An analysis of the supply process of loose materials to mountainous rivers and gullies as a result of dry debris avalanches

A dry debris avalanche will produce different volumes of colluviums or depositions (loose materials), which can have a significant impact on mountainous rivers or gullies. The loose material supply process caused by a debris avalanche is an important issue for understanding secondary disasters that form via the coupling of water flow and loose materials. Two flumes were designed for laboratory tests of the loose materials supply process to rivers/gullies, and the related impact factors were analyzed. Experimental results show that the supply of loose materials is a continuous process that directly relates to the avalanche’s mass movement processes. The sliding masses with smaller particle sizes are more sensitive to the flume slope and exhibited a longer supply time. The time-consuming for the debris avalanche travel in the flume decreased with the increasing particle size (such as flume B, time-consuming is decreased 0.2 s when the particle size increased from <1.0 to 20–60 mm), landslide volume and flume slope (flume A, consuming 1.6–2.1 s when flume slope is 29° decreased to consuming 1.3–1.5 s when flume slope is 41°), which means the increasing mobility of loose materials. The total supply time increased with the increasing landslide volume or decreasing particle size and flume slope. An empirical model for the process is presented based on numerous laboratory tests and numerical simulations, which can successfully describe the supply process for loose materials to a river/gully. The supply process of loose materials to mountainous gully from a dry debris avalanche is controlled by the material compositions of sliding masses, topographical conditions, landslide volume and bed friction, where large-volume debris avalanches that occur in mountainous river regions are more likely to obstruct the river flow and form a landslide-dammed lake.     

Two-dimensional Dynamics Simulation of Two-phase Debris Flow

To 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.     

基于EDEM的碎屑流运动规律及冲击性能研究

泥石流运动规律及其冲击性能对于泥石流灾害的影响范围及严重程度具有重要决定意义。出于泥石流这类多相介质的复杂性,本文采用离散元仿真软件EDEM 2018对碎屑流冲击流槽试验进行了数值模拟研究,考虑流槽坡度、底部拦挡结构角度以及颗粒级配的影响,在已有研究成果的基础上对固体颗粒运动过程及冲击性能展开了系统研究。本文将数值模拟结果与现存试验数据进行了对比分析,验证了数值模拟方法的可靠性,在此基础上得出了以下结论:(1)在拦挡结构角度与颗粒级配相同的情况下,流槽坡度越大,对应的碎屑流运动速度与冲击力的峰值也越大;(2)在流槽坡度与级配相同的情况下,拦挡结构越陡,与其相互作用的固体颗粒数量越多,碎屑流越快达到速度和冲击力峰值,且对应的速度与冲击力峰值也越大;(3)在运动过程中,各颗粒级配的碎屑流均出现反序现象,且细颗粒含量的提升可提高碎屑流运动速度,但同时冲击力降低,而粗颗粒含量的提升可增大碎屑流对拦挡结构的冲击力,对于运动速度的影响较小。     

泥石流冲击弧形拦挡坝动力响应研究

Dam foundation is subjected to a larger impact force when debris flow runs up, causing stress concentration and local impact failure. To address this problem, in this study the vertical structures are optimized into arc-shaped dams. Based on the principle of momentum and energy conservation, the theoretical calculations of the impact process of debris flow and arc-shaped dam are carried out, and the formulas of impact force and maximum run-up height of debris flow are deduced. The theoretical formulas are verified through a series of physical model tests of debris flow impact arc-shaped dam. The results show that the results of the physical model are highly consistent with those of the theoretical calculations, indicating that the proposed theoretical formulas are applicable in the calculation of the impact of debris flow on arc-shaped dam. The debris velocity, impact force and the maximum run-up height are proportional to the flume slope of debris flow. The impact force and the maximum run-up height are mainly controlled by Froude number(Fr), flume slope(?), and arc-shaped radius(R). Both the impact force and the maximum run-up height have quadratic relationships with the Froude number, and are inversely proportional to the cosine of the flume slope. Compared with the rigid vertical structures, the arc-shaped dams have no signicicant influence on the maximum run-up height, but it can reduce the normal impact force on the dam considerably, and the structure strength can also be enhanced by the strengthening of local structure. This study provides a theoretical and technical support for the dam structure design.     

