Particle size characterization and sources of sediments in the Uzhumqin sand dunes

Sediment constitutes the fundamental basis for forming and evolving aeolian geomorphology.The characteristics of sediment particle size offer insights into the development and evolution of sandy terrain,making their study critical to understanding aeolian geomorphology and sand control.In this study,we combined high-density collection of surface sediments in the Uzhumqin sand dunes and GIS spatial analysis to analyze the particle size parameters and changes in the spatial distribution of surface sediments in this region.In addition,we used an end-member analysis to identify the potential sources of the sediments.The results showed that surface sediments in the Uzhumqin sand dunes had distinct spatial distributions.Medium and coarse grain sands dominated the sediments in the dunes,and the mean grain size and the sorting coefficient generally increased along the prevailing wind direction,with high values in individual areas related to factors such as material sources and vegetation cover.Skewness was strongly influenced by factors such as landform change and human activity,and spatial variability became more complex.Kurtosis and the soil fractal dimension showed generally decreasing trends along the prevailing wind direction.With dune fixation,the contents of clay and powder particles in the soil increased;the mean particle size,the sorting coefficient,and the fractal dimension of the soil gradually increased,and the skewness and kurtosis gradually decreased.The end-member analysis results indicated the existence of five end-members(EM)in the dune sediments.EM 1 was a mixed component of wind-deposited fine sands and nearby fluvial sediments.EM 2 was the main component of sediments in the study area and was the result of sorting lake sediments by wind action and by the local topography.EM 3 may be a product of river flood deposition.EM 4 and EM 5 had coarser grain sizes.EM 4 was a lake-phase sediment product influenced by topographic and vegetation cover factors,and EM 5 was primarily a river and lake sediment product modified by weathering.The sediment particle size results from the study area indicate that the sediment in the sandy region is generally coarse due to multiple factors,including topography,climate,hydrology,and human activity.Sandy material in the study area originated from nearby,with very little sand being transported from long distances.

Morphological change and migration of revegetated dunes in the Ketu Sandy Land of the Qinghai Lake,China

Alpine revegetated dunes have been barely researched in terms of morphological change and migration within its regional aeolian environments. To reveal the sand-fixing and land-reforming mechanisms of artificial vegetation, we observed the morphology and migration of four dunes with four revegetated types(Hippophae rhamnoides Linn., Salix cheilophila Schneid., Populus simonii Carr., and Artemisia desertorum Spreng.) using unpiloted aerial vehicle images and GPS(global positioning system) mapping in 2009 and 2018. Spatial analysis of GIS(geographic information system) revealed that the revegetated dunes exhibited a steady progression from barchan dune shapes to dome or ribbons shapes mainly through knap planation, wing amplification, and slope symmetrization. Generally, conditions of northern aspects, smaller slope degree, and larger altitude of unvegetated dunes would suffer more serious wind erosion. The southward movement of dune wings with a migration speed of 2.0–5.0 m/a and the alternating motion of sand ridges in eastwestern directions led greater stability in revegetated dunes. The moving distances of revegetated dunes remarkably changed in patterns of quadratic or linear function with depositional depth. Compared with unvegetated dunes, the near-surface wind velocity of revegetated dunes decreased by 20%–30%, which led to heavy accumulation in low-flat dunes and erosion in high-steep dunes, but all vegetation species produced obvious sand-fixing benefits(100%–450% and 3%–140% in the lower and higher dune scales of revegetated dunes, respectively) with decreasing sand transport rates and increasing coverages. In practice, the four vegetation species effectively anchored mobile dunes by adapting to regional aeolian environment. However, future revegetation efforts should consider optimizing dune morphology by utilizing H. rhamnoides as a pioneer plant, S. cheilophila and P.microphylla in windward and northward dune positions, and A. desertorum in a sand accumulative southward position. Also, we should adjust afforestation structure and replant some shrub or herbs in the higher revegetated dunes to prevent fixed dune activation and southward expansion.

