Interdune Spacing Research Articles

A trans-scale simulation of aeolian sand flow – dune – dune field based on actual environmental wind field

The present work employs a predictive model of fluctuating wind velocities constructed on the basis of long-term field observations to perform trans-scale simulations of aeolian sand flow, dune morphology and migration characteristics, and the overall evolution of dune fields. On the premise of comparing the simulated basic statistics of turbulent wind field and sand flow, and dune dynamic characteristics with experimental results to verify the reliability of our dune field model, the simulation results at different scales are compared with those of mean-flow based simulations to explore the effects of turbulent fluctuations. Analyses on sand transport rate of sand flow and average saltation length of sand particles indicated that turbulent fluctuations enhance the sand transport capacity of wind field, which promotes the initial morphological growth of dunes, dune migration and inter-dune interactions; and thus, more small size dunes with typical morphologic features are formed earlier within dune fields. The cumulative impacts of turbulence during long-term evolution accelerate the coarsening of dune field pattern, causing the dune number density to decline more rapidly, the dune height and inter-dune spacing to be larger in dune fields. Additionally, the turbulence effects enhanced with increasing wind velocities have more significant influences on small inertial particles with better following features. These findings contribute to insights into the effects of natural turbulent wind field on aeolian dune systems.

The impact of inter-dune space and volume ratio on barchan dune collision patterns: A numerical simulation study

Collision among barchan dunes is a main cause for the dune swarm to achieve size and speed selection, and it can be divided into two patterns: coalescence and ejection. Here, a three-dimensional (3D) cellular automaton model is applied to collectively study the effects of inter-dune space and relative dune size on these collision patterns. A parameter space is established that includes a transition surface distinguishing coalescence and ejection. Confirmed by field observations and subaquatic experiments, a comprehensive depiction of transition between coalescence and ejection is provided, which may bring new constraints on the morphodynamics of barchan dune fields.

Large-eddy simulation of turbulent flow over spanwise-offset barchan dunes: Interdune vortex stretching drives asymmetric erosion

The coupling between turbulent flow physics and barchan dune geometry is important to dune migration, morphology of individual dunes, and the morphodynamics of merging and separating proximal dunes. Large-eddy simulation was used to model turbulent, inertial-dominated flow over a series of static barchan dune configurations. The dune configurations were carefully designed to capture realistic stages of a so-called ``offset interaction,'' wherein a small dune is placed upflow of a relatively larger dune, thereby guaranteeing interaction since the former migrates faster than the latter. Moreover, as interaction proceeds, the morphology of the small dune is mostly preserved, while the large dune undergoes dramatic transformation with greater erosion downflow of the interdune space. Simulations reveal that the wake centerline---determined here as the spanwise location at which the momentum deficit associated with each dune exhibits a minimum---veers due to dune geometry. Visualization of vortex identifiers reveals that hairpin vortices are produced via separation across the crestline of dunes, and these hairpins are advected downflow by the prevailing, background flow. The legs of hairpins emanating from the upflow dune contain streamwise vorticity of opposite sign, wherein the hairpin leg within the interdune space exhibits positive streamwise vorticity. This positive streamwise vorticity is supplied to the interdune space, where flow channeling induces acceleration of streamwise velocity. An assessment of right-hand side terms of the Reynolds-averaged streamwise vorticity transport equation confirm, indeed, that vortex stretching is the dominant contributor to sustenance of streamwise vorticity. With this, we can conclude that asymmetry of the large, downflow dune is a consequence of scour due to the interdune roller, and scouring intensifies as the spacing between dunes decreases. A structural model outlining this process is presented.

Turbulent Flow Structure Associated With Collision Between Laterally Offset, Fixed‐Bed Barchan Dunes

