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The Yangtze River is one of the most important rivers in China due to its large basin size, the large population along the river, and the numerous large dams and reservoirs on the river. The Jinsha River, the upper reach of the Yangtze River, was dammed twice recently at Baige, Tibet, one on 10 October 2018 and the other on 3 November 2018 (UTC + 8). Accordingly, two large landslide dams, 61 m and 96 m in height to the lowest dam crest, were formed in a 3-week interval. Due to the large inflow rates at the time of damming, the barrier lake level rose rapidly, posing huge risks to the downstream residents and properties. In managing the landslide dam risk, one of the important tasks is to predict the dam breaching flood beforehand. This paper focuses on rapid prediction of the dam breaching hydrograph and breach geometric parameters of the two landslide dams. The predictions were made timely before the breaching of the two landslide dams using both erosion-based empirical equations and numerical simulation and were refined based on detailed field investigation at the site after breaching. Comprehensive field investigations were conducted to determine the geological structures of the landslide dams, characterize the erodibility of dam materials, and measure the final beach dimensions. The simulated dam breaching processes, outflow hydrographs, lake water level changes, and final breach dimensions were validated by field observations. Compared with the hypothetical scenario without a diversion channel on the second landslide dam, a diversion channel 15 m in depth successfully lowered the peak flood discharge by about one third and helped to mitigate the flood risk significantly. The analysis outcome serves as basis for warning and evacuation of the downstream residents and making appropriate engineering risk mitigation plans.

Monitoring small rivers during extreme events is challenging, especially in remote areas. This study assesses the accuracy of the Surface Water and Ocean Topography (SWOT) mission in characterizing an extreme hydrological event on a narrow river (∼60 m wide), using a landslide‐induced flood on the Chilcotin River (Canada) as a case study. SWOT data, validated against optical imagery and elevation data sets, were used to estimate water surface elevation and slope changes. SWOT was able to detect lake impoundment, dam breach, and downstream slope adjustments induced by the landslide. Results suggest the utility of SWOT's spaceborne interferometric data to quantify spatial and temporal impacts of extreme events on river dynamics, even for small and narrow rivers.

Landslide dams are extremely dangerous because dammed rivers can inundate upstream areas with rising water levels and flood downstream areas after dam breaching. The longevity of landslide dams, which is uncertain, is of great significance for dam failure prevention and mitigation since it determines the time available to take mitigation measures. In this study, the full longevity of landslide dams is divided into three stages (infilling, overflowing and breaching) for better estimation. The influences of dam characteristic parameters (triggers, dam materials and geometric/hydrological parameters) on the full longevity of landslide dams (the period from landslide dam formation to the end of dam failure) as well as on each of the three stages are analysed based on the database. Based on eight dimensionless variables, regression models for estimating the full longevity of landslide dams are developed with a R2 value of 0.781, and regression models for the three-stage longevity (the longevity as the sum of the periods of the three stages) by considering infilling, overflowing and breaching are established with a R2 value of 0.938. It is found that the landslide dam longevity cannot be predicted by one or two influencing factors since it is affected by multiple factors. The relative importance of each control variable is evaluated based on sensitivity analysis: the trigger is the most significant variable in the breaching stage since it affects the size of dam particles, the water content and the inflow rate (e.g. the rainfall trigger results in a larger inflow rate); the lake volume coefficient is more significant in the overflowing stage because it indicates the potential volume of water eroding the dam; and the average annual discharge coefficient is the most important factor in the infilling stage because it controls the time to impound water. The longevity predicted by different models are compared. The models developed in this paper show better accuracy due to the consideration of more parameters based on more cases. In particular, the three-stage longevity regression model shows better accuracy than that of other models because it considers the particular influencing factors for each stage. Three case studies (the “10·10” Baige, Hsiaolin and Tangjiashan landslide dams) are presented to show the application of the regression models developed in this paper. The dam longevity can be predicted more precisely if the timely inflow rate can be estimated by site monitoring or multi-temporal remote sensing images and pre-event digital elevation model (DEM).

Successive major landslides during October and November 2018 in Baige village, eastern Tibet, dammed the Jinsha River on two occasions, and the subsequent dam breaches instigated a multi-hazard chain that flooded many towns downstream. Analysis of high-resolution aerial images and field investigations unveiled three potentially unstable rock mass clusters in the source area of the landslides, suggesting possible future failures with potential for river-damming and flooding. In order to evaluate and understand the disaster chain effect linked to the potentially unstable rock mass, we systematically studied the multi-hazard scenarios through an integrated numerical modelling approach. Our model begins with an evaluation of the probability of landslide failure, including runout and river damming, and then addresses the dam breach and resultant flood—hence simulating and visualising an entire disaster chain. The model parameters were calibrated using empirical data from the two Baige landslides. Then, we predict the future cascading hazards via seven scenarios according to all possible combinations of potential rock mass failure. For each scenario, the landslide runouts, dam-breaching, and flooding are numerically simulated with full consideration of uncertainties among the model input parameters. The maximum dam breach flood extent, depth, velocity, and peak arrival time are predicted at sequential sites downstream. As a first attempt to simulate the full spectrum of a landslide-induced multi-hazard chain, our study provides insights and substantiates the value provided by multi-hazard modelling. The integrated approach described here can be applied to similar landslide-induced chains of hazards in other regions.

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.

