Flow dynamics and sediment transport over a reversing barchan, Changli, China

Yuxiang Dong 1, Patrick Hesp1, S.L. Namikas1
1School of Geography and Planning, Sun Yat-Sen University, Guangzhou,Guangdong, China, 2Faculty of Science and Engineering, School of the Environment, Flinders University, Bedford Park, South Australia, Australia, 3Department of Geography and Anthropology, Louisiana State University, Baton Rouge,Louisiana State, USA

The flow and sediment transport over a reversing barchan dune is examined at the Changli Gold Coast in Hebei Province, China. Wind speeds over the dune crest relative to those over the interdune flat increased by 354.54% and 222.58% at 5 and 30cm respectively. The windward slope is significantly greater than most "normal" windward slopes, and the magnitude of speedup is significantly greater due to the fact that the reversing flow is incident to a slipface. The ratio of the aeolian sediment transport rate in the 0~4cm height range decreases from the bottom to the crest on the windward slope, increases from the dune crest  to the middle of the slope on the leeward slope, and decreases from the middle of slope to the bottom of the slope on leeward slope. The leeward slope sedimentation rate is strongly influenced by the sediment plume streaming upwards and downwind from the dune crest. In contrast, the ratio of aeolian sediment transport rate in the 10~30cm height range at different sites over the dune shows the reverse trend. The vertical distribution of aeolian sediment transport rate can be expressed by an exponential function at the height range of 0~10cm and by a power function at the height of 10~30cm at all sites over the dune. Dunes in the process of reversing display different sedimentation patterns than when the dune is in ‘normal' flow conditions. Once the flow moves over the dune crest, diffusion and separation take place across the leeward slope, but the rate of transport is high from the dune crest to the mid lee slope in the 0-10cm height range as the sediment plume streaming upwards and downwind of the dune crest deposits across the crest to midslope apron. As the lee slope displays a relatively gradual gradient, the transport rate above 10cm again increases to the lower lee slope.