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.     

海滩湿润沙面起动摩阻风速的风洞实验

风蚀是一种重要的地貌过程和地质灾害,它影响海岸沙丘的增长,加速土地沙漠化并危害沿岸建筑。沙面湿度强烈影响沙粒的临界起动风速和沙面稳定性,因此,也是影响风蚀过程的一个重要因子。本项风洞实验使用华南热带湿润海岸的海滩沙,研究了表面湿度（1 mm 深）对海滩沙风蚀起动的影响,建立了一个新的预测热带湿润海滩湿沙起动摩阻风速的模型,该模型指明给定粒径下,湿沙的起动摩阻风速随ln100 M（M,重量湿度）线性增加。初步评价了7个预测湿沙起动摩阻风速的模型。实验结果表明,各模型的预测结果间存在着很大差异。在0.0124(M1.5)的湿度下,不同模型预测的湿沙起动摩阻风速比观测的干沙起动摩阻风速大了34%~195%。在湿度小于0.0062(0.5 M1.5)时,Chepil 和Saleh的理论模型和实验数据很吻合;在湿度大于0.0062(0.5 M1.5)时,Belly的实验模型和实验数据更趋一致。     

Wind erodibility of major soils in the farming-pastoral ecotone of China

Wind erosion and desertification are severe problems in China's farming-pastoral ecotone. In this study, wind erodibility of five major soils in both uncultivated and simulated cultivated conditions, were determined through wind tunnel tests at nine wind speeds ranging from 10 to 26 m s⁻¹. The average wind erosion rate (g m⁻² min⁻¹) under the uncultivated condition (q₀) for the five soils could be set in the order: chestnut soil (28.5)>brown soil (24.8)>sierozem (21.8)>chernozem (19.9)>fixed sandy soil (11.4). The highest natural wind erosion might take place in the semi-arid steppe zone where the Chestnut soils predominate. Cultivation can significantly accelerate wind erosion, the mean wind erosion rate under the cultivated condition (q_c) for all five soils was 743.7 g m⁻² min⁻¹ in the following order: sandy soil (3313.2)>brown soil (227.2)>chernozem (221.8)>sierozem (85.1)>chestnut soil (81.2). For both the uncultivated and cultivated soil samples, the relationship between wind erosion rate (q) and wind speed (U) could be expressed in general as q=A e^BU (A and B are constant coefficients). There was a critical wind speed for each soil type except for the sandy soil. Below the critical wind speed, cultivation reduced wind erosion rate possibly due to soil clodiness and roughness effects. Above the critical speed, cultivation greatly intensified wind erosion rates due to the break down of the original soil structure. The critical wind speed measured at 20 cm above the soil surface was 20 m s⁻¹ for the brown soil, 14 m s⁻¹ for chernozem and the chestnut soils, and 10 m s⁻¹ for the sierozem. Among the five tested soils, the high wind erosion rate of the cultivated sandy soil showed its extreme sensitivity to cultivation, possibly because of the structureless nature of the loose sand. The “effect of cultivation on wind erosion” index, η (=q_c/q₀), increased exponentially with the increase of wind speed, indicating that under higher wind speed conditions, cultivation could result in more severe wind erosion.     

