Estimation of debris flow discharge coefficient considering sediment concentration

A sabo dam has a purpose to block the path of debris flow. However, when overflow occurs, a sabo dam works as a weir, a vertical obstruction, where the fluid must flow over. Many empirical formulas and discharge coefficients for weirs relating flow depth to discharge have been proposed to calculate overflow discharges. However, only a few studies about overflow discharge coefficients are available in the case of debris flow. In this paper, experiments and numerical simulations were done to estimate debris flow discharge coefficients by considering the sediment concentration. In the numerical simulation, a complete overflow equation and a free overfall equation were implemented to calculate debris overflow discharges at a sabo dam. To determine the discharge coefficients for each equation, single factor regression analysis was used. Laboratory experiments were done to calibrate and to compare with the simulation. Study results showed that the discharge coefficients increase as the sediment concentration increases. This finding suggests debris flow discharge coefficients are derived to calculate the debris overflow discharges at a sabo dam.     

Hydraulic model tests for evaluating sediment control function with a grid-type Sabo dam in mountainous torrents

There are two kinds of Sabo dams in order to control sediment transport by debris flow and flash floods in mountainous area, which are closed and open-type＇s dams. In Japan, open-type＇s Sabo dams are constructed taking into account the continuity of sediment routing from upstream to downstream reach in a basin. A plan to construct a 20 m high grid-type Sabo dam which can capture a sediment volume of 400,000 m3 is proposed in the Amahata river basin in Japan. Hydraulic model tests are conducted to decide on the section for a dam （Section A, B） and the grid size such as clearance of vertical/horizontal bars for evaluating the plan. Several runs of flume tests are conducted and the sediment control function of the Sabo darn is discussed using several experimental data such as dimensionless sediment runoff rate from Sabo dam, temporal changes of bed profile and mean diameter and so on. It was found that sediment deposition in sediment storage area of Sabo dam was affected by curved channel, and that next the grid size of steel bars and thirdly the section of a dam was able to capture sediment in storage area of Sabo dam. Sediment was controlled well in the section B and in the grid size of 1.0×d95, and the problems related to sediment runoffafter sediment capturing in Sabo dam are pointed out.     

Experimental study on blocking and self-cleaning behaviors of beam dam in debris flow hazard mitigation

Blocking is one of the important features when a beam dam intercepts debris flow, while self-cleaning is another when managing suspended debris flow. Both features determine the debris flow control benefits of beam dam but the latter often is not considered in practical engineering design. In this paper, a series of specially designed flume experiments were done to simulate blocking and self-cleaning processes. The blocking ratio and deposition features were measured to contrast the blocking and self-cleaning performance before and after artificial self-cleaning. The experimental results reveal that the beam dam net opening, particle diameter of sediment, sediment concentration, and gradient of the channel are the main factors affecting blocking performance. A new criterion of blocking performance of beam dams that considers the interaction of multiple factors and can provide guidance to practical project design is proposed. For all three types of blocking, sediment deposited upstream of a beam dam can be effectively transported downstream by erosion from post-debris-flow floods, Self-cleaning performance is most efficient for temporary blocking, followed by partial-blocking, and total-blocking. The efficiency of self-cleaning largely depends on the change of the sediment deposit due to erosion. Finally, a discussion is given for the optimal design of an open-type check dam and the feasibility of synergistic effects of self-cleaning in combination with artificial cleaning. Some supporting artificial silt-cleaning should be implemented in practice. A beam dam will, thus, have more storage capacity with which to control the next debris flow event.     

Study on hydraulic characteristics of sabo dam with a flap structure for debris flow

The front part of the flow is very important and complex in the case of debris flow where there is an accumulation of large boulders. It is important to control or dampen the energy of the frontal part of a debris flow for the safety of the downstream area because the impact pressure of debris flow is much greater than that of clear fluid. The main objective of this study is to analyze the hydraulic characteristics of the proposed dam (i.e. closed-type dam with flap). The vertical pressure distribution of this type is compared with conventional dam types. In the experiments, the total pressure associated with major debris flows was recorded in real time by a system consisting of four dynamic pressure sensors installed on different types of dam. The results from experimental data clearly show that the dam with the flap has advantages of capturing the debris flow with large boulders and controls the total pressure by flow circulation due to presence of the flap structure compared to a closed-type dam without flap. Further-more, the empirical coefficients of hydrodynamic and solid collision models were proposed and com-pared with available coefficients.     

