Shape Of Dunes Research Articles

Morphology and maintenance of steep dunes near dune asymmetry transitional areas on the shallow shelf (Beibu Gulf, northwest South China Sea)

Symmetrical dunes can be found in areas where dune asymmetry changes sense, but in areas where dunes are not expected to be moving the development and maintenance of symmetrical dunes are poorly understood. In the southeastern Beibu Gulf, we have identified five typical dune asymmetry transitions at the depths of shallower than 50 m basing on the latest bathymetric data. Those regions are separated into two classes according to their different shapes and locations. Sand transport directions are deduced by dune migrations between surveys, simulated regional currents, and sand size trends. The two classes of the regions are found to experience opposing net sand transports from the north and south. Around the asymmetry transitions, the dunes have impressive steep sections, relatively symmetrical shapes, and almost straight crests. They have a general Height-Length (H-L) relation: H = 0.0575 L1.0836 (R2 = 0.834), which is steeper than dunes in other seas. Some of the dunes have extraordinary large heights exceeding a quarter of water depth, which is attributed to the preserved sand body, bidirectional sand accretion, erosion in troughs, and absence of significant erosion on crests. The steepness and straight crests of dunes adapt to the nearly symmetrical or weak asymmetrical reversing currents, which induce flow separations and reversals on both flanks of the dunes. It also suggests that dunes develop differently under sufficient and limited sand supply. Dunes will gradually grow in size when being continually supplied with sand. However, if sand supply is restricted, dunes may separate or turn into more isolated individuals to sustain the H-L relation. The H-L relation of dunes can be maintained during either growth or separation. Overall, the nearly symmetrical or weak asymmetrical reversing currents maintain the steep shapes of dunes while variations in sand supply affect their sizes. To work out the detailed transports of sediment, future observations of flow structures and related models of such two-sided steep dunes are still required.

Species-specific functional morphology of four US atlantic coast dune grasses: Biogeographic implications for dune shape and coastal protection

Species-specific functional morphology of four US atlantic coast dune grasses: Biogeographic implications for dune shape and coastal protection

Higher-order moments with turbulent length-scales and anisotropy associated with flow over dune shapes in tidal environment

The purpose of this research paper is to investigate the third-order moments of velocity fluctuations, turbulent kinetic energy (TKE) fluxes, turbulence dissipation rate, turbulent length scales, and Reynolds stress anisotropy in the wake regions over two artificial upstream-facing bed forms. For the structure having a smaller flow-facing slope (slanting forward-facing structure, named as SFFS), the maximum flow intermittency is 0.6% with a maximum flow reversal frequency of 0.24 Hz, whereas for the greater flow-facing sloped structure (vertical forward-facing structure, named as VFFS), up to 70% flow intermittency is noticed just downstream of the corresponding crest location with a maximum of 28 Hz flow reversal frequency. A strong negative M30 (stream-wise flux of stream-wise normal stress) and a strong positive M03 (vertical flux of vertical normal stress), implying the occurrence of ejection events, take place above the flow separation region along the generated shear layer. However, near the lee-surface, positive M30 and negative M03 imply the presence of sweep events. For both the cases, maximum of normalized TKE (κ) is generated in the vicinity of crest location, which spread over the retarding flow region near-bed of the trough and wake-flow region downstream of the crest. The production of turbulence is mainly concentrated at the interface of intense ejection and sweep dominated zones over the trough region for the VFFS case, and for the other bed form, i.e., SFFS case, maximum turbulence production is concentrated at the near-bed crest location. Near the bed, the length-scale of eddies in the inertial sub-range decreases substantially compared to that in the upstream. It is seen that in the region of negative kinetic energy production, local maximum anisotropy is found, especially for b11,22,33S and b11,22,33V components. Interestingly, at the corresponding crest points for SFFS and VFFS, maximum affinity toward the one-component isotropy is measured almost throughout the vertical height. However, a clear increasing trend in the affinity to a better isotropy; i.e., the three-dimensional isotropy is identified as the vertical height increases up to z ≈ 3 cm above the bed when a fair downstream location for the VFFS case is considered. A special look toward the investigation of possible connection of the flow region with negative TKE production with the stream anisotropy shows that a substantial increase in the degree of anisotropy (Ad) in the anisotropy invariant maps of the negative TKE production takes place for both the cases, when compared to that for the positive TKE production region.