Effect of sand-fixing vegetation on the hydrological regulation function of sand dunes and its practical significance

Soil water content is a key controlling factor for vegetation restoration in sand dunes.The deep seepage and lateral migration of water in dunes affect the recharge process of deep soil water and groundwater in sand dune ecosystems.To determine the influence of vegetation on the hydrological regulation function of sand dunes,we examined the deep seepage and lateral migration of dune water with different vegetation coverages during the growing season in the Horqin Sandy Land,China.The results showed that the deep seepage and lateral migration of water decreased with the increase in vegetation coverage on the dunes.The accumulated deep seepage water of mobile dunes(vegetation coverage<5%)and dunes with vegetation coverage of 18.03%,27.12%,and 50.65%accounted for 56.53%,51.82%,18.98%,and 0.26%,respectively,of the rainfall in the same period.The accumulated lateral migration of water in these dunes accounted for 12.39%,6.33%,2.23%,and 7.61%of the rainfall in the same period.The direction and position of the dune slope affected the soil water deep seepage and lateral migration process.The amounts of deep seepage and lateral migration of water on the windward slope were lower than those on the leeward slope.The amounts of deep seepage and lateral migration of water showed a decreasing trend from the bottom to the middle and to the top of the dune slope.According to the above results,during the construction of sand-control projects in sandy regions,we suggest that a certain area of mobile dunes(>13.75%)should be retained as a water resource reservoir to maintain the water balance of artificial fixed dune ecosystems.These findings provide reliable evidence for the accurate assessment of water resources within the sand dune ecosystem and guide the construction of desertification control projects.

Comparisons between Movement Speed of Main Types of Dunes:A Case Study of Desert Areas in Hexi Region of Gansu Province

In desert areas in Hexi region,11 barchan dunes,6 chains of barchan dunes,6 pyramid dunes,6 parabolic dunes and 6 accumulated sand-belts were chosen to study their movement speed based on images obtained by Google Earth in various periods.The results show that among the dunes,the movement of the barchan dunes was the fastest,followed by the chains of barchan dunes,and only the tops of the pyramid dunes swayed,while the parabolic dunes and accumulated sand-belts hardly moved forward.The higher the barchan dunes(or the chains of barchan dunes) were,the slower the movement was.On the contrary,the higher the pyramid dunes were,the faster the swing was.There was a positive correlation between the movement speed of the barchan dunes and the average wind speed of sandstorms.

Contribution of groundwater to the formation of sand dunes in the Badain Jaran Desert, China

The tallest sand dune worldwide is located in the Badain Jaran Desert(BJD),China,and has been standing for thousands of years.Previous studies have conducted limited physical exploration and excavation on the formation of sand dunes and have proposed three viewpoints,that is,bedrock control,wind dominance,and groundwater maintenance with no unified conclusion.Therefore,this study analyzed the underlying bedding structure of sand dunes in the BJD.Although the bedrock of sand dunes is uplifted and wind controls the shape of dunes,the main cause of dune formation is groundwater that maintains the deposition of calcareous sandstone and accumulation of aeolian sand.According to water transport model and vapor transports in the unsaturated zone of sand dunes,capillary water transport height is limited with film water constituting the main form of water in dunes.Chemical properties and temperature of groundwater showed that aquifers in different basins receive relatively independent recharge from deep sources in the crater.Result of dune formation mechanism is of considerable importance in understanding groundwater circulation and provides a new perspective on water management in arid desert areas.

Characteristics of soil seed banks at different geomorphic positions within the longitudinal sand dunes of the Gurbantunggut Desert, China

Understanding the characteristics of soil seed banks in sand dunes is crucial to stabilize the dune systems and maintain the plant populations in deserts. In this study, we conducted a survey investigation in the field and a seed germination experiment in the laboratory to explore the characteristics of soil seed banks at various geomorphic positions of longitudinal sand dunes in the Gurbantunggut Desert, China. Totally, 17 plant species belonging to 17 genera and 9 families were identified in soil seed banks, and 35 plant species belonging to 34 genera and 17 families were identified in aboveground vegetation. Plant species richness in soil seed banks decreased with increasing soil depth. The highest species richness was presented in the upper slope of the windward slope and the lowest species richness was presented in the base of the windward slope. There was no significant difference in seed density of soil seed banks among the examined seven geomorphic positions. The highest seed density occurred in the lower slope of the leeward slope while the lowest occurred in the crest. Moreover, seed density decreased with increasing soil depth, being the highest in the upper soil layer （0-2 cm）. For both soil seed banks and aboveground vegetation, there was no significant difference in Simpson＇s diversity index among the seven geomorphic positions; however, Shannon-Wiener diversity index and Pielou＇s evenness index showed significant differences among the seven geomorphic positions. Those results showed that although there was no significant difference in seed density of soil seed banks among the seven geomorphic positions, the geomorphic positions significantly affected the species richness, diversity and distribution of soil seed banks. Therefore, understanding the characteristics of soil seed banks at different geomorphic positions of sand dunes is essential to vegetation restoration or reestablishment. Furthermore, the Jaccard＇s similarity coefficients of plant species between soil seed banks and aboveground vegetation at the seven geomorphic positions were low, suggesting that vegetation restoration or reestablishment processes should be promoted through adding seeds to surface layers.

