Velocity attenuation of debris flows and a new momentum-based load model for rigid barriers

Effective design of mitigation measures against debris flow hazards remains a challenging geotechnical problem. At present, a pseudo-static approach is commonly used for the calculation of impact load acting on a rigid debris-resisting barrier. The impact load is normally calculated based on the maximum velocity observed in the transportation zone under free-field conditions without considering debris-barrier interaction. In reality, the impact load acting on a barrier varies with the change of debris momentum flux but this is seldom considered in barrier design. To provide a scientific basis for assessing debris momentum flux during impact, this paper presents results from a study of debris-barrier interaction using physical flume modelling. This study showed that, following the first stage of impact, the accumulated debris behind a barrier formed a stationary zone and caused the remaining debris to slow down in a run-up process. In the experiments, the peak debris momentum was 30 % lower compared to that observed under free-field conditions. A new momentum-based model was developed to take into account attenuation of momentum flux for predicting debris impact load on rigid barriers. The new rationalised model was assessed using data from the notable Yu Tung Road debris flow in Hong Kong. The assessment showed that the design bending moment at the base of the barrier wall could be reduced more than 30 % using the proposed model, compared with the current design approach. The adoption of the proposed model could offer a new opportunity for practitioners to optimise the design of rigid barriers.     

Landscape and sedimentary response to catastrophic debris avalanches, western Taranaki, New Zealand

Catastrophic volcanic debris avalanches reshape volcanic edifices with up to half of pre-collapse cone volumes being removed. Deposition from this debris avalanche deposit often fills and inundates the surrounding landscape and may permanently change the distribution of drainage networks. On the weakly-incised Mt. Taranaki ring-plain, volcanic debris avalanche deposits typically form a large, wedge shape (in plan view), over all flat-lying fans. Following volcanic debris avalanches a period of intense re-sedimentation commonly begins on ring-plain areas, particularly in wet or temperate climates. This is exacerbated by large areas of denuded landscape, ongoing instability in the scarp/source region, damming of river/stream systems, and in some cases inherent instability of the volcanic debris avalanche deposits. In addition, on Mt. Taranaki, the collapse of a segment of the cone by volcanic debris avalanche often generates long periods of renewed volcanism, generating large volumes of juvenile tephra onto unstable and unvegetated slopes, or construction of new domes with associated rock falls and block-and-ash flows. The distal ring-plain impact from these post-debris avalanche conditions and processes is primarily accumulation of long run-out debris flow and hyperconcentrated flow deposits with a variety of lithologies and sedimentary character. Common to these post-debris avalanche units is evidence for high-water-content flows that are typically non-cohesive. Hence sedimentary variations in these units are high in lateral and longitudinal exposure in relation to local topography. The post-collapse deposits flank large-scale fans and hence similar lithological and chronological sequences can form on widely disparate sectors of the ring plain. These deposits on Mt. Taranaki provide a record of landscape response and ring-plain evolution in three stages that divide the currently identified Warea Formation: 1) the deposition of broad fans of material adjacent to the debris avalanche unit; 2) channel formation and erosion of Stage 1 deposits, primarily at the contact between debris avalanche deposits and the Stage 1 deposits and the refilling of these channels; and 3) the development of broad tabular sheet flows on top of the debris avalanche, leaving sediments between debris avalanche mounds. After a volcanic debris avalanche, these processes represent an ever changing and evolving hazard-scape with hazard maps needing to be regularly updated to take account of which stage the sedimentary system is in.     

Dynamical evolution properties of debris flows controlled by different mesh-sized flexible net barriers

Because the flexible net barrier is a gradually developed open-type debris-flow counter-measure, there are still uncertainties in its design criterion. By using several small-scale experimental flume model tests, the dynamical evolution properties of debris flows controlled by large and small mesh-sized (equal to D₉₀ and D₅₀, respectively) flexible net barriers are studied, including the debris flow behaviors, segregation, and permeability of sediments, as well as the energy absorption rates and potential overtopping occurring when debris flows impact the small mesh-sized one. Experimental results reveal that (a) two sediment deposition patterns are observed depending on variations in debris flow textures and mesh sizes; (b) the aggregation against flexible net barriers is dominated by flow dynamics; (c) the segregation and permeable functions of the barrier are determined by the mesh size, concentration, and flow dynamics; and (d) the smaller mesh-sized flexible net barrier tends to be more efficient in restraining more turbulent debris flows and can absorb greater rate of kinematic energy, and finally, the great kinematic energy dissipation that occurs when secondary debris flows interact with the post-deposits in front of the small mesh-sized flexible net barrier is believed to cause the failure of overtopping phenomenon. The mesh size is concluded to be the decisive parameter that should be associated with debris flow textures to design the control functions of flexible net barriers.     

