The effect of surface slope on saltation threshold

A new wind tunnel has been constructed to study the mass transport properties of wind-blown sand. This report on the first set of experiments in the new wind tunnel concerns the effect of slope on threshold friction speed. The results of this series of static threshold experiments (and one dynamic threshold experiment) for a range of particle diameters and bed slope angles show that, provided the effects of Reynolds number variation and interparticle cohesive force are accounted for, the static friction angle α is independent of slope and close in value to the measured static angle of response.     

Characteristics of particle size for creeping and saltating sand grains in aeolian transport

Creep and saltation are the primary modes of surface transport involved in the fluid‐like movement of aeolian sands. Although numerous studies have focused on saltation, few studies have focused on creep, primarily because of the experimental difficulty and the limited amount of theoretical information available on this process. Grain size and its distribution characteristics are key controls on the modes of sand movement and their transport masses. Based on a series of wind tunnel experiments, this paper presents new data regarding the saltation flux, obtained using a flat sampler, and on the creeping mass, obtained using a specifically designed bed trap, associated with four friction velocities (0·41, 0·47, 0·55 and 0·61 m sec^?1). These data yielded information regarding creeping and saltating sand grains and their particle size characteristics at various heights, which led to the following conclusions: (i) the creeping masses increased as a power function (q = ?1·02 + 14·19u_*³) of friction wind velocities, with a correlation (R²) of 0·95; (ii) the flux of aeolian sand flow decreases exponentially with increasing height (q = a exp(–z/b)) and increases as a power function (q = ?26·30 + 428·40 u_*³) of the friction wind velocity; (iii) the particle size of creeping sand grains is ca 1·15 times of the mean diameter of salting sand grains at a height of 0 to 2 cm, which is 1·14 times of the mean diameter of sand grains in a bed; and (iv) the mean diameter of saltating sand grains decreases rapidly with increasing height whereas, while at a given height, the mean diameter of saltating sand grains is positively correlated with the friction wind velocity. Although these results require additional experimental validation, they provide new information for modelling of aeolian sand transport processes.     

Aerodynamic grain‐size distribution of blown sand

Aeolian sand entrainment, saltation and deposition are important and closely related near surface processes. Determining how grains are sorted by wind requires a detailed understanding of how aerodynamic sand transport processes vary within the saltating layer with height above the bed. Grain‐size distribution of sand throughout the saltation layer and, in particular, how the associated flux of different grain size changes with variation in wind velocity, remain unclear. In the present study, a blowdown wind tunnel with a 50 cm thick boundary layer was used to investigate saltating sand grains by analyzing the weight percentage and transport flux of different grain‐size fractions and the mean grain size at different wind velocities. It was found that mean grain size decreases with height above the sand bed before undergoing a reversal. The height of the reversal point ranges from 4 to 40 cm, and increases with wind velocity following a non‐linear relationship. The content of the finer fractions (very fine and fine sand) initially increases above the sand bed and then decreases slightly with height, whereas that of the coarser fractions (medium and coarse sand) exhibits the opposite trend. The content of coarser grains and the mean grain size of sand in the saltation layer increase with wind velocity, indicating erosional selectivity with respect to grains in multi‐sized sand beds; but this size selectivity decreases with increasing wind velocity. The vertical mass flux structure of fine sand and very fine sand does not obey a general exponential decay pattern under strong wind conditions; and the coarser the sand grain, the greater the decrease rate of their transport mass with height. The results of these experiments suggest that the grain‐size distribution of a saltating sand cloud is governed by both wind velocity and height within the near‐surface boundary layer.     

