Debris Flow Initiation Research Articles

A supply-limited torrent that does not feel the heat of climate change

Debris-flow activity in the Alps is anticipated to undergo pronounced changes in response to a warming climate. Yet, a fundamental challenge in comprehensively assessing changes in process activity is the systematic lack of long-term observational debris-flow records. Here, we reconstruct the longest, continuous time series (1626-2020) of debris flows at Multetta, a supply-limited torrential system in the Eastern Swiss Alps. Relying on growth-ring records of trees that were damaged by debris flows, we do not detect significant changes in the frequency or magnitude over time. This seeming absence of a direct climatic influence on debris-flow initiation aligns with the regular distribution of repose time patterns, indicating a dependence of local process activity on sediment discharge and recharge. This stark difference in process behavior between our supply-limited site and transport-limited catchments has implications for assessing torrential hazard and risk mitigation in a context of global warming.

Initiation and Kinematic Process of Debris Flow with the Existence of Terraced Fields at the Sources

Initiation and Kinematic Process of Debris Flow with the Existence of Terraced Fields at the Sources

Determining the debris flow yield strength of weathered soils: a case study of the Miryang debris flow in the Republic of Korea

Debris flow hazards are often interpreted through back-calculated simulation analysis or empirical methods. The mobility of a debris flow is greatly influenced by mechanical and hydrological parameters. The strength parameters play important roles in the debris flow initiation and flow stages. In particular, the rheological parameters of yield strength and plastic viscosity directly affect the debris flow runout distance and velocity. One of the most important parameters to consider when evaluating debris flow hazards is the shear strength. This strength is called the residual shear strength in the failure stage and the yield strength in the post-failure stage. The residual shear strength obtained from ring shear tests can be related to the initiation of mass movements; the yield strength obtained from rheological tests can be related to the mobilization of debris flows. The residual shear stresses obtained from ring shear tests of weathered soils typically range between 10 and 100 kPa and strongly depend on the normal stress and shear velocity. When progressive slope failure (i.e., strain-softening behavior) occurs at a relatively shallow slope depth (e.g., < 1 m), the soil strength ranges from approximately 5–10 kPa. If the liquid limit state (i.e., solid‒liquid transition) is reached, the shear strength of the soil is approximately 2 kPa. Once the soil fails and mixes with ambient water along the slip surface, the yield strength decreases dramatically, resulting in high mobilization. A suggestion on how strength parameters can be applied to estimate debris flow mobility is presented by considering the 2011 Miryang debris flow, which occurred in weathered soil deposits in Miryang city, Republic of Korea. The best approach for debris flow yield strength estimation would be to consider the residual shear strength in the initiation stage, the yield strength in the flow stage, and the reduction in yield strength with the entrainment effect of the flow in the rapid fluidization stage.

Evaluation of highway debris flow hazard based on geomorphic evolution theory coupled with material response rate

Assessments of highway feasibility frequently lack the detailed data and geological information necessary to conduct hazard evaluations of debris flows. This study discusses the processes of debris flow development when regional rainfall meets the critical level required for debris flow initiation. It utilizes geomorphic evolution theory and establishes a regional risk assessment matrix for debris flow by combining information about gullies and source sensitivity. Considering the location relationship between the highway and debris flow gullies, a rapid evaluation method for debris flow risk assessment along the G318 highway in Sichuan Province is proposed by modifying the judgment matrix. The four debris flow gullies constructed during the upgrading project in Yajiang County, stretching from the west of the city to the Shearer Bay section, were analyzed via examples. The results show that, among the four selected debris flow gullies, two had medium hazard levels, and two had high hazard levels. The validation results are consistent with the actual results, implying that the evaluation method used in this study is accurate and feasible. This method is suitable for the rapid evaluation of debris flow disaster hazards in the feasibility assessment stage of a highway because it relies on readily available data sources, and the evaluation results are fast and convenient. The highway passes through four debris flow gullies, which directly impact the alignment of this particular section of the geological route and the engineering layout. Based on current specifications, the maximum impact range of a one-time debris flow under the given frequency conditions was calculated using the "rainfall method." The results showed that the maximum impact ranges of a debris flow, occurring once in 100 years, for four gullies would be 9.08 m, 9.09 m, 10.86 m, and 10.08 m. The safe clearance heights of bridges over the four gullies should be 14.58 m, 14.59 m, 16.36 m, and 16.3 m. Additionally, the safety clearance width for all gullies should be 5.0 m.

