Local Scour and the Laws of Scour Pit's Shape Downstream of Debris Flow Sabo Dam

The erosion shape and the law of development of debris flow sabo dam downstream is a weak part in the study on debris flow erosion. The shape and development of scour pit have an important effect on the stability and safety of debris flow sabo dam, which determines the foundational depth of the dam and the design of protective measures downstream. Study on the scouring law of sabo dam downstream can evaluate the erosion range and reasonably arrange auxiliary protective engineering. Therefore, a series of flume experiments are carried out including different debris flow characteristics （density is varying from 1.5 t/m3 to 2.1 t/m~） and different gully longitudinal slopes. The result shows that the scour pit appears as an oval shape in a plane and deep in the middle while superficial at the ends in the longitudinal section, the position of the maximum depth point moves towards downstream with an increase of flume slope angle. The maximum depth of scour pit is mainly affected by the longitudinal slope of gully, density of debris flow, and the characteristics of gully composition （particle size and the viscosity of soil）. The result also indicates that the viscosity of soil will weaken the erosion extent. The interior slopes of scour pit are different between the upstream and the downstream, and the downstream slope is smaller than the upper one. For the viscous and non-viscous sands with the same distribution of gradation, the interior slope of non- viscous sand is smaller than the viscous sand.According to tbe regression analysis on the experimental data, the quantitative relationship between the interior slope of scour pit, slope of repose under water and the longitudinal slope of gully is established and it can be used to calculate the interior slope of scour pit. The results can provide the basis for the parameter design of the debris flow control engineering foundation.

Long runout mechanism of the Shenzhen 2015 landslide:insights from a two-phase flow viewpoint

A catastrophic landslide occurred at Hongao dumpsite in Guangming New District of Shenzhen, South China, on December 20, 2015. An estimated total volume of 2.73×106 m3 of construction spoils was mobilized during this event. The landslide traveled a long distance on a low-relief terrain. The affected area was approximately 1100 m in length and 630 m in width. This landslide made 33 buildings destroyed, 73 people died and 4 people lost. Due to the special dumping history and other factors, soil in this landfill is of high initial water content. To identify the major factors that attribute to the long runout character, a two-phase flow model of Iverson and George was used to simulate the dynamics of this landslide. The influence of initial hydraulic permeability, initial dilatancy, and earth pressure coefficient was examined through numerical simulations. We found that pore pressure has the most significant effect on the dynamic characteristics of Shenzhen landslides. Average pore pressure ratio ofthe whole basal surface was used to evaluate the degree of liquefaction for the sliding material. The evolution and influence factors of this ratio were analyzed based on the computational results. An exponential function was proposed to fit the evolution curve of the average pore pressure ratio, which can be used as a reasonable and simplified evaluation of the pore pressure. This fitting function can be utilized to improve the single-phase flow model.

泥石流松散固体物源研究进展与展望

泥石流形成的三大基本条件中,固体物源条件是泥石流研究最直观的对象,也是最容易研究但同时最难定量研究的对象。首次从物理机制角度讨论了泥石流固体物质的定义,并给出了度量方法;此外,对泥石流松散固体物质储量、动储量的调查手段与方法,固体物质冲出总量,以及固体物质起动临界深度模型的研究现状进行了梳理分析。结果表明:目前常应用于度量泥石流固体物质的指标——动储量及固体物质冲出总量等,其估算方法多是建立数学模型或是拟合公式,缺乏明晰物理机制,普适性和度量精度较低;基于目前存在的问题而提出的可移动固体物质概念则恰好规避这一问题,且现有的研究成果可为其度量方法的建立提供理论支撑。最后,探讨了泥石流固体物源未来研究可开展的工作:深入开展泥石流可移动固体物质物理力学机制判别方法研究;开展泥石流固体物源空间分布规律研究;开展泥石流固体物质岩土特性随时间的推移所呈现的演变规律研究。

恒定渗流作用泥石流碎屑物质起动判别模型研究

恒定渗流条件是泥石流碎屑物质起动的主要水动力条件之一,目前鲜有学者对其起动判别模型和坡体内水力学参数变化规律进行研究。初步选取结构简单的碎屑物质作为地质模型进行理论分析,建立起动判别模型,借助模型试验对坡体内水力学参数的变化规律进行探究,同时对判别模型进行验证,并与其他模型进行对比分析。结果表明,恒定渗流过程对碎屑物质起动的作用力主要包括渗透力及渗流冲刷力;渗流速度随底床坡度、入水流量的增大而增大,但随入水流量增加的幅度较小;平行于水流方向的总水头损失随底床坡度的增大呈先减小后增大趋势,随入水流量增大而增大;稳定系数与底床坡度间呈幂函数关系,关系系数因判别模型不同而有所差异;与其他模型相比,模型经严格推导,满足量纲分析和试验验证要求,精度更高。

