RELATION BETWEEN PRECIPITATION AND INITIATION OF DEBRIS FLOWS IN THE JIANGJIA RAVINE,YUNNAN PROVINCE, SOUTHWEST CHINA

1 INTRODUCTION In the watershed of the Jiangjia Ravine, the frequency of occurrence of rainstorms which can mobilize debris flows is high, and there are abundant unconsolidated materials deposited in the upstream area, these resulted in frequent eruption …     

新疆泥火山发育区场地震陷灾害机理探讨

粒度组分是影响黄土震陷的重要因素之一.通过采集新疆艾其沟泥火山泥质碎屑喷出样品和泥火山发育区域出露的沉积剖面泥质碎屑样品,利用激光粒度分析测试的实验方法对样品进行粒度测试,分别对样品中值(Md)、均值(Mz)、标准偏差(σφ)、偏度(SK1)、峰度(KG)等粒度参数进行分析.研究结果显示:艾其沟泥火山泥质沉积物粒度组分...     

Possible effect of ENSO on annual sediment discharge of debris flows in the Jiangjia Ravine based on Morlet wavelet transforms

The multi-time-scale structures of an annual sediment discharge series of debris flow in the Jiangjia Ravine and the Southern Oscillation index are analysed using the method of Morlet wavelet transformations. The possible effects of E1 Nirio episodes on the annual sediment discharge are discussed by comparing the period variations of ENSO and the discharge. The results show that the annual sediment discharge series of debris flow is related to E1 Nifio episodes. Generally, the annual sediment discharge of debris flow is less than usual during an E1 Nifio episode and debris flow is less active. On the contrary, the annual sediment discharge of debris flows is greater than usual during a La Nifia episode and debris flows are more frequent. There is a relationship between the annual sediment discharges of debris flow in the Jiangjia Ravine and the summer Southern Oscillation index, with both having quasi-periodic variations of 2 and 5-6 years.     

大渡河长河坝水电站区域泥石流特征及危险性评价

2009年7月23日,四川省甘孜州康定县舍联乡长河坝水电站施工区响水沟发生特大泥石流灾害,造成重大人员伤亡和财产损失。响水沟是大渡河上游段右岸一级支沟,近百年来泥石流活动不明显。由于之前近一个月的连续降雨,沟谷内的不稳定堆积物已基本处于饱水状态,当日的强降雨直接诱发了此次泥石流灾害。本文根据现场调查资料,就该沟泥石流的形成条件、灾害成因及特征进行分析,并应用灰色系统理论关联分析方法和沟谷泥石流危险度计算分析综合评价其危险性。结果表明,在极端气象条件下,响水沟仍有可能发生中等至大规模暴雨型粘性泥石流。     

Real‐time measurement and preliminary analysis of debris‐flow impact force at Jiangjia Ravine,China

Impact forces associated with major debris flows (Jiangjia Ravine, China, August 25, 2004) were recorded in real time by a system consisting of three strain sensors installed at different flow depths. This provides the first real‐time and long‐duration record of impact forces associated with debris flows. A comprehensive approach including low‐pass filtering and moving average methods were used to preprocess the recorded signals. The upper limit of impact frequency in the debris flows was estimated at 188?66 Hz under the assumption that only coarse grains cause effective impact loadings. Thus, a low‐pass filter with a 200 Hz cut‐off frequency was needed to denoise the original data in order to extract the impact force. Then the moving average method was applied to separate long‐term and random components of the filtered data. These were interpreted as, respectively, the fluid pressure and grain impact loading. It was found that the peak grain impacts at different depths were non‐synchronous within the debris flows. The impact loadings were far greater than, and not proportional to the fluid pressures. Analysis of the impact force of 38 debris flow surges gives an empirical value for the ratio of the hydrodynamic pressure to the momentum flow density, i.e. the product of debris‐flow density and mean velocity square, which provides a very valuable basis for understanding debris flow dynamics and designing debris flow management systems. Copyright © 2011 John Wiley & Sons, Ltd.     