Quantitative analysis on the dynamic characteristics of megadunes around the Crescent Moon Spring, China

The Crescent Moon Spring is a precious natural heritage. However, the dynamic characteristics of megadunes around the Crescent Moon Spring are not well known. This paper quantitatively studied the character- istics and changes of megadunes around the Crescent Moon Spring by interpreting aerial photographs taken in 1985 and 2004 and analysing the dune crestlines and the wind data collected from 2011 to 2012. Results revealed that pyramid dunes were formed by a complex wind regime. The Crescent Moon Spring was not buried by shifting sands because of the stable wind regime and relative stability of pyramid dunes. The crestlines of the dunes around the spring moved northward between 1985 and 2004. The south-facing slip faces were also exposed to wind ero- sion, whereas the other faces were under deposition, thus indicating that the southerly wind was relatively en- hanced. Limiting the scale of tall windbreaks and architectures in the Dunhuang oasis at the north of the spring was necessary to maintain the dynamic equilibrium of the wind regime and sand transport.

Estimation of Sand Transportation Rate for Fixed and Semi-Fixed Dunes Using Meteorological Wind Data

Taibus County, Inner Mongolia, China, lies in a farming-pastoral ecotone, where severe wind erosion and various aeolian sand hazards are prevalent and fixed and semi-fixed sand dunes occur frequently. This study was conducted to investigate the relationships between sand transportation rate and wind speed for the fixed and semi-fixed sand dunes based on field measurements. The annual quantity of soil erosion by wind was estimated using meteorological wind data. The results indicated that the sand transportation rate in Taibus County in 2000 was 57.38 kg cm-1 year-1 for the semi-fixed dunes and 4.56 kg cm-1 year-1 for the fixed dunes. The total duration of erosive winds covered 12.5% of the time of the year, and spring posed the highest potential of sand transportation. Wind with low speed (≤ 17 m s-1) and high frequency plays a dominant role in sand transportation, while strong wind (≥ 17 m s-1) with low frequency significantly enhanced the sand transportation. Erosive wind speed, directions, and frequency were three crucial dynamic factors influencing sand hazards in the farming-pastoral ecotone. The dominant factors intensifying sand and dust storms in Taibus County might be related to the favorable wind condition in combination with the durable drought, which led to land desertification and vegetation degradation.

Impinging Cooling with a Crescent Surface Inspired by Barchan Dunes in a Simplified Gas Turbine Transition Section

For the enhancement of heat transfer efficiency,a novel turbulator inspired by the morphology of barchan dunes,called the mimetic barchan dune(MBD)turbulator,is designed and evaluated in the simplified gas turbine transition section.By using computational fluid dynamics(CFD),the numerical simulations for comparison have been carried out,concluding the smooth thermal surface,a thermal surface with riblet-shaped turbulator and a thermal surface with MBD turbulator.Then,two indicators are investigated for evaluating the coolant performance which are the heat transfer efficiency(η)on the outlet and the pressure loss(ΔP)in the coolant chamber.The numerical results show that the coolant has the best heat transfer efficiency with less pressure loss in the coolant chamber with the MBD turbulator.Then,the effect of the MBD turbulator sizes on heat transfer efficiency is investigated.When the height of the MBD turbulator(h)is set at 8 mm,the maximum amount of heat that could be transfered by the coolant is up to566.2 K and the corresponding heat transfer efficiency is 26.62%.The detail flows have been shown to elucidate the function of the MBD surface which may greatly arouse more design for solving harsh circumstance.

Study of Sand Dunes and Their Effect on Desertification of Cultivated Lands in Shaanxi Province, China Using Remote Sensing Techniques

About half of the arid and semi-arid lands in the world are deserts thatcomprise various types of aeolian sand dunes deposits. In Shaanxi Province, aeolian sand dunes coverconsiderable areas of the Yulin desert and northern Jinbian. Sand dunes are moving in the main winddirection and converting some agricultural area to wasteland. Remote sensing of sand dunes helps inthe understanding of aeolian process and desertification. Remote sensing data combined with fieldstudies are valuable in studying sand dunes, regional aeolian depositional history. In particular,active and inactive sand dunes of the north Shaanxi Province were studied using remote sensing andgeographic information system. In this study, we describe the Landsat thematic mapper (TM) images,covering north Shaanxi Province, which were used to study the distribution, shape, size, trends,density and movement of sand dunes and their effect on desertification of cultivated lands.Estimation was made depending on soil erodibility factor (Ⅰ) and local climatic factor (C) duringthe period (June to September). The result indicates that soil erosion caused sand drift of 8.957 5,7. 03 ton for Yulin and Jinbian, respectively. The mean sand dunes movement rate were 4.37, 3.11 m,whereas, monthly sand dune advance rate were 1. 092 5, 0. 777 5m, for the two locations,respectively. The study reveals that cultivated lands extended obliquely to the direction of sanddune movement are extremely affected, while other segments that extend parallel to the direction ofthe movement are not affected. Accordingly the north Shaanxi Province was divided into areas ofdifferent classes of potential risk. Moreover, blown sands and sand movement from neighboringhighlands also affect the area of western desert.