Morphodynamics of barchan and dome dunes under variable wind regimes

Research Article| July 19, 2018 Morphodynamics of barchan and dome dunes under variable wind regimes Xin Gao; Xin Gao 1State Key Laboratory of Desert and Oasis Ecology, Xinjiang Institute of Ecology and Geography, Chinese Academy of Sciences, 818 South Beijing Road, Urumqi 830011, Xinjiang, China Search for other works by this author on: GSW Google Scholar Cyril Gadal; Cyril Gadal 2Institut de Physique du Globe de Paris, Sorbonne Paris Cité, Université Paris Diderot, UMR 7154 CNRS, 1 rue Jussieu, 75238 Paris Cedex 05, France Search for other works by this author on: GSW Google Scholar Olivier Rozier; Olivier Rozier 2Institut de Physique du Globe de Paris, Sorbonne Paris Cité, Université Paris Diderot, UMR 7154 CNRS, 1 rue Jussieu, 75238 Paris Cedex 05, France Search for other works by this author on: GSW Google Scholar Clément Narteau Clément Narteau 2Institut de Physique du Globe de Paris, Sorbonne Paris Cité, Université Paris Diderot, UMR 7154 CNRS, 1 rue Jussieu, 75238 Paris Cedex 05, France Search for other works by this author on: GSW Google Scholar Author and Article Information Xin Gao 1State Key Laboratory of Desert and Oasis Ecology, Xinjiang Institute of Ecology and Geography, Chinese Academy of Sciences, 818 South Beijing Road, Urumqi 830011, Xinjiang, China Cyril Gadal 2Institut de Physique du Globe de Paris, Sorbonne Paris Cité, Université Paris Diderot, UMR 7154 CNRS, 1 rue Jussieu, 75238 Paris Cedex 05, France Olivier Rozier 2Institut de Physique du Globe de Paris, Sorbonne Paris Cité, Université Paris Diderot, UMR 7154 CNRS, 1 rue Jussieu, 75238 Paris Cedex 05, France Clément Narteau 2Institut de Physique du Globe de Paris, Sorbonne Paris Cité, Université Paris Diderot, UMR 7154 CNRS, 1 rue Jussieu, 75238 Paris Cedex 05, France Publisher: Geological Society of America Received: 01 May 2018 Revision Received: 06 Jul 2018 Accepted: 07 Jul 2018 First Online: 19 Jul 2018 Online Issn: 1943-2682 Print Issn: 0091-7613 © 2018 Geological Society of America Geology (2018) 46 (9): 743–746. https://doi.org/10.1130/G45101.1 Article history Received: 01 May 2018 Revision Received: 06 Jul 2018 Accepted: 07 Jul 2018 First Online: 19 Jul 2018 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Xin Gao, Cyril Gadal, Olivier Rozier, Clément Narteau; Morphodynamics of barchan and dome dunes under variable wind regimes. Geology 2018;; 46 (9): 743–746. doi: https://doi.org/10.1130/G45101.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract Dome dunes are traditionally interpreted as a transient or an independent class of bedforms because of their rounded and smooth shape without slipfaces. Here we show that they can also reach a steady state and form dome dune fields, which entail the same interactions between flow, bed topography, and moving sediment as other dune types in the absence of grain-size segregation and vegetation. We study the transition from barchan to dome dunes by increasing the standard deviation of a normal distribution of sand flux orientation in a numerical model. We find that, under steady-state conditions, barchan and dome dunes exhibit the same relationships between their height and migration rate. As shown in Earth’s deserts, dome dune shape and migration rate can then be used to estimate sand flux properties, including the variability in transport directionality. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Migration and Morphology of Asymmetric Barchans in the Central Hexi Corridor of Northwest China

