A preliminary study of the failure mechanisms of cascading landslide dams

Landslide dams commonly form when mass earth or rock movements reach a river channel and cause a complete or partial blockage of the channel.Intense rainfalls can induce upstream flows along a sloping channel that significantly affect downstream landslide dams.If a series of landslide dams are collapsed by incoming mountain torrents(induced by intense rainfall),large debris flows can form in a very short period.Furthermore,the failure of these dams can amplify the magnitude and scale of debris flows in the flow direction.The catastrophic debris flows that occurred in Zhouqu County,China on 8 August 2010 were caused by intense rainfall and the upstream cascading failure of landslide dams along the gullies.Incorporating the role of outburst floods associated with the complete or partial failure of landslide dams is an interesting problem usually beyond the scope of analysis because of the inherent modeling complexity.To understand the cascading failure modes of a series of landslide dams,and the dynamic effect these failures have on the enlargement of debris flow scales,experimental tests are conducted in sloping channels mimicking field conditions,with the modeled landslide dams distributed along a sloping channel and crushed by different upstream flows.The failure modes of three different cascades of landslide dams fully or partially blocking a channel river are parametrically studied.This study illustrates that upstream flows can induce a cascading failure of the landslide dams along a channel.Overtopping is the primary failure mechanism,while piping and erosion can also induce failures for different constructed landslide dams.A cascading failure of landslide dams causes a gradually increasing flow velocity and discharge of the front flow,resulting in an increase in both diameter and percentage of the entrained coarse particles.Furthermore,large landslide blockages can act to enhance the efficiency of river incision,or conversely to induce aggradation of fluvial sediments,depending on the blockage factor of the landslide dams and upstream discharge.     

A non-traditional approach to the analysis of flood hazard for dams

The traditional and still prevailing approach to characterization of flood hazards to dams is the inflow design flood (IDF). The IDF, defined either deterministically or probabilistically, is necessary for sizing a dam, its discharge facilities and reservoir storage. However, within the dam safety risk informed decision framework, the IDF does not carry much relevance, no matter how accurately it is characterized. In many cases, the probability of the reservoir inflow tells us little about the probability of dam overtopping. Typically, the reservoir inflow and its associated probability of occurrence is modified by the interplay of a number of factors (reservoir storage, reservoir operating rules and various operational faults and natural disturbances) on its way to becoming the reservoir outflow and corresponding peak level—the two parameters that represent hydrologic hazard acting upon the dam. To properly manage flood risk, it is essential to change approach to flood hazard analysis for dam safety from the currently prevailing focus on reservoir inflows and instead focus on reservoir outflows and corresponding reservoir levels. To demonstrate these points, this paper presents stochastic simulation of floods on a cascade system of three dams and shows progression from exceedance probabilities of reservoir inflow to exceedance probabilities of peak reservoir level depending on initial reservoir level, storage availability, reservoir operating rules and availability of discharge facilities on demand. The results show that the dam overtopping is more likely to be caused by a combination of a smaller flood and a system component failure than by an extreme flood on its own.     

美国密歇根州洪水溃坝灾害机制分析及对我国灾害风险防范的启示

大坝作为水利水电工程设施发挥效用的同时,其安全性对上下游及周边区域生产生活至关重要。除去恐怖袭击等人为破坏因素,大坝面临气象灾害、地震地质灾害等各类灾害风险。 “七下八上”历来是我国南北方主汛期,防御强降雨及其引发的滑坡、泥石流、堰塞湖等次生灾害,对大坝构成的单一、复合、链式灾害风险,是当前刻不容缓的任务。美国洪水大坝应急管理体系建立较早,此次密歇根州洪水溃坝事件发生在美国疫情严峻时期,暴露出诸多问题值得整理与研究,并对当前疫情常态化下我国大坝抵御综合性灾害风险具有良好的启示意义。      

