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1.
The effect of wind speed and bed slope on sand transport   总被引：7，自引：0，他引：7  
This paper reports on a wind tunnel study of the effects of bed slope and wind speed on aeolian mass transport. The use of a sloping wind tunnel has enabled estimation of the friction angle α to be about 40° for saltating particles in the range 170–540 μm. A formula relating dimensionless mass transport to friction speed and bed slope is proposed, and mass transport data for five uniform sand samples and one non-uniform sand sample are shown to fit the equation well. In particular, the relationship reveals an overshoot in mass transport slightly above threshold collisions, a feature also evident when previous experimental data is re-examined. As the number of mid-air collisions between the saltating particles increases greatly with wind speed, the overshoot may occur as a result of increasing energy losses resulting from the collisions. Finally, it is demonstrated that data for saltating snow shows a similar overshoot in the dimensionless transport rate.  相似文献   

2.
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.  相似文献   

3.
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.  相似文献   

4.
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.  相似文献   

5.
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.  相似文献   

6.
Field measurements of the flux and speed of wind-blown sand   总被引：13，自引：0，他引：13  
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.  相似文献   

7.
南疆铁路沿线风沙危害与工程防治   总被引：4，自引：0，他引：4  
文章通过对南疆铁路沿线的概况、地质地貌、植被分布及气象状况,特别是风信状况的调查,阐明了沿线的自然状况。20世纪90年代,研究工作者在沿线风沙活动最为剧烈的巴楚地区设置定位观测站,对典型的活动沙丘特征进行观测,获得了沙丘移动的定量数据。同时在各种类型典型戈壁地段,设置积沙仪,在不同的风速情况下收集动态流沙,总结出当地的输沙率公式,为风沙防护提供了直接的设计依据。通过对各种工程防治措施的调查研究、特别是新材料试验,归纳出各种工程防沙结构的功能效果,提出适宜当地条件的新工程防沙结构模式。  相似文献   

8.
Doklady Earth Sciences - The saltating particle aleurite mode has been discovered in the wind–sand flux over a desertified area. The approximation of the measured saltating particle...  相似文献   

9.
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.  相似文献   

10.
The characteristics of wind-blown sand on Gobi/mobile sand surface have been investigated through field observation and wind tunnel experiments. On moving sand surface, the pattern of wind speed profile in sand flow follows the power function and its power exponent is about 0.20. While on Gobi surface, due to collision of sand grains with gravels, the structure of sand flow differs from that on moving sand surface, which decreases exponentially with height. The height of blown sand activities on Gobi surface is mainly concentrated below 20 cm. In addition, the structure of sand flow can reach a peak at a certain height and increases with inlet wind velocity.  相似文献   

11.
Three sets of Landsat? satellite images for the years 1993, 1998, and 2003 show that the sand dunes at the southwestern Desert of Egypt are generally moving towards southeast direction with a mean annual creeping speed over ground attaining 15 m/year. The manual-stickled field measurements show that the net annual extension of the longitudinal dunes in the coastal area is between 4 and 5 m/year, while the inland longitudinal dunes showed a net movement ranging between 5 and 6 m/year. Seasonal variations of drift potential and sand movement refer to a strongly high energy wind desert environment in the spring season, high energy wind desert environment in the summer season, and relatively high to intermediate in the autumn and winter seasons, respectively. The total annual estimated volume of transported sand which falls down into Lake Nasser basin attains 16,225,808 m3 as calculated by Bagnold's equation and quantities of sand collected from the sand traps. Comparing this value with the total volume of Lake Nasser Basin, which attains 120?×?109 m3, we can conclude that the sand sheets or sand accumulations may represent serious natural hazards to Lake Nasser in some locations. However, the sand drifting towards the lake may be obstructed by high contour topography hindrance, and the mean grain size of the sand sheets is bigger than 0.25 mm, which needs high wind velocity more than 4 m/s. In addition, the direction of the prevailing wind is N-NNW to S-SSE, and this direction sometimes is parallel to Lake Nasser in some places according to the meandering of the lake. The total lengths of hazardous areas along the western bank of Lake Nasser, which receive the most amounts of the drifted sands, attain 43.6 km only.  相似文献   

