Debris Flow Formation Research Articles

Evolution of a giant debris flow in the transitional mountainous region between the Tibetan Plateau and the Qinling Mountain range, Western China: Constraints from broadband seismic records

The catastrophic Sanyanyu and Luojiayu debris flows, which were induced by heavy rainfall, occurred at approximately midnight, August 7th, 2010 (Beijing time, UTC+8) and claimed 1,765 lives. Most seismic stations located within 150km did not detect the debris flows except for the closest seismic station, ZHQ, indicating that the seismic signals generated by the debris flows decayed rapidly. We analyzed broadband seismic signals from the ZHQ seismic station, beginning approximately 20min before the outbreak of the Sanyanyu debris flow, to rebuild its evolution processes. Seismic signals can detect development of the Sanyanyu debris flow approximately 20min after a heavy rain started falling in its initiation area; this time was characterized by a gradual increase in seismic amplitude accompanied by a series of spike signals that were probably generated by rock collapses within the catchment. The frequency contents and the characteristics of seismic signals before and after 23:33:15 (T1) are distinctively different, which we interpret as being generated by a large quantity of flowing material entering the main channel, marking the formation of the Sanyanyu debris flow. We attribute seismic amplitude increases between 23:33:15 (T1) and 23:34:26 (T2) and between 23:35:40 (T3) and 23:36:49 (T4) to entrainment of the deposit material after initiation of the debris flow and to its flow through a colluvial deposit area, respectively. The main frequency band broadening of seismic signals after 23:37:30 (T5) is believed to have been induced by impacts between the flowing material and check dams.

Risk assessment and control method of debris flow in CheGou of ShenNongJia forest area

<p>In July, 2013, large debris flows occurred in CheGou of ShenNongJia forest area of Hubei province three times, and caused huge economic losses. The debris flow is likely to break out again, and poses a threat to residents‘s life, properties and transportation economy. Debris flow risks are evaluated based on fuzzy comprehensive evaluation method. Based on the results of debris flow risk evaluation, and in conjunction with debris flow formation process and development characteristics, reasonable and efficient comprehensive control engineering measures are adopted in order to control debris flow. As is concluded through risk evaluation of CheGou debris flow. CheGou debris flow has moderate risks. Because in the CheGou debris flow formation region, there are a lot of loose solid matters, rainfall conditions contributing to formation and outbreak of debris flow, scouring and cutting effect of centralized flow in the source of the debris flow, etc., the adopted comprehensive control measures include upstream water control in platform area of the formation region, arrangement of check dam in the circulation area and dredging in the accumulation area.</p>

Debris flows monitoring and localization using infrasonic signals

Infrasonic waves (frequency ≤ 20 Hz) are generated during the formation and movement of debris flows, traveling in air with a speed far higher than that of the debris-flow movement. Infrasound monitoring and localization of infrasonic waves can serve as warning properties for debris-flows. Based on the characteristics of infrasonic signals, this study presents a three-point array of infrasound sensors as time-synchronous multiple sensors for acquiring signals. In the meantime, the signals are sorted by mutual correlation of signals to figure out their latency, and by means of array coordinating to locate the sound source to realize the monitoring and positioning of a debris-flows hazard. The method has been in situ tested and has been proven to be accurate in monitoring debris-flow occurrences and determining their positions, which is particularly effective for pre-event warning of debris-flow hazards.

Study on the Formation and Initial Transport for Non-Homogeneous Debris Flow

Non-homogeneous debris flows generally occur during the rainy seasons in Southwest China, and have received considerable attention in the literature. Regarding the complexity in debris flow dynamics, experimental approaches have proven to be effective in revealing the formative mechanism for debris flow, and quantifying the relations between the various influencing factors with debris-flow formation and subsequent transport processes. Therefore, a flume-based and experimental study was performed at the Debris Flow Observation and Research Station of Jiangjia Gully in Yunnan Province, to theoretically analyze favorable conditions for debris-flow formation and initial transport by selecting the median particle size d50, flow rate Q, vertical grading coefficient ψ, slopes S, and the initial soil water contents W as the five variables for investigation. To achieve this, an optimal combination of these variables was made through an orthogonal experimental design to determine their relative importance upon the occurrence and initial mobilization behavior of a debris flow and to further enhance our insight into debris-flow triggering and transport mechanisms.