Crescent-shaped barchan dunes often display an asymmetric shape, with one limb longer than the other. As shown in previous studies, asymmetric bimodal winds constitute one major cause of barchan asymmetry, but the heterogeneous conditions of sand availability or flux, as well as topographic influences, may be also important. Understanding the morphology and dynamics of asymmetric barchans may have an impact in a broad range of areas, particularly as these dunes may serve as a proxy for planetary wind regimes and soil conditions in extraterrestrial environments. However, in addition to the existing theories and numerical models that explain barchan asymmetry, direct measurements of migration rates and morphologic changes of real asymmetric barchans over a time span of several years would be beneficial. Therefore, here we report such measurements, which we have acquired by investigating asymmetric barchans in the Hexi Corridor, northwest of China. We have found that dune interactions and asymmetric influx conditions are the most important causes of barchan asymmetry in this field. Particle size distributions in the Hexi Corridor display strong variations over different parts of the asymmetric barchans, as well as over different dunes, with gravel particles being incorporated from the substrate as the dunes migrate. Our observations have shown that upwind sediment sources are important for dune formation in the Hexi Corridor, and that interdune interactions affect dune shape in different ways, depending on their offset. The asymmetric barchans in the Hexi Corridor are active, with an average migration rate (MR) between 8 and 53 m year−1, in spite of the different asymmetric shapes. Our data for dune migration rates can be described well by a scaling of MR = A/(W + W0), where W is the barchan cross-wind width, A ≈ 2835 m2 s−1, and W0 ≈ 44 m. A similar scaling fits very well the migration rate as a function of dune along-wind width L, (i.e., MR = B/(L + L0), with B ≈ 1722 m2 s−1 and L0 ≈ 13 m). Linear relations are also found between both dune widths and the average limb and windward side lengths, thus indicating that the morphometric relations that are predicted from models for steady-state, symmetric crescent-shaped dunes can be applied to different transitional morphologies of interacting, asymmetric barchans.

Experimental study on the formation of subaqueous barchan dunes in closed conduits

The present paper reports the formation of subaqueous barchan dunes by analyzing the temporal evolution of their main geometrical characteristics (width W, length L and horn lengths Lh). After certain time, the dunes reach an equilibrium state and it is possible to study the relation between W versus L, and the dependence of the dune velocity on L. The barchan dunes were formed from spherical glass and zirconium beads. An initial conical heap of beads was placed on the bottom wall of a rectangular channel and it was entrained by a water turbulent flow. The evolution of the dunes was filmed with a CCD camera placed above the channel and mounted on a traveling system. Our results show that after a characteristic time the dune shape does not change and it travels with a roughly constant velocity. Once the equilibrium state is reach, W and L are measured, showing linear dependence. Furthermore, we show that the dune velocity Vd scales with the inverse of the dune length.

Parabolic dune development modes according to shape at the southern fringes of the Hobq Desert, Inner Mongolia, China

