Quantifying Grain Fall and Avalanche Dynamics Under Changing Wind Conditions on Barchan Dunes

Abstract:

Dunes are constantly moved and shaped by the wind but avalanche patterns and grain fall dynamics are particularly difficult to quantify without disturbing the lee slope. Here we use terrestrial laser scanning (TLS) to measure surface change over minutes, hours and a week where wind directions vary and lee slope reworking occurs. Changes in topography are linked to high resolution measurements of three dimensional airflow patterns at the brink of the dune. Grain fall is identified within a zone approximately 0.9 m from the brink that is constantly eroded. Several different avalanche patterns occur that can be related to slope characteristics, wind direction and slope reworking. We find that during oblique winds, horn reworking can reduce the lee slope. When normal winds return, this slope takes longer to start avalanching, but when avalanches are finally initiated, they are catastrophic. Conversely, avalanches in the centre of the dune are typically discrete and follow similar trajectories. Our findings help elucidate dune mobility mechanics and pattern modifications at the storm event scale.