Barchan Slipface Grainflows: Characteristics and Kinematics

• Published in: Geophysical research letters 2022-08, Vol.49 (15), p.n/a
• Main Authors: Sherman, Douglas J., Zhang, Pei, Pelletier, Jon, Ellis, Jean, Farrell, Eugene, Li, Bailiang
• Format: Article
• Language: English
• Subjects: Aeolian; avalanche; Avalanches; dune migration; Dunes; Field study; Gravity; Hypotheses; Kinematics; Laboratories; Landslides; Lasers; Mars; Mars environment; Sand; Sand transport; Scaling; Sediment transport; Transport rate; Wind measurement; Wind speed
• ISSN: 0094-8276; 1944-8007
• DOI: 10.1029/2021GL095257

Gravity‐driven grainflows on aeolian dunes are important agents of sand transport on Earth and Mars but have been the focus of few field studies. We present results from the first comprehensive field study to evaluate hypotheses posed by previous studies: (a) grainflow frequency depends on the sand transport rate; (b) grainflow magnitude is independent of sand transport rate; and (c) grainflow speed depends on its area. A barchan slipface was monitored with a terrestrial laser scanner and video camera, with measurements of wind speed and sand transport rate. More than 1,600 grainflows were detected and measured. Key findings support the first hypothesis, refute the second hypothesis, and support the third hypothesis. We also found that grainflow speeds measured in laboratory studies are substantially slower than comparable examples measured in this field study, and the grainflow speed and area relationships found for field and laboratory data are significantly different. Plain Language Summary Gravity‐driven grainflows, or avalanches, occur in many environments on Earth, Mars, and elsewhere. Many aspects of their characteristic behavior have been revealed through laboratory and field studies. Some findings have been, however, inconclusive or contradictory. We present the results from the first comprehensive field study of 1609, gravity‐driven grainflows, with measurements of their magnitude, frequency, area, speed, and related wind speeds and sand transport rates, made on the slipface of a 21 m high barchan dune. Key findings support previous research, with relatively small sample sizes from shorter slipfaces, indicating that grainflow frequency increases with sand transport rate and grainflow speed increases with its area. We also found that grainflow magnitude increases with transport rate, which contradicts a commonly made assertion that grainflow magnitude is independent of the sand transport rate. We found significant scaling differences between laboratory and field data concerning the relationship between grainflow area and speed, with laboratory speeds, per unit area, more than two orders of magnitude faster than speeds found in the field. The implications of this work point to the importance of additional field studies and the hazard of using laboratory data alone for modeling grainflow behavior on terrestrial and extraterrestrial dunes. Key Points Both the frequency and magnitude of grainflows are dependent on sand transport rates Grainflow speeds