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.     

巴丹吉林沙漠典型高大沙山迎风坡粒度特征

风沙地貌形态与地表风动力的互馈机制研究是近年来风沙地貌学研究的热点问题.以巴丹吉林沙漠高大沙山为研究对象,选取巴丹吉林沙漠东南部苏木巴润吉林以西高度约400 m的高大沙山,对其迎风坡沉积物粒度特征进行系统采样分析,研究了高大沙山迎风坡沙物质粒度由坡脚至丘顶以及表层和下层的变化规律.结果表明,沙山迎风坡表层沉积物的粒度组...     

Grain size and transport characteristics of non-uniform sand in aeolian saltation

Size frequency distributions of sediment particles in a wind tunnel containing a bed of non-uniform sand are investigated by re-interpreting existing experimental data using particle-size analysis. Each particle sample is classified into one of eight groups according to its size grading. The analysis reveals that the modal shape of the particle-size frequency distributions of the saltating sand at different elevations or longitudinal distances is similar to that of the mixed sand in the bed once the boundary layer is fully developed. The standard deviation of the grain-size frequency distribution increases with increasing elevation above the bed then stays constant, whereas its skewness decreases. The mean grain size decays exponentially with elevation. The aeolian sand mass flux is determined for each size grading at different vertical and horizontal measurement locations. The vertical profile of aeolian horizontal mass flux depends on the size grading. The distribution of the sand transport rate according to the mean grain size in each grading fits the normal distribution. A parameter w_i is defined to reflect the likelihood of saltation for sand particles of the i-th size grading, and the mean sand size corresponding to the maximum value of w_i is found to be 0.2 mm. In addition, wind velocity strongly influences the magnitudes of the particle-size distribution and the sand mass flux distribution in both vertical and longitudinal directions.     

Wind velocity and sand transport on a barchan dune

We present measurements of wind velocity and sand flux performed on the windward side of a large barchan dune in Jericoacoara, northeastern Brazil. From the measured profile, we calculate the air shear stress using an analytical approximation and treat the problem of flow separation by an heuristic model. We find that the results from this approach agree well with our field data. Moreover, using the calculated shear velocity, we predict the sand flux according to well-known equilibrium relations and with a phenomenological continuum saltation model that includes saturation transients and thus allows for nonequilibrium conditions. Based on the field data and theoretical predicted results, we indicate the principal differences between saturated and nonsaturated sand flux models. Finally, we show that the measured dune moves with invariant shape and predict its velocity from our data and calculations.     

Wind tunnel experimental investigation of sand velocity in aeolian sand transport

Sand velocity in aeolian sand transport was measured using the laser Doppler technique of PDPA (Phase Doppler Particle Analyzer) in a wind tunnel. The sand velocity profile, probability distribution of particle velocity, particle velocity fluctuation and particle turbulence were analyzed in detail. The experimental results verified that the sand horizontal velocity profile can be expressed by a logarithmic function above 0.01 m, while a deviation occurs below 0.01 m. The mean vertical velocity of grains generally ranges from − 0.2 m/s to 0.2 m/s, and is downward at the lower height, upward at the higher height. The probability distributions of the horizontal velocity of ascending and descending particles have a typical peak and are right-skewed at a height of 4 mm in the lower part of saltation layer. The vertical profile of the horizontal RMS velocity fluctuation of particles shows a single peak. The horizontal RMS velocity fluctuation of sand particles is generally larger than the vertical RMS velocity fluctuation. The RMS velocity fluctuations of grains in both horizontal and vertical directions increase with wind velocity. The particle turbulence intensity decreases with height. The present investigation is helpful in understanding the sand movement mechanism in windblown sand transport and also provides a reference for the study of blowing sand velocity.     

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.     