Risk analysis for clustered check dams due to heavy rainfall

Check dams are commonly constructed around the world for alleviating soil erosion and preventing sedimentation of downstream rivers and reservoirs.Check dams are more vulnerable to failure due to their less stringent flood control standards compared to other dams.Determining the critical precipitation that will result in overtopping of a dam is a useful approach to assessing the risk of failure on a probabilistic basis and for providing early warning in case of an emergency.However,many check dams are built in groups,spreading in several tributaries in cascade forms,comprising a complex network.Determining the critical precipitation for dam overtopping requires a knowledge of its upstream dams on whether they survived or were overtopped during the same storm,while these upstream dams in turn need the information for their upstream dams.The current paper presents an approach of decomposing the dam cluster into(1)the heading dam,(2)border dams,and(3)intermediate dams.The algorithm begins with the border dams that have no upstream dams and proceeds with upgraded maps without the previous border dams until all the dams have been checked.It is believed that this approach is applicable for small-scale check dam systems where the time lag of flood routing can be neglected.As a pilot study,the current paper presents the analytical results for the Wangmaogou Check Dam System that has 22 dams connected in series and parallel.The algorithm clearly identified 7 surviving dams,with the remaining ones being overtopped for a storm of 179.6 mm in 12 h,which is associated with a return period of one in 200 years.     

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.     

Prediction of the behaviour of landslide dams using a geomorphological dimensionless index

Landslide dams are a common phenomenon. They form when a landslide reaches the bottom of a river valley causing a blockage. The first effect of such a dam is the infilling of a lake that inundates the areas upstream, while the possibility of a sudden dam collapse, with a rapid release of the impounded waters, poses a higher flood risk to the downstream areas. The results of the main inventories carried out to date on landslide dams, have been examined to determine criteria for forecasting landslide dam evolution with particular emphasis on the assessment of dam stability. Not all landslides result in the blockage of a river channel. This only occurs with ones that can move a large amount of material with moderate or high‐velocities. In most cases, these landslides are triggered by rainfall events or high magnitude earthquakes. A relationship also exists between the volume of the displaced material and the landslide dam stability. Several authors have proposed that landslide dam behaviour can be forecast by defining various geomorphological indexes, that result from the combination of variables identifying both the dam and the dammed river channel. Further developments of this geomorphological approach are presented in this paper by the definition of a dimensionless blockage index. Starting with an analysis of 84 episodes selected worldwide, it proved to be a useful tool for making accurate predictions concerning the fate of a landslide dam. Copyright © 2002 John Wiley & Sons, Ltd.     

Quantifying differences between reservoir inflows and dam site floods using frequency and risk analysis methods

Reservoirs are the most important constructions for water resources management and flood control. Great concern has been paid to the effects of reservoir on downstream area and the differences between inflows and dam site floods due to the changes of upstream flow generation and concentration conditions after reservoir’s impoundment. These differences result in inconsistency between inflow quantiles and the reservoir design criteria derived by dam site flood series, which can be a potential risk and must be quantificationally evaluated. In this study, flood frequency analysis (FFA) and flood control risk analysis (FCRA) methods are used with the long reservoir inflow series derived from a multiple inputs and single output model and a copula-based inflow estimation model. The results of FFA and FCRA are compared and the influences on reservoir flood management are also discussed. The Three Gorges Reservoir (TGR) in China is selected as a case study. Results show that the differences between the TGR inflow and dam site floods are significant which result in changes on its flood control risk rates. The mean values of TGR’s annual maximum inflow peak discharge and 3 days flood volume have increased 5.58 and 3.85% than the dam site ones, while declined by 1.82 and 1.72% for the annual maximum 7 and 15 days flood volumes. The flood control risk rates of middle and small flood events are increased while extreme flood events are declined. It is shown that the TGR can satisfy the flood control task under current hydrologic regime and the results can offer references for better management of the TGR.     

