Study On The Wind Erosion Process Of Non-uniform Sand Bed

Soil wind erosion is the first step of arid and semi-arid region of desertification, and isone of the environmental problems of many parts of the world. This paper used the sevenkinds of non uniform sand samples: the average particle size range from0.068mm to0.344mm, sorting coefficient range from1.29to2.60, the range of density of888kg/m3~1670kg/m3,moisture content range of0.42%~2.38%. This paper based on the windtunnel simulation test of seven kinds non-uniform sand bed wind erosion, studies therelationship between wind erosion rate and influence factors, and the sand flow structurecharacteristics. Main results are the following:(1)Using the Bagnold’s transport rate formula, putting the transport rate Q and u*2（u*-u*t）for linear fitting, and the push to get the threshold friction velocity of all the samples.The fitting formulas of the correlation coefficient are all above0.990, so the results ofthreshold friction velocity are more accurate. Between threshold friction velocity and averageparticle size have quadratic polynomial correlation, the correlation coefficient of0.699. Withincreasing average particle size, the threshold friction velocity has the tendency of decreasingfirst and then increasing. Through contrast analysis, the threshold friction velocity and sortingcoefficient have no obvious correlation, including other granularity parameters.The threshold friction velocity and sand density is also the relation between quadraticpolynomial, the correlation coefficient of0.624.Sample density is about1240kg/m3,correspond to the minimum threshold friction velocity. With the increase of moisture, thethreshold friction velocity is decreasing with exponential function, and the correlationcoefficient of0.747.(2)Under the condition of6friction velocity, the sand beds are made up of differentparticle sizes, when the friction velocity from0.25m/s to0.68m/s. While with the increase ofwind velocity, wind erosion rates increase of all the sand bed. But the degrees of wind erosionrate increase different due to differences in sample characteristics. When friction velocity is in0.42m/s~0.68m/s, wind erosion rate increases with the increase of average particle size of thepower function, the correlation coefficient is0.800~0.914; with the increase of frictionvelocity, wind erosion rate increases the size of the non-uniform sand is different. The particle size of0.343mm sample’s wind erosion rate increases fastest, and under the same frictionvelocity, this wind erosion rate is the most. The particle size of0.068mm sample’s winderosion rate increases slowest, and under the same friction velocity, this wind erosion rate isthe least.When friction velocity is in0.42m/s~0.68m/s, wind erosion rate decreases with theincrease of the sorting coefficient of quadratic polynomial, the correlation coefficient is0.467~0.708; friction velocity is larger, the faster sand wind erosion rate decreases with theincrease of sorting coefficient.When friction velocity is in0.42m/s~0.68m/s, wind erosion rate increases with theincrease of the density of logarithmic function, the correlation coefficient is0.769~0.961.When the range of density are from888kg/m3to1044kg/m3, the wind erosion rate increasesslowly, with the increase of friction velocity. When the range of density are from1326kg/m3to1670kg/m3, the wind erosion rate increases fastest, with the increase of friction velocity. Atthe same time, the greater the friction velocity, the faster the wind erosion with the densityincrease.(3)On the base of factor analysis, they affect wind erosion process of non-uniform sand,which include stability factor and variable factor, two factors of contribution rate of93.25%.Stability properties include the sample average particle size, density and sorting coefficient,variable properties is the moisture. When the moisture of sample is higher, the wind erosionrate is low, the contribution rate of other properties stable factor declines obviously for theinfluence of wind erosion process. When the moisture of sample is lower, the contributionrate of stability property factor is higher. There is a reciprocal relation between two kinds offactors.(4) Aeolian deposit is collected at different wind velocity and different height by sandsampler, to obtain the corresponding of sand samples. The analysis results show that,sediment which is transported by air is below20cm from the surface, it proves that the sanddrift movement is a kind of the material handing process of close to the surface. The transportrates of all the samples increase according to the law of power function with the increase ofwind velocity, but different sand beds increase different degree of transport rate. Whenfriction velocity is in0.42m/s~0.68m/s, the percentage of sediment transport rate gradually reduce with the increase of the height, due to the composition of its grain diameters andmoisture, all the samples reduce according to the different function. With the increase of windvelocity, the absolute values of sand flux all increase at the height, but there are obviousdifferences at different level. Sand flux percentage of most sample gradually decrease below20cm from the surface, the degree of sand flux percentage is different.(5)Aeolian deposit is analyzed about grain-size distribution characteristics in the sandsamplers by Malvern laser particle size analyzer. The results show that, average particle sizeof Aeolian deposit of most samples less than the average particle size of bed. At the same time,average particle size of Aeolian deposit of little samples more than the average particle size ofbed, it may be low efficiency of sand device for fine particles. Because of the most of thesediment which transports by air concentrates in the0cm-10cm height, so the relationshipbetween average particle size and wind velocity are focused in the range of the height. Withthe increase of wind velocity, very fine sand and silts content in the Aeolian deposit increasein the0cm-10cm height, average particle sizes of Aeolian deposit of samples decrease. Whenthe wind velocity increases from0.34m/s to0.68m/s of sample1, percentage of very fine sandand silts increases from10.28%and6.48%to28.04%and26.93%respectively. The ranges ofparticle size of Aeolian deposit are similar under the different wind velocities, but theycentralized distribute in different range. The particle sizes of sample2, sample6and sample7concentrated distribute below0.30mm, the average particle sizes are below0.10mm, it showthat the characteristics of grain composition of Aeolian deposit is dominated by very fine sandand silts. The Aeolian deposit of sample3, sample4and sample5is dominated by mediumsand and coarse sand, the average particle sizes of Aeolian deposit is above0.25mm. Underthe wind velocity of0.51m/s~0.68m/s, with the increase of the height, the average particlesizes of Aeolian deposit decrease quickly at30cm height in the near surface, this is consistentwith the distribution along the height of sand flux, the average particle size of samplesincrease in the sand drift more than30cm height.