Xiaoqing Chen

Types and Causes of Debris Flow Damage to Drainage Channels in the Wenchuan Earthquake Area

Debris flows are among the most common geological disasters in China, and have been particularly frequent in Sichuan Province since the Wenchuan earthquake on 12 May 2008. The construction of debris flow drainage channels is a countermeasure used to distribute debris flow fans, and these channels play a critical role in the mitigation and prevention of damage resulting from debris flows. Under field conditions, the useful life of drainage channels can be greatly shortened as a result of strong abrasions to the drainage structure caused by the debris flow. Field investigations have shown that the types of damage to drainage channels include (a) erosion caused by hyper-concentrated silt flow, (b) impact fractures and foundation scour at the groundsills of the drainage channel, (c) destruction of the drainage channel outlet, and (d) destruction of the drainage channel caused by debris flow abrasion. In addition, based on the destruction of the drainage channel during the debris flow drainage process, a new type of drainage channel with energy dissipation components was proposed and applied in a steep, narrow gully for debris flow mitigation. Moreover, design and engineering repair recommendations for drainage channels are provided as a reference for repairing the damage to the channel. The results can provide an important reference for the effective repair and optimal design of drainage channels.

Characteristics of a drainage channel with staggered indented sills for controlling debris flows

The characteristics of a new type of drainage channel with staggered indented sills for controlling debris flows were studied. The intermediate fluid in the non-viscous debris flow exhibited a helical movement, whereas the fluid near the sidewall had a stop-start movement pattern; the viscous debris flow exhibited a stable structure between the indented sills. The experimental results indicate that the mean velocity of the debris flow increased with increasing channel gradients, and the debris flow velocity was slightly affected by the angle of the sills. The average velocity of the non-viscous debris flow increased in the range of (0.5–1.5) interval between the indented sills, whereas the average velocity of the viscous debris flow increased initially and then decreased in the range of (0.75–1.25) interval between the indented sills. The depth of the non-viscous debris flow tended to gradually increase as the channel gradients increased, whereas the depth of the viscous debris flow gradually decreased as the channel gradients increased. When the discharge of the debris flow was constant, the angle and the interval between the indented sills had a slight effect on the depth of the viscous debris flow, whereas the depth of the non-viscous debris flow exhibited a different trend, as the sill angles and intervals were varied.

Formation and Treatment of Landslide Dams Emplaced During the 2008 Wenchuan Earthquake, Sichuan, China

The M8.0 Wenchuan earthquake of 12 May 2008 triggered over 20,000 landslides in the earthquake affected area of Longmenshan Mountain. It was a common phenomenon that large scale landslides blocked river channels. In total, 257 landslide-dams were identified by field investigation and remote sensing inspection. The dams were distributed in a belt along the rupture zone and in clusters along the river valleys. Emergency risk assessment was carried out due to the rise of water levels behind the dams and catastrophic outburst potential of the landslide-dammed lakes. Under the emergency circumstances, the dam height, dam composition and storage capacity of the landslide-dammed lake were used as indices of the dam’s risk of collapse. Four risk levels were assigned. By analyzing 21 landslide dams in detail, the results of risk assessment were obtained; 1 dam had an extremely high danger risk (Tangjiashan Lake), 7 dams had a high danger, 5 dams medium danger and 8 dams were of low danger. The case of the Tangjiashan rockslide dam that formed the largest landslide-dammed lake, is discussed in detailed. The scenarios analysed were for 20, 25, 33, and 50% of the dam failing, yielding, respectively, flood heights of 4.6, 5.1, 5.7, and 6.2 m, inundation areas of 3.35, 3.84, 4.22, and 4.65 km2, and discharges of 6,106, 7,397, 9,062, and 11,260 m3/s, at the Fujiang Bridge in Mianyang, the second largest city in Sichuan Province. The outcome of this analysis was used by emergency managers to plan downstream evacuations. Sluiceways were designed to lower water level of the dammed lakes and to reduce the risk of the outburst floods. Emergency mitigation operations for risk-elimination proved to be effective and successful. On the other hand, debris flows will be the dominating disaster process in future and will likely deposit masses of debris in river channels forming new dams in the coming 5–10 years. It is thus of prime importance to take measures and prevent debris flows from damming main rivers and posing new threats to downstream locations.

Techniques of Debris Flow Prevention in National Parks

Abstract Most national parks in China are located in mountain areas and have been suffering from various disasters, especially debris flow. Techniques controlling debris flow in national parks are discussed in details. The basic goal is to protect the landscapes and eco-environment and to make engineering works be operated in harmony with landscapes. These techniques consist control work in the source areas and various structures for damming sediment and diverting flows. All works have been put into controlling debris flows in 14 tributaries of Jiuzhaigou, which have set up a model for debris flow prevention in national parks in China.

Efficiency of Slot-Check Dam Group on Debris Flow Control in Shengou Basin, Kunming, China

Slot-check dams are widely applied to mitigate debris flow hazards in China. This paper aims at providing an analysis of their performance in the field and establishing a verification to evaluate the efficiency of the slot-check dam group and a single slot-check dam in sediment control. The study area is Shengou basin in Dongchuan district, Kunming in China, where debris flows directly threats the safety of more than sixty thousand people living in the major city. The analysis is based on a relatively detailed field knowledge consisting of topographical, sediment data and geometry of the group of five slot-check dams at Shengou basin. The results suggest that the conserving sediment volume of each dam has a close relationship with its relative location in the group, gradually decreasing from upstream to downstream. The channel slope has a little influence on the conserving effect. The accumulated sediment volume upstream a check dam strongly depends on its sub-catchment characteristics and increases with the controlled catchment area of the dam and the distance from the dam to the upstream most. A semi-empirical relationship between the conserving efficiency for each subsystem of the group, defined as the percentage of its conserved sediment volume, and the subsystem’s catchment parameters is developed.