Experimental study on the moving characteristics of fine grains in wide grading unconsolidated soil under heavy rainfall

The initiation mechanism of debris flow is regarded as the key step in understanding the debris-flow processes of occurrence, development and damage. Moreover, migration, accumulation and blocking effects of fine particles in soil will lead to soil failure and then develop into debris flow. Based on this hypothesis and considering the three factors of slope gradient, rainfall duration and rainfall intensity, 16 flume experiments were designed using the method of orthogonal design and completed in a laboratory. Particle composition changes in slope toe, volumetric water content, fine particle movement characteristics and soil failure mechanism were analyzed and understood as follows: the soil has complex, random and unstable structures, which causes remarkable pore characteristics of poor connectivity, non-uniformity and easy variation. The major factors that influence fine particle migration are rainfall intensity and slope. Rainfall intensity dominates particle movement, whereby high intensity rainfall induces a large number of mass movement and sharp fluctuation, causing more fine particles to accumulate at the steep slope toe. The slope toe plays an important role in water collection and fine particle accumulation. Both fine particle migration and coarse particle movement appears similar fluctuation. Fine particle migration is interrupted in unconnected pores, causing pore blockage and fine particle accumulation, which then leads to the formation of a weak layer and further soil failure or collapses. Fine particle movement also causes debris flow formation in two ways: movement on the soil surface and migration inside the soil. The results verify the hypothesis that the function of fine particle migration in soil failure process is conducive for further understanding the formation mechanism of soil failure and debris flow initiation.

The role of bank erosion on the initiation and motion of gully debris flows

The erosion of sediment through the exchange of momentum and energy transfer within the debris flow affects the unstable motion of the debris flow. Many models have been established to study the impact of erosion on debris flow motion, but most of the models were based on bed erosion. This paper analyzed the process of unstable motion of debris flows through an experimental flume to contrast bank erosion-dominated conditions and bed erosion-only conditions. The experiments showed that bank erosion enhanced the formation and propagation of debris flows. The volume of sediments eroded by water and the debris flow mass for the bank erosion-dominated conditions was much greater than that for the bed erosion-only conditions. The height and velocity of the debris flows fluctuated, and the total basal normal stress and pore pressure increased unsteadily along the path under both conditions. However, bank erosion increased the velocity of the debris flow and made the motion fluctuation more obvious. Physical equations were established and the analyses suggested that bank erosion-dominated debris flows had increased resistance and gradient enhancement than that of bed erosion-only debris flows.

Analysis of the Correlation Between Strength and Fractal Dimension of Gravelly Soil in Debris-flow Source Areas

Particle size distribution of gravelly soil plays a crucial role in debris flow initiation. For better understanding the mechanism of debris flow formation, two crucial mechanical property parameters of the gravelly soil are required to be studied meticulously: hydraulic conductivity and strength. With the aim of measuring the composition of the gravelly soil, 182 soil samples were taken from debris flow prone areas. With the aid of a sieve test, the particle size distribution of the samples can be obtained and analyzed. Then fractal theory was employed to compute the fractal dimension of the soil samples. By analyzing the results of sieve test (particle size distribution curves) and the results of the fractal theory calculations, the relationship between fractal dimension and particle size distribution can be explored. The results illustrate that the particle compositions of the gravelly soil tends to remain uniform as the fractal dimension increases. Moreover, as the coarse particle content increases, the fractal dimension decreases. To better understand the formation mechanism of debris flows, direct shear tests were conducted. Subsequently the experimental results were analyzed. By analysis, the following conclusions can be drawn: the soil strength decreases as the fractal dimension increases, and for soils with lower moisture content and identical dry density, a linear relationship between fractal dimension and cohesion force was identified. Moreover, cohesion force and internal friction force both decrease as the fractal dimension increases, but the internal friction angle decreases slightly while the cohesion force decreases greatly. Therefore we concluded that soil strength decreased mainly due to the reduction in cohesion force.

