Pairing geotechnics and fluvial hydraulics for the prediction of the hazard zones of an exceptional flooding

The direct consequences of exceptional floods are usually considered to be limited to the maximum flooding zone created downstream. However, considering the magnitude of the flows, the morphology of the flooded zone could undergo deep changes. To predict the hazard zone on a river undergoing exceptional flooding, numerical simulations are widely used. In this article, the simulation of the evolution of river reaches resulting from such catastrophic events is performed by coupling the hydraulic and sediment transport numerical model GSTARS with a developed slope stability model based on the Bishop’s simplified method. This is a novel methodology for the delimitation of hazard zones along riverbanks by taking into consideration not only the flood risks but also the possible induced landslides. Indeed, each section of the river reach is subject to changes caused by the river hydraulics and the associated erosion or sediment deposition and also undergoes profile changes caused by possible landslides. The initial hydraulic and geotechnical characteristics are first defined and then used to test the stability of several slopes of representative sections of the river reaches before the dam break. Validation tests are performed on specific reaches of the Outaouais River (Quebec) undergoing a dam break flood.     

深水库区细颗粒淤积物重力驱动流动数值模拟——以三峡水库为例

针对三峡水库近坝区细颗粒泥沙淤积特征,从过程机理出发,提出了深水库区细颗粒泥沙淤积过程的物理图形。采用临界坡度作为细颗粒淤积物失稳流动的判别标准,借鉴浅水流动模式,描述失稳后细颗粒淤积物重力驱动流动过程,通过耦合三维水沙数学模型,建立了细颗粒淤积物重力驱动流动的数值模拟方法,在此基础上对三峡水库近坝区细颗粒泥沙淤积形态进行了模拟研究。研究结果表明:水库蓄水运行初期近坝区细颗粒泥沙主要淤积在断面深槽内,且呈水平状,考虑重力驱动流动后的模拟结果与实测结果吻合较好,从而为进一步深入研究深水库区细颗粒泥沙运动特征提供了技术支撑。     

土石坝漫顶溃决及洪水演进研究进展

土石坝漫顶溃决往往在其下游产生巨大的洪水灾难,研究坝体溃决及洪水演进是防灾减灾的需要。对土石坝漫顶溃决过程现象、机理及其模拟的研究进展进行了综述。讨论了物理模型试验的尺度设计,总结了不同尺度、不同类型、不同条件溃坝试验的研究成果;按照参数模型、简化物理模型、精细物理模型,分类总结了漫顶溃坝数学模型研究进展;阐述了溃坝洪水演进方面的试验与模拟研究。在此基础上,对该研究领域今后的研究工作提出了若干展望,包括河道边界对溃坝过程及溃坝洪水传播过程的影响、非均质土石坝溃决机理、溃坝过程中挟沙水流冲蚀规律、溃坝下游河床的冲淤调整及泥沙分选、溃坝对水生态环境的冲击影响等。     

土石坝漫顶溃决及洪水演进研究进展

土石坝漫顶溃决往往在其下游产生巨大的洪水灾难,研究坝体溃决及洪水演进是防灾减灾的需要。对土石坝漫顶溃决过程现象、机理及其模拟的研究进展进行了综述。讨论了物理模型试验的尺度设计,总结了不同尺度、不同类型、不同条件溃坝试验的研究成果;按照参数模型、简化物理模型、精细物理模型,分类总结了漫顶溃坝数学模型研究进展;阐述了溃坝洪水演进方面的试验与模拟研究。在此基础上,对该研究领域今后的研究工作提出了若干展望,包括河道边界对溃坝过程及溃坝洪水传播过程的影响、非均质土石坝溃决机理、溃坝过程中挟沙水流冲蚀规律、溃坝下游河床的冲淤调整及泥沙分选、溃坝对水生态环境的冲击影响等。     

