Numerical investigation of hydro-morphodynamic characteristics of a cascading failure of landslide dams

A cascading failure of landslide dams caused by strong earthquakes or torrential rains in mountainous river valleys can pose great threats to people’s lives,properties,and infrastructures.In this study,based on the three-dimensional Reynoldsaveraged Navier-Stokes equations(RANS),the renormalization group(RNG)k-εturbulence model,suspended and bed load transport equations,and the instability discriminant formula of dam breach side slope,and the explicit finite volume method(FVM),a detailed numerical simulation model for calculating the hydro-morphodynamic characteristics of cascading dam breach process has been developed.The developed numerical model can simulate the breach hydrograph and the dam breach morphology evolution during the cascading failure process of landslide dams.A model test of the breaches of two cascading landslide dams has been used as the validation case.The comparison of the calculated and measured results indicates that the breach hydrograph and the breach morphology evolution process of the upstream and downstream dams are generally consistent with each other,and the relative errors of the key breaching parameters,i.e.,the peak breach flow and the time to peak of each dam,are less than±5%.Further,the comparison of the breach hydrographs of the upstream and downstream dams shows that there is an amplification effect of the breach flood on the cascading landslide dam failures.Three key parameters,i.e.,the distance between the upstream and the downstream dams,the river channel slope,and the downstream dam height,have been used to study the flood amplification effect.The parameter sensitivity analyses show that the peak breach flow at the downstream dam decreases with increasing distance between the upstream and the downstream dams,and the downstream dam height.Further,the peak breach flow at the downstream dam first increases and then decreases with steepening of the river channel slope.When the flood caused by the upstream dam failure flows to the downstream dam,it can produce a surge wave that overtops and erodes the dam crest,resulting in a lowering of the dam crest elevation.This has an impact on the failure occurrence time and the peak breach flow of the downstream dam.The influence of the surge wave on the downstream dam failure process is related to the volume of water that overtops the dam crest and the erosion characteristics of dam material.Moreover,the cascading failure case of the Xiaogangjian and Lower Xiaogangjian landslide dams has also been used as the representative case for validating the model.In comparisons of the calculated and measured breach hydrographs and final breach morphologies,the relative errors of the key dam breaching parameters are all within±10%,which verify the rationality of the model is applicable to real-world cases.Overall,the numerical model developed in this study can provide important technical support for the risk assessment and emergency treatment of failures of cascading landslide dams.

Rapid prediction models for 3D geometry of landslide dam considering the damming process

The geometry of a landslide dam plays a critical role in its stability and failure mode,and is influenced by the damming process.However,there is a lack of understanding of the factors that affect the 3D geometry of a landslide dam.To address this gap,we conducted a study using the smoothed particle hydrodynamics numerical method to investigate the evolution of landslide dams.Our study included 17 numerical simulations to examine the effects of several factors on the geometry of landslide dams,including valley inclination,sliding angle,landslide velocity,and landslide mass repose angle.Based on this,three rapid prediction models were established for calculating the maximum height,the minimum height,and the maximum width of a landslide dam.The results show that the downstream width of a landslide dam remarkably increases with the valley inclination.The position of the maximum dam height along the valley direction is independent of external factors and is always located in the middle of the landslide width area.In contrast,that position of the maximum dam height across the valley direction is significantly influenced by the sliding angle and landslide velocity.To validate our models,we applied them to three typical landslide dams and found that the calculated values of the landslide dam geometry were in good agreement with the actual values.The findings of the current study provide a better understanding of the evolution and geometry of landslide dams,giving crucial guidance for the prediction and early warning of landslide dam disasters.

Controls on the regional distribution of landslide dams and implications for fluvial landform evolution in Bhutan and its surrounding area