Formation conditions of outburst debris flow triggered by overtopped natural dam failure

Natural dams formed by landslides may produce disastrous debris flows after dam outburst. However, studies on the critical conditions required for the formation of outburst debris flow resulting from natural dam failure are still at an early stage. In this paper, we present the results of a series of laboratory tests that assessed three different materials, five different flume bed slope angles (2°, 7°, 9°, 10°, and 13°), two in-flow rates, and four types of dam geometric shapes. The results showed that the unit weight of downstream fluid increased with increasing bed channel slope. Additionally, a critical flume bed angle was found for debris flow formation. Furthermore, the combination of lake volume and flume bed angle was found to influence the formation of debris flow. A nonlinear trend was observed between the unit weights of debris flow and the uniformity coefficients of solid material. Based on the theory of stream power, a critical condition for debris flow formation from natural dam failure was established. Based on two case studies, the results indicate that the condition that was established for debris flow formation following natural dam failure agrees well with reality.

Topographical and rainfall factors determining the formation of gully-type debris flows caused by shallow landslides in the Dayi area, Guizhou Province, China

Three groups of factors, topography, geology and hydrology have influence on the formation of gully-type debris flows triggered by shallow landslides. In this paper, a single representative factor (T factor) for the topography, and a single representative factor (R factor) for the rainfall (hydrology) are proposed, which can be used to define threshold values for debris flow formation. This study was carried out in the Dayi area, Guizhou Province, China. During a heavy rainfall event on June 5 and 6, 2011, 37 gully-type debris flows caused by shallow landslides were triggered. In some catchments no such debris flows were triggered even though these catchments were in the vicinity of gullies with debris flows. The triggering mechanism for gully-type debris flows is the transport of sediment provided by shallow landslides into the gully. We isolated and analyzed the influence of the topography on the formation of debris flows in gullies under almost identical hydrological and geological conditions and propose a T factor as a topographical indicator which is a combination of the catchment surface area, the ratio of the catchment area with a slope sensitive to trigger debris flows, and the average gradient of the drainage channel in the catchment. Additionally an R factor is proposed as a rainfall indicator which is a combination of the rainfall in 1 h before the debris flow was triggered, the cumulative rainfall before the debris flow was triggered, and the annual rainfall. Higher T factor values and higher R factor values are generally related to higher probabilities of debris flow formation. The primary probability factor P, which is the combination of T and R, gives an indication of the probability of debris flow formation. The T factor was successfully validated in debris flow gullies with the same initiation mechanism in the Cida River catchment, Sichuan, China.

Synthesising empirical results to improve predictions of post-wildfire runoff and erosion response

Advances in research into wildfire impacts on runoff and erosion have demonstrated increasing complexity of controlling factors and responses, which, combined with changing fire frequency, present challenges for modellers. We convened a conference attended by experts and practitioners in post-wildfire impacts, meteorology and related research, including modelling, to focus on priority research issues. The aim was to improve our understanding of controls and responses and the predictive capabilities of models. This conference led to the eight selected papers in this special issue. They address aspects of the distinctiveness in the controls and responses among wildfire regions, spatiotemporal rainfall variability, infiltration, runoff connectivity, debris flow formation and modelling applications. Here we summarise key findings from these papers and evaluate their contribution to improving understanding and prediction of post-wildfire runoff and erosion under changes in climate, human intervention and population pressure on wildfire-prone areas.

Monitoring and recognition of debris flow infrasonic signals

Low frequency infrasonic waves are emitted during the formation and movement of debris flows, which are detectable in a radius of several kilometers, thereby to serve as the precondition for their remote monitoring. However, false message often arises from the simple mechanics of alarms under the ambient noise interference. To improve the accuracy of infrasound monitoring for early-warning against debris flows, it is necessary to analyze the monitor information to identify in them the infrasonic signals characteristic of debris flows. Therefore, a large amount of debris flow infrasound and ambient noises have been collected from different sources for analysis to sum up their frequency spectra, sound pressures, waveforms, time duration and other correlated characteristics so as to specify the key characteristic parameters for different sound sources in completing the development of the recognition system of debris flow infrasonic signals for identifying their possible existence in the monitor signals. The recognition performance of the system has been verified by simulating tests and long-term in-situ monitoring of debris flows in Jiangjia Gully, Dongchuan, China to be of high accuracy and applicability. The recognition system can provide the local government and residents with accurate precautionary information about debris flows in preparation for disaster mitigation and minimizing the loss of life and property.

