12-fold increase compared to the average Holocene sedimentation rates in lake Plansee. Three types of event deposits were distinguished according to sedimentological criteria: flood-triggered debris flows, earthquake-induced subaquatic suspension flows and mega-events. The TOC/TN ratios of the sediment reveal a permanent influence of terrestrial carbon on the lake sediment and a mixed source of allochthonous and autochthonous organic matter. Large debris flow events can be distinguished from background sediments by increased d13C isotope ratios.

The results of this study reveal further scientific proof for the increase of debris flow activity in conjunction with increasing rainstorm activity. Here we show one of the first long-term archives of debris flow activity in the Northern Alps spanning the last 3 600 years and revealing cyclic shifts in debris-flow transport volumes by one order of magnitude.

How to cite: Kiefer, C., Krautblatter, M., Mayr, C., Oswald, P., and Strasser, M.: The Influence of Debris Flow Activity on the Sediment of the Lake Plansee over 3.6 ka (Tyrol, Austria), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21300, https://doi.org/10.5194/egusphere-egu2020-21300, 2020

D1978 |
EGU2020-11542
William Kane, Mallory Jones, and Lesley Firestein

In January 2018, deadly debris flows swept through the community of Montecito, California. The Santa Ynez Mountains above the town had been denuded by the Thomas Fire in Fall 2017. Twenty-three people were killed by the massive boulders and debris that swept down from the canyons during an intense rain event.

After the disaster, residents realized that waiting for protection from various agencies would take too long. Instead, they banded together and formed a non-profit organization, The Partnership for Resilient Communities (TPRC), to raise private funds and construct protective measures.
      
Research indicated that flexible debris nets designed by experienced geoengineers would be the quickest and most environmentally sound approach. Geobrugg AG, Romanshorn, Switzerland was selected as the supplier of the nets since it has a long research and implementation background. KANE GeoTech teamed with Access Limited Construction, Oceano, California to actualize the project. KANE and Access have designed and installed more Geobrugg debris nets in North America than any other firms and thus were the natural choices for this fast-track project.

Money was raised from private donors and nets constructed. Once nets were installed the purpose of TPRC had been realized. However, much knowledge was gained on aspects of permitting, locating, engineering, and constructing the nets. A new organization, Presilience Partners, was formed. The organization is composed of members of the TPRC, KANE GeoTech, Geobrugg AG, Access Limited, and the University of California, Santa Barbara. Its mission is to take the lessons learned and develop a response protocol for protection from future wildfire/debris flows.

This presentation will review the innovations developed in the past and describe ongoing work.

How to cite: Kane, W., Jones, M., and Firestein, L.: Development of a Post-Wildfire Response to Debris Flow Protection, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11542, https://doi.org/10.5194/egusphere-egu2020-11542, 2020

D1979 |
EGU2020-12809
Yanchao Gao, Songjiang Zhao, Jiazhu Wang, and Wei Xu

Strong earthquakes often induce a substantial rise in secondary geohazards. This problem has been studied more after the Great Kanto Earthquake in Japan and the Chichi Earthquake in Taiwan. In western China, after the 2008 Mw7.9 Wenchuan earthquake, large-scale regional debris flows occurred in 2008, 2009, 2010, 2011, 2013, 2014, and 2019 in the strong earthquake zone. Many control projects have been constructed, including more than 1,000 check dams. Part of the projects were damaged in the subsequent large debris flows. Debris flow after the earthquake is characterized by many loose sources, high frequency and large magnitude. Traditional design parameters and control engineering cannot meet disaster prevention requirements. In the 11 years after the Wenchuan earthquake, our research team continued to investigate the formation of the debris flow in the earthquake area, and summarized the reasons for the failure of the control projects, such as the low estimate of the loose sources and the insufficient design capacity of the check dam. In response to the above problems, we have proposed corresponding solutions, including the optimal combination of different control measures, the design of the dam site and storage capacity, and the structural form of the check dam. This optimization concept has been applied in debris flow prevention such as Qipan gully and Shaofang gully and has achieved good control results. The research provides a reference for subsequent disaster prevention and mitigation in similar earthquake areas.

How to cite: Gao, Y., Zhao, S., Wang, J., and Xu, W.: Key Problems on Debris Flow Control Engineering after Wenchuan Earthquake in China, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12809, https://doi.org/10.5194/egusphere-egu2020-12809, 2020

D1980 |
EGU2020-17017
Jacob Hirschberg, Simone Fatichi, Georgie Bennett, Brian McArdell, Stuart Lane, and Peter Molnar

Debris flows are rapid mass movements composed of a mixture of water and sediments and often pose a danger to humans and infrastructure. In the Alpine environment, they are mostly triggered by intense rainfall, snowmelt or a combination thereof, and conditioned by sediment availability. Their occurrence is expected to increase in a warmer climate due to changes in the hydrological regime (e.g. higher rainfall intensity, lower duration of snow cover). Furthermore, sediment production is likely to accelerate due to permafrost thawing and changes in freeze-thaw cycles, resulting in increased sediment availability. For the purpose of climate change impact assessment on sediment yield and debris-flow activity, interactions and feedbacks of climate and the aforementioned processes need to be considered jointly.

In the study presented here, we address this challenge by forcing a sediment cascade model (SedCas1) with precipitation and temperature from a stochastic weather generator (AWE-GEN2) producing ensembles of possible climate in the present and for the future. The chosen study site is the Illgraben, a debris-flow prone catchment in the Swiss Alps which currently produces 3-4 debris flows yearly on average. SedCas conceptualizes a geomorphic system in which hillslopes produce and store sediments from landslides and eventually deliver them to the channels. From there, sediments can be mobilized by concentrated surface runoff and transferred out of the catchment in form of bedload, hypreconcentrated flow, or debris flows, depending on the surface runoff magnitude and the sediment availability. AWE-GEN operates at the hourly scale and is trained for the current climate with observed data and for the future climate using the newest climate change projections for Switzerland CH2018 developed by the National Center for Climate Services3.

Preliminary results reveal a likely increase in debris-flow occurrence in the Illgraben in the future. Such an increase can be attributed to an extension in the debris-flow seasonal changes in the discharge regime. Furthermore, the number of landslides filling the sediment storage increases because they are affected by a shorter duration of snow cover and thus greater exposure to freeze-thaw weathering. However, projections are subject to large uncertainties, stemming not only from uncertainty in climate scenarios, but also from internal climate variability. Furthermore, the simplified hillslope weathering and debris-flow triggering mechanisms contribute to the overall uncertainty. Nevertheless, the methodology is thought to be transferable to any sediment-cascade-like catchment where dominant processes are driven by climate. Lastly, this work highlights the importance of considering stochasticity in climate and sediment history for projections of magnitudes and frequencies of relative rare events as debris flows. This allows us to explicitly separate climate change signals in geomorphic processes from fluctuations induced by internal natural variability.

REFERENCES

1 Bennett, G. L., et al. "A probabilistic sediment cascade model of sediment transfer in the Illgraben." Water Resources Research 50.2 (2014): 1225-1244. doi: 10.1002/2013WR013806

2 Fatichi, S., et al. "Simulation of future climate scenarios with a weather generator." Advances in Water Resources 34.4 (2011): 448-467. doi: 10.1016/j.advwatres.2010.12.013

3 CH2018 - Climate Scenarios for Switzerland. National Centre for Climate Services (2018): doi: 10.18751/Climate/Scenarios/CH2018/1.0

How to cite: Hirschberg, J., Fatichi, S., Bennett, G., McArdell, B., Lane, S., and Molnar, P.: Climate change impacts on sediment yield and debris-flow activity at the Illgraben, Switzerland, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-17017, https://doi.org/10.5194/egusphere-egu2020-17017, 2020

D1981 |
EGU2020-21379
| Highlight
Rong Chen, Peng Cui, Shengnan Wu, and Rongzhi Tan

The frequent occurrences of mountain disasters have posed a huge threat to the safety of life and property of settlement residents, which bring serious challenges to the post-disaster reconstruction and sustainable development of the affected area, especially in countryside resort areas. The countryside resort areas are populated with tourists whose risk perception and risk behaviours against mountain hazards are unpredictable, which has made the evacuation difficult or even worsened the situation when mountain hazards occur. How to evacuate evacuees to safety in mountain disasters is an important issue for disaster emergency management. By far, little attention has been given to emergency evacuation during mountain disasters in China. Based on mountain disaster events from 2008 to 2019, and 1385 households samples that obtained by stratified random sampling and questionnaire survey, this study has proved ‘Public Participation Monitoring and Warning System’ (PPMWS) is an essential tool to reduce related deaths. Furthermore, the roles and interfaces of different stakeholders in emergency evacuation process are discussed for the purpose to find out the unforeseen circumstances and vulnerable spots. The results show that the farmhouse owners and monitoring personnels play the key roles in emergency evacuation process. The evacuation model led by monitoring personnels is summarized and feasible measures to reduce risks and casualties of mountain disasters are proposed and applied in Longmenshan Town, Pengzhou, Sichuan. The results of this study will improve the efficiency of evacuation and provide scientific support for mountain disaster risk management in mountainous area.

How to cite: Chen, R., Cui, P., Wu, S., and Tan, R.: Community-based Mountain Disaster Risk Management during Emergency Evacuation Process: Evidence from China, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21379, https://doi.org/10.5194/egusphere-egu2020-21379, 2020

D1982 |
EGU2020-18219
| Highlight
Theresa Frimberger, Franziska Petry, and Michael Krautblatter

Lahars rank as one of the most destructive hazards at Cotopaxi volcano (5897 m asl) due to the presence of a massive glacier cap, the frequency of eruptions and the high population density in the surrounding, potentially inundated valleys. In 1877, Cotopaxi experienced the last major VEI 3-4 eruption, producing syneruptive lahars of 60-100 million m3 that travelled hundreds of km downstream.  Few lahar simulations based on empirical or fluid dynamic approaches exist for Cotopaxi, but here we introduce a calibrated numerical debris flow model capable of reproducing confluence and erosivity of flows.

In this study, we back-calculate the well documented 1877 lahar event using the 2D debris flow model RAMMS, which is based on the Voellmy-Salm friction approach and includes an entrainment algorithm. We first evaluate the sensitivity and range of possible model input parameters by systematically varying model inputs for release volume, density and frictional resistance (Coulomb type friction μ [-] and turbulent friction ξ [ms-2]). Supported by a probabilistic analysis, we find that a choice of historical and field-derived calibration metrics of the 1877 lahar event along the northern lahar trajectory can well constrain most likely input parameters for frictional resistance. Our results show that modelling large-scale primary lahars at Cotopaxi is strongly controlled by very small values for Coulomb friction μ (0.005-0.015). Finally, we apply the calibrated model to typical eruption scenarios of Cotopaxi (VEI 1 to >4) in order to enable a realistic lahar hazard representation.

Considering the rapid rise of the equilibrium-line altitude of tropical Andean glaciers together with reports on secondary lahars at the eastern flank of Cotopaxi without any clear trigger, we hypothesize a process-based link between the two phenomena.  Geoelectrical and refraction seismic field surveys near the glacier margin (5000- 5300 m asl) have been conducted in order to gain a better understanding of the structure, conditions and degree of freezing of the subsurface, which is dominated by loose pyroclastic material and interbedded lava layers. The tomography results are highlighted within the concept of permafrost degradation and accompanied material weakening as potential triggering mechanism for secondary lahars.

Here we show 1) a carefully calibrated numerical lahar model at Cotopaxi capable of reproducing previously non-respected effects such as confluence, erosion reach and propagation speed, and 2) first measurements addressing the role of glacier retreat on the formation of secondary lahars. Our results contribute to the multi-hazard risk assessment in the RIESGOS project funded by the German Ministry of Education and Research.

How to cite: Frimberger, T., Petry, F., and Krautblatter, M.: Assessing lahar hazards at Cotopaxi volcano (Ecuador) controlled by volcanic eruptions and glacier retreat, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18219, https://doi.org/10.5194/egusphere-egu2020-18219, 2020

D1983 |
EGU2020-11752
Alejandro Sanchez, Stanford Gibson, Cameron Ackerman, and Ian Floyd

The Hydrologic Engineering Center River Analysis System (HEC-RAS) is a free software developed by the United States Army Corps of Engineers for simulating hydraulics, sediment transport, and water quality.  We present on the recent and ongoing developments of non-Newtonian flow and mobile bed modeling within HEC-RAS. The numerical models solve the in one-dimensional (1D) St. Venant equation, and the two-dimensional (2D) Diffusion Wave and Shallow Water Equations with corrections and modifications for non-Newtonian flows and steep slopes. The equations are solved using a combination of Finite-Difference and Finite-Volume methods on unstructured grids (for 2D). Several flow resistance laws are implemented including the Bingham, Coulomb, Herschel-Bulkley, and Voellmy models. Sediment transport is simulated in 2D with a total-load advection-diffusion model with corrections for steep slopes and high concentrations. A subgrid modeling approach is utilized for hydraulics and sediment transport, which allows for larger computational cells while maintaining accuracy. The numerical models have been verified with analytical test cases, and validated with small and large scale physical experiments and field applications. The results demonstrate the applicability of HEC-RAS as a tool for natural hazard studies involving non-Newtonian flows.