Dynamic response of debris flows impacting curved joint check dams

The dynamic impact force of debris flow on dams with a curved upstream face curved was investigated using laboratory experiments and a theoretical approach. Equations describing the impact force and maximum run-up height were derived. The experiments and theoretical considerations reveal that the impact force and maximum run-up height are mainly controlled by the Froude number (Fr) reduced by the cosine of the channel slope angle (cosα). Both the impact force and the maximum run-up height have a quadratic relation with the modified Froude number (Fr/(cosα)^0.5). The experimental data and the results of the theoretical approach are in good agreement, indicating that the theoretical approach can be used in practical applications. It is concluded that the comparison between the curved-joint dam and the more conventional sharp-joint dam shows no differences in the maximum impact force and run-up height for the same modified Froude number. With the sharp-joint dam, the peak values of the impact force are reached more quickly than with the curved-joint dam.     

Modeling the effect of sediment concentration on the flow-like behavior of natural debris flow

The rheological behavior of natural slurries consisting of fine-grained, reconstituted debris-flow deposits on pyroclastic terrains having different solid concentrations (ranging from 30 to 42%) has been investigated using a rotational rheometer equipped with a vane rotor system. Experiments were done by increasing the applied shear stress step by step; then a decreasing stress ramp was applied following the same shear stress levels. The slurry mixtures exhibit a typical yield-stress fluid behavior with a static yield stress larger than the dynamic yield stress. In the range of the shear rate corresponding to the flow-like behavior the slurry mixtures behave as a dilatant fluid at lower grain concentrations and as a pseudoplastic fluid in correspondence with the higher grain content, showing a strong discrepancy from the Bingham idealization. The rheological behavior is better interpreted by a Herschel-Bulkley model, whose rheological parameters strongly depend on the granular concentration. Therefore, a generalized Herschel-Bulkley model accounting for the bulk sediment concentration effect is proposed.     

Experimental study of debris flow caused by domino failures of landslide dams

The formation of landslide dams is often induced by earthquakes in mountainous areas.The failure of a landslide dam typically results in catastrophic flash floods or debris flows downstream.Significant attention has been given to the processes and mechanisms involved in the failure of individual landslide dams.However,the processes leading to domino failures of multiple landslide dams remain unclear.In this study,experimental tests were carried out to investigate the domino failure of landslide dams and the consequent enlargement of downstream debris flows.Different blockage conditions were considered,including complete blockage,partial blockage and erodible bed（no blockage）.The mean velocity of the flow front was estimated by videos.Total stress transducers（TSTs）and Laser range finders（LRFs） were employed to measure the total stress and the depth of the flow front,respectively.Under a complete blockage pattern,a portion of the debris flow was trapped in front of each retained landslide dam before the latter collapsed completely.This was accompanied by a dramatic decrease in the mean velocity of the flow front.Conversely,under both partial blockage and erodible bed conditions,the mean velocity of the flow front increased gradually downward along the sloping channel.Domino failures of the landslide dams were triggered when a series of dams（complete blockage and partial blockage） were distributed along the flume.However,not all of these domino failures led to enlarged debris flows.The modes of dam failures have significant impacts on the enlargement of debris flows.Therefore,further research is necessary to understand the mechanisms of domino failures of landslide dams and their effects on the enlargement of debris flows.     