Barchan dunes are three‐dimensional, crescent‐shaped bedforms found in regions of unidirectional flow and limited sediment supply, and while most commonly associated with aeolian environments, they have recently been observed in subaqueous domains and on the surfaces of Mars and Titan. As barchans migrate in the direction of the flow, they interact with their neighbors, typically by way of collision. The morphodynamics of such collision processes are complex, where the role of the turbulent flow structure is strongly coupled to that of sediment transport and morphological change. Here we study the flow structure in a decoupled manner through measurements of turbulent flow over fixed‐bed barchan models in a series of configurations mimicking the early stages of a laterally offset collision between two dunes. Particle image velocimetry is used to measure the flow in a refractive‐index matched flume that enables uninhibited access to the flow field to study the role that the upstream barchan plays in modifying the turbulent structure over the downstream one. Flow sheltering by the upstream barchan results in a momentum deficit in its wake that is also marked by heightened levels of turbulence over the downstream barchan. Additionally, flow asymmetry induced by the lateral offset between barchans drives a flow channeling effect around the horn of the downstream barchan. Decomposition of Reynolds shear stresses in the interdune space via quadrant analysis reveals that near‐bed turbulent fluctuations capable of mobilizing sediment are particularly enhanced at mean flow reattachment where vortices within the shear layer impact the bed.

Sand transportation and reverse patterns over leeward face of sand dune

Sand saltation has complex interactions with turbulent flow and dune form. Most models of wind-blown sand consider ideal circumstances such as steady wind velocity and a flat surface, and the bulk of data on wind flow and sand transport over an individual dune has focused mostly on the influence of dune shape or inter-dune space on the wind flow, neglecting the effect of morphology on sand saltation, particularly airflow and sand transportation over the leeward slope. Wind flow structures over the leeward slope of sand dunes have a fundamental influence on the organization of sand dunes. In order to understand sand dune dynamics, lee face airflow and sediment transportation should be paid more attention. Previous field observations could not measure turbulent flow structure well because of the limited observation points and the influence of experiment structure on wind field. In addition, the reverse sand particles over leeward face could not be collected by sand trap in field. Numerous field observations could not measure turbulent flow structure because of the limited observation points and the influence of experimental structures on the wind field. In addition, the reverse transport of sand particles over leeward face could not be collected by sand traps in field. Therefore, this paper aims to investigate the turbulent flow structure and sand transport pattern over the leeward slope. A numerical model of sand saltation over slope terrain is constructed, which also considers the coupling effects between air flow and sand particles. The large eddy simulation method is used to model turbulent flow. Sand transport is simulated by tracking the trajectory of each sand particle. The results show that terrain significantly alters the turbulent air flow structure and wind-blown sand movement, especially over the leeward slope. Here, mass flux increases initially and then decreases with height in the reversed flow region in the direction of wind flow, and the mass flux decreases with height in the reversed direction. The height of 0.5H is the height of vortex core in the reversed flow region. The vortex core is a critical point in the flow region where few particles are transited. In the reversed region, the reversed mass flux of sand particles is 25% of the mass flux in the flow direction. This research may contribute to scientific understanding of the mechanisms of sand motion and wind flow over leeward of dune and it is likely to be significant in desertification control.

Temporal and spatial differences in gall induction on Haloxylon by Aceria haloxylonis (Acari: Eriophyidae) in the Gurbantünggüt Desert

Flower-like galls have been observed on Haloxylon ammodendron and H. persicum in the Gurbantünggüt Desert in northwest China. The galls were induced by Aceria haloxylonis, a new species of Eriophyidae. The galls began as small protuberances at the base of new stems and on small branches. As they matured, the galls changed color from green to dark brown. Some galls on H. persicum became red. At maturity, the galls and the infected branches became desiccated. Adult females of A. haloxylonis overwintered in galls or in branch crevices of H. ammodendron and H. persicum. There were more galls on H. ammodendron than on H. persicum. Several ecological factors influenced gall number, including terrain, tree size, branch direction and slope aspect. H. ammodendron trees in gravel desert had more galls than trees at sand dune edges. Trees in the interdune space had the fewest galls. Large H. ammodendron trees had significantly more galls than small trees. Branches on the south side of the tree had more galls than branches on the north, east, and west sides. Terrain * tree size had significant interaction on gall number on H. ammodendron. H. persicum trees on low sand dunes had more galls than trees on high sand dunes and trees on sunny slopes had more galls than trees on shady slopes. There were more galls on large H. persicum trees than on medium-sized trees. Few galls were observed on small H. persicum trees. The number of galls on H. persicum was significantly affected by terrain, tree size and slope aspect. The terrain * slope aspect interaction and tree size * terrain interaction were also significantly. This study is important for the conservation and recovery of the ecological environment in the Gurbantünggüt Desert.