Debris-flow volumes in northeastern Italy: Relationship with drainage area and size probability

A dataset of 809 debris flows that occurred in 537 basins in mountainous areas of northeastern Italy between the mid-19th century and 2015 is collected and analyzed. A remarkable increase in the number of events is observed in the last four decades and is mainly ascribed to more systematic data collection. The correlation between debris-flow volume and drainage basin area is obtained assuming a power-law relationship. The exponent of the power-law curve at the 50th percentile (0.67 ± 0.02) indicates negative allometry, which means that basin area grows out of proportion to debris-flow volume. In contrast, the exponents at the 98th and 99th percentiles are close to one, implying that debris-flow volumes grow linearly with basin areas. The isometric relationship between the largest debris flows and the corresponding basin areas is due to the enhanced debris supply provided by the activation of widespread sediment sources, the extent of which is proportional to the basin size. The probability density function of debris-flow volume for a subset of the collected dataset is calculated using the kernel density estimation function, which permits estimating the probability of the occurrence of debris flows that exceed a given threshold volume. The comparison with debris flows in other hydroclimatic regions shows that, although debris-flow volumes in northeastern Italy may attain high values, they are often exceeded by debris flows that occur in seismically active regions and/or are triggered by more intense rainstorms. © 2018 John Wiley & Sons, Ltd.     

Grain-energy release governs mobility of debris flow due to solid–liquid mass release

Debris flows often exhibit high mobility, leading to extensive hazards far from their sources. Although it is known that debris flow mobility increases with initial volume, the underlying mechanism remains uncertain. Here, we reconstruct the mobility–volume relation for debris flows using a recent depth-averaged two-phase flow model without evoking a reduced friction coefficient, challenging currently prevailing friction-reduction hypotheses. Physical experimental debris flows driven by solid–liquid mass release and extended numerical cases at both laboratory and field scales are resolved by the model. For the first time, we probe into the energetics of the debris flows and find that, whilst the energy balance holds and fine and coarse grains play distinct roles in debris flow energetics, the grains as a whole release energy to the liquid due to inter-phase and inter-grain size interactions, and this grain-energy release correlates closely with mobility. Despite uncertainty arising from the model closures, our results provide insight into the fundamental mechanisms operating in debris flows. We propose that debris flow mobility is governed by grain-energy release, thereby facilitating a bridge between mobility and internal energy transfer. The initial volume of debris flow is inadequate for characterizing debris flow mobility, and a friction-reduction mechanism is not a prerequisite for the high mobility of debris flows. By contrast, inter-phase and inter-grain size interactions play primary roles and should be incorporated explicitly in debris flow models. Our findings are qualitatively encouraging and physically meaningful, providing implications not only for assessing future debris flow hazards and informing mitigation and adaptation strategies, but also for unravelling a spectrum of earth surface processes including heavily sediment-laden floods, subaqueous debris flows and turbidity currents in rivers, reservoirs, estuaries, and ocean. © 2020 John Wiley & Sons, Ltd.     

EXPERIMENTAL STUDY ON EQUILIBRIUM CONCENTRATION OF DEBRIS FLOWS

Discussion open until 2002.EmunmL SwrY ONEQUII.thare CONCENTRATION OF DEBRIS rrOWSBin YU1AaSTsiCTThe paPe PresentS experimntal study of debris flows. The equiMm concentIation of solidparticle in the now is a hahon of the energy slope, density of solid Particle and kinetic ffichonange of paxtiles. The kinhc forhon angle is a funhon of intemal ffichon angle, the cOnCentrationof solid phocles and the mtalmum POssible concewhon. TO deteImin th6 hahon between thekinetic fficho…     

Hydraulic,physical and rheological characteristics of rain‐triggered lahars at Semeru volcano,Indonesia