Evaluation of rainfall threshold models for debris flow initiation in the Jiangjia Gully, Yunnan Province, China

Evaluation of rainfall threshold models for debris flow initiation in the Jiangjia Gully, Yunnan Province, China

The role of volcanic ash thickness on the hydraulic conductivity of the ground and the initiation of debris flows

The role of volcanic ash thickness on the hydraulic conductivity of the ground and the initiation of debris flows

Characteristics of debris-flow-prone watersheds and debris-flow-triggering rainstorms following the Tadpole Fire, New Mexico, USA

Abstract. Moderate- or high-severity fires promote increases in runoff and erosion, leading to a greater likelihood of extreme geomorphic responses, including debris flows. In the first several years following fire, the majority of debris flows initiate when runoff rapidly entrains sediment on steep slopes. From a hazard perspective, it is important to be able to anticipate when and where watershed responses will be dominated by debris flows rather than flood flows. Rainfall intensity averaged over a 15 min duration, I15, in particular, has been identified as a key predictor of debris flow likelihood. Developing effective warning systems and predictive models for post-fire debris flow hazards therefore relies on high-temporal resolution rainfall data at the time debris flows initiate. In this study, we documented the geomorphic response of a series of watersheds following a wildfire in western New Mexico, USA, with an emphasis on constraining debris flow timing within rainstorms to better characterize debris-flow-triggering rainfall intensities. We estimated temporal changes in soil hydraulic properties and ground cover in areas burned at different severities over &gt;2 years to offer explanations for observed differences in spatial and temporal patterns in debris flow activity. We observed 16 debris flows, all of which initiated during the first several months following the fire. The average recurrence interval of the debris-flow-triggering I15 is 1.3 years, which highlights the susceptibility of recently burned watersheds to runoff-generated debris flows in this region. All but one of the debris flows initiated in watersheds burned primarily at moderate or high soil burn severity. Since soil hydraulic properties appeared to be relatively resilient to burning, we attribute reduced debris flow activity at later times to decreases in the fraction of bare ground. Results provide additional constraints on the rainfall characteristics that promote post-fire debris flow initiation in a region where fire size and severity have been increasing.

Constructing Rainfall Threshold for Debris Flows of a Defined Hazardous Magnitude

Debris flow can cause damage only when its discharge exceeds the drainage capacity of the prevention engineering. At present, most rainfall thresholds for debris flows mainly focus on the initiation of debris flow and do not adequately consider the magnitude and drainage measures of debris flows. These thresholds are likely to initiate numerous warnings that may not be related to hazardous processes. This study proposes a method for calculating the rainfall threshold that is related to a defined level of debris flow magnitude, over which certain damage may be caused. This method is constructed by using the transient rainfall infiltration analysis slope stability model (TRIGRS) and the fluid dynamics process simulation model (MassFlow). We first use the TRIGRS model to analyze slope stability in the study area and obtain the distribution of unstable slopes under different rainfall conditions. Afterward, the MassFlow model is employed to simulate the movement process of unstable slope units and to predict the depositional processes at the mouth of the catchment. Lastly a rainfall threshold is constructed by statistically analyzing the rainfall conditions that cause debris flows flushing out of the given drainage ditch. This method is useful to predict debris flow events of a hazardous magnitude, especially for areas with limited historical observational data.

Experimental Investigation on Scouring v.s. Mass Failure of Unsaturated Soil Bed: Implications for Debris Flow Initiation and Erosion

AbstractScouring and mass failure are two common mechanisms used to describe soil bed erosion, but their combined effects are often not considered. To better understand how these mechanisms compete and under what conditions they prevail, it is essential to consider infiltration and a more realistic unsaturated soil bed. This study investigates soil bed erosion by considering unsaturated soil mechanics, a wetting front, and both erosion mechanisms of scouring and mass failure. Physical experiments were conducted on model water runoff over an unsaturated sand bed to investigate the effects of soil water content and flow velocity on erosion. Experimental results show that current understanding of soil bed erosion can be enhanced by adopting unsaturated soil mechanics and considering the combined effects of scouring and mass failure. The scouring rate is found to be independent of the bed water content because it only affects the uppermost soil particles, which immediately become saturated once water flows over them. Mass failure, on the other hand, is initiated at the wetting front when the rate of infiltration exceeds that of scouring. The depth of mass failure can be described by the net infiltration depth, which is defined as the difference between the infiltration and scouring depths. The net infiltration depth is jointly governed by the soil water content and flow velocity. The crucial role of the coupled effects of the hydro‐mechanical behavior of unsaturated soil in the realistic modeling of soil bed erosion is demonstrated. Outcomes present advancement toward improved hazard assessments of debris flows.