乌兰布和沙漠东北缘地表风沙流结构特征

在国家林业局磴口荒漠生态站长期监测的基础上,利用多种积沙仪,对乌兰布和沙漠东北缘流动沙丘、油蒿半固定沙丘、白刺半固定沙丘、油蒿固定沙丘、白刺固定沙丘5种典型下垫面近地面（0～100 cm）的风沙流输沙量进行了实地观测和对比分析。结果表明：（1） 输沙率（q）随高度（h）增加呈幂函数（q=ah^-b,R²≥0.8409）规律衰减,随风速（v）增大呈幂函数（q=av^b,R²≥0.9256）规律增加,42.8%～70.7%的输沙量分布在10 cm高度内,67.6%～90.0%的输沙量分布于30 cm高度内。当地表植被盖度达到40%以上时,输沙率下降至无植被覆盖地表输沙率的6.6%以下,可有效阻止地表风蚀。（2） 沙物质主要由粒径为50～250 μm的细沙和极细沙构成,各高度层风蚀物粒度组成服从单峰态分布,峰值在100～250 μm。随高度增加,风蚀物粒径范围趋于变窄,粒径趋于更细。（3） 起沙风多出现在WSW和NW方向,占全年起沙风的53.19%。风沙流中跃移输沙、蠕移输沙的空间分布在理论上应与风向频率分布基本一致,差异性主要由各方位风的强度和持续时间等因素导致。研究结果可为该区域防沙工程设计提供理论参考。     

不同砾石覆盖度戈壁床面风蚀速率定量模拟

通过风洞实验,利用称重传感器自动记录风蚀观测样方重量变化过程,对供沙条件下不同砾石覆盖度戈壁床面风蚀速率进行了定量模拟研究。结果表明:砾石覆盖度是影响戈壁风蚀速率的关键因子,戈壁床面风蚀速率随砾石覆盖度增加按指数规律递减。各实验风速下,砾石覆盖度>50%时,戈壁床面风蚀速率随砾石覆盖度增加而减小量有限,甚至无风蚀发生;而盖度从10%到50%时,风蚀速率显著减小。因此,两种实验粒径砾石(3 cm与4 cm)至少在50%盖度时才能达到较好的风蚀防治效果。戈壁风蚀防护机理主要是砾石覆盖度的增加增大了砾石间沙粒的临界起动剪切风速,而且减少了作用在砾石间可蚀地表的剪切压。与沙质对照床面相比,10%~90%砾石盖度戈壁床面沙粒临界起动剪切风速增大了0.8~3.4倍,只有0.5％~28%的剪切压作用在砾石间可蚀地表。     

The assessment of weathering stages in granites using an EC/pH meter

The sidewall effects of a wind tunnel on aeolian sand transport were investigated experimentally. A wind tunnel was used to conduct the experiments with a given channel height of 120 cm and varying widths (B) of 40, 60, 80, 100 and 120 cm. Both vertical profiles of wind velocity and sand mass flux were measured at different locations across the test section. The results show that the wind velocity with saltation first increases and then decreases to a minimum, from the sidewall to the central line of the wind tunnel. The discrepancy among wind velocities at different locations of the transverse section decreases with decreasing tunnel width. The wind friction velocity across the wind tunnel floor, with the exception of the region closest to the sidewalls, does not deviate strongly in wide wind tunnels from that along the central line, whereas it does vary in narrow tunnels. The sand mass fluxes, with the exception of some near-bed regions, are larger along the central line of the wind tunnel than they are at the quarter width location from the sidewall. Unlikely previously reported results, the dimensionless sand transport rate, Qg / (ρu³) (where Q is the total sand transport rate, g is the gravitational acceleration constant, ρ is the air density, and u is the wind friction velocity), first decreases and then increases with the dimensionless friction velocity, u / u_t (where u_t is the threshold friction velocity). The above differences may be attributed to the sidewall effects of the wind tunnel. A dimensionless parameter, F_B = u / (gB)^1/2, is defined to reflect the sidewall effects on aeolian sand transport. The flows with F_B of 0.33 or less may be free from the sidewall effects of the wind tunnel and can ensure accurate saltation tunnel simulation.     