Investigation of the elevation of saltwater wedge due to subsurface dams

Subsurface dams are rather effective and used for the prevention of saltwater intrusion in coastal regions around the world. We carried out the laboratory experiments to investigate the elevation of saltwater wedge after the construction of subsurface dams. The elevation of saltwater wedge refers to the upward movement of the downstream saltwater wedge because the subsurface dams obstruct the regional groundwater flow and reduce the freshwater discharge. Consequently, the saltwater wedge cannot further extend in the longitudinal direction but rises in the vertical profile resulting in significant downstream aquifer salinization. In order to quantitatively address this issue, field-scale numerical simulations were conducted to explore the influence of various dam heights, distances, and hydraulic gradients on the elevation of saltwater wedge. Our investigation shows that the upward movement of the saltwater wedge and its areal extension in the vertical domain of the downstream aquifer become more severe with a higher dam and performed a great dependence on the freshwater discharge. Furthermore, the increase of the hydraulic gradient and the dam distance from the sea boundary leads to a more pronounced wedge elevation. This phenomenon comes from the variation of the freshwater discharge due to the modification of dam height, location, and hydraulic gradient. Large freshwater discharge can generate greater repulsive force to restrain the elevation of saltwater wedge. These conclusions provide theoretical references for the behaviour of the freshwater–seawater interface after the construction of subsurface dams and help optimize the design strategy to better utilize the coastal groundwater resources.     

梯级筑坝对黑河水质时空分布特征的影响

为探究梯级大坝建设对河流水质变化规律的影响,将黑河上中游划分为坝上河段、坝下河段及自然河段,于2017年12月-2018年8月选取了24个主要控制断面进行水质调查,并采用多元统计的方法对比分析了不同时空尺度上的水质分布特征.结果表明:黑河上中游水质时空变化的主要影响因子为水温(WT)、pH值、溶解氧(DO)、电导率(EC)、总氮(TN)、总磷(TP)和五日生化需氧量(BOD_5).空间尺度上,WT、EC、BOD_5、高锰酸盐指数(CODMn)、TN等指标具有显著性差异,其中坝上河段受BOD_5、CODMn影响较大,自然河段WT、EC和TN为关键指标,而各个因子对坝下河段水质影响较小.时间尺度上,WT、EC、BOD_5、氨氮与季节变化存在明显相关性,是不同河段水质时间变化的控制因子,且大多数水质因子在非汛期变化最明显.降水、温度、水文条件等季节性影响因素和梯级水库联合运用模式是该区域水质时间差异的主要原因;空间差异主要受祁连、张掖地区外源性污染物排放以及筑坝环境下水动力条件改变而产生的沉积滞留效应和沿程累积效应影响.研究表明,外源性污染源依然是导致水质变差的主要因素,梯级筑坝则是导致水质变差的间接因素.因此控制该区域人类活动所造成的外源性污染源,并针对不同种类污染物的季节变化特征实施合理的水库运行方式是改善水电梯级开发河段水质状况的关键.     

Failures of Waterworks Facilities and Their Consequences

The causes of the failure of various dams, the existing approaches to determining the parameters of flood waves caused by dam failures, and the authors' method of calculation of these waves are considered. Characterization of the consequences of the dam-failure flood wave in the tailwater pool of the Volgograd hydroelectric station and some unfavorable aspects of possible evacuation of the Saratov reservoir are presented.     

Safety assessment for dams of the cascade reservoirs system of Lancang River in extreme situations

Numerous dams have been constructed in the midstream and downstream regions of Lancang River, which form a complex cascade reservoirs system. The safety of dams is critical for water resource management of the whole system. To check the safety of dams, this study used the MIKE 11 model to simulate flood routing along the Lancang River from Xiaowan dam to Jinghong dam under extreme situations of 100-, 500-, 1000-, 5000-, and 10,000-year design floods throughout the whole cascade reservoirs system. The design flood events used as the input for the MIKE 11 model contains the design flood hydrograph of the upstream reservoirs and corresponding flood hydrographs of the intermediate areas. The design flood hydrograph of the upstream reservoirs was obtained using the Equal Frequency Factor Method, and the corresponding flood hydrograph of the intermediate areas was obtained using the Equivalent Frequency Regional Composition Method. The results show that all dams are safe for the 100-, 500-, 1000-, and 5000-year design flood situations throughout the whole cascade reservoirs system, whereas the Manwan and Jinghong dams have a risk of overtopping under a 10,000-year design flood. The curves showing the relationship between the highest water level and return period for the dams are also presented.     

