Pseudo-feathery dunes in the Kumtagh Desert

The Kumtagh Desert is the last explored desert in China. The unique patterns on aerial photographs and satellite images have led previous researchers to the conclusion that the Kumtagh Desert is the only place over the world where typical feathery dunes are developed, and that some unique wind regimes are at work. Recent field investigation reveals that the feathery patterns in fact reflect the albedo contrasts between the seif dunes, bright dune-like drifts and the dark inter-dune lag sediments. The wind regime in the area containing the so-called feathery dunes is typical of the regime that produces seif dunes that were considered to be shafts of the feathery dunes, and the vanes of the feathery dunes are in fact bright dune-like drifts with indistinct height from the surroundings rather than true transverse dunes. Further analysis indicates that the sediments of seif dunes, dune-like drifts and dark inter-dune flats differ in grain size distribution, mineralogy, reflectance spectrum and particle micro-morphology. Sediments that constitute seif dunes and dune-like drifts are finer and poorly rounded, contain relatively more quartz, and hence have higher albedo, while the sediments that constitute the dark inter-dune flats are coarser and better rounded, contain far less quartz but relatively more andesite and dacite, and hence have lower albedo. As a result, what was originally perceived as unique feathery dunes are in fact only pseudo-feathery dunes composed primarily of seif dunes. This discovery also demonstrates that blowing sand drifts can form dune-like patterns that remain visible for long periods of time. It is important to distinguish between the patterns of true dunes and those of dune-like drifts (pseudo-dunes) that cause albedo contrasts in interpreting remote sensing images.     

Threshold wind velocity in the lee of two-dimensional transverse dunes

Secondary flow plays an important role in the development of dunes. To investigate this phenomenon, we simulated models of transverse dunes with different stoss slopes in a wind tunnel. We measured the threshold wind velocity at different locations in the lee of the dunes, and observed the corresponding particle movement characteristics under threshold conditions to assess the sedi-mentological significance of secondary airflow. Threshold wind velocity changed with both horizontal distance and stoss slope. Its variation with horizontal distance is characterized by a region in which a rapid reduction occurs, a region with a minimum value, and a region with high value. We confirmed the presence of reversal cells, irrespective of the stoss slope. Neither the variation in threshold wind velocity nor the variation in reattachment distance as a function of stoss slope followed a monotonic function. The 15° stoss slope had the greatest threshold wind velocity and reattachment distance.     

Measurements and numerical simulations of the degree of activity and vegetation cover on parabolic dunes in north-eastern Brazil

In this work we present measurements of vegetation cover over parabolic dunes with different degrees of activation along the north-eastern Brazilian coast. We extend the local values of the vegetation cover density to the whole dune by correlating measurements with the relative brightness index C of high resolution QuickBird panchromatic satellite images of the dune field. We then introduce the vegetation data into a continuous model for vegetated dunes, coupling sand erosion and vegetation growth, and perform simulations of the evolution of the morphology and vegetation cover of parabolic dunes. Finally, from the comparison of both, the measurements and the simulation results, we show that the model is able to predict the dune shape and the vegetation distribution of real parabolic dunes as result of the evolution of a blow-out.     

Magnitude and frequency of grain flows on a desert sand dune

Preliminary studies of the magnitude and frequency of lee side avalanches (grain flows) on a Namib crescentic dune show that the frequency of grain flows for a given segment of the lee face is dependent on the wind speed and sand transport rate for the period preceding their initiation; and the magnitude of the flows as described by their area is inversely proportional to the interval between flows and thus wind speed and sand transport rates. These studies indicate the potential of using a simple digital video camera technique to document the magnitude, frequency and geometry of grain flows on desert sand dunes.     

A second look at western Sinai seif dunes and their lateral migration

Tsoar et al. [Tsoar, H., Blumberg, D.G., Stoler, Y., 2004. Elongation and migration of sand dunes. Geomorphology 57, 293–302.] reported that seif dunes in the western Sinai Desert did not migrate laterally between 1973 and 1999. If the planform sinuosities of the dunes are removed by filtering, spatial averaging, or linear regression, however, it is evident that the dunes did, in fact, migrate laterally roughly 13 m during this 26-year period. The measured migration distance is 1–2 orders of magnitude greater than the rms co-registration error Tsoar et al. determined for the first and last air photos that were used to map the dunes. The western Sinai dunes provide another example demonstrating that linear dunes can migrate laterally, and they illustrate some of the difficulties in documenting systematic lateral motion.Lateral migration of a dune can be important geologically or geomorphologically, even where migration is too slow to detect from repeated topographic surveys. This article explains the wind conditions for the lateral migration of seif dunes in western Sinai and the possible wind occurrences that would not lead to such a migration.     