How to cite: Sanchez, A., Gibson, S., Ackerman, C., and Floyd, I.: 1D and 2D Debris Flow Modeling with HEC-RAS, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11752, https://doi.org/10.5194/egusphere-egu2020-11752, 2020

D1984 |
EGU2020-21752
Guillaume Meyrat

Since 2004, observations of shear and normal stresses have been collected at the base of naturally-triggered debris flows at the Illgraben observation station (Wallis, Switzerland) [1].   Because flow height and the normal force are simultaneously measured, and limited observations of basal fluid pore pressure are available, it is possible to investigate how the solid/fluid contents of the flow influence the measured shear stress.  The experimental results have emphasized two debris flow properties: (1) Debris flows are characterized by rocky or boulder-rich front, following by a fluidized tail. Consequently, the mass density varies from large values at the front of the flow to lower values towards the tail. A comparison between different debris flow events, however, likewise reveals that the streamwise change in density can vary dramatically between two different events. (2) The relationship between the measured shear and normal tress is highly non-linear. 

Operating on the assumption that the streamwise change in density (or equivalently change in streamwise composition) is primarily responsible for the observed non-linear stress behavior, we develop a rheological model describing two-phase debris flow motion. The underlying idea behind the model is that the granular content of the flow can dilate, changing the solid/fluid composition of the flow, and thereby alter the bulk flow density. The model allows us to estimate the correct debris flow composition for different classes of debris flow varying from granular to muddy fluid. Based on these results, we are then able to reproduce the measured shear stress data when we simulate the measured events numerically.  The results appear to confirm dilatant-type flow models proposed by Takahashi [2], and later developed in detail by Iverson and George [3]. The model is used to back-calculate recent debris flow events that occurred near Davos Switzerland in 2018/2019.

 

 

 

REFERENCES

  1. McArdell, B.W., Bartelt, P. and Kowalski, J. (2007): Field observations of basal forces and fluid pore pressure in a debris flow, Geophysical Research Letters, Vol. 34, No. L07406.

 

  1. Takahashi, T. (2007): Debris flows: mechanics, prediction and countermeasures, Taylor and Francis / Balkema, 448pp.

 

  1. George, D. L., & Iverson, R. M. (2011). A two-phase debris-flow model that includes coupled evolution of volume fractions, granular dilatancy, and pore-fluid pressure. Italian journal of engineering geology and Environment, 43, 415-424.

How to cite: Meyrat, G.: A two-phase dilatant debris flow model based on full scale shear stress and pore pressure measurements, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21752, https://doi.org/10.5194/egusphere-egu2020-21752, 2020

D1985 |
EGU2020-19245
Adel Albaba, Niels Hollard, Christoph Schaller, Massimiliano Schwarz, and Luuk Dorren

The increasing urbanization of mountainous areas increased the risk imposed on residential buildings and infrastructure. In Switzerland, shallow landslides and hillslope debris flows are responsible every year for high infrastructure damage, blocking of important highways, evacuations and deaths. Up till now, the assessment of these processes has been mainly based on the experience of experts, especially in the assessment of their run-out extent and expected damage. In this research we present a new computationally efficient Discrete Element Model (DEM) which has been developed for the aim of simulating the run-out of hillslope debris flows.

YADE-DEM open source code has been extended and an elasto-plastic adhesive contact law have been implemented, which partially account for the presence of the fluid composed of water and find material. This is achieved through the adhesive aspect of the contact law, which would indirectly take the presence of such fluid into account, as this fluid would increase the cohesion of the flowing mass. A parametric study has been carried out to define the most sensitive model parameters, which were found to be the microscopic basal friction angle (Φb) and the ratio between stiffness parameters (loading and unloading) of the flowing particles . Data of full-scale experiments of hillslope debris flows were used to compare the flow kinematics with the model’s prediction. A good agreement between the model and experiments was observed concerning the mean front velocity (average margin of error of 8%) and the maximum applied pressure (average margin of error of 5%), with less agreement of the flow height (average margin of error of 13%). Detailed comparisons of pressure evolution between different selected experiments and simulations revealed the model’s capability of reproducing observed pressure curves, especially during the primary loading phase, leading to maximum pressure.

In order to test the model’s prediction of run-out distance of hillslope debris flow, hundreds of past hillslope debris flow events in the Swiss Alps were analyzed and 30 cases were selected representing different situations (i.e. different release volumes, slopes and forest cover). Due to the discrete nature of results in YADE, a GIS algorithm was developed in order to create envelopes representing the temporal evolution of the simulated propagating processes, which were compared to the those of the historical events. Results of the comparison revealed that, with the calibration of the two sensitive parameters in YADE, a fair to very good agreement was observed between the envelopes of the model and those of historical events for 87% of the tested cases. Difficulties in reproducing the envelopes of the rest of the cases are linked to the uncertainties in the mapping of the envelopes of past events, the role of the forest which is not taken into account in the model, and the lack of direct representation of fluid in the model.

How to cite: Albaba, A., Hollard, N., Schaller, C., Schwarz, M., and Dorren, L.: Comparing a newly developed DEM-based runout model for hillslope debris flows with full-scale experiments and historical events, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19245, https://doi.org/10.5194/egusphere-egu2020-19245, 2020

D1986 |
EGU2020-7363
Nina Marlovits, Martin Mergili, Alexander Preh, and Thomas Glade

Some of the most destructive landslide events in history have evolved through cascading effects where, for example, a rock fall in High Alpine areas transforms into a flow of rock, debris, ice, or snow. Amplification effects often result in high velocities and energies. As a result, such events can destroy private properties, infrastructure or can even lead to loss of life even in areas distant from the source.

In order to reduce the negative consequences of cascading landslide processes, numerical modelling can enrich the efficiency of risk management strategies. Unfortunately, most landslide run-out simulation models are designed either for fall or flow processes. However, it is presumed that, at least in some cases, cascading effects cannot be properly represented by only one single process model. Due to the complexity of combining and comparing models for fall and flow processes, not many attempts to do so have been documented.

In an attempt to fill this gap, the primary goal of this study is to define a criteria-set on how and when to couple the models, based on appropriate key parameters. Hence, we analyse computer models for fall and flow processes and evaluate whether their combination can provide an appropriate description of cascading landslides. A set of well-documented fall-flow events is back-calculated. Fall and flow are first simulated separately, with some overlap, each with a tool tailored for the corresponding process, based on detailed information on the case study. The input and output parameters for the overlapping areas are then analysed to investigate how and when process chains are linked. Thereby, one of the key challenges consists in the spatial transformation of the output of fall models to the input of flow models.

The findings will be used to develop a simulation framework allowing for the automated combination of fall and flow models In order to efficiently perform simulations which can be used as input for the design of hazard and risk management measures.

How to cite: Marlovits, N., Mergili, M., Preh, A., and Glade, T.: A criteria-set for the construction of a model cascade for fall-to-flow landslide chains, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7363, https://doi.org/10.5194/egusphere-egu2020-7363, 2020

D1987 |
EGU2020-2797
Yong Kong, Jidong Zhao, and Xingyue Li

Flexible barriers have been increasingly used in the mitigation of destructive geophysical flows, including rock avalanches, debris avalanches, debris flood, muddy debris flows as well as muddy flows. No rigorous analytical tools are available for the design of flexible barriers to resist a wide spectrum of geophysical flows of different natures and over a broad Froude-number range. Responses of a flexible barrier to the impacts of geophysical flows are known to be exceedingly complicated, involving intricate multi-body, multi-phase interactions, mass exchange and transportation and energy transformation/dissipation which are challenging for both numerical and physical modelers. To investigate the complex interactions between channelized geophysical flows and a non-uniform flexible barrier, a unified hydro-mechanical modeling framework was developed based on the coupled computational fluid dynamics and discrete element method (CFD/DEM). Five typical geophysical flows were modeled, for instance, a muddy debris flow was considered as a mixture of a continuous viscous fluid phase and a discrete phase consisting of gap-graded frictional particles. A permeable flexible barrier consisting of deformable meshes, cables and energy dissipators was modeled by applying the DEM accounting for connections and contact in a realistic manner. The coupled CFD/DEM model was well validated by experimental data in the literature. Based on the simulations, we examined the dynamics of flow-barrier interactions, energy dissipation mechanism, regime quantification, peak-static load ratio, momentum reduction and the correlations between flow Froude number/solid fraction and the impact mechanism transitions. It was observed that the peak-static load ratio in a flexible barrier increases while the barrier-induced momentum reduction of overflow decreases with increasing flow Froude-number. The analyses of the peak-static load ratio showed that rock avalanches generate the largest one and muddy flows generate the lowest one. For the first time, the impact mechanism transitions from pile-up to run-up for five geophysical flows impacting on a non-uniform flexible barrier were quantitatively identified according to the approaching flow dynamics and solid fraction. (The study was supported by RGC/HK under T22-603/15N and GRF#16205418.​)

How to cite: Kong, Y., Zhao, J., and Li, X.: Assessing the performance of flexible barrier subjected to impacts of typical geophysical flows: a unified computational approach based on coupled CFD/DEM, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2797, https://doi.org/10.5194/egusphere-egu2020-2797, 2020

D1988 |
EGU2020-18157
Alessandro Leonardi, Andrea Pasqua, and Marina Pirulli

Debris flow barriers often feature one or more filter elements, i.e. narrow outlets that induce deposition of the coarsest sediments, while allowing water and fines to filter through. Slit dams and steel nets are examples of this type of barriers. The design of the filter elements must balance the need to trap boulders and to dissipate the flow energy, while keeping maintenance work as low as possible.

Filter barriers elude the traditional load model prescribed by guidelines. Under some conditions, the outlets can clog with large boulders. The time necessary for this to happen mainly depends on the relative size between boulder and outlet, and is a nonlinear function of the flow composition. In any case, the main clogging mechanism is the formation of granular arches. These can induce significant load also in directions different from the main direction of the incoming flow.

Unless the barrier is specifically designed to withstand this type of load, granular arches, but also prolonged flow through the outlet, can induce deterioration and loss of functionality of the structure. In this work, we estimate these effects employing a combination of discrete- and continuum-based numerical methods. We evaluate the performance of two types of debris-resisting barriers, comparing the results with laboratory measurements and with the outcome of a monitoring campaign on a real barrier located in the Italian alps.

 

 

References:

Leonardi, A., Goodwin, G. R., & Pirulli, M. (2019). The force exerted by granular flows on slit dams. Acta Geotechnica, 14(6), 1949–1963.

Leonardi, A., & Pirulli, M. (2020). Analysis of the load exerted by debris flows on filter barriers : Comparison between numerical results and field measurements. Computer & Geotechnics, 118, 103311.

How to cite: Leonardi, A., Pasqua, A., and Pirulli, M.: Debris flow interaction with structures: challenges to traditional load models, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18157, https://doi.org/10.5194/egusphere-egu2020-18157, 2020

D1989 |
EGU2020-10642
Miguel Angel Cabrera and Gustavo Pinzón

The granular column collapse is a simplified system of the complex dynamics observed in gravity-driven natural mass-movements (i.e., landslides, debris flows, rock avalanches) and industrial applications (i.e., pharmaceutics, concrete, and food industry). In this system, a granular column is built with an initial height and initial width and then is allowed to collapse by self-weight onto a horizontal plane, while observing the variation in runout as a function of its initial geometry. Despite its wide use in the study of mass-movements mobility, either dry or with a liquid, little is known on the internal physics during collapse and its variation when immersed in an ambient fluid. This work presents a planar setup that allows the study of fully and partially immersed granular columns, with little disturbance at release [1]. The use of a planar configuration allows the monitoring of the moving mass and its deformation patterns, providing a unique insight into the particle-fluid interactions at release and during collapse that were not possible before. These observations are of great importance for the understanding of particle-fluid interactions at a mesoscale and can shed light into larger processes like a submarine and subaerial landslides. This work addresses these interactions by varying the geometry and measuring the mobility in dry and immersed conditions. The associated deformation patterns are observed both at the column-scale and at the particle-scale, reflecting in the velocity scaling of a deformable and moving granular mass and the occasional ejection of particles at its surface. We observed that the area of the released portion decreases during collapse and converges toward an equivalent portion of surface particles with little influence by the initial column geometry. These observations validate the planar setup for the study of granular columns, provides a novel interpretation in the momentum transfer in particle-fluid systems, and sets a validation case for future numerical simulations.