基于有限元法的土石坝除险加固渗流问题分析

土石坝坝体在坝前水位作用下极易产生渗流,为研究坝体加固对渗透水压的影响,针对具体水库实例,采用有限元法对除险加固前/后的坝体进行渗透坡降、单宽渗流量、准流网等计算,分析3种工况下的渗流过程,为土石坝的除险加固设计提供参考。结果表明:加固后上游坡各工况下的安全系数明显提高,其中单宽渗流量最大,可达0.636m~3/d,远大于规范允许值。     

Consequences of implementing a reservoir operation algorithm in a global hydrological model under multiple meteorological forcing

We investigated dam behaviours during high-flow events and their robustness against perturbations in meteorological conditions using the H08 global hydrological model. Differences in these behaviours were examined by comparing simulation runs, with and without dams and using multiple meteorological datasets, at a case-study site, Fort Peck Dam on the Missouri River, USA. The results demonstrated that dam-regulated river flow reduced temporal variability over large time periods and also dampened inter-forcing discrepancies in river discharge (smoothing effects). However, during wet years, differences in peak flow were accentuated downstream of the dam, resulting in divergence in simulated peak flow across the meteorological forcing (pulsing effect). The pulsing effect was detected at other major dams in global simulations. Depending upon the meteorological forcing, the dams act as a selective filter against high-flow events. Synergy between a generic dam scheme and differences in meteorological forcing data might introduce additional uncertainties in global hydrological simulations.     

Hydraulic model tests for propagation of flow and sediment in floods due to breaking of a natural landslide dam during a mountainous torrent

During mountain torrents, large-magnitude floods may result from heavy rainfall and cause the breakage of landslide dams naturally formed by heavy rainfall, earthquakes, and so on. The characteristics of longitudinal spreading of clear water discharge and changes in flow depth must be clarified because the changes in peak depth have not yet been examined in steep-slope torrents and because there are few data on spreading of flash floods and related sedimentation in mountainous torrents. In the present study, experimental data were collected through hydraulic model tests over a rigid bed, and the spreading of water, fine sediment, bed load, and large boulders due to flooding are discussed assuming that flash flooding/debris flows occur in the upstream reach. The effects of changes in flow width, such as expansions and contractions in the flow width, as well as changes in meandering channels, sediment transportation, and spreading flow depth resulting from bores are examined using flume data for a steep-slope torrent. The data obtained in the present study reveal that fine sediment components are transported to the downstream reach if large-magnitude floods occur and that the spreading rate and peak lags of the fine sediment and water level indicate the occurrence of a flood in the upstream reach.     

Seepage driving effect on deformations of San Fernando dams

In the process of flow deformation of an earth dam, the seepage force inside the dam plays a role as a driving force. The seepage force acts just like the gravitational force in terms of pushing soils away from their original locations after liquefaction is triggered. This paper draws attention to this seepage driving effect by presenting a set of fully coupled finite element analyses on the well-known San Fernando dams, with the objective of evaluating the impact of this seepage effect. The results indicate that while this effect is always there, its practical significance depends on a number of factors. In the case of the upper San Fernando dam, which experienced a significant, but restricted, downstream movement during the 1971 earthquake, the seepage driving effect was indeed significant. On the contrary, for the lower dam, which failed and slid into the upstream reservoir during the same earthquake, this seepage effect was relatively less pronounced. The detailed results of the analyses reveal the likely mechanisms of failure and deformation of the two dams and the likely cause behind the difference between their responses during the earthquake.     