SPH model for fluid–structure interaction and its application to debris flow impact estimation

On 13 August 2010, significant debris flows were triggered by intense rainfall events in Wenchuan earthquake-affected areas, destroying numerous houses, bridges, and traffic facilities. To investigate the impact force of debris flows, a fluid–structure coupled numerical model based on smoothed particle hydrodynamics is established in this work. The debris flow material is modeled as a viscous fluid, and the check dams are simulated as elastic solid (note that only the maximum impact forces are evaluated in this work). The governing equations of both phases are solved respectively, and their interaction is calculated. We validate the model with the simulation of a sand flow model test and confirm its ability to calculate the impact force. The Wenjia gully and Hongchun gully debris flows are simulated as the application of the coupled smoothed particle hydrodynamic model. The propagation of the debris flows is then predicted, and we obtain the evolution of the impact forces on the check dams.     

泥石流浆体与大颗粒冲击力特征的试验研究

开展了密度为1 400～2 200 kg/m3的泥石流浆体、浆体与大颗粒混合流体的冲击力试验,获取了流速为2.8～4.9 m/s条件下31组冲击力试验数据。采用小波分析方法有效地去除了冲击力数据中的噪声信号,依据离散傅里叶变换（FFT）为基础的频谱分析结果,将低频泥石流浆体冲击和高频大颗粒冲击的临界频率值界定为2 Hz,实现了浆体和大颗粒冲击信号的分离。目前水动力学公式中待定系数α缺乏统一的确定方法,以不同地区157组泥石流观测和试验数据为基础,建立了待定系数?与流体Fr数的幂函数关系,形成可表征不同流态,且弱化尺度效应的浆体动压力计算公式。与泥石流浆体平滑信号相比,大颗粒冲击压力具有一定随机性。泥石流大颗粒冲击次数与频率随大颗粒的质量比增加而增大,其质量比从0.05增至0.21时,冲击总次数从1 305次增至2 838次,冲击频率从82次/s增至195次/s,且龙头段大颗粒的冲击频率高于后续泥石流体。测得大颗粒的压力约为60 kPa,是相同密度和流速下浆体动压力的3倍。随着大颗粒比例的增加,上部1#和2#传感器测得大颗粒冲击频率增加量明显高于下部3#～6#。说明随着流体中大颗粒比例上升,颗粒物质多集中于泥石流上部或表层运动,也佐证了泥石流运动中大颗粒多集中在龙头顶部的认识。对大颗粒和浆体冲击规律的分析可为固液两相流运动机制研究和防治工程设计以及承灾体易损性定量评估提供合理参数。     

A unified CFD‐DEM approach for modeling of debris flow impacts on flexible barriers

This paper presents a unified modeling framework to investigate the impacts of debris flow on flexible barriers, based on coupled computational fluid dynamics and discrete element method (CFD‐DEM). We consider a debris flow as a mixture of fluid and particles where the fluid and particle phases are modeled by the CFD and the DEM, respectively. The fluid‐particle coupling is considered by the exchange of interaction forces between CFD and DEM calculations. The flexible barrier is simulated by the DEM as a network of bonded particles with remote interactions. The proposed coupled CFD‐DEM approach enables us to conveniently handle the complicated three‐way interactions among the fluid, the particles, and the flexible barrier structure for debris flow impact simulations. The proposed approach is first used to investigate the influences of channel inclination and the volumetric solid fraction in a debris mixture on the impact force, the resultant deformation, and the retained mass in a flexible barrier. The predictions agree well with existing experimental and numerical studies. We further examine the possible failure modes of a flexible barrier under debris flow impact and their underlying mechanisms. The performance of different components in a flexible barrier system, including single wires, double twists and cables, and their load sharing mechanisms, are carefully evaluated. The proposed unified framework offers a novel, promising pathway towards physically based, quantitative analysis and design of flexible barriers for debris flow mitigation.     

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.     

Variational Laws of Debris Flow Impact Force on the Check Dam Surface Based on Orthogonal Experiment Design

Debris flow impact force is an important factor for controlling structural damage, and it is the key factor for engineering design and risk assessment. Variation laws of debris flow impact force play an important role in preventing check dam impact damage and providing technology, data and support for check dam construction. Many influencing factors exist in debris flow impact force with different influencing magnitudes. The three main factors, i.e. the debris flow bulk density, the drainage channel slope and the upstream surface gradient of the check dam, were selected to be analyzed. The purpose of the study was to analyze the influencing degree of the three factors. Three levels were set for each factor and nine text schemes were established based on the theory of orthogonal experimental design. What is more, the related miniaturized flume experiment was carried out to measure impact force of debris flow. Finally, taking the impact force mean values of key point as the evaluation index, the flume experiment results were analyzed in detail by extreme difference analysis and variance analysis. Research results indicate: among the three factors, the drainage channel slope has the most significant influence, the upstream surface gradient of the check dam is in the second place and the debris flow slurry density is the third. The form of impact force mean with the maximum value: the drainage channel slope is 15°, the debris flow bulk density is 18.1 kN/m³ and the upstream surface gradient of the check dam is 1:0.     