Study of the interaction between dry granular flows and rigid barriers with an SPH model

This study uses an incompressible smoothed‐particle hydrodynamics model to investigate the interaction between dry granular material flows and rigid barriers. The primary aim is to summarise some practical guidelines for the design of debris‐resisting barriers. The granular materials are modelled as a rigid‐perfectly plastic material where the plastic flow corresponds to the critical state. The coupled continuity equation and momentum equation are solved by a semi‐implicit algorithm. Compared with flows in controlled flume experiments, the model adequately reproduces both the kinetic of the flows and the impact force under various conditions. Then the numerical simulations are used to study the detailed interaction process. It is illustrated quantitatively that the interaction force consists of two parts, ie, the earth pressure force caused by the weight of the soil and a dynamic force caused by the internal deformation (flowing mass on top of a dead zone). For the estimation of impact load, this study suggests that an increased earth pressure coefficient depending on the Froude number should be incorporated into the hydrostatic model.     

Effects of particle size of mono-disperse granular flows impacting a rigid barrier

Understanding the interaction between complex geophysical flows and barriers remains a critical challenge for protecting infrastructure in mountainous regions. The scientific challenge lies in understanding how grain stresses in complex geophysical flows become manifested in the dynamic response of a rigid barrier. A series of physical flume tests were conducted to investigate the influence of varying the particle diameter of mono-dispersed flows on the impact kinematics of a model rigid barrier. Particle sizes of 3, 10, 23 and 38 mm were investigated. Physical tests results were then used to calibrate a discrete element model for carrying out numerical back-analyses. Results reveal that aside from considering bulk characteristics of the flow, such as the average velocity and bulk density, the impact load strongly depends on the particle size. The particle size influences the degree of grain inertial stresses which become manifested as sharp impulses in the dynamic response of a rigid barrier. Impact models that only consider a single impulse using the equation of elastic collision warrant caution as a cluster of coarse grains induce numerous impulses that can exceed current design recommendations by several orders of magnitude. Although these impulses are transient, they may induce local strucutral damage. Furthermore, the equation of elastic collision should be adopted when the normalized particle size with the flow depth, δ/h, is larger than 0.9 for Froude numbers less than 3.5.     

Dynamic response of flexible rockfall barriers under different loading geometries

Flexible steel barriers are commonly constructed on steep hillsides to mitigate rockfall. The evaluation of the dynamic response of proprietary flexible barriers is conventionally performed using full-scale field tests by dropping a weight onto the barriers in accordance with the European test standard ETAG 27. The weight typically has a spherical or polyhedral shape and cannot reproduce more complex rockfall scenarios encountered in the field. A rigid slab may load a barrier over a larger area and its effect has not been investigated. In this study, a calibrated three-dimensional finite-element model was developed to study the performance of vertically and horizontally orientated rockfall barriers under concentrated areal impact loads. A new bilinear force-displacement model was incorporated into the model to simulate the behavior of the energy-dissipating devices on the barriers. The effect of different weight geometries was studied by considering impacts by a rigid single spherical boulder and a rigid slab. Results reveal that areal loading induced by a rigid slab increases the loading on the barrier foundation by up to 40 % in both horizontally and vertically positioned barriers when compared to a concentrated load scenario with a single boulder. This indicates that barriers tested under the current test standard does not give the worst-case scenario in terms of foundation loads, and barrier designers should take into account the possible effect of increased foundation loads by reinforcing the barrier posts and/or increasing their spacing.     