Experimental analysis on the impact force of viscous debris flow

A miniaturized flume experiment was carried out to measure impact forces of viscous debris flow. The flow depth (7.2–11.2 cm), velocity (2.4–5.2 m/s) and impact force were recorded during the experiment. The impact process of debris flow can be divided into three phases by analyzing the variation of impact signals and flow regime. The three phases are the sudden strong impact of the debris flow head, continuous dynamic pressure of the body and slight static pressure of the tail. The variation of impact process is consistent with the change in the flow regime. The head has strong–rapid impact pressure, which is shown as a turbulent‐type flow; the body approximates to steady laminar flow. Accordingly, the process of debris flows hitting structures was simplified to a triangle shape, ignoring the pressure of the tail. In order to study the distribution of the debris flow impact force at different depths and variation of the impact process over time, the impact signals of slurry and coarse particles were separated from the original signals using wavelet analysis. The slurry's dynamic pressure signal appears to be a smooth curve, and the peak pressure is 12–34 kPa when the debris flow head hits the sensors, which is about 1.54 ± 0.36 times the continuous dynamic pressure of the debris flow body. The limit of application of the empirical parameter α in the hydraulic formula was also noted. We introduced the power function relationship of α and the Froude number of debris flows, and proposed a universal model for calculating dynamic pressure. The impact pressure of large particles has the characteristic of randomness. The mean frequency of large particles impacting the sensor is 210 ± 50–287 ± 29 times per second, and it is 336 ± 114–490 ± 69 times per second for the debris flow head, which is greater than that in the debris flow body. Peak impact pressure of particles at different flow depths is 40–160 kPa, which is 3.2 ± 1.5 times the impact pressure of the slurry at the bottom of the flow, 3.1 ± 0.9 times the flow in the middle, and 3.3 ± 0.9 times the flow at the surface. The differences in impact frequency indicate that most of the large particles concentrate in the debris flow head, and the number of particles in the debris flow head increases with height. This research supports the study of solid–liquid two phase flow mechanisms, and helps engineering design and risk assessment in debris flow prone areas. Copyright © 2015 John Wiley & Sons, Ltd.     

Threshold criterion for debris flow initiation in seasonal gullies

A series of flume experiments were done to investigate the effect of grain composition on the critical gradient and discharge of debris flows initiated in seasonal gullies. The results indicated that the critical gradient and discharge for debris flow initiation decrease initially, and then increase as the mass content of fine particles (<2 mm) increases. As the mass content of fine particles increases, the angle of repose, permeability of widely graded gravel soils, and the incipient motion conditions of the coarse grains in non-uniform sediments decrease at first, and then increase. The mass content of fine particles of all inflection points is the same. The theoretical model based on the combination of hydrodynamic force and shear stress is more applicable to the prediction of the critical gradient for debris flow initiation. The critical discharge model considering the effect of non-homogeneity of the soil and the size of coarse and fine grains provides a more accurate prediction of debris flow initiation than other models based on the mean diameter.     

Sensitivity analysis of influencing parameters on slit-type barrier performance against debris flow using 3D-based numerical approach

Extreme rainfall-induced debris flow can be catastrophic to an urban area,and installation of slit-type barriers can prevent such damage while minimizing negative impact on environments.However,the performance of slit-type barriers against debris flows remains poorly identified partly due to the innate complexity in interactions between debris flow and solid structure.This paper investigated the flow behaviors of debris affected by slit-type barriers using the computational fluid dynamics(CFD)method,in which the numerical model based on the volume of fluid method was verified using the physical modeling results.The sensitivity analysis was performed by building metamodels to determine the primary parameters influencing the barrier performance against debris flows among various variables,in which the effect of input properties and design parameters,particularly the soil concentration in fluidized debris,initial velocity and volume of debris,the barrier height,and the opening ratio,was evaluated from the perspectives of the flow energy reduction and debris trapping.The initial velocity and volume of debris were found to play a significant role in determining the debris flow characteristics.A decrease in the opening ratio in the channel primarily facilitated the energy reduction and trapping due to the reduced opening size.However,the barrier height exhibited a limited effect when the height was sufficiently high to block the debris flow volume.In addition,it was observed that the double barrier system effectively increased the energy reduction while keeping the benefit of open-type barrier.The developed simulation method and obtained results provide an effective tool and an insight that can contribute to an optimum design of the debris-flow barrier.     