Since the 1970s, parabolic dunes at the southern fringe of the Hobq Desert, Inner Mongolia, China have exhibited many different shapes (V-shaped, U-shaped, and palmate) each with a unique mode of development. In the study area, parabolic dunes are mainly distributed in Regions A, B, and C with an intermittent river running from the south to the north. We used high-resolution remote-sensing images from 1970 to 2014 and RTK-GPS measurements to study the development modes of different dune shapes; the modes are characterized by the relationship between the intermittent river and dunes, formation of the incipient dune patterns, the predominant source supply of dunes, and the primary formation of different shapes (V-shaped, U-shaped, and palmate). Most parabolic dunes in Region A are V-shaped and closer to the bank of the river. The original barchans in this region exhibit “disconnected arms” behavior. With the sand blown out of the riverbed through gullies, the nebkhas on the disconnected arms acquire the external sand source through the “fertile island effect”, thereby developing into triangular sand patches and further developing into V-shaped parabolic dunes. Most parabolic dunes in Regions B and C are palmate. The residual dunes cut by the re-channelization of river from transverse dune fields on the west bank are the main sand source of Region B. The parabolic dunes in Region C are the original barchans having then been transformed. The stoss slopes of V-shaped parabolic dunes along the riverbank are gradual and the dunes are flat in shape. The dune crest of V-shaped parabolic dune is the deposition area, which forms the “arc-shaped sand ridge”. Their two arms are non-parallel; the lateral airflow of the arms jointly transport sand to the middle part of dunes, resulting in a narrower triangle that gradually becomes V-shaped. Palmate parabolic dunes have a steeper stoss slope and height. The dune crest of the palmate parabolic dune is the erosion area, which forms a long and narrow trough between nebkhas by the “funnelling effect”. This process forces sand towards lee slopes, which transform from concave (original barchans) into convex, ultimately resulting in the formation of palmate parabolic dunes.

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.

Skin Friction Reduction Characteristics of Nonsmooth Surfaces Inspired by the Shapes of Barchan Dunes

A new type of nonsmooth surface inspired by the shape of barchan dunes has been proposed and is intended to reduce skin friction, a major cause of overall drag. Simulations were carried out to obtain skin friction reduction characteristics for the nonsmooth surface using the commercial computational fluid dynamics software Fluent. A realizable k‐ε model was employed to assess the influence of the nonsmooth structure on turbulent flow and velocity fields. The numerical simulation results showed that the new nonsmooth surface possesses the desired skin friction reduction effect and that the maximum skin friction reduction percentage reached 33.63% at a fluid speed of 30 m/s. Various aspects of the skin friction reduction mechanism were discussed, including the distribution of velocity vectors and shear stress contours and the variations in boundary layer thickness. The accuracy of the flow field for the nonsmooth unit was further verified by particle image velocimetry test results. The new bionic nonsmooth surface, which exceeds the limitations of existing nonsmooth bionic structures, can effectively reduce skin friction and should provide insights into engineering applications in the future.

A coupled CFD-DEM approach to model particle-fluid mixture transport between two parallel plates to improve understanding of proppant micromechanics in hydraulic fractures

Computational Fluid Dynamics coupled with Discrete Element Method (CFD-DEM) was used in this paper to model particle-liquid mixture transport between two parallel plates to improve the understanding of proppant micromechanics in a hydraulic fracture. The linear spring-dashpot model was used to model contact behavior between the particles in DEM code, and the interaction between the particles and fluid was coupled in the CFD code in terms of volumetric porosity and coupling force. The flow patterns and particle transport mechanisms were investigated based on which four developmental stages were divided from the beginning of injection to the formation of the final particle bank. The results show that when particles are transported in a thin fluid, they will quickly settle out of the fluid and accumulate at the bottom forming a particle dune. As the dune height increases, the flow stream is gradually hindered by the dune, and then the injected particles are vertically lifted and settle at the front of the dune. When the equilibrium height of the dune is reached, the dune develops to a bank, and then the particles injected later overshoot the bank and settle at the back side of the bank. The dune shape is significantly influenced by the erosion caused by the transported particles and flowing fluid, and the flow patterns of three-layers and two-layers are observed in different stages. Three particle transport mechanisms of settlement, fluidization and suspension, previously presented in early experimental studies, are observed in the CFD-DEM simulations. A new important transport mechanism of vorticity is also observed, which can control the motion direction of the injected particles during earlier and later injection stages.