[1] Pinzon & Cabrera, Planar collapse of a submerged granular column. Physics of fluids, v31, 2019.

How to cite: Cabrera, M. A. and Pinzón, G.: Internal kinematics in a planar granular column collapse, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10642, https://doi.org/10.5194/egusphere-egu2020-10642, 2020

D1990 |
EGU2020-6335
Yao Jiang and Gonghui Wang

The shear behavior of granular materials has drawn considerable attention due to its great potential for various geophysical processes such as landslides and debris flows. Field and remote sensing observations reveal that the progressive maturation of these geophysical events may involve different styles of movement, such as stable creep, periodic slow sliding or accelerative sliding. Laboratory experiments also suggest that the mechanical conditions of granular materials may play a significant role in controlling diverse frictional behaviors, such as shear-rate weakening or strengthening. Furthermore, the granular frictional processes may involve abrupt perturbations of internal forces and release of strain energy. Such energy release events are manifested in the generation of high frequency (kHz-MHz) elastic waves, termed acoustic emissions (AEs), which deliver important information concerning the physical processes of granular shearing deformation.

A significant, though still inconclusive, body of research has been directed toward revealing possible mechanisms of AEs occurring on rock or among granular materials in shear. These studies attributed the generation of AEs to the formation of microcracks in intact rocks, the breaking of asperities between solid surfaces or the rearrangement of grain contacts. In this study, we performed laboratory tests on granular analogues composed of spherical glass beads in a ring shear configuration under conditions of room temperature and atmospheric humidity to examine whether the AE events are correlated with mechanical response. For measurements of elastic waves, a high-frequency AE transducer was installed near the shear plane. AE signals and mechanical data were synchronously sampled at the rate of 1 MHz using an additional recoding system.

The results show that (1) there is a strong correlation between the stress drop and the main acoustic burst; (2) the primary frequency bands are in the tens of kHz ranges for acoustic signals generated during granular shearing; (3) the onset of AE amplitudes precedes the impending global mechanical failures by several milliseconds.

How to cite: Jiang, Y. and Wang, G.: Characteristics of mechanical response and acoustic emission during granular shearing, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6335, https://doi.org/10.5194/egusphere-egu2020-6335, 2020

D1991 |
EGU2020-20756
Alexander Taylor-Noonan, Natalie Arpin, Miguel Cabrera, Greg Siemens, and W Andrew Take
The granular column collapse experiment, which consists of the rapid removal of lateral support to a column of granular material, is an important benchmark case for the physical and numerical study of transitional mass flows. While other researchers have focussed on the link between the aspect ratio of the column to mobility of the flow, these experiments are also an important platform to evaluate frameworks for triggering of slope failures.
 
Critical state soil mechanics centers around the theory that initially dense soils will dilate, and initially loose soils will contract upon shearing. If the soil is sheared at a rate which exceeds the rate which fluids can be expelled or drawn into the pore space between particles, the shearing is considered to be occuring at constant volume and termed “undrained”. This state is associated with a rise in pore fluid pressure and a reduction in intergranular normal effective stress. The authors have conducted experiments varying the time scales of the volume change and dissipation processes. In these experiments, a novel transparent soil mixture comprised of quartz and mineral oil was utilized to visualize the saturation regime of soils during the granular column collapse experiment. Particular attention was paid to triggering mechanisms and the transition between the metastable state and avalanche regimes. The transparent material allowed visual confirmation of the volume change during shearing and important insights were gained into the role of the unsaturated soil condition in temporary strength. These observations have implications beyond the column collapse experiment, including the initiation of debris flow experiments as well as analysis of triggering mechanisms of unstable slopes in the field.

How to cite: Taylor-Noonan, A., Arpin, N., Cabrera, M., Siemens, G., and Take, W. A.: Mechanism of air entry during collapse of saturated and unsaturated columns of transparent granular soil , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20756, https://doi.org/10.5194/egusphere-egu2020-20756, 2020

D1992 |
EGU2020-1967
Yongbo Tie, Jintao Jiang, and Shuai Wang

The debris flow initiate by glacial till always dangers the local residents and facilities in alpine region in southwest China. The study of debris flow initiate from glacial till can help in understanding the mechanism of glacial till transfer to debris flow, in revealing the development of alpine mountainous topography. In this study, we designed analogue experiments that simulate the initiating process of glacial till eroded by the runoff. This research focuses on the relationship between the glacial till initiating and the critical value of flow velocity by performing analogue experiments with different flow velocity under a constant slope of landform.

A particle analysis of the modeled glacial till take from field allows understanding the structure of tested soil and standardizing the critical value of debris flow initiation. After the rush of flow with different velocity, the tested glacial till reaches a failure condition (i.e., the movement of certain particle, the undercutting of soil) which was assigned as the evidence for debris flow initiating. Results show that there are three types of erosion occurred during the experiment, the sheet erosion related to flood generation, the vertical erosion related to debris flow initiation, and lateral erosion related to the volume increasing of debris flow. Results show that the time duration of debris flow initiation are negative correlated with the velocity of flow. Because of the distribution of glacial till particle, the surface of the longitudinal profile showed corrugated form after the eroding of flow, this mainly depends on the infiltration zone where the water content of glacial till are saturated.

In the early period before the formation of debris flow, the main type of soil erosion was sheet erosion, the dual peak structure of glacial till made it easy to start up the soil with fine particles in the action of runoff scouring. Therefore, the sediment content in the flood could be improved, which provided a precondition for the formation of debris flow. In this process, the influence of runoff velocity was significant. According to the statistical results of the experiment, the faster the runoff velocity was, the faster the glacial till erosion rate was; and on the contrary, the slower the glacial till erosion rate was. We show that faster the flow velocity was, relatively shorter time the flood took to form, but relatively longer time the debris flow took to start. Finally, our results demonstrate the runoff scouring first leads to the removal of fine particles in glacial till, then the coarse grained soil was unstable due to the loss of foundation support and it initiated to form debris flows.

How to cite: Tie, Y., Jiang, J., and Wang, S.: The experimental research on initiation mechanism of debris flow from glacial till, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1967, https://doi.org/10.5194/egusphere-egu2020-1967, 2020

D1993 |
EGU2020-18935
Luca Sarno, Maria Nicolina Papa, and Yongqi Wang

The dynamics of granular media, involved in several hazardous geophysical phenomena such as debris flows and avalanches, is extremely complex and still represents a hot topic for the scientific community and specialists. When choosing a mathematical tool to describe such flows, depth-averaged models remain the first choice especially in large field-scale applications, while three-dimensional and discrete element models are more complete but very computationally expensive. However, the dynamics variations along the flow depth cannot be described by classical depth-averaged models. With the aim of getting a better insight into the dense regime of granular flows, which is the most common in nature, we report a laboratory investigation where a number of dense dry granular flows with different basal boundary conditions and flow rates are studied in a 2m-long Plexiglas flume. The employed granular medium consists of small spheroidal beads (d≈3mm), made of acetal resin (POM). The flume is instrumented with a high-speed digital camera and a no-flicker planar lamp, so that reliable measurements of the velocity and of the volume fraction at the side wall are obtained by using a multi-pass particle image velocimetry (PIV) approach [Sarno et al., Adv. Powder Tech., 2018] and a stochastic-optical method (SOM) [Sarno et al., Granul. Matter, 2016]. By iteratively decreasing the interrogation window in the PIV analysis down to approximately half the grain size, it is possible to estimate the magnitude of the fluctuation velocities along normal-to-bed and stream-wise directions. Small normal fluctuation velocities and relatively large volume fractions (≈0.6) are observed in the major part of the flow, where the chief resistance mechanism is frictional. At the uppermost region, close to the free surface, slightly larger values of the fluctuation velocities and lower values of the volume fraction are observed, due to the increasingly collisional behavior. These findings indicate that, owing to the particles non-penetration condition and weak collisionality, the mass exchanges from one layer to the neighboring ones are rather limited in the dense regime. Therefore, dense granular flows exhibit a clear stratified nature and, thus, they may be regarded as composed of different superimposed layers, partially coupled each other. It is worth noting that this behavior is considerably different from turbulent incompressible fluids and also from chiefly collisional granular flows, where mass and momentum exchanges are considerable along the entire flow depth. These experimental findings suggest that a multi-layer depth-averaged mathematical approach would be a suitable tool for improving the modeling of these flows without increasing significantly the computational costs.

How to cite: Sarno, L., Papa, M. N., and Wang, Y.: The stratified nature of dense granular flows supported by fluctuation velocities and volume fraction measurements from laboratory flume experiments, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18935, https://doi.org/10.5194/egusphere-egu2020-18935, 2020

D1994 |
EGU2020-134
Shaojie Zhang

Conventional sensors for debris flow monitoring suffer from several drawbacks including low service life, low reliability in long-distance data transfer, and stability in severe weather conditions. Recently, fiber Bragg grating (FBG)-based sensors have been developed to monitor debris flows. However, they can be easily damaged by the impact forces of boulders within debris flow. This paper presents a new FBG-based device to measure the strain induced by the impact force of debris flow with high reliability and effectiveness. The effects of the impact forces of debris flows have been investigated. Then, the relationship between the strain and the debris flow energy correlating with the damage to building structures has been established. It is shown that this new FBG-based device is capable of monitoring and warning about debris flows. The impact experiment results show that the peak value of dynamic strain on the fixed end of the new device is positively correlated with the external impact force. Using an impact force, we establish a relationship between the measured strain and the potential of a debris flow resulting in damage to structures was established. This follows the general rule that a larger measured strain corresponds to a higher level of debris flow. Using this relationship, we can quantify a dangerous level of debris flow using the monitored strain data. Our new device is capable of monitoring and warning about dangerous debris flows, allowing for more effective debris flow mitigation.

How to cite: Zhang, S.: An experimental evaluation of impact force on a fiber bragg grating (FBG)-based device for debris flow warning, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-134, https://doi.org/10.5194/egusphere-egu2020-134, 2019

D1995 |
EGU2020-3524
Zheng Chen, Siming He, and Dieter Rickenmann

Geophysical granular flows such as rock and snow avalanches, flow-like landslides, debris flows, and pyroclastic flows are driven by gravity and often impact on engineering structures located in gullies and slopes as they flow down, generating dynamic impact pressures and causing a major threat to infrastructures. It is necessary to understand the physical mechanism of such granular flows impacting obstacles to improve the design of protective structures and the hazard assessment related to such structures. In this study, the small-scale laboratory experiments were performed to investigate the dynamic impact caused by granular flow around a circular cylinder with variable radius of curvatures and the dynamic impact against a flat wall. Pressure sensors were used to measure the impact pressure of granular flows at both the upstream cylinder surface and at the bottom of the channel. Accelerometers were mounted on the underside of channel to record the seismic signals generated by the granular flows before and during the impact with the obstacle. Flow velocities and flow depths were determined by using high-precision cameras. The results show that a bow shock wave is generated upstream of the cylinder, causing dynamic pressures on both the obstacle and the bottom of the channel. The dimensionless standoff distance of the granular shock wave decreases nonlinearly or almost exponentially with increasing Froude number (Fr) in the range of 5.5 to 11.0. The dimensionless pinch-off distance and dimensionless run-up height grow linearly with increasing Fr, and they were significantly influenced by the radius of curvature of the structure at the stagnation point (RCSSP). The dimensionless impact pressure on the structure surface is sensitive to the RCSSP, while the differences decrease as Fr increases; Seismic signals generated at the underside of the channel and at the top of the cylinder were also recorded to assist in analyzing the effects of RCSSP.

How to cite: Chen, Z., He, S., and Rickenmann, D.: Effects of Obstacle’s Curvature on Shock Waves in Gravity-Driven Experimental Flows Impacting a Circular Cylinder or a Wall, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3524, https://doi.org/10.5194/egusphere-egu2020-3524, 2020

D1996 |
EGU2020-427
Raül Oorthuis, Marcel Hürlimann, Clàudia Abancó, José Moya, Antonio Lloret, and Jean Vaunat

Torrential flows, like debris flows and debris floods, can mobilize large volumes at high velocities in mountainous regions. Therefore, they represent an important erosional process and a significant hazard towards infrastructures and people (sometimes catastrophic).