Wind tunnel modeling and measurements of the flux of wind-blown sand

This paper presents a way to empirically fit experimental data for the horizontal flux of various sizes of wind-blown dry sand using data from wind tunnel experiments. We measured vertical wind profiles to derive threshold shear velocity and estimated shear velocity and the flux of sand mass as a function of the height for nine different grain sizes. We propose a fitting model based on the experimental data and a least-squares method and derive an explicit form of sand flux as a function of height and shear velocity for these grain sizes. We also obtained an explicit form of the empirical equation for the measurement of sand transport per unit width and unit time by integrating the empirical equation as a function of height. Finally, we compared the effectiveness of Bagnold's equation, Kawamura's expression and Lettau and Lettau's equation, for predicting sand transport with the results of our empirical equation. The results show that the transport predicted by all of the equations were always lower than the measured results from the empirical equation for all grain sizes and shear velocities. However, the empirical equation matched Bagnold's equation, Kawamura's equation, and Lettau and Lettau's equation if the coefficients in these equations were adjusted instead of using their original coefficients. The empirical equation for sand transport in the present study contradicts previous conclusions generated by Bagnold's equation, which predict that for a given wind drag, the transport of a coarse sand is greater than that of a fine sand.     

The harmonious character in equilibrium aeolian transport on mixed sand bed

Wind tunnel experiments were carried out to measure the spatial distribution in equilibrium transport of four types of mixed sand. The flux profiles of each grain size group were calculated. It is found that the vertical distribution of mean grain size has a close relation with the grain size distribution in the sand bed. In a log-linear plot, the flux profiles of main grain size groups are all straight lines and their slopes are nearly equal. It is also found that the ratio of transport rate of each size group to the whole transport rate is directly proportional to the mass ratio of each size group in the sand bed and the proportion value is only dependent on the grain size. This harmonious law is applicable to all four types of mixed sand used in the experiment.     

Deposition of sandstorms in a vegetation-covered sand dune in Ejin Oasis and its characteristics

“Ejin Section” found in a typical vegetation-covered sand dune in Ejin Oasis was investigated. In this study, 263 samples were taken from the section for grain-size analysis, 25 for chemical analysis, 11 for 14C dating and 6 for scanning electron microscope (SEM). The results of the study indicate that 3 types of the sediments in the section can be identified, YS, LS and ST. YS, homogeneous yellow-brown dune sands, is equal to those of inland deserts, LS, loess-like sandy soils, is the same as the sandy loess in the middle Yellow River and modern falling dusts, and ST, sandy sediments interbeded with the deadwood and defoliation of Tamarix spp, represents the depositional process of the section interrupted by abrupt changes in climate. The Ejin Section has recorded the repeated dust-storms or sandstorms since 2500 yr BP and the peak periods of the dust-storms or sandstorms revealed by the section are consistent with the records of “dust rains” in historical literatures, indicating that the change of climate is a key factor to increase sandstorms or dust-storms, whereas, “artificial” factor may only be an accelerating one for desertification.     

Movement of a small slipfaceless dome dune in the Namib Sand Sea, Namibia

The migration of a small slipfaceless dome dune close to the northern edge of the Namib Sand Sea has been measured by topographic survey. The dune dimensions are 45-m wide and 1-m high with a volume of 551 m³ that has been calculated as the difference between the dune's surface elevation and an interdune surface extrapolated from measurements around the edge of the dune. The migration direction, 64°, and distance moved, 90 m, are measured against stakes set out in 1976. The dune has moved about 90 m between 1976 and 1999. This is an average linear migration rate of around 4 m year⁻¹, and is equivalent to an annual sand transport rate of about 1.2 tonnes m⁻¹ year⁻¹. The calculated total potential sand flow in this part of the Namib Desert is 119 tonnes m⁻¹ year⁻¹, and the resultant potential sand flow is 63 tonnes m⁻¹ year⁻¹. The dune migration is therefore about 1% of the total potential sandflow and 2% of the resultant indicating that dune migration is only a small part of total potential sand transport. The results suggest that small slipfaceless dome dunes are very inefficient at trapping sand, and that winds blowing across the interdune in this area are undersaturated with sand.     

