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.     

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.     

A catastrophic debris flow in the Wenchuan Earthquake area,July 2013: characteristics,formation, and risk reduction

In the Wenchuan Earthquake area,many co-seismic landslides formed blocking-dams in debris flow channels. This blocking and bursting of landslide dams amplifies the debris flow scale and results in severe catastrophes. The catastrophic debris flow that occurred in Qipan gully(Wenchuan,Southwest China) on July 11,2013 was caused by intense rainfall and upstream cascading bursting of landslide dams. To gain an understanding of the processes of dam bursting and subsequent debris flow scale amplification effect,we attempted to estimate the bursting debris flow peak discharges along the main gully and analyzed the scale amplification process. The results showed that the antecedent and triggering rainfalls for 11 July debris flow event were 88.0 mm and 21.6 mm,respectively. The event highlights the fact that lower rainfall intensity can trigger debris flows after the earthquake. Calculations of the debris flow peak discharge showed that the peak discharges after the dams-bursting were 1.17–1.69 times greater than the upstream peak discharge. The peak discharge at the gully outlet reached 2553 m~3/s which was amplified by 4.76 times in comparison with the initial peak discharge in the upstream. To mitigate debris flow disasters,a new drainage channel with a trapezoidal V-shaped cross section was proposed. The characteristic lengths(h1 and h2) under optimal hydraulic conditions were calculated as 4.50 m and 0.90 m,respectively.     

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.     

Geomorphologic analysis and physico-dynamic characteristics of Zhatai-Gully debris flows in SW China

Zhatai gully is a typical debris flow channel in Butuo county of Sichuan province, southwestern China. The geomorphologic features are analyzed and the physical-dynamic characteristics are discussed on the basis of field investigation and laboratory tests. Geomorphologic analysis indicates that Zhatai-gully drainage in relation to debris flow can be divided into source area, transport area, and deposition area. The source area has a steep slope and has very limited vegetation cover, which favors runoff, allowing loose solid materials to be mobilized easily and rapidly. In the transport area, there are many small landslides, lateral lobes, and loose materials distributed on both banks. These landslides are active and constantly providing abundant source of soils for the debris flows. In the deposition area, three old debris-flow deposits of different ages can be observed. The dynamic calculation shows that within the recurrence intervals of 50 and 100 years, debris flow discharges are 155.77m³/s and 1y8.19m³/s and deposition volumes are 16.39 x 10⁴ m³ and 18.14 x 10⁴ m³, respectively. The depositional fan of an old debris flow in the outlet of the gully can be subdivided into six layers. There are three debris flow deposits on left and two on the right side of the gully. Grain-size tests of sediments from the soil, gulley bed deposits, and the fresh and old debris flow deposits showed that high amounts of clay and fine gravel were derived from the soil in the source area whereas much of the gravel fraction were sourced from the gully bed deposits. Comprehensive analysis indicates that Zhatai gully is viscous debris-flow gully with moderate to high frequency and moderate to large magnitude debris flows. The risk of a debris flow disaster in Zhatai-gully is moderate and poses a potential threat to the planned hydroelectric dam. Appropriate engineering measures are suggested in the construction and protection of the planned hydroelectric station.     

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.     

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.     

Early Holocene High Magnitude Debris Flow Events and Environmental Change as Illustrated by the Moxi Platform, Hengduan Mountains, SW China

Introduction Landscape evolution in glaciated high mountains environment in southwestern China is undergoing the coupled processes of monsoon- induced denudation and active tectonic. The debris fan development, in particular sediment transfer, is most int…     

The siltation of debris flow behind check dam in the midstream of Bailong River

Siltation gradient and siltation length are important parameters for designing gravity check dams for debris flows, which directly affect the accuracy of estimates of interception capacity. At present, siltation gradient calculations are based primarily on empirical values, and range from 0.4 to 0.95 times the channel slope coefficient. The middle reaches of the Bailong River are one of the four areas in China that are most severely affected by debris flow hazards. Gravity dams are widely employed in this mountainous area. However, field studies of their capacity are lacking. In this paper, the operations of check dams were investigated. Based on field investigation results and theoretical analysis, calculations for siltation gradient, siltation length, and dam storage capacity are established. The impact of debris flow density, channel slope, and particle size weight percentages are discussed. The calculations show that the theoretical values for siltation gradient are consistent with measured values with 83.6% accuracy; and theoretical values of siltation length are consistent with measured values with 91.6% accuracy. The results of this research are an important reference for optimal height and spacing of dams, estimation of dam storage capacity, and disaster prevention.     

