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.     

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.     

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.     

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.     

Evaluation of saltation flux impact responders (Safires) for measuring instantaneous aeolian sand transport intensity

The measurement of aeolian sand transport rates on small scales is of interest to the development and testing of detailed models of sand movement by wind. This paper reports on laboratory evaluations and preliminary field tests of a new design of a piëzo-electric impact responder, called a ‘Safire’, capable of measuring saltation impacts at a frequency of 20 Hz. The advantages of the Safire are: (1) that it provides high-frequency measurements, (2) that it presents a minimal obstruction to the wind flow (no scour observed in the field), and (3) that it is of a (relatively) low-cost.Laboratory calibrations were performed with a vertical gravity flume generating known sand grain fluxes using both mixed sand and specific size fractions. Initial tests investigated three fundamental characteristics: correspondence between digital and analogue signals generated by the instrument, directional response of the probe, and linearity of instrument response to mass flux.Instrument calibration included determination of the momentum threshold required for the sensor to register a grain impact. Based on this lower limit and the known distribution of grain size and speed at different fall heights, a prediction is made as to the sand grain flux the Safire ought to measure, which is then compared with the signal response. The result of this comparison is an assessment of the instrument's efficiency in counting saltating grains. These Safires were also deployed in the field as part of a larger investigation of spatio-temporal transport variability. This experiment provided the opportunity to compare the instrument's performance with traditional sand traps, and this paper develops methods and assumptions required to convert measurements from impact responders to traditional mass transport rates.The evaluations indicate that improvements to the instrument production process are required to ensure a standard momentum threshold among individual instruments. Furthermore, the sensor design needs to be reconsidered in order to eliminate the variation in response depending on azimuth direction, so that the sensor is uniformly omni-directional.     

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.     

A parameterisation for the threshold shear velocity to initiate deflation of dry and wet sediment

A new parameterisation for the threshold shear velocity to initiate deflation of dry and wet particles is presented. It is based on the balance of moments acting on particles at the instant of particle motion. The model hence includes a term for the aerodynamic forces, including the drag force, the lift force and the aerodynamic-moment force, and a term for the interparticle forces. The effect of gravitation is incorporated in both terms. Rather than using an implicit function for the effect of the aerodynamic forces as reported earlier in literature, a constant aerodynamic coefficient was introduced. From consideration of the van der Waals force between two particles, it was further shown that the effect of the interparticle cohesion force between two dry particles on the deflation threshold should be inversely proportional to the particle diameter squared. The interparticle force was further extended to include wet bonding forces. The latter were considered as the sum of capillary forces and adhesive forces. A model that expresses the capillary force as a function of particle diameter squared and the inverse of capillary potential was deduced from consideration of the well-known model of Fisher and the Young–Laplace equation. The adhesive force was assumed to be equal to tensile strength, and a function which is proportional to particle diameter squared and the inverse of the potential due to adhesive forces was derived. By combining the capillary-force model and the adhesive force model, the interparticle force due to wet bonding was simplified and written as a function of particle diameter squared and the inverse of matric potential. The latter was loglinearly related to the gravimetric moisture content, a relationship that is valid in the low-moisture content range that is important in the light of deflation of sediment by wind. By introducing a correction to force the relationship to converge to zero moisture content at oven dryness, the matric potential–moisture content relationship contained only one unknown model parameter, viz. moisture content at −1.5 MPa. Working out the model led to a rather simple parameterisation containing only three coefficients. Two parameters were incorporated in the term that applies to dry sediment and were determined by using experimental data as reported by Iversen and White [Sedimentology 29 (1982) 111]. The third parameter for the wet-sediment part of the model was determined from wind-tunnel experiments on prewetted sand and sandy loam aggregates. The model was validated using data from wind-tunnel experiments on the same but dry sediment, and on data obtained from simulations with the model of Chepil [Soil Sci. Soc. Am. Proc. 20 (1956) 288]. The experiments showed that soil aggregates should be treated as individual particles with a density equal to their bulk density. Furthermore, it was shown that the surface had to dry to a moisture content of about 75% of the moisture content at −1.5 MPa before deflation became sustained. The threshold shear velocities simulated with our model were found to be in good agreement with own observations and with simulations using Chepil's model.     