Sedimentological analyses of martian gullies: The subsurface as the key to the surface

Gullies are among the youngest landforms formed by liquid water on Mars, and therefore of critical importance in resolving the planets most recent hydrologic and climatic history. Water-free sediment flows, debris flows and fluvial flows have all been identified in gullies. These processes require very different amounts of liquid water, and therefore their relative contribution to gully-formation is of key importance for climatic inferences. We show that many gullies dominantly formed by debris flows, based on sedimentological analysis of outcrops in gully-fans in 51 HiRISE images widely distributed over the southern midlatitudes. The great majority (96%) of outcrop exposures in gully-fans fed by catchments which mainly comprise bedrock and thus host boulders, contain sedimentological evidence for debris-flow formation. These exposures contain many randomly distributed large boulders (>1m) suspended in a finer matrix and in some cases lens-shaped and truncated layering. Such diagnostic features are rare in gully-fan exposures mainly fed by catchments comprising abundant latitude dependent mantle deposits (LDM; a smooth, often meters-thick deposit consisting mainly of ice and dust), wherein boulders are largely absent. These LDM-fed gullies may have formed by fine-grained debris flows, but this cannot be determined from outcrop sedimentology alone because of the lack of boulders in these systems. The fan surface morphology, in contrast to the subsurface, is dominated by secondary, post-depositional, processes, mainly weathering, wind erosion, and ice-dust mantling. These processes have removed or severely reworked the original, primary, debris-flow morphology over time. This explains the controversy between previously published morphometric analyses implying debris-flow formation and observations of gully-fan surfaces, which are often interpreted as the product of fluvial flows because of the absence of surficial debris-flow morphology. The inferred debris-flow origin for many gullies implies limited and ephemeral liquid water during gully-formation.

Debris flow formation process and critical hydrodynamic conditions in the meizoseismal area of the Wenchuan earthquake

The Wenchuan earthquake generated strong surface disturbances and triggered a large number of loose deposits, resulting in the disaster-prone environment with special watershed hydrological characteristics. This paper was to propose a debris flow formation process and explore the permeability characteristics and critical hydrodynamic conditions of the loose deposits triggered by the earthquake. The Guo Juanyan gully (31°05′27″ N to 31°05′46″ N, 103°36′58″ E to 103°37′09″ E) in Du Jiangyan City, located in the meizoseismal areas of the Wenchuan earthquake, was chosen as the study area and the disaster-prone environment was analyzed. The formation process of the debris flow was first proposed using a stability analysis, and then, the permeability characteristics of loose deposits were determined via in situ permeability experiments. Finally, the critical 1 h rainfall was simulated through a distributed hydrological model and verified by field observations. The formation process of debris flow could be divided into three stages based on the relationship between the hydrodynamic force and loose deposit resistance. The critical 1 h rainfall amounts under three antecedent moisture conditions (I-dry, II-normal and Ill-wet) were 52 mm/h, 43 mm/h and 34 mm/h, respectively. This study proposed a debris flow formation process in the meizoseismal areas of the Wenchuan earthquake based on the stability analysis and defined the rainfall threshold for debris flow early warning at the local level, which is significant for debris flow mitigation and risk management.

Countermeasures and Scheme in Preventing Zhaohe River Debris Flow Disaster in Erlong Town, Zhenping County in Henan Province

Zhaohe debris flow gully is located in the south foothill of Funiu Moutain, serious debris flow disasters occurred here. Now it is still threatening downstream villagers’ life and property safety directly. The geology, landform, climate and other natural environment conditions of Zhaohe valley were analyzed. It shows that Zhao He have debris flow formation conditions. A comprehensive treatment scheme including fending dikes, cleaning river, suppressing foot engineering and biological measures, was put forward based on the hazard characteristic.

