降雨模式对震后泥石流起动模式影响的试验研究: 以九寨天堂沟为例

降雨过程中降雨强度的变化会影响土体渗透率及饱和过程, 从而改变土体的力学性质, 影响泥石流起动模式及破坏规模。为探究不同降雨模式对震后泥石流起动机制的影响, 自制了小比例模型槽, 结合可控雨型的降雨模拟系统, 进行了人工降雨诱发泥石流的室内模型试验; 基于不同降雨模式下泥石流的起动过程分析, 对坡体内部含水率和孔隙水压力的变化规律进行了研究。研究结果表明: 递增型降雨模式下泥石流发生突然, 呈整体滑坡转化为泥石流起动模式, 坡体破坏规模最大; 递减型降雨模式下表现为后退式溃散失稳起动模式; 均匀型降雨模式下则表现为溯源侵蚀起动模式; 中峰型降雨模式下以局部滑坡转化为泥石流起动模式; Ⅴ型降雨模式下则由坡面侵蚀加剧转化为泥石流启动模式, 破坏规模最小。研究结果可以为九寨沟地区泥石流的预报预警提供参考。      

Experimental analysis for the dynamic initiation mechanism of debris flows

Debris flow is one of the major secondary mountain hazards following the earthquake. This study explores the dynamic initiation mechanism of debris flows based on the strength reduction of soils through static and dynamic triaxial tests. A series of static and dynamic triaxial tests were conducted on samples in the lab. The samples were prepared according to different grain size distribution, degree of saturation and earthquake magnitudes. The relations of dynamic shear strength, degree of saturation, and number of cycles are summarized through analyzing experimental results. The findings show that the gravelly soil with a wide and continuous gradation has a critical degree of saturation of approximately 87%, above which debris flows will be triggered by rainfall, while the debris flow will be triggered at a critical degree of saturation of about 73% under the effect of rainfall and earthquake(M>6.5). Debris flow initiation is developed in the humidification process, and the earthquake provides energy for triggering debris flows. Debris flows are more likely to be triggered at the relatively low saturation under dynamic loading than under static loading. The resistance of debris flow triggering relies more on internal frication angle than soil cohesion under the effect of rainfall and earthquake. The conclusions provide an experimental analysis method for dynamic initiation mechanism of debris flows.     

南堡凹陷东北部东二段重力流砂岩储层发育规律

随着勘探程度的提高,深水重力流成因的浊积岩储层已成为我国东部断陷湖盆油气勘探开发的重要目标之一。因为沉积分异不足和成岩破坏,重力流砂岩的储层质量通常整体较差,优质储层的预测成为制约其有效油气勘探的关键地质因素。利用岩心、测井资料及储层物性、岩石薄片分析结果,研究了南堡凹陷东北部东营组二段重力流砂岩的岩相特征、成因类型、储层特征,以探索优质储层的控制因素和发育规律。研究表明,区内重力流沉积可细分为8种岩相,解释为滑动滑塌、砂质碎屑流、泥质碎屑流、浊流4类成因。储层物性参数统计分析证实,本区重力流砂岩储层非均质性强,储层质量受控于砂岩成因、砂-泥结构及其影响的溶蚀强度。从成因看,砂质碎屑流和浊流对重力流砂岩优质储层的发育贡献最大。砂质碎屑流成因的块状砂岩厚度较大、泥岩夹层较少、钙质胶结物的溶蚀程度高,储层质量最好;而浊流成因的砂岩厚度较薄,与泥岩呈互层或夹层产出,成岩环境封闭、钙质胶结物溶蚀程度低,储层质量较差。本研究为湖盆深水重力流砂岩油气的高效勘探开发提供了一种基于成因和结构的储层预测思路。     

Debris flows introduced in landslide deposits under rainfall conditions: The case of Wenjiagou gully