推移质翻越低坝输移特性的试验研究

对于山区河流低坝而言，平时淤积在坝前的推移质粗沙可能会在洪水期集中翻越坝顶，形成高强度输沙。本文开展水槽试验，研究推移质粗沙自上游起动、推进、再翻越坝顶后向下游输移的过程，分析了输沙参数的变化特性及数理规律，描述了翻坝输沙模式及运动特征，揭示了输沙规律与河床形态之间的自然联系。取得如下认识：①输沙量随时间大致以幂函数规律增长。②低坝附近区域河床形态终将趋于稳定，上游和下游均形成相对稳定的曲面斜坡淤积体。③在不同的水流强度下推移质翻坝输移模式存在差异。对于一般水流强度工况，上游淤积体曲面斜坡表面泥沙颗粒以滚动或滑动模式起动，推移至接近坝顶位置时再跃移翻坝，后向下游输移；对于更高水流强度工况，后期的翻坝输沙模式可能发生显著转变，周期性边壁漩涡成为翻坝输沙的主要动力来源。     

推移质翻越低坝输移特性的试验研究

对于山区河流低坝而言,平时淤积在坝前的推移质粗沙可能会在洪水期集中翻越坝顶,形成高强度输沙。本文开展水槽试验,研究推移质粗沙自上游起动、推进、再翻越坝顶后向下游输移的过程,分析了输沙参数的变化特性及数理规律,描述了翻坝输沙模式及运动特征,揭示了输沙规律与河床形态之间的自然联系。取得如下认识:①输沙量随时间大致以幂函数规律增长。②低坝附近区域河床形态终将趋于稳定,上游和下游均形成相对稳定的曲面斜坡淤积体。③在不同的水流强度下推移质翻坝输移模式存在差异。对于一般水流强度工况,上游淤积体曲面斜坡表面泥沙颗粒以滚动或滑动模式起动,推移至接近坝顶位置时再跃移翻坝,后向下游输移;对于更高水流强度工况,后期的翻坝输沙模式可能发生显著转变,周期性边壁漩涡成为翻坝输沙的主要动力来源。     

Stability of landslide dams and development of knickpoints

The Wenchuan earthquake triggered many landslides and numerous avalanches and created 100 odd quake lakes. The quake lakes may be removed or preserved. The removal strategy was applied to several large landslide dams, which were dangerous because massive amounts of water pooled up in the quake lakes. The dams could eventually fail under the action of dam outburst flooding, potentially endangering the lives of people in the downstream reaches. This paper studied the stability of landslide dams and the development of knickpoints by field investigations and experiments, and analyzing satellite images. The study concluded that if landslide dams were preserved, they would develop into knickpoints and act as a primary control of riverbed incision and, thus, reduce the potential of new landslide. The stability of landslide dams depends mainly on the development of the step-pool system and stream power of the flood flow. If a landslide dam consists of many boulders, a step-pool system may develop on the spillway channel of the dam, which would maximize the resistance, consume most of the flow energy and consequently protect the dam from incision. The development degree of the step-pool system is represented by a parameter S _p, which was measured with a specially designed instrument. A preservation ratio of landslide dams is defined as the ratio of preserved height after flood scouring to the original height of the dam. For streams with peak flood discharge lower than 30 m³/s, the preservation ratio is linearly proportional to S _p. For rivers with a peak flood discharge higher than 30 m³/s (30–30,000 m³/s), the minimum S _p value for stable channel increases with log p, in which p is the unit stream power. For a landslide dam with a poorly developed step-pool system, S _p is smaller than the minimum value and the outburst flood incises the spillway channel and causes failure of the dam. For preserved landslide dams, sediment deposits in the quake lakes. A landslide dam may develop into a knickpoint if it is stabilized by long-term action of the flow. Large knickpoints can totally change the fluvial processes and river morphology. Uplift of the Qinghai–Tibetan Plateau has caused extensive channel bed incision along almost all rivers. For many rivers, the incision has been partly controlled by knickpoints. Upstream reaches of a knickpoint have a new and unchanging base level. This brings about a transition from degradation to aggradation and from vertical bed evolution to horizontal fluvial process. Multiple and unstable channels are prominent in the reaches, upstream of the knickpoints. If hundreds of landslide dams occurred simultaneously on a reach of a mountain river, the potential energy of bank failure and the slope erosion would be greatly reduced and sediment yield from the watershed may be reduced to nearly zero. The quake lakes may be preserved long term and become beautiful landscapes. Streams with long-term unfilled quake lakes have good aquatic ecology.     