A landscape scale model to predict post-fire debris flow impact zones

Post-fire debris flows pose a significant hazard in mountainous regions, but accurate assessment of the associated risk at large scales, particularly in the context of mega-fires, is limited. Although numerous models exist, few are functional at the landscape scale, and none integrate assessments of downstream effects with the likelihood of flow initiation to define both the frequency and magnitude of potential flows. The aim of this work, therefore, was to develop a post-fire debris flow model that could characterise both the occurrence frequency and associated runout of debris flow events and which could be rapidly applied to fire events at the landscape scale. We achieve this using a model integration approach that included the development of a reduced-complexity debris flow runout model, which was calibrated and independently tested for its application at large scales with a dataset of 1377 individual post-fire flows across south-eastern Australia. This model was then integrated with previously published methods determining debris flow source areas and likelihood of initiation specific to our study area. The developed model is intentionally designed for rapid response hazard management. Therefore, it predominantly incorporates only the fundamental, lowest parameter form drivers of debris flow initiation and runout termination, thus minimising computational complexity while ensuring prediction accuracy sufficient for hazard assessment even at application scales as large as mega-fires. Accounting for debris flows initiated from single headwaters and cases where flows simultaneously converge from multiple initiation points, our integrated model effectively characterises debris flow hazard across a geomorphologically diverse landscape encompassing >200,000km2 (capturing ~640,000 potential debris flow initiation locations across Victoria, Australia). Model testing using an independent validation dataset resulted in debris flow runout predictions with root mean square error (RMSE) of 176 and 308 m for non-convergent and convergent flows respectively. Crucially, these predictions can be computed rapidly for large (up to landscape scale) fire events with minimal input requirements and the use of a remotely-sensed observation dataset for domain calibration. As such, the model provides a critical means of addressing the capacity gap in assessments of post-fire debris flow hazard by efficiently integrating measures of occurrence frequency and runout for functional application at large scales. This new work empowers land managers to rapidly predict the hazard arising from post-fire erosion processes and represents an important applied step towards better understanding and mitigating the impacts of wildfire-induced debris flows.

Prediction of debris flow initiation points using high resolution DEM and GIS analysis in South Korea

This study presents a GIS analysis model that utilizes high-resolution Digital Elevation Map data to predict the initiation points of debris flow through the overlap of slope stability analysis techniques and rainfall flow analysis. The debris flow that occurred on Mount Woomyun in Seoul, South Korea in 2011 was selected as the case study for comparing and analyzing the prediction model against the actual case. In South Korea, occurrences of debris flow mostly happen during the summer monsoon season. When a substantial amount of rainfall accumulates and heavy rainfall events occur, the pore water pressure within slopes reaches saturation. Consequently, flow is initiated along “temporary streams.” In this study, areas with a combination of low slope stability and abrupt changes in velocity vectors were assumed to have the highest probability of debris flow occurrence due to the overlapping of these factors. The research model was conducted under this assumption. To evaluate the performance of the model, 3D coordinates of the model’s result point, and actual occurrence points were obtained, and a Pearson correlation analysis was conducted to compare them. With these results, R2 values of X = 0.9147, Y = 0.8625, and H = 0.8942 were obtained. These high R2 indicate a strong correlation between the model’s predictions and actual occurrences.

