Mechanism Of Debris Flow Research Articles

Two‐dimensional Dynamics Simulation of Two‐phase Debris Flow

AbstractTo investigate the movement mechanism of debris flow, a two‐dimensional, two‐phase, depth‐integrated model is introduced. The model uses Mohr‐Coulomb plasticity for the solid rheology, and the fluid stress is modeled as a Newtonian fluid. The interaction between solid and liquid phases, which plays a major role in debris flow movement, is assumed to consist of drag and buoyancy forces. The applicability of drag force formulas is discussed. Considering the complex interaction between debris flow and the bed surface, a combined friction boundary condition is imposed on the bottom, and this is also discussed. To solve the complex model equations, a numerical method with second‐order accuracy based on the finite volume method is proposed. Several numerical experiments are performed to verify the feasibilities of model and numerical schemes. Numerical results demonstrate that different solid volume fractions substantially affect debris flow movement.

Submersion simulation in a typical debris flow watershed of Jianzhuangchuan catchment, Loess Plateau

Debris flow is one of the most serious and frequent geological disasters that occur in the Loess Plateau. The outbreak of a debris flow is sudden, ferocious, swift, and destructive. The characteristics and mechanism of debris flow were explored in this study via survey, numerical simulation, and simulation analysis in a Loess Plateau area (Huangling County, Shaanxi Province, China). Numerical models and formulas corresponding to the occurrence and movement mechanism were established based on the HEC-RAS, HEC-GeoRAS, and SWAT results. The range of debris flow deposition was determined through capturing the debris flow free surface. A hydrological model and critical rainfall threshold were determined in order to provide technical support for debris flow forecasting in the Loess Plateau. The results suggest that 10-year floods do not submerge any portion of the basin. One village area was affected by the 100-year flood (total area of 0.648 km2) while four villages areas were submerged by the 1000-year flood (total area of 1.39 km2). The method presented here may provide a reliable scientific basis for mitigating loss due to debris flow hazards.

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.

A fully coupled hydro-mechanical material point method for saturated dense granular materials

The stability of a dense granular assembly can be greatly reduced by a pore pressure of the interstitial fluid, and the body may fail and transit from a solid-like state to a fluid-like state. This process involves two major problems: large deformation and hydro-mechanical coupling. In this work, a three-dimensional fully coupled hydro-mechanical model using material point method (MPM) is developed. Darcy's law, considering the inertial effect, is adopted to govern the motion of interstitial water, and the conservation of momentum of the mixture is used to govern the motion of the solid, i.e., granular materials. The spatial discretization schemes for these equations are derived using the generalized integration material point method (GIMP), and the proposed coupled MPM formulation is implemented in a three-dimensional numerical code. The developed model is first quantitatively validated by comparing the simulation results of temporal evolution of spatial distribution of hydraulic pressure in a one-dimensional oedometer test with the analytical results. An experiment is designed to observe the failure of a saturated sand pile, in which the partial-saturated region is avoided by increasing the hydraulic head at the input boundary, and the kinetic energy of water is dissipated by a filtering cloth. The failure process is simulated with the MPM code. It is found that the location of the shear band in the simulation agrees with the location of the sliding surface in the experiment. The temporal evolutions of the spatial distributions of hydraulic pressure and the solid velocity distribution at a specific time are given to provide insight into the mechanism of the failure process. This work would be helpful in understanding the initiation mechanism of debris flows induced by rainfall, and sand production in gas hydrate-bearing sediments due to increasing fluid content associated with hydrate dissociation.

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.

Experimental analysis for the dynamic initiation mechanism of debris flows

Debris flow is one of the major secondary mountain hazards following the earthquake. This study explores the dynamic initiation mechanism of debris flows based on the strength reduction of soils through static and dynamic triaxial tests. A series of static and dynamic triaxial tests were conducted on samples in the lab. The samples were prepared according to different grain size distribution, degree of saturation and earthquake magnitudes. The relations of dynamic shear strength, degree of saturation, and number of cycles are summarized through analyzing experimental results. The findings show that the gravelly soil with a wide and continuous gradation has a critical degree of saturation of approximately 87%, above which debris flows will be triggered by rainfall, while the debris flow will be triggered at a critical degree of saturation of about 73% under the effect of rainfalland earthquake (M>6.5). Debris flow initiation is developed in the humidification process, and the earthquake provides energy for triggering debris flows. Debris flows are more likely to be triggered at the relatively low saturation under dynamic loading than under static loading. The resistance of debris flow triggering relies more on internal frication angle than soil cohesion under the effect of rainfall and earthquake. The conclusions provide an experimental analysis method for dynamic initiation mechanism of debris flows.

