Landslide dam breaching under combined hydraulic loads: Experimental insights into surge–overflow synergy

Landslide dams are usually formed by river blockages with massive amounts of materials from avalanches, landslides, and debris flows. The breaching of landslide dams may cause mega floods, posing great threats to people and infrastructure downstream. The important role in evaluating the breaching of landslide dams is the erodibility of dam deposits. In this study, ten grain size distribution with different characteristics (e.g. unit weight, fines content, d 50 ) were extracted based on 7 landslide dams. The laboratory flume apparatus, with a length of 5 m and a width of 0.4 m, were used to study the erosion rate of materials under six water flow rates. The test samples were collected, dried, and weighed before and after every single test to calculate the erosion rate. Results indicated that the erosion rate increases with the increase of flow velocity and shear stress. The relationship between erosion rate and flow velocity and shear stress is almost linear. Both the critical incipient velocity and critical shear stress have poor correlations with soil property, which means these two parameters cannot be predicted by one or two influence factors since it is affected by multi-factors. The work can serve as a basis to predict erosion rate of dam materials and analyze the breaching of landslide dams.

<p>Landslide dam breaching is the one of focus topics in the geophysical flows. The frequency of occurrence of landslide dam increases due to earthquake, climate change and mans activities in recent years. Once the dam breaks, it would trigger extreme flood downstream. A field experiment on landslide dam breach has been carried out on a small mountain river in Mianzhu, Sichuan Province, China from 23 November to 29 December, which aims to reveal impact of different diversion channel types on the dam breaching process as well as the resulting flood. The dam is of 4m high, 10~15m wide. the length of the dam crest is 5m, upstream downstream slopes of the dam are 1:2 and 1:5. Results show division channel can reduce the peak flood discharge obviously. The pilot vertical fall can trigger earlier back erosion and thus peak discharge appears earlier with smaller magnitude.</p>

The breach of landslide dam often causes significant disaster in the inundated area; the prediction of breach hydrograph is in high demand for the dam breach risk evaluation. In this study, according to the model tests and Tangjiashan landslide dam breach case, the surface erosion accompanied by intermittent mass failure is known as the key breaching mechanism for landslide dam due to overtopping failure. The downstream slope angle would gradually decrease during the dam-breaching process, whereas a planar wedge failure occurs when the breach slopes at the dam crest and downstream breach channel fail. Based on the breach mechanism, a numerical model for landslide dam breach due to overtopping is developed to simulate the coupling process of water and soil. The model focuses on the breach morphology evolution during the breaching for the sake of the improvement of breach hydrograph prediction. Furthermore, the model can handle one- and two-sided breach, as well as incomplete and base erosion at the vertical direction. The case study of Tangjiashan landslide dam-breaching feedback analysis testifies the rationality of the present model with the relative errors less than 10% for peak discharge, final breach widths, and time to peak. The sensitivity analysis indicates that the final breach depth and soil erodibility affect the breach flow prediction of the landslide dam significantly, whereas the one- or two-sided breach mode is less sensitive.

Spillway excavation is often adopted as a precautionary engineering measure for disaster mitigation before landslide dam breaching. Based on the landslide dam breach mechanisms, this paper focuses on developing a numerical model to comprehensively discuss the issue based on three documented landslide dam failures, such as Tangjiashan, Xiaogangjian, and Baige landslide dams. The spillway cross section morphologies were modeled with different sizes under common shape (i.e., an inverted trapezoid) and slope conditions. The influence of cross section on dam breach processes was analyzed under conditions of different depth, bottom width, slope ratio in the cross and longitudinal sections, with/without spillway. The following conclusions can be drawn: 1) excavation of a spillway can effectively reduce the peak breach flow, therefore delay the time to peak; 2) the peak breach flow dramatically decreases and the time to peak delays as the spillway depth increases; 3) the peak breach flow changes little and the time to peak occurs earlier with the increment in spillway bottom width; 4) the peak breach flow decreases and the time to peak delays with the decrease of slope ratio in cross section in the spillway; 5) the slope ratio in the longitudinal section has little influence on the breach process. Hence, if conditions permit, the spillway with large spillway depth, small bottom width, and gentle slope ratio in the cross section is the preferable section morphology for the emergency disposal of the landslide dam.

