Xueyong Zou

Short-term dynamics of wind erosion of three newly cultivated grassland soils in Northern China

This study examined the dynamic nature of short-term wind erosion processes of three grassland soils under simulated cultivation. In a laboratory wind tunnel, soil loss on a sandy loam, a loamy sand, and a sand soil was measured in six successive 10-, 3-, and 1-min exposures at the speeds of 10, 18, and 26 m s 1 , respectively. Erodible soil particle sizes of each interval were determined through direct dry sieving. It was observed that increasing blowing duration generally led to rapid decrease of soil loss, decrease of the erodible soil particle size from a definitely restricted area by any definite speed of wind, and the non-erodible particles remaining on the soil surface increased. From the first to the last blowing interval, soil loss decreased by 86.4–94.8% at 10 m s 1 in 60 min, 90.0–95.4% at 18 m s 1 in 18 min, and 71.5–97.8% at 26 m s 1 in 6 min. The results indicate that wind erosion on the newly cultivated soils is most intense during the initial blowing. The decrease of soil loss with time is presumed due to modification of soil surface by blowing wind, i.e. the concentration of non-erodible aggregates with the deflation of the erodible particles on the soil surfaces.

Cogitation on developing a dynamic model of soil wind erosion

Studies on soil wind erosion began with single factors affecting soil wind erosion; with increasing quantities of data being accumulated, the wind erosion equation (WEQ), the revised wind erosion equation (RWEQ), the wind erosion prediction system (WEPS), and other soil wind erosion models have been successively established, and great advances have been achieved. Here we briefly review the soil wind erosion research course and analyze the advantages and disadvantages of the current soil wind erosion models. From the perspective of the dynamics of wind erosion, we classified the factors affecting soil wind erosion into three categories, namely, wind erosivity factors (WEF), soil antierodibility factors (SAF), and roughness interference factors (RIF). We proposed the concept of a standard plot of soil wind erosion to solve the problem of uncertainty of the soil wind erosion modulus on a spatial scale, and provided methods to set similarity conditions in wind tunnel simulation experiments and to convert the spatial scale of the wind erosion modulus from the standard plot to a large scale field. We also proposed a conceptual model on the basis of the dynamics of soil wind erosion with the theoretical basis that wind produces a shear force on the soil surface. This shear force is partitioned by barely erodible soil surfaces and roughness elements on the ground, and the amount of soil loss by wind should be calculated by comparing the shear force of the wind on barely erodible soil surfaces with the anti-erosion force of the surface soil. One advantage of this conceptual model is that the calculated soil wind erosion modulus is not subject to changes of spatial scale. Finally, we recommended continual improvement of the existing models while also establishing new models.

Aeolian creeping mass of different grain sizes over sand beds of varying length

Creep is an important mode of aeolian sand transport, but it has received little attention in previous studies due to experimental difficulties and insufficient theory. In this study, we conducted 116 groups of experiments with three repeats for each group in a wind tunnel to measure the creeping mass of four different mean grain sizes (152, 257, 321, and 382 µm) over six bed lengths (2.0, 3.5, 5.0, 6.5, 8.0, and 10.0 m) at six different friction velocities (0.23, 0.35, 0.41, 0.47, 0.55, and 0.61 m/s). We attempted to develop a comprehensive model of the aeolian creeping mass by analyzing the effect of wind velocity, the particle size, and the sand bed length based on the experimental data. The primary conclusions are as follows: (1) the complex relationship among the wind velocity, the grain size, the length of the bed, and the surface shape determines sand creep. There was no unified formula to express the effect of particle sizes and the sand bed length on aeolian creeping masses, and their effects appeared to depend on each other and wind velocity, whereas the creeping mass increases with increasing wind velocity for any particle size with any length of sand bed. (2) This paper presented a predicting model to determine the aeolian creeping mass, whose calculating results can match to experimental data with correlation coefficients (R2) of 0.94 or higher. (3) The effect of grain size on creeping mass can be classified into three categories: the creeping mass increases with increasing grain size, the creeping mass initially decreases and subsequently increases with increasing grain size, and the creeping mass fluctuates with the grain size. (4) The effect of increasing bed length appears to depend on the grain size. For mean grain sizes of 152, 257, and 321 µm, creep initially increases with increasing bed length before decreasing above a certain value, while for a mean grain size of 382 µm, the creeping mass gradually increased with increasing bed length. The results help to elucidate aeolian creep and provide an intense foundation for advanced study.

Blown sand motion within the sand-control system in the southern section of the Taklimakan Desert Highway

Although scientists have performed many studies in the Taklimakan Desert, few of them have reported the blown sand motion along the southern edge of the Taklimakan Desert Highway, which differs significantly from the northern region in terms of aeolian sand geomorphology and formation environment. Based on the field observation data of airflow and aeolian sand transport, continuous monitoring data of erosional and depositional processes between 14 April 2009 and 9 April 2011 and data of surface sand grains from the classical section along the southern edge of the Taklimakan Desert Highway, this paper reported the blown sand motion within the sand-control system of the highway. The main results are as follows: 1) The existing sand-control system is highly effective in preventing and controlling desertification. Wind velocities within the sand-control system were approximately 33%–100% of those for the same height above the mobile sand surface. Aeolian sand fluxes were approximately 0–31.21% of those of the mobile sand surface. Sand grains inside the system, with a mean diameter of 2.89 φ, were finer than those (2.15 φ) outside the system. In addition, wind velocities basically followed a logarithmic law, but the airflow along the classical section was mainly determined by topography and vegetation. 2) There were obvious erosional and depositional phenomena above the surface within the sand-control system, and these phenomena have very consistent patterns for all observation points in the two observed years. The total thicknesses of erosion and deposition ranged from 0.30 to 14.60 cm, with a mean value of 3.67 cm. In contrast, the deposition thicknesses were 1.90–22.10 cm, with a mean value of 7.59 cm, and the erosion thicknesses were 3.51–15.10 cm, with a mean value of 8.75 cm. The results will aid our understanding of blown sand within the sand-control system and provide a strong foundation for optimizing the sand-control system.

