GRAINFALL AND GRAINFLOW DYNAMICS ON AEOLIAN DUNES (Invited Presentation)

Grainflow (or avalanching) is the main mechanism by which aeolian bedforms migrate, but it is difficult to measure the process dynamics of these sediment accumulations because the lee slopes that avalanches traverse are at or close to the angle of repose and therefore inherently unstable and difficult to instrument. Avalanches are initiated when grainfall deposited close to the dune brink as a ‘bulge’, exceeds an angle of repose and is transported down the lee slope. The placement of the bulge depends on the distribution of grainfall on the lee slope, which in turn, is related to wind speed. Avalanches that initiate further downslope, typically transport more sediment as they result in larger erosion scarps. Here we use terrestrial laser scanning (TLS) to measure avalanche dynamics on a 5 m high barchan dune under variable wind speeds, on the Skeleton Coast, Namibia. The TLS has the ability to remotely capture both surface change on the lee slope as the avalanches occur, and an indicative measure of the grainfall zone above the lee slope. We find that as the wind speed increases, the grainfall zone expands (by up to 0.45 m for a 3 m/s increase in wind speed). The increased grainfall distance shifts sand further from the brink resulting in dominant avalanche initiation point locations expanding from 0.3 m to 0.4 m for wind speeds above 6 m/s. This shift also corresponds to the appearance of secondary avalanches, which are initiated by primary avalanche lobe deposits extending outside of the main grainfall zone. Ultimately, under stronger winds the expansion of the grainfall distance contributes to the destabilisation and movement of increased sediment volumes down the lee slope and a reworking of the brink area. This increase in sand transport conditions produces wider, longer and thicker avalanche lobe deposits. Along with the increased avalanche size, stronger winds produce steeper slopes, greater avalanche initiation angles and an increase in avalanche frequency. This study provides a unique insight into the influence of grainfall on grainflow dynamics and ultimately aeolian dune mobility.