小麦玉米田耕作模式的防风蚀效果

通过风洞实验，在5个风速下对6种耕作方式下农田土壤风蚀速率、0~20 cm风沙流结构进行了模拟研究。结果表明：保护性耕作土壤风蚀速率较传统耕作平均降低20%~40%；保护性耕作和传统耕作条件下土壤风蚀速率均随风速的增大呈幂函数递增，但在传统耕作条件下递增较快；风速14 m·s^-1是荒漠绿洲农田土壤风蚀加剧转折点，当风速>14 m·s^-1时保护性耕作下风蚀速率较传统耕作明显降低；0~20 cm内，传统耕作和保护性耕作下输沙率与高度分别呈线性和指数关系，保护性耕作下0~4 cm输沙量和输沙量百分比（Q_0~4/Q_0~20）均低于传统耕作。     

Experimental investigation of the concentration profile of a blowing sand cloud

Detailed wind tunnel tests were carried out to establish the mean downwind velocity and transport rate of different-sized loose dry sand at different free-stream wind velocities and heights, as well as to investigate the vertical variation in the concentration of blowing sand in a cloud. Particle dynamic analyzer (PDA) technology was used to measure the vertical variation in mean downwind velocity of a sand cloud in a wind tunnel. The results reveal that within the near-surface layer, the decay of blown sand flux with height can be expressed using an exponential function. In general, the mean downwind velocity increases with height and free-stream wind velocity, but decreases with grain size. The vertical variation in mean downwind velocity can be expressed by a power function. The concentration profile of sand within the saltation layer, calculated according to its flux profile and mean downwind profile, can be expressed using the exponential function: c_z=ae^−bz, where c_z is the blown sand concentration at height z, and a and bare parameters changing regularly with wind velocity and sand size. The concentration profiles are converted to rays of straight lines by plotting logarithmic concentration values against height. The slope of the straight lines, representing the relative decay rate of concentration with height, decreases with an increase in free-stream wind velocity and grain size, implying that more blown sand is transported to greater heights as grain size and wind speed increase.     

毛乌素沙地植被覆盖率与风蚀输沙率定量关系

基于毛乌素沙地不同植被覆盖条件下的风蚀输沙率实地观测数据，借鉴国外最新的建模思想，采用Matlab5.3软件对数据进行非线性回归，建立毛乌素沙地植被覆盖率与风蚀输沙率之间的定量关系模型，比较系统地考察了植被覆盖对沙粒起动风速和风蚀输沙率的影响，确定了不同风速下的有效植被覆盖率。结果表明，在毛乌素沙地要有效减少和防治风蚀，植被覆盖率必须达到40－50％的水平，而要保证在最高风速下风蚀输沙得到有效控制，植被覆盖率必须达到60－70％的水平。     

The response of drainage basins to the late Quaternary tectonics in the Sicilian side of the Messina Strait (NE Sicily)

Despite more than 40 yr of research attributing temporal changes in streambank erosion rates to subaerial processes, little quantitative information is available on the relationships between streambank erodibility (k_d) and critical shear stress (τ_c) and the environmental conditions and processes that enhance streambank erosion potential. The study goal was to evaluate temporal changes in k_d and τ_c from soil desiccation and freeze–thaw cycling. Soil erodibility and τ_c were measured monthly in situ using a multiangle, submerged jet test device. Soil moisture, temperature, and bulk density as well as precipitation, air temperature, and stream stage were measured continuously to determine changes in soil moisture content and state. Pairwise Mann–Whitney tests indicted k_d was 2.9 and 2.1 times higher (p < 0.0065) during the winter (December–March) than in the spring/fall (April–May, October–November) and the summer (June–September), respectively. Regression analysis showed 80% of the variability in k_d was explained by freeze–thaw cycling alone. Study results also indicated soil bulk density was highly influenced by winter weather conditions (r² = 0.86): bulk density was inversely related to both soil water content and freeze–thaw cycling. Results showed that significant changes in the resistance of streambank soils to fluvial erosion can be attributed to subaerial processes. Water resource professionals should consider the implications of increased soil erodibility during the winter in the development of channel erosion models and stream restoration designs.     