Accurate and Intelligent Early Warning Method of Debris Flow Formation Based on IGWO-LSTM Algorithm

To improve the accuracy of debris flow forecasts and serve as disaster prevention and mitigation, an accurate and intelligent early warning method of debris flow initiation based on the IGWO-LSTM algorithm is proposed. First, the entropy method is employed to screen the early warning indicators. Then, the improved grey wolf algorithm (IGWO) is obtained by optimizing the grey wolf algorithm by combining elite reverse learning and adaptive convergence factors. Finally, the IGWO-LSTM algorithm is obtained by using IGWO to improve the total connection layer weight and bias parameters of LSTM, which takes the screened early warning indicators as input and outputs the early warning results of the debris flow formation risk level. In comparison with the methods introduced in earlier studies, the results demonstrate that the proposed method achieves superior outcomes in terms of assessing a single warning of multiple debris flow gullies, a multi-year warning of a single debris flow gully, and a multi-year warning of multiple debris flow gullies. The mean absolute error and root mean square error of the early warning results of the ANN model and PEEM method show low values, while the early warning hit rate shows high values, surpassing 90%. Also, the other two methods developed in the previous studies show low values of the early warning coverage rate, reaching 90% at most. Moreover, the triggered traffic model and MLPG method show high values in the early warning coverage rate, exceeding 90%, and low values in the early warning hit rate of less than 90%, and the average absolute error and root mean square error are high. On the other hand, the results of the proposed method show that the overall early warning hit rate is higher than 95%, the coverage rate is close to 100%, and the error is less than 1.5. Thus, the comprehensive analysis results show that the proposed method has better performance and higher reliability than other studied methods.

Research on Rainfall Intensity Threshold of Occasional Debris Flow Based on Infiltration

The rainfall warning method for debris flows usually uses rainfall intensity and duration to establish an I-D relationship internationally and determine the rainfall warning threshold for debris flows. This method requires extensive rainfall data from debris flow events in the study area to establish the I-D relationship. However, some areas with occasional debris flows lack sufficient debris flow events to establish I-D relationships to determine rainfall warning thresholds. Therefore, this study uses the infiltration effect of water flow on gravel soil and establishes a rainfall intensity threshold judgment formula for debris flow initiation based on the limit equilibrium method. Taking the Taiqing debris flow that occurred in Laoshan, China, on June 13, 2018, as an example, the rainfall intensity and characteristics of the debris flow are analyzed. The maximum rainfall intensity during this rainfall process far exceeds the rainfall intensity threshold determined by the judgment formula. Using the judgment formula, it can be determined that the rainfall process will cause debris flow. The judgment result is consistent with the actual situation (where a debris flow occurred during the rainfall process). To further verify the accuracy of the judgment formula, the rainfall process of Typhoon Lichma on August 11, 2019, in the study area was analyzed. The rainfall process has a long history. Still, the rainfall intensity is much lower than the threshold of rainfall intensity for the initiation of debris flow, so this rainfall will not cause the occurrence of debris flow. The judgment result is consistent with the actual situation (no debris flow occurred during rains). Doi: 10.28991/CEJ-2023-09-09-02 Full Text: PDF

The Reason for the Rise in Critical Shear Stress on Sloping Beds

In sediment mechanics, the conditions for the onset of sediment movement are of particular importance. However, despite decades of research, questions remain unanswered. Thus, physical logic suggests that sediments move more easily on beds inclined in the direction of flow than on horizontal beds and that transport rates are correspondingly increased. However, numerous studies have shown that sediments require increased rather than decreased shear stress to move on sloping beds and that transport rates are lower instead of increased. Since an early publication on this issue by Ashida and Michiue in 1973, many possible effects have been suggested for this apparent contradiction. The literature almost consistently concludes that high relative roughness (d/h), i.e., low water cover (h/d), is mainly responsible for this contradictory effect. This is true for current-induced sediment transport and for the initiation of debris flow. In this paper, an analytical solution for the effect of small water coverage on the transport process is developed. Effects of low coverage also occur on beaches during wave runup and runoff and thus control the formation of steep or less steep beaches. The present paper shows that the effect of turbulence damping occurring under low water coverage plays a decisive role here.