Debris flows often occur in landslide deposits during heavy rainstorms. Debris flows are initiated by surface water runoff and unsaturated seepage under rainfall conditions. A physical model based on an infinitely long, uniform and void-rich sediment layer was applied to analyze the triggering of debris-flow introduced in landslide deposits. To determine the initiation condition for rainfall-induced debris flows, we conducted a surface water runoff and saturated-unsaturated seepage numerical program to model rainfall infiltration and runoff on a slope. This program was combined with physical modeling and stability analysis to make certain the initiation condition for rainfall-introduced debris flows. Taking the landslide deposits at Wenjiagou gully as an example, the initiation conditions for debris flow were computed. The results show that increase height of surface-water runoff and the decrease of saturated sediment shear strength of are the main reasons for triggering debris-flows under heavy rainfall conditions. The debris-flow triggering is affected by the depth of surface-water runoff, the slope saturation and shear strength of the sediment.     

Effects of loose deposits on debris flow processes in the Aizi Valley,southwest China

Loose deposits, rainfall and topography are three key factors that triggering debris flows.However, few studies have investigated the effects of loose deposits on the whole debris flow process.On June 28, 2012, a catastrophic debris flow occurred in the Aizi Valley, resulting in 40 deaths.The Aizi Valley is located in the Lower Jinsha River,southwestern Sichuan Province, China. The Aizi Valley debris flow has been selected as a case for addressing loose deposits effects on the whole debris flow process through remote sensing, field investigation and field experiments. Remote sensing interpretation and laboratory experiments were used to obtain the distribution and characteristics of the loose deposits, respectively. A field experiment was conducted to explore the mechanics of slope debris flows, and another field investigation was conducted to obtain the processes of debris flow formation, movement and amplification. The results showed that loose deposits preparation, slope debris flow initiation,gully debris flow confluence and valley debris flow amplification were dominated by the loose deposits.Antecedent droughts and earthquake activities may have increased the potential for loose soil sources in the Aizi Valley, which laid the foundation for debris flow formation. Slope debris flow initiated under rainfall, and the increase in the water content as well as the pore water pressure of the loose deposits were the key factors affecting slope failure. The nine gully debris flows converged in the valley, and the peak discharge was amplified 3.3 times due to a blockage and outburst caused by a large boulder. The results may help in predicting and assessing regional debris flows in dry-hot and seismic-prone areas based on loose deposits, especially considering large boulders.     

Debris flow in metropolitan area — 2011 Seoul debris flow

A large number of debris flows occurred simultaneously at around 8:30 to 8:50 a.m. on July 27, 2011, at the center of Seoul, Korea. This area is located in the southern part of Seoul and is a densely populated district. As a result of the debris flow event, 16 people were killed, 30 houses were buried, and 116 houses were damaged around Umyeon Mountain, a relatively small mountain with a height of 312.6 m. Since the debris flow event, field investigations on the initiation and transportation zones of debris flows have been carried out. Rainfall data were collected from the automatic weather stations (AWSs) which are operated by the Korea Meteorological Administration (KMA). Video files recorded by residents were also acquired and used to analyze the flow characteristics of the debris flow. Field investigation shows that about 40 debris flows occurred around Umyeon Mountain and most of the debris flows were initiated by small slope failures. The effects of the precipitation that triggered the debris flows were analyzed as well. A landslide hazard map which considers slope gradient and aspect, strength of soil, hazard record, rainfall conditions, and vegetation, was constructed and compared with the initiation zones of debris flows.     

Field observations of debris-flow initiation processes on sediment deposits in a previous deep-seated landslide site

Although information regarding the initiation processes of debris flows is important for the development of mitigation measures,field data regarding these processes are scarce.We conducted field observations of debris-flow initiation processes in the upper Ichinosawa catchment of the Ohya landslide,central Japan.On 19 June 2012,our videocamera monitoring systems recorded the moment of debris-flow initiation on channel deposits(nine surges) and talus slopes(eight surges).The initiation mechanisms of these surges were classified into three types by analyzing the video images: erosion by the surface flow,movement of deposits as a mass,and upward development of the fluid area.The first type was associated with the progress of surface flow from the upper stream on unsaturated channel deposits.The second type was likely caused by an increase in the pore water pressure associated with the rising in the groundwater level in channel deposits;a continuous water supply from the upper stream by the surface flow might have induced this saturation.The third type was associated with changes in the downstream topography caused by erosion.The flow velocity of most surges was less than 3 m s~(-1) and they usually stopped within 100 m from the initiation point.Surges with abundant pore fluid had a higher flow velocity(about 3- 5 m s~(-1)) and could travel for alonger duration.Our observations indicate that the surface flow plays an important role in the initiation of debris flows on channel deposits and talus slopes.     

