Inertial Grain Research Articles

Evolution of debris flow properties and physical interactions in debris-flow mixtures in the Wenchuan earthquake zone

The 2008 Wenchuan earthquake triggered an exceptional number of landslides. The large amount of loose landslide materials on the steep terrains can easily turn into debris flows by heavy rainfall. During the wet seasons from 2008 to 2011, five destructive debris flows occurred repeatedly in Yingxiu, the study area near the epicenter. This paper presents the evolution of debris flow properties and physical interactions in the debris flow mixtures in the past few years. Seven dimensionless numbers are used to describe the physical interactions. The evolution of the physical interactions over time was influenced by changes in debris-flow compositions, which in turn affected the runout characteristics of the debris flows significantly. Grain contact friction was the most important physical interaction for the five debris flows in the study area. With gradual loss of fine particles over time, the viscosity of the debris-flow mixtures became lower and lower, leading to a change from the dominance of viscous shear over solid–fluid interactions and inertial grain collision to the dominance of solid–fluid interactions and inertial grain collision over viscous shear. With inertial grain collision being more and more important, the flow resistance of the debris flows increased. The mobility of the future debris flows may become smaller and smaller.

Signature of coseismic decarbonation in dolomitic fault rocks of the Naukluft Thrust, Namibia

Unequivocal geological signatures of seismic slip are rare, exceptionally so in carbonate-hosted faults where carbonate minerals dissociate at temperatures lower than those required for producing a friction melt. This thermal dissociation leads to significant fault weakening by increased fluid pressure and/or nanoparticle lubrication, preventing further heating of the fault surface. Pseudotachylyte is therefore unlikely to form in carbonate-hosted faults, and other evidence for seismic slip must be identified.We studied the lower Cambrian Naukluft Thrust which crops out in central Namibia. It contains a cataclastic dolomite fault rock, referred to as “gritty dolomite”, which we interpret as a signature of coseismic carbonate dissociation and subsequent fluid–rock interactions. The fault was active at ambient temperatures below 200°C.“Gritty dolomite” contains: rounded, low aspect ratio dolomite clasts with a uniform Fe-rich dolomite coating, euhedral to subhedral magnetite, quartz, and K-feldspar in a fine-grained, massive to laminated carbonate matrix of particulate dolomite and crystalline calcite cement. The fault rock texture, combined with evidence of injectites of gritty dolomite into the wallrock, indicates the cataclasite deformed as a fluidized granular flow. At seismic slip velocities, frictional heating caused dissociation of dolomite to CO2 and Ca-, Fe- and Mg-oxides. This release of CO2 decreased the pH of the pore fluid in the fault, causing dissolution and rounding of dolomite clasts within an inertial grain flow, and precipitation of carbonate coatings and euhedral silicates and oxides during subsequent cooling and CO2 escape.Examples of similar rocks having some, if not all of these characteristics have been described from other carbonate-hosted faults. The geological setting of the Naukluft Thrust is unique in spatial extent and quality of exposure, allowing us to eliminate alternative hypotheses for sources of CO2 to drive fluidization.

Mechanisms and runout characteristics of the rainfall-triggered debris flow in Xiaojiagou in Sichuan Province, China

The 2008 Wenchuan earthquake induced a large number of landslides, and a vast amount of loose landslide materials deposited on steep hill slopes or in channels. Such loose materials can become sources of deadly debris flows once triggered by storms. On 13 August 2010, a storm swept Yingxiu and its vicinity, triggering a catastrophic debris flow with a volume of 1.17 million m3 in Xiaojiagou Ravine. The debris flow buried 1,100 m of road, blocked a river and formed a debris flow barrier lake. A detailed field study was conducted to understand the initiation mechanisms and runout characteristics of this debris flow. Two types of debris flows are identified, namely hill-slope debris flow and channelized debris flow. The hill-slope debris flow was triggered in the forms of firehose effect, rilling and landsliding, whereas the channelized debris flow was triggered in the form of channel-bed failure. This debris flow was a water–rock flow since most particles were gravel, cobble or larger rocks and the fraction of silt and clay was less than 2%. Grain contact friction, pore-pressure effects and inertial grain collision were the three most important physical interactions within the debris flow. Such interactions yielded a smaller runout distance (593 m) compared with those of mud–rock flows of similar size.