Monitoring-based analysis is a crucial task to improve the understanding of the mechanisms triggering torrential flows and its propagation, which are necessary to implement early warning systems. The monitoring of triggering conditions generally focusses on rainfall measurements and the characterization of the critical rainfall conditions. However, rainfall data do not provide a complete picture of the physical processes involved. Very few studies include soil moisture and/or pore water pressure measurements to define the hydrologic response at the natural slopes of the catchment. In that respect, this study analyses both rainfall and soil moisture data at a Mediterranean-influenced torrential basin located in Central Pyrenees (the Rebaixader site).

The Rebaixader site has a high torrential activity, with 11 debris flows and 24 debris floods detected since 2009. The temporal distribution of rainfall episodes and torrential flows shows a clear shift between the most frequent rainfall episodes (beginning of June) and torrential flows (mid-July). This suggests that soil moisture conditions, depending on antecedent rainfall and/or snowmelt, affect the triggering of torrential flows. Regarding critical rainfall conditions, a previously published rainfall threshold was updated including total rainfall duration and mean intensity of 2009-2019 rainfalls. On the other hand, measured volumetric water content (VWC) was analysed for triggering and non-triggering rainfall events. Preliminary results show lower VWC increment on wetter soils at the beginning of rainstorms that triggered torrential flows. This indicates that soil saturates with lower rainfall amount if the soil is initially wetter; which subsequently generates higher runoff rate and therefore a higher erosion and transport energy that may trigger torrential flows. In addition, a slight trend was observed when comparing rainfall intensity and soil moisture; generally larger rainfall intensity is necessary to trigger torrential flows when soil is drier.  

The analysis of VWC data was more complicated in contrast to the one of rainfall data, since the time series are shorter (2013-2019) and the physical interpretation is not straightforward. Therefore, additional data are necessary to confirm and define soil moisture thresholds triggering torrential flows.

How to cite: Oorthuis, R., Hürlimann, M., Abancó, C., Moya, J., Lloret, A., and Vaunat, J.: Rainfall and soil moisture conditions for the triggering of torrential flows at the Rebaixader catchment (Central Pyrenees), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-427, https://doi.org/10.5194/egusphere-egu2020-427, 2019

D1997 |
EGU2020-2118
Jiaoyang Li

A debris flow occurred in Shiyang gully, located between Hebei Province and Beijing, on 8 June 2017, resulting in 6 people dead or injured. Short-term heavy rainfall is the main factor that triggered this event, however, the meteorological agency didn’t forecast this event very well. In this study, numerical simulation using FLO-2D was performed to reproduce the debris flow event (flow depths, flow velocities, and sediment depositions)occurred in 2017. The results of the field survey showed that the influential range of debris flow is consistent with the simulation results. Simulated depth accuracy is greater than 70%. Then, we used FLO-2D is calibrated to simulate debris flows disasters under different rainfall scenarios. The results showed that, the Beijing needs to be warned when the accumulated precipitation is 40mm at the rainfall intensity of 1mm/min. As cumulative rainfall and rainfall intensity increase, the risk of Shiyang gully is increasing.  This study used FLO-2D simulated process of debris flows triggered by rainfall. The results showed the early warning time and influential range for different intensity ,accumulated precipitation, and rain area, which is beneficial to the debris flow management in the western mountainous areas of Beijing.

How to cite: Li, J.: Simulating debris flows triggered by rainfall in Shiyang gully, China, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2118, https://doi.org/10.5194/egusphere-egu2020-2118, 2020

D1998 |
EGU2020-2827
| Highlight
Yuan Fang Tsai, Jia Hao Pan, and I Chia Hsieh

The outreach road of mountain community has been interrupted by disasters such debris flow, flood and landslides, resulting in the interruption of the outreach road of the mountain community, forming a state like an island, which can be regarded as an isolation effect. In recent years, extreme events caused by extreme weather. The special geographical conditions in Taiwan, coupled with the increase in the frequency of natural disasters, have been heard by isolated island news. In 2015, Typhoon Soudelor hit Taiwan, and Wulai, New Taipei City caused severe disasters. Debris flow and landslides occurred, causing the interruption of Xinwu Road, the main liaison road in Wulai, and the isolation effect in Wulai. If we can integrate the historical data and research of isolation effect, and combine the theory of isolated prediction with instant rainfall and disaster prevention information, and finally visualize the information by alert system, it will help the general public's disaster prevention awareness and related disaster prevention unit decision-making reference.

Therefore, this research builds an isolation alert system. The three main information functions of this system include 1. disaster island geographic information function 2. isolated accident village identification function and 3. immediate isolated warning function. The d isolated geographic information display function is mainly to display the historical information about the isolation effect. The information of the village has been published, including the village's geography, social information and disaster history, and the risk map is presented by the vulnerability and resilience indicators. The village identification function of the isolated incident is realized by the Common Alerting Protocol of the road, and based on this, the identification in the immediate isolated village is carried out. The immediate disaster isolated warning function combines real-time rainfall information and integrates the Rainfall Triggering Index, Machine Learning's Supervised Learning algorithm, and the Common Alerting Protocol for the road. In the end, it was verified by the 2017 Typhoon Nepartak incident, and the results were all given the correct warning level for the isolated village.

How to cite: Tsai, Y. F., Pan, J. H., and Hsieh, I. C.: Establishing an Isolation Alert System for Mountain Community, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2827, https://doi.org/10.5194/egusphere-egu2020-2827, 2020

D1999 |
EGU2020-3410
Tobias Schöffl, Georg Nagl, and Johannes Hübl

Comparison of the surface velocity of a debris flow at the Gadria creek using pulse compression radar and digital particle image velocimetry (DPIV).

 

Tobias Schöffl, Georg Nagl, Johannes Hübl

Institute of Mountain Risk Engineering, University of Natural Resources and Life Sciences, Vienna, Austria

 

A central aspect of protection against debris flows is the understanding of the process. The flow velocity is an important parameter which is used, for example, in the dimensioning of protective structures, for technical building protection and for early warning systems. The measurement of the surface velocity which is regarded as the maximum velocity occurring within a debris flow, is therefore an essential link in the chain of fundamental process research and applied protection against natural hazards.

Due to the further development of various technologies such as video technology and high-frequency radar technology, the non-contact measurement of the surface speed of a debris flow has improved significantly in recent years. Radar technology provides a wide aspect of applications in alpine mass movements such as debris flows, avalanches and rockfall and is able to detect such processes up to a range of 2500 meters in distance. An additional beneficial feature is the possibility of non-contact measurement of the surface velocity. In the catchment area of the Gadria basin (South Tyrol, Italy), the measuring station, which has been in operation since 2016, has been extended by a pulse compression radar and a new HD video camera. On July 26, 2019 a debris flow consisting of several surges was recorded with both the radar and the HD video camera. To obtain surface velocity data from the video material, the material was analyzed and evaluated using digital particle image velocimetry by making use of the MATLAB software and its freely accessible ADD-On "PIVlab".

The results of the compared surface velocity data showed a value of up to 0.74 according to the statistical mean of the coefficient of determination. The results demonstrate the high effectiveness of the pulse compression radar and the DPIV analysis in a wide range of the assessment of surface velocity of natural debris flows. There is great potential in both measuring systems and the chosen comparative analysis provides a blueprint for future recorded debris flows.

How to cite: Schöffl, T., Nagl, G., and Hübl, J.: Comparison of the surface velocity of a debris flow at the Gadria creek using pulse compression radar and digital particle image velocimetry (DPIV)., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3410, https://doi.org/10.5194/egusphere-egu2020-3410, 2020

D2000 |
EGU2020-3706
| Highlight
Victor Carvalho Cabral, Fernando Mazo D'Affonseca, Marcelo Fischer Gramani, Agostinho Tadashi Ogura, Claudia Santos Corrêa, Carolina Martinez Mendoza, Vinicius Veloso, and Fábio Vieira Reis

Debris flows represent great hazard to communities and infrastructures, since they move quickly and are very destructive. In Brazil, debris flows mainly occur in the Serra do Mar Mountain Range, where thousands of casualties were reported in the last two decades due to these phenomena. This study aims at estimating the magnitude of a debris-flow event that occurred in Serra do Mar on February 2017, at the Pedra Branca watershed in the State of Paraná. Debris-flow magnitude refers to the volume of material discharged during an event and is an important aspect of debris-flow hazard assessment. The Pedra Branca event was initiated by rainfall-triggered shallow landslides, damaging local oil pipelines and farms. The magnitude estimation is based on the combination of empirically based equations and the geomorphic features of the debris flow, acquired from in situ and aerial investigation. 28 cross-sections were made along the river channel, considering post-event channel width, erosion and accumulation depth, as well as depositional features. Sediment sources and accumulation areas were identified and delimitated based on high-resolution (1:500) aerial drone photographs. The results indicate that the landslides that initiated the event released approximately 26,884.5 m3 of sediments (Vi) into the main channel of Pedra Branca and that the volume eroded (Ve) and accumulated (Vd) along the channel are, respectively, 82,439 m3 and 22,012 m3. The estimated total solids volume (Vs) is 87,274 m3, assuming that Vs = Vi + Ve - Vd. Moreover, considering a solids concentration of 57% calculated according to empirically-based equations for Serra do Mar, the debris flow had a total magnitude of 153,113 m3. These estimations suggest that the February 2017 debris flow mobilised great volume of material and that 15% of the total volume accumulated on the channel bed, which can be remobilised by future events. Further research on debris-flow dynamics and recurrence at the Serra do Mar Mountain Range is recommended to mitigate future hazards.

How to cite: Carvalho Cabral, V., Mazo D'Affonseca, F., Fischer Gramani, M., Tadashi Ogura, A., Santos Corrêa, C., Martinez Mendoza, C., Veloso, V., and Vieira Reis, F.: Magnitude estimation of a landslide-triggered debris flow in the Serra do Mar Mountain Range, Brazil, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3706, https://doi.org/10.5194/egusphere-egu2020-3706, 2020

D2001 |
EGU2020-4838
Yu-Wen Su, Yen-Hsiu Lin, Yu-Chao Hsu, Ji-Shang Wang, and Chih-Hsiang Hong

INTRODUCTION

In 2009, the typhoon Morakot caused many multimodal sediment disasters in Taiwan. The Soil and Water Conservation Bureau invested a lot of resources in the reconstruction project. To accelerate the stability of soil in the catchment area, and reduce the possibility of secondary disasters. After a period of time, appropriate review and governance benefit assessment should be carried out. In this study, 2 major areas with heavy sediment disasters within the jurisdiction of the Tainan Branch, Soil and Water Conservation Bureau were chose to do hazard and risk assessment.

METHODS

This study collected the documents of the erosion and sediment control engineering over the years from 2009. Then, matched with the field survey, digital elevation model analysis, and using the evaluation matrix to assess the level of hazard and risk of selected major disaster areas. The row and column of the evaluation matrix including “function of structures” and “environmental condition (EC)”. Function of structures are divided into 4 levels: Nice, Good, Poor, and Bad. Environmental condition is assessed by four factors “landslide rate of watershed (%)”, “upstream channel slope (degree)”, “river erosion or siltation change (m)”, “preservation factor”. Landslide rate of watershed (LA) means the percent of landslide in the watershed. Upstream channel slope (US) means the slope of the channel from the middle to the top of watershed. River erosion or siltation change (CD) means the maximum vertical height change of river bed. Preservation Factor considered the protected targets and the preservation distance. According to the individual scores of the four factors, the weighted average is taken and divided EC into 4 levels. The hazard and risk assessment work can be done according to the evaluation results of “function of structures” and “situation of environment”.

RESULTS AND DISCUSSION

In this study, we chose two sites, i.e., Cianghuangkeng (Tainan City), Henansiang (Kaohsiung), to practice hazard and risk assessment, in 2018. 2009, typhoon Morakot caused 1.96 ha of landslide, and brought about 160,000 m3 of sediment at Cianghuangkeng. From 2009 to 2018, the Soil and Water Conservation Bureau practiced 6 erosion and sediment control engineering. According to the results of assessment, the level of function of environment is good, and the level of environmental condition is A. Therefore, the result of hazard and risk assessment is low. Cianghuangkeng has low potential for hazard and risk. In this way, the evaluation result of hazard and risk assessment in Henansiang is also low.

According to the results of evaluation matrix, the potential of hazard and risk could divide into three levels: high (H), middle (M), low (L).

CONCLUSIONS

This study use evaluation matrix method to assess the hazard and risk of the major disaster areas caused by the typhoon Morakot event. According to the assessment results, we can review whether the remediation strategies and directions of key disaster areas need to be revised. It will help improve related technologies, provide reference for future related governance planning strategies, and effectively promote the improvement of soil and water conservation.