The regional distribution of landslide dams can provide valuable insights into the interactions among various factors,including lithology,topography,climate,and fluvial landforms in tectonically active mountains.Himalayan rivers are frequently impacted by large-scale landslide damming,which profoundly influence fluvial geomorphology.In this study,we identified 1652 landslide dams in four major rivers of Bhutan and its surrounding area by remote sensing interpretation.Notably,approximately 71%of these landslide dams are found in regions composed of quartzite or gneiss.Fault-related tectonic activity plays a significant role in governing the distribution of these landslide dams,as approximately 83%of the mapped landslide dams are found within a 10 km radius of the nearest fault.The majority of the identified landslide dams are situated in areas with relatively modest local relief,ranging from 227 m to 327 m.These dams tend to cluster in the tributaries,and the stream power of almost 95%of them is typically below 1×10^(6) kg m^(2) s^(-3).Our data,combining the erosion rate and kernel density map of the landslide dams,reveals that regions with high erosion rates do not consistently align with the major high-density distribution of landslide dams.It is shown that the distribution of landslide dams is strongly influenced by the valley form.In comparison to U-shaped valleys,V-shaped valleys exhibit a higher density of landslide dams.Intriguingly,we also find a positive correlation between the landslide-dam distribution density and the erosion rate only in relatively arid regions with mean annual rainfall less than 500 mm.Moreover,the length of the upstream reach protected by the knickpoint associated with both lithology and landslide damming is about three times longer than that protected by the knickpoint associated only with landslide damming.

Laboratory-scale investigation of the material distribution characteristics of landslide dams in U-shaped valleys

Material distribution characteristics during sliding and depositing is particularly significative to investigate the internal structure and spatial variation of landslide dams,which are fundamentally determining the mechanical and hydraulic behavior and the susceptibility to cause dam failure.However,limited by longevity shortages and special geographic environments,the material distribution characteristics and their formation mechanisms are difficult to observe in the field.Therefore,an experimental apparatus modeling a landslide dam was developed in this paper,designing three sampling methods with two valley states.The internal deposit characteristics,void ratio variation and relative content of the particle size range(PSR)were analyzed,and the mechanics of deposit structure were also delicately ascertained.The results indicate that granular material deposited in valley shows a structure of inverse grain size accumulation in both vertical and horizontal directions,exhibiting spatial variability of particle gradation and void ratio.The characteristic PSR decreases from 22-30 mm in the two-dimensional state to 10-14 mm in the threedimensional state.Vibration excitation and vibration sieve are the intrinsic mechanisms of granular flow segregation,intrinsically inducing the formation of inverse grading deposit structures.Consequently,spatial variability in size is mainly trig gered by segregation,whereas coarse particle content and deposition boundaries merely exacerbate the difference degree.

Insights into the long-term stability of landslide dams on the eastern margin of the Tibetan Plateau, China——A case study of the Diexi area

Landslide dams,especially long-term stable landslide dams,have been recognized as important contributors to regional geomorphological evolution.Here,the Diexi area,a long-term stable dam-prone area located in upstream of the Minjiang River on the eastern Tibetan Plateau,was adopted to reveal reasons that landslide dams are concentrated in this area and maintain long-term stability via detailed field investigations,landslide dam sampling,unmanned aerial vehicle(UAV)images,and digital surface models(DSM).The results show the controlling factors that the slopes are prone to sliding and rock mass structure deterioration including lithological combination mode,slope structure,topographic conditions,a series of NNE-trending radial fissures and hydrological conditions.Fault activities,which have caused many earthquakes,are the main inducing factor.Landslide dams are prone to occurrence in the Diexi area owing to the combined effect of the narrow channels,the large landslide dam volume and the rock fragments.The river flow,and the landslide dam volume,material,structure,and parameters control the stability of landslide dams.The landslide dam consists of various sizes of boulders and all landslide dams exhibit an obvious inverse grading sequence,and this size combination could consume most of the flow energy,and consequently protect the dam from incision.Additionally,a total of seven knickpoints were formed by landslide dams,and the longitudinal gradient upstream of every landslide dam was found to decrease by the action of knickpoint.In the eastern margin of the Tibetan Plateau,there are numerous landslide dams existed for hundreds or thousands of years.Studies on the long-term stable landslide dams in the Diexi area could provide experience for studying similar kinds of landslide dams in this region.