Numerical and experimental study of flow over stages of an offset merger dune interaction

Results of a coordinated research effort on unidirectional turbulent flows over canonical barchan dunes at high Reynolds number are presented. Large-eddy simulations (LES) and experiments are conducted under inertial-dominated flow conditions, thus capturing dynamics essential to fully understanding processes responsible for observed patterns of asymmetric erosion and migration. A series of dune field topographies have been considered wherein a small “impactor” dune is positioned at a series of positions upflow of a large “parent” dune, from a spanwise-offset position; this interaction constitutes a so-called “offset interaction” Kocurek and Ewing [14]. The small dune is geometrically similar, but one-eighth the volume of the large dune. Since migration rate is inversely proportional to volume, the prescribed volumetric ratio ensures that the synthetic topographies replicate instantaneous configurations exhibited during actual dune interactions in a laboratory or natural setting. In this sense, the static dune configurations provide a means to understand flow processes responsible for patterns of erosion and deposition that induce interactions. Experimental measurement and LES are both used to study these configurations, with strong agreement reported between resultant datasets. We report that flow channeling in the interdune space between the upflow and downflow dune induces a mean flow heterogeneity – termed “wake veering” – in which the location of maximum momentum deficit in the dune wake is spanwise displaced. We also report elevated turbulent stresses in the channeling region. These results are used to explain the mechanisms responsible for simultaneous merging and ejection between the small and large dunes. Finally, spatial distributions of surface stress from LES have been used to identify locations of elevated erosion and, therefore, to predict bedform migration patterns. Results show that locations of minimal erosion – whether associated with upflow sheltering or with vanishing spatial gradients of dune height – constitute spatial “junctions” of coalescing, proximal dunes. In the absence of upwind sheltering, or in locations with the least upwind sheltering, turbulent mixing facilitates downward fluxes of high momentum fluid. This augments downwind dune erosion, cumulatively accelerating the migration of downwind dunes.

Interstadial inland dune slacks in south-west Slovakia: a multi-proxy vegetation and landscape reconstruction

Palaeoecological studies spanning back to the Bølling/Allerød (GI-1) Interstadial of the Late Glacial in the whole Western Carpathians and adjacent lowlands are extremely scarce. To improve our knowledge about the landscape structure and diversity of wetland habitats, we analysed two palaeoecological interstadial sequences of pollen and macrofossils (bryophytes, vascular plants and molluscs) from the sandy landscape of the Borská lowland (southwest Slovakia). The deposits differed between the two sequences analysed, consisting predominantly of lacustrine chalk or brown-moss sedge fen peat, respectively. The results showed that, during wetter and warmer conditions during the Interstadial, open large dunes had been stabilised by pine woodlands admixed by birch. Contrary to the Podunajská lowland, which is situated eastwards towards the centre of the Pannonian Plain, broadleaf thermophilous trees such as Ulmus, Quercus and Tilia were almost absent in the study region, which suggests an importance of the bedrock for Late-Glacial forest vegetation. Salix, Alnus and Betula nana scrubs were probably scattered across wetlands. Active dunes were occupied by Hippophaë rhamnoides and steppe vegetation with grasses, Artemisia and Helianthemum. Low abundance of Chenopodiaceae pollen suggests that the sandy bedrock was probably unfavourable for the development of salt-rich habitats. Wetland habitats were diverse and abundant in the inter-dune space, partly resembling recent coastal dune slack vegetation or alluvial salty grasslands in continental central Asia. The wetland habitats comprised aquatic habitats with Myriophyllum, Potamogeton filiformis, P. pectinatus, P. pussilus, Sparganium minimum and planktonic algae (Pediastrum and Botryococcus), strongly continental calcareous-fens with Blysmus rufus and Triglochin maritima, boreocontinental brown-moss sedge fen fens with Drepanocladus trifarius, Carex lasiocarpa, Menyanthes trifoliata and Scorpidium scorpioides) and reed beds with Cicuta virosa, Phragmites and Schoenoplectus tabernaemontani. Remains of these unique wetland mosaics have recently disappeared because of human activities including peat extraction and dune stabilisation by pine plantations, which have transformed the modern landscape.