Entrainment effects and the dynamical evolution of debris avalanche/flow on substrate materials

A pair of flumes with variable inclinations were employed to investigate the entrainment mechanics and dynamical evolution of a debris avalanche/flow. A fixed quantity of solid and water mixture was released from a constant elevation and accelerated along a higher chute to impact substrate materials with different water contents and particle size distributions in the lower chute. Two high-speed cameras, pore and earth pressure detecting devices, were placed in the substrate materials where severe scouring occurs in order to collect multiple measurements of dynamical and mechanical parameters. The entrainment dynamics were verified by geometrical analysis and quasi-static simulation. The results show that wet and fine materials that are placed in the lower chute with steeper slopes are easily entrained during debris flow initiation, the pattern of which can be described by Coulomb friction and the Mohr-Coulomb law. Elaborate measurements of dynamical parameters enable the results of an elementary computational framework to predict the time-dependent scouring depth ht, which provides insight into rapidly determining debris flow propagation. Finally, the post-entrainment dynamics were studied. The results indicate that the propagation and the amplification of debris flows along erodible beds are dominated by the velocity and the solid volume fraction of the mixed substrate, and the coarse particle group of the substrate is a key feature affected by momentum changes.     

Changes in runout distances of debris flows over time in the Wenchuan earthquake zone

A large number of debris flows occurred in the Wenchuan earthquake zone after the 12 May 2008 earthquake.The risks posed by these debris flows were rather high.An appropriate model is required to predict the possible runout distance and impacted area.This paper describes a study on the runout characteristics of the debris flows that occurred in the Wenchuan earthquake zone over the past four years.A total of 120 debris flows are analyzed.Separate multivariate regression models are established for the runout distances of hill-slope debris flows and channelized debris flows.The control variables include type of debris flow,debris flow volume,and elevation difference.Comparison of the debris flows occurring before and after the earthquake shows that the runout distance increased after the earthquake due to sufficient material supply and increased mobility of the source materials.In addition,the runout distances of annual debris flow events in 2008,2010 and 2011 are analyzed and compared.There is a tendency that the runout distance decreases over time due to the decreasing source material volume and possible changes of debris flow type.Comparison between the debris flows in the earthquake zone and the debris flows in Swiss Alps,Canada,Austria,and Japan shows that the former have a smaller mobility.     

Mechanism of downcutting erosion of debris flow over a movable bed

The phenomenon of debris flow is intermediate between mass movement and solid transport. Flows can be sudden, severe and destructive. Understanding debris flow erosion processes is the key to providing geomorphic explanations, but progress has been limited because the physical-mechanical properties, movement laws and erosion characteristics are different from those of sediment-laden flow. Using infinite slope theory, this research examines the process and mechanism of downcutting erosion over a moveable bed in a viscous debris flow gully. It focuses specifically on the scour depth and the critical slope for viscous debris flow,and formulas for both calculations are presented.Both scour depth and the critical conditions of downcutting erosion are related to debris flow properties(sand volume concentration and flow depth) and gully properties(longitudinal slope,viscous and internal friction angle of gully materials,and coefficient of kinetic friction). In addition, a series of flume experiments was carried out to characterize the scouring process of debris flows with different properties. The calculated values agreed well with the experimental data. These theoretical formulas are reasonable, and using infinite slope theory to analyze down cutting erosion from viscous debris flow is feasible.     