How to cite: Su, Y.-W., Lin, Y.-H., Hsu, Y.-C., Wang, J.-S., and Hong, C.-H.: Hazard and risk assessment of watershed in South Taiwan., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4838, https://doi.org/10.5194/egusphere-egu2020-4838, 2020

Chat time: Tuesday, 5 May 2020, 16:15–18:00

Chairperson: Marcel - Roland (Velio - Xiaojun)
D2002 |
EGU2020-4345
Yuichi Sakai and Norifumi Hotta

The fluidity of a debris flow varies by grain size. Flows containing principally coarse grains are considered to be laminar and those featuring largely incohesive fine grains turbulent. The transition from laminar to turbulent flow depends on the ratio of flow depth to grain size (i.e., the relative flow depth). Debris flows with relative flow depths of approximately 10 are entirely laminar; those with relative flow depths over approximately 20 exhibit transitional flow behavior from entirely laminar to partially turbulent. This transitional flow has been investigated in the laboratory using the resistance law and the vertical distribution of streamwise velocity. The flow exhibits a two-layer structure; the lower layer remains laminar but the upper layer becomes turbulent. However, transition modeling remains incomplete given the lack of data on the internal stresses associated with transitional flow. Here, we studied the laminar-turbulent transitions of debris flows by measuring basal pore fluid pressures using flume tests.

We flowed saturated monodisperse granular materials over an open-channel rigid bed; we used sediment particles of diameters 2.9, 2.2, 1.3, 0.8, 0.5, and 0.2 mm. When the debris flow attained the steady state, the flow depth and basal pore fluid pressure were measured using an ultrasonic sensor and pressure gages respectively, and the basal total normal stress estimated using the bulk density of the debris flow assessed at the downstream end.

The relative flow depths ranged from 5 to 130. Comparisons among the measured pore fluid pressures and the hydrostatic and total normal stresses indicated that a pore fluid pressure of 0.2 mm differed greatly from the hydrostatic pressure, equaling, in fact, the total normal stress, and indicating fully turbulent flow. In contrast, pore fluid pressures of 2.9, 2.2, and 1.3 mm were slightly higher than the hydrostatic pressures, indicating that the Reynolds stresses of the pore fluid due to the strong shears imparted by the sediment particles were in play; flow was entirely laminar. Pore fluid pressures of 0.8 and 0.5 mm were intermediate between the hydrostatic and total normal stresses, indicating the transition from fully laminar to partially turbulent flow.

By analogy with the Reynolds number for Newtonian fluid, we investigated the transition based on the non-dimensional number for debris flows (thus, the ratios of inertial to dynamic stresses caused by interparticle collisions and the Reynolds stresses of the debris flow pore fluid). This identified the critical Reynolds number in terms of transition commencement. We describe the transitional flow behavior of monodisperse granular debris flows using a two-layered model in which the position of the between-layer interface is estimated based on that critical Reynolds number.

How to cite: Sakai, Y. and Hotta, N.: Laminar-turbulent transition in debris flow: measurement of basal pore fluid pressure in an open channel flow experiment, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4345, https://doi.org/10.5194/egusphere-egu2020-4345, 2020

D2003 |
EGU2020-5382
Xingzhang Chen and Hui Chen

Abstract: Uplift pressure is crucial for the stability of debris flow dam because of its reducing the effective pressure on the dam foundation and the anti-slide force of the dam. This study investigates the spatial and temporal variations of the uplift pressures during the debris flow impact processes, through a series of flume experiments. Before the debris flow impacting on the dam, the uplift pressure keeps stable due to the steady water level, and then it decreases slightly at the instant of debris flow impacting on the dam which lasts for no more than 1 s, and then increases sharply within a time lag no more than 2 s, and then decreases sharply soon afterwards. The maximal increasing ratio is 6.4 and the average value is 3, comparing with the uplift pressure before the impacting. The peak pressure occurs before the dam and decreases with the distance from the dam with a nearly linear tendency. The increment of uplift pressure also presents a similar tendency with the distance from the dam. In addition, the uplift pressure is found to be strongly influenced by the permeability of debris flow deposits, especially by the fine content of grain composition, and by the properties of the flow, such as the flow density, runoff volume and hydraulic gradient, and the pressure rises in a nearly linear form with the properties.

Keywords: debris flow, uplift pressure, check dam, flume experiments

How to cite: Chen, X. and Chen, H.: Variation of uplift pressure of debris flow on the dam bottom, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5382, https://doi.org/10.5194/egusphere-egu2020-5382, 2020

D2004 |
EGU2020-4654
Xiaojun Guo

Abstract: Debris flow monitoring provides valuable data for scitienfic research and early warning, however, it is of difficulty to sucessfully achive because of the great damage of debris flows and the high cost. This report introduces monitoring systems in two debris flow watersheds in western China, the Jiangjia gully (JJG) in Yunnan Province and the Ergou valley in Sichuan Province. JJG is loacted in the dry-hot valley of Jinsha River, and the derbis flows are frequent due to the semi-arid climate, deep-cut topography and highly weathered slope surface. A long-term mornitoring work has been conducted in JJG and more than 500 debris flows events has been recorded since 1965. The monitoring system consists of 10 rainfall gauges and a measuring section, with instruments to measure the flow depth and velocity; and flow density is measured through sampling the fresh debris flow body. Ergou lies in the Wenchuan earthquake affected area and the monitoring began in 2013 to investigate the characteristics and development tendency of post-earthquake debris flows. Three stations were set up in the mainstream and tributaries, with instruments to measure the flow depth, velocity, and density. Over 10 debris flow events were recorded up to date.

Based on the monitoring output, the rainfall spatial distribution and thresholds for debris flows are proposed. The debris flow dynamics characteristics are analyzed, and the relations between the parameters, e.g. density, velocity, discharge and grain compositions are presented. The debris flow formation modes and the mechanisms in different regions are discriminated and simulation methods are suggested. It is anticipated that the monitoring results will promote understanding of debris flow characteristics in the western China.

Keywords: Debris flow, monitoring, rainfall, discharge, formation. 

How to cite: Guo, X.: Debris flow monitoring in China, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4654, https://doi.org/10.5194/egusphere-egu2020-4654, 2020

D2005 |
EGU2020-7805
| Highlight
Philipp Aigner, Leonard Sklar, Markus Hrachowitz, and Roland Kaitna

Processes like flash floods or debris flows, which typically occur in small headwater catchments, represent a substantial natural hazard in alpine regions. Due to the entrainment of sediment, the discharge of debris flows can be up to an order of magnitude larger compared to 100-year fluvial flood events in the same channel, which poses a great threat to affected communities. Besides the triggering rainfall, the initiation of debris flows depends on the watershed’s hydrological and geomorphological susceptibility, which makes it hard to predict and understand where and when debris flows occur.

In this study we aim to quantify the influence of geomorphologic characteristics and long-term sediment dynamics on debris flow activity in the Austrian Alps. Based on a database of debris-flow events within the last 60+ years, a geomorphological assessment of active and non-active sub-catchments in different study regions is carried out. In a first step, we derive geomorphological characteristics, such as terrain roughness, Melton number as well as weathering potential of geological units found within the watersheds. Based on the findings of the terrain shape analysis, a set of representative watersheds will be selected for systematic monitoring of surface elevation changes over the project period of three years. This will be achieved by comparing digital surface models based on photogrammetric UAV surveys and monitoring of channel reaches with cameras.

In order to project these findings onto a larger regional scale, the derived terrain parameters will be used to integrate and extend a previously designed hydro-meteorological debris-flow susceptibility model (Prenner et al., 2018) with a sediment-disposition-model. This will form the basis for an advanced debris flow forecasting tool and help to better assess the impact of climate change on the magnitude and frequency of future debris flows.

 

References:
Prenner, D., Kaitna, R., Mostbauer, K., & Hrachowitz, M. ( 2018). The value of using multiple hydrometeorological variables to predict temporal debris flow susceptibility in an Alpine environment. Water Resources Research, 54, 68226843.

 

How to cite: Aigner, P., Sklar, L., Hrachowitz, M., and Kaitna, R.: Why are some alpine catchments debris-flow active and others not? - the influence of geomorphology on debris-flow initiation, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7805, https://doi.org/10.5194/egusphere-egu2020-7805, 2020

D2006 |
EGU2020-6992
Seungjun Lee, Hyunuk An, and Minseok Kim

The shallow landslide-generated debris flow on hillside catchment plays a critical role in the change of landscape features caused by natural hazards. Numerous studies has been conducted on the analysis of the transported and deposited sediments by debris flows that were developed at the hillside catchments. Among these researches, the debris flow numerical modeling approach has an advantage of being able to predict and simulate the movement of the flow over irregular topographic terrains. A number of modeling approaches have been studied to explore the process of debris flow development. However, there are still a lot of uncertainties in the erosion-entrainment process, although several erosion models have been proposed to simulate debris flow. The objective of this study is to test and analyze several erosion models for debris flow simulation. Deb2D model, a two-dimensional debris flow simulation software based on quadtree-grid, is used to simulate the debris flow. The study case was 2011 Mt. Umyeon landslide in the Republic of Korea. The total debris flow volume, maximum velocity and inundated depth generated from Deb2D were compared with the field validation data. In particular, the spatial distribution of erosion depth was extracted from the LiDAR-based DEM data gauged before and after the event to compare the performance of the erosion model. The research showed each erosion model accuracy and shortcomings through comparison with field validation data.

Keywords : debris flow, numerical simulation, entrainment, erosion model, Deb2D

How to cite: Lee, S., An, H., and Kim, M.: Evaluation of different erosion models for debris flow modeling, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6992, https://doi.org/10.5194/egusphere-egu2020-6992, 2020

D2007 |
EGU2020-7406
| Highlight
Holly Chubb, Andrew Russell, Alejandro Dussaillant, and Stuart Dunning

Landslides and mass flows are dynamic processes that involve the movement of rock, debris and earth down a slope. As a result of the 2017 catastrophic mass flow, these processes have been further established as a significant risk to the population of Chile, and further afield. Through field site investigations, it is possible to develop a greater insight into the mechanisms and conditions that influence the dynamics of these phenomena.

On Saturday 16 December 2017, a catastrophic debris flow (aluvión) partially destroyed the village of Villa Santa Lucía and a 5 km long reach of the Panamerican Highway resulting in 22 fatalities. The apparent trigger was an intense rainfall event of 124 mm in 24h associated with an elevated 0˚C isotherm (1600 m.a.s.l.) that led to the failure of 5.5 - 6.8x106m3  mountainside in the uppermost catchment of Rio Burritos near the SE end of the Cordón Yelcho Glacier. The landslide transformed rapidly into a highly mobile debris flow as it entrained water from the Rio Burritos river and glacier ice from the Cordón Yelcho.

This study characterises the geomorphological impacts and dynamics of the 2017 mass flow. Post-event DEMs, aerial photos and satellite imagery provided the basis for geomorphological mapping and terrain analysis. Fieldwork in January 2019 allowed sampling of mass flow deposits, logging of sedimentary sections and dGPS surveys.

Both erosion and deposition occurred over the Villa Santa Lucía flow path. Erosion occurred more frequently in the first 7.9km of the flow path due to high slope angles and presence of the Rio Burritos that channelised flow. A high proportion of coarse particles in the flow enhanced basal scouring and erosion of the valley sides, resulting in significant flow bulking. A total of 7.6x106m3 – 7.7x106m3  of material was deposited across the latter 6.3km of the flow path.

Sediment sample analysis showed that the flow began as cohesive and viscous in nature in spite of a lack of clay particles and high proportions of sands and gravels. The addition of water from the Rio Burritos reduced the viscosity of the flow as the flow propagated downstream. This resulted in enhanced lobe spreading and particle interactions in the depositional zone. In spite of this water entrainment, the flow remained both sediment and debris rich over its duration.

Catastrophic mass flows like the event at Villa Santa Lucía are likely to become more common around the world in the future as intense rainfall events become more frequent due to the dominance of El Nino Southern Oscillation (ENSO) events. By studying recent catastrophic mass flow events, an insight into the relationship between mass flow triggers and flow composition will be developed. This will allow for greater understanding of how these influence mass flow behaviours. As a result, it may then be possible to predict the rheology and routes of future flows. These predictions have the ability to be used to protect communities from such events in the future.