Sand on salt: Controls on dune subsidence and determining salt substrate thickness

Lenticular sand bodies in the Upper Jurassic Norphlet Sandstone in the northeastern Gulf of Mexico basin have been interpreted as preserved large linear dunes that subsided into the underlying Louann Salt layer. Due to imaging difficulties, the thickness of the underlying salt is largely unknown. We performed physical experiments and created a mathematical model to understand dune and salt deformation and to determine the controls on the final preserved dune topography. Nonuniform dune geometry results in unequal loading by sand that drives locally varying subsidence into the salt substrate, resulting in varying degrees of dune flattening and dune relief. Our results indicate that initial salt thickness relative to dune size governs the initial rate of subsidence, thus affecting the final dune topography. However, interdune spacing and dune geometry are also important because they set the pressure gradient and determine the maximum possible subsidence. We conclude that preserved dunes with high relief indicate areas of relatively thin salt thickness and/or close dune spacing, and that dunes with low relief indicate areas of relatively thick salt thickness and/or wide dune spacing. For the case of the Norphlet Formation, dune spacing and salt thickness likely played the major roles in controlling dune subsidence.

Large eddy simulation of interacting barchan dunes in a steady, unidirectional flow

We have performed large-eddy simulations of turbulent flow 4 over barchan dunes in a channel with different interdune spacings in the downstream direction at Reynolds number, Re∞ ≃ 26000 (based on the free 6 stream velocity and channel height). Simulations are validated against ex-perimental data (at Re∞ = 55460); the largest interdune spacing (2.38λ, where λ is the length of the barchan model) presents similar characteristics to the isolated dune in the experiment, indicating that at this distance the sheltering effect of the upstream dune is rather weak. We examine 3D realizations of the mean and instantaneous flow to explain features of the flow field relevant to sediment transport. Barchan dunes induce two counter-rotating streamwise vortices, along each of the horns, which direct high-momentum fluid toward the symmetry plane and low-momentum fluid near the bed away from the centerline. The flow near the bed, upstream of the dune, diverges from the centerline plane, decelerates and then rises on the stoss side of the dune while accelerating; the flow close to the centerline plane separates at the crest and reattaches on the bed. Away from the centerline plane and along the horns, flow separation occurs intermittently. The flow in the separation bubble is routed towards the horns and leaves the dune at their tips. The separated flow at the crest reattaches on the bed, except on the centerline symmetry plane of the dune, where a weak saddle point of separation ap- pears at the bed. The distribution of the bed shear-stress, characteristics of the separation and reattachment regions, and instantaneous wall turbulence are discussed. Characteristics of the internal boundary layer developing on the bed after the reattachment region are studied. The interdune spacing isfound to affect significantly the turbulent flow over the stoss side of the downstream dunes; at smaller interdune-spacings, coherent high- and low- speed streaks are shorter but stronger, and the spanwise normal Reynolds stress is larger. The turbulent kinetic energy budgets show the importance of the pressure transport and mean-flow advection in transporting energy from the overlying wake layer to the internal boundary layer over the stoss side of the closely-spaced dunes. The characteristics of the separated-shear layer are altered slightly at smaller interdune spacing; the separation bubble is smaller, the separated-shear layer is stronger, and the bed shear-stress is larger. Away from the dunes, typical wall-turbulence structures are observed, but coher- ent eddies generated in the separated-shear layer due to the Kelvin-Helmholtz instability are dominant near the dune. Coherent structures are generated more frequently at smaller interdune spacing; they move farther away from the bed, towards the free surface, and remain in between the horns. At larger interdune spacings, these coherent structures are advected in the spanwise direction with the mean streamwise vortices and can be observed outside of the dunes.

The dune size distribution and scaling relations of barchan dune fields

Barchan dunes emerge as a collective phenomena involving the generation of thousands of them in so called barchan dune fields. By measuring the size and position of dunes in Moroccan barchan dune fields, we find that these dunes tend to distribute uniformly in space and follow an unique size distribution function. We introduce an analytical mean-field approach to show that this empirical size distribution emerges from the interplay of dune collisions and sand flux balance, the two simplest mechanisms for size selection. The analytical model also predicts a scaling relation between the fundamental macroscopic properties characterizing a dune field, namely the inter-dune spacing and the first and second moments of the dune size distribution.