Lahars (volcanic debris flows) have been responsible for 40% of all volcanic fatalities over the past century. Mount Semeru (East Java, Indonesia) is a persistently active composite volcano that threatens approximately one million people with its lahars and pyroclastic flows. Despite their regularity, the behaviour and the propagation of these rain‐triggered lahars are poorly understood. In situ samples were taken from lahars in motion at two sites in the Curah Lengkong River, on the southeast flank of Semeru, providing estimates of the particle concentration, grain size spectrum, grain density and composition. This enables us to identify flow sediment from three categories of lahars: (a) hyperconcentrated flow, (b) non‐cohesive, clast‐ and matrix‐supported debris flow, and (c) muddy flood. To understand hyperconcentrated flow sediment transport processes, it is more appropriate to sample the active flows than the post‐event lahar deposits because in situ sampling retains the full spectrum of the grain‐size distribution. Rheometrical tests on materials sampled from moving hyperconcentrated flows were carried out using a laboratory vane rheometer. Despite technical difficulties, results obtained on the <63, <180, and <400 µm fractions of the sampled sediment, suggest a purely frictional behaviour. Importantly, and contrary to previous experiments conducted with monodisperse suspensions, our results do not show any transition towards a viscous behaviour for high shear rates. These data provide important constraints for future physical and numerical modelling of lahar flows. Copyright © 2010 John Wiley & Sons, Ltd.     

Experimental investigation of the effects of shrub filter strips on debris flow trapping and interception

Ecological engineering plays an increasingly significant role in mountain hazard control, but the effect of species selection and arrangement (e.g., row spacing and stem spacing) on debris flow suppression is still unclear. To further understand the interception efficiency of shrub arrangement parameters on debris flow and explore the difference with slow hydraulic erosion, sixteen sets of small-scale flume experiments with different stem and row spacings were done to study the effects of shrubs on debris flow severity, flow rate, velocity, and particle size. The results suggest that, for a dilute debris flow, sediment interception effectiveness (27.4%–60.9%) decreases gradually as stem spacing increases. Moreover, as row spacing increases, flow velocity reduction (34.4%–44.9%) and flow reduction (18.5%–47.4%) gradually decrease; and the bulk density reduction (0.5%–5.3%) and sediment interception increase initially and then decrease. In contrast, for a viscous debris flow, the flow reduction, flow velocity reduction, and sedimentation interception decrease gradually as the stem spacing increases. As row spacing increases, the flow velocity reduction, flow reduction, and sediment interception all increase initially and then decrease. A formula for the flow velocity of dilute debris flow after the filter strip was derived based on the energy conservation law and Bernoulli's equation, confirming that debris flow movement is closely related to the degree of vegetation cover. This research strengthens the current understanding of the effectiveness of vegetation in debris flow disaster prevention and control and can guide practical applications.     

Particle size analysis of the sediment suspended in a proglacial stream: Glacier De tsidjiore Nouve,Switzerland

Hourly, at-a-point samples of suspended sediment taken from the outflow stream of Glacier de Tsidjiore Nouve, Switzerland, over a 60 day sampling period (n = 1440) are shown to be dominantly composed of silt-sized particles. Particle size, SEM, and XRD analyses indicate a subglacial provenance for the suspended sediment. Temporal variations in particle size and sorting correspond poorly to fluctuations in water discharge, being dominated by erratic hour-to-hour fluctuations and clockwise hysteresis over diurnal flow events. Examination of grain size and sorting dynamics over snowmelt- and icemelt-related ablation events, during precipitation events, and during glacier drainage events enables some inferences to be drawn regarding sediment source areas and supply regimes. We conclude that although the bulk of the suspended sediment in the proglacial stream of Glacier de Tsidjiore Nouve is derived directly from subglacial sources (with occasional contributions from the valley train during rapid snowmelt and heavy rainfall periods), a portion of the suspended load undergoes intermittent ‘flush-fall’ transfer through the proglacial zone, which acts as a sediment source during rising flows and as a sink during periods of waning flow.     

The downstream gradation of particle sizes in the Squamish river,British Columbia

The exponential downstream decline in particle size predicted by Sternberg (1875) is examined within a highly active alpine fluvial environment, the gravel-bed Squamish River in southwestern British Columbia. Transect survey procedures for sampling within the coarse alluvial gravels are described in detail. Downstream trends for various particle size statistics, plotted with distance from a major sediment source, are considered according to two scenarios. Evaluation of overall trends indicates that the downstream expression derived by Sternberg (1875) is not the most appropriate; rather, curves are better described by power functions. This reflects a very rapid decline in particle size immediately downstream of the major sediment source. Secondly, evaluation of downstream trends in relation to channel planform determines that exponential functions with different coefficients describe adequately relations within individual planform reaches. There is insufficient evidence to suggest which of these two scenarios may be appropriate. Several lines of evidence suggest that the trends found are more the result of selective sediment transport phenomena than of abrasion, a condition attributed to channel competence within a system dominated by sediment-supply conditions.     

