Zhengcai Zhang

Aeolian transport over a developing transverse dune

The spatial and temporal changes in aeolian transport over a dune are fundamental factors that control the morphology of the dune. In the present study, we obtained direct field observations of aeolian transport over a developing transverse dune at the Shapotou Aeolian Experiment Site in the southeastern part of China’s Tengger Desert. The transport rate versus wind speed relationship is complicated over a developing dune compared with the relationships over flat surfaces and over dunes that are in equilibrium with the wind. We obtained trend lines for transport rate over the transverse dune versus distance. The transport rate generally increased from the toe to the crest above the stoss slope, but the difference in transport rate between the crest and the toe was smaller than those that have been proposed for taller dunes. The crest/toe ratio for transport rates therefore seems to depend greatly on dune height. Flux density profiles for different points above the dune at different wind speeds were well described by the exponential decay law, as has been proposed for saltation flux density profiles. Coefficients in the flux density profile function can be defined in terms of the transport rate and wind speed. However, the dependence of relative decay rate with height and average saltation height on wind speed was weaker than that observed in a wind tunnel and above a flat surface. The preliminary results obtained in this study require more evidence from field observations to fully describe aeolian transport above developing dunes.

Observations of Gobi aeolian transport and wind fetch effect

The wind fetch effect is important to wind erosion and aeolian transport and controls aeolian flux. It is useful to study the wind fetch effect in determining the aeolian transport mechanism and improving our knowledge of aeolian physics and wind erosion. In this paper, multichannel samplers measure aeolian transport at different heights above an artificial Gobi surface in the southeastern region of the Tengger Desert. The results show that aeolian transport flux can be expressed as an exponential function of height. Wind fetch obviously affects aeolian flux and aeolian transport. The coefficients and relative decay rate of aeolian flux decrease and then increase with increasing wind fetch distance. Aeolian transport depends on the height and fetch distance; aeolian transport increases and then decreases with increasing fetch distance, reaching a maximum at a fetch distance of about 34 m at the very near surface. The fetch distance of maximum aeolian transport tends to increase with height.

High-altitude aeolian research on the Tibetan Plateau

Aeolian processes and their role in desertification have been studied extensively at low elevations, but have been rarely studied at high elevations in areas such as the Tibetan Plateau, where aeolian processes were active in the geologic past and remain active today. In this review, we summarize research that improves our understanding of aeolian processes on the Tibetan Plateau, including the distribution, characteristics, and provenance of aeolian sediments; the history of aeolian activity; aeolian geomorphology; and wind-driven land degradation. Contemporary aeolian processes primarily occur in dry basins, in wide river valleys, on lakeshores, on mountain slopes, and on gravel pavements. Sediment characteristics suggest a local origin, and provide interesting contrasts with those of Chinas Loess Plateau. The history of aeolian activity and its paleoclimatic implications, reconstructed based on aeolian archives, is short (mostly since the Late Glacial) and shows wide regional differences. Aeolian geomorphology is simple and suggests short formation time. Wind-driven land degradation is less severe than previously thought, driven by different factors in different areas, and exhibited complex interactions with freeze–thaw processes. Aeolian research has been conducted within the general framework of aeolian science, but addresses issues specific to the Tibetan Plateau that arise due to the low air temperature, low air density, and the presence of a cryosphere. We propose six priorities for future research: aeolian physics, the effect of freeze-thaw cycles, comparisons with other areas, regional differences, effects of wind-driven land degradation, and integrated observation and monitoring.

Field observations of the vertical distribution of sand transport characteristics over fine, medium and coarse sand surfaces