Sand Dune Attributes Estimated from SAR Images

Dunes (such as linear dunes oriented in a constant direction), their size, and interdune spacing are very regular in the deserts. Therefore, they manifest a periodic pattern in terms of spatial distribution. There are several modulation transfer functions between SAR images and the actual periodic surface parameters, and the image brightness is sensitive to the local incidence angle. According to previous studies, a linear mapping process can describe these modulation processes. This study proposes an algorithm for estimating the linear dune features including linear dune direction (the prevailing wind direction), interdune spacing, and rms slope and dune height from SAR images. To estimate rms slope, two algorithms were developed, a local incidence angle method and a modulation transfer function method. The linear dune height was then calculated based on the rms slope of the surface. Finally, the obtained results were compared with the truth data from field investigations. The estimated physical quantities did not completely agree with the truth data, and possible error sources are discussed.

The maintenance of species diversity by miniature dunes in a sand-depletedHaloxylon salicornicumcommunity in Kuwait

Dwarf shrubs play a predominant role in the natural vegetation cover of Kuwait, with the chenopodHaloxylon salicornicumwidespread in northern parts of the country. A number of influences, largely anthropogenic in nature, have led to serious land degradation in many areas. The present study deals with species diversity and other floristic attributes of the ephemeral vegetation in a sand-depletedHaloxylon salicornicumcommunity, contrasting the situation of miniature dunes formed at the base of shrubs with that of the interdune space. The importance of such dunes and their associated shrub cover as a protection against further land deterioration is examined.

Two types of longitudinal dune fields and possible mechanisms for their development

AbstractLongitudinal dune fields characterized by nearly uniform interdune spacing are distinguished from longitudinal dune fields characterized by fairly variable interdune spacing and high frequencies of dune coalescence. The empirical and theoretical evidence indicating that the former may be due to helical air currents aligned with the dunes is reviewed. Hypotheses arguing that the latter may arise indirectly from horizontal pressure gradients or bidirectional wind regimes are discussed. Evenly spaced linear sand banks aligned with tidal currents may be shown mathematically to result from energy optimalization within two‐dimensional, sand‐transporting flow regimes, and a similar simple or non‐rotational flow model is considered for the problem of desert longitudinal dunes. An initial complex or rotational flow analysis is undertaken to discern the likely significance of roll vortices in desert sediment transport. An ‘evolutionary timescale’ is estimated for the formation of desert longitudinal dune fields. A simple analysis is performed for the effect of regional sand mass change on longitudinal dune field ordering. Recommendations are made for future empirical and theoretical research.

Spaceborne and airborne imaging radar observations of sand dunes

Seasat and aircraft radar imagery of five areas of sand dunes in the southwestern United States and northwestern Mexico have been studied and compared to Landsat imagery and air photos. Radar imaging system parameters included two wavelengths (23.5 cm and 3.0 cm) and incidence angles ranging from 0° through 70°. Horizontal polarization was used on both transmit and receive for all radar imagery. Resolution ranged from 12 m for the best aircraft imagery to about 40 m for Seasat. Sand dunes studied behave as smooth surfaces for both 23.5 cm and 3.0 cm wavelength radars and the reflection of the radar energy from the dunes is specular. Measured surface irregularities of sand dunes are inadequate to cause either Bragg or incoherent scattering of the incident radar beam in the 3.0 to 23.5 cm wavelength region. This results in dunes that appear dark when imaged at incidence angles greater than the angle of repose because none of the dune faces are oriented properly (normal to the incident radar beam) to give a specular return to the antenna. A scattering model based on specular return from randomly oriented flat dune faces was used and experimentally verified using aircraft data. Imaging radar resolution must be adequate to define the changing distribution of dune faces that characterize the dune morphology. Linear dune features with interdune spacing of less than 3–4 resolution elements are not recognizable. Distributed dune types such as barchans or stars must be more than about 10 resolution elements in each direction to be defined on the imagery. Illumination direction of the radar beam is important; directional dune features must be oriented within about 60° of perpendicular to the radar illumination direction in order to be imaged. If such features have considerable topographic relief, the direction of radar illumination is less important. Because directional trends in unexplored regions cannot be predicted, availability of radar imagery from two directions greatly facilitates interpretation of dune morphology and hence conclusions about causative wind regimes.