Abstract
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.
Highlights
Avalanching or grainflow, where sediment accumulation from grainfall leads to exceedance of the repose angle and results in downslope transport by gravity, is a fundamental process in sedimentary bedform development
The plateauing of this curve reveals that at higher levels of QSz (>2 kg m–2 s–1), the GL95 does not increase downslope very rapidly, so that the increasing QGm becomes more concentrated on the upper lee slope as QSz increases
While our data show a similar disconnect between wind speed and net surface change within the top 1 m of the lee slope, we find that net surface change scales well with wind speed across the entire slip face
Summary
Avalanching or grainflow, where sediment accumulation from grainfall leads to exceedance of the repose angle and results in downslope transport by gravity, is a fundamental process in sedimentary bedform development. In aeolian environments, avalanching is the principal mechanism for dune dynamism, and the cross-strata that grainflows produce can provide a record of dune accumulation and formation. Lee slope depositional processes are problematic to measure in large fluvial systems where sediment transport is greatest close to the bed (Kostaschuk et al, 2009). Avalanche deposits are a significant component of preserved aeolian stratigraphy, an understanding of avalanche processes provides important information for interpreting the rock record (Eastwood et al, 2012; Hunter, 1977)
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