Study of damage to a high concrete dam subjected to underwater shock waves

To study the effect of a strong underwater shock wave on a concrete dam, this research aims to improve hammer impact methods in model tests. Six 1:200 scale models were designed and tested under distributed impact loads. A device was deployed for a direct measurement of the impact force at the upstream face of the dams. The model dam bases were anchored to prevent displacement. The experimental results indicate that the top part of the concrete dam is a weak zone, and the impact failure initiates with a fracture on the top of the dam. The peak value of impact stress increases when the second crack appears in the concrete dam from the upstream face to the downstream face. And, the level of the second crack in the dam body is lower as the peak value of impact stress increases. In this study, dynamic analysis was conducted by calculating the results to verify the effectiveness of a device to directly measure the impact force. This method may be used to approximately forecast the damage of concrete dam and may also be useful in other engineering applications.     

Large-scale shaking table model tests on seismically induced failure of Concrete-Faced Rockfill Dams

An increasing number of high Concrete-Faced Rockfill Dams (CFRDs) have been and are being built in highly seismic hazardous regions. Because there are few examples, the failure mechanisms for each damaged CFRD still remain unclear, which prevents a rational evaluation of aseismic performance of a CFRD. In this paper, a series of large-scale shaking table tests are conducted to determine the dynamic failure modes of CFRDs, especially the failure mechanism of the face slab. A type of model slab material is developed according to similitude rules. Particle Image Velocimetry (PIV) is used and improved to track the motion of each grain and subsequently measure the entire velocity field of the deforming cross-sections of dam models. The failure processes of the dams, the failure mechanisms of the slab fractures and the slab dislocations are discussed based on the experimental results. The experiments indicate that large deformations of the uppermost part of the dam caused the face slab to curve upward and crack. Another important fracture morphology of the face slab is the dislocation of its joint. The change in the granular fabric caused the granular slope to slide down and thrust outward, eventually dragging the upper half of the face slab down against its lower half; this motion is accompanied by the formation of a joint dislocation. Therefore, the stability and anti-deformation ability of both the downstream slope and the crest region of the upstream slope should be improved to withstand earthquake. The results of the shaking table model tests are consistent with the numerical simulation, and both results support the prototype dam׳s seismically induced failure characteristics.     

土石坝振动台模型试验颗粒流数值模拟分析

目前一般采用振动台试验、离心振动台试验和有限元动力分析来获得土石坝在设计地震荷载作用下的形态和抗震性能。本文结合孔宪京等的土石坝振动台试验结果开展了颗粒流细观数值模拟研究,克服了传统连续介质力学的宏观连续性假设,形象而直观地表现出坝体在动力荷载作用下的破坏特征。数值模拟规律与振动台试验规律基本一致。同时还分析了坝体颗粒粘结强度和地震峰值加速度变化对坝体破坏特征的影响。数值结果表明,当颗粒间粘结强度较低时,表现为坝体表面颗粒的滑动破坏,粘结强度稍大时,会出现局部的小块颗粒团整体滑动破坏;随着峰值加速度的增大,坝顶沉降量在增大,坝体破坏特征不变。     

An investigation into the seismic behaviour of dams using dynamic centrifuge modelling

Dynamic response of dams is significantly influenced by foundation stiffness and dam-foundation interaction. This in turn, significantly effects the generation of hydrodynamic pressures on upstream face of a concrete dam due to inertia of reservoir water. This paper aims at investigating the dynamic response of dams on soil foundation using dynamic centrifuge modelling technique. From a series of centrifuge tests performed on model dams with varying stiffness and foundation conditions, significant co-relation was observed between the dynamic response of dams and the hydrodynamic pressures developed on their upstream faces. The vertical bearing pressures exerted by the concrete dam during shaking were measured using miniature earth pressure cells. These reveal the dynamic changes of earth pressures and changes in rocking behaviour of the concrete dam as the earthquake loading progresses. Pore water pressures were measured below the dam and in the free-field below the reservoir. Analysis of this data provides insights into the cyclic shear stresses and strains generated below concrete dams during earthquakes. In addition, the sliding and rocking movement of the dam and its settlement into the soil below are discussed.