VERTICAL-DISTRIBUTION OF WIND-BLOWN SAND FLUX IN THE SURFACE-LAYER, TAKLAMAKAN DESERT, CENTRAL-ASIA

The vertical distribution of the wind-blown sand flux in a 40-cm flow layer above the ground surface was investigated through laboratory win d-tunnel tests and field measurements on the mobile dune surface durin g sand storms in the Taklamakan Desert of China. Results show that ver tical distribution of the horizontal mass flux of drifting sand is a d iscontinuous function of height. More than 90% of the total material i s transported in the flow layer from the surface to 14 cm. From 2 to 4 cm above the surface, a distinct transition zone occurs wherein mixed transport by creep, saltation, and suspension becomes saltation and s uspension. The flow layer from 14 to 15 cm represents a further transi tion from saltation to suspension, where the distribution curves of th e transport rate against height converge. The basic natural exponentia l function cannot describe well the vertical distribution of the salta tion mass flux in the Taklamakan Desert. As a function of height, salt ation mass flux follows a function q(salt) = a'Z(-bz), and the distrib ution of suspension mass flux fits the power function very well. A tot al transport rate from surface creep to saltation and suspension in th e measured flow layer, which is directly proportional to the effective wind speed squared (V - V-t)(2), can be predicted by integrating Q = a'Z-(bZ) + cZ(-d). The height distribution of the average quantities o f transported materials varies as an exponential function of wind spee d, and deceases with the increase in total transport quantity. Higher wind speed results in a higher transport rate and a higher vertical gr adient for the particle concentration. The increment of relative trans port quantity in the higher flux layer increases as wind speed increas es, which generates a higher concentration of drifting particles in th e upper flow layer.