A comparison of methods used to estimate the height of sand dunes on Mars

The collection of morphometric data on small-scale landforms from other planetary bodies is difficult. We assess four methods that can be used to estimate the height of aeolian dunes on Mars. These are (1) stereography, (2) slip face length, (3) profiling photoclinometry, and (4) Mars Orbiter Laser Altimeter (MOLA). Results show that there is good agreement among the methods when conditions are ideal. However, limitations inherent to each method inhibited their accurate application to all sites. Collectively, these techniques provide data on a range of morphometric parameters, some of which were not previously available for dunes on Mars. They include dune height, width, length, surface area, volume, and longitudinal and transverse profiles. The utilization of these methods will facilitate a more accurate analysis of aeolian dunes on Mars and enable comparison with dunes on other planetary surfaces.     

Geomorphology of sand dunes in the Northeast Taklimakan Desert

Three types of sand dunes exist in the Taklimakan Desert, namely compound/complex crescent dunes and crescent chains, compound dome dunes and compound/complex linear dunes. Besides these three compound/complex types, single simple dunes are also distributed throughout the sand sea. The compound/complex linear dunes are developed under acute bimodal wind regimes. Though the ratios of the resultant drift potential (RDP) and the drift potential (DP) are the same as that near the border and adjacent area of the sand sea, the compound/complex crescent and dome dunes are developed, respectively, because of divergence of the sand available, the stress of the sand-moving winds and the time scales of dune formation. The sand supply for the dunes is not from Lopo Nor in the east as previous studies suggested but mainly from local alluvial or lacustrine deposits. The grain size component does not correlate evidently to the morphology parameters of the sand dunes. Analyses of the DP and drift direction suggest that the northeast Taklimakan is an area of low wind energy and the resultant drift direction (RDD) coincides well with the distribution, morphology and scales of the dunes.     

Variable vegetation cover and episodic sand movement on longitudinal desert sand dunes

Longitudinal (linear) sand dunes of the Simpson and Strzelecki dunefields in eastern central Australia present a paradox. Low levels of activity today stand in contrast to luminescence dating which has repeatedly shown deep deposits of sand on dune crests dating to within the late Holocene. In order to investigate the nature of dune activity in the Simpson–Strzelecki dunefield, vegetation and sand mobility were investigated by detailed vegetation survey and measurement of rippled area and loose sand depth of dunes at three sites along a climatic gradient. The response of both vegetation and sand movement to inter-annual climate variability was examined by repeat surveys of two sites in drought and non-drought conditions. Projected plant cover and plant + crust cover were found to have inverse linear relationships with rippled area and the area of deep loose sand. No relationship was found between these measures of sand movement and the plant frontal area index. A negative exponential relationship between equivalent mobile sand depth on dune surfaces and both vascular plant cover and vascular + crust cover was also found. There is no simple threshold of vegetation cover below which sand transport begins. Dunes with low perennial plant cover may form small dunes with slip faces leading to a positive feedback inhibiting ephemeral plant growth in wet years and accelerating sand transport rates. The linear dunefields are largely within the zone in which plant cover is sufficient to enforce low sand transport rates, and in which there is a strong response of vegetation and sand transport to inter-annual variation in rainfall. Both ephemeral plants (mostly forbs) and crust were found to respond rapidly to large (> 20 mm/month) rainfall events. On millennial time-scales, the level of dune activity is controlled by vegetation cover and probably not by fluctuations of wind strength. Land use or extreme, decadal time-scale, drought may destabilise dunes by removing perennial plant cover, accelerating wind erosion.     

Particle size and sorting characteristics of sand in transport on the stoss slope of a small reversing dune

Studies of the particle size and sorting characteristics of sand on the stoss slope of a 6-m high reversing dune show that the sand in transport is generally finer and better sorted than surface sand at the same position on the slope. The sand in transport becomes coarser and more poorly sorted as wind speed and rates of mass transport increase toward the dune crest. These patterns reflect changes in the competence of the wind, which is capable of transporting larger grains and a wider range of grain sizes as its speed increases in space and time. Our field observations suggest that the particle size and sorting characteristics of surface sand are highly dependent on antecedent wind conditions and are not an invariant property of the dune, as is widely assumed. The wide range of particle sizes on the surface, as well as its change through time, also has important implications for modeling sediment transport on dunes. Transport thresholds may vary by as much as 30% on the stoss slope of the study dune.     