鄂尔多斯盆地南部彬长区块延长组长6-长7段深水重力流沉积特征分析

鄂尔多斯盆地南部彬长区块的延长组长6-长7段发育厚层无沉积构造的块状砂岩,具有良好的油气显示和开发效益,然而关于该套砂岩的形成机制尚不清晰。确定长6-长7段砂岩的沉积相及沉积模式,对于该套低渗砂岩储层"甜点"形成机制的理解,"甜点"分布模式的预测,以及后续勘探开发都具有重要的指导意义。对彬长区块36口取心井的长6-长7段1 024 m长的岩心进行了沉积学特征描述,结合粒度分析资料及地质制图分析,确定了该套厚层砂岩的沉积相及沉积模式。结果表明:鄂尔多斯盆地南部彬长区块的延长组长6-长7段砂岩共发育15种岩相和3种主要沉积微相类型,即:砂质碎屑流、浊积岩和震积滑塌岩微相,以及它们在空间上的3类组合关系。其深水重力流沉积模式可以概括为扇根（坡折带斜坡上半部分）的震积滑塌相-砂质碎屑流亚相（沉积组合）、扇中（斜坡中下部位-坡脚）的砂质碎屑流-浊积岩沉积亚相（沉积组合）和扇端（坡脚-盆底）浊积砂等亚相（沉积组合）。通过对彬长区块延长组长6-长7段发育的致密砂岩沉积特征的分析与讨论,确定了该厚层块状砂岩的主要沉积相及沉积微相的特征及分布,为致密砂岩储层的高效开发及"甜点"预测提供了科学依据与良好借鉴。      

Debris flow susceptibility assessment using a probabilistic approach: A case study in the Longchi area,Sichuan province,China

The Longchi area with the city of Dujiangyan, in the Sichuan province of China, is composed of Permian stone and diorites and Triassic sandstones and mudstones intercalated with slates. An abundance of loose co-seismic materials were present on the slopes after the May 12, 2008 Wenchuan earthquake, which in later years served as source material for rainfall-induced debris flows or shallow landslides. A total of 48 debris flows, all triggered by heavy rainfall on 13th August 20l0, are described in this paper. Field investigation, supported by remote sensing image interpretation, was conducted to interpret the co-seismic landslides in the debris flow gullies. Specific characteristics of the study area such as slope, aspect, elevation, channel gradient, lithology, and gully density were selected for the evaluation of debris flow susceptibility. A score was given to all the debris flow gullies based on the probability of debris flow occurrence for the selected factors. In order to get the contribution of the different factors, principal component analyses were applied. A comprehensive score was obtained for the 48 debris flow gullies which enabled us to make a susceptibility map for debris flows with three classes. Twenty-two gullies have a high susceptibility, twenty gullies show a moderate susceptibility and six gullies have a low susceptibility for debris flows.     

Gravel accumulation in deposits of viscous debris flows with hyper-concentration

According to the observational data of viscous debris flows with hyper-concentration, debris flows can be classified into three types： high-viscous, viscous, and sub-viscous debris flows. Distinct formation mechanism of different graded bedding structures in deposits of viscous debris flows was analyzed in this paper by using their yield-stress ratio and flow plug ratio. This paper specially analyzed the effect of Weissenberg which the gravels in squirm condition of hyper-concentration viscous flows would tend to move vertically, and the formation mechanism of the gravels accumulated at surface was also studied. The analysis in this paper can establish a foundation for the studies on differentiation of bedding structures of debris flow deposits and studies on dynamic parameters of debris flows.     