How to cite: Chubb, H., Russell, A., Dussaillant, A., and Dunning, S.: The dynamics and impacts of the December 2017 catastrophic mass flow Villa Santa Lucia, Chile, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7406, https://doi.org/10.5194/egusphere-egu2020-7406, 2020

D2008 |
EGU2020-8199
Muneyuki Arai


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<title>    A research on initial formation process of intermittent debris flow   </title>
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<body>
Some debris flows are intermittent surges flows with discontinuous changes in water depth.
These phenomena are considered to be a kind of nonlinear wave phenomena based on flow instability.
The authors show a KdV-Burgers equation as an equation representing the change in water depth.
When the phase velocity in the equation is long wave velocity, the equation becomes a Burgers equation.
The characteristic of the roll wave is that it has a discontinuous change in water depth and flows down intermittently as many surges. 

<br>
Roll wave experiments were performed using a straight channel with a length of 56m, a width of 10cm, a depth of 15cm and a channel gradient of &theta;=2.5 deg.
The experimental conditions are by a plane water and a flow containing 42% (C=0.42) solid particles.
The particles are cylindrical particles with a typical particle size of d = 3 mm and a density of &sigma;=1.04 g/cm<sup>3</sup> made of polystyrene. 
The flow conditions of the plane water are as follows: mean discharge Q = 1112cm<sup>3</sup>/s, 
mean depth h<sub>0</sub> = 1.23cm at the downstream end of the channel, 
mean flow velocity u<sub>0</sub> = 90.2cm/s, 
and the flow conditions with solid particles Q=1193cm<sup>3</sup>/s, h<sub>0</sub> = 1.35cm, u<sub>0</sub> = 88.4cm/s. 
To supply water to the flume, a water tank storing 0.5m<sup>3</sup> is placed on the upstream side of the flume, 
and water or a mixture of water and particles is supplied from the upstream end of the flume.
The water tank is closed and the inside is kept at a constant pressure according to the Marriott bottle principle. As a result, the water supply is constant.

<br>
In the case of plane water, the period is T = 1.12sec at x = 14m from the upstream end of the flume, 
and at the downstream end x = 56m T = 2.25sec. 
In the flow including solid particles, T = 1.78sec and T = 3.06sec at the same position.
In each case, the surge period becomes longer as the flow goes down. 
The wave velocity of the surge here in the experimental results is different. 
Looking at the details of the waveform, the subsequent surge may catch up.
Therefore, it is considered that the period of the surge becomes longer as it flows down and combines with other surges. 
In the Burgers equation, the initial condition waveform is integrated into a waveform with wave number k = 1 by the initial condition of a non-integer multiple waveform and non-fixed boundary conditions at both ends.
From the above, it is considered that the initial waveform formation of the roll wave is contributed by the weak shock wave equation such as the Burgers equation.
In this Burgers equation, when the waveform is integrated into a waveform with wavenumber k = 1, the analytical solution shows that the phase velocity v<sub>p0</sub> is no longer c<sub>0</sub>.
Therefore, the governing equation returns to the KdV-Burgers equation.


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How to cite: Arai, M.: A research on initial formation process of intermittent debris flow, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8199, https://doi.org/10.5194/egusphere-egu2020-8199, 2020

D2009 |
EGU2020-8720
| Highlight
Velio Coviello, Matteo Berti, Lorenzo Marchi, Francesco Comiti, Giulia Marchetti, Ricardo Carrillo, Shusuke Miyata, and Pierpaolo Macconi

The complete understanding of the mechanisms controlling debris-flow initiation is still an open challenge in landslide research. Most debris-flow models assume that motion suddenly begins when a large force imbalance is imposed by slope instabilities or the substrate saturation that causes the collapse of the channel sediment cover. In the real world, the initiation of debris flows usually results from the perturbation of the static force balance that retains sediment masses in steep channels. These perturbations are primarily generated by the increasing runoff and by the progressive erosion of the deposits. Therefore, great part of regional early warning systems for debris flows are based on critical rainfall thresholds. However, these systems are affected by large spatial-temporal uncertainties due to the inadequate number and distribution of rain gauges. In addition, rainfall analysis alone does not explain the dynamics of sediment fluxes at the catchment scale: short-term variations in the sediment sources strongly influence the triggering of debris flows, even in catchments characterized by unlimited sediment supply.

In this work, we present multi-parametric observations of debris flows at the headwaters of the Gadria catchment (eastern Italian Alps). In 2018, we installed a monitoring network composed of geophones, three soil moisture probes, one tensiometer and two rain-triggered videocameras in a 30-m wide steep channel located at about 2200 m a.s.l. Most sensors lie on the lateral ridges of this channel, except for the tensiometer and the soil moisture probes that are installed in the channel bed at different depths. This network recorded four flow events in two years, two of which occurred at night. Specifically, the debris flows that occurred on 21 July 2018 and 26 July 2019 produced remarkable geomorphic changes in the monitored channel, with up to 1-m deep erosion. For all events, we measured peak values of soil water content that are far from saturation (<0.25 at -20 cm, <0.15 at -40 cm, <0.1 at -60 cm). We derived the time of occurrence and the duration of these events from the analysis of the seismic signals. Combining these pieces of information with data gathered at the monitoring station located about 2 km downstream, we could determine the flow kinematics along the main channel.

These results, although still preliminary, show the relevance of a multi-parametric detection of debris-flow initiation processes and may have valuable implications for risk management. Alarm systems for debris flows are becoming more and more attractive due the continuous development of compact and low-cost distributed sensor networks. The main challenge for operational alarm systems is the short lead-time, which is few tens of seconds for closing a transportation route or tens of minutes for evacuating settlements. Lead-time would significantly increase installing a detection system in the upper part of a catchment, where the debris flow initiates. The combination of hydro-meteorological monitoring in the source areas and seismic detection of channelized flows may be a reliable approach for developing an integrated early warning - alarm system.

How to cite: Coviello, V., Berti, M., Marchi, L., Comiti, F., Marchetti, G., Carrillo, R., Miyata, S., and Macconi, P.: Multi-parametric observations of debris-flow initiation at the headwaters of the Gadria catchment (eastern Italian Alps), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8720, https://doi.org/10.5194/egusphere-egu2020-8720, 2020

D2010 |
EGU2020-8022
Yi-Chao Zeng, Chyan-Deng Jan, Mu-Jung Lin, Ji-Shang Wang, Hsiao-Yuan Yin, and Li-Hsing Kuo

Due to climate change, precipitation characteristics have been significantly variation and rainfall patterns are presented more concentrated, high-intensity and long-duration trend in the past two decades. Catastrophic debris-flow disaster threaten lives and property of residents. For mitigation impact of debris-flow, SWCB (Soil and Water Conservation Bureau, Taiwan) has had a leading role in sponsoring debris-flow research and developing a rainfall-based debris-flow warning model. Early warning criteria for debris-flow triggered are also determined depending on the historical rainfall data, and the observational data of rain-gauge are adopted to issue debris-flow warning. However, application of rain-gauge rainfall data has some disadvantages such as low density in mountain area, observation failure to properly represent actual rainfall condition, and data transmission likely interrupted during heavy rainfall or Typhoon. In order to improve the efficiency of debris-flow warning system, two types of gridded precipitation are analyzed and discussed in this study, which are the spatial interpolation rainfall of rain-gauge and the radar-estimated rainfall (QPESUMS). For comparison the differents of multiple rainfall data mentioned above with rain-guage, the third quartile is firstly applied to calculate the regional representative rainfall from grid cells within warning issued area. The results show that the spatial interpolation rainfall underestimates the rainfall intensity and cumulative rainfall owing to the influence of complex topography. By contrast, the radar-estimated rainfall has the advantage in comprehension of the rainfall spatial variability and provide a more complete spatial coverage. Besides, for assessing the appropriate and feasibility of multiple rainfall data applied to debris flow warning, the disaster–capture ratio has been proposed which is defined as the number of debris-flow hazards after issuing warning divided by total number of debris- flow hazards. According to analyis results of historical disaster records from 2012 to 2016, the disaster–capture ratio are 47.6%, 38.1% and 61.9% as warning issued refer to rain gauge, the spatial interpolation rainfall and the radar-estimated rainfall respectively. By the aforementioned process, we realize that the application of radar-estimated rainfall to debris flow warning is obviously increasing efficiency of debris-flow warning ,and gives assistance for reducing uncertainty of rainfall observational data, especially in mountain area.

How to cite: Zeng, Y.-C., Jan, C.-D., Lin, M.-J., Wang, J.-S., Yin, H.-Y., and Kuo, L.-H.: Study on multiple rainfall data applied to debris flow warning in Taiwan, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8022, https://doi.org/10.5194/egusphere-egu2020-8022, 2020

D2011 |
EGU2020-12406
Natsumi Katayama and Takashi Yamada

It is known that woody debris in a debris flow is concentrated near the flow front. However, the actual state of transportation of woody debris hasn’t been revealed. Accordingly, the purpose of our study is to clear characteristics of transportation of woody debris in debris flows by video footage analysis.

We collected and analyzed video footage of woody debris carried on debris flows and sediment flows. As a result, qualitative characteristics that woody debris was concentrated near the flow front and a lot of woody debris was carried on debris flows were revealed. In our study, the part of woody debris entangled by each other near the flow front is called “woody debris mass”. Woody debris that forms woody debris mass moved with little change in the relative position. When a sediment flow reached the widening part of stream channel and the flow was spread laterally, woody debris mass was broken down and the height of woody debris mass was reduced. Moreover, we measured stream channel width, velocity, flow depth, average length of woody debris, height of woody debris mass, and so on by using video footage. Consequently, a positive correlation was found between “the ratio of the average length of woody debris to the stream channel width” and “the height of woody debris mass”.

Besides, we carried out hydraulic flume experiment on the transportation of woody debris by debris flows. As a result, woody debris mass was formed near the flow front of debris flows. Furthermore, a positive correlation was found between “the ratio of the length of woody debris model to the flume width” and “the height of woody debris mass”. This result was harmonious with that of video footage analysis results.

How to cite: Katayama, N. and Yamada, T.: A study on characteristics of movement of woody debris mass in debris flows by video footage analysis, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12406, https://doi.org/10.5194/egusphere-egu2020-12406, 2020

D2012 |
EGU2020-13778
Hyeong-Jin Kim, Dae-Ho Yun, and Yun-Tae Kim

A debris flow, a mass movement of soil and water mixture, is generally occurred by heavy rainfall during the rainy season in Korea. Because of climate change, the amount and frequency of rainfall has continually increased these days. Populated areas located in debris flow-prone mountainous areas are commonly subject to debris flow hazards. For this reason, it is necessary to analyze the characteristics of the debris flow behavior for the hazard mitigation. In this study, for two samples from Hwangnyeong Mt. and Umyeon Mt. in Korea, the vane-type rheometer test were performed to estimate the rheological property such as viscosity and yield stress and small-scale flume experiment was carried out to evaluate the characteristics of debris flow behaviors such as front velocity, runout distance and deposition volume. From the experimental results, rheological properties decrease with decreasing volumetric sediment concentration, and debris flow behavior gradually increased with decreasing rheological properties in the experiment. Additionally, in case of Hwangnyeong Mt. which has a high silt and clay fraction, the experimental results show that the amount of the front velocity, runout distance and deposition volume tend to increase higher than Umyeon Mt. as viscosity and yield stress decreased.

How to cite: Kim, H.-J., Yun, D.-H., and Kim, Y.-T.: A Study of Debris Flow Behaviors According to Rheometer Properties: Focused on the Hwangnyeong Mt. and Umyeon Mt., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13778, https://doi.org/10.5194/egusphere-egu2020-13778, 2020

D2013 |
EGU2020-15074
| Highlight
Marc-Henri Derron, Valérie Baumann, Tiggi Choanji, François Noël, Ludovic Baron, Simon Hiscox Baux, Aurelien Ballu, Emmanuel Nduwayezu, and Michel Jaboyedoff

Debris flows triggered by heavy rain are common and can cause huge damages in Alpine valleys. In this case we documented the changes occurred in the Losentsé valley after the 11 August 2019 event, which caused two death and several damages to the village of Chamoson. The Chamoson basin is located in the Alps on the right side of the Rhône valley. Three main rivers drain the Chamoson basin, the Losentsé, the Cry and the Tsené. The main debris flow event occurred in the Losentsé sub-basin. The Losentsé River is 9 km long from the sources at 3000 m until the alluvial cone apex at 600 m. In the upper part of the Chamoson basin thick loose debris cones and glacial deposits lie on steep slopes, the geology of the middle basin is formed by unstable clayey shales with several active landslides on both lateral valley slopes.

The village of Chamoson is located on the huge alluvial cone built with torrential events from the three main rivers. Since the XIX century, several big debris flow events (1898, 1923, 2003, 2018) were recorded in this area and mitigation measures were built in the principal rivers. Unfortunately, the 2019 debris flows overflowed the channels limit when the flows reached the alluvial cone apex, reaching the road and took a car with 2 persons inside. Upstream in the middle basin 2 wood bridges were destroyed and many concrete or stone walls (mitigation measures) along the river were damaged.