Intra- and extra-caldera volcaniclastic facies and geomorphic characteristics of a frequently active mafic island–arc volcano, Ambrym Island, Vanuatu

Ambrym is one of the most voluminous active volcanoes in the Melanesian arc. It consists of a 35 by 50 km island elongated east–west, parallel with an active fissure zone. The central part of Ambrym, about 800 m above sea level, contains a 12 kilometre-wide caldera, with two active intra-caldera cone-complexes, Marum and Benbow. These frequently erupting complexes provide large volumes of tephra (lapilli and ash) to fill the surrounding caldera and create an exceptionally large devegetated plateau “ash plain”, as well as sediment-choked fluvial systems leading outward from the summit caldera. Deposits from fall, subordinate base surge and small-volume pyroclastic (scoria) flows dominate the volcaniclastic sequences in near vent regions. Frequent and high-intensity rainfall results in rapid erosion of freshly deposited tephra, forming small-scale debris flow- and modified grain flow-dominated deposits. Box-shaped channel systems are initially deep and narrow on the upper flanks of the composite cones and are filled bank-to-bank with lapilli-dominated debris flow deposits. These units spill out into larger channel systems forming debris aprons of thousands of overlapping and anastomosing long, narrow lobes of poorly sorted lapilli-dominated deposits. These deposits are typically remobilised by hyperconcentrated flows, debris-rich stream flows and rare debris flows that pass down increasingly shallower and broader box-shaped valleys. Lenses and lags of fines and primary fall deposits occur interbedded between the dominantly tabular hyperconcentrated flow deposits of these reaches. Aeolian sedimentation forms elongated sand dunes flanking the western rim of the ash-plain. Outside the caldera, initially steep-sided immature box-canyons are formed again, conveying dominantly hyperconcentrated flow deposits. These gradually pass into broad channels on lesser gradients in coastal areas and terminate at the coast in the form of prograding fans of ash-dominated deposits. The extra-caldera deposits are typically better sorted and contain other bedding features characteristic of more dilute fluvial flows and transitional hyperconcentrated flows. These outer flank volcaniclastics fill valleys to modify restricted portions of the dominantly constructional landscape (lava flows, and satellite cones) of Ambrym. Apparent maturity of the volcanic system has resulted in the subsidence of the present summit caldera at a similar rate to its infill by volcaniclastic deposits.     

Transition from debris flow to hyperconcentrated flow in a submarine channel (the Cretaceous Cerro Toro Formation, southern Chile)

ABSTRACT It is important to understand the exact process whereby very large amounts of sediment are transported. This paper reports peculiar conglomerate beds reflecting the transition of submarine debris flows into hyperconcentrated flows, something that has been well documented only in subaerial debris-flow events until now. Voluminous debris flows generated along a Cretaceous submarine channel, southern Chile, transformed immediately into multiphase flows. Their deposits overlie fluted or grooved surfaces and comprise a lower division of clast-supported and imbricated pebble–cobble conglomerate with basal inverse grading and an upper division of clast- to matrix-supported, disorganized conglomerate with abundant intraformational clasts. The conglomerate beds suggest temporal succession of turbidity current, gravelly hyperconcentrated flow, and mud-rich debris flow phases. The multiphase flows resulted from progressive dilution of gravelly but cohesive debris flows that could hydroplane, in contrast to the flow transitions in subaerial environments, which involve mostly non-cohesive debris flows. This finding has significant implications for the definition, classification, and hazard assessment of submarine mass-movement processes and characterization of submarine reservoir rocks.     

A unified CFD‐DEM approach for modeling of debris flow impacts on flexible barriers

This paper presents a unified modeling framework to investigate the impacts of debris flow on flexible barriers, based on coupled computational fluid dynamics and discrete element method (CFD‐DEM). We consider a debris flow as a mixture of fluid and particles where the fluid and particle phases are modeled by the CFD and the DEM, respectively. The fluid‐particle coupling is considered by the exchange of interaction forces between CFD and DEM calculations. The flexible barrier is simulated by the DEM as a network of bonded particles with remote interactions. The proposed coupled CFD‐DEM approach enables us to conveniently handle the complicated three‐way interactions among the fluid, the particles, and the flexible barrier structure for debris flow impact simulations. The proposed approach is first used to investigate the influences of channel inclination and the volumetric solid fraction in a debris mixture on the impact force, the resultant deformation, and the retained mass in a flexible barrier. The predictions agree well with existing experimental and numerical studies. We further examine the possible failure modes of a flexible barrier under debris flow impact and their underlying mechanisms. The performance of different components in a flexible barrier system, including single wires, double twists and cables, and their load sharing mechanisms, are carefully evaluated. The proposed unified framework offers a novel, promising pathway towards physically based, quantitative analysis and design of flexible barriers for debris flow mitigation.     

