Gravity-Driven Grainflows on Barchan Slipfaces: Field Data

Introduction

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.

Here we present the data (the four spreadsheets in Natural grainflow dataset.xlsx) and metadata used to support a manuscript reporting our results and conclusions. The study site was on the slipface of a 20+m high barchan dune near Jericoacoara, Ceará, Brazil, and measurements made on November 7 and 9, 2013. The slipface was scanned repetitively with a Leica C10 terrestrial Laser scanner (TLS) on those two days. Details of TLS data processing and limitations are reported in Pelletier et al. (2015). Wind speeds were measured with Gill-type, DC-generating, cup anemometers. Sand transport rates were measured with sets of mesh-type traps described in Sherman et al. (2014). Grainflows that occurred near the apex of the slipface were video-recorded. More detail is provided for each of the individual spreadsheets to describe the measurement or derivation of data types.

Spreadsheet 1: Grainflow Area and Speed

Columns A and B report the date and time of each observation. Column C reports grainflow area (m²) as derived from Difference of Digital Elevation Models using the methods described in Zhang (2021). Column D reports grainflow speeds (ms^-1) derived from DEM data (grainflow length) and video recordings (grainflow duration).

Spreadsheet 2: Grainflow Frequency and Magnitude

Columns A and B report the date and time of each observation. Column C reports the duration of a particular scan. Most scans required 7 minutes to complete and then a new scan began. Column D reports number of grainflows detected in an individual scan. Grainflow frequency, Column E, is normalized to one-minute equivalents. Average grainflow magnitude (m³) in Column F is derived from measurements of the volumes of the accretionary tails of all grainflows divided by the number of grainflows occurring during a scan.

Spreadsheet 3: Wind Speeds

Wind speed was measured with a set of three anemometers installed at 20 m spacing at the center of the top of the barchan slipface. Measurements at each anemometer were made at 1 Hz frequency. Columns A and B report the date and time of each observation. Column C reports average wind speeds (ms^-1) obtained by block-averaging the 1 Hz data to one minute equivalent, and then averaging those data from the three anemometers.

Spreadsheet 4: Sand Transport Rates

Columns A and B report the date and time of each observation and Column C reports the duration of deployments of pairs of traps. Average transport rates in Column D were obtained by first averaging the trapped sand masses of the trap pair and then converting that value to an equivalent transport rate (gm^-1s^-1) for each sample period. The irregularly spaced point data were linearly interpolated to produce a time series with one-minute increments.

References:

Pelletier, J. D., Sherman, D. J., Ellis, J. T., Farrell, E. J., Jackson, N. L., Li, B., et al. (2015). Dynamics of sediment storage and release on aeolian dune slip faces: A field study in Jericoacoara, Brazil. Journal of Geophysical Research: Earth Surface, 120(9), 1911–1934.

Sherman, D. J., Swann, C., & Barron, J. D. (2014). A high-efficiency, low-cost aeolian sand trap. Aeolian Research, 13, 31–34.

Zhang, P. (2021). An algorithm for objective analysis of grainflow morphology, Aeolian Research, 50, 100686.