A Depth‐Averaged Material Point Method for Shallow Landslides: Applications to Snow Slab Avalanche Release

AbstractShallow landslides pose a significant threat to people and infrastructure. Despite significant progress in the understanding of such phenomena, the evaluation of the size of the landslide release zone, a crucial input for risk assessment, still remains a challenge. While often modeled based on limit equilibrium analysis, finite or discrete elements, continuum particle‐based approaches like the Material Point Method (MPM) have more recently been successful in modeling their full 3D elasto‐plastic behavior. In this paper, we develop a depth‐averaged Material Point Method (DAMPM) to efficiently simulate shallow landslides over complex topography based on both material properties and terrain characteristics. DAMPM is a rigorous mechanical framework which is an adaptation of MPM with classical shallow water assumptions, thus enabling large‐deformation elasto‐plastic modeling of landslides in a computationally efficient manner. The model is here demonstrated on the release of snow slab avalanches, a specific type of shallow landslides which release due to crack propagation within a weak layer buried below a cohesive slab. Here, the weak layer is considered as an external shear force acting at the base of an elastic‐brittle slab. We verify our model against previous analytical calculations and numerical simulations of the classical snow fracture experiment known as Propagation Saw Test. Furthermore, large scale simulations are conducted to investigate cross‐slope crack propagation and the complex interplay between weak layer dynamic failure and slab fracture. In addition, these simulations allow us to evaluate and discuss the shape and size of avalanche release zones over different topographies. Given the low computational cost compared to 3D MPM, we expect our work to have important operational applications in hazard assessment, in particular for the evaluation of release areas, a crucial input for geophysical mass flow models. Our approach can be easily adapted to simulate both the initiation and dynamics of various shallow landslides, debris and lava flows, glacier creep and calving.

The role of thermokarst evolution in debris flow initiation (Hüttekar Rock Glacier, Austrian Alps)

Abstract. A rapid sequence of cascading events involving thermokarst lake outburst, local rock glacier front failure, debris flow development, and river blockage hit Radurschl Valley (Ötztal Alps, Tyrol) on 13 August 2019. Compounding effects from permafrost degradation and drainage network development within the rock glacier initiated the complex process chain. The debris flow dammed the main river of the valley, impounding a water volume of 120 000 m3 that was partly drained by excavation to prevent a potentially catastrophic outburst flood. We present a systematic analysis of destabilizing factors to deduce the failure mechanism. The identification and evaluation of individual factors reveals a critical combination of topographical and sedimentological disposition, climate, and weather patterns driving the evolution of a thermokarst drainage network. Progressively changing groundwater flow and storage patterns within the frozen sediment accumulation governed the slope stability of the rock glacier front. Our results demonstrate the hazard potential of active rock glaciers due to their large amount of mobilizable sediment, dynamically changing internal structure, thermokarst lake development, and substantial water flow along a rapidly evolving channel network.

Triggering conditions, runout, and downstream impacts of debris flows following the 2021 Flag Fire, Arizona, USA

Debris flows pose a serious threat to communities in mountainous areas, particularly in the years following wildfire. These events have been widely studied in regions where post-wildfire debris flows have been historically frequent, such as southern California. However, the threat of post-wildfire debris flows is increasing in many regions where detailed data on debris-flow physical properties, volume, and runout potential are sparse, such as the Southwest United States (Arizona and New Mexico). As the Southwest becomes more vulnerable to these hazards, there is an increasing need to better characterize the properties of post-wildfire debris flows in this region and to identify similarities and differences with nearby areas, particularly southern California, where there is a greater abundance of data. In this paper, we study the characteristics and downstream impacts of two post-wildfire debris flows that initiated following the 2021 Flag Fire in northern Arizona, United States. We gathered data regarding soil hydraulic properties, rainfall characteristics, watershed response, and debris-flow initiation, runout, volume, grain size, and downstream impacts during the first two monsoon seasons following the containment of the Flag Fire. We also applied established debris-flow runout and volume models that were developed in southern California to our study watershed and compared the output with observations. In the first monsoon season following the fire, there were two post-wildfire debris flows, one of which resulted in damage to downstream infrastructure, and one major flood event. We found that, while more intense rainfall is required to generate debris flows at our study site compared to southern California, burned watersheds in northern Arizona are still susceptible to debris flows during storms with low recurrence intervals in the first year following fire. During the second monsoon season, there were no major runoff events, despite more intense storms. This indicates that the temporal window for heightened debris-flow susceptibility at our study area was less than one year, due to the recovery of soil hydraulic properties and vegetation regrowth. We also found that the debris-flow properties at our study site, such as volume, mobility, and grain size distribution, may differ from those in other regions in the western United States, including southern California, potentially due to regional differences in rainfall characteristics and sediment supply. Differences in rainfall characteristics and sediment supply may have also influenced the performance of the debris-flow runout and volume models, which overpredicted the observed runout distance by 400 m and predicted a volume more than 17 times greater than what was observed.

