Sand Avalanches Research Articles

Morphological dynamics of a trough-shaped blowout on a fixed dune in China

• There is a strong link between blowout morphology and the pattern of airflow. • The pattern of airflow within blowout is related to the regional wind direction. • Airflow undergoes separation and deflection in the leeward of the deflation basin. • Corkscrew vortices are the determinant of upwind expansion of the deflation basin. The formation and development of blowouts is an important surface indication of sand drift activity in semiarid grassland areas; thus, an accurate understanding of their morphological evolution characteristics and dynamic processes is of significance for grassland desertification control. This study analyzed the long-term topographic change of a trough blowout developed on a fixed sand dune in the Otindag sandy land of China via ground measurements from 2011 to 2020 and examined the short-term airflow structure through field observations. The results indicated that the area of the deflation basin shows a state of continuous growth. The expansion of the deflation basin was most obvious on the western, southwestern and southern slopes, which is contrary to the regional prevailing wind direction. As airflow passes over the blowout, there is significant airflow steering with the change in topography, and the degrees of airflow steering and acceleration are determined by the direction of the approaching wind. Airflow expands and decelerates on the leeward side of the deflation basin, causing flow separation and producing a recirculation zone on the sheltered lee slopes. Based on a detailed analysis of the results, we suggest that the soil collapse and sand avalanches caused by vortices are the main reasons for the upwind expansion of the deflation basin. Collectively, these findings reveal a strong link between the blowout morphology and the airflow pattern.

Characteristics of sand avalanche motion and deposition influenced by proportion of fine particles

Characteristics of sand avalanche motion and deposition influenced by proportion of fine particles

Cellular automata modeling of rockfill dam failure caused by overtopping or any other extreme throughflow

A cellular automaton for the study of the progressive failure of rockfill dams caused by overtopping flows is presented. The celebrated Bak, Tang and Wiesenfeld’s model, proposed in 1987 to reproduce sand avalanches on the surface of a sand heap is adapted to a rockfill dam with an impervious central core that is subjected to extreme throughflow caused by overtopping flow or by failure of the watertightness of the core, or the dam foundation. The downstream rockfill shoulder is represented by a set of cells whose states (elevation and submergence) evolve according to a set of fixed transition rules. These rules, which are affected by the instantaneous throughflow, are based on widely used hydraulic and geotechnical approaches to mass sliding and particle dragging mechanisms. The CA model has been validated with laboratory experiments performed on a U-shaped channel. Albeit the apparent simplicity of the underlying formulation, this model not only sheds light on a complex phenomenon but also provides a way for quantifying the overtopping throughflow needed to threaten the central core or upstream face, which can lead to a catastrophic failure.

Elucidating the importance of sediment avalanches in scour at obstacles

ABSTRACT An experimental analysis of the dynamics of avalanches for different stages of development and turbulent flow conditions is introduced. Experiments were conducted in a laboratory flume using clear-water conditions and a video camera placed within an acrylic cylinder. The results of a proper orthogonal decomposition of the pre-processed image sequences highlighted the existence of two types of sand avalanches: Type I corresponding to a narrow and vertically long-advancing slide, and Type II characterized by an elongated and short-advancing pattern. Temporally speaking, sand avalanches were triggered aperiodically, resembling the behaviour of the horseshoe vortex system. The frequency of occurrence of avalanches decreased with time, while the estimated volume of mobilized sand increased in the same period. The sand slide magnitudes and associated inter-avalanche time were Log-Pearson 3 distributed. Sand avalanches are an important scour mechanism, which depending on the flow intensity contribute to about 40% to 60% of the scoured volume.

The self-organized critical multiverse

Recently a dynamical selection mechanism for vacua based on search optimization was proposed in the context of false-vacuum eternal inflation on the landscape. The search algorithm, defined by local vacuum transitions, is optimal in regions of the landscape where the dynamics are tuned at criticality, with de Sitter vacua having an average lifetime of order their Page time. The purpose of this paper is to shed light on the nature of the dynamical phase transition at the Page lifetime. We focus on a finite region of the landscape, which exchanges volume with the rest of the landscape and as such acts as an open system. Through a change of variables the master equation governing the comoving volume of de Sitter vacua is mapped to a stochastic equation for coupled overdamped stochastic oscillators—the well-known Ornstein-Uhlenbeck process. The rest of the landscape, which acts as an environment, is assumed to result in a non-vanishing driving term for all sites in the region with uncorrelated, white noise fluctuations (though not necessarily Gaussian). We first show that the static susceptibility of the oscillators diverges as the average lifetime of de Sitter vacua approaches the Page time. Thus, optimal regions of the landscape are most susceptible to volume influx from their environing landscape. We then show that the displacement fluctuations for the oscillators exhibit a 1/f power spectrum over a broad range of frequencies, precisely at the critical Page lifetime distribution. A 1/f power spectrum is a hallmark of non-equilibrium systems at criticality. In analogy with sand avalanches in the abelian sandpile or neuronal avalanches in the brain, de Sitter vacua at criticality can be thought of as undergoing scale invariant volume fluctuation avalanches.

