Effect of rock fragment eccentricity on eolian deposition of atmospheric dust

Abstract

The effect of rock fragment eccentricity on the deposition of airborne dust was examined in a wind tunnel. Eccentricity was defined as the ratioa/b, wherea is the longest andb the intermediate axis of the rock fragments. The effect of eccentricity on the deposition of dust on the pebbles, on the deposition of dust between and underneath the pebbles, and on total dust deposition (pebbles + interpebble space) was measured separately, and this for different values of pebble size, pebble flattening, cover density and wind speed. The more pebbles are elongated, the larger dust deposition on them will be. This effect is more pronounced as pebbles become larger, as they become less flattened, and as wind speed increases, but no correlation is observed with cover density. It also holds that the more pebbles are elongated, the less dust settles between and underneath them. This effect is more pronounced as pebbles become smaller and cover density becomes larger, but there is no relationship with pebble flattening. Whether or not there is a relationship with wind speed is unclear. Pebble eccentricity indirectly influences total dust deposition on a pebble field, since it determines whether or not (and to what extent) total dust deposition will be a function of pebble size. For circular pebbles, total dust deposition is independent of pebble size, but once pebbles become elongated, total dust deposition will be smaller on pebble fields composed of small pebbles than on pebble fields composed of large pebbles. It was also demonstrated that the more pebbles are elongated, the more the dust will tend to settle on the pebbles rather than between or underneath them. Air flow separation plays a primary role in the deposition of dust on a rock fragment field, since it determines the location and size of the low-sedimentation areas that are connected to the air flow separation bubbles. Highly-elongated pebbles oriented parallel to the wind catch much more dust (also per unit surface) than circular pebbles, the surface of which is largely located in a zone of very low deposition. Dust deposition density on highly-elongated pebbles normal to the wind, on the contrary, is equal to dust deposition on circular pebbles. It should be noted that all data in this study refer to initial dust deposition (not accumulation), since only wind velocities below the dust deflation threshold were tested.