Changes of hydro-meteorological trigger conditions for debris flows in a future alpine climate

Debris-flow activity is strongly controlled by hydro-meteorological trigger conditions, which are expected to change in a future climate. In this study we connect a regional hydro-meteorological susceptibility model for debris flows with climate projections until 2100 to assess changes of the frequency of critical trigger conditions for different trigger types (long-lasting rainfall, short-duration storm, snow-melt, rain-on-snow) in six regions in the Austrian Alps. We find limited annual changes of the number of days critical for debris-flow initiation when averaged over all regions, but distinct changes when separating between hydro-meteorological trigger types and study region. Changes become more evident at the monthly/seasonal scale, with a general trend of critical debris-flow trigger conditions earlier in the year. The outcomes of this study serve as a basis for the development of adaption strategies for future risk management.

Feasibility of satellite-based rainfall and soil moisture data in determining the triggering conditions of debris flow: The Jiangjia Gully (China) case study

Heavy precipitation is the main trigger for debris-flow hazards. The antecedent moisture condition, which is usually represented by antecedent precipitation, is another hydrometeorological contributing factor to debris flows that originate from shallow landslides. Satellite techniques are an economical and effective way to access rainfall and soil wetness information to determine the triggering conditions of debris flow. However, satellite-based thresholds need to be compared with ground-based thresholds and adjusted to the data source prior to their application. In this study, rainfall intensity-duration (I-D) thresholds were derived from the early and final run products of the Integrated Multi-satellitE Retrievals for Global Precipitation Measurement (IMERG-E and IMERG-F) and gauge measurements using logistic regression for debris flows in the Jiangjia Gully, Yunnan Province, southern China, which has a dense rain gauge network. Data from both the IMERG-E and IMERG-F covered the entire study period. Our evaluation revealed that the I-D threshold derived from the IMERG-E deviated from the gauge-based thresholds. Although the threshold determined by the IMERG-F was comparable to the ground-based ones, the presence of substantial false positives and false negatives indicated that its performance was weaker than that of the gauge-based ones. Furthermore, the IMERG-F was suitable if the nearest available gauge was farther than 10 km from the debris-flow initiation zone. We evaluated the feasibility of the surface soil moisture product provided by the Climate Change Initiative program of the European Space Agency (CCI-SM), root-zone soil moisture derived from the CCI-SM using an exponential filter (SM-RZ), and antecedent precipitation for improving the performance of the thresholds by separately using them as the third explanatory variable in addition to rainfall intensity and duration in logistic regression. Satellite soil moisture data were available for 54% of the study period. The results suggested that including antecedent precipitation effectively improved the performance of the thresholds. In contrast, the performance of the thresholds increased only slightly when the CCI-SM or SM-RZ was included. Although these findings are valid only for this study area and need to be assessed for other regions, they present new insights for using satellite rainfall and soil moisture estimates to define thresholds for debris flow.

Hydrometeorological triggering of periglacial debris flows using a Bayesian approach: a case study of the Hailuogou Gully region, China

Many debris flows have occurred in small basins with a small glacier cover or snow cover in periglacial areas, suddenly caused high mortality and great damage. The hydrometeorological conditions that caused such debris flows are complex, making forecasting and early warning difficult. Previous studies for these small-glacial-covered basins have primarily considered rainfall as the only inducing factor of debris flows, and often the effects of temperature are neglected. Thus, we carried out a probabilistic analysis of variables derived from hydrometeorological factors for the Mount Gongga region, Sichuan, China, where debris flows were recorded on 14 days between 1988 and 2019. By analyzing hydrological characteristics when debris flows occurred, three distinct dominant trigger types could be identified. The results show that seven (50%) of the observed debris flow events during the study period, high-intensity rainfall was the dominant trigger, snowmelt by high temperature was identified as the dominant trigger for 2 (14%). Furthermore, five (36%) debris flow events could be attributed to the combined effects of long-lasting (or short-medium) rainfall and sustained higher temperatures. We find that the differences between the trigger types are statistically significant, and a susceptibility prediction differentiating between trigger types can outperform simple rainfall-only situations. This study contributes to an improved understanding of the hydrometeorological impact on debris flow initiation in high elevation watersheds.