Hua-li Pan

Debris flow formation conditions and optimal characteristics of drainage canal following Wenchuan earthquake

The debris flow formation conditions changed after the Wenchuan earthquake. For studying the optimal characteristics of the drainage canal under the changed formation conditions, the Zhangjia gully in Beichuan County, a typical debris flow gully, was selected as the study area. Two debris flow events occurred on September 24, 2008 and July 17, 2009, respectively, which threatened the safety of the inhabitants and destroyed the properties and transportation routes on the alluvial fan. Huge amounts of solid loose materials triggered by the earthquake and the effective antecedent precipitation were responsible for the debris flow occurrence. For eliminating the damage caused by debris flows, the construction of a drainage canal on the alluvial fan became necessary. A design method for the optimal cross-section characteristics of a debris flow drainage canal is discussed in this paper. Using the standard measurements for the depth (h) and the width (b), the cross-section size parameter (F) and the cross-section configuration parameter (M) under optimal hydraulic conditions are defined and deduced. Subsequently, formulas for calculating the cross-section measurements are deduced under optimal hydraulic conditions when peak discharge (Q), grain composition (D50 and D10), and longitudinal slope (I) are known. Finally, this method is applied for the cross-section design of the debris flow drainage canal in Zhangjia gully.

High-speed ring shear tests to study the motion and acceleration processes of the Yingong landslide

In this paper, the motion and acceleration process, as well as the mechanism of a high speed and long run landslide are investigated by adopting high speed ring shear test and taking the landslide occurred at Yigong River in Bomi, Tibet on April 9, 2000 as the background. According to the motion characteristics of high-speed and long distance motion landside, the mechanism is studied under different conditions such as shear speed, consolidated drained and consolidated undrained status. Results show that high speed shearing process hinders and delays the dissipation of pore pressure, and drives pore water migrating to shear zone slowly. Both of water content and fine particle content at shear zone are obviously higher than those in other layers; and soil liquefaction occurs at shear zone in the saturated consolidated undrained ring shear tests. The effective internal friction angle of the consolidated undrained soil is much lower than that of the consolidated drained soil under ring shearing. The results also indicate that the shearing speed affecting the strength of soil to some extent. The higher the ring shearing speed is, the lower the strength of soil is. This investigation provides a preliminary interpretation of the mechanism of the motion and acceleration process of the Yigong landslide, occurred in Tibet in 2000.

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.

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.

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/m3) 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 the 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 of the 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.

The hydraulic characteristics in compound channels of viscous debris flow: A case study on the Dabaini debris flow gully in Xiaoj fang Basin of Yunnan province, China

The channels of natural viscous debris flows are all compound. The hydraulic characteristics (wetted perimeter, overflowing area and hydraulic radius) are different under different width ratio of beach to groove and different slurry depth of debris flow. By using the hydraulic calculating method of natural river, this paper discusses the hydraulic characteristics in compound channels of viscous debris flow. The results indicate that the width ratio of beach to groove has a great influence on overflowing capacity and discharge distribution of beach and groove. When the width ratio is the same, the groove discharge decreases with the increase of slurry depth, while the beach discharge increases; when debris flow with the same scale passes by channels which have different width ratio, the overflowing capacity in channels with bigger width ratio is much lager than the smaller ones; the influence between beach and groove in debris flow compound channels can be expressed by COH and it decreases with the increase of debris flow discharge; when debris flow with the same scale passes by compound channels, the bigger the width ratio is the greater the influence between beach and groove is.