Effect of soil crusting on the emission and transport of wind-eroded sediment: field measurements on loamy sandy soil

Field data are reported for the horizontal and vertical flux of wind-eroded sediment on an agricultural field in northern Germany. Measurements were made during a windstorm that hit the region on 18 May 1999. The magnitude of both fluxes was significantly affected by the presence of a surface crust covering the test field. Measuring the physical crust strength at 45 locations with a torvane, the relationships between crust strength (τ) and the horizontal (F_h) and vertical (F_v) sediment fluxes were investigated. Both fluxes decreased as the surface crust became stronger. The decay behaved as an exponential function for both types of flux. The horizontal sediment flux over a crusted surface can be accurately predicted by completing Marticorena and Bergametti's [Journal of Geophysical Research 100 (1995) 16415] erosion model with a crust function. The vertical particle flux over crusted soil can be calculated by adding a similar function to Alfaro and Gomes's [Journal of Geophysical Research 106D (2001) 18075] dust production model. The study also suggests that the gradual bombardment of a surface crust by impacting particles does not immediately result in a decay of the crust's protective effect, provided that the crust has a minimum thickness. However, once the crust becomes perforated, its protective effect disappears very quickly, leading to much higher horizontal and vertical sediment fluxes than predicted for undamaged crusted soil.     

Mobility of a remobilised parabolic dune in Kennemerland, The Netherlands

A parabolic dune in the Netherlands was remobilised in December 1998 by removing vegetation and soil. The main aim of the experiment was ecological: to investigate whether permanent rejuvenation at the landscape scale is possible by restoration of natural processes. If processes can be reactivated at coarse scale, periodic rejuvenation of the landscape over the long term is possible, without the need for managers to interfere further. The experiment provides the opportunity to address another important question: can large parabolic dunes in the Netherlands be mobile in the present climate? Mobility of the dune is investigated by means of erosion pins, aerial photography and measurement of cross sections. Activity indices are derived from erosion pin recordings and correlated to weather conditions. From 1999 to 2001, displacement of the dune ranged from 0 to 12 m in east–northeasterly direction. Activity of the dune is related to wind conditions, but the relationship is strongly influenced by precipitation and therefore differs for wet and dry periods. Periods with extreme wind speeds resulted in much less geomorphic change than expected.     

Sand transport under increased lateral jetting of raindrops induced by wind

Wind tunnel experiments for ‘Raindrop Detachment and Wind-Driven Transport’ (RD–WDT) process were conducted under improved lateral jetting induced by wind velocities of 6.4, 10, and 12 m s^− 1 at nozzle operating pressures of 75, 100, and 150 kPa. Wind-driven rainfalls were also incident on the windward and leeward slopes of 4° and 9° to have a broad variation in the angle of incidence. The objective of this experimental set-up was to distinguish the roles of both impact components of obliquely striking wind-driven raindrops on RD and wind on WDT. Raindrop impact components and reference horizontal wind were quantified by normal (E_tz) and horizontal (E_tx) kinetic energy fluxes and wind shear velocity (u), respectively, to physically model the process of RD–WDT. The results showed, at each level of u, differential sand transport rates by RD–WDT (q_m(RD–WDT)) occurred depending on the magnitude of raindrop impact components, and q_m(RD–WDT) increased as the relative contribution of E_tz increased. Although E_tx was more correlated with q_m(RD–WDT) than E_tz, the extreme increases in E_tx at the expense of E_tz brought about no increases but decreases in q_m(RD–WDT). An RD–WDT model was built under the process of examining the discrete effects of E_tz and E_tx on RD together with u and resulted in a better coefficient of determination (R² = 0.89) than only total kinetic energy (E_t) did alone with u (R² = 0.84). In this study, E_tx was strongly related to u and not to E_tz, which was the principal difference from the previous rainsplash studies, which relied on the compensatory lateral jet development by the compressive pressure build-up at the raindrop–soil interface. Including E_tx in the RD–WDT model both separated the distinct role of each raindrop impact component in RD and improved the performance of u in WDT by better distinguishing its interaction with E_tx, which was not explicitly separated in previous models of RD–WDT.     