Mineral composition and particle size distribution of river sediment and loess in the middle and lower Yellow River

Hyperconcentrated flows often occur in the middle and lower Yellow River(MLYR)and its tributaries,within which the main sediment source originates from the Loess Plateau of China due to serious water erosion.Little is known about the properties of river sediment that is transported by hyperconcentrated flows,particularly with respect to the mineral composition and size distribution.Samples of sediment and loess were collected in the northern,middle,and southern Loess Plateau and the mainstream and tributaries of the MLYR.A total of 18 loess samples and 24 river sediment samples were analyzed to determine their sediment size distribution and mineral composition.The bottom loess samples reflected the original sedimentary features of the Loess Plateau,and the median particle size reduced,and the clay content increased from the north to the south of the study region.The surface loess has been weathered under the action of wind and rainfall,and the clay particle content in the surface loess samples was higher than that in the undisturbed bottom loess.Erosion of the surface soil due to rainfall and surface runoff means that fine particles(mostly clay)have been washed away.The median diameter of surface loess particles was a little larger than that of the bottom loess particles where water erosion dominates.The particle size became coarser with increasing distance from the estuary in the MLYR,which reflects depositional sorting in the river channel.Significant logarithmic relations were found between the median diameter of the sediment particles and the i)non-clay mineral content and ii)clay mineral content.Thus,clay and non-clay mineral compositions can be conveniently estimated from the particle size distribution.     

Response of nephelometric turbidity to hydrodynamic particle size of fine suspended sediment

Turbidity is used as a surrogate for suspended sediment concentration (SSC), and as a regulatory tool for indicating land use disturbance and environmental protection. Turbidity relates linearly to suspended material, however, can show non-linear responses to particulate organic matter (POM), concomitant with changes in particle size distribution (PSD). In the paper the influence of ultra-fine particulate matter (UFPM) on specific turbidity and its association with POM in suspended sediment are shown for alpine rivers in the Southern Alps of New Zealand. The approach was two-fold: a field-based investigation of the relations between SSC, POM, and turbidity sampled during event flow; and experimental work on hydrodynamic particle size effects on SSC, POM, PSD, and turbidity. Specific turbidity changes over event flow and are sensitive to increasing proportional amounts of sand, UFPM, and POM in suspension. Furthermore, the UFPM is the size fraction (<6 μm) where POM increases. The implications of the current study are that the slopes of turbidity-SSC relations are undesirable in locations that may be dominated by cyclic release of POM or distinct pulses of fine-grained material. At locations where the turbidity-SSC slopes approximate 2, the POM proportion is usually <10% of the total suspended load. However, when turbidity-SSC slopes are <1 this is likely caused by high amounts of side-scatter from UFPM concomitant with higher proportions of POM. Thus, the use of turbidity as a proxy for determining SSC may have serious consequences for the measurement of representative suspended sediment data, particularly in locations where POM may be a significant contributor to overall suspended load.     