Sliding and damming properties of granular debris with different geometric configurations and grain size distributions

Granular debris plays a significant role in determining damming deposit characteristics. An indepth understanding of how variations in grain size distribution(GSD) and geometric configurations impact the behavior of granular debris during the occurrence of granular debris is essential for precise assessment and effective mitigation of landslide hazards in mountainous terrains. This research aims to investigate the impact of GSD and geometric configurations on sliding and damming properties through laboratory experiments. The geometric configurations were categorized into three categories based on the spatial distribution of maximum volume: located at the front(Type Ⅰ), middle(Type Ⅱ), and rear(Type Ⅲ) of the granular debris. Our experimental findings highlight that the sliding and damming processes primarily depend on the interaction among the geometric configuration, grain size, and GSD in granular debris. Different sliding and damming mechanisms across various geometric configurations induce variability in motion parameters and deposition patterns. For Type Ⅰ configurations, the front debris functions as the critical and primary driving component, with energy dissipation primarily occurring through inter-grain interactions. In contrast, Type Ⅱ configurations feature the middle debris as the dominant driving component, experiencing hindrance from the front debris and propulsion from the rear, leading to complex alterations in sliding motion. Here, energy dissipation arises from a combination of inter-grain and grain-substrate interactions. Lastly, in Type Ⅲ configurations, both the middle and rear debris serve as the main driving components, with the rear sliding debris impeded by the front. In this case, energy dissipation predominantly results from grainsubstrate interaction. Moreover, we have quantitatively demonstrated that the inverse grading in damming deposits, where coarse grain moves upward and fine grain moves downward, is primarily caused by grain sorting due to collisions among the grains and between the grain and the base. The impact of grain on the horizontal channel further aids grain sorting and contributes to inverse grading. The proposed classification of three geometric configurations in our study enhances the understanding of damming properties from the view of mechanism, which provides valuable insights for related study about damming granular debris.

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.

Experimental study of breach process of landslide dams by overtopping and its initiation mechanisms

The present paper studies the physics of the breach erosion process, particularly, the breach initiation process in over- topped landslide dams. Due to great complexities involved, only homogeneous landslide dams are considered. The flume experime- nts of dam overtopping are conducted to study the breach growth process. And in order to reveal the effects of the seepage during the breach development, the permeability characteristics of the dam materials are also taken into consideration. With the experimental observation, the details of the breach growth are examined, and the whole breach process could be distinguished into five stages, i.e., Stage I, the seepage erosion, Stage II, the formation of the initial breach, Stage III, the erosion toward the head, Stage IV, the expan- sion and incision of the breach, and Stage V, the re-equilibration of the river channel through the breach. It is shown that once trigge- red the entire breach process goes continually without stop, which highlights the significant impact of the early stages on the later deformation of the dam. Evidence shows that the initial breach of the dam is most likely to take place in the downstream slope of the dam, near the upper edge of the seepage face. The experimental results show a ＂headcut＂ mechanism of the breach initiation.

EXPERIMENTAL STUDY OF LANDSLIDE DAM-BREAK FLOOD OVER ERODIBLE BED IN OPEN CHANNELS

Large-scale landslide dams may block the river flow and cause inundation upstream, and subsequently fail and result in severe flooding and damage in the downstream. The need for enhanced understanding of the inundation and flooding is evident. This article presents an experimental study of the inundation and landslide dam-break flooding over erodible bed in open channels. A set of automatic water-level probes is deployed to record the highly transient stage, and the post-flooding channel bed elevation is measured. New experimental data resources are provided for understanding the processes of landslide-induced flooding and for testing mathematical rivers models.

Sequential Damming Induced Winter Season Flash Flood in Uttarakhand Province of India

204 persons were killed while two hydropower projects located in close proximity at Rishiganga(13.2 MW),and Tapoban(520 MW)were damaged in Dhauliganga flood of February 7,2021 in the Indian Himalaya.This incidence occurred during the winter season when the discharge of the glacier fed rivers is minimal,and no rain was experienced in the region around the time of the flood.Despite discharge of the main river,Rishiganga,not involved in the flood due to damming upstream of its confluence with Raunthi Gadhera,based on field evidences massive volume of around 6 million cu m water involved in this flood is attributed to sequential intermittent damming at three different places;(i)Raunthi Gadhera was dammed first in its upper reaches,(ii)Rishiganga river was then dammed to the north of Murunna,and(iii)finally Dhauliganga river was dammed around Rini village to the upstream of its confluence with Rishiganga river.Lacking warning system only enhanced the flood-induced devastation.Legally binding disaster risk assessment regime,together with robust warning generation,and dissemination infrastructure are therefore recommended for all major infrastructure projects.