Wind tunnel experiments on vertical distribution of wind-blown sand flux and change of the quantity of sand erosion and deposition above gravel beds under different sand supplies

Wind tunnel experiments were carried out with respect to the vertical distributions of wind-blown sand flux and the processes of aeolian erosion and deposition under different wind velocities and sand supplies above beds with different gravel coverage. Preliminary results revealed that the vertical distribution of wind-blown sand flux was a way to determine whether the gobi sand stream was the saturated one or not. It had different significances to indicate characteristics of transport and deposition above gobi beds. Whether bed processes are of aeolian erosion or deposition was determined by the sand stream near the surface, especially within 0–6 cm height, while the sand transport was mainly influenced by the sand stream in the saltating layer above the height of 6 cm. The degree of the abundance of sand supply was one of the important factors to determine the saturation level of sand stream, which influenced the characteristic of aeolian erosion and deposition on gravel beds. Given the similar wind condition, the sand transport rates controlled by the saturated flow were between 2 and 8 times of the unsaturated one. Those bed processes controlled by the saturated flow were mainly of deposition, and the amount of sand accumulation increased largely as the wind speed increased. In contrast, the bed processes controlled by the unsaturated flow were mainly of aeolian erosion. Meanwhile, there was an obvious blocking sand ability within the height of 0–2 cm, and the maximal value of sand transport occurred within the surface of 2–5 cm height.     

Air density effects on aeolian sand movement: Implications for sediment transport and sand control in regions with extreme altitudes or temperatures

Aeolian sand transport results from interactions between the ground surface and airflow. Previous research has focused on the effects on sand entrainment and mass transport of surface features and wind velocity, but the influence of air density, which strongly constrains airflow characteristics and the resulting sand flow, has not been widely considered. In the present study, entrainment, saltation characteristics and transport rates were examined at nine experimental sites ranging in elevation from ?154 m below sea‐level (Aiding Lake) to 5076 m above sea‐level (Tanggula Mountain pass on the Qinghai–Tibetan plateau). At each site, a portable wind tunnel and high‐speed camera system were set up, and the friction wind velocity, threshold friction velocity and sand flow structure were observed systematically. For a given volumetric airflow, lower air density increases the wind velocity. Low air density also creates a high threshold friction velocity. The Bagnold wind erosion threshold model remains valid, but the value of empirical parameter A decreased with decreasing air density and ranged from 0·10 to 0·07, the smallest values reported in the literature. For a given wind velocity, increased altitude reduced total sand transport and creeping, but the saltation rate and saltation height increased. The present results provide insights into the fundamental mechanisms of the initiation and transport of sand by wind in regions with an extreme temperature or altitude (for example, alpine deserts and low‐lying lake basins) or on other planets, including Mars. These results also provide theoretical support for improved sand‐control engineering measures. The data and empirical equations provided in this paper improve the ability to estimate threshold and transport conditions for wind‐blown sand.     

Wind tunnel and field testing of a simple sand catcher for sampling inhomogeneously saltating sand in desertified environments

Problems of vertical alignment and vibrations of disposable coffee cups used in a modified version of the De Ploey saltating sand catcher were solved. The new version was tested in a sediment wind tunnel. Its catches appeared linearly related to amounts of eroded sand, largely independent of wind speed and wind direction, and depended logarithmically on height. The catch efficiency may therefore be taken to be approximately independent of wind speed and direction. The instrument performed well under conditions of inhomogeneously saltating sand in a strongly desertified environment in Central Sudan. Use and results show the improved simple catcher to be easy to assemble, reproducible and cheap, suitable for multipoint use to cover all inhomogeneities in outdoor saltating sand fields.     

The impact process in aeolian saltation: two-dimensional simulations

ABSTRACT
A critical event in the trajectory of a sand grain saltating in air is its interaction with the surface. We examine the phenomenon of grain-bed impacts in two dimensions using a combination of dynamical computer simulations, analytical models and physical reasoning. The results indicate that the grain-bed collisions can be treated as two-body collisions with the bed particle assuming an effective mass greater than its true mass. Also, the presence of geometrical surface irregularities has a strong bearing on the interaction between saltating and surface grain populations, as well as on the formation of small-scale bedforms.     