Dynamic processes of a simple linear dune—a study in the Taklimakan Sand Sea, China

Samples of dune sands, surveys of the morphology and field measurements of wind velocity and direction of a simple linear dune in Taklimakan Sand Sea show that the airflow and sand flux vary with the change of wind direction on the dune surface. Decrease of the airflow stress on the lee flank does not result in much decrease of the sand flux because of the low threshold shear velocities and the airflow conditions. There are no significant relations between the sand flux on the lee flank and the angle of incidence of the airflow. The low threshold shear velocities and the maintenance of the sand flux at the lee flank are the main mechanisms keeping the linear shape of the dunes. Measurements of the sand flux shows that it reaches a maximum on the crest of the dune. The grain size of the transported sands has some differences compared to that of the dune surface. The sands transported are finer than that on the dune surface, but better sorted under the influence of the medium to low wind activity. The field experiment results exhibit that it is possible for the dunes to be shaped as linear dunes during the processes of accumulation and elongation.     

古尔班通古特沙漠树枝状沙丘沉积物粒度和微形态特征的空间分异

树枝状沙丘是古尔班通古特沙漠独特的沙丘地貌类型.分析了古尔班通古特沙漠树枝状沙丘表层沉积物的粒度和微形态特征.结果表明:(1)树枝状沙丘以中砂为主,其次是细砂和粗砂;平均粒径与分选系数有较好的线性关系,平均粒径越小,分选系数越好;沉积环境以河流湖泊环境为主;石英颗粒形态以圆形和长形为主,磨圆度适中,碟形坑分布广泛,同时...     

Variations of horizontal and vertical velocities over two-dimensional transverse dunes: A wind tunnel simulation of the effect of windward slope

The changes in wind velocity induced by dune topography have an important significance in dune dynamics. In this paper, the horizontal and vertical velocities over six transverse dune models were measured non-intrusively by means of Particle Image Velocimetry in a wind tunnel. The windward slope angle and the free-stream wind velocity both affected the horizontal and vertical velocity components. On the windward side, the acceleration of horizontal velocity depended mainly on the windward slope angle and the height above the dune surface, but was also affected by the free-stream wind velocity. The speed-up ratio increased with increasing slope angle but decreased with increasing height. The ascending vertical velocities also increased with increasing slope angle and free-stream wind velocity. The maximum values moved upper along the dune when the windward angle became steeper. In the leeward sides, the horizontal velocity decreased and reversed because of airflow separation; the maximum reverse velocity in the separation cell was about 17% of the free-stream wind velocity. Behind the dune crest, the airflow moves downwards, and its maximum downward velocity is found near the flow reattachment point. Finally, we discussed the significance of these velocity variations for sediment transport and dune dynamics.     

荒漠人工固沙植被区土壤结皮斥水性发展特征

土壤结皮深刻影响绿洲边缘固沙植被区的土壤水文过程。土壤结皮斥水性是量化上述影响的潜在指标。采用滴水穿透时间（WDPT）法,研究了河西走廊荒漠边缘不同建植年限梭梭植被区固定沙丘土壤结皮斥水性的时间演变规律及影响因素。结果表明：建植20 a后,丘顶与丘间地土壤结皮出现显著斥水性（WDPT>5 s）,而丘坡土壤结皮无斥水性（WDPT<5 s）。丘顶土壤结皮斥水性与总有机碳、微生物量碳、有机碳C-H组分以及电导率显著相关（P<0.01）;而丘间地土壤结皮斥水性与黏、粉粒含量显著相关（P<0.01）。植被形成的“碳岛”及“盐岛”效应是丘顶土壤结皮斥水性形成的主要原因,而黏、粉粒在土壤表层的积聚是丘间地土壤结皮斥水性形成的主要原因。     

察尔汗盐湖区线形沙丘下伏湖相地层的释光年代及其环境意义

本研究尝试使用钾长石多步升温红外后红外法(MET-pIRIR)对察尔汗盐湖北侧线形沙丘下伏古湖相沉积物进行年代测定,并初步探讨其古环境意义.结果表明:钾长石MET-pIRIR250法能够得到等效剂量坪区,说明长石信号能够充分晒退并且异常衰减对测年结果的影响可以忽略,从而保证了测年结果的可靠性;古湖相沉积顶部200-18...     