Evaluation of geophysical mass flow models using the 2006 block-and-ash flows of Merapi Volcano, Java, Indonesia: Towards a short-term hazard assessment tool

The dynamics and depositional processes associated with block-and-ash flows (BAFs) are most commonly inferred to be a function of granular or inertial grain flow, similar to debris flows and cold rock avalanches. Existing geophysical mass flow models are either based on frictional (Mohr–Coulomb) behavior (the Titan2D model developed at the State University of New York at Buffalo, USA) or another rheological law (i.e., a constant retarding stress), eventually adding some viscous and turbulent components (the VolcFlow model developed at the Laboratoire Magmas et Volcans, Clermont-Ferrand, France). The 2006 BAFs of Merapi present a rare opportunity to test these two well-established models against a well-constrained field example. Integration of high-resolution field-based data into numerical simulations allows the validity of these models to be tested and rapid quantification of best-fit input parameters. We first show that with the incorporation of spatially varying bed friction angles, Titan2D is capable of reproducing the paths, runout distances, areas covered and deposited volumes of the 2006 Merapi flows over highly complex topography. However, some discrepancies with field data are noted and the velocity and travel time of the flows do not match entirely. Using a single free parameter (a constant retarding stress), simulations obtained with the VolcFlow model also reproduce the morphology and distribution of the natural deposits as well as the time of emplacement and velocities of the flows. The results suggest that the performance of these models in simulating actual events is critically dependent on: (1) the calibration of the model by using extensive field-based data such as deposit distribution, and processes of flow generation, transport and deposition; (2) the incorporation of a suitable numerical topographic dataset (i.e., high-resolution digital elevation model), and (3) the choice of input parameters, such as location and volume of the initial pile of material and source characteristics (single or multiple dome-collapse, dome-collapse duration and total volume of collapsed material). Sensitivity analyses and inundation maps based on the probability of impact were used to produce a suite of potentially inundated areas from future gravitational dome-collapse events affecting the Gendol valley and adjacent areas on the southern flank of the volcano. Our results provide the basis for defining hazard zonations of key areas at risk from BAFs which will be generated during future comparable eruptions at Merapi.

A mechanical model for Merapi-type pyroclastic flow

A numerical simulation model of Merapi-type pyroclastic flow for predicting a potential hazardous area is given, and its applicability is examined by numerically reproducing actual phenomena that occurred in 1991 at Unzen Volcano, Kyushu Japan. Merapi-type pyroclastic flow arises from collapse of the lava dome whereby large lava blocks are crushed into smaller particles during movement down the steep slope. The early stage of flow (it may be called the debris avalanche stage), in which the dominant particles are coarser than about 1mm in diameter, is considered as a grain (granular) flow. In an inertial grain flow particle collision stress plays an important role in the mechanism of flow, and the effect of gas emitted from the lava blocks is minimal. Most of the particles composing the inertial grain flow deposit over a comparatively short range on steep slope due to marked resistance within the flow. The remainder which is composed of mainly fine particles smaller than 1mm continues to run down by a support of the upward flow of gas ejected from the material itself, thereby the resistance to flow becomes small. Thus, the pyroclastic flow stage in a narrow sense appears. The main body of the pyroclastic flow is composed of lower insufficiently fluidized layer (bottom layer) and upper fluidized layer in which the entire weight of the particles is supported by the upward gas flow. Sometimes, the fluidized layer cannot exist if gas emission is insufficient. As the slope down which the pyroclastic flow moves becomes milder downstream, the bottom layer deposits some solids because the driving force due to gravity within this layer becomes smaller than the resistance due to inter-particle contact. Then, a part of the fluidized layer, if exists, changes to a part of the bottom layer because of the shortage of the upward gas flow that is caused by deposition of the gas-emitting particles upstream. Thus, the entire main body stops when it arrives at a gently sloping area. Smaller particles and gas escape from the main body, generating a hot ash cloud layer above the main body. The hot ash cloud layer can travel independent of the main body, but its development or attenuation depend on the conditions of the supply of particles and gas from the main body. When the hot ash cloud lacks the supply of particles and gas due to the stopping of the main body upstream, or by its swerving from the course of the main body, it becomes soon weakened and stops. The entrainment of ambient air, the escape of air and ash from the upper boundary as a buoyant plume, and the particle settling also affect the behaviors of the hot ash cloud. Fundamental mechanics of the grain (granular) flow stage, the pyroclastic flow stage and the transition from the former to the latter stages are discussed theoretically and experimentally, and mathematical formulae describing the phenomena are obtained. These one-dimensional equations are extended to fit the planar two-dimensional system. Behaviors of the main body as well as the hot ash cloud are simulated using the obtained system of equations. The results of simulation are very satisfactory in comparison to the actual phenomena.

