Landslide Dam Failure Research Articles

Rare and dangerous: Recognizing extra-ordinary events in stream channels

Extreme-value statistics has taught us that flood flows can be estimated reasonably well, and that while extreme flows are rare, they are certain to occur. The physical process of flooding is reasonably well understood. However, this knowledge does not extend to steep creeks with potentially highly mobile beds. Most infrastructures on such creeks have been designed for clearwater floods with return periods of up to 200 years. This does not account for hydrogeomorphic processes such as debris floods and debris flows in which parts of, or the entire, channel bed sediments are mobilized and lead to massive erosion of channel bed and banks and debris inundation on terminal alluvial fans. Similarly, the potential for outburst floods – many times larger than normal floods – related to failure of landslide, glacier, moraine, beaver or man-made dams is not systematically included in standard hazard assessments. This paper has the objective of bridging science and practice by highlighting some of the most threatening hydrogeomorphic hazards to which people and infrastructure in mountain regions are exposed, and provides suggestions on how practice can be improved to properly diagnose and analyze the potential for such unusual floods. It is hoped that this will reduce potential losses in spite of the continued encroachment of urban and industrial development into mountain terrain.

Modeling of natural dam failure modes and downstream riverbed morphological changes with different dam materials in a flume test

Landslide dam failure modes are influenced by dam geometry, material characteristics, and riverbed conditions. The different characteristics of earthquake- and rainfall-induced natural dams and the effects of riverbed conditions on dam failure have not attracted much attention from researchers in the past. This study used flume tests to simulate earthquake- and rainfall-induced landslide dam failures for various combinations of dam geometry, materials, and riverbed conditions. The 64 flume test combinations thus included two discharge rates, two dam shapes (triangular and trapezoidal) with two heights, two types of riverbed conditions, and two types of dam material with two different water contents. The experimental results show that dam geometry, the amount of inflow, hydraulic conductivity of the dam material, and riverbed conditions have considerable effect on dams' lifespan and their corresponding failure mode. The hydraulic conductivity of the dam material affected the dam lifespan and its corresponding failure mode. The four observed failure modes of dam breaks in rigid and mobile riverbeds constitute progressive erosion, overtopping erosion, slope slide, and complex failure. The complex failure mode, which involves headward erosion, slope slide, and overtopping, can occur in highly hydraulically conductive dams with high discharge. Progressive erosion occurs in highly hydraulically conductive dams with low discharge. Dam material with low hydraulic conductivity tends to fail by pure overtopping. Regardless of the dam shape and material, dams in a mobile riverbed had a longer lifespan than those in a rigid riverbed. The composition of the dam material affected the peak outflow discharge after dam break. Compared with rigid riverbeds, dams in mobile riverbeds delay the arrival time of peak sediment discharge. Failure modes triggering flash floods and sediment transport after a dam break involved a series of sandbars and an armoring layer in the downstream riverbed. Earthquake-induced dry material forming highly hydraulically conductive dams generally fail through complex when failure was subjected to a high discharge. Dams composed of wet materials tend to fail by overtopping. In flume tests, dam failure mode and inflow changed downstream mobile riverbed morphology in ways similar to those seen in full-scale model tests conducted in the field.

地形や地質の違いが天然ダムの堆積形状に与える影響

Landslide dams are caused by large-scale slope collapse. Predicting their failure is important because hazardous flooding may result downstream when landslide dams burst. Accurate prediction of a landslide-dam formation which means height and length via landslide maps that show two-dimensional figure but does not consider landslide depth is needed for a countermeasure against the sediment disaster caused by landslide dam. In this study, past landslide dam failures were investigated. Relationships between landslide dam formation and the topographical and geological characteristics of the region were established to improve landslide dam analysis for more accurate prediction of dams’ structural integrity.