The formation and development of debris flows in large watersheds after the 2008 Wenchuan Earthquake

The Wenchuan earthquake has caused abundance of loose materials supplies for debris flows. Many debris flows have occurred in watersheds in area beyond 20 km(2), presenting characteristics differing from those in small watersheds. The debris flows yearly frequency decreases exponentially, and the average debris flow magnitude increases linearly with watershed size. The rainfall thresholds for debris flows in large watersheds were expressed as I = 14.7 D (-0.79) (2 h < D < 56 h), which is considerably higher than those in small watersheds as I = 4.4 D (-0.70) (2 h < D < 37 h). A case study is conducted in Ergou, 39.4 km(2) in area, to illustrate the formation and development processes of debris flows in large watersheds. A debris flow develops in a large watershed only when the rainfall was high enough to trigger the wide-spread failures and erosions on slope and realize the confluence in the watershed. The debris flow was supplied by the widely distributed failures dominated by rill erosions (14 in 22 sources in this case). The intermittent supplying increased the size and duration of debris flow. While the landslide dam failures provided most amounts for debris flows (57 % of the total amount), and amplified the discharge suddenly. During these processes, the debris flow velocity and density increased as well. The similar processes were observed in other large watersheds, indicating this case is representative.

Experimental study on the triggering mechanisms and kinematic properties of large debris flows in Wenjia Gully

Debris flows are typically caused by natural terrain landslides triggered by intense rainfalls. If an incoming mountain torrent flows along sloping channels at high velocity, huge amounts of sediment (from landslide dams and eroded channel beds) will be entrained into the flows to form debris flows. It is likely that large debris flows are due to the failure of many landslide dams of different scales (due to bank slides or collapses), bed erosion, and solid transport. The catastrophic debris flows that occurred in Wenjia Gully (Wenchuan Earthquake Area), China on August 13, 2010 (two years after the mega earthquake), were caused by intense rainfall and the serious erosion of sloping channels. In the wake of the incident, experimental tests were conducted to better understand the process of sediment erosion and entrainment on the channel bed and the formation of debris flows. The results show that the bed erosion, bank collapses and channel widening caused by erosion accounted for the triggering and scale amplification of downstream debris flows in the Wenjia Gully event. This study illustrates how the hazardous process of natural debris flows can begin several kilometers upstream, and how such a complex cascade of geomorphic events (failure of landslide dams and erosion of the sloping bed) can lead to catastrophic discharges. Neglecting recognition of these hazardous geomorphic and hydrodynamic processes may result in high cost.

Topographical factors in the formation of gully-type debris flows in Longxi River catchment, Sichuan, China

There are three groups of factors: topography, geology and hydrology which have influence on the formation of gully-type debris flows. In this paper, a single representative factor (T-factor) for the topography is proposed, which can be used to define threshold values for debris flow formation. This study was carried out in the Longxi River catchment, located in the 2008 Wenchuan earthquake hit area. During a heavy rainfall event on August 13, 2010, 34 gully-type debris flows were triggered. In some catchments, no debris flows were triggered even though these catchments were in the vicinity of gullies with triggered debris flows. The formation mechanism of the triggered debris flows is the runoff-induced mechanism. The influence of the topography on the formation of debris flows in gullies with almost identical hydrological and geological conditions was isolated and analyzed. A new T-factor is proposed in this study as a topographical indicator which is a combination of the form factor, the average gradient of the stream, the catchment size, and the percentage of the catchment area with a slope gradient sensitive to trigger debris flows. Higher T-factor values are generally related to higher probabilities of debris flow formation. The T-factor can be used to predict the formation of debris flows in the sub-catchments of the Longchi River catchment. The T-factor was successfully validated in debris flow gullies with the same initiation mechanism in the Cida River catchment, Sichuan, China.

