Guoqiang Ou

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, were used 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.

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.

Debris flow hazard assessment by combining numerical simulation and land utilization

Debris flow has caused serious human casualties and economic losses in the main earthquake-hit areas affected by the Wenchuan earthquake. As one of the important and effective means of non-engineering disaster mitigation, debris flow hazard assessment is a key issue for the sustainable economic and social development of earthquake-hit areas. This article illustrates a new method to quantify the debris flow hazard by combining debris flow simulated results with different land utilization within the influence area. The Guo Juanyan gully in Dujiangyan city, Sichuan province, China, located in the meizoseismal area of the Wenchuan earthquake, was selected as the study area. The rainfall characteristics, including the rainfall pattern and the 10-min, 1-h, and 24-h critical rainfalls, were fully explored first. Then, the numerical simulation method was applied as a modeling tool to simulate debris flow influence area and final buried depth under rainfall with different return periods. The simulated results under a 100-year return period rainfall were validated based on field measurements. Finally, the debris flow hazard maps under different return periods were overlapped by combining the simulated results and the types of land utilization. The proposed method can enhance the accuracy of debris flow hazard assessment and can be widely used for debris flow mitigation, which has important application value.

The analysis of regional slope stability based on gis: a case study in daqu watershed from donggu to ranchong of the first stage of the west line of water diversion project from south to north of China

Slope failure hazard usually causes great damages in mountain area. So, the regional slope stability study is very important and essential to engineering construction, regional planning, disaster mitigation and economic development in mountain area. This paper takes Daqu watershed from Donggu to Ranchong of the first stage of the west line of water Diversion Project from South to North of China as a study area. It chooses gradient, slope type, slope aspect, elevation difference, lithology, earthquake, fault and land use as factors to analyze the stability of regional slope. The factors are classified, quantified and overlapped by using GIS method. After analysis, the regional slope stability of the study area is divided into stable area, middle-stable area and unstable area, and the areas liable to slope failure hazards are delimited based on the possibility of happening slope failure hazards. At last, through the field investigation of slope failure hazards in the study area, the rationality of analyzing results is tested.

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.

Effects of land use on soil physical and chemical characters at debris flow bottomland

Understanding of the characteristics of soil nutrients at the field and catchment scale is important for refining agricultural management practices and for improving sustainable land use. In order to analyze soil nutrient differences among different land use types and their relationships between land uses, 9 sampling sites including 3 land uses were selected in the Daqiaohe catchment on Yunnan of China. Significant differences in soil nutrients among these land uses were found. Higher values of soil nutrient in crop land soil, but lower values in forest soil and virgin soil. Nutrient contents decreased with soil profile depth increasing. All nutrient contents were poor in debris flow bottomland. At the same time, relationship between nutrient contents and grade contents was analyzed. The result showed that TP, AP and AK were significantly correlated with clay contents and slit contents at 0.01 level, while other nutrients were not correlated with clay and slit contents. There existed significant relationship between TP and sand contents, but for TN, the condition was found. However, nutrient was relation to many factors, thus relationship between nutrient contents and grade contents should he studied in future.