Antlions are sensitive to subnanometer amplitude vibrations carried by sand substrates.

The antlion larvae (Myrmeleontidae) are ambush predators. They detect substrate-borne vibrations induced by the movement of the prey. European pit-building antlions (Myrmeleon inconspicuus) are studied for their ability to perceive vibrations generated by the locomotion of an ant (Cataglyphis cursor) outside the pit. These strides have been recorded and copied in detail in their time sequences. The signal created was emitted by piezoelectric transducers placed several centimeters outside the peripheries of the pits: the ant movements create waves with particle accelerations that are three orders of magnitude less than g, alleviating any possibility of sand avalanche towards the bottom of the pit. Depending on the amplitude of the vibrations, the antlions answer back, generally by sand tossing. One remarkable feature is the time delay between the start of the cue and the predatory behaviour induced by this cue. This time delay is studied versus the cue amplitude. We found that antlions answer back within minutes to cues with amplitudes of nanometer range, and within seconds to these same cues if they are preceded by a sequence of signals at the Ångström amplitude. This difference in latency is used to evidence the sensitivity to vibrations at an extremely low level.

Influence of Sediment and Drift Woods in Flood Disaster and Sand Avalanches

Influence of Sediment and Drift Woods in Flood Disaster and Sand Avalanches

Physics of particulate flows: From sand avalanche to active suspensions in plants

Abstract Flows of granular media in air or in a liquid have been a research field for physicists for several decades. Sometimes solid, sometimes liquid, these particulate materials exhibit peculiar behaviors, which have motivated many studies at the frontiers between nonlinear physics, soft matter physics and fluid mechanics. This paper presents a summary of the recent advances in the field, with a focus on the development of continuous approaches, which make it possible to treat granular media as a complex fluid and to develop a granular hydrodynamics. We also discuss how the better understanding of granular flows we have today may help to address more complex materials, such as colloidal suspensions or some biological systems.

Craters and Granular Jets Generated by Underground Cavity Collapse.

We study experimentally the cratering process due to the explosion and collapse of a pressurized air cavity inside a sand bed. The process starts when the cavity breaks and the liberated air then rises through the overlying granular layer and produces a violent eruption; it depressurizes the cavity and, as the gas is released, the sand sinks under gravity, generating a crater. We find that the crater dimensions are totally determined by the cavity volume; the pressure does not affect the morphology because the air is expelled vertically during the eruption. In contrast with impact craters, the rim is flat and, regardless of the cavity shape, it evolves into a circle as the cavity depth increases or if the chamber is located deep enough inside the bed, which could explain why most of the subsidence craters observed in nature are circular. Moreover, for shallow spherical cavities, a collimated jet emerges from the collision of sand avalanches that converge concentrically at the bottom of the depression, revealing that collapse under gravity is the main mechanism driving the jet formation.

The abelian sandpile model on randomly rooted graphs and self-similar groups

The Abelian sandpile model is an archetypical model of the physical phenomenon of self-organized criticality. It is also well studied in combinatorics under the name of chip-firing games on graphs. One of the main open problems about this model is to provide rigorous mathematical explication for predictions about the values of its critical exponents, originating in physics. The model was initially defined on the cubic lattices ℤ d , but the only case where the value of some critical exponent has been established so far is the case of the infinite regular tree—the Bethe lattice. This paper is devoted to the study of the abelian sandpile model on a large class of graphs that serve as approximations to Julia sets of postcritically finite polynomials and occur naturally in the study of automorphism group actions on infinite rooted trees. While different from the square lattice, these graphs share many of its geometric properties: they are of polynomial growth, have one end, and random walks on them are recurrent. This ensures that the behaviour of sandpiles on them is quite different from that observed on the infinite tree. We compute the critical exponent for the decay of mass of sand avalanches on these graphs and prove that it is inversely proportional to the rate of polynomial growth of the graph, thus providing the first rigorous derivation of the critical exponent different from the mean-field (the tree) value.

