Aeolian sand transport along beaches

Storm‐generated ephemeral transverse sand ridges were observed developing along the beach fronting Sir Richard Peninsula, South Australia during 25 March 1984. The ridges displayed a mean height of 10 cm and a wavelength of 12 m; their breadth was approximately 2 m, and length varied with beach width but ranged up to 40 m over 10 km of coastline. The steeper sides of the ridges faced upwind due to erosion after initial ridge development. Damp sand in the swales inhibited wind scour and restricted sand supply, but provided a firm substrate over which the sand could saltate. Approximately 5000 m³ of sand were incorporated into the ridges by westerly winds blowing at velocities between 45 and 69 km/hr. This observation emphasises the role of alongshore winds in transporting beach sediments and developing essentially ephemeral forms, which might, nevertheless, be preserved in the geological record. The significance of these forms varies with coastal orientation and local wind regimes.     

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.     

稳态风沙流中瞬态输沙特征

风沙流中沙粒运动在来流风速不变时也会表现出非稳态特征。在风洞内利用粒子图像测速系统（PIV）测量了风沙运动的时间序列,并基于PIV测量技术提出风沙流中沙粒平均直径、数密度、平均水平速度和输沙通量等参数在某一时刻的计算方法,其中输沙通量的计算考虑沙粒大小垂向分布的影响。结果表明：来流风速不变时,沙粒平均直径、数密度、平均水平速度和输沙通量随时间具有明显的波动性;沙粒平均直径和平均水平速度的标准偏差一般随高度增加而增加,沙粒数密度和输沙通量标准偏差随高度增大而减小;这些参数的相对标准偏差均随高度增加而增大。     

On the rate of aeolian sand transport

A simple explicit formula for the transport rate of windblown sand is derived based on physical reasoning. The curve relating the dimensionless transport rate to the dimensionless friction speed exhibits the empirically well-established peak. In fact, the theory developed in the paper can explain the peaked shape of this curve. Three empirically determined coefficients in the formula can be given a physical interpretation. The formula is shown to fit transport rates measured in wind-tunnel experiments well, except for very coarse sand. Formulae for the wind profile in the saltation layer and for the grain dislodgement rate in dependence on the friction speed are obtained as part of the theory.     

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.     

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.     

Patterns in the sand: From forcing templates to self-organization

The nearshore region exhibits many striking morphological patterns with a variety of spatial and temporal scales. The formation of these rhythmic features has been initially ascribed, depending on the pattern in question, to spatial structures in the flow or geological constrains. These forcing templates have been hypothesized to provide the spatial structure that becomes imprinted on the shoreline or seabed morphology. More recently, new explanations for rhythmic patterns have involved interactions between fluid flow and sediment transport that create morphological feedbacks and lead to pattern self-organization. While forcing-template models do not explicitly treat transport of the sediment that makes up the pattern, self-organization models focus on the strong couplings within flow/sediment systems, and on interactions between emergent structures. We illustrate the sweeping shift from template explanations to self-organization by discussing four nearshore patterns: beach cusps, surfzone crescentic sandbars, inner-shelf sorted bedforms, and large-scale cuspate shorelines. Models involving self-organization show that local interactions between flow and sediment transport can collectively give rise to patterns with large-scale coherence and that the driving feedbacks can be associated with either topographical or grain-size composition instabilities. An approach based on self-organization also allows researchers to establish limitations in the predictability of the occurrence of rhythmic patterns and characteristics as well as to study mechanisms leading to the observed variability of pattern or lack of regularity.     

Effects of fetch and surface texture on aeolian sand transport on two nourished beaches

Aeolian sand transport on two nourished beaches was related to the fetch of the wind over the beach sand and to surface characteristics. Meteorological and hydrological conditions were recorded for 2 months. The fetch of wind over beach sand was estimated from wind direction, water level, wave height and beach topography. Aeolian sand transport was determined with sand traps. Sediment flux was found to increase with fetch, although this relation was especially affected by the variability in surface characteristics. On one of the beaches sediment supply was limited as a result of shells, forming a lag deposit.     