Sedimentology of a sandur formed by multiple jökulhlaups,Kverkfjöll,Iceland

The Kverkfjöll sandur in north Iceland is the furthest upstream of a suite of fluvial landforms extending for 200 km along the Jökulsá á Fjöllum river. Incision of the sandur exposes over 3 km of sedimentary sections, up to 15 m in height. A sandur wide, well-bedded succession of matrix-rich cobble-gravel and pebble/granule gravel, with individual beds 0.2 to 0.5 m thick indicates that the sandur is primarily the product of sandur-wide sheet-floods, with sediment-rich hyperconcentrated flows and also some debris flows and channelised turbulent flows. This interpretation is evidenced by bedded hyperconcentrated flow deposits occurring as laterally extensive tabular depositional units that dominate the entire sandur, reflecting the unconfined nature of the flow. Clast-supported boulder-gravel units interpreted as the product of macroturbulent flow occur in relatively narrow, but deep channels. The sedimentary succession is interpreted as the product of at least six volcanically generated catastrophic outburst floods (jökulhlaups) during the Little Ice Age. The sedimentology of these Little Ice Age flood deposits, on a small, high-gradient sandur, contrasts strongly with the deposits of volcanically-generated jökulhlaups on large, low-gradient coastal sandar, and sandar associated with retreating glaciers which have been the basis for most previous models of jökulhlaup sedimentation.     

Volcanic natural dams: identification,stability, and secondary effects

Volcanic activity can enhance several secondary effects, including the formation of one or more natural dams. A common example is from volcanic collapse, where huge mass volumes are rapidly emplaced, obstructing the drainage around a volcano. Their duration depends on the volume of the obstructing mass, inflow rate, and on its textural characteristics. A block facies of a debris avalanche produces durable and permeable dams that consist of decimeter to meter-sized blocks without matrix, whereas a mixed facies is easily eroded after overflowing. Analysis of the sedimentological characteristics of different volcaniclastic deposits that formed natural dams indicate that a mean grain size (Md) equal to −1 phi divides the field of debris avalanche dams (Md < −1 phi) from that formed from other types of volcanic deposits. In addition, the matrix proportion of dams formed by debris avalanches are less than the 50% and the percentage of mud fraction is highly variable, up to 30%. Combining the granulometric textures with duration time of the dam shows no clear relation. Dam durability is probably more dependent on the volume of the lake and the inflow rate. Only in some cases, as mud fraction increases is the blockage also less durable because the lower permeability favors rapid infilling. The texture of the dam also determines the types of secondary flows that originate by their breakdown. These vary from cohesive debris flow to hyperconcentrated flow, representing different hazards due to their magnitude and their different behavior downstream.     

Recognition, classification and mechanical description of debris flows

Various types of flow or mass movement involving water and sediments occur on steep slopes in mountainous areas. Among them, debris flows are peculiar events during which a large volume of a highly concentrated viscous water-debris mixture flows through a stream channel. Throughout the world these phenomena cause considerable damage but remain poorly understood although a basic knowledge is already available concerning their recognition and propagation.

Firstly, a synthesis of the useful practical criteria of recognition is proposed. Debris flows must be seen as intermediate phenomena between hyperconcentrated flows (intense bed load transport) and landslides separated from them by sharp transitions of some characteristics (celerity, deposit nature and flow type). Two parameters, solid fraction and material type, thought to be appropriate for a sound and practical classification, are brought out, and the corresponding complete classification of flow and mass movements in mountain areas is presented. Two extreme debris flow types are thus distinguished: muddy debris flows and granular debris flows. A critical review of recent advances in debris flow dynamic is then proposed. It is pointed out that adequate work must be carried out in the field of non-Newtonian fluid mechanics. In particular, one fundamental rheological property of debris flow materials is the yield stress, which explains thick deposits on steep slopes and can be inferred from field measurements. Furthermore it can be used to estimate viscous dissipation within the bulk during flow. Relevant models predicting muddy debris flow dynamics are already available whereas further progress is needed concerning granular flows.     