Simulation of Effects of the Saffman Force and the Magnus Force on Sand Saltation in Turbulent Flow

The effects of both the Saffman force and Magnus force on sand saltation are investigated. Turbulent flows in a channel and over a barchans dune are considered with sand particles injected into the flow. The results show that both of the forces increase the height and skipping distance of sand saltation, with the Magnus force giving more significant effect on the height. These forces can also increase the sand settling at the lee side of the barchans dune.Copyright

Experimental study of aeolian sand ripples in a wind tunnel

The topographic parameters and propagation velocity of Aeolian sand ripples reflect complex erosion, transport, and deposition processes of sand on the land surface. In this study, three Nikon cameras located in the windward (0-1 m), middle (4.5-5.5 m), and downwind (9-10 m) zones of a 10-m long sand bed are used to continuously record changes in sand ripples. Based on the data extracted from these images, this study reaches the following conclusions: 1) The initial formation and full development times of sand ripples over a flatbed decrease with wind velocity. 2) The wavelengths of full development sand ripples are approximately twice the wavelengths of initially formed sand ripples. Both wavelengths increase linearly with friction velocity. During the developing stage of sand ripples, the wavelength increases linearly with time. 3) The propagation velocity of full development sand ripples is approximately 0.6 times that of the initially formed sand ripples. Both wavelengths increase as power functions with respect to friction velocity. During the developing stage of sand ripples, the propagation velocity decreases with time following a power law. These results provide new information for understanding the formation and evolution of aeolian sand ripples and help improve numerical simulations.

Wind tunnel tests of the dynamic processes that control wind erosion of a sand bed: The dynamic processes responsible for wind erosion

Aeolian sand transport is a complicated process that is affected by many factors (e.g. wind velocity, sand particle size, surface microtopography). Under different experimental conditions, erosion processes will therefore produce different results. In this study, we conducted a series of wind tunnel experiments across a range of wind velocities capable of entraining sand particles (8.0, 10.0, 12.0, and 14.0m s) to study the dynamic changes of the shear velocity, aerodynamic roughness length, and sand transport. We found that the shear velocity and aerodynamic roughness length are not constant; rather, they change dynamically over time, and the rules that describe their changes depend on the free-stream air velocity. For wind tunnel experiments without feeding sand into the airflow, the sand bed elevation decreases with increasing erosion time, and this change significantly affected the values of shear velocity and aerodynamic roughness length. A Gaussian distribution function described the relationships between the sand transport rate (qT) and the duration of wind erosion (T). It is therefore necessary for modelers to consider both deflation of the bed and the time scale used when calculating sand transport or erosion rates.

Experimental Investigation on Shear-Stress Partitioning for Flexible Plants with Approximately Zero Basal-to-Frontal Area Ratio in a Wind Tunnel

Shear-stress partitioning is investigated for one type of flexible plant for very small values of the basal-to-frontal area ratio σ (0.001–0.007). The plant model is made of plastic with irregular structures, which are different from previously investigated rigid regular or flexible roughness elements with larger σ values. The distribution of the surface shear stress and the total shear stress at four plant densities with five plant heights are measured in a wind tunnel using Irwin-type sensors and a load cell, respectively. The wind-tunnel experiments prove that, for these flexible plants, the plant height and lateral cover usually decrease with increasing friction velocity, especially for taller plants, while the plant coverage generally increases. However, these characteristics may be inconsistent with flexible roughness elements with very large σ values (and usually very low aspect ratios) because these elements are less flexible. The present flexible plants generally result in lower shear-stress ratios compared with other roughness elements, which is also proven by the higher values of β (the ratio of the drag coefficient of an isolated roughness element to that of the bare surface) and a constant m (accounting for the difference between the average and peak surface shear stresses) from the present experiments (β = 184–210 and m = 0.68–0.79). The peak mean stress ratio of the present flexible plants is not a constant (1.07–1.54) because it is affected by the lateral cover, which is different from previous studies that consider the ratio to be constant without regard for the lateral cover.

Influence of dust storms on atmospheric particulate pollution and acid rain in northern China

Northern China is the area with the highest incidence of dust storms in the world, which are the main sources of its soil dust emissions. In addition, the region consumes huge amounts of fossil fuels and has serious atmospheric particulate pollution. Existing observation results show that a single dust storm has significant influence on atmospheric particulate pollutant concentrations and precipitation acidity. Proving the influence of dust storms on atmospheric particulate pollution, acid rain, and the acid rain ratio and determining whether there is a causal relationship among them on a longer time scale will help us recognize the impact of dust storms on the atmospheric environment. This paper proves that dust storms are the direct cause of the variations in the number of acid rain days and acid rain ratio, as well as the changes in atmospheric particulate pollution, in spring by using the Granger Causality Test and correlation analysis methods based on 1993 to 2007 data, including the number of days of dust storms, atmospheric particulate pollution, and acid rain. Atmospheric particulate pollution is the direct cause of variations in the number of acid rain days and the acid rain ratio in spring; for the other seasons, additional data combined with atmospheric particulate pollution are needed to explain the causes of the acid rain day and ratio changes.