Volume estimation of small scale debris flows based on observations of topographic changes using airborne LiDAR DEMs

This paper describes a geographic information system(GIS)-based method for observing changes in topography caused by the initiation, transport, and deposition of debris flows using highresolution light detection and ranging(LiDAR) digital elevation models(DEMs) obtained before and after the debris flow events. The paper also describes a method for estimating the volume of debris flows using the differences between the LiDAR DEMs. The relative and absolute positioning accuracies of the LiDAR DEMs were evaluated using a real-time precise global navigation satellite system(GNSS) positioning method. In addition, longitudinal and cross-sectional profiles of the study area were constructed to determine the topographic changes caused by the debris flows. The volume of the debris flows was estimated based on the difference between the LiDAR DEMs. The accuracies of the relative and absolute positioning of the two LiDAR DEMs were determined to be ±10 cm and ±11 cm RMSE, respectively, which demonstrates the efficiency of the method for determining topographic changes at an scale equivalent to that of field investigations. Based on the topographic changes, the volume of the debris flows in the study area was estimated to be 3747 m3, which is comparable with the volume estimated based on the data from field investigations.     

Artificial Neural Network-based prediction of glacial debris flows in the ParlungZangbo Basin,southeastern Tibetan Plateau,China

Accurate prediction on geological hazards can prevent disaster events in advance and greatly reduce property losses and life casualties.Glacial debris flows are the most serious hazards in southeastern Tibet in China due to their complexity in formation mechanism and the difficulty in prediction.Data collected from 102 glacier debris flow events from 31 gullies since 1970 and regional meteorological data from 1970 to 2019 in ParlungZangbo River Basin in southeastern Tibet were used for Artificial Neural Network(ANN)-based prediction of glacial debris flows.The formation mechanism of glacial debris flows in the ParlungZangbo Basin was systematically analyzed,and the calculations involving the meteorological data and disaster events were conducted by using the statistical methods and two layers fully connected neural networks.The occurrence probabilities and scales of glacial debris flows(small,medium,and large)were predicted,and promising results have been achieved.Through the proposed model calculations,a prediction accuracy of 78.33%was achieved for the scale of glacial debris flows in the study area.The prediction accuracy for both large-and medium-scale debris flows are higher than that for small-scale debris flows.The debris flow scale and the probability of occurrence increase with increasing rainfall and temperature.In addition,the K-fold cross-validation method was used to verify the reliability of the model.The average accuracy of the model calculated under this method is about 93.3%,which validates the proposed model.Practices have proved that the combination of ANN and disaster events can provide sound prediction on geological hazards under complex conditions.     

Slope debris flows in the Wenchuan Earthquake area

Avalanches and landslides, induced by the Wenchuan Earthquake on May 12, 2008, resulted in a lot of disaggregated, solid material on slopes that could be readily mobilized as source material for debris flows. Rainstorms triggered numerous slope debris flows with great damage to highways and rivers over the subsequent two years. Slope debris flows (as opposed to channelized debris flows) are defined as phenomena in which high-concentration mixtures of debris and water flow down slopes for short distances to highways and river banks. Based on field investigations and measurements of 19 slope debris flows, their main characteristics and potential mitigation strategies were studied. High rainfall intensity is the main triggering factor. Critical rainfall intensities for simultaneous occurrence of single, several and numerous slope debris flow events were 20 mm/day, 30mm/day, and 90 mm/day, respectively. Field investigations also revealed that slope debris flows consist of high concentrations of cobbles, boulders and gravel. They are two-phase debris flows. The liquid phase plays the role of lubrication instead of transporting medium. Solid particles collide with each other and consume a lot of energy. The velocities of slope debris flows are very low, and their transport distances are only several tens of meters. Slope debris flows may be controlled by construction of drainage systems and by reforestation.     

RECENT DEVELOPMENTS IN DEBRIS FLOW RESEARCH IN ITALY

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Deposition pattern of stony and muddy debris flow at the confluence area