A study of the debris flow activity on the one-stepped channel slope

The purpose of this study is to evaluate the behaviour and mechanism of debris flow on various slopes through numerical simulation. The numerical simulation was performed using the Finite Difference Element Method based on the equation of mass conservation and momentum conservation. In order to measure the behaviour of the debris flow, the debris flow of the straight rectangular channel slope and that of the one-stepped channel slope were compared. Firstly the water flow discharge, the water flow depth, and the sediment volume concentration at the slope downstream of the channel, depending on the different inflow water flow discharges at the upstream of the channel, were analyzed. The smaller the supply from the upstream of the channel, the water flow discharge and the water flow depth surged only at the point after the debris flow reached the downstream of the channel, and showed a tendency to decrease thereon after. On the other hand, when the supply at the upstream of the channel increased, the curve of the water flow discharge and the water flow depth was unsteadily high. Through the Root Mean Square ratio (RMS) comparison, the water flow discharge and water flow depth of the one-stepped channel slope was lower than that of the straight rectangular channel slope. Secondly, the water flow discharge, water flow depth, and sediment volume concentration depending on the change in the slope of the one-stepped channel slope were analyzed. The larger the slope, the larger the fluctuation in amplitude of the curve and this resulted in a higher water flow discharge distribution as well as in a wider fluctuation bandwidth. In the results of the study at each point, in the case of the straight rectangular channel, the water flow discharge and depth increased as it went downstream. This is because more erosion than deposition occurs when debris flow occurs at the upstream of the channel.

가능 최대강수량에 의한 토석류 범람 예측

In recent years, debris flow disaster has occurred in multiple locations between high and low mountainous areas simultaneously with a flooding disaster in urban areas caused by heavy and torrential rainfall due to the changing global climate and environment. As a result, these disasters frequently lead to large-scale destruction of infrastructures or individual properties and cause psychological harm or human death. In order to mitigate these disasters more effectively, it is necessary to investigate what causes the damage with an integrated model of both disasters at once. The objectives of this study are to analyze the mechanism of debris flow for real basin, to determine the PMP and run-off discharge due to the DAD analysis, and to estimate the influence range of debris flow for fan area according to the scenario. To analyse the characteristics of debris flow at the real basin, the parameters such as the deposition pattern, deposit thickness, approaching velocity, occurrence of sediment volume and travel length are estimated from DAD analysis. As a results, the peak time precipitation is estimated by 135 mm/hr as torrential rainfall and maximum total amount of rainfall is estimated by 544 mm as typhoon related rainfall.

Model Tests of Catastrophic Initiation Mode and Transport Mechanism of Viscous Mudflows

Most previous studies on the initiation mechanism of debris flows have achieved some significant results by means of traditional linear methods, statistical analyses, and qualitative and single-factor analyses. In fact, however, a large number of field observation analysis indicate that the initiation of debris flows may be a complicated nonlinear process subject to many influencing factors, with its initiation mode showing typical and complex characteristics such as sudden happening and slow happening and so on, which we could not explain clearly till now. In other words, if the initiation conditions could be regulated and controlled, the prevention and control of debris flows should achieve the practical effect. Based on catastrophe theory, the characteristics and mechanisms of the initiation, transport, and sedimentation associated with viscous mudflows, which can be taken as representative of natural debris flows, are analyzed, described, and verified using flowing model tests on the well-designed test apparatus. The results show that not only does the initiation mode of viscous mudflows belong to the slow-starting mode, but also its transport mechanism is in accordance with the general characteristics of the cusp catastrophe model. This is of great significance not only with regard to the forecasting and controlling of viscous debris flows that occur frequently in mountainous areas, but to the enriching of the nonlinear theoretical system of debris flows initiation mechanism as well.

