Vertical profiles of aeolian mass flux above different sand surfaces and sand surfaces covered with pebbles

Abstract

The vertical distribution of aeolian mass flux is an important issue in aeolian geomorphology. It has been studied mainly on sand surfaces and much less often on gravel surfaces, particularly under natural conditions. We present data of measurements made in 3 beach settings that differed in petrographic composition of the beach sand (quartz, bioclastic, and bioclastic-basaltic sand) and density of pebble coverage (5–70%). We aimed to show the differences/similarities in vertical mass flux profiles induced by surface properties resulting from sediment composition. All measurements were made under conditions of maximum sand transport rate on a dry sand bed by means of passive segmented 0.5-m-high sand traps. The results showed that (i) regardless of the surface type, all vertical mass flux profiles were well fitted by an exponential decay function, but the regression coefficients differed greatly between those for sand surfaces and surfaces with pebble covers of different densities, (ii) changes in these coefficients with wind speed were much more pronounced in the case of sand surfaces than on surfaces with pebbles, (iii) the exponential model underpredicted mass flux in the near-bed region in the case of sandy surfaces, whereas in the case of pebble surfaces, the departures from the model were insignificant, and (iv) a pebble cover with a low density between 5% and 10% strongly affected the concentration of sand in the vertical profile. All mass flux profiles showed that as wind speed increased, the proportion of sand transported in the near-bed region decreased and the proportion of sand transported at higher elevations increased. For each surface type, the height at which the constant proportion of sand was transported may be defined irrespective of wind speed. This height was equal to 3–4 cm in the case of sand surfaces and changed from 6 cm to 11 cm as the density of pebble coverage increased from 5 to 10% to 50–70%. This was also reflected in the average saltation height, which increased slightly with wind speed and significantly with the density of gravel coverage.