Aerodynamic entrainment thresholds and dislodgement rates on impervious and permeable beds

Williams, Jon J.; Butterfield, Graeme R.; Clark, David G.

doi:10.1002/esp.3290150307

Cited by 10 publications

(5 citation statements)

References 18 publications

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“…In order to simulate more accurately the aerodynamic and sedimentary characteristics of a flat sediment bed, it was necessary to create artificially a rough, granular surface on the leading edge and working sections of the plate. This also provided a fixed surface of uniform geometry over the entire working area upon which loose grains could be placed.. As previous research had demonstrated that, for well sorted sediments, bed permeability has no detectable effect on entrainment dynamics of threshold shear velocity (Williams et al, 1990b), it was concluded that a single layer of grains would provide the necessary consistency of particle packing whilst not adversely affecting threshold determinations. Covering the surface of the plate with fixed grains, however, meant acceptance that boundary layer development would be influenced significantly by roughness elements and that, consequently, its precise properties could not be predicted numerically.…”

Section: Experimental Hardwarementioning

confidence: 99%

Aerodynamic entrainment threshold: effects of boundary layer flow conditions

1994

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Experimental data are presented demonstrating the influence of boundary layer flow conditions on aerodynamic entrainment of grains in the absence of intersaltation collisions. New methods are proposed for (I) the unambiguous determination of aerodynamic threshold for any grain population and (2) approximation of the probability density function (PDF) distributions of threshold shear velocity for aerodynamic entrainment.In wind tunnel experiments, the orderly spatial development of flow conditions within a developing boundary layer over the roughened surface of a flat plate constrains the aerodynamic threshold condition in terms of both mean and fluctuating values. Initial grain dislodgements and subsequent erosion from narrow strips of loose, finely fractionated ballotini were recorded photographically as wind speed was increased. Boundary layer parameters, including average threshold shear velocity ( U.,), were calculated using the momentum integral method.Direct observations show that sporadic oscillation of grains preceded dislodgement. At slightly higher velocities most grains rolled over their neighbours before entering saltation. Initial entrainment in spatially semi-organized flurries of 50 or more grains was followed by quiescent periods at airflow velocities close to threshold. These observations provide strong circumstantial evidence linking both the nature and spatial pattern of initial grain motions to sweep events during the fluid bursting process.For each grain fraction, values of U., were found to span an unexpectedly wide range and to decrease downwind from the leading edge of the plate as turbulence intensity increased. A probabilistic entrainment model is applied to the aerodynamic threshold condition so as to incorporate the effects of changing turbulent flow regimes over the plate. Analysis of strip erosion curves gives both an objective definition of the threshold condition and usable approximations of the PDF for U,, required by the model and for future stochastic treatment of the threshold condition.

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Section: Experimental Hardwarementioning

confidence: 99%

Aerodynamic entrainment threshold: effects of boundary layer flow conditions

1994

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“…These early studies identified fundamental relationships and concepts governing sand and dust transport by the airstream which now form the basis for many contemporary investigations into aeolian sediment transport. Technological advances allowing the observation of fine grain transport by the airstream through the use of high technology wind tunnels (Nickling, 1988;Williams et al, 1990), in conjunction with computer simulations of fine grain transport, continue to advance the science. However, research into aeolian grain transport is still largely confined to the semi-arid/arid desert and prairie environments, such as that outlined by Jones et al (1986), Lougeay et al (1987), Brazel and Nickling (1987), Wheaton and Chakravarti (1990), Wheaton (1992) and Lee et al (1993).…”

Section: Introductionmentioning

confidence: 99%

Characteristics of aeolian grain transport over a fluvio-glacial lacustrine braid delta, Lake Tekapo, New Zealand

McGowan

Sturman

1997

Earth Surf. Process. Landforms

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This paper presents results from one of the few scientific studies to examine the physical characteristics of aeolian sediment transport in an alpine area, where topographically reinforced foehn winds initiate dust storm events. The major objective of this study is to improve knowledge of aeolian processes in mid-latitude alpine regions experiencing extreme wind speeds. Of particular interest is the role of surface characteristics in contributing to the unusually deep saltation layer which is seen to form over fluvio-glacial deposits in the Southern Alps of New Zealand. Sediment was collected at several heights (0·5, 1, 2 and 4 m) and locations over a large alpine braided river delta, and standard laboratory techniques used to examine grain size characteristics. An image processing technique was also used to evaluate grain roundness.Grains filtered from the airstream at 0·5 m and 1m above such surfaces were found to display a mean grain size of approximately 300 to 435 µm, resembling grain size characteristics of saltation clouds previously observed in high latitude, cold climate locations, in contrast to desert and prairie environments. Samples collected at 2 and 4 m above the surface were found to consist of 60 to 65 per cent sand-sized material, with some grains exceeding 1-1·5 mm in diameter. Grain shape analysis conducted on silt-and clay-sized grains filtered from the airstream above mixed sand and gravel surfaces showed such grains to display an increase in grain roundness with height. This characteristic is thought to reflect the airstream's shape-sorting ability and has important implications with respect to the often observed increase in grain roundness in aeolian deposits with increasing distance from source areas. Namely, if more rounded grains are preferentially carried higher into the airstream and therefore into regions of higher wind speed, they should theoretically be transported further from the entrainment zone before being deposited.The high wind speeds observed, often exceeding 30 m s −1 , are seen to transport significantly larger sediment than reported in the literature for desert and prairie environments. In addition, the mixture of grain sizes, and especially the pebble-and cobble-sized clasts that dominate the fluvio-glacial deposits associated with the braided rivers in this mountain region, also appear to increase significantly the trajectory height of saltating sand grains. As a result of these two factors, the depth of the saltation cloud often exceeds 1m.Observations made in this study therefore highlight the need for field and laboratory aeolian process studies to be extended to examine grain transport over coarse-grained beds during much higher wind velocities than typically reported in the literature. Such studies would provide a valuable insight into aeolian processes in high latitude/altitude environments, such as loess genesis.

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“…It has been shown that aerodynamic entrainment is primarily a function of the mean grain size of the particles involved combined with the erosivity of the wind, with u * the preferred measure (Williams, Butterfield and Clark, 1990). Bagnold (1941) studied these relationships and derived values of critical threshold shear velocity (u * ct ) for a wide range of particle sizes, using the square root of the grain diameter as the principal determinant: Chepil, 1945).…”

Section: Grain Entrainmentmentioning

confidence: 99%

Sediment Mobilisation by the Wind

Wiggs¹

2011

Arid Zone Geomorphology

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Wind can be a particularly effective medium for sediment movement in drylands where a relatively sparse vegetation cover and thin soils combine to create highly erosive conditions on highly erodible surfaces. There is little evidence to suggest that dryland winds are any stronger than their counterparts in humid regions, but the sparse vegetation cover in deserts allows the winds to contact the surface more effectively, and the lack of root systems and moisture to bind sediment together renders it far more susceptible to erosion (Pye and Tsoar, 1990). The efficiency of dryland winds is underlined by the continuous advance and development of desert dunes in the Earth's sand seas (e.g. Bristow, Duller and Lancaster, 2007

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Aerodynamic entrainment thresholds and dislodgement rates on impervious and permeable beds

Cited by 10 publications

References 18 publications

Aerodynamic entrainment threshold: effects of boundary layer flow conditions

Aerodynamic entrainment threshold: effects of boundary layer flow conditions

Characteristics of aeolian grain transport over a fluvio-glacial lacustrine braid delta, Lake Tekapo, New Zealand

Sediment Mobilisation by the Wind

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