Erosion and deposition by debris flows at Mt Thomas,North Canterbury,New Zealand

The grass-covered slopes on the southern flank of Mt Thomas, an upfaulted block of highly sheared sandstone and argillite 40 km NW of Christchurch, New Zealand, are presently undergoing severe erosion by a combination of mass-wasting processes. Gully erosion, soil slips, and debris flows have carved out a number of steep, deeply incised ravines, from which coarse debris is transported (primarily by debris flows) to alluvial fans below. Geologic and historical evidence indicates that debris flows have been episodically active here for at least the last 20,000 years and have been the dominant process in fan building. This demonstrates that catastrophic geomorphic processes, rather than processes acting at relatively uniform rates, can be dominant in humid-temperate areas as well as in arid and semi-arid regions. In April 1978, debris flows were triggered in one of two unstable ravines in the Bullock Creek catchment by a moderate intensity, long duration rainstorm with a return period in excess of 20 years. Surges of fluid debris, moving at velocities up to 5 m/s, transported a dense slurry of gravel, sand, and mud up to 3·5 km over a vertical fall of 600 m. Deposition on the alluvial fan occurred when the flows left the confines of an entrenched fan-head channel and spread out as a 0·16 km² sheet averaging 1·2 m thick. In all, 195,000 m³ were deposited, roughly a third of that being reworked sediments from the head of the fan. Sediment yield from this one event would be equivalent to several thousand years worth of erosion at average sediment discharge rates for small South Island mountain catchments. Samples of viscous fluid debris during surges contained up to 84 per cent solids, composed of 70 per cent gravel, 20 per cent silt, and 4 per cent clay. Fluid density of the material ranged between 1·95 and 2·13 g/cm³, and it was extremely poorly sorted. Between surges the fluid was less viscous, less dense, and unable to carry gravel in suspension. Severe fan-head entrenchment of the stream channel (approximately 10 m in less than 24 hours) was accomplished by the erosive action of the surges. Tectonic uplift of the Mt Thomas block and the weak, crushed condition of the bedrock appear to be ultimately responsible for the catastropic erosion of slopes in the Bullock Creek catchment. However, forest clearing within the last few centuries appears to have greatly increased the rate of mass wasting and gully erosion on these slopes.     

A Novel Approach for Estimating Debris Flow Velocities from Near-Field Broadband Seismic Observations

On August 7^th, 2010, Sanyanyu and Luojiayu debris flows triggered by a heavy rain have lashed Zhouqu City around midnight, leading to catastrophic destruction which killed 1 765 people and resulted in enormous economic loss. The ZHQ Seismic Station is located approximately 170 m west of the outlet of the Sanyanyu Gully. The seismometer deployed at the seismic station started recording seismic signals of ever-enlarging amplitude around 10 minutes before the debris flow rushed out of the Sanyanyu Gully, showing ever approaching seismic source, i.e. the debris flow. In this study, we analyze this seismic event and propose an inversion algorithm to estimate the velocity of the debris flow by searching the best-fitting pairs of envelopes in the synthetic seismograms and the corresponding field seismic records in a least-square sense. Inversion results reveal that, before rushing out of the outlet, the average velocity of the debris flow gradually increased from 6.2 m/s to 7.1 m/s and finally reached 15 m/s at approximately 0.5 km above the outlet and kept this value since then. Obviously, the ever-increasing velocity of the debris flow is the key factor for the following disasters. Compared with other studies, our approach can provide the velocity distribution for the debris flow before its outbreak; Besides, it has the potential to provide technological support for a better understanding of the disaster process of a debris flow.     

Predicting post-fire debris flow grain sizes and depositional volumes in the Intermountain West,United States

Post-fire debris flows represent one of the most erosive consequences associated with increasing wildfire severity and investigations into their downstream impacts have been limited. Recent advances have linked existing hydrogeomorphic models to predict potential impacts of post-fire erosion at watershed scales on downstream water resources. Here we address two key limitations in current models: (1) accurate predictions of post-fire debris flow volumes in the absence of triggering storm rainfall intensities and (2) understanding controls on grain sizes produced by post-fire debris flows. We compiled and analysed a novel dataset of depositional volumes and grain size distributions (GSDs) for 59 post-fire debris flows across the Intermountain West (IMW) collected via fieldwork and from the literature. We first evaluated the utility of existing models for post-fire debris flow volume prediction, which were largely developed for Southern California. We then constructed a new post-fire debris flow volume prediction model for the IMW using a combination of Random Forest modelling and regression analysis. We found topography and burn severity to be important variables, and that the percentage of pre-fire soil organic matter was an essential predictor variable. Our model was also capable of predicting debris flow volumes without data for the triggering storm, suggesting that rainfall may be more important as a presence/absence predictor, rather than a scaling variable. We also constructed the first models that predict the median, 16th percentile, and 84th percentile grain sizes, as well as boulder size, produced by post-fire debris flows. These models demonstrate consistent landscape controls on debris flow GSDs that are related to land cover, physical and chemical weathering, and hillslope sediment transport processes. This work advances our ability to predict how post-fire sediment pulses are transported through watersheds. Our models allow for improved pre- and post-fire risk assessments across diverse ranges of watersheds in the IMW.     