The formation and development of debris flows in large watersheds after the 2008 Wenchuan Earthquake

The Wenchuan earthquake has caused abundance of loose materials supplies for debris flows. Many debris flows have occurred in watersheds in area beyond 20 km(2), presenting characteristics differing from those in small watersheds. The debris flows yearly frequency decreases exponentially, and the average debris flow magnitude increases linearly with watershed size. The rainfall thresholds for debris flows in large watersheds were expressed as I = 14.7 D (-0.79) (2 h < D < 56 h), which is considerably higher than those in small watersheds as I = 4.4 D (-0.70) (2 h < D < 37 h). A case study is conducted in Ergou, 39.4 km(2) in area, to illustrate the formation and development processes of debris flows in large watersheds. A debris flow develops in a large watershed only when the rainfall was high enough to trigger the wide-spread failures and erosions on slope and realize the confluence in the watershed. The debris flow was supplied by the widely distributed failures dominated by rill erosions (14 in 22 sources in this case). The intermittent supplying increased the size and duration of debris flow. While the landslide dam failures provided most amounts for debris flows (57 % of the total amount), and amplified the discharge suddenly. During these processes, the debris flow velocity and density increased as well. The similar processes were observed in other large watersheds, indicating this case is representative.

Experimental study on the triggering mechanisms and kinematic properties of large debris flows in Wenjia Gully

Debris flows are typically caused by natural terrain landslides triggered by intense rainfalls. If an incoming mountain torrent flows along sloping channels at high velocity, huge amounts of sediment (from landslide dams and eroded channel beds) will be entrained into the flows to form debris flows. It is likely that large debris flows are due to the failure of many landslide dams of different scales (due to bank slides or collapses), bed erosion, and solid transport. The catastrophic debris flows that occurred in Wenjia Gully (Wenchuan Earthquake Area), China on August 13, 2010 (two years after the mega earthquake), were caused by intense rainfall and the serious erosion of sloping channels. In the wake of the incident, experimental tests were conducted to better understand the process of sediment erosion and entrainment on the channel bed and the formation of debris flows. The results show that the bed erosion, bank collapses and channel widening caused by erosion accounted for the triggering and scale amplification of downstream debris flows in the Wenjia Gully event. This study illustrates how the hazardous process of natural debris flows can begin several kilometers upstream, and how such a complex cascade of geomorphic events (failure of landslide dams and erosion of the sloping bed) can lead to catastrophic discharges. Neglecting recognition of these hazardous geomorphic and hydrodynamic processes may result in high cost.

Experimental study of debris flow caused by domino failures of landslide dams

The formation of landslide dams is often induced by earthquakes in mountainous areas. The failure of a landslide dam typically results in catastrophic flash floods or debris flows downstream. Significant attention has been given to the processes and mechanisms involved in the failure of individual landslide dams. However, the processes leading to domino failures of multiple landslide dams remain unclear. In this study, experimental tests were carried out to investigate the domino failure of landslide dams and the consequent enlargement of downstream debris flows. Different blockage conditions were considered, including complete blockage, partial blockage and erodible bed (no blockage). The mean velocity of the flow front was estimated by videos. Total stress transducers (TSTs) and Laser range finders (LRFs) were employed to measure the total stress and the depth of the flow front, respectively. Under a complete blockage pattern, a portion of the debris flow was trapped in front of each retained landslide dam before the latter collapsed completely. This was accompanied by a dramatic decrease in the mean velocity of the flow front. Conversely, under both partial blockage and erodible bed conditions, the mean velocity of the flow front increased gradually downward along the sloping channel. Domino failures of the landslide dams were triggered when a series of dams (complete blockage and partial blockage) were distributed along the flume. However, not all of these domino failures led to enlarged debris flows. The modes of dam failures have significant impacts on the enlargement of debris flows. Therefore, further research is necessary to understand the mechanisms of domino failures of landslide dams and their effects on the enlargement of debris flows.

Predictive uncertainty of peak outflow relations for landslides dam breach

Risk assessment development considering the failure of landslide dams often requires the estimation of peak outflow through the breach. The empirical equations based on data from case studies tend to be the first direct approach. This paper conducted an uncertainty analysis when these empirical relations were utilized to predict the peak outflow of a breached landslide dam. The results suggest that the relations derived from manmade dams or embankments typically overestimate the peak outflow about 1/5 to 3/4 of an order of magnitude; and the relations derived from the database of landslide dams have much smaller mean prediction errors and also exhibit broad uncertainty bands. Application of the uncertainly analysis was illustrated by the Tangjiashan landslide dammed lake, formed during 2008 Wenchuan earthquake. In addition, the predicted results from Eq. 1 deduced herein were considered to be the reliable estimate of peak outflow through the breach of landslide dam.