Lee slope sediment processes leading to avalanche initiation on an aeolian dune

In order to detail the governing conditions through which a slipface matures to the point of failure, dry sand avalanches were observed in the Dune Simulation Wind Tunnel on a 1:1 replica transverse dune with a crest height of approximately 1.2 m. Areal distributions of grainfall and reptation were measured using traps. Changes in the slipface elevation were observed using 3‐D laser scanning with a vertical accuracy of 0.096 mm for approximately every 1 mm2 of surface area. Grainfall decayed exponentially from the brink with a constant rate across all wind velocities. Reptation removed sediment from areas close to the brink and deposited it downslope, creating low amplitude, cross‐slope ripples on the slipface. A critical length scale separating grainscale and bulk sediment behavior is identified, and it defines the lower limit to the validity of angle of repose measurements. Avalanche initiation occurred in an area of steep surface slope below a sediment bulge, with distance from the brink independent of wind velocity. The time between avalanches was found to be constant for constant wind velocity.

Singing‐sand avalanches without dunes

Singing‐sand dunes have attracted curiosity for centuries and are now the subject of controversy. We address here two aspects of this controversy: first the possible link between the frequency heard and the shear rate (for a gravity avalanche on a dune slip‐face, scaling as , with d the ‘mean’ grain diameter), and second, the assumed necessity of a layered dune structure under the avalanche that acts as a resonator. Field recordings of singing dunes over the world reveal that they can present very different spectral characteristics: a dune with polydisperse grains produces a very broad and noisy spectrum, while a dune with sorted grains produces a well‐defined frequency. Performing laboratory avalanches on a hard plate with singing‐dune sand shows that there is no need for a dune below the sand avalanche to produce the singing sound, anda fortiorineither for the dune's layered structure nor for its particular sound transmission. By sieving the polydisperse grains, the same well‐defined frequency is obtained to that of the dune with sorted grains, with the same diameter‐frequency relation. The various frequencies heard in the field avalanches match the shear rates not calculated from the average size, but from the various peaks of the grain size distributions.

Wind may have driven avalanches on Martian dunes

In the sand dunes in the north polar sand sea of Mars, sand avalanches that appear as wedge‐shaped alcoves can be seen above fan‐shaped deposits. The avalanches, which are typically several meters across, are currently actively forming, with new alcoves showing up in recent images taken in consecutive Mars years. Some scientists have proposed that these sand avalanches occur when frozen carbon dioxide (CO2) sublimates, triggering the downslope flow (mass wasting) of sand grains and rock that had been lying on top of the CO2 frost. However, Horgan and Bell now suggest that these alcoves may actually have been formed by wind‐driven movement of sand, not by CO2 sublimation.

Numerical forward modelling of ‘fluxoturbidite’ flume experiments using Sedsim

Delta front ‘fluxoturbidity deposits’ in rift basin margins, as well as sand avalanches, are influenced by topography, water level fluctuation and wave action. Instability of prior sediments deposited on the delta front is a prerequisite for the generation of ‘fluxoturbidity’. In this study Sedsim, a three-dimensional numerical stratigraphic forward model is used to replicate and extend a set of physical flume tank experiments investigating the formation of ‘fluxoturbidites’ from different initial conditions.This study has investigated the influence of topographic slope, relative water level change and wave action on the formation of ‘fluxoturbidites’, and hopefully improves our understanding and insight into the dynamic processes of ‘fluxoturbidity’ resulting from different initiation mechanisms. The study also illustrates the value of numerical modelling in complementing and extending physical flume tank studies.We show that, at least at the scale of a flume tank, there exists an optimum window of topographic slopes within which ‘fluxoturbidites’ arise due to slumping. This window, ranging from 9° to 18°, may be useful in distinguishing the effects of topographic slope from other ‘fluxoturbidite’ causal mechanisms. The amplitude and frequency of water level oscillation appears to be a significant control on ‘fluxoturbidite’ thickness, while wave attributes, especially wave angles, appear to affect ‘fluxoturbidite’ locality relative to the sediment source.

Do partially buried dune plants grow in optimal trajectories?