Effects of river flow velocity on the formation of landslide dams

Natural dams are formed when landslides are triggered by heavy rainfall during extreme weather events in the mountainous areas of Taiwan.During landslide debris movement, two processes occur simultaneously: the movement of landslide debris from a slope onto the riverbed and the erosion of the debris under the action of high-velocity river flow. When the rate of landslide deposition in a river channel is higher than the rate of landslide debris erosion by the river flow, the landslide forms a natural dam by blocking the river channel. In this study, the effects of the rates of river flow erosion and landslide deposition(termed the erosive capacity and depositional capacity, respectively) on the formation of natural dams are quantified using a physics-based approach and are tested using a scaled physical model.We define a dimensionless velocity index vde as the ratio between the depositional capacity of landslide debris(vd) and the erosive capacity of water flow(ve).The experimental test results show that a landslidedam forms when landslide debris moves at high velocity into a river channel where the river-flow velocity is low, that is, the dimensionless velocity index vde 54. Landslide debris will not have sufficient depositional capacity to block stream flow when the dimensionless velocity index vde 47. The depositional capacity of a landslide can be determined from the slope angle and the friction of the sliding surface, while the erosive capacity of a dam can be determined using river flow velocity and rainfall conditions. The methodology described in this paper was applied to seven landslide dams that formed in Taiwan on 8 August 2009 during Typhoon Morakot,the Tangjiashan landslide dam case, and the YingxiuWolong highway K24 landslide case. The dimensionless velocity index presented in this paper can be used before a rainstorm event occurs to determine if the formation of a landslide dam is possible.     

Identification of potential sites of debris flows in the upper Min River drainage,following environmental changes caused by the Wenchuan earthquake

The Wenchuan earthquake caused numerous landslides and collapses that provide abundant unconsolidated material for future mobilization as debris flows.Debris flows will be very active and cause considerable damage for some time in the affected area.Because of environmental changes related to the earthquake,many potentially dangerous debris flow gullies have yet to be identified.This paper selects the upper Min River from Yinxiu to Wenchuan as the study area,interprets the unconsolidated deposits,and discusses their relationship to distance from the fault.Then,applying that information and the values of other factors relating to debris flow occurrence,the locations of potential debris flows are analyzed by multi-factor comprehensive identification and rapid identification.The multi-factor comprehensive identification employs fuzzy matter-element extension theory.The volume of unconsolidated material in the study area is about 3.28 × 108 m3.According to the analysis by multi-factor comprehensive identification,47 gullies have a high probability for potential debris flow,8 gullies have a moderate probability,and 1 gully has a low probability.     

Influence of housing and urban development on debris flow flooding and deposition

Debris flows form deposits when they reach an alluvial fan until they eventually stop.However,houses located in the alluvial fan might affect the debris flow flooding and deposition processes.Few previous studies have considered the effects of houses on debris flow flooding and deposition.This study conducted model experiments and numerical simulations using the Kanako2D debris flow simulator to determine the influence of houses on debris flow flooding and deposition.The model experiments showed that when houses are present,the debris flow spreads widely in the cross direction immediately upstream of the houses,especially when the flow discharge is large or the grain size is small.Houses located in the alluvial fan also influence the deposition area.The presence of houses led to flooding and deposition damage in some places and reduced the damage in others.The simulation also demonstrated the influence of houses.Both the model experiment and the simulation showed that houses change the flooding and deposition areas.     

Erosion and transport mechanisms of mine waste along gullies

Mine waste debris flows continue to occur in China, and the disaster prevention and mitigation of these flows faces severe challenges since the mechanisms determining erosion and transport of mine waste along gullies are not yet fully understood. The erosion and delivery process of mine waste heaps was reproduced through flume experiments with the method based on field survey data of the Daxicha mine waste debris flow gully in the Xiaoqinling gold mining area. The results showed that the erosion and movement of mine wastes could be divided into three modes: minimal sediment movement, sediment sorting and delivery, and a large amount of sediment transfer. Moreover, there was an obvious amplification effect on peak discharge along with the formation and failure of temporary landslide dams during the erosion process. The correlation between the coefficient of peak discharge amplification and three dimensionless influencing factors, flume gradient, dimensionless volume, and dimensionless particle size, were comprehensively analyzed. An empirical formula for the coefficient of peak discharge amplification was proposed and verified based on 16 sets of experimental data. These preliminary results can provide a scientific reference for future research on disaster prevention and mitigation of mine waste debris flows.     

Monitoring and warning of landslides and debris flows using an optical fiber sensor technology

Landslides and debris flows are typical geo-hazards which occur in hilly or mountainous regions. Debris flows may result from landslides. Geotechnical instrumentation plays an important role in monitoring and warning of landslides and resulted debris flows. Traditional technologies for monitoring landslides and debris flows have certain limitations. The new optical fiber sensors presented in this paper can overcome those limitations. This paper presents two new optical fiber sensor systems: one is the Fiber Bragg Grating (FBG)-based in-place inclinometer for monitoring landslides and the other is the FBG-based column-net system for monitoring debris flows. This paper presents the calibration results of FBG-based in-place inclinometers in laboratory. It is found that the calibration results are in good agreement with theoretical results. Both the FBG-based in-place inclinometers and the FBG-based column-net system have been installed at a site in Weijiagou valley, Beichuan County, Sichuan Province of China. Some preliminary results have been obtained and reported in the paper. The advantages of the FBG monitoring systems and their potential applications are also presented.     