Aerodynamic dislodgement of multiple-size sand grains over time

An experimental study was undertaken in a large-scale wind tunnel to investigate sand particle dislodgement by wind over time in the absence of grain-bed collisions. Aerodynamic dislodgement was measured for six groups of sand particles under two known wind velocity profiles. The results show that the dislodgement rate for both fine and coarse particles decreases rapidly during the transition of the particle surface from a non-wind-worked condition to a wind-worked condition, and that the dislodgement rate continues to decay under a wind-worked condition even though the mean grain size of surface particles remains nearly the same. A previously developed theoretical method for calculating the number of particles left on the bed by wind was developed further. The derived method was used to calculate the time-decay of the dislodgement rate and the length of time required for the dislodgement rate to reach an equilibrium. The length of time for dislodgement rate to reach an equilibrium in this study is of the order of 10–15 min. This not only provides further observation of the second, long stage of aeolian sediment transport system development reported previously but also indicates a potentially large variation in the time-decay of transport rate under different conditions. The results indicate that the time-decay of the particle dislodgement rate is related to sorting processes. Because of the artificial method of preparation of the grain surface and the wind velocity profiles, the results of this study should be applied with caution to natural conditions.     

The effect of unsteady winds on sediment transport on the stoss slope of a transverse dune, Silver Peak, NV, USA

Rapid (10 s) measurements of sediment transport and wind speed on the stoss slope of a transverse dune indicate that the majority of sand transported is associated with fluctuations in wind speed with a periodicity of 5–20 min duration. Increases in the sediment transport rate towards the dune crest are associated with a small degree of flow acceleration. The increase in wind speed is sufficient, however, to greatly increase values of the intermittency index ( γ ), so that the duration of saltation is extended in crestal regions of the dune. The pattern of sediment transport on the stoss slope and, therefore, the locus of areas of erosion and deposition change with the regional wind speed. Erosion of the crest occurs during wind speed events just above transport threshold, whereas periods of higher magnitude winds result in deposition of sand upwind of the crest, thereby increasing dune height. Although short-term temporal and spatial relations between sand transport and wind speed on the stoss slope are well understood, it is not clear how these relations affect dune morphology over longer periods of time.     

Field measurements of the flux and speed of wind-blown sand

A field experiment was conducted to measure the flux and speed of wind-blown sand under known conditions in a natural setting. The experiment, run at Pismo Beach, California, involved a tract 100 m long (parallel with the wind) by 20 m wide. The site was instrumented with four arrays of anemometers to obtain wind velocity profiles through the lower atmospheric boundary-layer, temperature probes to determine atmospheric stability and wind vanes to determine wind direction. From these measurements, wind friction speeds were derived for each experimental run. In order to measure sand saltation flux, a trench 3 m long by 10 m wide (transverse to the wind direction) by 0·5 m deep was placed at the downwind end of the tract and lined with 168 collector bins, forming an ‘egg-box’ pattern. The mass of particles collected in each bin was determined for four experimental runs. In order to assess various sand-trap systems used in previous experiments, 12 Leatherman traps, one Fryberger trap and one array of Ames traps were deployed to collect particles concurrently with the trench collection. Particle velocities were determined from analysis of high-speed (3000 and 5000 frames per second) motion pictures and from a particle velocimeter. Sand samples were collected from the trench bins and the various sand traps and grain size distributions were determined. Fluxes for each run were calculated using various previously published expressions, and then compared with the flux derived from the trench collection. Results show that Bagnold's (1941) model and White's (1979) equation most closely agree with values derived from the trench. Comparison of the various collector systems shows that the Leatherman and Ames traps most closely agree with the flux derived from the trench, although these systems tended to under-collect particles. Particle speeds were measured from analysis of motion pictures for saltating particles in ascending and descending parts of their trajectories. Results show that particle velocities from the velocimeter are in the range 0·5–7·0 m s^?1, compared to a wind friction velocity of 0·32–0·43 m s^?1 and a wind velocity of 2·7–3·9 m s^?1 at the height of the particle measurements. Descending particles tended to exceed the speeds of ascending particles by ～ 0·5 m s^?1.     