Debris flow fan affects the river profile and landscape evolution.The propagation of multiple debris flows along a river can cause inundation and breaching risk,which can be exemplified by the Min River after the Wenchuan earthquake,Sichuan province,China.In this work,large flume tests were conducted to examine the interactions between debris flows and water current with the fan geometry,momentum,runout distance,deposited width,the relative water level upstream and dominated stress.The results reveal that stony flow commonly travels at a high speed and forms a long rectangle shape fan,while the muddy flow generally travels at a low speed and forms a fan-shaped depositional area.The stony flow can block a river even when the momentum is close to the water current;the muddy flow can block a river when the momentum is lower than that of water current.In case of complete river damming,the relative water level upstream indicates that the inundation risk from the muddy flow damming river would be higher than the inundation risk of stony flow.The diversion ratio of muddy flow decreases as damming ratio.Comparison of dimensionless numbers reveals that stony flow is dominated by grain collision stress combined with turbulent mixing stress,while the muddy flow is dominated by viscous shear stress over friction stress.The fan geometry,damming ratio,diversion ratio,and the dominated stress all together indicate that stony flow strongly interacts with water current while the muddy flow does not.The results can be helpful for understanding the physical interactions between water current and various debris flows,and debris flow dynamics at the channel confluence area.     

栖霞市生木树泥石流隐患点发育特征及发展趋势分析评价

栖霞市生木树泥石流隐患点为烟台市471处地质灾害隐患点其中1处,曾于1979年7月因暴雨引发泥石流灾害,给当地村民造成严重经济损失。以该泥石流沟流域作为研究区,并以区内泥石流发育的自然环境、基本特征及形成泥石流的地质条件、物源条件和水源条件等勘察成果为基础,综合分析区内泥石流发育特征、类型、形成机理、引发因素,并选取相关参数对泥石流基本特征值进行计算,为同类型泥石流的防治提供科学依据。综合研究确定区内泥石流易发程度为轻度易发,现阶段泥石流沟发展阶段为发展期,泥石流灾害趋于相对稳定,但一旦遭遇暴雨至特大暴雨,可能会再次引发泥石流地质灾害。     

Characteristics, causes and mitigation of catastrophic debris flow hazard on 21 July 2011 at the Longda Watershed of Songpan County, China

Debris flow is a common natural hazard in the mountain areas of Western China due to favorable natural conditions,and also exacerbated by mountainous exploitation activities.This paper concentrated on the characteristics,causes and mitigation of a catastrophic mine debris flow hazard at Longda Watershed in Songpan County,Sichuan Province,on 21 July 2011.This debris flow deposited in the front of the No.1 dam,silted the drainage channel for flood and then rushed into tailing sediment reservoir in the main channel and made the No.2 dam breached.The outburst debris flow blocked Fu River,formed dammed lake and generated outburst flood,which delivered heavy metals into the lower reaches of Fu River,polluted the drink water source of the population of over 1 million.The debris flow was characterized with a density of 1.87~2.15 t/m 3 and a clay content of less than 1.63%.The peak velocity and flux at Longda Gully was over 10.0~10.9 m/s and 429.0~446.0 m 3 /s,respectively,and the flux was about 700 m 3 /s in main channel,equaling to the flux of the probability of 1%.About 330,000m 3 solid materials was transported by debris flow and deposited in the drainage tunnel(120,000~130,000 m 3),the front of No.1 dam(100,000 m 3) and the mouth of the watershed(100,000~110,000 m 3),respectively.When the peak flux and magnitude of debris flow was more than 462 m 3 /s and 7,423 m 3,respectively,it would block Fu River and produce a hazard chain which was composed of debris flow,dammed lake and outburst flood.Furthermore,the 21 July large-scale debris flow was triggered by rainstorm with an intensity of 21.2 mm/0.5 h and the solid materials of debris flow were provided by landslides,slope deposits,mining wastes and tailing sediments.The property losses were mainly originated from the silting of the drainage tunnel for flash flood but not for debris flow and the irrational location of tailing sediment reservoir.Therefore,the mitigation measures for mine debris flows were presented:(1) The disastrous debris flow watershed should be identified in planning period and prohibited from being taken as the site of mining factories;(2) The mining facilities are constructed at the safe areas or watersheds;(3) Scoria plots,concentrator factory and tailing sediment reservoir are constructed in safe areas where the protection measures be easily made against debris flows;(4) The appropriate system and plan of debris flow mitigation including monitoring,remote monitoring and early-warning and emergency plan is established;(5) The stability of waste dump and tailing sediment reservoir are monitored continuously to prevent mining debris flows.