额济纳绿洲沙尘暴沉积特征

1 Introduction Oases are unique ecosystems and important residences for our ancestors in arid and semiarid regions of the world, and have deeply been imprinted by human actions, such as Jericho oasis of Jordan Valley, Jarmo oasis of the Euphrates River an…     

腾格里沙漠沙丘沉积物粒度特征及其空间差异

沉积物粒度特征对于研究物源、衡量搬运能量具有重要意义。分析了腾格里沙漠45个沙丘顶部样品的粒度特征,并计算了30 a输沙势。结果表明：（1）沙丘沙以细沙（71%）和中沙（20%）为主,平均粒径2.38 Φ,分选系数0.40 Φ;平均粒径与分选系数、偏度呈良好的线性关系。（2）分选为“极好”和“好”的样品整体分布在沙漠中部和西南部,前者概率累计曲线趋于近对称、中等峰态,为二段式,后者趋于正偏、中等或尖窄峰态,为三段式;而沙漠边缘分选较差,概率累计曲线为多段式。（3）盛行起沙风为西北风。西北和东北部属中风能环境,风况属单峰型和复杂型;南部和东南部分别属低、中风能环境,均双峰型风况。研究区沙丘沙源应以下伏河湖相沉积物就地起沙为主。沙漠边缘河湖相沉积物应该为中部区沙丘沙提供了物源,西南部沙丘沙可能源自石羊河下游的冲洪积物,风力分选作用较长,边缘沙丘沙则应来源于该区河湖相沉积物就地起沙,风力分选不充分。粒度参数空间分布特征与沙丘类型和规模之间存在一定的耦合关系。     

The dynamic effects of moisture on the entrainment and transport of sand by wind

There is an increasing awareness of the influence of surface moisture on aeolian entrainment and transport of sediment. Existing wind tunnel studies have shown the impact of a limited range of moisture contents on entrainment thresholds but similar investigations are lacking in the field. The research reported here investigated the influence of changes in surface moisture content on sand entrainment and transport on a meso-tidal beach in Anglesey, North Wales.High frequency (1 Hz) wind velocities measured with hot-wire anemometers were combined with grain impact data from a Sensit monitor and mass flux measurements from a standard sand trap. Surface and near-surface moisture contents were assessed gravimetrically from surface sand scrapes and also directly by using a ThetaProbe. Critical threshold values for entrainment were specified using a modified form of the time fraction equivalence method (Stout, J.E., Zobeck, T.M., 1996a. Establishing the threshold condition for soil movement in wind-eroding fields. Proceedings of the International Conference on Air Pollution from Agricultural Operations. MWPS C-3, Kansas City, 7–9 February 1996, pp. 65–71).Results indicate a time-dependent change in dominant control of the sand transport system from moisture to wind speed, dependent upon the moisture content of the surface sediment. This interchange between controlling parameters on both entrainment and transport was very sensitive to prevailing moisture conditions and took place over a period of minutes to hours. Under conditions experienced in the experiments presented here, the critical moisture threshold for sediment entrainment was determined to be between 4% and 6%, higher than the 1–4% specified in previous wind tunnel experiments. Furthermore, a moisture content of nearly 2% (where moisture was adhered to transported sediment) appeared to have little or no impact on the rate of sand flux.     

黄河源区玛多盆地沙漠化土地粒度特征

为探讨黄河源区玛多盆地风沙沉积物分异特征,采集玛多盆地不同沙漠化程度、不同地貌部位、不同动力条件的沉积物样品进行粒度分析。结果表明：（1）玛多盆地沉积物随沙漠化程度增加,平均粒径增大,粗颗粒组分增多,分选性变好,偏度向负偏靠拢,峰态逐渐转向平坦,频率分布曲线也由双峰态转变为单峰态。（2）流动沙丘以中沙和细沙组分为主,分选性好,偏度近对称,峰度为中等尖锐。各个沙丘不同地貌部位沉积物粒度参数分异受风向改变的影响,变化规律不明显。流动沙丘沉积物相较于流动沙地沉积物的粒径更粗,分选性更好,可能是沙丘形成过程中风对沉积物颗粒的再次分选所致。（3）河流阶地剖面沉积物的主要粒级为中沙和细沙,分选中等偏差,偏度为正偏,峰尖锐,河流阶地沉积物的粒度组成与周围风成作用形成的流动沙丘（地）的粒度组成高度相似,极有可能是当地风沙活动的主要物源。     