Investigating sediments and rock structures beneath a river using underwater ERT

This article presents hydrogeophysical investigations performed in a well-developed, long-term hydrogeological gypsum karst research site where subsurface evaporite dissolution has led to the subsidence of a river dam and an adjacent highway; both constructed on gypsum-containing rock, southeast of Basel, Switzerland. An observation system was set up to improve the protection of surface and subsurface water resources during remedial construction measures of the highway and in order to understand the processes, as well as the temporal evolution, of rock water interaction (flow and dissolution). However, no detailed hydrogeological information beneath the river could be derived from the previous investigations. To supplement the basic knowledge on this area, underwater Electrical Resistivity Tomography (ERT) measurements were conducted in the river bed upstream of the dam. The ERT-data are interpreted together with drill-core information and a conceptual 3D-Model of the area behind the dam and beneath the river. Results help to delineate weathered zones, associated faults and the thickness of sediment deposits behind the dam, as well as to locate voids within the local karst system. The combination of the ERT and modeling allows the optimization of future site-specific remedial construction measures.     

陆相湖盆大型重力流发育的成因机制及其优质储层特征研究:以歧口凹陷沙河街组一段为例

陆相重力流沉积是现今油气勘探领域的研究热点.发育在歧口凹陷滨海斜坡古近系沙河街组一段的大型重力流沉积体（沉积范围达到1 700 km2）具有多物源供给、长距离搬运、多级断坡传输、沿路沉积的发育过程与沉积机理特征.以渤海湾盆地富烃凹陷-歧口凹陷的大型重力流为研究对象,在构造-高精度层序地层格架下,以"源-渠-汇"的整体过程为思路,（1）研究多物源的形成、持续供给及匹配关系;（2）分析古地貌（断控陡坡带+多级断阶坡折带）的演化,及其对沉积物搬运、堆积、发育过程的控制作用;（3）针对该重力流具有大规模、多期次的沉积特征,解剖其平面展布及空间分布、沉积模式、时空演化规律;（4）综合探讨陆相重力流沉积体的成因机制、控制因素.综上可知:（1）歧口凹陷大型陆相重力流沉积体发育受多个物源体系的影响与控制,湖盆中心的重力流沉积体与供给物源之间有明确的匹配关系.在古近系沙一段沉积时期,滨海斜坡沉积区主要受北部燕山物源区的大神堂物源、茶淀物源以及西部沧县隆起物源区的葛沽物源和小站物源4个物源共同供给,持续的物源供给使得在歧口凹陷的陆相湖盆中,发育了大规模的重力流沉积体.（2）沙一段同沉积期,歧口凹陷滨海斜坡周缘整个古地貌格局主要表现为断控陡坡带与多级断阶坡折带复合体,断控陡坡带主要发育于滨海斜坡北部,多级断阶带主要呈近东西向发育于斜坡西部.这些断坡带既是物源水下搬运通道又是沉积物堆积的可容纳空间分布区,再加上这些断坡带差异性的持续沉降,对砂分散体系和相带展布具有关键的控制作用.（3）歧口凹陷沙一段重力流沉积过程机制主要表现为重力滑塌沉积、砂质碎屑流、泥质碎屑流、浊流等多种成因,具有横向连片,纵向叠置的沉积样式.      

Assessment of potential outburst floods from the Tsho Rolpa glacial lake in Nepal

Flood and sediment disasters caused by glacial lake outbursts have occurred frequently in recent years in the Himalayas of Nepal. Glacial lake outburst floods (GLOFs) can cause catastrophic flooding in downstream areas with serious damage to lives and property. It is thus important to investigate outburst floods from potentially dangerous glacial lakes. In this study, the characteristics of potential outburst floods from the Tsho Rolpa glacial lake due to two types of moraine dam failure caused by seepage flow or water overtopping were analyzed with various scenarios by using integrated modeling system of three numerical models: (1) the flow and bed-surface erosion model, (2) the seepage model and (3) the slope stability model. Flood inundation areas were also identified by using the numerical model of the flow and moraine dam failure and geographical information system tools. Possible threats and damages due to the potential GLOF events from the lake were also analyzed based on numerical results, flood inundation maps and field investigations.     