AUTOMATIC EXTRACTION OF DUNES FROM GOOGLE EARTH IMAGES NEW APPROACH TO STUDY THE DUNES MIGRATION IN THE LAÂYOUNE CITY OF MOROCCO

Abstract. In desert environments, the sandy dune movements are considered as the main threat to the livelihoods and productivity of people living in Laâyoune city, South of Morocco. Studying dunes at a large scale requires intensive fieldwork and a huge dataset. But this task is not always easy because of the high cost and the limited availability of data. In this paper we present a new approach to study and evaluate the dune dynamic using Google earth archive image acquired from 2005, 2011, 2012, and 2013. To achieve this goal, an area with 3577 meter square including 40 barchan dunes, were selected, and the barchan dunes were automatically extracted from these images using Geographic Information System (GIS) and spatial analysis techniques provided by image processing software. The result shows not only the dune shape but also direction of dunes movements, and distance separating dunes. The obtained direction is ranging from North east to South west. The average rate of movement of the extracted dunes between 2005 and 2013 was calculated and is estimated at 32m per year. Validation of these dunes movement of the Laâyoune city was confirmed by the field data and previous studies.

A review of Computational Fluid Dynamics (CFD) airflow modelling over aeolian landforms

Abstract Aeolian landforms occur on all earths’ continents as well as on Mars, Titan and Venus and are typically formed where sediment is eroded and/or deposited by near surface wind flow. As wind flow approaches an aeolian landform, secondary flow patterns are created that cause wind to deviate in both speed and direction, producing complex patterns of sediment erosion, deposition and transportation. Computational Fluid Dynamics (CFD) modelling of wind flow has become a common tool to predict and understand secondary wind flow and resulting sediment transport. Its use has progressed from simulating wind flow over simple two dimensional dune shapes, to calculating a multitude of flow parameters over a range of increasingly complex landforms. Analysis of 25 peer reviewed journal articles, found that CFD has been crucial to providing additional insight to flow dynamics on the stoss slope of dunes, the structure and nature of wind flow separation in the lee of landforms and information on localised wind flow variations in large-scale dune fields. The findings of this assay demonstrate that further research is required regarding the parameterisation and modelling of surface roughness, the incorporation of accurate sediment transport to wind flow models, and the prediction of topographic surface changes. CFD is anticipated to be increasingly utilised in aeolian geomorphology and this work aims to be a starting point for aeolian geomorphologists wishing to better understand and review the utilisation of the technique to date.

Improvements of film cooling effectiveness by using Barchan dune shaped ramps

In the present study, an experimental investigation was conducted to explore a novel film cooling design with Barchan-dune-shaped-ramp (BDSR) concept to augment the effectiveness of film cooling injection from discrete holes. Inspired by the unique shape of Barchan dunes commonly seen in deserts to prevent sand particles on the ground from being blown away by the oncoming airflow, the BDSR concept was proposed to enable the coolant streams exhausted from coolant injection holes to stay more firmly on the surface of interest for improved film cooling performance. During the experiments, while coolant streams were injected from discrete circular holes on a flat test plate at an injection angle of 35°, a row of BDSRs were mounted on the test plate at either downstream or upstream of the coolant injection holes for improved film cooling effectiveness. While a high-resolution Particle Image Velocimetry (PIV) system was used to conduct detailed flow field measurements to quantify the dynamic mixing process between the coolant jet streams and the mainstream flows over the test plate, a Pressure Sensitive Paint (PSP) technique was used to map the corresponding adiabatic film cooling effectiveness on the surface of interest based on a mass-flux analog to traditional temperature-based cooling effectiveness measurements. The cooling effectiveness data of the BDSR design were compared quantitatively against those of a conventional film cooling design without BDSR (i.e., baseline case) under the same test conditions in order to evaluate the effects of the BDSRs on the film cooling effectiveness over the surface of interest. The effects of the locations and height of the BDSRs in relation to the coolant injection holes on the film cooling effectiveness at different blowing ratios (i.e., the coolant-to-mainstream mass flux ratios) were examined quantitatively based on the PIV and PSP measurements. The detailed flow field measurements were correlated with the measured film cooling effectiveness distributions to elucidate the underlying physics in order to explore/optimize design paradigms for better film cooling protection of turbine blades from harsh environments.