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.

Endophytic fungi of wild legume Sesbania bispinosa in coastal sand dunes and mangroves of the Southwest coast of India

Evaluation of 450 surface sterilized tissue segments of a seasonal wild legume, Sesbania bispinosa （Jacq.）, of coastal sand dunes and mangroves of southwest India yielded 546 isolates comprising 39 endophytic fungi with six dominant taxa （Aspergillus flavus, Aspergillus niger, Cladosporium tenuissimum, Fusarium moniliforme, Penicillium chrysogenum and morpho sp. 1）. A consortium of saprophytic, pathogenic and toxigenic fungi exists as endophytes in S. bispinosa. Number of segments colonized, number of isolates obtained, species richness and diversity were higher in S. bispinosa in mangroves compared to coastal sand dunes. Seeds yielded more fungal isolates, but species richness and diversity were low. In spite of low fungal colonization in root segments, the diversity was high. Up to 30-40 % endophytic fungi of S. bispinosa differed between coastal sand dunes and mangroves revealing partial host- and habitat-specificity. As S. bispi- nosa is extensively used as green manure and forage in southwest India, further studies especially on the bioactive compounds of its endophytic fungi might broaden its range of uses. In addition to conventional morphologicaltechniques, molecular tools would provide precise insight on the endophytic fungi of coastal sand dunes and mangroves.

Large eddy simulation of water flow over series of dunes

Large eddy simulation was used to investigate the spatial development of open channel flow over a series of dunes. The three-dimensional filtered Navier-Stokes （N-S） equations were numerically solved with the fractional-step method in sigma coordinates. The subgrid-scale turbulent stress was modeled with a dynamic coherent eddy viscosity model proposed by the authors The computed velocity profiles are in good agreement with the available experimental results. The mean velocity and the turbulent Reynolds stress affected by a series of dune-shaped structures were compared and analyzed. The variation of turbulence statistics along the flow direction affected by the wavy bottom roughness has been studied. The turbulent boundary layer in a complex geographic environment can be simulated well with the proposed large eddy simulation （LES） model.

Drag Reduction Characteristics of Microstructure Inspired by the Cross Section of Barchan Dunes under High Speed Flow Condition

A new type of microstructure inspired by the cross section of barchan dunes was proposed to reduce windage,which was considered as a passive drag reduction technology in aerospace manufacturing field.Computational fluid dynamics method was carried out to discuss the effect of the microstructure on the skin friction reduction under high velocity flow condition.Different microstructure heights were employed to survey the reduction of drag.The results illustrated that the appearance of microstructure led to a generation of pressure drag in non-smooth model(with microstructures inspired by cross section of barchan dune)in contrast to smooth model.However,the microstructure significantly increased the thickness of the low-speed fluid by 11.4%in the near-wall flow field,causing the low-speed fluid to rise and decreasing the velocity gradient near the wall,thereby reducing viscous resistance.In addition,high-speed fluid flowed above the microstructure units instead of along the inner side of the units due to the influence of micro-vortex,resulting in a reduction of friction near the surface.Further-more,micro-vortex was considered to be the significant internal factor to achieve turbulent drag reduction since it could not only reduce the viscous resistance by promoting the fluid flow above the microstructure but also pro-vide a reverse thrust force.The understanding of the mechanism of drag reduction provides theoretical guidance for further fabrication of drag reduction coatings using renewable materials.

Determination of actual evapotranspiration and transpiration in desert sand dunes (Negev Desert) using different approaches

In an arid environment, especially in sandy areas where surface runoff is of no practical importance in the hydrological budget, it is rainfall, dewfall and evapotranspiration that become the most important variables. To assess actual evapotranspiration, several methods （flux-gradient, BREB, eddy correlation） were applied to data from the Nizzana experimental site in the northwestern Negev Desert. Additionally, a model specifically designed for arid environments is introduced in this paper. This zero plane model shows the most reasonable results compared with the other methods, which overestimate evapotranspiration to a large degree. It is shown that plant transpiration is the dominant process in total evapotranspiration while advective processes do not play a major role in the near-ground boundary layer, although the study area is influenced by a sea breeze. Actual transpiration ofArtemisia monosperma was measured in a field experiment to validate the calculated evapotranspiration. The vegetation contributed 41% of the calculated total evapotranspiration in a single month.