Research on Motion Characteristics and Formation Mechanism of Debris Flow

On July 9 2013, debris flows occurred around Longchi town with large scale and wide harm, which was a great threat to people's life and property as well as reconstruction work. Debris flow ditch in the surrounding town was studied. This paper focused on loose materials, topography and rainfall characteristics, and explored the formation mechanism of debris flow in Longchi town. The result shows that: a small catchment area in valleys also have the risk of large range of accumulation of debris flow, the debris flow is caused by a lot of loose materials in mountains after earthquake and extreme rainfall. Research results contribute to a better understanding of trigger condition of debris flow after earthquake.

Experimental study on initiation of gully-type debris flow based on artificial rainfall and channel runoff

Debris flows usually occur abruptly and rapidly in mountainous areas and it is difficult to observe their occurrence progress. Therefore, an experiment has become an important method to study debris flow initiation mechanism in recent years. In this paper, taking Xiongjia Gully in SW China as an example and on the basis of artificial rainfall experiment, the author has studied the initiation of gully-type debris flow. Experiment results indicate some relation between rainfall intensity and gully erosion, failure mode of soil mass, initiation mechanism and characteristics of debris flow. Based on several groups of experiments, conclusions were drawn as follows: (1) Under strong precipitation, the infiltration rate and soil water content in different depths are inversely proportional to the rainfall intensity. Intense rainfall favors the overland flow, gully runoff and erosion, but is not conducive to water infiltration. (2) Slope failure modes and initiation mechanism of debris flow vary with different rainfall and runoff conditions. Under the condition with the rainfall intensity of 55 mm/h, the slope failure mode presents a soil liquefaction and landslide. Accordingly, the debris flow initiation mechanism belongs to landslide transformation. However, under the condition of intense rainfall and runoff, gully beds are easy to be eroded and slopes are prone to collapse. Then, debris flows occurred with initiation mechanism of entrainment. (3) In terms of debris flow characteristics, the debris flow occurrence process consists of several intermittent flows. In addition, the debris flow magnitude and flow viscosity are not consistent with rainfall intensity. On the contrary, under condition of intense rainfall of 65 and 75 mm/h, debris flows tend to be watery. However, under the rainfall condition of 55 mm/h, the flow viscosity is higher. The experiment results are consistent well with the natural debris flow occurrence from Xioangjia Gully.

A regional-scale method of forecasting debris flow events based on water-soil coupling mechanism

A debris flow forecast model based on a water-soil coupling mechanism that takes the debris-flow watershed as a basic forecast unit was established here for the prediction of disasters at the watershed scale. This was achieved through advances in our understanding of the formation mechanism of debris flow. To expand the applicable spatial scale of this forecasting model, a method of identifying potential debris flow watersheds was used to locate areas vulnerable to debris flow within a forecast region. Using these watersheds as forecasting units and a prediction method based on the water-soil coupling mechanism, a new forecasting method of debris flow at the regional scale was established. In order to test the prediction ability of this new forecasting method, the Sichuan province, China was selected as a study zone and the large-scale debris flow disasters attributable to heavy rainfall in this region on July 9, 2013 were taken as the study case. According to debris flow disaster data on July 9, 2013 which were provided by the geo-environmental monitoring station of Sichuan province, there were 252 watersheds in which debris flow events actually occurred. The current model predicted that 265 watersheds were likely to experience a debris flow event. Among these, 43 towns including 204 debris-flow watersheds were successfully forecasted and 24 towns including 48 watersheds failed. The false prediction rate and failure prediction rate of this forecast model were 23% and 19%, respectively. The results show that this method is more accurate and more applicable than traditional methods.

A model of debris flow forecast based on the water-soil coupling mechanism

Debris flow forecast is an important means of disaster mitigation. However, the accuracy of the statistics-based debris flow forecast is unsatisfied while the mechanism-based forecast is unavailable at the watershed scale because most of existing researches on the initiation mechanism of debris flow took a single slope as the main object. In order to solve this problem, this paper developed a model of debris flow forecast based on the water-soil coupling mechanism at the watershed scale. In this model, the runoff and the instable soil caused by the rainfall in a watershed is estimated by the distributed hydrological model (GBHM) and an instable identification model of the unsaturated soil. Because the debris flow is a special fluid composed of soil and water and has a bigger density, the density estimated by the runoff and instable soil mass in a watershed under the action of a rainfall is employed as a key factor to identify the formation probability of debris flow in the forecast model. The Jiangjia Gulley, a typical debris flow valley with a several debris flow events each year, is selected as a case study watershed to test this forecast model of debris flow. According the observation data of Dongchuan Debris Flow Observation and Research Station, CAS located in Jiangjia Gulley, there were 4 debris flow events in 2006. The test results show that the accuracy of the model is satisfied.