Numerical simulation on a tremendous debris flow caused by Typhoon Morakot in the Jiaopu Stream, Taiwan

In August 2009, Typhoon Morakot brought a large amount of rainfall with both high intensity and long duration to a vast area of Taiwan. Unfortunately, this resulted in a catastrophic landslide in Hsiaolin Village, Taiwan. Meanwhile, large amounts of landslides were formed in the Jiaopu Stream watershed near the southeast part of the Hsiaolin Village. The Hsiaolin Village access road (Provincial Highway No. 21 and Bridge No. 8) was completely destroyed by the landslide and consequent debris flow. The major scope of this study is to apply a debris flow model to simulate the disaster caused by the debris flow that occurred in the Jiaopu Stream during Typhoon Morakot. According to the interviews with local residents, this study applied the destruction time of Bridge No. 8 and Chen’s house to verify the numerical debris flow model. By the spatial rainfall distributions information, the numerical simulations of the debris flow are conducted in two stages. In the first stage (before the landslide-dam failure), the elevation of the debris flow and the corresponding potential damages toward residential properties were investigated. In the second stage (after the landslide-dam failure), comparisons of simulation results between the longitudinal and cross profiles of the Jiaopu Stream were performed using topographic maps and satellite imagery. In summary, applications of the adopted numerical debris flow model have shown positive impact on supporting better understanding of the occurrence and movement of debris flow processes.

Analysis on failure of slope and landslide dam

A three-dimensional (3D) seepage-flow numerical simulation model was developed for seepage analysis of a landslide dam. A 3D seepage-flow numerical simulation model coupled with a two-dimensional (2D) surface flow and erosion/deposition model was developed for seepage analysis of a slope due to a rainfall event. The conventional water-phase (one-phase) seepage-flow model assumed only water-phase flow in seepage analysis, which was inadequate for unsaturated soil domains. A water-air two-phase seepage-flow model that considers both the water and air phases in the seepage-flow process was also used for the seepage analysis. The pore-water pressure and moisture-content data obtained from the seepage-flow model were used to analyze the stability. Janbu’s simplified method and the extended Spencer method were used for the stability analysis. The numerical simulation results almost compared well with laboratory experimental measurements.

Formation, failure, and consequences of the Xiaolin landslide dam, triggered by extreme rainfall from Typhoon Morakot, Taiwan

An extreme rainfall event on August 9, 2009, which was close to setting a world record for 48-h accumulated rainfall, induced the Xiaolin deep-seated landslide, which was located in southwestern Taiwan and had volume of 27.6 × 106 m3, and caused the formation of a landslide dam. The landslide dam burst in a very short time, and little information remained afterward. We reconstructed the process of formation and failure of the Xiaolin landslide dam and also inferred the area of the impoundment and topographic changes. A 5 × 5-m digital elevation model, the recorded water stage of the Qishan River, and data from field investigation were used for analysis. The spectral magnitude of the seismic signals induced by the Xiaolin landslide and flooding due to failure of the landslide dam were analyzed to estimate the timing of the dam breach and the peak discharge of the subsequent flood. The Xiaolin landslide dam failure resulted from overtopping. We verified the longevity of the Xiaolin landslide dam at about 2 h relying on seismic signals and water level records. In addition, the inundated area, volume of the impoundment behind the Xiaolin landslide dam, and peak discharge of the flood were estimated at 92.3 ha, 19.5 × 106 m3, and 17 × 103 m3/s, respectively. The mean velocity of the flood-recession wave front due to the dam blockage was estimated at 28 km/h, and the peak flooding velocity after failure of the dam was estimated at 23 km/h. The Xiaolin landslide provides an invaluable opportunity for understanding the mechanism of deep-seated landslides and flooding processes following a landslide dam failure.

