The influence of grainfall and grainflow dynamics on barchan dune mobility under variable wind conditions

Joanna Nield 1, Giles Wiggs2, Matthew Baddock3, Martin Hipondoka4
1University of Southampton, Southampton, UK, 2University of Oxford, Oxford, UK, 3Loughborough University, Loughborough, UK, 4University of Namibia, Windhoek, Namibia

Aeolian dune morphology responds dynamically to changing wind conditions. The lee slope avalanche dynamics of dunes are particularly sensitive to prior morphological conditions as well as the varying intensity and duration characteristics of sand transport events. Here we use terrestrial laser scanning (TLS) to measure both grainfall zone characteristics and associated surface change of a 5 m high barchan dune during two different transport events. Additionally, wind and saltation at the brink were recorded by 3D sonic anemometers and wenglors. Whilst saltation and grainfall patterns are strongly controlled by wind speed (R2 = 0.65 and 0.71 respectively), avalanche dynamics are better explained by the extent of the area over which grainfall is distributed, rather than the transport rate of saltation moving over the brink. Grainfall dynamics also influence the way in which the lee slope is modified under different wind conditions. During weaker winds, much of the grainfall sedimentation is focused within a few centimetres of the brink. Avalanche frequency and size are reduced and limited to the upper section of the lee slope. This results in lee slope reworking, but does not contribute significantly to dune mobility within the transport event. Conversely, during stronger winds (interdune winds > 6 m/s at 1.5 m) the migration rate of the dune is as much as 1.5 cm/hr. We show that under these higher winds the distance over which significant grainfall is deposited downwind of the brink expands (0.21 to 0.59 m). This contributes to a greater destabilisation area on the lee slope, which, combined with an increase in avalanche size and frequency, leads to continuous erosion in the upper section of the lee slope and the reshaping of the dune cross-section. Our findings help elucidate dune mobility mechanics and pattern modifications at the wind storm event scale.