12.
A sand budget for the Alexandria coastal dunefield, South Africa   总被引：5，自引：0，他引：5  
The sand in the Alexandria coastal dunefield is derived from the sandy beach which forms the seaward boundary of the dunefield. Sand is blown off the beach onto the dunefield by the high-energy onshore-directed dominant wind. The dunefield has been forming over the past 6500 years. Sand transport rates calculated from dune movement rates and wind data range from 15 to 30 m3 m -1 yr-1 in an ENE direction. The sand transport rate decreases with increasing distance from the sea due to a reduction in wind speed resulting from the higher drag imposed upon the wind by the land surface. Aeolian sand movement rates of this order are typical of dunefields around the world. The total volume of sand blown into the dunefield is 375 000 m3 yr-1. Sand is being lost to the sea by wave erosion along the eastern third of the dunefield at a rate of 45 000 m3 yr -1. The dunefield thus gains 330 000 m3 of sand per year. This results in dunefield growth by vertical accretion at about 1.5 mm yr-1 and landward movement at about 0.25 m yr-1. The dunefield is a significant sand sink in the coastal sand transport system. The rate of deposition in coastal dunefields can be 10 times as high as rates of deposition in continental sand seas. The higher rate of deposition may result from the abundant sand supply on sandy beaches, and the higher energy of coastal winds. Wind transport is slow and steady compared to fluvial or longshore drift transport of sediment, and catastrophic aeolian events do not seem to be significant in wind-laid deposits.  相似文献   

13.
Based on the detailed wind data and in situ observation of wind-blown sand in the section of Wudaoliang-Tuotuo River along the Qinghai-Tibet Railway, the dynamic environment of sand flow, involving sand-laden wind, drift potential, sand transport and their time variation were investigated. The prevailing direction of sand-laden wind obviously varies seasonally. Sand-laden wind presents unidirectional characteristics from winter to the next spring and its prevailing direction is westerly. In summer, northeasterly wind begins to increase and lasts for a short period. The annual drift potential along the Qinghai-Tibet Railway reaches 970.54 Vector Units (VU), which belongs to a high-energy wind environment. Directional variability of wind regime (RDP/DP, RDP is the resultant drift potential (RDP) and DP is drift potential) is 0.88. The RDP is 854.31VU in the direction of 89.7°, which indicates that westerly sand-laden wind prevails in this region. Sand transport is well correlated to the frequency of sand-laden wind and increases with drift potential logarithmically.  相似文献   

14.
Thresholds of aeolian sand transport: establishing suitable values   总被引：4，自引：0，他引：4  
This paper assesses the practical use and applicability of the time fraction equivalence method (TFEM; Stout & Zobeck, 1996) of calculating a wind speed threshold for sand grain entrainment in field situations. A modification of the original method is used and is applied to 1 Hz measurements of wind speed and sand transport on a beach surface. Calculated grain entrainment thresholds are tested in terms of the percentage of sand transport events that they explain. It was found that the calculated thresholds offered a poor representation of the occurrence of saltation activity, explaining only about 50% of the measured transport events. Results are discussed in terms of system response time, wind speed measurement height, undetected events and sampling period. A shear velocity threshold for grain entrainment was also calculated, but this also failed to explain a high proportion of the sand transport events. The best results (67–91% of transport events explained) were found by calculating a threshold based on time‐averaged (≈ 40 s) wind velocity measurements. The applicability of a single threshold to a natural grain population is discussed. A natural surface is likely to possess a range of thresholds varying over short time scales in response to parameters such as grain rearrangement and changes in moisture conditions. The results show that calculated thresholds based on 40 s time‐averaged data consistently explain a high proportion of the recorded sand transport events. This is because such a time‐averaged approach accounts for higher frequency variability inherent in the sand transport system.  相似文献   