Inviscid behaviour of fines-rich pyroclastic flows inferred from experiments on gas-particle mixtures

Experiments were carried out on granular flows generated by instantaneous release of gas-fluidised, bidisperse mixtures and propagating into a horizontal channel. The mixture consists of fine (< 100 μm) and coarse (> 100 μm) particles of same density, with corresponding grain size ratios of ∼ 2 to 9. Initial fluidisation of the mixture destroys the interparticle frictional contacts, and the flow behaviour then depends on the initial bed packing and on the timescale required to re-establish strong frictional contacts. At a fines mass fraction (α) below that of optimal packing (∼ 40%), the initial mixtures consist of a continuous network of coarse particles with fines in interstitial voids. Strong frictional contacts between the coarse particles are probably rapidly re-established and the flows steadily decelerate. Some internal friction reduction appears to occur as α and the grain size ratio increases, possibly due to particle rolling and the lower roughness of internal shear surfaces. Segregation only occurs at large grain size ratio due to dynamical sieving with fines concentrated at the flow base. In contrast, at α above that for optimal packing, the initial mixtures consist of coarse particles embedded in a matrix of fines. Flow velocities and run-outs are similar to that of the monodisperse fine end-member, thus showing that the coarse particles are transported passively within the matrix whatever their amount and grain size are. These flows propagate at constant height and velocity as inviscid fluid gravity currents, thus suggesting negligible interparticle friction. We have determined a Froude number of 2.61 ± 0.08 consistent with the dam-break model for fluid flows, and with no significant variation as a function of α, the grain size ratio, and the initial bed expansion. Very little segregation occurs, which suggests low intensity particle interactions during flow propagation and that active fluidisation is not taking place. Strong frictional contacts are only re-established in the final stages of emplacement and stop the flow motion. We infer that fines-rich (i.e. matrix-supported) pyroclastic flows propagate as inviscid fluid gravity currents for most of their emplacement, and this is consistent with some field data.     

Bed scour by debris flows: experimental investigation of effects of debris‐flow composition

Debris flows can grow greatly in size by entrainment of bed material, enhancing their runout and hazardous impact. Here, we experimentally investigate the effects of debris‐flow composition on the amount and spatial patterns of bed scour and erosion downstream of a fixed to erodible bed transition. The experimental debris flows were observed to entrain bed particles both grain by grain and en masse, and the majority of entrainment was observed to occur during passage of the flow front. The spatial bed scour patterns are highly variable, but large‐scale patterns are largely similar over 22.5–35° channel slopes for debris flows of similar composition. Scour depth is generally largest slightly downstream of the fixed to erodible bed transition, except for clay‐rich debris flows, which cause a relatively uniform scour pattern. The spatial variability in the scour depth decreases with increasing water, gravel (= grain size) and clay fraction. Basal scour depth increases with channel slope, flow velocity, flow depth, discharge and shear stress in our experiments, whereas there is no correlation with grain collisional stress. The strongest correlation is between basal scour and shear stress and discharge. There are substantial differences in the scour caused by different types of debris flows. In general, mean and maximum scour depths become larger with increasing water fraction and grain size, and decrease with increasing clay content. However, the erodibility of coarse‐grained experimental debris flows (gravel fraction = 0.64) is similar on a wide range of channel slopes, flow depths, flow velocities, discharges and shear stresses. This probably relates to the relatively large influence of grain‐collisional stress to the total bed stress in these flows (30–50%). The relative effect of grain‐collisional stress is low in the other experimental debris flows (<5%), causing erosion to be largely controlled by basal shear stress. Copyright © 2016 John Wiley & Sons, Ltd.     

TEMPORAL AND SPATIAL DYNAMICS OF DIFFERENT PARTICLE SIZE OF SUSPENDED SEDIMENT IN THE LIU RIVER CATCHMENTS,CHINA

1 INTRODUCTION The particle size of sediment eroded from basins can provide basic information about erosion processes (Meyer et al., 1980), which can be divided into sheet wash sediment processes on hill slopes and fluvial sediment processes in rivers. In…