兰新高铁烟墩风区戈壁近地表风沙流跃移质垂直分布特性

跃移质作为风沙流的主体,其近地表垂直分布规律是风沙物理学的重要研究内容,对防沙工程具有重要的指导意义。受研究条件与观测仪器限制,戈壁特别是极端大风区近地表风沙流结构特性研究较为薄弱。利用多梯度风蚀传感器与阶梯式集沙仪对兰新高铁烟墩风区戈壁近地表风沙流跃移质的垂直分布特性进行了观测研究。结果表明：兰新高铁烟墩风区戈壁沙粒发生跃移运动的2 m高临界风速达12 m·s^-1;戈壁近地表风沙流具有明显的阵性特征,沙粒跃移发生的时间比例在50%以下,与平均风速成正相关关系,与风速脉动强度无显著相关关系;2 m高阵风7级风速下,戈壁跃移沙粒主要集中于地表50 cm范围内,近地表风沙流结构呈"象鼻效应",跃移质最大质量通量出现在地表2.5~5 cm高度处,沙粒最大跃移高度可达2 m,且沙粒跃移高度随2 m高风速的增加呈指数规律递增。因此,兰新高铁烟墩风区2 m高阻沙栅栏不足以完全阻截戈壁风沙流,是造成烟墩风区兰新高铁轨道积沙的重要原因之一。     

The dynamic effects of moisture on the entrainment and transport of sand by wind

There is an increasing awareness of the influence of surface moisture on aeolian entrainment and transport of sediment. Existing wind tunnel studies have shown the impact of a limited range of moisture contents on entrainment thresholds but similar investigations are lacking in the field. The research reported here investigated the influence of changes in surface moisture content on sand entrainment and transport on a meso-tidal beach in Anglesey, North Wales.High frequency (1 Hz) wind velocities measured with hot-wire anemometers were combined with grain impact data from a Sensit monitor and mass flux measurements from a standard sand trap. Surface and near-surface moisture contents were assessed gravimetrically from surface sand scrapes and also directly by using a ThetaProbe. Critical threshold values for entrainment were specified using a modified form of the time fraction equivalence method (Stout, J.E., Zobeck, T.M., 1996a. Establishing the threshold condition for soil movement in wind-eroding fields. Proceedings of the International Conference on Air Pollution from Agricultural Operations. MWPS C-3, Kansas City, 7–9 February 1996, pp. 65–71).Results indicate a time-dependent change in dominant control of the sand transport system from moisture to wind speed, dependent upon the moisture content of the surface sediment. This interchange between controlling parameters on both entrainment and transport was very sensitive to prevailing moisture conditions and took place over a period of minutes to hours. Under conditions experienced in the experiments presented here, the critical moisture threshold for sediment entrainment was determined to be between 4% and 6%, higher than the 1–4% specified in previous wind tunnel experiments. Furthermore, a moisture content of nearly 2% (where moisture was adhered to transported sediment) appeared to have little or no impact on the rate of sand flux.     

Splash-saltation trajectories of soil particles under wind-driven rain

It is usually recognized that relatively large amounts of soil particles cannot be transported by raindrop splashes under windless rain. However, the splash-saltation process can cause net transportation in the prevailing wind direction since variations in splash-saltation trajectory due to the wind are expected in wind-driven rain. Therefore, determining the combined effect of rain and wind on the process should enable improvement of the estimation of erosion for any given prediction technique. This paper presents experimental data on the effects of slope aspect, slope gradient, and horizontal wind velocity on the splash-saltation trajectories of soil particles under wind-driven rain. In a wind tunnel facility equipped with a rainfall simulator, the rains driven by horizontal wind velocities of 6, 10, and 14 m s⁻¹ were allowed to impact three agricultural soils packed into 20×55 cm soil pans placed at both windward and leeward slopes of 7%, 15%, and 20%. Splash-saltation trajectories were measured by trapping the splashed particles at distances downwind on a 7-m uniform slope segment in the upslope and downslope directions, respectively, for windward and leeward slopes. Exponential decay curves were fitted for the mass distribution of splash-saltation sediment as a function of travel distance, and the average splash-saltation trajectory was derived from the average value of the fitted functions. The results demonstrated that the average trajectory of a raindrop-induced and wind-driven soil particle was substantially affected by the wind shear velocity, and it had the greatest correlation (r=0.96 for all data) with the shear velocity; however, neither slope aspect nor slope gradient significantly predicted the splash-saltation trajectory. More significantly, a statistical analysis conducted with nonlinear regression model of C₁(u_*²/g) showed that average trajectory of splash saltation was approximately three times greater than that of typical saltating sand grain.     