The formation and breaching of a short-lived landslide dam at Hsiaolin Village, Taiwan — Part II: Simulation of debris flow with landslide dam breach

Prediction of the overtopping-induced breach process of the landslide dam

Cascading breaching of the Tangjiashan landslide dam and two smaller downstream landslide dams

On October 10 and November 3, 2018, two successive landslides occurred at Baige village, the border between Sichuan Province and Tibet Autonomous Region, in China, which totally dammed the Jinsha River on both occasions. Due to the rapid rise in water level in the “10·10” dammed lake, on October 12, the landslide dam breached naturally with the peak breach flow of about 10,000 m3/s. The residual landslide dam was stacked by the subsequent landslide on November 3, resulting in an even larger dammed lake. Fortunately, the height from the water level in the lake to the dam crest made it possible to construct a spillway to drain the water in the dammed lake to a relatively low level. On November 12, the drainage process began with the peak breach flow of 31,000 m3/s. In this study, based on the detailed records of the breach process of the “11·03” Baige landslide dam and using the developed physically based numerical method, a back analysis was conducted. The numerical method was developed based on the overtopping-induced breach mechanism of landslide dams. An iterative time step algorithm was used to simulate the breach evolution and the hydrograph coupling. The major highlights of the numerical method are the consideration of the breach mechanism of landslide dam, such as the breach morphology evolution process along the streamwise and transverse directions, as well as the variation of soil erodibility with depth and the influence of the presence and absence of a spillway. Comparison of the measured and the calculated results indicated that the numerical method developed in this study can reproduce reasonable breach hydrograph and breach evolution process. The sensitivity analysis showed that the soil erodibility coefficient and the residual dam height significantly influenced the landslide dam breaching process. In addition, it was determined that constructing a spillway before landslide dam breaching is an effective flood hazard mitigation measure for large dammed lakes. However, the availability of the construction conditions and the shape of the spillway should be judged comprehensively according to the rising rate of water level and construction capacity.

<p>The failure of landslide dams is a sudden geological disaster, with its formation and failure greatly threatening the security of local people’s lives and property. In this research, we conducted 12 sets of model experiments, considering the influence of different angle of flume bed, dam heights, and downstream slopes on the process of overtopping breaching of noncohesive landslide dams. Based on these experimental results, we analyzed the characteristics of the longitudinal and transverse evolution, and outburst discharge of landslide dams in detail. At first, we divided the failure process of landslide dams into four stages, including initiation, headward erosion, downcutting erosion, and riverbed rebalancing. In addition, the quantitative analysis of breaching evolution model and the numerical method for simulating landslide dam failure due to overtopping has also been introduced in this research. This paper provides the research basis for the following two papers, which also provide a scientific reference for the prevention and mitigation of landslide dams.</p>

Landslide dams are common geological features in mountainous areas, which may have serious consequences due to sudden breaching of the dam. An effective emergency response requires rapid and accurate forecasts regarding the landslide dam breach process. However, most existing models use physical, mechanical, and erosion properties of the mean or characteristic grain sizes to represent the landslide deposits. The grain size distribution and variations in soil erodibility with the depth in the landslide dam are not considered, resulting in an incorrect estimation of the breach flow hydrograph. In this paper, a simplified landslide dam classification is presented based on the formation mechanism and grain size distribution of landslide dams. Additionally, the influences of grain size distribution on the residual dam height and breach process of landslide dams are analyzed. This paper proposes a numerical method to rapidly obtain the breach hydrographs and breach morphology evolution of landslide dams. The new method can quickly classify landslide dams according to geological survey data and predict the landslide dam breach process. Three types of representative landslide dams in China are simulated to validate the proposed method. The breach flow discharge is significantly affected by spillway excavation. This contribution can provide rapid prediction of the landslide dam breach process and can be used for the emergency response planning before dam breaching.

Landslide dam are naturally formed dams with the characteristics of the loose structure, poor stability and strong permeability. Once a landslide dam is formed, its high probability of breaching and the flood caused by its breaching may threaten the safety of people and property in local area. Therefore, a systematic and comprehensive study needs to be conducted to analyze the breaching characteristics of landslide dam. In this article, a detail review from model experimental studies on dam break of landslide dam is conducted. The results and limitations are analyzed and summarized. Moreover, the factors of particle size distribution, compactness, water content and bed slope of single dam break are also analyzed. Finally we discuss the relationship among the dam break factor and the failure mode, the characteristics of dam breaching, peak discharge, and downcutting process. On the basis of the systematic analysis of the breaching of landslide dams, it's suggested that further studies of model similarity theory, scaling effect, testing methods and the disposal of landslide dam may be carried out in the future.