Evaluation of potential landslide damming: Case study of Urni landslide,Kinnaur, Satluj valley,India

This work aims to understand the process of potential landslide damming using slope failure mechanism,dam dimension and dam stability evaluation. The Urni landslide, situated on the right bank of the Satluj River, Himachal Pradesh(India) is taken as the case study. The Urni landslide has evolved into a complex landslide in the last two decade(2000-2016) and has dammed the Satluj River partially since year 2013,damaging ～200 m stretch of the National Highway(NH-05). The crown of the landslide exists at an altitude of ～2180-2190 m above msl, close to the Urni village that has a human population of about 500.The high resolution imagery shows ～50 m long landslide scarp and ～100 m long transverse cracks in the detached mass that implies potential for further slope failure movement. Further analysis shows that the landslide has attained an areal increase of 103,900 ± 1142 m^2 during year 2004-2016. About 86% of this areal increase occurred since year 2013. Abrupt increase in the annual mean rainfall is also observed since the year 2013. The extreme rainfall in the June, 2013; 11 June(～100 mm) and 16 June(～115 mm),are considered to be responsible for the slope failure in the Urni landslide that has partially dammed the river. The finite element modelling(FEM) based slope stability analysis revealed the shear strain in the order of 0.0-0.16 with 0.0-0.6 m total displacement in the detachment zone. Further, kinematic analysis indicated planar and wedge failure condition in the jointed rockmass. The debris flow runout simulation of the detached mass in the landslide showed a velocity of ～25 m/s with a flow height of ～15 m while it(debris flow) reaches the valley floor. Finally, it is also estimated that further slope failure may detach as much as 0.80 ±0.32 million m^3 mass that will completely dam the river to a height of 76±30 m above the river bed.     

2D numerical modelling of meandering channel formation

A 2D depth-averaged model for hydrodynamic sediment transport and river morphological adjustment was established. The sediment transport submodel takes into account the influence of non-uniform sediment with bed surface armoring and considers the impact of secondary flow in the direction of bed-load transport and transverse slope of the river bed. The bank erosion submodel incorporates a simple simulation method for updating bank geometry during either degradational or aggradational bed evolution. Comparison of the results obtained by the extended model with experimental and field data, and numerical predictions validate that the proposed model can simulate grain sorting in river bends and duplicate the characteristics of meandering river and its development. The results illustrate that by using its control factors, the improved numerical model can be applied to simulate channel evolution under different scenarios and improve understanding of patterning processes.     

低坝影响下的流场与淤积形态特性试验研究

为研究径流式低坝影响下的水流流动与泥沙淤积特性,开展水槽试验,基于图像测量技术,获取并解析坝附近区域流场信息及典型淤积形态。结果表明:坝前附近流段纵向流速在垂线上出现衰减区,减幅随水流强度增大而减小;坝顶断面纵向和垂向流速沿垂线的分布均呈现显著的分区特性,分区界限几乎不受水流强度的影响;随坝顶水深增加,坝下游漩涡涡心向下游及河底移动,面积和强度皆增大;坝上游淤积形态特性对水流强度的变化非常敏感,在较低强度来流下,呈接近坝体的稳定曲面斜坡,而在高强度来流下,不形成稳定淤积体;坝下游形成动态稳定的淤积斜坡,纵剖表面线呈抛物线规律,随来流强度变异程度小。     

低坝影响下的流场与淤积形态特性试验研究

为研究径流式低坝影响下的水流流动与泥沙淤积特性,开展水槽试验,基于图像测量技术,获取并解析坝附近区域流场信息及典型淤积形态。结果表明：坝前附近流段纵向流速在垂线上出现衰减区,减幅随水流强度增大而减小;坝顶断面纵向和垂向流速沿垂线的分布均呈现显著的分区特性,分区界限几乎不受水流强度的影响;随坝顶水深增加,坝下游漩涡涡心向下游及河底移动,面积和强度皆增大;坝上游淤积形态特性对水流强度的变化非常敏感,在较低强度来流下,呈接近坝体的稳定曲面斜坡,而在高强度来流下,不形成稳定淤积体;坝下游形成动态稳定的淤积斜坡,纵剖表面线呈抛物线规律,随来流强度变异程度小。     