The vertical distribution of sand transport characteristics is an important issue in aeolian research. Surface characteristics affect sand transport processes, but their effects are not yet fully understood. To provide more data on this subject, we observed sand transport in 16 field experiments above surfaces covered by fine, medium and coarse sand. The sand transport rate over relatively coarser-grained medium and coarse surfaces could be expressed as a Gaussian peak function: qz = a + b exp (-0.5[(|z – Ch|)/d]e), where qz is the measured sediment transport at height z above the bed and a, b, Ch, d, and e are regression coefficients. The measured sand transport flux peak values (Hh) were linearly related to Ch, and both values were significantly related to the mean surface grain size. However, for the relatively finer-grained medium and fine sand surfaces, the sediment transport could be expressed as an exponential function. The cumulative sand transport below 0.1 m was directly related to the mean surface grain size, and the relationship could be expressed as the following exponential function: Cz = f + g exp –Mz/i, where Cz is the cumulative sand transport at height z above the bed, Mz is the mean grain size and f, g, and i are regression coefficients. Above 0.1 m, there were no significant relationships between the cumulative sand transport and the mean surface grain size. The mean grain size decreased with increasing height below the peak height and then increased with increasing height. The surface grain size distribution and proportions of the particles in different grain size categories controlled the mean grain size as a function of height. The observed changes in the sand transport rate and grain size with height will provide support for sand disaster mitigation, numerical modelling and studies of dune formation. This article is protected by copyright. All rights reserved.

Pattern analysis of a linear dune field on the northern margin of Qarhan Salt Lake, northwestern China

In terms of formation mechanisms of linear dunes, there are open arguments for their widespread distribution and multi-morphological diversities. In order to clarify the formation mechanism of linear dunes of Qarhan Salt Lake, we used pattern analysis method to analyze the statistical characteristics and spatial variation of their pattern parameters. Except at the west-northwest margin, the pattern parameters showed regular spatial variation from the up-middle part towards the downwind end of the dune field. Based on the cumulative probability plots for inter-crest spacing and crest length, we divided the linear dunes into three groups, which corresponding to the three evolution stages of these dunes. The first group comprises erosional relics, with shorter crests, smaller inter-crest spacing and more random dune orientation. The second group comprises dunes whose sand supply is just sufficient to maintain stability and these dunes are approaching the net erosion stage. The crest length and inter-crest spacing of these dunes are much larger than those of the first group, and dune orientation is closer to the resultant drift direction (RDD). The last group comprises linear dunes that are still undergoing vertical accretion and longitudinal elongation, which follows the RDD of the modern wind regime. The presence of regular spatial variation of pattern parameters and a similar geometry with the vegetated linear dunes suggest that deposition and erosion coexist in the development and evolution of linear dunes of Qarhan Salt Lake, i.e. deposition predominates at the downwind end of linear dunes in the vertical accretion and longitudinal elongation stage, whereas erosion mainly occurs at the upwind end of linear dunes in the degradation stage. Therefore, the formation mechanism of linear dunes in Qarhan Salt Lake can be reasonably explained by the combination of depositional and erosional theories.

Grain-size characteristics of linear dunes on the northern margin of Qarhan Salt Lake, northwestern China

In order to clarify the formation mechanism of linear dunes on the northern margin of Qarhan Salt Lake, northwestern China, we analyzed the grain-size and sorting parameters of the dune and interdune sands. The surface sands (0–30 mm) from the dune base to the crest of both flanks and interdune corridors were sampled along transects from upwind to downwind through the dune field. The results indicated that the grain-size distribution differed at different positions between and within the dunes. The frequency curve for dune sands mainly showed a bimodal distribution, while the interdune sediments showed a trimodal distribution. The grain size distribution of the linear dunes showed a finer crest pattern, i.e. the crests were composed of sands that were generally finer, better sorted than those of base sands. In addition, at the dune field scale, the dune crest sands were tending to become much finer but sorting became worse along the downwind transects. However, the grain-size parameters of sediments in the interdune corridors showed no clear pattern. The results demonstrated that the grain size and sorting parameters exhibited a systematic change not only at the individual scale but also at the dune field scale. Our results quantitatively estimate the limited role of cohesive sediments on the formation of linear dune under unidirectional wind regime. More attention should be paid to a long-term wind regime observation, internal sedimentary structures and their formation ages.