直压立式纱网沙障对近地表输沙量及风速的影响

评价直压立式纱网沙障对流动沙地的固沙效果是其应用的前提。采用阶梯式集沙仪和HOBO风速仪野外测定了纱网沙障的输沙量及风速变化。结果表明:与对照相比,设置纱网沙障后,0~10 cm或0~20 cm高度范围内,带宽2、3、4、5 m带状沙障输沙量分别降低91.4%或89.7%、97.0%或96.4%、98.1%或96.7%、85.9%或84.2%;而2 m×2 m、3 m×3 m、 4 m×4 m、5 m×5 m网格沙障输沙量分别降低92.1%或91.2%、70.4%或65.6%、65.3%或65.2%、39.9%或30.0%。与对照相比,10、30、50 cm高度,带宽2、3、4 m带状沙障平均降低风速分别为26.7%、17.3%、11.8%,2 m×2 m、3 m×3 m、4 m×4 m、5 m×5 m网格沙障平均降低风速分别为55.3%、30.4%、23.0%。综合考虑,设置带宽3~4 m带状或网格纱网沙障能够满足工程固沙减少风沙危害的需求。     

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.     

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.     

雅鲁藏布江曲水-泽当段风沙活动动力条件分析与风沙灾害防治建议

雅鲁藏布江河流宽谷区是青藏高原风沙活动最为强烈地区之一.本文选择雅鲁藏布江曲水-泽当段为研究区,分析该地区1981-2010年以来的风动力条件变化特征.结果表明:1981年以来年平均风速明显降低,月平均风速2-4月最大.受河流-山地复合系统的影响,风向复杂且具有明显的空间差异性,贡嘎以西风和东风为主,泽当包括西风、南风...     

The dependency of sand transport rate by wind on the atmospheric stability: A numerical simulation

A theoretical model of the process for wind–sand flow is developed through consideration of the coupling between wind flow and the motion of sand particles under different atmospheric stability conditions. Using this model, we studied the effects of atmospheric stability on the sand transport rate, the number of sand particles per unit area and time, and the duration before a steady state is achieved in detail. The results show that atmospheric stability has a strong effect on the movement of the wind–sand flow, and produces results with different characteristics from those previously reported in the literature which apply only to conditions of neutral stability. Under unstable conditions, the wind–sand flow reached equilibrium more quickly, with a higher total sand flux and sand flux at all heights than under neutral or stable conditions.     

An aeolian transport model for flat shifting sand fields under dynamic-limiting conditions

Estimating aeolian transport rates in the field is a difficult task even though various transport equations have been proposed. In this study, we used 5 years of field data on aeolian transport from an experimental plot for aeolian research located in the southeastern Tengger Desert to develop a practical aeolian transport equation capable of estimating transport rates in flat shifting sand fields. The resulting equation relates transport to the average wind velocity during a given period through a modified power function. The transport rate increases linearly with the cube of the average wind velocity. The equation can be generalized rate to a form that contains a proportionality coefficient and a threshold average wind velocity. The proposed model has an advantage over previous models in that it is based on a much larger sample size and corrected to account for variability in wind direction, sampling efficiency, and the height of the wind measurements.     

雅鲁藏布江中游不同地表输沙量特征

雅鲁藏布江河流宽谷区是青藏高原风沙活动最为强烈地区之一.输沙量是评价区域风沙活动的最主要指标之一,但目前对该地区输沙量的研究几乎是空白.利用旋转多路方口集沙仪对雅鲁藏布江流域中游河岸沙地、河漫滩沙地和山麓沙地3种地表类型、6个点2020年9-12月的输沙量进行野外实地观测,对比不同下垫面的输沙量,分析地表特征对输沙量的...     