Characterization of Sand Dunes to Detect the Sand Source and their Stabilization, Abardej, Iran

A dune field was investigated to detect dunes morphology, physiochemical and mineralogical specifications in Abardej region in the southeast of Tehran, Iran. The border of dunes was determined using aerial photographs and field observations, and then the various shapes of dunes were found. The results have shown that dune shape is mostly transverse, indicating the existence of a dominant wind in one direction. Also, there are some Nebka dunes infrequently. Based on the chemical analysis of dunes, there isn’t any limitation for cultivation of desert plants, and the biologic stabilization of dunes is possible and suitable. The morphoscopic study has shown that the sand grains almost have steep angles; therefore, the dunes may have local sources. The sand grains were almost shiny and semi-shiny in microscope’s light, indicating that the source of dunes is generally the river-beds. Quartz, Feldspar and Calcite were the main minerals of sand grains. The mean of sampled sand grains diameter was about 200 microns and 27 percent of them had a diameter of 250 microns or more; these reveal the existence of a local source for the dunes. Based on the obtained results, it would be concluded that the source of sand dunes is floodplains in east and south- east of the dunes.

Phase diagrams of dune shape and orientation depending on sand availability.

New evidence indicates that sand availability does not only control dune type but also the underlying dune growth mechanism and the subsequent dune orientation. Here we numerically investigate the development of bedforms in bidirectional wind regimes for two different conditions of sand availability: an erodible sand bed or a localized sand source on a non-erodible ground. These two conditions of sand availability are associated with two independent dune growth mechanisms and, for both of them, we present the complete phase diagrams of dune shape and orientation. On an erodible sand bed, linear dunes are observed over the entire parameter space. Then, the divergence angle and the transport ratio between the two winds control dune orientation and dynamics. For a localized sand source, different dune morphologies are observed depending on the wind regime. There are systematic transitions in dune shape from barchans to linear dunes extending away from the localized sand source, and vice-versa. These transitions are captured fairly by a new dimensionless parameter, which compares the ability of winds to build the dune topography in the two modes of dune orientation.

Modeling river dune development and dune transition to upper stage plane bed

AbstractLarge asymmetric bedforms known as dunes commonly dominate the bed of sand rivers. Due to the turbulence generation over their stoss and lee sides, dunes are of central importance in predicting hydraulic roughness and water levels. During floods in steep alluvial rivers, dunes are observed to grow rapidly as flow strength increases, undergoing an unstable transition regime, after which they are washed out in what is called upper stage plane bed. This transition of dunes to upper stage plane bed is associated with high transport of bed sediment in suspension and large decrease in bedform roughness. In the present study, we aim to improve the prediction of dune development and dune transition to upper stage plane bed by introducing the transport of suspended sediment in an existing dune evolution model. In addition, flume experiments are carried out to investigate dune development under bed load and suspended load dominated transport regimes, and to get insight in the time scales related to the transition of dunes to upper stage plane bed. Simulations with the extended model including the transport of suspended sediment show significant improvement in the prediction of equilibrium dune parameters (e.g. dune height, dune length, dune steepness, dune migration rate, dune lee side slope) both under bed load dominant and suspended load dominant transport regimes. The chosen modeling approach also allows us to model the transition of dunes to upper stage plane bed which was not possible with the original dune evolution model. The extended model predicts change in the dune shapes as was observed in the flume experiments with decreasing dune heights and dune lee slopes. Furthermore, the time scale of dune transition to upper stage plane bed was quite well predicted by the extended model. Copyright © 2015 John Wiley & Sons, Ltd.

Discrete Element Method simulations of the saturation of aeolian sand transport

AbstractThe saturation length of aeolian sand transport (Ls), characterizing the distance needed by wind‐blown sand to adapt to changes in the wind shear, is essential for accurate modeling of the morphodynamics of Earth's sandy landscapes and for explaining the formation and shape of sand dunes. In the last decade, it has become a widely accepted hypothesis that Ls is proportional to the characteristic distance needed by transported particles to reach the wind speed (the “drag length”). Here we challenge this hypothesis. From extensive numerical Discrete Element Method simulations, we find that, for medium and strong winds, , where Vs is the saturated value of the average speed of sand particles traveling above the surface and g is the gravitational constant. We show that this proportionality is consistent with a recent analytical model, in which the drag length is just one of four similarly important length scales relevant for sand transport saturation.