Experimental investigation of the concentration profile of a blowing sand cloud

Detailed wind tunnel tests were carried out to establish the mean downwind velocity and transport rate of different-sized loose dry sand at different free-stream wind velocities and heights, as well as to investigate the vertical variation in the concentration of blowing sand in a cloud. Particle dynamic analyzer (PDA) technology was used to measure the vertical variation in mean downwind velocity of a sand cloud in a wind tunnel. The results reveal that within the near-surface layer, the decay of blown sand flux with height can be expressed using an exponential function. In general, the mean downwind velocity increases with height and free-stream wind velocity, but decreases with grain size. The vertical variation in mean downwind velocity can be expressed by a power function. The concentration profile of sand within the saltation layer, calculated according to its flux profile and mean downwind profile, can be expressed using the exponential function: c_z=ae^−bz, where c_z is the blown sand concentration at height z, and a and bare parameters changing regularly with wind velocity and sand size. The concentration profiles are converted to rays of straight lines by plotting logarithmic concentration values against height. The slope of the straight lines, representing the relative decay rate of concentration with height, decreases with an increase in free-stream wind velocity and grain size, implying that more blown sand is transported to greater heights as grain size and wind speed increase.     

基于DEM数据的巴丹吉林沙漠沙丘分布规律及其形态参数

沙丘形态特征是风沙地貌研究的重要内容。数字高程模型（DEM）因卓越的三维地形表达能力被广泛应用于沙丘形态研究中。长期以来,受交通条件和研究方法的制约以及高大沙山分布位置的限定,对巴丹吉林沙漠沙丘形态的研究主要集中在沙漠东南部,缺乏对整个沙漠沙丘分布规律及其形态参数特征的系统认知。而对整个沙漠沙丘形态特征的研究,是区域风沙地貌形成与演化研究的重要组成部分。以巴丹吉林沙漠内所有独立沙丘为研究对象,利用研究区DEM数据和一种新的算法,研究了沙漠内两种主要类型沙丘（横向沙丘、星状沙丘）的分布规律及其形态参数特征。结果表明：沙漠内独立沙丘大约有6 033座,高度9~433 m,高度超过350 m的巨型沙山共有53座,其中星状沙山7座,最大高度383 m;横向沙山46座,最大高度433 m。横向沙丘高度与等效沙丘厚度呈正相关的线性函数关系,星状沙丘高度与等效沙丘厚度呈正相关的对数函数关系,表明在现代气候和环境条件下,横向沙丘高度仍在增加,而星状沙丘高度增长已经趋缓。两类沙丘的主轴向大都垂直于研究区主风向NW-SE。横向沙丘起沙风风向单一,沙丘轴向集中在NE-SW和NNE-SSW方向;星状沙丘起沙风风向略为复杂,沙丘存在多个次级轴向。     

The airflow field and dynamic processes of pyramid dunes

Dune sedimentary structure and wind tunnel experimental results show that the pyramid dunes on the cliff top surface of Dunhuang Mogao Grottoes are formed under three wind directions (NE, SW and NW); the deformation and separation variety of chain dune. The local air circulation (SW) is key to the development of the pyramid dunes. Analysing the morphological dynamic processes indicates that inherent relations exist between trihedral pyramid dunes and other complex sand dune types (linear, reticulate and polyhedral pyramid dunes). The wind regime of multidirection, in conjunction with the connected main and subsidiary sand ridges, resulted in the geomorphological forms adapted to the near-surface air streams. It is concluded that integrated studies, including sedimentary structures and wind tunnel experiments, can make more efficient contributions to an overall, systematic and thorough understanding of the developmental processes of the pyramid dunes than previously used single-study methods.     

古尔班通古特沙漠蜂窝状沙丘沉积物理化特征及沉积环境

采样分析了古尔班通古特沙漠蜂窝状沙丘表层沉积物的粒度、元素和微形态特征,讨论了其沉积环境.结果表明:(1)古尔班通古特沙漠蜂窝状沙丘表层沉积物以细沙为主;与上部陆壳平均化学组成(UCC)比,常量元素产生亏损,微量元素Co富集明显;化学蚀变指数(CIA)与上部陆壳平均值接近;石英沙颗粒磨圆度适中,微形态特征以碟形坑与不规...     

海岸沙丘形态对季风/台风的协同响应研究现状与展望

海岸沙丘形态变化及其动力学过程是认识和研究海岸风沙地貌的基础.中国海岸沙丘受季风和台风共同驱动,但梳理总结海岸沙丘形态对季风/台风的响应研究、海岸沙丘形态-动力学过程研究及其技术和方法等发现,现有研究对中国海岸沙丘受季风/台风协同驱动的特点认识不足,对季风和台风的影响分别进行研究,与海岸沙丘形成与演化过程并不相符,季风...