15.
R. D. SARRE 《Sedimentology》1988,35(4):671-679
Sand transport rates were measured using a vertical sand trap along the intertidal zone of a beach in North Devon, England, together with simultaneous monitoring of the wind speed on the beach and moisture levels in the surface layers of sand. The results of 88 sand trap samples in a wide range of wind speeds showed that moisture levels up to 14%, in the top 1–2 mm of the beach sand, have no discernible effect on the transport rates. Transport rates measured from areas of the beach where the moisture was below this level are compared with the rates predicted by seven expressions based on theoretical and wind tunnel research together with the empirical results of other published research. Measured transport rates range from 0.0001 to 0.22 kg m-1 s-1. The results indicate that expressions based on a power relation between the wind speed and the transport rate, and which include a threshold velocity term, provide the best estimates of the observed transport rates.  相似文献   

16.
Serious hazards have taken place in urban areas and road construction in Saudi Arabia because of the presence of accumulations of drifting sand dunes. Several researches, which carried out investigative work to solve this problem, were reviewed. Three locations of dune fields along the area between Jeddah and Al-Lith were investigated. The dune forms was identified. Detailed field investigations showed that barchan dunes are dominant in the area. The sands from the studied locations were found to be similar in grain size and shape parameters. Mineralogically, the sand reflects the composition of the surrounding igneous and metamorphic rocks. Regression analyses were performed and empirical relationships between dune height, width, windward length, slip-face length, and rate of movements were developed. Relatively strong relations exist between these parameters. The most important geometric parameter controlling dune movement with wind speed and direction is determined to be the dune height. A reasonable similarity occurs between the barchan dunes in the study area and those existing in Al Nufud, Al Jafurah sand seas, and Khulays area. The studied dune fields pose some natural hazards on the roads, and the surrounding buildings and constructions in the villages along the area between Jeddah and Al-Lith, especially during wind storms.  相似文献   

17.
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.  相似文献   

18.
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.  相似文献   

19.
Quasi-horizontal trajectories of salting sand grains were found using high-speed video-recording in the desertified territory of the Astrakhan region. The sizes and displacement velocities of the saltating sand grains were determined. A piecewise logarithmic approximation of the wind profile in a quasi-stationary wind–sand flow is suggested, which is consistent with the data of observations and modeling. It was established that, in the regime of stationary saltation, the wind profile in the lower saltation layer of the wind–sand flow depends only slightly on the wind profile variations in the upper saltation layer. The vertical profiles of the horizontal wind component gradient in a quasi-stationary wind–sand flow were calculated and plotted. It was shown using high-speed video recording of the trajectory of a sand grain with an approximate diameter of 95 μm that the weightlessness condition in the desertified territory of the Astrakhan region in a stationary wind–sand flow is satisfied at a height of approximately 0.15 mm. The electric parameters of a wind–sand flow, which can provide for compensation of the force of gravity by the electric force, were estimated. In particular, if the specific charge of a sand grain is 100 μC/kg, the force of gravity applied to the sand grain can be compensated by the electric force if the vertical component of the electric field in a wind–sand flow reaches approximately 100 kV/m. It was shown that the quasi-horizontal transport of sand grains in the lower millimeter saltation layer observed in the desertified territory can be explained by the joint action of the aerodynamic drag, the force of gravity, the Saffman force, the lift force, and the electric force.  相似文献   

20.
Assessment of sand encroachment in Kuwait using Geographical Information System (GIS) technology has been formulated as a Multi-Criteria Decision Making problem. The Delphi method and Analytical Hierarchy Process were adopted as evaluating techniques, in which experts’ judgments were analyzed for objectively estimating and weighting control factors. Seven triggering factors, depicted in the form of maps, were identified and ordered according to their priority. These factors are (1) wind energy; (2) surface sediment; (3) vegetation density; (4) land use; (5) drainage density; (6) topographic change and (7) vegetation type. The factor maps were digitized, converted to raster data and overlaid to determine their possible spatial relationships. Applying a susceptibility model, a map of sand encroachment susceptibility in Kuwait was developed. The map showed that the areas of very high and high sand encroachment susceptibility are located within the main corridor of sand pathway that coincides with the northwesterly dominant wind direction.  相似文献   

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