Dune deformation in a multi‐directional wind regime: White Sands Dune Field, New Mexico

AbstractAs with most dune fields, the White Sands Dune Field in New Mexico forms in a wind regime that is not unimodal. In this study, crescentic dune shape change (deformation) with migration at White Sands was explored in a time series of five LiDAR‐derived digital elevation models (DEMs) and compared to a record of wind direction and speed during the same period. For the study period of June 2007 to June 2010, 244 sand‐transporting wind events occurred and define a dominant wind mode from the SW and lesser modes from the NNW and SSE. Based upon difference maps and tracing of dune brinklines, overall dune behavior consists of crest‐normal migration to the NE, but also along‐crest migration of dune sinuosity and stoss superimposed dunes to the SE. The SW winds are transverse to dune orientations and cause most forward migration. The NNW winds cause along‐crest migration of dune sinuosity and stoss bedforms, as well as SE migration of NE‐trending dune terminations. The SSE winds cause ephemeral dune deformation, especially crestal slipface reversals. The dunes deform with migration because of differences in dune‐segment size, and differences in the lee‐face deposition rate as a function of the incidence angle between the wind direction and the local brinkline orientation. Each wind event deforms dune shape, this new shape then serves as the boundary condition for the next wind event. Shared incidence‐angle control on dune deformation and lee‐face stratification types allows for an idealized model for White Sands dunes. Copyright © 2015 John Wiley & Sons, Ltd.

Morphodynamic modeling of aeolian dunes: Review and future plans

Sand dunes are ubiquitous in deserts, on coasts, on the sea bottom, and on the surface of Mars, Venus and Titan. The quantitative understanding of dune dynamics is thus of relevance for a broad range of physical, geological and planetary sciences. A morphodynamic model for dunes, which combines an analytical description of the average turbulent wind field over the topography with a continuum saltation model, has proven successful to quantitatively reproduce the shape of aeolian dunes of different types. We present a short review on the physics of dune formation and the model development, as well as some future plans for further developments and applications.

The geomorphology and evolution of aeolian landforms within a river valley in a semi-humid environment: A case study from Mainling Valley, Qinghai–Tibet Plateau

This paper systematically analyzes a valley's aeolian landforms in a semi-humid region and presents a model of its contemporary evolution. Mainling Valley of the Yarlung Zangbo River on the Qinghai–Tibet Plateau was chosen as the case study for the analysis of morphometric characteristics and the evolution sequence of aeolian landforms via field data and remote sensing images. The aeolian landforms were primarily composed of aeolian sand belts on river terraces and dunes (sheets) on hillside slopes. Three types of aeolian sand belts were identified based on their dune types. In type I belts, an erosive air stream combined with relatively high vegetation cover (10%) produced sparsely distributed parabolic dunes with a high variability of dune heights; in type II belts, the continual reworking by the erosive air stream in combination with low vegetation cover (3%) formed more densely distributed barchans and transitional dunes with a moderate variability of dune heights; and in type III belts, the gradual evolution from an erosive sand-laden air stream to a saturated sand-laden air stream in combination with low vegetation cover (2%) produced the densest crescentic dunefields but with the least variability in dune heights. Dune sizes increase, dune shapes become uniform, and dune distribution becomes close from type I to III belts. Lateral linking and merging of the dunes were also observed within the belts. Together this evidence indicates that an evolution sequence may exist. Aeolian dunefields in the belt appear to evolve from embryonic parabolic dunefields to adolescent barchan dunefields and, subsequently, to mature compound crescentic dunefields. As the aeolian sand belt evolves into the mature stage, sand accumulations at the foot of the mountain valley can be steps for sand accumulation on valley-side slopes.