Field determination for roughness length above the different non-erodible surfaces

Non-erodible elements, for its disturbance to the near-surface airflow, have been widely used in arid and semi-arid regions to protect the surface from wind erosion. Roughness length was usually used to evaluate the protection effect of non-erodible elements from wind erosion. In this study, the wind profiles above five types of non-erodible surfaces including gravel, wheat straw checkerboard barriers, cotton stem checkerboard barriers, shrubs, and herbs were measured and analyzed. The wind velocities above these surfaces increased with height approximately in logarithmic functions. The roughness length of different non-erodible surfaces was calculated by the functions of wind profiles. The results reveal that:(1) Roughness length increased with wind velocity in given wind velocity ranges. (2) On vegetative surfaces, wind did not effectively bend the stems. The threshold wind velocity for bending the stems of Achnatherum splendens was 4 m/s, 10 m/s for Agropyron cristatum, and for Artemisia ordosica, no obvious bending of stems even for wind velocity reaching 12 m/s. (3) Correlation analysis results show that the vegetation''s coverage and frontal area affect the roughness length more significantly than the other parameters. (4) The protective results of these non-erodible elements were evaluated. The checkerboard sand barriers made of cotton stem could provide more effective protection than that made of wheat straw. In the same coverage conditions, vegetation could provide more effective protection from wind erosion than gravel, and the blending of different non-erodible elements especially the combination of blending of vegeation and checkerboard sand barriers could provide more effective protection to the surface.     

北京南部沙地风蚀物理特征

1 Introduction Wind erosion, which occurred in arid, semiarid and sub-humid areas, is a critical process in land desertification and degradation. It is a global environmental problem (Dong et al., 1996; Gomesa et al., 2003). Previous researches on process…     

沙石质建筑垃圾不同覆盖方式防风固沙效益

为了解沙石质建筑垃圾不同覆盖方式防风固沙效益,在野外对不同覆盖方式的风蚀风积量、风沙流特征、土壤水分、植被生长状况进行了观测研究。结果表明：（1）不同覆盖方式年风蚀量较流沙减少了91%以上,其中平铺覆盖方式固沙效果好于带状和方格覆盖方式。平铺覆盖方式和流沙风沙流结构呈幂、指数函数分布,带状、方格覆盖方式呈多项式函数分布;（2）不同覆盖方式较流沙显著提高了土壤容积含水率（P<0.05）,平铺方式保水效果好于方格和带状覆盖方式。覆盖度30%平铺、1.5 m×1.5 m方格和宽1.5 m带状土壤各层平均容积含水率比流沙分别提高了3.1%、2.8%和1.5%;（3）不同的覆盖方式较流沙显著增加了植被盖度（P<0.05）,其中平铺方式群落多样性指数最高。覆盖度30%平铺植被盖度较流沙提高了5%。1.5 m×1.5 m方格覆盖方式植被群落多样性指数最低,植被盖度最高,较流沙提高了14%。     