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。     

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...     

Wind tunnel investigation of the influence of surface moisture content on the entrainment and erosion of beach sand by wind using sands from tropical humid coastal southern China

Wind erosion has major impacts on dune growth, desertification, and architecture on sea coasts. The deflation threshold shear velocity is a crucial parameter in predicting erosion, and surface moisture greatly affects this threshold and thus sand stability. Wind tunnel studies have shown that reduced moisture contents decrease entrainment thresholds and increase wind erosion, but field and wind tunnel test data is lacking for tropical humid coastal areas. In this study, we investigated the influence of surface moisture contents (at 1 mm depth) on sand entrainment and erosion using tropical humid coastal sands from southern China. Shear velocities were deduced from velocity profiles above the sand. The threshold shear velocity increased linearly with increasing ln100M (M, gravimetric moisture content). The increase was steepest below a moisture content of 0.0124 (i.e., at M_1.5, the moisture content in the sand at a matric potential of − 1.5 MPa). We compared several popular models that predict threshold shear velocity of moisture sediment, and found substantial differences between their predicted results. At a surface moisture content of 0.0124, the predicted increase in the wet threshold shear velocity compared with the dry threshold shear velocity ranged from 34% to 195%. The empirical model of Chepil and Selah simulated the data well for M < 0.0062 (i.e., 0.5M_1.5), whereas Belly's empirical model simulated the data best for M > 0.0062. Wind erosion modulus increased with increasing effective wind velocity following a power function with a positive exponent at all moisture contents, but decreased with increasing surface moisture content following a power function with a negative exponent. When wind speed and moisture content varied simultaneously, wind erosion modulus was proportional to the 0.73 power of effective wind velocity, but inversely proportional to the 1.48 power of M. The increase in resistance to erosion at low moisture contents probably results from cohesive forces in the water films surrounding the sand particles. At a moisture content near M_1.5, wind erosion ceases nearly for all wind velocities that we tested.     

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.     

Numerical simulation of aeolian dusty sand transport in a marginal desert region at the early entrainment stage

Aeolian dusty sand transport in the marginal region of a desert is described numerically from first suspension motion at the early entrainment stage to the unsteady state within a moderate range (1000 m long × 500 m high). A two-dimensional model is built for wind-blown dust flow, and the calculations are carried out using Fluent software. The simulation results describe an integrated picture of aeolian dusty sand transport including uplift, suspension, diffusion, deposition and its space–time concentration. According to the features of a sand–dust storm near the surface, a volume concentration expression of ejection grains is developed as a boundary condition in the simulation approach. The model is verified by comparing results with both experimental data from a wind tunnel and an analytic solution. Uniform dust sizes and R–R distributions are used in the simulation. Gas–solid two-phase flow patterns with these grains are obtained in the downwind space, including the turbulence intensity, gas phase stream functions and solid volume concentration distributions. The influence of wind velocities and grain sizes is analyzed. From the simulation results, spatial distributions of dust volume concentration in the early entrainment stage are described clearly. Different from coarse sands, there is a clear band of uniformly saturated dust concentration in the region directly above the surface.     