The present work addresses the influence of internal drainage systems on failure by overtopping of earth dams. It is based on data collected in six homogeneous dam breach laboratory experiments. Different drainage systems were tested: (a) toe drain only and (b) complete internal drainage system—chimney filter, horizontal blanket and toe drain. The tests were performed with the same embankment body material, reservoir dimensions and inflow conditions but different relative compactions. We analyze the breach hydrograph and the stages of dam breaching. The failure of dams with complete and fully functional drainage systems exhibits marked differences, the most conspicuous of all being the longer duration of the initial erosion stage. This is accompanied by a lower increase rate of its breach discharge. We argue that this is the combined result of the upstream migration of the locus of hydraulic control and of a higher resistance to erosion of the downstream shell, kept unsaturated by the drainage system. Hydraulic control moves upstream, to a less morphologically active cross‐section, as a result of channel deepening once the chimney drain is uncovered and eroded. Once the erosion merges with the cavity where the chimney filter was, marking the end of the initial erosion stage, the rate of erosion increases and becomes comparable to that of dams with toe drain only. Our argument about the longer duration of the initial stage is not applicable to latter stages—the total time before full collapse is not necessarily increased.

Risk assessment of cascade reservoir dams is not only the key to ensure the safety of the basin, but also the objective requirement of dam risk management. Based on the development status of cascade reservoirs in China, the complexity of dam risk management of cascade reservoirs compared with a single reservoir was analyzed. By reviewing the advances on the studies of dam risk in cascade reservoirs, this paper summarized their limitations in terms of scientificity and practicability. Moreover, some concepts and methods were proposed on the risk assessment of cascade reservoirs: (1) The dam risk of a cascade reservoir was decomposed into own risk and additional risk, the consequence of its dam breach was decomposed into direct loss and potential loss, and an influence coefficient was defined to reflect the risk transmission and superposition degree among cascade reservoirs; (2) The related concepts and formulas for the calculation of dam risk probability and consequence of cascade reservoirs were proposed, which realized the transition of dam risk assessment method from a single reservoir to cascade reservoirs; (3) A project rank classification method for cascade reservoirs was proposed, which took into account not only the project scale and benefits in socioeconomic development, but also the successive dam breaches possibility and consequences. This study is of great significance to clarify the focus of future research and promote the practical application of dam risk management in cascade reservoirs.

One of the most perilous natural hazards is flooding resulting from dam failure, which can devastate downstream infrastructure and lead to significant human casualties. In recent years, the frequency of flash floods in the northern part of Nicosia, Cyprus, has increased. This area faces increased risk as it lies downstream of the Kanlikoy Dam, an aging earth-fill dam constructed over 70 years ago. In this study, we aim to assess potential flood hazards stemming from three distinct failure scenarios: piping, 100-year rainfall, and probable maximum precipitation (PMP). To achieve this, HEC-HMS hydrologic model findings were integrated into 2D HEC-RAS hydraulic models to simulate flood hydrographs and generate flood inundation and hazard maps. For each scenario, Monte Carlo simulations using McBreach software produced four hydrographs corresponding to exceedance probabilities of 90%, 50%, 10%, and 1%. The results indicate that all dam breach scenarios pose a significant threat to agricultural and residential areas, leading to the destruction of numerous buildings, roads, and infrastructures. Particularly, Scenario 3, which includes PMP, was identified as the most destructive, resulting in prevailing flood hazard levels of H5 and H6 in the inundated areas. The proportion of inundated areas in these high hazard levels varied between 52.8% and 57.4%, with the number of vulnerable structures increasing from 248 to 321 for exceedance probabilities of 90% and 1%, respectively. Additionally, the number of flooded buildings ranged from 842 to 935, and 26 to 34 km of roads were found to be inundated in this scenario. These findings revealed the need for authorities to develop comprehensive evacuation plans and establish an efficient warning system to mitigate the flood risks associated with dam failure.

The sudden and unpredictable breach of landslide dams in the Yarlung Tsangpo river basin usually causes megafloods, posing great risks to human lives and infrastructures in the downstream areas. This study proposed an efficient and quantitative risk assessment framework of breach floods caused by landslide dam failures in the mainstream and tributaries of the Yarlung Tsangpo river basin with limited data. The impact of dam breach floods on human risks was evaluated. The flood attenuation along rivers, strategies for mitigating overlapping floods, and sensitivity analysis of human risks were also discussed. The results show that the developed framework successfully assessed flood risks caused by the breach of landslide dams. The flood attenuation ratio increased with river length but decreased with the peak discharge at dam site. A higher peak discharge and a larger inundated area downstream were predicted when the breach floods of two landslide dams, one in mainstream and the other in a tributary, overlapped at the confluence. The overlapping flood could be mitigated by reducing peak discharges of the two landslide dams or increasing time interval between the two peaks. The simulations also outlined the downstream peak discharge resulting from the cascading breach was larger than that of a single dam. However, it was smaller than the combined peak discharges of two separate dams, because the erosion during the breach of the downstream dam incurred energy dissipation. The human risks in the Pasighat village were greater when overlapping flood occurred due to the increased water depth and more hazardous inundated buildings. In the case of multi-peak floods, the warnings for the former peak flood would also warn the peak flood thereafter when individuals were notified multiple peaks. Otherwise, individuals might be misled by the warning of the previous peak flood, resulting in catastrophic flood impacts. A parametric analysis indicated that early evacuation warnings were needed to avoid serious loss of life and flood damages, especially in cases of dam breaches occurring at nighttime or for areas in close proximity to the dam site.

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.