An instrumented flume to investigate the initiation mechanism of the post-earthquake huge debris flow in the southwest of China

The 2008 Wenchuan Earthquake in SW China in the Sichuan Province generated many co-seismic landslides, which delivered a huge amount of loose co-seismic landslide deposits. It caused a dramatic increase in debris flow occurrence in the subsequent years. The mechanism of these runoff debris flows has not been completely understood. The study of the complexity of the phenomenon, such as runoff-induced erosion in initially unsaturated granular deposits, and damming and breaching effects caused by instability in gully banks due to erosion, requires the use of a well-equipped flume. To this aim, an instrumented flume was built at Chengdu University of Technology for the study of the initiation mechanism. Advanced equipment such as the 3D laser scanner was used to monitor the change in tomography of the slope during the test. And some sensors for unsaturated soils such as TDR and tensiometers were installed to capture the change of hydraulic properties of soil. Especially, the erosion was analyzed quantitatively by collecting the washing-out material every 20 s. The preliminary experimental results are very encouraging for investigating the initial mechanism of debris flow. The erosion curve was obtained, and the damming and breaching effect was found to be of key importance to the initiation process.

A Study on the Flowing Mechanism of Debris Flow Induced by Slope Failure and Flowing

The purpose of this paper was to study the characteristics of debris ∞ow which was transferred from slope failure. Three kinds soil materials that were composed of difierent grain sizes were used to observe the behaviors of slope failure under difierent channel slopes and water content by laboratory experiments. The movement of soil body appeared from sliding or ∞owing to deposition, if the slope gradient of channel was below 32%. And the experimental results were shown as follows. The scale of deposition and the types of soil body movement were relative with channel slope, water content and the composition of soil material. The slope stability was in∞uenced by clay ∞occulation and water cohesion. Sandy loam soil contained more clay content than the sand soil and loamy sand soil, which could keep maintain the slope earth steepness vertically as water content was 10»25%. The shape of deposition area became narrow and long with the channel slope increasing.

Flume tests to study the initiation of huge debris flows after the Wenchuan earthquake in S-W China

In the Wenchuan area in the southwest of China, a huge amount of loose co-seismic landslide material was deposited on slopes during the Wenchuan earthquake of May 2008. These loose deposits formed the source material for rainfall-induced debris flows or shallow landslides in the years after the earthquake. On August 13, 2010, about 20 large debris flows were triggered by heavy rainfall in the area around the epicenter of the Wenchuan earthquake. Field reconnaissance revealed that the initiation of these post-earthquake debris flows was closely related to severe erosion of the loose deposits.Flume tests were carried out to study the initiation mechanism of these post-earthquake debris flows and the related influencing factors. The flume was instrumented with ten combination sets of suction and pore-pressure sensors. These sensors were accompanied by TDR probes to measure the soil water content. The flume has a length of 2.5m and a width of 1.5m.A series of 26 tests was conducted to study the influence of slope gradient and discharge on the initiation mechanism and scale of the debris flows. The amount of debris-flow discharge was obtained by collecting the washed out deposits every 20s.The experimental results showed that the initiation mechanism of debris flows for gentle slopes and steep slopes was different. On steeper slope, incision by run-off water was very rapid initiating directly the start of a debris flow. The debris flow volume increased rapidly by a chain of subsequent cascading processes starting with collapses of the side walls, damming and breaching leading to a rapid widening of the erosion channel. At gentler slopes less intensive run-off incision caused an accumulation of material down slope, which, after saturation, failed as shallow slides transform in a second stage into debris flows. It was demonstrated that the slope was the dominant factor in controlling the scale of the debris flows while the effect of discharge on erosion and the size of the debris flows was less clear.