Sedimentology of proximal to distal volcaniclastics dispersed across an active foldbelt: Ellensburg Formation (late Miocene), central Washington

Late Miocene volcaniclastics of the Ellensburg Formation (Washington, northwestern USA) are exposed in basins within an integrated palaeodrainage over the depositional reach from 15 to 120 km eastward from a Cascade Range source area. Two facies associations are recognized. The first is composed of laterally restricted, well-sorted, polymictic conglomerates representing a gravel-bedload regime during inter-eruption periods. The second comprises laterally extensive sheets, bounded by deeply-developed paleosols, composed of monomictic pebbly, pumiceous dacitic sandstones with intercalated debris-flow and hyperconcentrated flood-flow deposits. These sheets aggraded in response to eruptive events that are sometimes recorded by air-fall tephras at the bases of sequences. Debris-flow and hyperconcentrated flood-flow deposits occur as far as 120 km from source, but are uncommon beyond 50 km. Hyperconcentrated flood-flow deposits initially increase in abundance away from source as debris-flow deposits diminish, suggesting formation of hyperconcentrated flood flows by dilution of debris flows. Sandy facies form broad sheets dominated by scour-fill bedding, in proximal and medial settings, and grade to narrower, trough cross-bedded sheets in distal settings, suggesting moderation of flood discharges with distance. Base-level changes associated with episodic sediment influx caused incision-aggradation cycles in mainstream settings, and episodic impoundment of tributaries to form lakes or rapidly aggrading, poorly drained floodplains. Although volcanism was the primary control on depositional style, concurrent development of the Yakima fold belt produced a structure-consequent drainage pattern that determined sediment dispersal, and basin subsidence permitted preservation of both syn-eruption and inter-eruption facies. Detritus from rising anticlines was generally diluted by the volumetrically superior extrabasinal volcaniclastics, but dominates deposits of small tributary streams flowing toward, rather than from, the Cascades.     

New impact equation using barrier Froude number for the design of dual rigid barriers against debris flows

Landslides - In the design of multiple rigid barriers, the height of the first barrier governs the impact dynamics of debris flow on the next barrier in a channel. However, current design...     

开口柔性防护网调控泥石流性能试验研究

柔性防护网是防治泥石流灾害的重要工程措施之一,现有防治结构以闭口防护网形式为主,易发生堵塞且调控能力欠佳。为此,提出一种开口柔性防护网结构,并基于理论分析、物理模型试验开展了开口柔性防护网对泥石流的调控性能研究,推导泥石流速度衰减率、爬升高度和柔性防护网拦挡率的理论公式。结果表明：与闭口柔性防护网结构相比,该结构起到了良好的自清洁作用,且可有效调控泥石流速度峰值;所推导的无量纲理论公式计算结果与物理试验结果具有较好的一致性;速度衰减率、爬升高度和拦挡率主要由相对开口高度、无量纲流深、泥石流相对密度和弗罗德数控制;速度衰减率和拦挡率与相对开口高度呈负相关,与泥石流相对密度呈正相关,爬升高度与相对开口高度、相对密度均呈负相关。上述研究可为开口柔性防护网在泥石流防治工程中的应用提供理论及技术支持。     

Effect of slit-type barrier on characteristics of water-dominant debris flows: small-scale physical modeling

Slit-type barriers, one of open-type barriers, are widely used as active measures to mitigate potential risk and damage by debris flows, and those are designed and installed to reduce the flow energy by only passing relatively small debris. However, the mechanisms of slit-type barriers in reducing the debris flow velocity and debris volume remain poorly understood because of the lack of well-controlled and reliable physical modeling results. This study explored the influence of various arrangements of slit-type barriers, including P-type barriers in which each rectangular barrier was placed in parallel and V-type barriers where the barriers were placed in a V-shape, on characteristics of water-dominant debris flows via small-scale model experiments. The debris flow events were reproduced against the slit-type barriers, where the velocity reduction and trap ratio were monitored, varying the angle and shape of barrier arrangements. The velocity reduction and trap ratio appeared to increase as the angle of the barrier wall decreased because of the decreased opening ratio. The V-type barriers resulted in higher velocity reduction and trap ratio than the P-type, primarily because of the smaller effective opening ratio and the more backwater effect. In addition, as the debris contained more boulders, the extent of velocity reduction and debris trap became greater in all barrier types. Two types of opening ratios, the projected and effective opening ratios, were correlated to the interactions between debris and walls. The obtained results provide baseline data for the optimum design of slit-type barriers against debris flow and suggest that the slit-type barriers can effectively manage the risk of damage by debris flows.     