干旱沙区植被恢复过程中土壤颗粒分形特征

统计分析了干旱沙漠地区植被恢复过程中土壤颗粒分形特征及其与土壤沙粒(>0.05mm)、粉粒(0.05~0.002mm)和粘粒(<0.002mm)含量和土壤养分状况间的关系。结果表明:土壤颗粒分形维数随恢复时间延长有增大的趋势,土壤颗粒分形维数大小与土壤质地的细粒化有一致的变化趋势。0.05mm粒径成为土壤各粒径的分界值--即土壤分形维数的临界粒径,大于这一粒径颗粒含量越高,土壤分形维数越小;而小于这一粒径的颗粒含量越高,分形维数越大。土壤各养分状况均与土壤颗粒分形维数有极显著的线性正相关关系(p<0.0001),表明了土壤颗粒分形维数能客观地反映土壤肥力特征。在干旱沙漠地区,伴随着植被恢复和流沙固定,土壤颗粒分形维数可作为一个评价土壤演变程度的综合定量指标。     

颗粒分选与有机质因子对复合指纹识别泥沙来源的影响

泥沙来源研究作为土壤侵蚀研究的关键内容之一,对水土保持措施的科学布设具有重要意义.复合指纹识别法是目前研究泥沙来源的较先进方法之一,在泥沙来源研究中具有较大的应用潜力.然而,颗粒分选作用对运用复合指纹识别法判别沉积物来源的准确性有重大影响.加之,有机质等代表的土壤属性(指纹因子)随颗粒分选在不同粒径范围发生富集或贫化也...     

Effects of sand burial on dune plants: a review

Burial of different growth stages of plants （e.g., adult plants, seedlings and seeds） is frequent in dune ecosystems. The soil micro-environment, which differs from surface conditions, influences the survival and growth of dune plants. To sum up knowledge about the survival mechanisms of plants under sand burial and to promote practical rehabilitation of dune vegetation, we reviewed relevant published literature and concluded that： （1） Focus in recent years has been on impacts of sand burial on seed germination and seedling emergence. Generally, shallow burial increased seed germination and seedling emergence, but deeper burial was negative. Buried at the same depth, large seeds showed higher germination and seedling emergence rates, attributed to larger energy reserves. （2） Survival, growth and reproduction rates of dune plants show plasticity in response to sand burial. Long-term deep burial is fatal because it creates a physical barrier which overcomes the vertical growth of plants, reduces photosynthetic leaf area, and limits oxygen availability to roots. Modest burial, on the other hand, is advantageous for growth and reproduction of many dune plants, due to protection from excessive temperature and drought. （3） There are few reports concerning effects of sand burial on plant physiology, but a limited number of studies indicate that partial burial increases water use efficiency, chlorophyll content, transpiration rate and net photosynthetic rates. The antioxidant protective enzyme system and osmolyte balance were reported to be involved in the mechanisms of dune plant resistance to burial.     

Dune sand transport as influenced by wind directions, speed and frequencies in the Ordos Plateau, China

The Ordos Plateau in China is a region with extensive wind erosion, severe desertification and various aeolian sand hazards. In order to determine aeolian sand transport in this region, the relationship between the sand transport rate and wind speed at 10 min frequencies was established by field observation in both the Qubqi Sand Desert and the Mu Us Sandy Land. Threshold wind speeds (2 m above the ground) for mobile, semi-fixed and fixed dune surfaces were estimated by field observations. The sand transport rate increased with the increase of the bare land ratio and near-bed wind speed. High-resolution meteorological 10 min average wind velocity data at 10 m above the ground were converted into velocity values at a height of 2 m to calculate sand transport potential based on three specific parameters decisive for sand transport: wind speed, duration and direction. The quantity of aeolian sand transported was calculated for various wind speed levels and directions, and the overall characteristics of sand transport on different dune surface types were determined by vector operation techniques. Sand transporting winds took place mainly in springtime. The prevailing wind directions were W, WNW and NW, with a frequency of more than 60% in total, and sand transport in these directions made up more than 70% of the total transport, corresponding to a general southeastward encroachment of aeolian sand in the study area. The relationship between wind frequency and speed can be expressed by a power function. High magnitude strong winds had a low frequency, but they played a dominant role in aeolian sand transport.     

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...