中国公路泥石流研究

公路泥石流是指发育于公路沿线并对公路桥涵、路基路面及相应防护结构具有冲击毁损和淤埋破坏的病害类型。丰富的物源、具有焚风效应的气象条件以及泥石流沟轴线与区域新构造应力场主压应力方向一致等是形成大型泥石流的宏观背景。将泥石流概化为固、液两相流体,运用两相流理论、泥沙运动力学、Bingham流变方程和Bagnold颗粒相互作用试验结果等,初步建立了泥石流固-液分相流速计算方法、基于泥石流在防治结构表面及泥石流沟岸产生的冲击形迹建立的反求泥石流冲击力计算方法以及泥石流磨蚀力计算方法。开发了速流结构、泥石流隧道及翼型墩汇流结构等10余种防治技术,集成了拦-汇-排等多种综合治理模式。据此撰写了《公路泥石流防治工程设计、施工指南》。实施了60余个防治工程。效果显著。研究成果初步构建了公路泥石流理论及技术体系。     

基于旋转水槽试验的泥石流阻力坡降速率效应研究

使用自主研发的旋转水槽制作稳定循环流动的泥石流,通过实测泥石流阻力坡降和流动速度,探索泥石流阻力坡降的速率效应。在旋转水槽试验中,通过控制线速度v,制作出稳定循环运动的泥石流,其阻力坡降J等于槽底坡降tanθ,可以实测。试验再现了野外的泥石流运动特性,观察到“龙头”、颗粒弹跳、低阻力坡降、固液相分离、剪切速率效应等现象。试验结果表明,在一定的流动速度范围内（例如试验中1.88m·s-1≥v>0.47m·s-1）,泥石流的流动速度越高,阻力坡降越大;如果泥石流运动速率过低（例如试验中v<0.47m·s-1）,泥石流会出现固液两相分离现象,砾石（粒径>2mm）运动明显落后于液相运动。本项目的试验研究成果为泥石流制作和阻力坡降测量提供新途径,为泥石流阻力坡降的速率效应理论研究提供实验支持。     

Substantiation of debris flow velocity from super-elevation: a numerical approach

The use of super-elevations that a forced vortex flow leaves on the valley walls of a curved flume is a plausible approach toward estimating debris flow velocities in earthquake-induced geo-hazard studies. The centrifugal force of a speeding flow is responsible for a higher flow depth on the outer bend. However, in reality, a flow is not steady, and only the highest flow-marks are left at the outer and inner bends of the flow, which can lead to an inaccurate estimation of the actual velocity. Seeing the real scenario of the field, a series of numerical flume tests using smoothed particle hydrodynamics (SPH) is conducted to validate the estimation of debris flow velocities from flow-marks. Velocities estimated from flow-marks are lower than real velocities near the source region, but they converge to real velocities as the distance to the source increases. Based on several simulations, a best-fit line is proposed for adjusting debris flow velocity from mud-marks, and it is used to estimate flow velocities of the well-documented debris event called “Shiraito river debris flow,” which happened near the rim of the Hakone Crater, Kanagawa Prefecture, Japan, ensuing from the 1923 Great Kanto earthquake.     

Velocity attenuation of debris flows and a new momentum-based load model for rigid barriers

Effective design of mitigation measures against debris flow hazards remains a challenging geotechnical problem. At present, a pseudo-static approach is commonly used for the calculation of impact load acting on a rigid debris-resisting barrier. The impact load is normally calculated based on the maximum velocity observed in the transportation zone under free-field conditions without considering debris-barrier interaction. In reality, the impact load acting on a barrier varies with the change of debris momentum flux but this is seldom considered in barrier design. To provide a scientific basis for assessing debris momentum flux during impact, this paper presents results from a study of debris-barrier interaction using physical flume modelling. This study showed that, following the first stage of impact, the accumulated debris behind a barrier formed a stationary zone and caused the remaining debris to slow down in a run-up process. In the experiments, the peak debris momentum was 30 % lower compared to that observed under free-field conditions. A new momentum-based model was developed to take into account attenuation of momentum flux for predicting debris impact load on rigid barriers. The new rationalised model was assessed using data from the notable Yu Tung Road debris flow in Hong Kong. The assessment showed that the design bending moment at the base of the barrier wall could be reduced more than 30 % using the proposed model, compared with the current design approach. The adoption of the proposed model could offer a new opportunity for practitioners to optimise the design of rigid barriers.     