The enormous Chillos Valley Lahar: an ash-flow-generated debris flow from Cotopaxi Volcano, Ecuador

 The Chillos Valley Lahar (CVL), the largest Holocene debris flow in area and volume as yet recognized in the northern Andes, formed on Cotopaxi volcano's north and northeast slopes and descended river systems that took it 326 km north–northwest to the Pacific Ocean and 130+ km east into the Amazon basin. In the Chillos Valley, 40 km downstream from the volcano, depths of 80–160 m and valley cross sections up to 337 000 m² are observed, implying peak flow discharges of 2.6–6.0 million m³/s. The overall volume of the CVL is estimated to be ≈3.8 km³. The CVL was generated approximately 4500 years BP by a rhyolitic ash flow that followed a small sector collapse on the north and northeast sides of Cotopaxi, which melted part of the volcano's icecap and transformed rapidly into the debris flow. The ash flow and resulting CVL have identical components, except for foreign fragments picked up along the flow path. Juvenile materials, including vitric ash, crystals, and pumice, comprise 80–90% of the lahar's deposit, whereas rhyolitic, dacitic, and andesitic lithics make up the remainder. The sand-size fraction and the 2- to 10-mm fraction together dominate the deposit, constituting ≈63 and ≈15 wt.% of the matrix, respectively, whereas the silt-size fraction averages less than ≈10 wt.% and the clay-size fraction less than 0.5 wt.%. Along the 326-km runout, these particle-size fractions vary little, as does the sorting coefficient (average=2.6). There is no tendency toward grading or improved sorting. Limited bulking is recognized. The CVL was an enormous non-cohesive debris flow, notable for its ash-flow origin and immense volume and peak discharge which gave it characteristics and a behavior akin to large cohesive mudflows. Significantly, then, ash-flow-generated debris flows can also achieve large volumes and cover great areas; thus, they can conceivably affect large populated regions far from their source. Especially dangerous, therefore, are snow-clad volcanoes with recent silicic ash-flow histories such as those found in the Andes and Alaska. Received: 28 April 1997 / Accepted: 19 August 1997     

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.     

Closure relations for mobile bed debris flows in a wide range of slopes and concentrations

Debris flows are flows of water and sediment driven by gravity that initiate in the upper part of a stream, where the slope is very steep, allowing high values of solid concentration (hyperconcentrated flows), and that stop in the lower part of the basin, which is characterized by much lower slopes and reduced speeds and concentrations. Modelling these flows requires mathematical and numerical tools capable of simulating the behavior of a fluid in a wide range of concentrations of the solid phase, spanning from hyperconcentrated flows to flows in the fluvial regime. According to a two-phase approach, the depth integrated equations of mass and momentum conservation for water and sediments, under the shallow water hypothesis, are employed to solve field problems related to debris flows. These equations require suitable closure relations that in this case should be valid in a very wide range of slopes. In the hypothesis of absence of cohesive material, we derived these closure relations properly combining the relative relations valid separately in the fluvial and in the hyperconcentrated regimes. In the intermediate regime, the shear stress is due to the combined effect of the deformation of the liquid phase (grain roughness turbulence) and of inter-particle collisions. Therefore, an approach based on the sum of the effects of the two causes has been proposed, combining the Darcy–Weisbach equation and the Bagnoldian grain-inertia theory.A similar treatment has been made for the transport capacity relations, combing the Bagnold expression of the collisional regime with a transport capacity monomial formula valid in the fluvial regime.The closure relations are expressed in non-dimensional form as a function of the Froude number, of the solid concentration, of the relative submergence, and of the slope.In order to test the closure relation, a set of experiments with mixtures of non-cohesive sediments and water have been carried out in a laboratory flume under steady uniform flow conditions, with different solid and liquid discharges and different grain size distributions. The closure equations are satisfactorily tested against experimental investigation.     