Aeolian saltation threshold: the effect of density ratio

Saltation threshold data from three wind tunnels and from hydraulic flumes are presented to show that the dimensionless threshold friction speed for small particles is a continuous function of particle-to-fluid-density ratio. In addition, the dimensionless threshold speed is a function of the grain-friction Reynolds number and an interparticle force term. The variation with density ratio seems to be due to the relative energy with which particles impact other particles to initiate saltation.     

An experimental study of multiple grain-size ejecta produced by collisions of saltating grains with a flat bed

Collision data are presented from coloured high-speed films of three size fractions of sand grains saltating over a bed of the total grain population. Each fraction was colour tagged and the proportion of each size ejected by grains colliding with the surface was recorded on a number of films taken as the bed was progressively eroded. The results confirm earlier findings that V₃/V₁?0.5–0.6, V_n/V₁?.08 and the rebound angle increases with decreasing grain size. Ejected grains are examined in relation to their size, the impactor size, ejection speed and angle and the number of ejecta per collision. In addition, changes in grain parameters are observed with time. For fine impactors, ejection speeds generally increase with a decrease in ejecta size, but the fine fraction does not follow this trend for the coarse and medium impactors. Ejection angles are typically between 40° and 60°, with coarse grains having shallower mean angles than fine ejecta. The number of ejections per collision increases with a decrease in particle size for each impactor size. The general tendency for coarse particles to be ejected at lower speeds and shallower angles than fine particles will lead to sorting of the grain sizes. There is poor correlation between the forward momentum loss of the saltating grams at collision and both the forward momentum of the ejected grains and the number of ejected grains. Much of the forward momentum of the saltating grains is transfered to creeping grains. The composition and geometry of the bed are considered to be important factors in the evolution of the saltation cloud.     

Steady state saltation in air

Coupled equations of motion for steady state saltation over an infinite plane are derived and solved for a simplified model of the grain-surface impact process. Experimentally observed features of the wind velocity profile in saltation are qualitatively reproduced, including a diminution of the sub-saltation layer mean wind speed, as the friction speed increases. In this model the surface impact velocity of the saltating grains remains relatively constant over a wide range of free-stream shear stresses, and the grain mass flux increases with friction speed u_f* less rapidly than u_f³.     

Particle size characteristics of aeolian ripple crests and troughs

Ripples are prevalent in aeolian landscapes. Many researchers have focused on the shape and formation of sand ripples, but few have studied the differences in the particle size of sand on crests and in troughs along bed, especially the variations caused by changes in friction velocity and the wind‐blowing duration. A particle size of 158 μm (d ) was used to create aeolian ripples in a wind tunnel under four friction velocities (u *) with different wind duration times (t ). Samples were collected from the surfaces of ripple crests and troughs, respectively, at seven sites, and particle sizes were measured using a Malvern Mastersizer 2000. The main results were: (i) The particle size distributions of sand in troughs are unimodal with slight variations of particle size parameters, including mean particle size, standard deviation, skewness and kurtosis, etc., under different conditions, while these particle size parameters of sand on crests change with friction velocity and deflation time. Moreover, some of the particle distribution curves for the sand on crests do not follow typical unimodal curves. (ii) With increasing friction velocity or deflation duration, the sand on the crests shows a coarsening process relative to those on the bed surface. The particle size of sand on crests at a 1 m bed increases linearly with friction velocity (d  =  344·27 + 34·54 u *) at a given wind‐blowing duration. The particle sizes of sand on crests at 1 m, 2 m and 4 m beds increase with a power‐law relationship (d  = a + t ^b, where a and b are fitting parameters) with deflation time at a given friction velocity. (iii) The probability cumulative curves of sand showed a three‐section pattern in troughs and on most of the crests but a four‐section pattern at crest locations due to increased influence by friction velocity and deflation time. The proportions of the sediment moved by suspension, saltation and creep in the three‐section pattern were within the ranges of 0·2% to 2·0%, 97·0% to 98·9%, and 0·8% to 3·0%, respectively. For the four‐section pattern, suspension accounted for 0·3% and 3·0%, and the proportion of creep increased with friction velocity and deflation time, while saltation decreased accordingly. Although these results require additional validation, they help to advance current understanding of the grain‐size characteristics of aeolian ripples.     