Coarse-grained debris-flow deposits in the Miocene fan deltas, SE Korea: a scaling analysis

Abstract The viscoplastic and inertial grain-flow models have been widely used as tools for description and interpretation of ancient debris-flow deposits, providing a basis to estimate yield strength, viscosity, cohesion, and internal friction angle. Recent studies suggest, however, that a debris flow is an intimate mixture of solid and fluid, in which a number of momentum-transfer mechanisms operate. It is therefore necessary to evaluate the relative importance of different momentum-transport processes to properly describe a debris flow. Scaling analysis may be useful to this end, using the Bagnold number (the ratio of inertial grain stress to viscous shear stress), Savage number (the ratio of inertial grain stress to shear stress borne by sustained grain contacts), friction number (the ratio of frictional shear stress to viscous shear stress), and Darcy number (the ratio of grain–fluid interaction stress to inertial grain stress). Scaling analyses of Miocene gravelly debris-flow deposits in Korea suggest that different types of debris flows can be better distinguished by the analyses, providing some implications for debris-flow processes.

Flooding and sediment transport in a small alpine drainage basin in Colorado

A short-duration, high-intensity rainfall on a 0.43-km 2 (105-acre) alpine drainage basin in the central Colorado mountains produced a peak discharge, including water and sediment, of 108 to 134 m 3 /s (3,810 to 4,730 cfs). This exceeds by an order of magnitude the discharge of the 100-yr flood as predicted by the procedures recommended by the U.S. Soil Conservation Service and exceeds by a factor of two the largest discharge ever reported in a basin of similar size. The Manning formula and superelevation calculations were two methods used to compute velocities and discharges. These two methods depend upon independent sets of assumptions, but they produced similar results. Calculations of the fluid-dynamic stresses required to transport boulders in the upper channel also suggest flood depths and velocities within the range of values computed through use of the Manning formula and superelevation calculations. In the lower channel, bed load moved as inertial grain flows. One flow studied in detail transported sediment onto a part of the alluvial fan not reached by deep flood water. The larger clasts were transported to the top and front of the grain flow. This type of sorting supports a theory of grain-flow deformation first suggested by Bagnold.

Explosive eruptions and pyroclastic avalanches from Ngauruhoe in February 1975

Pyroclastic eruptions of Ngauruhoe volcano on February 19, 1975, were observed closely. A 1.5-hour period of voluminous gas-streaming emission was followed by a series of violently explosive cannon-like eruptions, which threw blocks at least 2.8 km from the vent. Initial ejecta velocities of 400 m s −1 indicate high explosion gas pressures, possibly caused by magmatic intrusions rapidly heating confined meteoric water, the explosions being driven largely by phreatic steam rather than magmatic gases. Pyroclastic avalanches were generated by both the gas-streaming and cannon-like eruptions; in the latter they were formed by collapse of dense eruption columns onto the summit of the volcano. Downslope transport appears to have been by inertial grain flow mechanisms; erosion of chutes and short runout distances indicate that the avalanches were neither highly fluidised nor of air-layer lubricated type. A minimum bulk volume of 3.4 × 10 6 m 3 of pyroclastic material was erupted. Although of smaller scale, the Ngauruhoe explosions appear to have analogies to the 1968 Arenal and Mayon eruptions.

Transportation of detrital materials on the lunar surface: evidence from Apollo 15

ABSTRACTThe thickness frequency distribution of stratigraphic layers intersected by the Apollo 15 deep core suggests that the majority of impact events reworking the lunar soil are small and produce ejecta blankets with an average thickness of less than 1·D5 cm. The energy frequency distribution of the meteorites producing the layers may be bimodal. The impacting meteorites produce both normal and reverse graded beds which appear to be the end products of two depositional mechanisms. First, the normally graded beds appear to be produced in base surges as escaping gases fluidize the flowing debris and larger particles move downwards in response to Stokes Law. Second, if the gas loss from the base surge is excessive the fluidization may cease and inertial grain flow dominates. In this situation the beds are reverse graded as larger particles move under dispersive pressure to the region of minimum shear stress at the upper boundary of the base surge. The same processes also produce measurable shape sorting of the particles in the beds.