Dynamic decision making for dam-break emergency management – Part 2: Application to Tangjiashan landslide dam failure

Abstract. Tangjiashan landslide dam, which was triggered by the Ms = 8.0 Wenchuan earthquake in 2008 in China, threatened 1.2 million people downstream of the dam. All people in Beichuan Town 3.5 km downstream of the dam and 197 thousand people in Mianyang City 85 km downstream of the dam were evacuated 10 days before the breaching of the dam. Making such an important decision under uncertainty was difficult. This paper applied a dynamic decision-making framework for dam-break emergency management (DYDEM) to help rational decision in the emergency management of the Tangjiashan landslide dam. Three stages are identified with different levels of hydrological, geological and social-economic information along the timeline of the landslide dam failure event. The probability of dam failure is taken as a time series. The dam breaching parameters are predicted with a set of empirical models in stage 1 when no soil property information is known, and a physical model in stages 2 and 3 when knowledge of soil properties has been obtained. The flood routing downstream of the dam in these three stages is analyzed to evaluate the population at risk (PAR). The flood consequences, including evacuation costs, flood damage and monetized loss of life, are evaluated as functions of warning time using a human risk analysis model based on Bayesian networks. Finally, dynamic decision analysis is conducted to find the optimal time to evacuate the population at risk with minimum total loss in each of these three stages.

Scale amplification of natural debris flows caused by cascading landslide dam failures

Debris flows are typically caused by natural terrain landslides triggered by intense rainfalls. If an incoming mountain torrent collapses a series of landslide dams, large debris flows can form in a very short period. Moreover, the torrent can amplify the scale of the debris flow in the flow direction. The catastrophic debris flows that occurred in Zhouqu, China, on 8 August 2010 were caused by intense rainfall and the upstream cascading failure of landslide dams along the gullies. In the wake of the incident, a field study was conducted to better understand the process of cascading landslide dam failures and the formation of debris flows. This paper looks at the geomorphic properties of the debris-flow gullies, estimates the peak flow discharges at different locations using three different methods, and analyzes the key modes (i.e., different landslide dam types and their combinations) of cascading landslide dam failures and their effect on the scale amplification of debris flows. The results show that five key modes in Luojiayu gully and two modes in Sanyanyu gully accounted for the scale amplification of downstream debris flows in the Zhouqu event. This study illustrates how the hazardous process of natural debris flows can begin several kilometers upstream as a complex cascade of geomorphic events (failure of landslide dams and erosion of the sloping bed) can scale to become catastrophic discharges. Neglecting recognition of these hazardous geomorphic and hydrodynamic processes may result in a high cost.

Experimental study on cascading landslide dam failures by upstream flows

Landslide dams in mountainous areas are quite common. Typically, intense rainfalls can induce upstream flows along the sloping channel, which greatly affects the stability and failure modes of landslide dams. If a series of landslide dams are sequentially collapsed by an incoming mountain torrent (induced by intense rainfall), large debris flows can be formed in a short period of time. This also amplifies the magnitude of the debris flows along the flow direction. The catastrophic debris flows, which occurred in Zhouqu, China on August 8, 2010, were indeed caused by intense rainfall and the upstream cascading failure of landslide dams along the gullies. Experimental tests were conducted in a sloping channel to understand the dynamic process of cascading landslide dam failures and their effect on flow scale amplification. Similar to the Zhouqu conditions, the modeled landslide dams were distributed along a sloping channel and breached by different upstream flows. For each experiment, the front flows were sampled, the entrained grain sizes were analyzed, and the front discharge along the channel was measured. The results of these experiments show that landslide dams occurring along the channel can be destroyed by both high and low discharge flows, although the mechanisms are quite different for the two flow types. Regardless of flow type, the magnitude of the flows significantly increases after a cascading failure of landslide dams, resulting in an increase in both the diameter and the entrained coarse particles percentage.