Wind tunnel test of snow loads on a stepped flat roof using different granular materials

An accurate prediction of wind-induced redistribution of snow load on roof surfaces is vital to structural design. To represent the pattern of snow distribution caused by snowdrift in wind tunnel test, appropriate modeling particles should be selected. The particle density is the key to determine the values of several important similarity parameters. In this study, the redistribution of snow load on a stepped flat roof was simulated by means of wind tunnel test using low-density saw wood ash, medium-density polyfoam, and high-density silica sand, respectively. To ensure the comparability of the test results of the three modeling particles, the wind tunnel test results for comparison were performed under almost the same conditions of dimensionless wind velocity and dimensionless time. Then, the results of the present study were compared with those from field observations of prototypes in previous studies. The effects of wind duration, wind velocity, and roof span on the redistribution of snow on roof surfaces were investigated. The characteristics of erosion/deposition range and the location of maximum quantities of erosion/deposition under independent effects of wind duration, wind velocity, and roof span were also studied.     

Determination of efficiency of Vaseline slide and Wilson and Cooke sediment traps by wind tunnel experiments

The trap efficiency of a catcher in wind erosion measurements plays a significant role, and in many cases suspension trap efficiencies at high wind velocities are still unknown. The sediment trap efficiency generally changes with particles size and with wind speed. In this study, the efficiency of Vaseline Slide (VS) and Modified Wilson and Cooke (MWAC) catchers were determined with different sand particle sizes (<50, <75, 50–75, 200–400, and 400–500 μm) at a fixed wind speed (13.3 ms⁻¹) and with different soil textures at different wind velocities (10.3, 12.3, and 14.3 ms⁻¹) in the wind tunnel of the International Center for Eremology (ICE), Ghent University, Belgium. The traps were placed at different heights (4, 6.5, 13, 20, 120, and 192 cm for VS and 1.5, 3, 5, 8, 11, and 30 cm for MWAC) to catch saltating and suspended sediments in a 12-m long, 1.2-m wide and 3.2-m high working section of the wind tunnel. In the sand particle experiments, the efficiency of the VS catcher was 92% for particles smaller than 50 μm and decreased with increasing particles size, falling to 2.2% for 400–500 μm particle size at 13.4 ms⁻¹. However, the MWAC’s efficiency was 0% for particles smaller than 50 μm and increased with increasing particle size to 69.5% at 400–500 μm. In the experiments with different soil textures, the efficiency of each catcher significantly changed with soil and with wind speed. It also considerably varied with the catchers: for instance, for sand (S), the MWAC efficiency was very high (67.4, 113.4, and 90.5% at 10.3, 12.3, and 14.4 ms⁻¹, respectively) while the efficiency of VS was relatively very low (5.2, 4.4, and 1.9% at 10.3, 12.3, and 14.4 ms⁻¹, respectively). Results indicated that the efficiency depends critically on the particle size, type of catcher, and wind speed, and these could be helpful to increase the robustness of wind erosion measurements.     