Risk assessment of highways affected by debris flows in Wenchuan earthquake area

Since the Wenchuan earthquake in China on May 12^th, 2008, highways in earthquake-affected areas have been frequently interrupted by debris flows. We analyzed the hazard effect modes and damage processes along highways and developed three key indexes, scale of debris flows, deposits on highways and river blockage, to describe quantitatively the highway disasters. By combining the empirical methods and the actual terrain conditions, we proposed new methods to determine the value of hazard indexes. In addition, we used the economic value and resistance of highway as vulnerability assessment indexes, then determined the specific subindexes for the subgrade, bridges and culverts, and developed a way for the quantified vulnerability zoning. Moreover, we proposed the assessment and mapping methods for highway risk. The risk is described into 5 grades: extremely low risk, low risk, middle risk, high risk and extremely high risk. We applied these methods in a case study carried out on provincial highway S303 from Yingxiu Town to Wolong Town, in Wenchuan County. Analysis of debris flow risk for the whole highway, showed that the total length of highway in extremely low risk area was 28.26 km, 4.83 km in low risk area, 8.0 km in middle risk area, 3.65 km in high risk area, and 3.06 km in extremely high risk area. The assessment results are consistent with the field survey data which reflected the disaster situation. This risk method can be used objectively to evaluate the debris-flow risk along highways, and is useful for highway reconstruction in mountainous areas suffering from active debris flows.     

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.     

Mutual Interference of Two Debris Flow Deposits Delivered in a Downstream River Reach

We investigate experimentally the depositions of two contiguous debris flows flowing into a main river reach.The aim of the present experimental research is to analyze the geometry and the mutual interactions of debris flow deposits conveyed by these tributaries in the main channel.A set of 19 experiments has been conducted considering three values of the confluence angle,two slopes of the tributary,and three different triggering conditions（debris flows occurring simultaneously in the tributaries,or occurring first either in the upstream or in the downstream tributary）.The flow rate along the main channel was always kept constant.During each experiment the two tributaries had the same slope and confluence angle.The analysis of the data collected during the experimental tests indicates that the volume of the debris fan is mainly controlled by the slope angle,as expected,while the shape of the debris deposit is strongly influenced by the confluence angle.Moreover,in the case of multiple debris flows,the deposit shape is sensitive to the triggering conditions.Critical index for damming formation available in literature has been considered and applied to the present case,and,on the basis of the collected data,considerations about possible extension of such indexes to the case of multiple confluences are finally proposed.     

中朝板块南缘的滑塌堆积及其构造环境

陕西商洛及河南卢氏、栾川地区上元古界和下古生界地层中发育有滑塌堆积物,其主要类型包括岩崩、滑动、滑塌和沉积物重力流,其中后者主要见到碎屑流、颗粒流和浊流沉积。它们多从下到上构成了由粗到细由浅水到深水的退积序列,呈海相扇三角洲沉积在晚元古代冰川边缘及早古生代碳酸盐台地边缘的两种大陆斜坡上,其沉积特征反映了中朝板块南缘从晋宁期到早加里东期呈强烈抗张断陷的大陆边缘环境。     

Evidence of the formation of landslide-dammed lakes in the Zagros Mountains range,Iran

Alternation of high and low resistance sedimentary beds,active tectonics,large rivers,and slope erosion in valleys consequently resulted in landslide in dammed lakes within the Zagros range.This study presents the results in the analysis of geological and topographic data,satellite imageries,morphotectonics and hydrodynamics of drainage networks about the landslide dammed lakes.There are four landslides in central Zagros(Zagros FoldThrust Belt,ZFTB)which have formed five dammed lakes named,Seymareh,Jaidar,Shur,Shimbar,and Godar.According to the results,damming landslides occurred in the active-tectonic regions on the slopes of anticlines and in valleys with undercutting effects of rivers on their slopes consisting of alternations of loose and resistant beds.The studied landslide dams in narrow valleys are formed as a result of blocking river by sliding debris slopes and rock slides.This study also indicates the formation of Jaidar and Godar in one stage and the presence of Lake Terrace sequence in Seymareh,Shur and Shimbar lakes.The observed sequences of terrace formation in these lakes are caused by four Seymareh Landslides followed by the three-stage excavation of Shur andShimbar lakes.     

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.