Aeolian deposition of dust over hills: the effect of dust grain size on the deposition pattern

Abstract

Wind tunnel experiments were conducted to investigate the effects of topography on the grain size characteristics of aeolian dust deposits. Experiments were performed on three isolated hills having various size and aspect ratios. The longitudinal profile of the median grain diameter was investigated for each hill. The longitudinal dust deposition profile was also studied for nine grain size classes of between 10 and 104 µ µ µ µ µm, as were wind and dust concentration profiles in the atmosphere upwind of, over and downwind of a hill. The wind tunnel experiments show that the grain size characteristics of aeolian dust deposits are affected by topography. Most apparent is the occurrence of a zone of reduced grain size on the leeside of hills, which extends from just upwind of the summit to a distance of several times the height of the hill. Slightly coarser than normal dust is deposited on the concave windward hill slope and in a zone downwind of the area of reduced grain size, but the increase in grain size in these zones remains very small. Although the normalized dust deposition profile for a hill does not vary substantially as a function of grain size, systematic trends are observed. The most important tendencies are: (1) a progressive extension, in the downwind direction, of a zone of decreased dust deposition on the leeside of a hill (the coarser the grains, the further downwind the zone of reduced deposition extends); (2) a progressive increase in dust deposition immediately upwind of a hill (the finer the grains, the higher the deposition value upwind of a hill becomes). Both tendencies are explained by the difference in inertia of the grains, which is controlled by grain size.

Effects of grazing and topography on dust flux and deposition in the Xilingele grassland, Inner Mongolia

“…The greatest erosion rates were observed on the windward slopes close to the summits while on the leeward slopes, erosion rates were considerably lower (Goossens and Offer, 1997). Dust deposition was greatest on the windward slopes, followed by the troughs, leeward slopes, and summits with rates at the windward slopes being up to four times higher than at the summits (Goossens, 2006;Zufall et al, 1999). In models, the highest long-term accumulations of loess were usually found behind obstacles (Goossens, 1988;Mason et al, 1999).…”

Calibration of the MDCO dust collector and of four versions of the inverted frisbee dust deposition sampler

“…Goossens (2006) recently investigated how the dust shadow created by obstacles in the flow changes as a function of particle size. It was found that shadow length increases as particle size increases, up to approximately 50 μm, after which it remains more or less constant.…”

Using 137Cs to estimate wind erosion and dust deposition on grassland in Inner Mongolia-selection of a reference site and description of the temporal variability

“…Topographical effects are relevant if other surface characteristics are homogenous and/or deposition processes are considered over a long period with a predominant transport direction (Goossens 1997;Goossens 2006). Topography influences the wind velocity close to the ground, which may result in the typical deposition patterns on summits, windward or leeward orientated slopes related to the prevailing transport direction (Goossens and Offer 1997;Goossens 1988;Zufall et al 1999).…”

“…The selection of an appropriate reference site in a wind erosion/dust deposition area was based on fluid dynamical principles as shown in previous wind tunnel and field studies by Goossens (1988Goossens ( , 1996Goossens ( , 2006. Summit positions have the lowest dust depositions because of the local increase of the wind speed.…”