A hypoplastic constitutive model for debris materials

Debris flow is a very common and destructive natural hazard in mountainous regions. Pore water pressure is the major triggering factor in the initiation of debris flow. Excessive pore water pressure is also observed during the runout and deposition of debris flow. Debris materials are normally treated as solid particle–viscous fluid mixture in the constitutive modeling. A suitable constitutive model which can capture the solid-like and fluid-like behavior of solid–fluid mixture should have the capability to describe the developing of pore water pressure (or effective stresses) in the initiation stage and determine the residual effective stresses exactly. In this paper, a constitutive model of debris materials is developed based on a framework where a static portion for the frictional behavior and a dynamic portion for the viscous behavior are combined. The frictional behavior is described by a hypoplastic model with critical state for granular materials. The model performance is demonstrated by simulating undrained simple shear tests of saturated sand, which are particularly relevant for the initiation of debris flows. The partial and full liquefaction of saturated granular material under undrained condition is reproduced by the hypoplastic model. The viscous behavior is described by the tensor form of a modified Bagnold’s theory for solid–fluid suspension, in which the drag force of the interstitial fluid and the particle collisions are considered. The complete model by combining the static and dynamic parts is used to simulate two annular shear tests. The predicted residual strength in the quasi-static stage combined with the stresses in the flowing stage agrees well with the experimental data. The non-quadratic dependence between the stresses and the shear rate in the slow shear stage for the relatively dense specimens is captured.     

Modelling and numerical simulation of two-phase debris flows

Gravity-driven geophysical mass flows often consist of fluid–sediment mixtures. The contemporary presence of a fluid and a granular phase determines a complicated fluid-like and solid-like behaviour. The present paper adopts the mixture theory to incorporate the two phases and describe their respective movements. For the granular phase, a Mohr–Coulomb plasticity is employed to describe the relationship between normal and shear stresses, while for the fluid phase, the viscous Newtonian fluid is taken into account. At the basal topography, a Coulomb sliding condition for the solid phase and a Navier’s sliding condition for the fluid phase are satisfied, while the top free surface is traction-free for both the phases. For the interactive forces between the phases, the buoyancy force and viscous drag force are included. The established governing equations are expressed in a curvilinear coordinate system embedded in a curvilinear reference basal surface, above which an arbitrary shallow basal topography is permitted. Taking into account the typical length characteristics of such geophysical mass flows, the “thin-layer” approximation is assumed, so that a depth integration can be performed to simplify the governing equations. The resulting strongly nonlinear partial differential equations (PDEs) are first simplified and then analysed for a steady state in a travelling coordinate system. We find the current model can reproduce the characteristic shape of some flow fronts. Additionally, a stability analysis for steady uniform flows is performed to demonstrate the development of roll waves that means instabilities grow up and become clearly distinguishable waves. Furthermore, we numerically solve the resulting PDEs to investigate general unsteady flows down a curved surface by means of a high-resolution non-oscillatory central difference scheme with the total variation diminishing property. The dynamic behaviours of the granular and fluid phases, especially, the effects of the drag force and the fluid bed friction are discussed. These investigations can enhance the understanding of physics behind natural debris flows.     

Impact force to a rigid obstruction from a granular mass sliding down a smooth incline

The impact force to a rigid obstruction from a granular mass sliding down a smooth incline provides insights into the solid-like and fluid-like behaviors of granular avalanches and useful information for risk assessment and engineering design against landslides. In this study, a series of 2-D flume tests were performed to systematically investigate the effects of inclination angle, sliding distance, and initial relative density on the flow front velocity and impact force on a rigid obstruction. The experimental results show that for inclination angles smaller than the critical state friction angle of sand, an increase in the sliding distance and/or initial relative density results in smaller impact forces; for higher inclination angles, the trend is reversed. Based on the experimental results, an analytical equation is proposed to estimate the flow front velocity and an empirical approach is presented to estimate the maximum impact force based on elastic solid and hydrodynamic methods. The proposed equations are found to provide more accurate predictions for the maximum impact force than similar equations in the literature.     