After the event we acquired pictures with a drone from the sources area and the Losentsé river valley in order to have a post event image. With this image we could analyse and map the source areas and the inundated areas in the Losentsé channel. We did also field observation along the river.

After comparing the pre- and post-event images we mapped the middle and upper basin inundated areas by the 2019 event and the described the deposits and eroded sections along the river. We calculated the peak discharge of 1000 m3/s for this event using the inundated transversal profile area near the cone apex and the flow velocity obtained from a movie. The peak discharge corresponds to 4 in the size classification for debris flows (Jacob et al., 2005).

Reference:

Jakob, M. (2005). A size classification for debris flows. Engineering geology, 79(3-4), 151-161.

How to cite: Derron, M.-H., Baumann, V., Choanji, T., Noël, F., Baron, L., Hiscox Baux, S., Ballu, A., Nduwayezu, E., and Jaboyedoff, M.: Assessment of the 2019 Chamoson debris flow event (Swiss Alps) , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-15074, https://doi.org/10.5194/egusphere-egu2020-15074, 2020

D2014 |
EGU2020-18203
Giacomo Belli, Emanuele Marchetti, Fabian Walter, and Brian McArdell

Debris flows are episodic strongly impacting gravitational currents of generally high density, consisting of mixtures of water and debris in varying proportions and occurring in steep mountain torrents with volumes commonly exceeding thousands of m3. Despite the observation that debris flows are among the most dangerous processes in mountain environments, the moderate flow velocities (typically < 10 m/s) make early warning in principle possible if the flows are detected early upon formation.

Seismic and infrasound studies of debris flows rapidly increased in the last decade but focused mostly on event detectability and application as early-warning systems. Seismic networks, arrays of infrasound sensors and the combined use of a collocated single seismic and infrasound sensors, have turned out to be efficient systems for detecting the occurrence of debris flows in near-real time with a good reliability.

However, open questions remain about the possibility to infer source characteristics and event magnitude from recorded geophysical signals. This requires theoretical source models of elastic energy radiated both in the ground, in the form of seismic waves, and in the atmosphere, in the form of infrasound.  Seismic waves are believed to be generated by both large sediment particles impacts on the channel bed and by turbulent structures within the debris flow. Infrasound is instead believed to be generated by standing waves at the free surface of the flow, but their source processes are not yet fully understood.

Here we present preliminary results of a study performed at the Illgraben catchment (Switzerland), in the 2017-2019 period, combining infrasound and seismic signals with direct in-torrent measurements of flow depth and velocity. Seismic and infrasound signals are analyzed using both spectral analysis and array techniques, in order to achieve an improved understanding of the dynamics of the source mechanisms of the two wavefields. Comparison with in-situ measurements is used to extrapolate empirical relationships between signal features, e.g. amplitude, spectral content or waveform characteristics of both seismic signals and infrasound, and flow characteristics.

The results obtained will possibly be used to develop an efficient monitoring system for remote detection and the early warning of debris flows using seismic signals and infrasound generated by the process.

How to cite: Belli, G., Marchetti, E., Walter, F., and McArdell, B.: Seismo-acoustic analysis of Debris Flows events at the Illgraben catchment, Switzerland, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18203, https://doi.org/10.5194/egusphere-egu2020-18203, 2020

D2015 |
EGU2020-15609
Michele Amaddii, Vincenzo D'Agostino, Leonardo Disperati, and Pier Lorenzo Fantozzi

During the June 19th of 1996 a storm involved the Tyrrhenian sector of northern Tuscany (Italy), especially hitting the Versilia and Garfagnana areas. Major consequences and damages, due to the extremely intense precipitation (about 500 mm/13 h and 158 mm/h peak intensity), occurred in the surrounding of the Cardoso village (Versilia river basin, Stazzema, LU), with 14 casualties. At 1.20 p.m., the rainfall peak intensity coupled with the development of a large number of shallow landslides, triggered rapid flows and caused severe flooding in the Cardoso area, which was covered by hundred thousand of cubic meters of deposits.
The aim of this study was the characterization of the rapid flows occurred during the event and their back analysis numerical modelling by using a hydrological-hydraulic software. First of all, the amount of mobilized solid volume was assessed, differentiating between materials collapsed from the slopes and those eroded from the low-order drainage network. This goal was obtained by visual interpretation of post-event orthophotos and by morphometric analysis. Subsequently, starting from the rainfall data of the event, the hydrological modelling was performed by the Curve Number method, in order to define flood hydrographs along the drainage network of the Cardoso sub-basins. For the hydraulic modelling, the liquid discharge data were used to calculate debris-graphs of rapid flows, by implementing empirical correlations based on peak discharge, debris volume and channel slope. Different rheological parameters were tested to perform numerical modelling.
Back analysis results allow to infer that the mass movements initially started as hyperconcentrated flows in the upper parts of the sub-basins and after evolved into muddy debris flows, which caused flooding of the Cardoso valley. The results are in good agreement with the flooded area extent, as estimated by visual interpretation of both archive photos and aerial orthophotos acquired immediately after the event.

How to cite: Amaddii, M., D'Agostino, V., Disperati, L., and Fantozzi, P. L.: Back analysis numerical modelling of the 19/06/1996 Cardoso (Stazzema, LU - Italy) flood: from gravitational movements to their evolution in rapid flows, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-15609, https://doi.org/10.5194/egusphere-egu2020-15609, 2020

D2016 |
EGU2020-19470
| Highlight
Markus Moser and Susanne Mehlhorn

Recent years have repeatedly witnessed natural disasters throughout Austria, e.g. the catastrophic debris flows of 2012, 2013, 2016, 2017 and 2019 which caused enormous damage and losses in some areas. The impacts of climate change on these events is rather unclear in many cases, it must be assumed that the intensity and frequency of extreme events and natural hazards is likely to increase in future.

Management of bedload/debris flow processes to ensure the protective function is a major challenge. Observing the historical development shows the constant change of design types and constructions in the course of time. Hand in hand with technical progress, lessons learned from events in the light of climate change as well as a higher process understanding the constructions were constantly improved. Other reasons for the development of fitted systems with an integrative catchment-view down to the receiving stream are the high and still rising maintenance and clearance costs. On the basis of these findings and results, recommendations were derived to improve the function fulfilment of the technical protection measures. Furthermore, integrative concepts focus on the adaptation of the alpine forests to climate change. Under the principle, “fit for the future” the recommendations are summarized and presented in this contribution.

How to cite: Moser, M. and Mehlhorn, S.: Debris flow events in Austria - regional strategies for mitigation and adaptation in the light of climate change, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19470, https://doi.org/10.5194/egusphere-egu2020-19470, 2020

D2017 |
EGU2020-16936
Andrea Pasqua, Alessandro Leonardi, and Marina Pirulli

Debris flows consist of mixtures of poorly sized sediments mixed with mater, moving with high speed within natural channels. They pose a constant threat to settlements located on mountainous terrains, with casualties and economic losses reported every year. An efficient numerical model, able to aid in the design of mitigation structures, is a long-sought tool by the community of practitioners.

One of the challenging aspects of debris flows is their complex multiscale nature. Typically, events are characterized by long runouts, with debris transported for kilometres after their initial mobilization. At the same time, the scale of interaction between flow and obstacles is much smaller, because debris-resisting structures are seldom taller than a few meters. For this reason, numerical methods typically focus on one of two aspects: the runout simulation, or the flow-structure interaction problem. This is however problematic: the type of interaction is a function of the equilibrium conditions achieved by the flow during runout, which can hardly be reconstructed if the phenomenon is not reproduced in its entirety.

In an effort to bypass this problem, we present here a coupling strategy between RASH3D, a depth-averaged model based on the shallow-water equation, and the Lattice-Boltzmann Model (LBM), an innovative 3D Navier-Stokes solver. RASH3D is employed for simulating the initial mobilization and flow runout. Before impact with a barrier, the flow variables are converted from their depth-averaged values into full 3D fields, inverting the depth-averaging procedure. The 3D flow-structure interaction is then solved with LBM. The most important and innovating point about this strategy consists in saving computational time using RASH3D without losing any important information (velocity, pressure, volume etc…) at interaction between structures and flow thanks to LBM, thus reconstructing with good precision and efficiency the whole problem.

References:

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

Thorimbert, Y., Lätt, J., & Chopard, B. (2019). Coupling of lattice Boltzmann shallow water model with lattice Boltzmann free-surface model. Journal of Computational Science, 33, 1-10.

How to cite: Pasqua, A., Leonardi, A., and Pirulli, M.: Multi-scale numerical modelling of debris flow: coupling 2D and 3D simulation strategies, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-16936, https://doi.org/10.5194/egusphere-egu2020-16936, 2020

D2018 |
EGU2020-20382
Andreas Schimmel, Matteo Cesca, Pierpaolo Macconi, Velio Coviello, and Francesco Comiti

With the rapid socio-economic development of European mountain areas, the automatic detection and identification of mass movements like landslides, debris flows, and avalanches become more and more important to mitigate related risks by means of early warning systems. Past studies showed that such processes induce characteristic seismic and acoustic signals, the latter mostly in the infrasonic spectrum which can thus be used for event detection. Several investigations have already addressed signal processing and detection methods based on seismic or infrasound sensors. However, for developing an efficient warning system, not only the detection of events is important but also the identification of the event type (e.g. debris flow vs debris flood) and the estimation of its magnitude. So far, no method for such objectives has been developed which is based on the combination of both seismic and infrasonic signals.

This work presents a first approach to identify debris flows and debris floods magnitude based on the integration of infrasound and seismic data. First analysis shows that, for peak discharge, the use of infrasound amplitudes with a power curve fitting offers a good approach for finding an initial relationship between the recorded signals and this event parameter. For an estimation of the total volume, the discharge calculated with the relationship for peak discharge is integrated over the entire detection time of an event. Calculation of the peak discharge based on infrasound data offers a good approximation, but, for the calculation of the total volume, this method shows still a wide variance.

The method will be applied to seismic and infrasound data collected on three different test sites in the Alps: Gadria (South Tyrol, Italy), Lattenbach (Tyrol, Austria), and Cancia (Belluno, Italy).

How to cite: Schimmel, A., Cesca, M., Macconi, P., Coviello, V., and Comiti, F.: Debris flow magnitude estimation based on infrasound and seismic signals, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20382, https://doi.org/10.5194/egusphere-egu2020-20382, 2020

D2019 |
EGU2020-21082
Rajesh Kumar Dash and Debi Prasanna Kanungo

Debris flows are one of the most frequently occurring and destructive hazards in Indian Himalayas which are often initiated by rainfall.  To minimize the losses due to the destructive power of the debris flows, demarcation of debris flow risk zones is an effective practice for risk reduction. In the present study, site specific debris flow risk assessment has been carried out based upon runout behaviour modeling. Tangni debris flow is an active debris flow in the Chamoli district of Garhwal Himalayas, India which is responsible for disrupting the traffic by blocking the road for days. This debris flow is repetitive in nature and occurs many a times every year in the monsoon during the months between June to September. The Tangni debris flow is categorized as a hill slope debris flow and the failure is considered as a block failure. Runout modeling of Tangni debris flow has been carried out using a Voellmy approach based continuum model. Quantitative information on debris flow intensity parameters such as flow velocity, height and pressure was obtained from the numerical simulation. The calibration of model input parameters was done by back analysis of an event from a particular source area that took place in 2013. Depending upon the amount of materials present in different source areas in the entire source zone and using the calibrated model input parameters, several simulations were performed to assess the flow behaviour of at different possible scenarios. Thus, Tangni debris flow risk assessment has been carried out based on its runout effect modeling. This study revealed that there may be a possibility of damming of river as well as blocking of the National Highway which are located at the downstream of the debris flow.

Key words:  Debris flow, Risk assessment, Runout modeling, Garhwal Himalayas, Voellmy model

How to cite: Dash, R. K. and Kanungo, D. P.: Runout modeling based debris flow risk assessment: a case study from Garhwal Himalaya, India, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21082, https://doi.org/10.5194/egusphere-egu2020-21082, 2020

D2020 |
EGU2020-1469
Kaiheng Hu, Xiaopeng Zhang, and Jinbo Tang

Several high-magnitude glacial debris flows happened at Sedongpu, a tributary of Yarlung Tsangpo river in the southeastern Tibet in 2018. The hazards blocked the main river twice and inundated the road and bridge to Jiala village on the foot of Namche Barwa massif. The glacial dammed lake with an impounded water of 0.6 billion m3 broke out and caused an outburst flood of peak discharge ~ 30,000 m3/s on October 19. A comprehensive methodology was developed to assess the potential hazard of the glacial-debris-dammed lake before the outburst. Multi-temporal remote sensing image interpretation was used to obtain the frequency-magnitude relationship. The debris-flow deposition and dam height were estimated via numerical simulation of 2-D shallow water equations. Then, the impoundment area and potential inundation were analysed by GIS spatial analysis. We also test different hydrological empirical models of calculating the peak discharge of glacial-debris lake outburst floods. With regard to the Sedongpu event, the 1985 Costa’s model shows best agreement with the measured data.