Variability in rainfall threshold for debris flow after the Chi-Chi earthquake in central Taiwan, China

The purpose of this study is to analyze variability in rainfall threshold for debris flow （critical rainfall for debris flow triggering） after the ML 7.3 Chi-Chi earthquake in central Taiwan in 1999. Two study sites with different geological conditions were surveyed in the earthquake area. Streambed surveys were conducted to continuously monitor debris flows between 1999 and 2006. During the 7-year study period, every debris flow event was identified, and the streambed characterized. Results show that the rainfall threshold for debris flow was remarkably lower just after the Chi-Chi Earthquake, but gradually recovered. To date, this rainfall threshold is still lower than the original level prior to the earthquake. This variability in rainfall threshold is closely related to the mount of sediment material in the initiation area of debris flow, which increased rapidly due to landslides resulting from the earthquake. With the increase in sediment material, the rainfall threshold was lowered severely during the first year following the Chi-Chi earthquake. However, heavy rainfalls mobilized the sediment material, causing debris flows and transporting sediment downstream. With the decrease in sediment material, the rainfall threshold recovered gradually over time. Furthermore, debris flows occurred only in the subbasins that had sufficient sediment material to cause significant movement. Hence, these results confirm that the sediment material in the initiation area of debris flow is a crucial component of the rainfall threshold for debris flow.     

GIS-based risk analysis of debris flow： an application in Sichuan, southwest China

Debris flow is a serious geologic hazard in China. It is estimated that nationally debris flows cause up to 2 billion RMB （250 million US$） in damages and 300-600 deaths and injuries annually. To mitigate debris flow hazards, it is necessary to map, model, and identify zones of debris flow hazards and vulnerability as to inform the local people about the potential risk with a geographic information system. This research presents a regional scale case study modeling debris flow risk （hazard and vulnerability） in Sichuan Province, Southwestern China. In this area, 3,290 debris flows have been identified and the spatial-temporal distribution and activity characteristics of them have been documented. Based on the available meteorological data, a Digital Elevation Model with the rate of 1：250,000 and a regional geological map, the 24-hr rainfall threshold （y） for debris flow occurrence is closely related （significant at 99% confidence level） to the index （x） defined using a geology factor （rock hardness： a） and a topographical factor （channel gradient： d） where y = 21 ＋ 10200 / x, in which x = 2.7 × e^a ＋ 1000 × d. The discipline is constructive in developing the rainfall threshold for debris flow activity in remote mountainous areas that lack data. For a given watershed, a four-level debris flow hazard map is developed by comparing the rainfall threshold to the design rainfall intensities with 50-, 20-, and 5-year average recurrence intervals, respectively. The degree of debris flow vulnerability is determined by the watershed socio-economic conditions. A four-class debris flow risk map, at the final phase of the research, is generated by combining debris flow hazards and vulnerability. With the debris flow risk assessment, the Sichuan Province is classified into the slight, moderate, severe and very severe regions, which accounts for 36%, 19%, 20% and 25% of total area respectively.     

Single-grain quartz OSL dating of debris flow deposits from Men Tou Gou,south west Beijing,China

Debris flows in the mountainous regions south west of Beijing, China occur frequently and often result in considerable mass movements with disastrous consequences for human life, infrastructure and agriculture. Obtaining chronological information on such events is important for the prediction of the return frequency of these debris flows, risk assessment and climate change research. In this project, we use quartz single-grain optically stimulated luminescence (OSL) methods to determine the burial ages of five debris flow samples from the Zhai Tang region ∼60 km west of Beijing. OSL characteristics were found to be acceptable despite the low inherent brightness of quartz extracted from these samples. Single-grain thermal transfer was determined to be negligible and beta dose recovery experiments were satisfactory. The quartz single-grain dose distributions strongly indicate that the samples were poorly bleached prior to deposition; relative over-dispersions are larger than 60%. Minimum age modelling indicates that all five samples were deposited within the past few hundred years, indicating that catastrophic debris flows are occurring under the historically-recent land-use pattern.     