Measurement of the movement parameters of saltating sand over a flat sand bed using a high-speed digital camera

The movement parameters of saltating sand are basic physical characteristics in aeolian physics, but remain poorly understood because of limitations in the available measurement technology. High-speed photography provides an efficient method to track the trajectories of saltating particles. From a digital recording of the trajectory, movement parameters can be derived. However, accurate and quick reconstruction of particle trajectories from video images is not yet possible, as this requires the use of prohibitively laborious manual methods. In addition, light reflection from the bed decreases contrast between near-surface particles and the bed. This makes saltation information near the surface difficult to obtain, even though this data is vitally important for studying particle entrainment processes and the mechanisms by which particles leave the surface. In this paper, we describe a novel method for tracking the trajectories of saltating particles. Using motion-detection algorithms based on the subtraction of consecutive images and dynamic thresholds, we developed a new series of high-contrast images from the original images. We then tracked particles by combining a manual operation with a computerized operation, after which a polynomial fitting method could be used to obtain various movement parameters of the saltating particles, such as the lift-off velocity and angle, impact velocity and angle, characteristic length and height, and instantaneous velocity vector at each point in the trajectory. The new method can quickly and accurately calculate the movement parameters of saltating sand particles in air and near the bed surface.

GRAIN-SIZE CHARACTERISTICS OF DUNE NETWORKS IN CHINA'S TENGGER DESERT

Abstract Grain‐size characteristics of dune networks in the Tengger Desert of northwestern China were investigated. Grain‐size parameters (mean, standard deviation, skew, and kurtosis) were determined on dune surfaces at windward toe, stoss, crest and leeward toe locations. Multiple discriminant analyses were applied to distinguish deposition environments. Results indicated that the aeolian sediment is mainly composed of very fine and fine sand in the dune networks of the Tengger Desert. Sorting improves as grain size becomes finer. However, the relationships between mean grain size, skew, and kurtosis vary in space. There is a negative relationship between skew and sorting; similarly, the relationships between sorting and skew, and skew and kurtosis change in space. The sediment deposition environment includes aeolian sediment and lacustrine sediment. The lacustrine sediment provides the source material for dune windward toe and stoss formation and development, but the aeolian sediment provides the source material for dune crest and leeward toe formation and development. According to the log‐probability of grain size distributions, aeolian sediments in dune networks are composed of two distinct saltation populations.

Airflow patterns upwind of obstacles and their significance for echo dune formation:A field measurement of the effects of the windward slope angle

The velocities in front of five topographic obstacles with windward slope angles between 45° and 80° were measured using ultrasonic anemometers in the field at the southeastern margin of the Tengger Desert. The characteristics of the horizontal and vertical velocities indicated that the airflow pattern was significantly influenced by the obstacle’s windward slope angle. The horizontal flow decelerated but the vertical flow accelerated upwind of the obstacles when the windward slope was less than 60°, but a reversed flow formed in front of steeper obstacles. The size of the area occupied by the reversed horizontal component increased with increasing windward slope, but the region of reversed vertical velocity became narrower and taller. Airflow turbulence was strengthened as the wind approached the obstacle, and low-frequency high-energy velocity fluctuations were observed upwind of the obstacle, and could affect the entrainment and saltation processes of sand particles. Turbulence intensities for both the horizontal and vertical components of the airflow increased with increasing windward slope and with decreasing distance from the obstacle. The deceleration and downward movement of airflow upwind of the obstacle would result in supersaturation of the air-sand flow, leading to deposition of moving sand, and these deposited grains would become the material basis for the formation of echo dunes. The reversed airflow would both determine the initial position of the echo dune and provide the necessary motive power to subsequently shape the echo dune.

Dune types and their distribution in the Kumtagh Sand Sea, northwestern China

Integrated regional studies are important tools for understanding dune geomorphology. As a part of an integrated regional study of dune geomorphology in the Kumtagh Sand Sea, northwestern China, we described the dune types and their distribution. In this paper, we propose a tentative dune classification scheme for these dunes based on previous schemes. We identified 11 main types of dunes, described their distribution, and estimated the areas they covered. Dunes generally became more complex moving from the north to the south of the study area, and from the edges to the inner areas of the sand sea. The wind regime also became more complex moving towards the south due to the physical blocking and thermodynamic effects of the mountains and the Qinghai-Tibetan Plateau, as well as the increasingly complex underlying topography. The combination of these factors appears to be responsible for the observed spatial variation of the dune types. Dune heights were estimated from 1:100,000 topographic maps and remote-sensing images. More than 80% of the dunes in the Kumtagh Sand Sea were less than 100 m tall, but with a maximum height exceeding 300 m. Dune height increased from north to south, with the tallest dunes occurring near the southern mountains. This general distribution of dune heights suggests that dune height was determined by a combination of sediment availability, airflow patterns, the blocking effect of mountains, and the underlying topography.