扇三角洲形成过程及演变规律

应用实验沉积学的方法对扇三角洲的形成过程及演化规律进行了较为详细的研究。指出扇三角洲的形成是突发性洪流与常态水流交替作用的结果,洪水期以碎屑流或泥石流沉积为主,平水期以牵引流沉积为主。随着沉积作用的延续,在基准面下降过程中,扇三角洲形态在平面上不同步增长,展宽速率大于伸长速率,扇三角洲的形态呈边缘圆滑的舌状。辫状河道的迁移摆动是导致扇三角洲演化的根本内在原因,而构造运动的强度与辫状河道的迁移摆动速率呈近似正相关关系。湖水深度、入湖坡降、辫状河道的形态、流量变化率以及粒径大小是影响扇三角洲演化的重要因素。     

论河流的弯曲机理

基于能量守恒原理,通过实体模型试验的方法,对河流发生弯曲的机理进行了探讨。研究认为,河流具有弯曲的自然属性,其弯曲程度主要取决于水流能量的大小,与流量、比降有很大关系;河流发生弯曲是水流为补偿能量损失得以保持运动而形成的一种自然造床过程,河流在运动过程中,通过弯曲形成上游壅水,增加势能,使下游比降相对增大,动能得以增加,从而保持河段内的水沙输移达到相对平衡,此即"动能自补偿"的弯曲机理;河流弯曲程度与需要补偿的动能多少有关,河段上下断面的动能差越大,河段弯曲系数就越大。"动能自补偿"的机理得到了实验室观测结果的验证和野外原型定位观测资料的佐证。     

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.     

Numerical simulation of landslide dam breaching due to overtopping

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.     

松散土坡结构特征与滑坡堰塞体堆积特征之间的关系试验研究

通过模型实验,探讨了松散土坡3种不同结构特征条件下(均匀坡体(坡体物料均匀混合)、平行坡体(土层成层且层面与滑面平行)和斜交坡体(土层成层且层面与滑面斜交))形成滑坡堰塞体的动力过程和堆积特征,通过分区域取样及三维扫描技术研究了堰塞体的物质分布规律与形态特征。研究结果表明：堰塞体堆积特征与坡体特征存在紧密联系,堰塞体纵向(沿主沟道方向)和横向(沿滑坡运动方向)上的物质分布与坡体纵向和横向的物质分布特征基本对应;在垂向(表层到底部)上,由于不同坡体条件下滑坡的动力过程和机理不同,从而导致堰塞体堆积特征存在一定区别。均匀坡体和平行坡体呈整体启动模式,运动过程中物料间存在垂向渗透和交换作用,导致堰塞体物质在垂向上呈明显的上粗下细反粒序分布特征,堰塞体横剖面多呈平坦型和倾斜型。斜交坡体呈分层启动模式,运动堆积过程中保持原有层序,粗、细颗粒先后启动条件下颗粒间存在推挤、爬升和水平渗透作用,使得堰塞体更加密实且垂向上也呈现反粒序分布特征,横剖面多呈起伏型。本研究为滑坡堰塞体稳定性快速评估和复原滑坡初始状态提供依据。     

感潮河网区水沙运动的数值模拟

根据感潮河网区水沙运动特征,建立了一维感潮河网区非恒定水沙数学模型,对河网区水沙运动和河床变形进行了数值模拟,并在水沙模拟的范围内着重对河网区内动边界的处理、挟沙力公式的选用、内边界含沙量的确定等问题进行了讨论,提出了解决问题的途径和技术处理方法。采用东江下游感潮河网区的水文和河道地形资料对所建模型进行了检验,模拟结果较好地反映了感潮河网区水沙运动的往复输运特征和河道冲淤的宏观效应。