Coupling leeside grainfall to avalanche characteristics in aeolian dune dynamics

Abstract

Avalanche (grainflow) processes are fundamental drivers of dune morphodynamics and are typically initiated by grainfall accumulations. In sedimentary systems, however, the dynamism between grainfall and grainflow remains unspecified because simple measurements are hampered by the inherent instability of lee slopes. Here, for the first time, terrestrial laser scanning is used to quantify key aspects of the grainfall process on the lee (slip face) of a barchan sand dune. We determine grainfall zone extent and flux and show their variability under differing wind speeds. The increase in the downwind distance from the brink of peak grainfall under stronger winds provides a mechanism that explains the competence of large avalanches to descend the entire lee slope. These findings highlight important interactions between wind speed, grainfall, and subsequent grainflow that influence dune migration rates and are important for correct interpretation of dune stratigraphy.

Morphometric analysis of slipface processes of an aeolian dune: Implications for grain‐flow dynamics

“…While the Maspalomas observations did not include a study of grain‐flow behaviour under a variety of wind conditions, the data agree well with the behaviour observed by Nield et al . () during stronger wind velocities. The wind velocity at the study site, adjusted to a height of 0·5 m, was approximately 8·6 m sec −1 and exhibited a high frequency of grain‐flow activity, where some grain flows occurred simultaneously or shortly after one another, or in ‘multiple families of failures’.…”

“…For example, Nield et al . () observed that with increasing wind velocities, the length and thickness of grain flows also increased. Additionally, the entrainment of sediment as grain flows travel down the slipface may vary.…”

“…In contrast, Nield et al . () argued that there was a correlation between grain‐flow frequency as well as magnitude with greater wind velocity, where weaker winds produced smaller grain flows near the dune brink and stronger winds triggered avalanches further downslope resulting in more sediment being mobilized upslope. Additional field observations are needed to resolve the inconsistency of the relationship between wind velocity and grain‐flow magnitude and frequency.…”

“…Failure occurs near the lower inflection point of the sediment bulge from which a tongue of sediment subsequently flows downslope (e.g. McDonald & Anderson, , ; Tischler et al ., ; Cupp et al ., ; Dasgupta & Manna, ; Sutton, ; Sutton et al ., ,b; Nield et al ., ). As sediment is removed, a scarp forms and simultaneously migrates upslope, spreading laterally (e.g.…”

“…() characterized two types of grain flows based on flow triggers during field observation. The first was named primary flow and was observed to initiate by sediment accumulation on the slipface near the brink, consistent with previous descriptions of localized slope over‐steepening with the formation of an alcove and sediment tongue (Allen, , ; Hunter, ; Borówka, ; Fryberger & Schenk, ; Hunter, ; Anderson, ; McDonald & Anderson, , ; Tischler et al ., ; Nickling et al ., ; Walker & Nickling, ; Cupp et al ., ; Dasgupta & Manna, ; Kok et al ., ; Sutton, ; Sutton et al ., ,b; Nield et al ., ). The second, referred to as a secondary flow, was thought to be initiated by disturbances on the slipface due to primary flows and were generally smaller and occurred soon after primary flows.…”