Evaluation of Sand Dunes Stabilization Techniques in Baiji District,Iraq

This research was conducted at the sand dunes stabilization research s tation in Baiji district, Iraq. Three techniques for sand dunes stabilization ar e selected: the first method is stabilization by clayey block barriers; the seco nd method is stabilization by dry planting of tamarix (tamarix articulata) cutti ngs and the third is stabilization by using cane branch barriers. Randomized sam ples were taken from the surface and subsurface layers of the stabilized and shi fting sand dunes to evaluate the effect of the three techniques on wind erosion parameters. The results indicate high significant differences between the wind e rosion parameters in the surface and subsurface layers in the stabilized sand du nes, while there are insignificant differences between the subsurface layer of t he stabilized dunes and the surface and subsurface layers in the active sand dun es. The results clarify the fact that there is an increase in the percentage of clay, silt, organic matter, mean weight diameter and the percentage of the dry a ggregates (>0.84 mm). A decrease is found in the rate of disaggregation for the dry aggregates in the samples of the surface layer of stabilized dunes when comp ared with the subsurface layer of stabilized dunes and the surface layer of the shifting sand dunes. There is a positive high significant correlation among the aggregate stability parameters and the percentage of clay and silt, and the cont ent of organic matter in the studied layers.

Sand Dunes Fixation in Baiji District, Iraq

This study was carried out at Sand Dunes Stabilization Researches Station in Baiji district (230 km north of Baghdad, Iraq) to evaluate the effects of local soil conditioners manufactured from oil derivatives and plant residuals on sand dunes fixation as the first step for sand dunes stabilization. The results indicate that the fuel oil has the first place in improving wind erosion parameters in the study area, such as increasing mean weight diameter, dry aggregates percentage, the needed time for complete disaggregation by dry sieving, and decreasing the disaggregation rates. Bitumen emulsion occupies the second place, while the plant residuals occupies the third place and has slight effects on the studied parameters. Effects of conditioners on natural vegetation cover are negative in oil derivatives treatments, while positive in plants residuals treatments.

Change Detection of Desert Sand Dunes: A Remote Sensing Approach

Deserts are one of the major landforms on the Earth. While deserts occupy about one-fifth of Earth’s land surface, they have been studied to a much lesser extent. All over the world, desert landforms are expanding ever rapidly and more and more human settlements are finding place in desert regions for habitation. Thus, quantifying and monitoring dunes becomes more relevant from a managerial perspective. Analyzing desert areas using satellite imagery is a challenging task due to weak textural differences and nearly homogeneous spectral responses in most parts of the terrain. In this paper, a post-clustering methodology for change detection of desert sand dunes is proposed. Features based on Radon spectrum are used to cluster dunes of various orientations. These clustered boundaries are used to detect if there are any changes occurring in the dune regions. In the experiments, remote sensing data covering various dune regions of the world are observed for possible changes in dune orientations. In all the cases, it is seen that there are no major changes in desert dune orientations since three decades.

Morphological Profiles of Sand Dunes from ICESat-2 Geolocated Photons

Aeolian process leads to the transportation and accumulation of sand particles that result in sand dune landforms. The structure and shape of the sand dunes are driven by the parameters of interacting wind force and the material composition of sand within. Cross-section profiles over the sand dunes will essay the geomorphological parameters through which the steady state and rate of sand transport can be computed. National Aeronautics and Space Administration’s novel satellite namely Ice, Cloud and land Elevation Satellite-2 (ICESat-2) hosts a solo sensor namely Advanced Topographic Laser Altimeter System (ATLAS) which is a photon counting instrument that measures the round-trip time of the light pulse being emitted and reflected back from the surface determines the true height of the topographic feature on the Earth. In this article, cross-section profiles generated from the beams of ICESat-2 ground-tracks acquired over sand dunes of the Thar Desert region were analysed for detecting the geomorphological parameters. Observations from the cross-section profiles have resulted in giving unprecedented details about the shapes and morphological settings of various types of sand dunes like barchanoids, parabolic, longitudinal, and transverse dunes. Morphological parameters of sand dunes like the length of the stoss slope, crest height, slip face details, inter-arms spacing, height of the trailing arms, length of the depositional lobes, and sinuosity of the recurring crest lines were retrieved with ease from the Level-2A data product namely ATL03 of ICESat-2/ATLAS.

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 ram） 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.