风力作用下沙粒蠕移概率的转化特征

建立了沙粒从蠕移状态向静止、跳跃转化以及继续滚动的概率模型.根据滚动沙粒简化的运动方程,结合两个关键随机变量-沙床表面风速和床面位置参数,求出了静止概率、滚动概率、跳跃概率随时间变化的表达式.在此基础上通过对时间求极限得到稳定情况下沙粒处于三种运动状态的概率.利用沙粒蠕移概率除蠕移概率和跃移概率之和,得到沙粒相对蠕移概率,并以此反映蠕移输沙量占总输沙量的比例.计算结果表明,三种转移概率是时间、沙床表面风速和沙粒直径的函数.静止概率随时间增大而增大,但随粒径增大而减小;滚动概率随时间的变化根据粒径的不同表现出不同的特点.粒径较小,滚动概率随时间的增大先增大后减小,存在极大值;粒径较大时,滚动概率随时间的增大逐渐增大,不存在峰值.跳跃概率随时间的增大而增大.三种概率达到稳定状态所需要的时间随粒径的增大而减少.同粒径的沙粒在相同的时间内,三种转移概率会趋于定值.稳定概率和相对蠕移概率由沙床表面风速分布和粒径大小来确定,具有很大的取值范围;静止稳定概率和跳跃稳定概率随粒径的增大分别增大和减小;而滚动稳定概率随粒径的增大逐步先增大后减小.在一定风速条件下,相对蠕移概率随风速和方差的变化都不大,沙粒粒径是最主要的影响因素.     

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.     

基于COSI-Corr技术的龙羊峡库区1987—2019年风沙输移特征及潜在入库量估算

风沙活动威胁着龙羊峡水库的安全运营,查清沙害来源和入库量对于防治水患和沙害具有重要意义。基于1987、1995、2003、2013、2019年的Landsat卫星影像,利用COSI-Corr技术监测了龙羊峡库区不同时空的沙丘移动特征,并重新评估库区近32 a的潜在风沙入库量。结果显示：（1）1987—2019年龙羊峡库区沙丘平均移动速率为5.81 m·a^-1,呈先加速（1987—2003年）后减速（2003—2013年）再加速（2013—2019年）趋势;沙丘移动方向在132.81°—165.82°范围内,与该区主风向一致。（2）近32 a向龙羊峡水库输送的潜在风沙量可达7.82×10⁷ m3（1.20×10⁸ t）。上风向塔拉滩潜在输送量为7.38×10⁷ m3（1.14×10⁸ t）,下风向木格滩仅贡献了0.44×10⁷ m3（0.68×10⁷ t）。（3）库区内风沙输移受风况、气候、植被等多种因素的影响,在未来全球变暖条件下,青藏高原的风沙活动将会持续发展,风沙入库量的长期累计效应将对水库安全构成严重威胁,必须引起足够重视。     

Modeling the aeolian sand transport for the desert of Kuwait: Constraints by field observations

The Kuwaiti desert is swept by strong winds, particularly by northwesterly wind, during the summer months (April through September), causing mass-scale sand transport that poses a serious environmental hazard. Understanding that accurate transport rate data are very difficult to obtain either by field measurements or by using any existing sand transport formulae, this paper presents a mathematical model to predict the sand-transport rate for Kuwaiti desert by following a well-known theoretical approach of Lettau and Lettau (1978) and by taking into account the effect of seasonal wind variability. The validity of the model is next examined using the field data from the study of Al-Awadhi and Al-Sudairawi (2002). The examination showed that the model underestimates the rate of sand transport by an average ratio of 0.15. The application of the model, at some seven locations that are evenly distributed across the Kuwaiti desert, indicated that about 40% of the amount of sand transport in Kuwait prevails from the NW during the peak summer months (June and July). The resulting map of sand transport rate indicated that the zone of higher sand transport rate is located within the areas covered by most extensive mobile sand sheets and sand dunes.