泥石流拦砂坝下游局部冲刷研究现状及展望

下游局部冲刷是影响泄水建筑物、泥石流拦砂坝安全运行的共同危害形式之一.对泄水建筑物与泥石流拦砂坝下游局部冲刷的准确认识,是防治下游局部冲刷的依据.以泄水建筑物与拦砂坝下游局部冲刷为研究对象,对下游局部冲刷坑深度计算、冲刷动力来源—水舌和泥舌的研究现状进行阐述与分析.总结得出:对泄水建筑物下游局部冲刷坑深度的计算已有较多计算公式及预测模型,关于其冲刷动力来源—水舌的水力特性也有较全面研究;拦砂坝下游局部冲刷的研究大多借鉴水利工程上的研究成果,然而泥石流属于非牛顿流体、不同于水流,并不能将泄水建筑物下游局部冲刷的研究成果简单应用于泥石流中。此外,对拦砂坝下游局部冲刷的动力来源—泥舌的水力特性认识不够充分.鉴于拦砂坝下游局部冲刷研究中存在的不足,探讨了未来可开展的工作:①基于物质组成和流速分布的泥舌水力特性研究;②基于动力演变过程的泥舌冲击力特征研究;③基于泥石流动力学过程的拦砂坝下游冲刷坑参数计算方法研究.

Debris Flow Formation Process and Critical Hydrodynamic Conditions in the Meizoseismal Area of the Wenchuan Earthquake

Abstract： 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 （31005＇27＂ N to 31005＇46＂ N, 103036＇58＂ E to 103037＇09＂ 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 Ⅲ-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.

Mechanism of Downcutting Erosion of Debris Flow over a Movable Bed

The phenomenon of debris flow is intermediate between mass movement and solid transport. Flows can be sudden, severe and destructive. Understanding debris flow erosion processes is the key to providing geomorphic explanations, but progress has been limited because the physical-mechanical properties, movement laws and erosion characteristics are different from those of sediment-laden flow. Using infinite slope theory, this research examines the process and mechanism of downcutting erosion over a moveable bed in a viscous debris flow gully. It focuses specifically on the scour depth and the critical slope for viscous debris flow,and formulas for both calculations are presented.Both scour depth and the critical conditions of downcutting erosion are related to debris flow properties(sand volume concentration and flow depth) and gully properties(longitudinal slope,viscous and internal friction angle of gully materials,and coefficient of kinetic friction). In addition, a series of flume experiments was carried out to characterize the scouring process of debris flows with different properties. The calculated values agreed well with the experimental data. These theoretical formulas are reasonable, and using infinite slope theory to analyze down cutting erosion from viscous debris flow is feasible.

A Modified Certainty Coefficient Method(M-CF) for Debris Flow Susceptibility Assessment:A Case Study for the Wenchuan Earthquake Meizoseismal Areas

In the meizoseismal areas hit by the China Wenchuan earthquake on May 12, 2008, the disasterprone environment has changed dramatically, making the susceptibility assessment of debris flow more complex and uncertain. After the earthquake, debris flow hazards occurred frequently and effective susceptibility assessment of debris flow has become extremely important. Shenxi gully in Du Jiangyan city, located in the meizoseismal areas, was selected as the study area. Based on the research of disaster-prone environment and the main factors controlling debris flow, the susceptibility zonations of debris flow were mapped using factor weight method(FW), certainty coefficient method(CF) and geomorphic information entropy method(GI). Through comparative analysis, the study showed that these three methods underestimated susceptible degree of debris flow when used in the meizoseismal areas of Wenchuan earthquake. In order to solve this problem, this paper developed a modified certainty coefficient method(M-CF) to reflect the impact of rich loose materials on the susceptible degree of debris flow. In the modified method, the distribution and area of loose materials were obtained by field investigations and postearthquake remote sensing image, and four data sets, namely, lithology, elevation, slop and aspect, wereused to calculate the CF values. The result of M-CF method is in agreement with field investigations and the accuracy of the method is satisfied. The method has a wide application to the susceptibility assessment of debris flow in the earthquake stricken areas.

Numerical simulation for different motion forms of sliding soil along slope with PFC3D

The motion forms（deformation） of the sliding soil along the slope include inviscid particle flow,viscous particle flow,and mass sliding.The motion forms were mainly influenced by the mechanical properties of the viscous mud medium filling.The effect of the medium filling on the motion forms of the sliding soil is worth discussing.The sliding soil can be seen as the mixture which is composed of the medium filling and coarse particles.The Chengdu clay with different moisture contents were selected as the medium filling.Their mechanical parameters were experimentally measured for simulating the moving process of the sliding soil.Different motion forms of the sliding soil were numerical simulated based on PFC3D by changing the specific mechanical parameters.The mechanical parameters of the Chengdu clay are experimentally determined.The numerical results show that with the moisture content of the filling slurry medium in the range of 〉302%,101%~182%,and 〈80%,the motion form of the sliding soil belongs to the inviscid particles flow（similar with the sliding sand）,viscous particles flow（seen as the viscous flows）;and masssliding（seen as the block sliding along the slope surface）,respectively.