Plant adaptations minimising costs of burial responses are vital in mobile dune ecosystems. Conventionally, the burial responses of dune plants have been measured as vertical growth. However, a model developed here shows that growth normal to accumulating non-horizontal dune surfaces requires up to 18% less stem production than vertical growth. To determine whether dune plants grow with this optimal geometry a field survey of growth trajectories was made for three coastal plants, and a greenhouse experiment tested whether plants could actively change growth trajectories away from vertical. In the field and greenhouse partial burial resulted in shifts towards vertical growth, indicating that negative gravitropism is the major response to burial, and reduces the amount of stem needed to respond to burial relative to unburied growth trajectories. However, for two species in the field many buried stems grew closer to optimal (shorter) than vertical, mostly on northward, sun-facing dunes. Thus phototropism is the major stimulus for non-vertical growth, and coincides with optimal trajectories on north-facing dunes. Arctotheca populifolia also displayed non-vertical growth after burial on steep south-facing dunes, but responded to burial in the greenhouse with vertical growth. Measurements of the pressure of sand avalanches occurring on dune slipfaces and stem elasticity indicated that deep avalanches were sufficient to orient the flexible shoots of A. populifolia towards optimal trajectories. Thus, dune plants respond to burial by actively modulating growth towards vertical via negative gravitropism, and passively towards non-vertical—more optimal—trajectories via phototropic growth and the influence of sand avalanches.

Booming Dune Instability

Sand avalanches flowing down the leeward face of some desert dunes spontaneously produce a loud sound with a characteristic vibrato around a well-defined frequency, a phenomenon called the "song of dunes." Here, we show through theory that a homogenous granular surface flow is linearly unstable towards growing elastic waves when a localized shear band forms at the interface between the avalanche and the static part of the dune. We unravel the nature of the acoustic amplifying mechanism at the origin of this booming instability. The dispersion relation and the shape of the most unstable modes are computed and compared to field measurements.

Coupling the dynamics of boundary layers and evolutionary dunes

A theoretical formulation and corresponding numerical solutions are presented for fluid flow and sediment transport past evolutionary sand dunes. Time-dependent curvilinear coordinates are employed to fully couple flow aloft with the developing landform. The differential conservation law that defines shape of the lower boundary depends on details of local surface stress, thereby favoring the large eddy simulation of the boundary layer. To shrink the gap between the time scales characteristic of planetary boundary layer flows O(10(3)) s and sand dune evolution O(10(6)) s, a hypothetical "severe-wind scenario" is adopted with the saltation flux amplified up to 3 orders of magnitude. While the results are largely insensitive to the rescaling, the efficacy of computations is greatly improved. The flux-form partial differential equation for the interface profile--via saltation and sand avalanches--is formulated as an advection-diffusion equation, to facilitate discrete integrations. Numerical experiments verify the adopted theoretical framework by reproducing scaling results reported in the literature. The versatility of the approach is illustrated with evolution of a sandhole--an example of application likely never addressed in the literature, yet realizable in nature.

Solving the mystery of booming sand dunes

Desert booming can be heard after a natural slumping event or during a sand avalanche generated by humans sliding down the slip face of a large dune. The sound is remarkable because it is composed of one dominant audible frequency (70 to 105 Hz) plus several higher harmonics. This study challenges earlier reports that the dunes' frequency is a function of average grain size by demonstrating through extensive field measurements that the booming frequency results from a natural waveguide associated with the dune. The booming frequency is fixed by the depth of the surficial layer of dry loose sand that is sandwiched between two regions of higher compressional body wave velocity. This letter presents measurements of the booming frequencies, compressional wave velocities, depth of surficial layer, along with an analytical prediction of the frequency based on constructive interference of propagating waves generated by avalanching along the dune surface.

Dynamics of Impact Cratering in Shallow Sand Layers

When a solid sphere impacts a shallow layer of sand deposited on a solid surface, a crater can be obtained. The dynamics of the opening of the crater can be followed accurately. During this opening, the radius of the crater can be conveniently modeled by an exponential saturation with a well-defined time constant. The crater then closes up partially once the opening phase is over as the sand avalanches down the slope of the crater. We here present a detailed study of the full dynamics of the crater formation as well as the dynamics of the corrola formed during this process. A simple model accounts for most of our observations.

Numerical simulation of sand dune evolution in severe winds

AbstractWe present a novel approach for numerical simulation of fluid flow and sediment transport past evolutionary landforms, with emphasis on sand dunes developing in extreme winds. Time‐dependent curvilinear coordinates are employed to fully couple flow aloft with the developing landform. The conservation law that defines shape of the lower boundary depends on details of local surface stress, thereby favouring a large eddy simulation of the atmospheric boundary layer. The flux‐form partial differential equation for the interface profile—via saltation and sand avalanches—is formulated as an advection–diffusion equation. The latter facilitates discrete integrations using nonoscillatory algorithms—consistent with the nonoscillatory forward‐in‐time solver, based on MPDATA transport methods, employed to simulate the boundary layer flow aloft. Theoretical discussions are illustrated with numerical experiments that explore the interactive evolution of the dune/atmosphere system. Copyright © 2005 John Wiley & Sons, Ltd.