Post-analysis simulation of the collapse of an open sabo dam of steel pipes subjected to boulder laden debris flow

The objective of the proposed method is to utilize a site investigation of a debris flow disaster and verify a real scale analysis to evaluate the impulsive load on an open sabo dam. The Nagiso debris flow disaster which occurred in Nagano in 2014, where damage caused by Typhoon Neogri was studied. The verification result of the site investigation demonstrated the weak components of the open Sabo dam experienced damage owing to the debris flow. A discrete element method is normally applied to a solid body to calculate an interaction function force with respect to the contact point between boulders and the dam. The numerical method initially concatenates elements that model the open Sabo dam. Moreover, the stiffness coefficient of flanges and coupling joints between pipes was expressed to utilize the sectional partition method to determine the structural characteristics. The method was improved to separate from the connecting elements beyond the boundary conditions. The debris flow model uses a water flow distribution model, and the debris flow flowed from 200 m upstream of the open sabo dam. Accordingly, the proposed method was examined to verify the primary cause of damage to the open sabo dam and used to reproduce the circumstances that evaluated the impulsive load occurrence mechanism in the case of a real disaster. In addition, the coupling joints between the hollow steel pipes utilized a ‘reproduction analysis’ for a real sabo dam and a ‘reinforced analysis’ for a reinforced sabo dam were applied to assess the weak point of the dam.     

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

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

鹅掌河泥石流对四川邛海影响的初步研究

沉积作用在湖泊的演化和消亡过程中至关重要,洪水泥石流的淤积作用对我国西南地区广泛分布的构造断陷湖泊是一个普遍的环境问题．通过调查鹅掌河泥石流发育背景,人类的活动对泥沙进入邛海的影响和对比1988年与2003 年邛海水下地形图得出:在近30年的时间内,鹅掌河洪水和泥石流改沿固定河堤流入邛海,在湖底形成浊流,将更大量泥沙带到湖中．浊流在鹅掌河河口的水下扇陡坡上形成一水下冲沟,在湖底沉积区形成一条长2 km的水下堤．计算发现, 鹅掌河泥石流带入邛海的泥沙远大于一般土壤侵蚀产沙量,其中以1996,1997和1998三年泥石流输入的泥沙量最多．鹅掌河洪水和泥石流对邛海的影响以及水下地形、湖泊基本特征和湖泊环境的变化,应引起人们对邛海保护的高度关注．     

Topographic reflection of the Socompa debris avalanche,Chile

One of the most remarkable features of the exceptionally well preserved 26 km³ Socompa debris avalanche deposit is the evidence for topographically driven secondary flow. The avalanche formed by sector collapse of Socompa stratovolcano and spread 40 km across a pre-existing basin, forming a sheet of ∼50 m average thickness. As the avalanche impinged on the western and northern margins of the basin, it was reflected back, forming a secondary flow that continued to travel 15 km down a gentle slope at an oblique angle to the primary flow, the front of the return wave being preserved frozen on the surface of the deposit as a prominent escarpment. Satellite images, aerial photos, digital elevation models and field observations were used to reconstruct the sequence of events during avalanche emplacement, and in particular during secondary flow. The avalanche sheet was divided into distinct terrane groups, each believed to have experienced a particular strain history during emplacement. Evidence for avalanche reflection includes clearly recognizable secondary slide masses, sub-parallel sets of curvilinear shear zones, headwall scarps separating the (primary) levée from the secondary terranes, extensional jigsaw breakup of surface lithologies during return flow, and cross cutting, or deflection, of primary flow fabrics by secondary terranes. Reflection off the basin margin took place in an essentially continuous manner, most major return motions being simultaneous with, or shortly following, primary flow. The secondary flow occurred as a wave that swept obliquely across the primary avalanche direction, remobilizing the primary material, which was first compressed, then stretched, as it passed over and rearward of the wave front. As return flow occurred, surface lithologies were rifted in a brittle manner, and the slabs were sheared pervasively as they glided and rotated back into the basin; some sank into the more fluidal interior of the avalanche, which drained out into a prominent distal lobe. Extension by factors of up to 1.8 took place during return flow. Secondary flow took place on slopes of only a few degrees, and the distal lobe flowed 8 km on a slope of ∼1°. Overall the avalanche is inferred to have slid into place as a fast-moving sheet of fragmental rock debris, with a leading edge and crust with near-normal friction and an almost frictionless, fluidal interior and base. The avalanche emplacement history deduced from field evidence is consistent with the results of a previously published numerical model of the Socompa avalanche.