How to cite: Hu, K., Zhang, X., and Tang, J.: Risk analysis of the 2018 Sedongpu glacial debris flows in the southeastern Tibet, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1469, https://doi.org/10.5194/egusphere-egu2020-1469, 2019

D2021 |
EGU2020-2791
Huayong Chen, Chunran Cao, Xiaoqing Chen, and Jiangang Chen

Besides the numerous artificial dams, there are some other kind of dams distribute such as the glacier dams, moraine dams, landslide dams, and the debris flow dams in China. Especially, the landslide dams and debris flow ones widely distribute in southwest of China after the M8.0 Wenchuan earthquake. Much attention has been paid to the formation, stability, breach process, and the peak discharge prediction of a landslide dam. However few achievements are obtained on the debris flow dams even if the failure of a debris flow dam has posed great threat to the property and life of residents downstream. In this paper, based on the main difference between a landslide and debris flow dam, experiments were conducted by considering different clay content, the initial water content, and incoming water flow. It indicated that the failure duration of a debris flow dam was about 1.60 times as long as that than that of a landslide dam. The peak discharge at the debris flow dam breach was 5.38 L/s. However, the peak discharge at the landslide dam was 7.50 L/s, which was 1.39 times as big as that of a debris flow dam. Finally, by modifying the existing critical initialization condition for the landslide dams, the critical initialization condition for a debris flow dam was proposed.

How to cite: Chen, H., Cao, C., Chen, X., and Chen, J.: Comparison on the failure process and mechanism between a debris flow and a landslide dam, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2791, https://doi.org/10.5194/egusphere-egu2020-2791, 2020

D2022 |
EGU2020-2813
Shuai Li, Xiaoqing Chen, Chong Peng, and Jiangang Chen

Drainage channel with step-pool systems are widely used to control debris flow. However, the blocking of debris flow often gives rise to local damage at the steps and baffles. Hence, the estimation of impact force of debris flow is crucial for design step-pools channel. This paper presents a numerical study on the impact behavior of debris flows using SPH (Smoothed Particle Hydrodynamics) method. Some important parameters, such as the baffle shape (square, triangle, and trapezoid) and the densities of debris flows are considered to examine their influence on the impact force. The results show that the largest peak impact force is obtained at the second last baffle, rather than the first baffle. Moreover, the square baffle gives rise to the largest impact force whereas the triangle baffle bears the smallest one among the three baffles. Generally, the peak impact force increases with increasing the inflow density. However, a threshold density, beyond which the peak impact force will decrease, is suggested by the simulations. Based on the numerical results, an improved expression to predict the impact force considering the inclined angle of baffle is proposed.

How to cite: Li, S., Chen, X., Peng, C., and Chen, J.: Numerical study on debris flow in step-pools channel using smoothed particle hydrodynamics method, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2813, https://doi.org/10.5194/egusphere-egu2020-2813, 2020

D2023 |
EGU2020-2865
Xiaoqing Chen, Jiangang Chen, and Wanyu Zhao

UNESCO designated 1121 properties with outstanding universal value, including 869 cultural sites, 213 natural sites and 39 mixed sites, from 167 states parties as world heritage sites at the end of 2019. Some of them are threatened by geological disasters, especially, the landslides and debris flows become the most frequent hazard type at world heritage sites. Until 2019, China has 55 world heritage sites and ranks first in the world, with 24 places under threat from different types of geological disasters and these disasters directly or indirectly threaten the security of heritage points. The forest coverage rate in Jiuzhaigou valley is more than 80%, and the collapse, rock fall, landslide and other disasters induced by the Jiuzhaigou earthquake on August 8, 2017 have caused extensive forest destruction. We found that there are a lot of large wood (LW) in Jiuzhaigou valley that can be transported. According to previous study results, the process of blocking-outburst in gullies will appear with a large number of LW when transported along with debris flows. Compared with the discharge amplification effect of the debris flow in natural gully, the blocking-outburst effect of LW also intensifies the damage. The process of blockage and outburst with LW movement causes the discharge amplification of debris flow, while the discharge amplification coefficient determines the accuracy of discharge calculation, in further it affects the accuracy of engineering design parameters. Moreover, the LW carried in the debris flow may cause strong impact damage to check dams and other engineering measures. Therefore, we take the debris flow occurred in the Jiuzhaigou valley as an example to investigate the characteristics of the magnitude amplification ratio.

How to cite: Chen, X., Chen, J., and Zhao, W.: The clogging and outbreak processes of debris flow with large wood in Jiuzhaigou Valley, China, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2865, https://doi.org/10.5194/egusphere-egu2020-2865, 2020

D2024 |
EGU2020-4306
| Highlight
Ningsheng Chen

Abstract: In recent years, the increasing frequency of debris flow demands enhanced effectiveness and efficiency are essential not only from an economic point of view but are also considered as a frontline approach to alleviate hazards. Currently, the key issues are the imbalance between the limited lifespan of equipment, the relatively long period between the recurrences of such hazards, and the wide range of critical rainfall that trigger these disasters. This paper attempt to provide a stepwise multi-parameter debris flow warning system after taking into account the shortcomings observed in other warning systems. The whole system is divided into five stages. Different warning levels can be issued based on the critical rainfall thresholds. Monitoring starts when early warning is issued and it continues with debris flow near warning, movement warning and hazard warning stages. For early warning, historical archives of earthquake and drought are used to choose a debris flow susceptible site for further monitoring, Secondly, weather forecasts provide an alert of possible near warning. Hazardous precipitation, model calculation and debris flow initiation tests, pore pressure sensors and water content sensors are combined to check the critical rainfall and to publically announce a triggering warning. In the final two stages, equipment such as rainfall gauges, flow stage sensors, vibration sensors, low sound sensors and infrasound meters are used to assess movement processes and issue hazard warnings. In addition to these warnings, community-based knowledge and information is also obtained and discussed in detail. The proposed stepwise, multi-parameter debris flow monitoring and warning system has been applied in Aizi valley China which continuously monitors the debris flow activities.

How to cite: Chen, N.: Outlining a stepwise, multi-parameter debris flow monitoring and warning system: an example of application in Aizi Valley, China, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4306, https://doi.org/10.5194/egusphere-egu2020-4306, 2020

D2025 |
EGU2020-6243
Yong Li

Granular effects in debris flows are usually assessed by dimensionless numbers, such as numbers of Savage, Bagnold, and Iverson, which measure the relative significance of granular interaction, and the values indicate that the granular effects are generally ignorable. But observations suggest robust phenomena pertain to grain composition in many ways. This implies that the dimension analysis does not apply to the recognition of granular behaviors in debris flows, partly because we have not really a direct description of changes in grain compositions of debris flows. We have proposed and confirmed that for debris flows the material grain size distribution (GSD) satisfies a unified function, P(D) = C*power(D, – μ)*exp(–D/Dc), where P(D) is the exceedance percentage of grains beyond size D (mm), and C, μ, and Dc are parameters, with a semi-log relationship between C and μ. Then the grain composition is characterized by the GSD parameters μ, and Dc, respectively representing the fine and coarse content of the materials. In this study we present a variety of appearances to illustrate how grain compositions impact on the initiation, formation, motion, and deposition of debris flow. Results indicate that debris flow occurs through a selection mechanism in which soil or sediment blocks of different grain compositions initiate in different ways and form separate surges in different flow regimes. The flow properties (X), such as the speed, the discharge, the density, are all dependent on the GSD parameters in power laws: X ~ power(μ, –m) and X ~ power (Dc, n); and the power laws impose constraints on the fluctuation of the dynamical quantities. In particular, the GSD evolves from the randomly aggregated grains to the fluid with some self-organized constitute.

 

How to cite: Li, Y.: granular effects in debris flows , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6243, https://doi.org/10.5194/egusphere-egu2020-6243, 2020

D2026 |
EGU2020-6686
Chih-Hao Hsu, Chuan-Yi Huang, Ting-Chi Tsao, Hsiao-Yuan Yin, Hsiao-Yu Huang, and Keng-Ping Cheng

This study added the dams and retain basin according to their dimensions measured with UAV onto the original 5m-resolition DEM to compare the effect of mitigation structures to debris flow hazard. The original and the modified DEMs were both applied to simulate the consequences by using RAMMS::Debris Flow (RApid Mass Movement Simulation) model.

Hazard map is the best tool to provide the information of debris flow hazard in Taiwan. It has an important role to play in evacuating the residents within the affected zone during typhoon season. For the reason, debris flow hazard maps become a useful tool for local government to execute the evacuation. As the mitigation structure is constructed, the intensity of debris flow hazard reduces.

The Nantou DF190 debris flow potential torrent is located in central Taiwan. In 1996 when Typhoon Herb stroke, 470,000 cubic-meter of debris were washed out and deposited in 91,200 square-meter area (Yu et al., 2006), and the event caused the destruction of 10 residential houses with 2 fatalities. After the event the Soil and Water Conservation Bureau constructed a 100-meter long sabo dam and sediment retain basin with capacity of 60,000 cubic-meters. In order to compare the difference of affected zone before and after the construction of mitigation structures, the study applies RAMMS to simulate the above-mentioned event.

The result shows when large-scale debris flow occurs, most of the sediments still overflow and deposit on the fan with shape similar to the 1996 Typhoon Herb event. However, the intensity has reduced significantly with 50% less in area, several meters less in inundation depth and 50% less in flow velocity approximately. The comparison shows the effect of mitigation structures and could provide valuable information for debris flow hazard mapping.

Key Words: Debris flow, RAMMS, Hazard map, Mitigation, Taiwan

How to cite: Hsu, C.-H., Huang, C.-Y., Tsao, T.-C., Yin, H.-Y., Huang, H.-Y., and Cheng, K.-P.: Debris flow hazard mapping considering the effect of mitigation structure – a case study in Taiwan based on numerical simulation, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6686, https://doi.org/10.5194/egusphere-egu2020-6686, 2020

D2027 |
EGU2020-9461
| Highlight
Li Xiang

Abstract: Based on 2246 research articles focusing on the topic of “debris flow” derived from Science Citation Index (SCI) database which were published from 2010 to 2019, this article presents a comprehensive review on the global scientific outputs in this research field. By adopting bibliometric analysis, the most productive journals, authors, institutions and countries were identified. Combining with the visual software, the temporal change of the cooperation scope, degree and intensity on country- and institution- level were discussed. It also provides in-depth investigations on the co-occurrence of author key words, which may contribute to reveal the current research hotspots and future development trends. The results of this study can provide a broad insight for scientific community devoting to debris flow related research field and support for the development of other related research work.

How to cite: Xiang, L.: An overview of the evolution of global debris flow related research from 2010 to 2019 — A bibliometric analysis on based on Web of Science Core Collection, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9461, https://doi.org/10.5194/egusphere-egu2020-9461, 2020

D2028 |
EGU2020-17251
Georg Nagl, Johannes Hübl, and Roland Kaitna

The internal deformation behavior of natural debris flows is of interest for model development and model testing for debris-flow hazard mitigation. The pulsing nature of debris flows can increase the runout length by remobilization of deposited material. Up to now, only a view attempts were made to measure internal deformation behavior in natural debris-flow surges due to the low predictability and high destructive power of these flows.  In this contribution we present recent advances of measuring in-situ velocity profiles together with flow parameters like flow height, basal normal stress, and pore fluid pressure. For that a fin-shaped monitoring barrier was constructed in the Gadria creek (IT), laterally carrying an array of paired conductivity sensors. We present results from a debris-flow event in 2019 with 20 surges and flow heights up to 2 m. We observe changing velocity profiles during the passage of the surges and identify deposition and remobilization domains in the flow. The flows exhibited significant longitudinal changes of flow properties like flow height and density. The liquefaction ratios reached values up to unity in some sections of the flows. Between surges, the lower levels of the flow deposited and were subsequently overridden by the next surge and reactivated. These measurements gain new insights of the dynamics of surges of a real-scale debris flow.

How to cite: Nagl, G., Hübl, J., and Kaitna, R.: Insights into a pulsing debris flow, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-17251, https://doi.org/10.5194/egusphere-egu2020-17251, 2020