Characteristics and hazard prediction of large-scale debris flow of Xiaojia Gully in Yingxiu Town, Sichuan Province, China

The Wenchuan earthquake of May 12, 2008 produced large amounts of loose solid material that can lead to the development of devastating debris flows. In 5years since the earthquake, large numbers of large-scale debris flows occurred and caused great damage to the resettled population and the reconstruction efforts. The reconstructed Yingxiu Town suffered debris flow and its secondary disaster in the past 5years, and will be in the risk range of debris flow hazard in the future. It is very important to predict debris flow and its secondary hazard (especially the dam-breaking flood) near the town. The debris flow of the Xiaojia Gully was described and analyzed as a typical large-scale case in the Yingxiu area. The study aimed to describe the debris flow event, investigate the sediment supply conditions, analyze the characteristics of the debris flow deposition, calculate the river-blocking parameters, and predict the damage of dam-breaking flood to Yingxiu Town under different debris flow frequencies. Debris flows in Xiaojia Gully were well developed and occurred several times, resulting in frequent interruption of traffic on the S303 road. The interpretation of satellite images showed that landslide deposition, channel deposition, and soil erosion sediment were three kinds of sediment supply types for debris flow formation. 188 sediment supply sites in total were identified in the catchment. The total area for the supply of sediment for debris flows is about 0.99km2, giving a total volume of 532.09×104m3. If the Yuzi River is blocked by debris flow material, the dam-breaking flood can cause potential damage to the newly constructed Yingxiu Town. The calculation results reveal that debris flows with a frequency of 5% and 10% are impossible to cause river-blockage, and consequently, the Yingxiu Town is safe in these situations. However, debris flows with a frequency of 1% and 2% may cause river blockage and the dam-breaking flood can cause damage to the Yingxiu Town.

Monitoring Geologic Hazards and Vegetation Recovery in the Wenchuan Earthquake Region Using Aerial Photography

On 12 May 2008, the 8.0-magnitude Wenchuan earthquake occurred in Sichuan Province, China, triggering thousands of landslides, debris flows, and barrier lakes, leading to a substantial loss of life and damage to the local environment and infrastructure. This study aimed to monitor the status of geologic hazards and vegetation recovery in a post-earthquake disaster area using high-resolution aerial photography from 2008 to 2011, acquired from the Center for Earth Observation and Digital Earth (CEODE), Chinese Academy of Sciences. The distribution and range of hazards were identified in 15 large, representative geologic hazard areas triggered by the Wenchuan earthquake. After conducting an overlay analysis, the variations of these hazards between successive years were analyzed to reflect the geologic hazard development and vegetation recovery. The results showed that in the first year after the Wenchuan earthquake, debris flows occurred frequently with high intensity. Resultantly, with the source material becoming less available and the slope structure stabilizing, the intensity and frequency of debris flows gradually decreased with time. The development rate of debris flows between 2008 and 2011 was 3% per year. The lithology played a dominant role in the formation of debris flows, and the topography and hazard size in the earthquake affected area also had an influence on the debris flow development process. Meanwhile, the overall geologic hazard area decreased at 12% per year, and the vegetation recovery on the landslide mass was 15% to 20% per year between 2008 and 2011. The outcomes of this study provide supporting data for ecological recovery as well as debris flow control and prevention projects in hazard-prone areas.

Debris-flows scale predictions based on basin spatial parameters calculated from Remote Sensing images in Wenchuan earthquake area

Debris flow is a common hazard in the Wenchuan earthquake area. Collapse and Landslide Regions (CLR), caused by earthquakes, could be located from Remote Sensing images. CLR are the direct material source regions for debris flow. The Spatial Distribution of Collapse and Landslide Regions (SDCLR) strongly impact debris-flow formation. In order to depict SDCLR, we referred to Strahler's Hypsometric analysis method and developed 3 functional models to depict SDCLR quantitatively. These models mainly depict SDCLR relative to altitude, basin mouth and main gullies of debris flow. We used the integral of functions as the spatial parameters of SDCLR and these parameters were employed during the process of debris-flows scale predictions. Grouping-occurring debris-flows triggered by the rainstorm, which occurred on September 24th 2008 in Beichuan County, Sichuan province China, were selected to build the empirical equations for debris-flows scale predictions. Given the existing data, only debris-flows runout zone parameters (Max. runout distance L and Lateral width B) were estimated in this paper. The results indicate that the predicted results were more accurate when the spatial parameters were used. Accordingly, we suggest spatial parameters of SDCLR should be considered in the process of debris-flows scale prediction and proposed several strategies to prevent debris flow in the future.