云南贡山818特大泥石流灾害调查分析与启示

本文是在野外调查的基础上,结合遥感解译数据,通过对地质环境背景分析,详细阐述了东月各河泥石流灾害的成因与特征:远距离、大坡降、窄峡谷的地形条件以及平直的沟道,为本次泥石流的发生提供了强有力的地势条件; 流域上游大量的松散堆积物为本次泥石流的暴发提供了充足的物源; 泥石流历时2273s,流速4.62ms-1,流量1008m3s-1,在沟口形成长度约300m、最大宽度约400m的堆积扇,冲出方量约60.5104m3,一次冲出固体物质总量41.95104m3,泥石流流体具结构性,直进性强,运动中无垂直交换,浆体浓稠,浮托力大,具有明显的辅床减阻作用和阵性运动特征,泥石流容重2.04gcm-3,属于特大型黏性泥石流; 泥石流在距沟口2.5km的弯道处超高现象比较明显,其高度为4.1m,泥石流整体冲击力为64.01kN,单块块石最大撞击力为491.27tf; 东月各河泥石流具有高落差、大坡降、窄峡谷、大体量、高速度、远距离的特点,其隐蔽性、突发性和破坏性极强,属于典型的高位泥石流。对该泥石流的形成机理和运动特征分析,可为整个怒江流域,乃至西南高山峡谷区高位泥石流灾害的早期识别、监测预警与风险评价等提供科学依据。     

Assessment of debris flow multiple-surge load model based on the physical process of debris-barrier interaction

Debris-flow impact load is one of the key parameters for design of engineering countermeasures. The multiple-surge load model is a remarkable progress in estimating the debris-flow impact load, which clearly delineates the contribution of each surge to the total impact load and the corresponding acting points. In order to better understand the impact process of channelized debris flow against flexible barrier, a series of medium-scale flume experiments with varying debris-flow volumetric solid concentration (0.40/0.50/0.55) were conducted. Especially, surge impact behavior is focused so that the predictability of the multiple-surge load model could be assessed. The flume and model flexible barrier were instrumented so that both the barrier dynamic response and the debris-flow properties (flow regime) could be correlated to facilitate the assessment. The results show that multiple-surge load model well predicts the total impact load. However, due to the simplification in the impact process, the interaction between the mobile phase (surge) and the deposited phase is ignored, resulting in discrepancy in the load distribution between the model prediction and experimental result. The remixing of deposited debris by the subsequent surges leads to downward momentum transfer to the lower section of barrier, which should be regarded as an adverse scenario of the design of flexible-barrier anchor capacity.

     

冰湖溃决泥石流形成的临界条件

随着全球气候的变暖, 在世界上许多高山峡谷区的冰湖溃决及其溃决洪水引发的泥石流, 经常对下游居民及其他基础设施造成极为严重的危害. 使用水槽试验的方法, 从单宽流量和库容、沟道纵坡、堆积物粒径3个方面探讨了冰湖溃决泥石流形成的影响因素和临界条件. 结果显示: 冰湖溃决泥石流形成与否不仅与溃决洪水提供的能量有关, 还与参与泥石流活动的沟床物质特性紧密相关. 通过对试验数据的分析, 当泥石流形成的特征参数K>2.66时, 冰湖溃决洪水可以演化为泥石流. 该种方法可以对危险性冰湖的预测提供理论参考.     

A dilatant,two-layer debris flow model validated by flow density measurements at the Swiss illgraben test site

We propose a dilatant, two-layer debris flow model validated by full-scale density/saturation measurements obtained from the Swiss Illgraben test site. Like many existing models, we suppose the debris flow consists of a matrix of solid particles (rocks and boulders) that is surrounded by muddy fluid. However, we split the muddy fluid into two fractions. One part, the inter-granular fluid, is bonded to the solid matrix and fills the void space between the solid particles. The combination of solid material and inter-granular fluid forms the first layer of the debris flow. The second part of the muddy fluid is not bonded to the solid matrix and can move independently from the first layer. This free fluid forms the second layer of the debris flow. During flow the rocky particulate material is sheared which induces dilatant motions that change the location of the center-of-mass of the solid. The degree of solid shearing, as well as the amount of muddy fluid and of solid particles, leads to different flow compositions including debris flow fronts consisting of predominantly solid material, or watery debris flow tails. De-watering and the formation of muddy fluid washes can occur when the solid material deposits in the runout zone. After validating the model on two theoretical case studies, we show that the proposed model is able to capture the streamwise evolution of debris flow density in time and space for real debris flow events.