Experimental investigation on the longitudinal evolution of landslide dam breaching and outburst floods

Outburst flood of the dammed lake poses a great threat to the down-stream residents and properties, in addition, the process of dam erosion and flood routing is very complicated. Thus, the research which using numerical method to simulate and repeat the typical landslide dam breaching, has a great significance for disaster prevention and mitigation in the downstream. The Jinsha River, was dammed twice recently at Baige, Tibet, one on 10 October 2018 and the other on 3 November 2018. Accordingly two large landslide dams were formed in a three-week interval, and breached subsequently, causing a major loss of property and damage in the downstream. This study focuses on the “10.11” Baige landslide-dammed lake. A three-dimensional numerical model of “10.11” Baige landslide dam is established with the terrain data acquired by UAV, and the actual process of dam breaching is simulated and reproduced using the Flow-3D software. The Flow-3D software could be powerful, which has the ability to simulate the phenomenon of turbulence flow and sediment movement under complex terrain conditions. The parameters required for this study, such as hydrological data and particle composition of dam body, are directly referred to the field data. A flow monitoring section is set up near the downstream of the dam, and the simulated peak discharge process is in good agreement with the measured values. Furthermore, the velocity distribution and sediment scour in the natural spillway are analyzed. The results show that: overtopping scour of dam can be divided into four phases, i.e. before the outburst, rapid expansion phase, peak discharge phase and phase with breach develops steadily. In second phase, the sediment erosion rate is large and the terrain changes fast, so the breach expands rapidly. After the flood peak, the upstream water level and discharge decrease gradually, and the development speed of breach becomes slow, finally the terrain tends to be stable. The velocity of flow on the ramp of spillway is large during the releasing period, which leads to the maximum scour depth. On the contrary, the flow velocity in the front and middle of the spillway is small and the scour depth is relatively small. The terrain becomes flat at the downstream of the dam body, so the sediment sinks under the influence of weight and friction with the riverbed, then siltation occurs. There was obvious headward erosion in the process of first dam breaking: with the undercutting of spillway, the bed drop-off moved to the upstream continuously. Headward erosion is an important scour mode of dam breaching and the dominant force of channel expansion, which can produce strong scour effect in a short period of time. This study is significant for understanding the process and mechanism of the “10.11” Baige landslide dam breaching, which could provide technical reference to the management and accommodation of emergency.

Outburst flooding after a landslide dam breach causes global fatalities and devastation. Information on the timing, magnitude, and location of the landslide dam is crucial to hazard assessment. Despite recent efforts, successful real-time detection of landslide dams in mountain valleys and dam breakages is rare. Here, we present a series of seismic analysis including landslide detection, identification of landslide dam formations, and monitoring of dam breaches. We show the working of our analysis on a recent landslide dam that occurred in eastern Taiwan. The results indicate that our seismic analysis provides important information on the location and magnitude of landslides and the dam forming based on data acquired from a regional broadband seismic network. Furthermore, we see that the failure of the landslide dam is directly caught by the riverside seismic signals. To provide warning times for impending floods to downstream areas, we believe that proximal high-quality seismic signals along the river channel are viable options for an operational real-time monitoring system, for landslide dams occurring in mountain valleys. Our work can be a starting point to raise awareness in the community.

Accurate prediction of the hydrographs of outburst floods induced by landslide dam overtopping failure is necessary for hazard prevention and mitigation. In this study, flume model tests on the breaching of landslide dams were conducted. Unconsolidated soil materials with wide grain size distributions were used to construct the dam. The effects of different upstream inflow discharges and downstream bed soil erosion on the outburst peak discharge were investigated. Experimental results reveal that the whole hydrodynamic process of landslide dam breaching can be divided into three stages as defined by clear inflection points and peak discharges. The larger the inflow discharge, the shorter the time it takes to reach the peak discharge, and the larger the outburst flood peak discharge. The scale of the outburst floods was found to be amplified by the presence of an erodible bed located downstream of the landslide dam. This amplification decreases with the increase of upstream inflow. In addition, the results show that the existence of an erodible bed increases the density of the outburst flow, increasing its probability of transforming from a sediment flow to a debris flow.