Defining the threshold wind velocity for moistened sediments

Abstract

[1] The moisture of surface sediments is one of the most significant factors governing the initiation of particle movement by the wind and hence the aeolian transport rate. This paper develops an equation for the threshold shear velocity of moistened sediments based more soundly on physics by means of the moment balance method, taking account of the interparticle cohesive forces produced by moisture. The equation relates threshold shear velocity directly to moisture content and contains three simple coefficients that need be determined from experiments. The threshold equation for moistened sediments basically follows Bagnold's threshold equation for loose, dry sediments but with a proportionality coefficient that accounts for the effect of moisture. Previously published data on the threshold shear velocities of moistened sediments from wind tunnel tests are used to determine the coefficients contained in the threshold equation, and it proves that the new equation describes the threshold velocities of moistened sediments reasonably well. Comparing the threshold shear velocity predicted using the new equation with the results of Belly's and Hotta et al.'s empirical equations reveals that the predicted results differed greatly. These differences can be attributed to differences in the definition of threshold velocity and moisture content in the physical and chemical properties of the sediment samples and in the experimental methods. Further theoretical and experimental investigation is required to fully understand the effects of sediment moisture content on threshold wind velocity. We also suggest that methodological improvement itself is also a topic of future research.Citation: Dong, Z., Q. Mu, and X. Liu (2007), Defining the threshold wind velocity for moistened sediments,

Discrete Element Simulation on Sand-Bed Collision Considering Surface Moisture Content

“…It could have been caused by the unequally distributed force chains and liquid bridges in the unsaturated sand bed surface as shown in Figure 6. The average velocity and ejected numbers significantly decrease when the sand surface becomes wet, and constant with numerous experimental studies [20,21,23,[25][26][27]. This work reproduces the process of surface moisture restraining the saltation, in which the liquid bridges increases kinetic energy consumption of ejected grains and decreases the total number of particles ejected from the sand-bed collision.…”

“…A large amount of sand grains originally belong to the surface are ejected into the air, and the total number decreases with the increasing surface moisture. This is reasonable because the combination of moisture and sand grains produces a dense and hard system, resulting in the underlying saltation cascade phenomenon observed by numerous experimental studies [20,21,23,[25][26][27]. Simultaneously, largish impact craters are generated near the collision positions similar to craters on Earth or Mars.…”

“…It is interesting to note that the shape of crater is wide and shallow when the surface moisture is low, and higher moisture content significantly decreases the area and slightly increases the depth of the crater. The shape and volume changes of crater not only affect the ejected grain number from there but also turn the rebound direction of incident grain, which may be an important influencing factor for the structure of drifting sand flux [20,23]. In order to investigate the deep reason for the changes of crater affected by moisture, we draw the distribution of contact forces among sand grains as shown in Figure 5, and the simulation cases are same with Figure 4.…”

“…A relatively small number of researchers have been reported to explain the saltation behaviour considering surface moisture in field and wind tunnel environments [19][20][21][22][23][24][25][26][27]. Many researchers suggest that small amounts of surface moisture content restrain the saltation in two ways: reducing the number of incipient motions of sand particles due to wind stress and decreasing the total number of particles ejected from the sand-bed collision subsequently.…”

Effect of the surface water content on dry saltation cloud dynamics: A wind tunnel simulation with particle tracking velocimetry

“…Not surprisingly, the effect of water on aeolian transport has been addressed in a proliferation of models of various forms (Azizov, 1977; Belly, 1964; Bolte et al, 2010; Cornelis, Gabriels & Hartmann, 2004; Cornelis & Gabriels, 2003; Dong et al, 2007; Horikawa et al, 1982; Hotta et al, 1985; McKenna Neuman & Nickling, 1989), as well as intensively examined in field settings (refs provided in paragraph above), and wind tunnel simulation (Chen et al, 1996; Cornelis & Gabriels, 2003; Dong & Liu, 2002; McKenna Neuman & Maljaars Scott, 1998; McKenna Neuman & Nickling, 1989; McKenna Neuman & Sanderson, 2008). The overarching goal in all this work has been to establish a relationship between the sediment texture, water content and wind speed required for the initiation of aeolian transport; that is, identification of the conditions surrounding the fluid threshold.…”