Evaluation of the Hydraulic Hysteresis of Unsaturated Pyroclastic Soils by in Situ Measurements

The triggering mechanism of debris flows and mudflows involving unsaturated soils has been extensively covered in the literature. It is associated with heavy rainfalls occurring during wet periods, when the pore water pressure regime is critical for landsliding. Having said this, full understanding of slope failure conditions needs a fair analysis of groundwater flow and, therefore, a proper hydraulic soil characterization as fundamental basis of the analysis. At this regard, laboratory characterization could be not sufficient: the soil water retention curve (SWRC) is hysteretic and the paths followed in situ, in terms of water content and matric suction, depend on the preceding hydraulic history. This aspect has to be accurately taken into account for analysis and prediction. In order to explore the hydraulic hysteretic behaviour of pyroclastic soils, in this paper pairs of water content and matric suction measurements registered at the same depth in an instrumented unsaturated pyroclastic slope are compared to a number of retention curves obtained in the laboratory.

Mechanism of Debris Flows in Mountain Streams

This paper suggests the mechanism of occurrence of debris flow in mountain streams. It is found that the sliding of the undermined stream banks follows the exponential probability distribution function. This indicates that the major amount of debris is from an intensive sliding in the initial stage of flood flow development. This can be otherwise proved by the viscous flow boundary layer theory such as the shear stresses on the wall are initially very large. Through a relative dimensional analysis, a special dimensionless parameter K is grouped which has a very important effect on the occurrence of the acceleration of the heavy debris by the flow. It interprets, during the flood flow development, the amount of debris on the stream bed can only be accelerated when the kinetic energy of the flow is greater than the apparent work of the debris against the flow. Of course, as a transient behavior, once it starts, the sudden acceleration of a huge amount of debris flow bore will usually cause an unpredictable downstream disaster.

Particle Routing Segregation of Debris Flow Mechanisms Near the Erodible Bed

Debris flows represent a natural hazard in mountainous terrain around the world. Numerous catastrophic events have occurred in recent years. In contrast to other types of landslide activity, debris flows may be considered to be a ‘low- magnitude high frequency’ hazard. They result in damage to property and, in some circumstances, may also result in loss of life. This paper presents and discusses about the debris particles routing segregation characteristics near the erodible bed at 25° slope angle. The particle movement was visually analyzed by using high speed video camera (HSVC) to capture the movement characteristics of the individual particle grain. The footage of particle routing characteristics can be used as a proof for further investigation study. Large boulders running at the front carry enormous amount of energy so that debris is quite hazardous in terms of destructive power and the size of disaster area. The resulting destruction of debris flow characterized first by massive impact force of the front and second, the burying capacity of valuable land, assets and infrastructures.

Study of Formation Mechanism and Blocking in Giant Debris Flow at Hongchun Gully Yingxiu Town Wenchuan County

From Aug. 12th to 14th 2009, the catastrophic giant debris flow occurred in Hongchun gully Yingxiu town Wenchuan County, Du-Wen higyway in the period of heavy storm. It has cut off the Du-wen highway traffic totally and weir of Ming River became the damming body, which diverted the river plug body and rushed into Yingxiu doxens of new houses. The overflow river isolated Yingxiu towm serious and threat the post-disaster reconstruction triumph.It have important significance that to study the basic features，the deformation and failure mechanism ，the types and countermeasures to prevent and control the scale, the prevention and cure of high seismic intensity of debris flow in Hongchun gully. Base on the researches of geo-hazards background, conditions and basic features with the example of Hongchun gully debris flow, the author Analyzed the formation mechanism of debris flow and specific reason of blocking river and breaking highway, forecasted that causes of development of debris flow, also analyzed the stability of material sources in Hongchun gully’s debris flow. Then the author put forward that the formation of debris flow in Hongchun gully and blocking of Minjiang River are related closely to rich loose material sources induced by Wenchuan earthquake, the heavy rainfall, and its unique landform. Researches show that the giant debris flow in Hongchun gully started from Ganxipu lateral ditch in the upstream, the materials collected and dam-broke, It collected the material sources of the main ditch, crushed into Minjiang River, joined with the material sources of Shaofan gully debris flow in the river-upstream, blocked river and became dam lake. In a word,it is necessary to strengthen the control of the debris flow because that it is very rich material sources in Hongchun gully, and prevent hazard of the debris flow burst again.