Analysis of human risks due to dam break floods—part 2: application to Tangjiashan landslide dam failure

The Tangjiashan landslide dam was formed during the Ms8.0 Wenchuan earthquake in 2008 and posed high risks to 1.2 million people downstream the dam. A human risk analysis model (HURAM) reported in the companion paper is applied to evaluate the human risk in the Tangjiashan landslide dam failure. The characteristics of this landslide dam are introduced first. The breaching parameters in two cases (i.e., the actual case and a high erodibility case) are predicted with a physically based model, and the flood routing processes in these two cases are simulated using numerical analysis. The population at risk downstream of the landslide dam is then obtained based on the results of the flood routing simulations. Subsequently, the human risks are analyzed with HURAM using Bayesian networks. Fourteen influence parameters and their interrelationships are considered in a systematic structure in the case study. A change in anyone of them may affect the other parameters and leads to loss of life. HURAM allows not only cause-to-result inference, but also result-to-cause inference by updating the Bayesian network with specific information from the study case. The uncertainties of the parameters and their relationships are studied both at the global level using multiple sources of information and at the local level by updating the prior probabilities.

Initiation process of debris flows on different slopes due to surface flow and trigger-specific strategies for mitigating post-earthquake in old Beichuan County, China

The 12 May 2008 Wenchuan earthquake (Ms 8.0) in China, produced an estimated volume of 28 × 108 m3 loosened material, which led to debris flows after the earthquake. Debris flows are the dominant mountain hazards, and serious threat to lives, properties, buildings, traffic, and post-earthquake reconstruction in the earthquake-hit areas. It is very important to understand the debris flow initiation processes and characteristics, for designing debris flow mitigation. The main objective of this article is to examine the different debris flow initiation processes in order to identify suitable mitigation strategies. Three types of debris flow initiation processes were identified (designated as Types A, B, and C) by field survey and experiments. In “A” type initiation, the debris flow forms as a result of dam failure in the process of rill erosion, slope failure, landslide dam, or dam failure. This type of debris flow occurs at the slope of 10 ± 2°, with a high bulk density, and several surges following dam failure. “B” type initiation is the result of a gradual increase in headward down cutting, bank and lateral erosion, and then large amount of loose material interfusion into water flow, which increases the bulk density, and forms the debris flow. This type of debris flow occurs mainly on slopes of 15 ± 3° without surges. “C” type debris flow results from slope failures by surface flow, infiltration, loose material crack, slope failure, and fluidization. This type of debris flow occurs mainly on slopes of 21 ± 4°, and has several surges of debris flow following slope failure, and a high bulk density. To minimize the hazards from debris flows in areas affected by the Wenchuan earthquake, the erosion control measures, such as the construction of grid dams, slope failure control measures, the construction of storage sediment dams, and the drainage measures, such as construction of drainage ditches are proposed. Based on our results, it is recommend that the control measures should be chosen based on the debris flow initiation type, which affects the peak discharge, bulk density and the discharge process. The mitigation strategies discussed in this paper are based on experimental simulations of the debris flows in the Weijia, Huashiban, and Xijia gullies of old Beichuan city. The results are useful for post-disaster reconstruction and recovery, as well as for preventing similar geohazards in the future.

Experimental investigation of the failure of cascade landslide dams

This paper presents results of model tests for the landslide dam failure of a single dam and cascade dams in a sloping channel. The dams were designed to be regular trapezoid with fine sand. A new measuring method named the labeled line locating method was used to digitalize the captured instantaneous pictures. Under two different inflow discharges, the morphological evolution and the flow patterns during one dam failure and the failure of cascade dams were investigated. The results indicate that when the inflow discharge is large, the deformation pattern of the downstream dam is similar to that of the upstream dam, and both dams are characterized with the overtopping scour throughout the dam failure process. When the inflow discharge is small, the upstream dam is scoured mainly through a sluice slot formed by the longitudinal incision, and the downstream dam is characterized with the overtopping scour. The data set presented in this paper can be used for the validation of numerical models and provide a reference for the flood risk management of cascade landslide dams.