Estimation of PM20 emissions by wind erosion: main sources of uncertainties

The physics of the two processes (saltation and sandblasting) leading to fine mineral dust emissions by wind erosion in arid or semi-arid areas has been detailed and modeled. The combination of these two models has led to a physically explicit Dust Production Model (DPM). In this work, sensitivity tests are performed with the DPM to determine the nature of the main soil parameters that control dust emissions by sandblasting. It is found that the soil roughness length and the dry size distribution of the soil aggregates constituting the loose wind erodible fraction of the topsoil have the greatest influence on the soil potential for mineral dust production. Contrary to what is often assumed, soil texture is not a relevant parameter.In the light of these new findings, results of vertical flux measurements performed over a wide variety of sources in Niger and the US south west (14 soils) have been reanalyzed. Results show (1) that for the tested soils the DPM, and hence sandblasting, explain all dust emissions, and (2) that 13 of the 14 soils that had been selected a priori for their high potential for dust emissions contained a fine soil-aggregate component. This is consistent with the sensitivity tests indicating that the presence of such a component could enhance dust emissions by one order of magnitude. Finally, it can be concluded that most of the apparent scatter in the experimental results was in large part due to an inappropriate choice of soil parameters to interpret them.     

A wind tunnel investigation on the transverse motion of aeolian sand

A wind tunnel experiment was performed to investigate aeolian grain motions in the transverse direction, which is perpendicular to the incoming flow and parallels the sand bed. The trajectories in the horizontal plane were recorded by high-speed camera. Statistical analysis of 630 trajectories shows that both the motion orientation and the time-averaged speed follow Gaussian distributions. An exclusive method was used to analyze the driving mechanism. It was concluded that the three-dimensional turbulent air flow, rather than the spin of grain or grain–bed collisions, controls the transverse motion.     

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.     

Wind tunnel test of the influence of moisture on the erodibility of loessial sandy loam soils by wind

Loessial sandy loam soils are the major soil categories in the northern Loess Plateau, China. Owing to a dry, windy climate and sparse surface cover, wind erosion is a serious problem and dust (sand) storms occur frequently. Soil moisture is one of the most important factors influencing resistance to wind erosion. The influence of moisture content on the erodibility of sandy loam soils was investigated through wind tunnel simulations. Results showed that the threshold velocity for soil particle movement by wind increases with increasing soil moisture by a power function. The intrinsic factor in the increase in soil resistance due to moisture content is the cohesive force of soil water. Cohesive forces of the film and capillary water are different; the influence of soil moisture on threshold velocity was shown to follow a step-like pattern. The wind erosion modulus of sandy loam was directly proportional to the cube of the wind velocity or the square of the effective wind velocity (V−V_t). There existed a negative exponential relationship between the wind erosion rate and soil moisture content. Initially, as soil moisture increased the decrease in the wind erosion rate was rather rapid. When the moisture content reached more than 4%, the rate of decrease in erosion slowed and became almost constant with successive increments of moisture. This suggests that different soil moisture contents can prevent wind erosion at different levels. Four percent soil moisture could only reduce the erodibility of the sandy loam soil by a small degree.     

Temporal patterns of arid-humid transitions in the south-eastern Arabian Peninsula based on optical dating

The Rub Al Khali aeolian system of the Arabian Peninsula is the world's largest erg. While the region is presently hyper-arid, evidence for multiple past humid periods has been documented in previous studies. Radiocarbon dating of lacustrine, travertine and palaeogroundwater deposits suggests that the climate was humid at 10–6 and 35–25 ka. These phases have been associated with summer rainfall increases occurring in tandem with an intensification and northward migration of the monsoon system during Northern hemisphere precessional maxima. Published optical dating studies have focused on preserved sand dune and aeolianite deposits to establish ages for the intervening arid phases. Here we present a summary of this work and additional optical dating results for samples of aeolian sediment from a deep drill core, which provides insights into the style of barchan dune accretion in the Liwa region of the United Arab Emirates. The large dune accreted rapidly during the mid-late Holocene. The results suggest that the transitions from humid to arid conditions and resulting accumulation of aeolian sediment in the form of large bedforms occurred abruptly at around 6 ka. Further aeolian sedimentation has not been constant since that time, with initial gradual dune growth followed by rapid vertical accretion, and possible termination of accumulation around c. 2 ka.