输沙势计算中的“时距”问题

风力作用是干旱区风蚀过程和风沙地貌形成发展过程的动力基础,我们在探讨区域的风力作用时,通常是利用风速资料。风速是评价区域风沙活动的基础,但不同方法采集的风速资料对评价结果的影响并不一样。利用野外实测的风速资料进行不同时距处理,旨在说明时距在评价区域风沙作用的影响。结果表明,利用数据中值计算的结果要比平均值大11.31%,这说明以往研究中对输沙势的评价有偏大的现象。但是,由于以往所用数据的时距比较长,又造成计算的输沙势减小的趋势,随着时距增加,数据最大值/峰值降低,数据偏离中值的程度越明显,不论是利用中值还是平均值来计算输沙势,随着时距的增加,计算的DP值逐渐越小。对于用风速平均值计算的DP,1 min的计算结果要比5 min的计算大4.94%,比10 min的计算结果大16.90%,比15 min的计算结果大17.78%;而对于用中值计算的DP,1 min的计算结果要比5 min的计算大4.84%,比10 min的计算结果大16.70%,比15 min的计算结果大17.38%。     

海岸沙丘沙运动特征若干问题的研究——以闽江口南岸为例

在海岸带,沙粒粒径和水分含量是决定沙粒起动风速的最主要因素。闽江口南岸海滩沙的平均粒径为2.0Φ,水分含量多在0.3%~1.0%之间。运用Johnson的经验公式,可计算出海滩沙的起动风速为6~7m/s。这一结果与实际观测值十分吻合。由于海岸环境因素的影响,使得同等条件下海岸带沙粒的起动风速明显大于内陆沙漠沙,而风沙流的输沙率明显小于内陆沙漠沙。海岸带是风速剧变带,在引用陆地气象台站的风速记录分析海岸风况和研究风沙运动规律时,必须对风速进行订正。     

新疆策勒沙漠-绿洲过渡带不同下垫面地表蚀积变化特征

沙漠-绿洲过渡带天然植被具有良好的防风阻沙效益,对绿洲内部农田起到重要生态保护作用。在新疆策勒沙漠-绿洲过渡带流沙地、半固定沙地、绿洲边缘固定沙地6个不同植被覆盖度样地风蚀、风积变化观测基础上,结合地表风速数据,探讨流沙地、半固定沙地、固定沙地不同植被覆盖度和地形下地表风蚀风积变化特征及其影响因素。结果表明：流沙地表现出较强烈的地表风蚀;半固定沙地整体表现出强烈的地表风积;固定沙地上植被覆盖度越高、植株越高和排列方式越均匀、整体地势越低,单位面积风积量也就越大、风蚀量越小,风蚀主要发生在灌丛沙堆的上风向、侧翼、背风风向的裸低凹沙地表面,较高沙堆侧翼的地表风蚀量最大。植被覆盖度与单位面积风蚀量呈多项式或指数函数关系递减,植被覆盖度与单位面积风积量不呈函数分布,说明除了植被覆盖度外,植株类型、高度、排列方式、地形等都会对地表风积量产生一定影响。     

The flux profile of a blowing sand cloud: a wind tunnel investigation

The flux profile of a blowing sand cloud, or the variation of blown sand flux with height, is the reflection of blown sand particles that move in different trajectories, and also the basis for checking drifting sand. Here we report the wind tunnel results of systematic tests of the flux profiles of different sized sands at different free-stream wind velocities. The results reveal that within the 60-cm near-surface layer, the decay of blown sand flux with height can be expressed by an exponential function: q_h=aexp(−h/b), where, q_h is the blown sand transport rate at height h, a and b are parameters that vary with wind velocity and sand size. The significance of coefficient a and b in the function is defined: a represents the transport rate in true creep and b implies the relative decay rate with height of the blown sand transport rate. The true creep fraction, the ratio of the sand transported on the surface (h=0) to the total transport varies widely, decreasing with both sand size and wind speed. The flux profiles are converted to straight lines by plotting sand transport rate, q_h, on a log-scale. The slope of the straight lines that represents the relative decay rate with height of sand transport rate decreases with an increase in free-stream wind velocity and sand grain size, implying that relatively more of the blown sand is transported to greater heights as grain size and wind speed increase. The average saltating height represented by the height where 50% of the cumulative flux percentage occurs increases with both wind speed and grain size, implying that saltation becomes more intense as grain size and/or wind velocity increase.