Performance of ethylene-vinyl acetate foam as cushioning material for rigid debris-resisting barriers

This paper reports an investigation on the performance of ethylene-vinyl acetate (EVA) foam when used as a cushion layer for rigid barriers used to resist debris flow. Large-scale pendulum impact tests were conducted to study the effects of layer thickness on cushion performance under six successive impacts. Results show that for the first impact at 70 kJ, the peak contact force with the EVA foam thickness of 500 mm is about twice larger compared to that of 1000 mm. Results also reveal that the cushion mechanism of elastic collapse of cell walls in the EVA foam results in peak contact forces and maximum transmitted loads that are up to 30 and 50% lower compared to gabions for the first impact at 70 kJ, respectively. The elastic behavior of EVA foam provides consistent cushioning efficiency. Furthermore, EVA foam is found to be susceptible to degradation by ultraviolet light so that a suitable coating layer is required for outdoor use. Polyurea was identified as a suitable coating material and a small-scale coating trial was performed to confirm this. Findings presented in this paper will have direct implications on the future design of cushion layers for rigid barriers used to intercept debris flows.     

Hydraulic behavior of tsunami backflows: insights from their modern and ancient deposits

Tsunamis are unpredictable, catastrophic events, and so present enormous difficulties for direct studies in the field or laboratory. However, their sedimentary deposits yield evidence of a wide variety of hydrodynamic conditions caused by flow transformations on a spatial and temporal scale. Tsunami deposits ranging from the Miocene to modern times identified at different localities along the Chilean coast are described to provide a database of their characteristics. Among the typical features associated with tsunami deposits are well-rounded megaclasts eroded from coastal alluvial fans or beaches by very dense, competent flows. Sand injections from the base of these flows into the substrate indicate very high dynamic pressures, whereas basal shear carpets suggest hyperconcentrated, highly sheared flows. Turbulence develops in front of advancing debris flows, as indicated by megaflutes at the base of scoured channels.     

长白山火山1000年前火山泥流堆积及其灾害

长白山火山１０００年前大喷发,火山泥流堆积物沿松花江中上游分布。距火山较近的是火山泥流的岩屑流堆积,远离火山变为火山泥流的超高密度流堆积。探讨了火山泥流的成因,并指出长白山火山一旦再次爆发,火山泥流将是主要的火山灾害,沿松花江中上游可能造成巨大破坏,并可能危及鸭绿江、图们江中、上游。     

Assessment of debris flow multiple-surge load model based on the physical process of debris-barrier interaction

Debris-flow impact load is one of the key parameters for design of engineering countermeasures. The multiple-surge load model is a remarkable progress in estimating the debris-flow impact load, which clearly delineates the contribution of each surge to the total impact load and the corresponding acting points. In order to better understand the impact process of channelized debris flow against flexible barrier, a series of medium-scale flume experiments with varying debris-flow volumetric solid concentration (0.40/0.50/0.55) were conducted. Especially, surge impact behavior is focused so that the predictability of the multiple-surge load model could be assessed. The flume and model flexible barrier were instrumented so that both the barrier dynamic response and the debris-flow properties (flow regime) could be correlated to facilitate the assessment. The results show that multiple-surge load model well predicts the total impact load. However, due to the simplification in the impact process, the interaction between the mobile phase (surge) and the deposited phase is ignored, resulting in discrepancy in the load distribution between the model prediction and experimental result. The remixing of deposited debris by the subsequent surges leads to downward momentum transfer to the lower section of barrier, which should be regarded as an adverse scenario of the design of flexible-barrier anchor capacity.