Wind Trasportation

Topics: Sand, Wind, Dune Pages: 6 (2074 words) Published: March 24, 2013
Movement of particles by the wind takes place by a combination of direct wind shear stress and atmospheric turbulence. There are three modes of sediment transport by wind: creep or reptation; saltation, and suspension (Fig. 1). The mode of transport depends primarily on the ratio between particle settling velocity, and hence particle size, and wind shear stress and turbulence intensity. Very small particles (<20 microns) are transported in suspension (tens of km or greater) and are kept aloft by turbulent eddies in the wind. True suspension occurs when the particle settling velocity is very small compared to the turbulence intensity of the wind. Larger particles (20–70 microns) undergo short-term suspension for distances of tens to hundreds of meters; material of sand size (70–1000 microns) is transported mainly in a series of short hops (saltation), in which the vertical component of wind velocity (turbulence) has a minimal effect on particle trajectories. Material coarser than 500 microns in diameter (coarse sand) is transported on surface by reptation and creep. The modes of transport are interdependent: saltating sand particles eject silt- and clay-sized particles into the wind and impact coarse grains that are rolled along the bed. Grains begin to move and sediment is entrained by the wind when fluid forces (lift, drag, moment) exceed the effects of the weight of the particle, and any cohesion between adjacent particles as a result of moisture, salts, or soil crusts. The threshold wind speed at which grains begin to move is strongly dependent on particle size. For quartz sand, the minimum threshold velocity is associated with fine sand (~100 microns diameter). The mass flux or transport rate of sand has been determined by numerous laboratory wind tunnel and field studies to be proportional to the cube of wind shear velocity above a threshold value. For any wind shear velocity, there is a potential rate of sand transport or transport capacity, which is only reached when the availability of sediment is unrestricted (e.g., most loose sand surfaces). In these conditions, the wind is saturated with respect to transport capacity. Very fine grains (silt and clay size) are inherently resistant to entrainment, yet are readily transported by the wind. Recent studies have shown the critical role of impacting sand grains in the mobilization of silt- and clay-size particles and demonstrated the close relations between the horizontal flux of sand-size particles and the vertical flux of fine particles. In these situations, the horizontal mass transport rate is directly related to shear velocity so dust emissions scale to the fourth power of wind shear velocity. Where there is a limited supply of particles able to abrade soil clods or playa crusts, dust emissions are limited by the supply of particles rather than the wind shear velocity, and the vertical flux of dust is almost independent of wind shear velocity.

Figure 1. Modes of sediment transport by the wind.

Erosion by wind involves two linked processes: abrasion mechanical wearing of coherent materials, including playa crusts and clods created by tillage) and deflation (removal of loose material). Considerable attention has been devoted to the processes and rates of wind erosion because of their impact on agriculture, especially in semi-arid regions, and the implications of dust emissions for air quality. Wind erosion abrades crops, removes organic matter, nutrients and fertilizer, and changes soil texture. The products of wind erosion (especially dust particles) impact air quality, atmospheric radiative properties, and human health, causing respiratory illnesses. Rates of wind erosion vary widely and for a given wind shear velocity are dependent on soil or sediment texture and the degree of crusting and cohesion. The highest emission rates for fine-grained sediment are associated with soils of loamy...
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