Observations from the large, rapid Yigong rock slide – debris avalanche, southeast Tibet

The Yigong rock slide – debris avalanche (YRA), which occurred on 9 April 2000, received worldwide attention as one of the largest nonseismic landslides in recent years, with a volume of 0.3 × 109 m3. Sixty-two days after this landslide event, a catastrophic flood happened because of landslide dam failure. One of the special features of this debris avalanche is liquefaction, which plays an important role in the entrainment and long run-out distance and high-speed movement of the debris avalanche. Numerous sand boils were found in the deposition zone, providing strong evidence for liquefaction. The YRA provides the first actual evidence for a theoretical model where the mechanisms of excess pore pressure and liquefaction induced by undrained loading, and entrainment and dissipation control the run out and deposition of the debris avalanche. The damage mode to trees and the presence of debris cones or molards with a rounded top is proven to be the result of strong air waves and eddies. These features all imply that the YRA is a solid–liquid–air mixed-debris avalanche.

Flood hydraulics due to cascade landslide dam failure

AbstractLandslide dams are usually unconsolidated and when they act to impound water, they are subjected to failure that triggers catastrophic flooding downstream. However, the flood hydraulics due to landslide dam failure has remained poorly understood and cannot be reliably predicted at present. Inevitably, this militates against effective flood risk management in areas prone to landslides. Extending the recent investigation of a single landslide dam failure and the resulting flood, here, we present an experimental and computational study on the flood flow induced by cascade landslide dam failure. A total of 12 runs of experiments are conducted in a flume of 80 m × 1.2 m × 0.8 m, with differing inflow discharge and dam composition. An array of 12 automatic water‐level probes is deployed to measure the stage hydrographs along the flume. The coupled shallow water hydrodynamic model calibrated for cases of a single landslide dam failure is demonstrated to perform reasonably well for cascade landslide dam failure. Most notably, under appropriate conditions, the occurrence of streamwise progressive enhancement of the flood induced by cascade landslide dam failure is demonstrated. This features a higher risk of flooding downstream and therefore warrants careful consideration in flood risk management.

Landslide dam failure and flood hydraulics. Part I: experimental investigation

Landslide dam failure can trigger catastrophic flooding in the downstream. However, field observation of such flooding is rarely available, while laboratory experimental studies are sparse. The mechanism of landslide dam failure and the flood has so far remained insufficiently understood. Here, we present an experimental investigation of landslide dam failure and the flood. A total of 28 runs of experiments are carried out in a flume of 80 m × 1.2 m × 0.8 m, with differing inflow discharge, dam composition, dam geometry, and initial breach dimension. An array of twelve automatic water-level probes is deployed to measure the stage hydrographs along the flume, and the video recording of the dam failure processes facilitates an estimation of the widening of initial breach. Under the present experimental conditions with dams composed of homogeneous materials, landslide dam failure is primarily caused by erosion of overtopping flow, and lateral mass collapse is also considerable during the cause of breach widening. Cohesive clay may act to mitigate the seepage through the dam and thus its subsidence and appreciably modulate the dam failure process and the flood. However, the impacts of clay may be readily overwhelmed by a large inflow discharge and initial breach. Gravels in the dam may appreciably depress the rate of the dam failure process and thus modify the flood. The present work provides new experimental data set for testing mathematical models of the flood flow due to landslide dam failure.

Landslide dam failure and flood hydraulics. Part II: coupled mathematical modelling

A coupled 2D mathematical modelling study of landslide dam failure and flood is presented, complementing our experimental investigation presented in the companion paper. The model is built upon the shallow water hydrodynamic equations. The governing equations are numerically solved using the total-variation-diminishing version of the second-order weighted-average-flux method along with the HLLC (Harten, Lax and van Leer with Contact wave restored) approximate Riemann solver. Two parameters related to bed-load sediment transport and critical slope stability are calibrated using the measured stage hydrographs from two runs of the flume experiments. The calibrated model is then applied to other independent runs of the experiments featuring different inflow discharges, dam geometry, dam composition and initial breach dimensions. It is found to be able to satisfactorily reproduce the measured stage hydrographs and the widening of initial breach. The experimental observation of the prime role of the inflow discharge and initial breach in dictating the dam failure process and flood is unequivocally resolved, along with the impacts of dam geometry as well the content of cohesive clay and gravel in the dam. Interestingly, the downstream peak discharge and stage of the flood are substantially reduced by initial breach, which clearly exemplifies its role in modulating the flooding.