Kaiheng Hu

Characteristics and triggering mechanism of Xinmo landslide on 24 June 2017 in Sichuan, China

At 5: 39 AM on 24 June 2017, a huge landslide-debris avalanche occurred on Fugui Mountain at Xinmo village, Diexi town, Maoxian county, Sichuan province, China. The debris blocked the Songpinggou River for about 2 km, resulting in a heavy loss of both human lives and properties (10 deaths, 3 injuries, 73 missing, and 103 houses completely destroyed). The objectives of this paper are to understand the overall process and triggering factors of this landslide and to explore the affecting factors for its long term evolution before failure. Post event surveys were carried out the day after the landslide occurrence. Information was gathered from literature and on-site investigation and measurement. Topography, landforms, lithology, geological setting, earthquake history, meteorological and hydrological data of the area were analysed. Aerial photographs and other remote sensing information were used for evaluation and discussion. Eye witnesses also provided a lot of helpful information for us to understand the process of initiation, development and deposition. The depositional characteristics of the moving material as well as the traces of the movement, the structural features of the main scarp and the seismic waves induced by the slide are presented and discussed in detail in this paper. The results show that the mechanism of the landslide is a sudden rupture of the main block caused by the instability of a secondary block at a higher position. After the initiation, the failed rock mass at higher position overloaded the main block at the lower elevation and collapsed in tandem. Fragmentation of the rock mass occurred later, thus forming a debris avalanche with high mobility. This landslide case indicates that such seismic events could influence geological hazards for over 80 years and this study provides reference to the long term susceptibility and risk assessment of secondary geological hazards from earthquake.

Characteristics of clustering debris flows in Wenchuan earthquake zone

Clustering debris-flow events, namely many debris flows simultaneously triggered by a regional rainstorm in a large-scale mountainous area, occurred in four regions of Wenchuan earthquake stricken areas in 2008 and 2010. The characteristics of the clustering debris flows are examined with regard to triggering rainfall, formation process, and relationship with the earthquake by field survey and remote sensing interpretation. It is found that the clustering events occurred nearly at the same time with the local peak rainstorms, and the rainfall intensity-duration bottom limit line for clustering debris flows is higher than the worldwide line. It means that more rainfall is needed for the occurrence of the clustering debris flows. Four kinds of major formation processes for these debris flows are summarized: tributary-dominated, mainstream-dominated, transformation from slope failures, and mobilization or liquefaction of landslide. The four regions has a spatial correlation with the strong-quake-influenced zone with the peak ground acceleration = 0.2 g and the seismic intensity > X.

Susceptibility mapping of landslides in Beichuan County using cluster and MLC methods

Cluster analysis and maximum likelihood classification (MLC) are exploited to map the post-earthquake landslide susceptibility in Beichuan County that was affected by the Ms 8.0 Wenchuan earthquake. The methodology is applicable even if there is short of training data. Six effective factors are chosen for mapping the susceptibility, including land use, seismic intensity, average annual rainfall, relative relief, slop gradient and lithology. Four clusters are grouped from sampling grid cells by k-means clustering approach. MLC classifies all the cells in the study area into the four clusters according to their statistical characteristics. Four susceptibility classes (extreme low, low, moderate and high) are assigned to these clusters applying expert experience and hazard density. The final map gives a reasonable assessment of post-earthquake landslide susceptibility in Beichuan County. Comparing with the pre-earthquake susceptibility map made in Beichuan County geological disaster survey project, the result t using cluster and MLC classification has a better agreement with the dot density value of post-earthquake landslides in Beichuan County. The susceptibility map can be used to identify safety spots within the high danger area, which are suitable for habitations and facilities. It is also found that more landslides are densely concentrated at the boundary between high and moderate regions, and between high and extreme low regions.

Mean Velocity Estimation of Viscous Debris Flows

The mean velocity estimation of debris flows, especially viscous debris flows, is an important part in the debris flow dynamics research and in the design of control structures. In this study, theoretical equations for computing debris flow velocity with the one-phase flow assumption were reviewed and used to analyze field data of viscous debris flows. Results show that the viscous debris flow is difficult to be classified as a Newtonian laminar flow, a Newtonian turbulent flow, a Bingham fluid, or a dilatant fluid in the strict sense. However, we can establish empirical formulas to compute its mean velocity following equations for Newtonian turbulent flows, because most viscous debris flows are turbulent. Factors that potentially influence debris flow velocity were chosen according to two-phase flow theories. Through correlation analysis and data fitting, two empirical formulas were proposed. In the first one, velocity is expressed as a function of clay content, flow depth and channel slope. In the second one, a coefficient representing the grain size nonuniformity is used instead of clay content. Both formulas can give reasonable estimate of the mean velocity of the viscous debris flow.

Comparison of rheometric devices for measuring the rheological parameters of debris flow slurry

Soil samples with clay content ranging from 15% to 31%, were taken from three debris flow gullies in Southwest China. Three debris flow slurry samples were prepared and tested with four measuring systems of an Anton Paar Physica MCR301 rheometer, including the concentric cylinder system, the parallel-plate system, the vane geometry, and the ball measuring system. All systems were smoothwalled. Flow curves were plotted and yield stress was determined using the Herschel-Bulkley model, showing differences among the different systems. Flow curves from the concentric cylinder and parallelplate systems involved two distinct regions, the low shear and the high shear regions. Yield stresses determined by data fitting in the low shear region were significantly lower than the values from the inclined channel test which is a practical method for determining yield stress. Flow curves in the high shear region are close to those from the vane geometry and the ball measuring system. The fitted values of yield stress are comparable to the values from the inclined channel test. The differences are caused by wall-slip effects in the low shear region. Vane geometry can capture the stress overshoot phenomenon caused by the destruction of slurry structure, whereas end effects should be considered in the determination of yield stress. The ball measuring system can give reasonable results, and it is applicable for rheological testing of debris flow slurries.