Fine particle deposition to initially starved,stationary, planar beds

Reported here are results from new flume experiments examining deposition and entrainment of inert, silt‐sized particles (with spherical diameters in the range from 20 to 60 μm) to and from planar, impermeable and initially starved beds underlying channel flows. Bed surfaces comprised smooth or fixed sand‐size granular roughness and provided hydraulically smooth to transitionally rough boundaries. Results of these experiments were analysed with a simple model that describes the evolution of vertically averaged concentration of suspended sediment and accommodates the simultaneous delivery to and entrainment of grains from the bed. The rate of particle arrival to a bed diminishes linearly, and the rate of particle entrainment increases by the 5/2 power, as the value of the dimensionless Saffman parameter S = u_*³/g’ν approaches a threshold value of order unity, where u is the conventional friction velocity of the turbulent channel flow, g’ is the acceleration due to gravity adjusted for the submerged buoyancy of individual particles and ν is the kinematic viscosity of the transporting fluid. This transport behaviour is consistent with the notion that non‐cohesive, silt‐sized particles can neither reach nor remain on an impermeable bed under flow conditions where mean lift imposed on stationary particles in the viscous sublayer equals or exceeds the submerged weight of individual particles. Within the size range of particles used in these experiments, particle size and the characteristic size of granular roughness, up to that of medium sand, did not affect rates of dimensionless arrival or entrainment to a significant degree. Instead, a new but consistent picture of fine‐particle transport is emerging. Silt‐sized material, at least, is subject to potentially significant interaction with the bed during intermittent suspension transport at intermediate flow speeds greater than the value required for initiation of transport (ca 20 cm sec^?1) but less than the value (ca 50 cm sec^?1) required by the Saffman criterion ensuring transport in fully passive suspension or, equivalently, ‘wash‐load’.     

沙漠公路风沙土路基风蚀破坏试验研究

以沙漠公路风沙土路基为研究对象,通过室内风蚀风洞试验研究路基的风蚀破坏规律,以及路基不同断面对风沙流运动的影响。以路基高度、路基边坡坡率和路基宽度作为路基断面主要设计参数,研究不同路基断面下风沙流扰动、增速、减速、恢复的过程,以及路基周围风速流场的变化特征,分析路基病害较未病害时路基周围流场的变化。试验结果表明：路基高度和边坡坡率对风沙流运动的影响较大。随路基高度增加,路基对风沙流流场扰动增强,迎风坡坡顶处吹蚀破坏和背风坡坡底处堆蚀破坏越显著,在确定的路基边坡坡率下,路基模型高度为250 mm较模型高度为60 mm时,迎风坡坡顶风速增加1.13倍,背风坡坡底风速减小2.53倍,建议沙漠公路路基高度宜小于2.5 m。进一步,在确定的路基高度下,比较不同的边坡坡率对路基沿程风速的影响,发现当路基边坡坡率为1:1.75时,路基沿程风速变化不明显,沙漠公路风沙土路基不宜被风蚀破坏。     

The effects of atmospheric stability intensity on the movement of windblown sand

Using a model that couples wind flow with the motion of sand particles under different atmospheric stability intensities, this paper studied the effects of atmospheric stability on the trajectory and velocity of sand particles in the saltation layer, and the duration before a steady state was achieved. The vertical velocity, horizontal distance, and the maximum height of saltating sand particles increased with increasingly negative stability intensity under unstable conditions. The wind–sand flow reached equilibrium more quickly with increasingly negative stability intensity under unstable conditions, but reached equilibrium more slowly with increasing stability intensity under stable conditions.     

RETRACTION: Numerical simulation of a wind-sand flow under different conditions of atmospheric stability

A theoretical model for wind‐sand flow is developed by considering the coupling between wind flow and sand particle motion, the latter subject to the Magnus effect, under different atmospheric stability conditions. Using this model, the characteristics of the wind‐sand flow are discussed in detail. The results show that the atmospheric stability and the Magnus effect both have a strong influence on wind profiles and on the trajectories of sand particles. This approach produces results with characteristics that differ from those previously reported; the latter only applying to atmospheric conditions of neutral stability. The saltating sand reaches a greater height under non‐neutral stability than under neutral stability, while the maximum horizontal distance is greater under unstable conditions and is smaller under stable conditions than under conditions of neutral stability.