Assessment of prospective hazards resulting from the 2017 earthquake at the world heritage site Jiuzhaigou Valley, Sichuan, China

On August 8, 2017, a Ms = 7.0 magnitude earthquake occurred in the Jiuzhaigou Valley, in Sichuan Province, China (N: 33.20°, E: 103.82°). Jiuzhaigou Valley is an area recognized and listed as a world heritage site by UNESCO in 1992. Data analysis and field survey were conducted on the landslide, collapse, and debris flow gully, to assess the coseismic geological hazards generated by the earthquake using an unmanned aerial vehicle (UAV), remote-sensing imaging, laser range finders, geological radars, and cameras. The results highlighted the occurrence of 13 landslides, 70 collapses, and 25 potential debris flow gullies following the earthquake. The hazards were classified on the basis of their size and the potential property loss attributable to them. Consequently, 14 large-scale hazards, 30 medium-sized hazards, and 64 small hazards accounting for 13%, 28%, and 59% of the total hazards, respectively, were identified. Based on the variation tendency of the geological hazards that ensued in areas affected by the Kanto earthquake (Japan), Chi-chi earthquake (Taiwan China), and Wenchuan earthquake (Sichuan China), the study predicts that, depending on the rain intensity cycle, the duration of geological hazard activities in the Jiuzhaigou Valley may last over ten years and will gradually decrease for the following five to ten years before returning to pre-earthquake levels. Thus, necessary monitoring and early warning systems must be implemented to ensure the safety of residents, workers and tourists during the construction of engineering projects and reopening of scenic sites to the public.

Characteristics and interpretation of the seismic signal of a field-scale landslide dam failure experiment

Outburst floods caused by breaches of landslide dams may cause serious damages and loss of lives in downstream areas; for this reason the study of the dynamic of the process is of particular interest for hazard and risk assessment. In this paper we report a field-scale landslide dam failure experiment conducted in Nantou County, in the central of Taiwan. The seismic signal generated during the dam failure was monitored using a broadband seismometer and the signal was used to study the dam failure process. We used the short-time Fourier transform (STFT) to obtain the time–frequency characteristics of the signal and analyzed the correlation between the power spectrum density (PSD) of the signal and the water level. The results indicate that the seismic signal generated during the process consisted of three components: a low-frequency band (0–1.5 Hz), an intermediate-frequency band (1.5–10 Hz) and a high-frequency band (10–45 Hz). We obtained the characteristics of each frequency band and the variations of the signal in various stages of the landslide dam failure process. We determined the cause for the signal changes in each frequency band and its relationship with the dam failure process. The PSD sediment flux estimation model was used to interpret the causes of variations in the signal energy before the dam failure and the clockwise hysteresis during the failure. Our results show that the seismic signal reflects the physical characteristics of the landslide dam failure process. The method and equipment used in this study may be used to monitor landslide dams and providing early warnings for dam failures.

Dam-break risk analysis of the Attabad landslide dam in Pakistan and emergency countermeasures

The Attabad landslide dam caused significant property losses and many human casualties in Pakistan, and also greatly affected the operation of the China-Pakistan Karakoram Highway (KKH). This paper discusses the risk of dam breach and hazards to the KKH project construction site following a dam breach. The paper examines the following three topics. (1) The geomorphologic dimensionless blockage index (DBI) and the analogy method were used to analyze the stability of the Attabad landslide dam. The long-term behaviors of landslide dams downstream of the Attabad landslide dam indicate that the risk of a dam breach exists, but the probability of a total dam failure is low. (2) The peak discharge of a potential breach of the Attabad landslide dam was calculated for scenarios in which 1/4, 1/3, 1/2, and total failure of the dam was breached. The potential breach discharge decreases with the downstream distance. (3) The potential impacts of the landslide dam breach on the KKH project construction site were analyzed. Based on the composition of the landslide dam, the probability of a 1/3 dam breach is high. To ensure the safety of downstream areas, disaster preparedness plans that correspond to the 1/2 dam breach scenario should be developed. Based on experience in addressing the landslide dam that was caused by the Wenchuan Earthquake, artificial controlled drainage measures are suggested and provide a technical reference for addressing the Attabad landslide dam and achieving recovery and normal operation of KKH.

Fractal Structure of Debris Flow

One of the most remarkable characteristics of debris flow is the competence for supporting boulders on the surface of flow, which strongly suggests that there should be some structure in the fluid body. This paper analyzed the grain compositions from various samples of debris flows and then revealed the fractal structure. Specifically, the fractality holds in three domains that can be respectively identified as the slurry, matrix, and the coarse content. Furthermore, the matrix fractal, which distinguishes debris flow from other kinds of flows, involves a hierarchical structure in the sense that it might contain ever increasing grains while the total range of grain size increases. It provides a possible mechanism for the boulder suspension.

Application of incomplete similarity theory to the estimation of the mean velocity of debris flows

The mean velocity of debris flow is one of the most important parameters in the design of mitigation structures and in quantitative risk analysis. This study develops a model to predict the mean debris flow velocity observed in the field by applying the incomplete similarity argument. An equation for estimating the Darcy-Weisbach resistance coefficient for debris flows with a volumetric sediment concentration larger than 0.19 is accordingly derived using 128 sets of observation data from nine Chinese gullies, in which both the effect of the volumetric sediment concentration and channel slope on resistance are considered. The derived equation is then verified and compared against five previously published equations by using 61 sets of published observation data from six gullies located in four countries. The applications of the proposed equation are discussed, and the improvements made using the proposed equation are clearly very significant when compared with the previously published equations.