Effect of Bed Sediment Entrainment on Debris-Flow Resistance

Entrainment of sediment from hillslope and channel usually has a significant influence on resistance of debris flow. This effect is studied by flume experiments of debris flow over rigid and erodible beds. Fifty contrast flume tests show that the flow resistance over the erodible bed is significantly larger than that over the rigid bed because of higher energy consumption resulting from sediment transport. A new formula of debris-flow resistance involving the entrainment effect is obtained from the erodible-bed data by nondimensional multiple regression analysis.

Experimental and numerical analysis on the responses of slope under rainfall

Rainfall is an important factor to trigger the slope failure such as landslides and debris flows. First, the relationship between rainfall duration with the initiation of debris flow and rainfall intensity was mainly studied by the series tests in a box model. Then, the rainfall induced responses of slopes and the initiation of slope failure were simulated by using the software FLAC2D based on the soil parameters in Weijia Gully, Beichuan County, Sichuan Province. The effects of the slope angle, rainfall intensity, soil parameters on the development of the stress, and pore pressure in the soil of the slope were analyzed. It indicates that largest displacements in the slope are mainly located near the toe. With the increase of the rainfall intensity, the effective stress in the slope decreases and the displacement increases.

A decision support system for debris-flow hazard mitigation in towns based on numerical simulation: a case study at Dongchuan, Yunnan Province

The hazard mitigation decision support system is an efficient method to avoid severe human damage by debris flow. Using modern technology, the multi-functional synthetical mitigation decision support system integrates debris flow monitoring, information transmission, disaster forecast and alarm, disaster estimate, evacuation and rescue plan to provide support for making mitigation strategic decision in all aspects before and after the disaster. The system was successfully utilised in debris flow hazard mitigation in the Dongchuan suburb in Yunnan Province. According to the data of monitoring and predicting, the system can simulate the movement of debris flow, and define the range and hazard zone of debris flow disaster. The result can be used to conduct disaster estimation, and prepare evacuation and rescue plan, which increases the degree of disaster mitigation.

Assessment of Secondary Mountain Hazards along a Section of the Dujiangyan－Wenchuan Highway

Conducting a hazard assessment for secondary mountain hazards is the technical basis for reconstructing destroyed highways and for disaster prevention. It is necessary to consider the role and influence of structural engineering measures as an important assessment factor. In this study, based on six substantial field investigations conducted between July 2008 and July 2012, a 2 km wide zone along both sides of the Dujiangyan-Wenchuan (Du-Wen) Highway was selected as the study area. Microgeomorphic units and small watersheds in the study area were extracted with GIS software and used as basic assessment units. Through field investigations, remote sensing surveys and experimental analysis, a structural engineering effectiveness assessment was conducted using the technique of principal component analysis. The results showed the following: 1) A total of 491 collapses, 12 landslides, 32 slope debris flows and 17 gully debris flows were scatted across the study area. The total overall areal density of all mountain hazards was 25.7%. The distribution of secondary hazards was influenced mainly by seismic intensity, active fault zones, lithology, slope and altitude. More than 70% of secondary hazards occurred in zones with a seismic intensity of XI, a distance to the fault zone of between 0 and 25 km, a slope between 25° and 50°, and an altitude of between 1,000 m and 1,800 m. 2) Different structural engineering measures play different roles and effects in controlling different types and scales of secondary mountain hazards. 3) With a secondary mountain hazard area of 128.1 km2 and an areal density of 34.9%, medium, high and very high hazard zones accounted for 74% of the study area and were located on the high, steep slopes along both sides of the highway. The low hazard zone was located mainly in the valley floor, on gentle slope platforms and at locations 1.5 km away from the highway ? the hazard area was 45 km2 and the areal density was 3.3%. 4) The methodology for hazard assessment of secondary mountain hazards, which is based on five factors, solves such key technical problems as the selection of assessment units, multi-source data fusion, and the weight calculation for each assessment index. This study provides a new and more effective method for assessing secondary mountain hazards along highways, and the proposed models fit well with validation data and field observations. The findings were applied to reconstruction and disaster mitigation in the case of the Du-Wen Highway and proved to be feasible.

Influence of Flow Width on Mean Velocity of Debris Flows in Wide Open Channel

Debris flow in a wide open downstream channel has a significant transverse velocity component that strongly influences its mean longitudinal velocity. Investigation of observation data of debris-flow surges at Jiangjia Ravine in China shows the dependency of Manning resistance of debris flows on the ratio of flow width to depth. Regression fit reveals a power function relationship between the Manning resistance coefficient and the width-to-depth ratio. This derives a new formula of mean velocity incorporating the influence of flow width. The result indicates that the width-to-depth ratio can be viewed as a kind of shape roughness analogous to grain roughness in the Darcy-Weisbach resistance coefficient expression. DOI: 10.1061/(ASCE)HY.1943-7900.0000648