Grainflows (or avalanches) are fundamental mechanisms associated with the evolution and migration of dunes. This study addresses grainflow morphology on the 21.3 m and 54.5 m high slipfaces of two barchans. A series of metrics to characterize shape developed with the perspective that this approach may provide a key to recognizing different grainflow morphodynamic regimes. We provide the first detailed and objective measurements of grainflow morphometry based on terrestrial laser scanning and subsequent image analysis of 1243 grainflows on the larger dune and 1609 from the smaller dune. Grainflow shape was classified based on five representative morphometric attributes: average length, average width, rectangularity, triangularity, and elongation. Cluster analysis indicated three recognizable grainflow types: elongated, narrow grainflows (Type 1); long, triangular grainflows (Type 2); and short, rectangular grainflows (Type 3). The distributions of grainflow types were substantially different on the larger and smaller dune, and different from distributions found on smaller dunes in other studies.

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Gravity-driven grainflows or granular flows (Russell et al., 2019) or avalanches (Sutton et al., 2013a(Sutton et al., , 2013b are ubiquitous on the lee slopes of migrating sand dunes on Earth and elsewhere. Grainflows have been the focus of substantial laboratory research and related modeling efforts to address the fundamental physics of particle flow (

<p>Barchans represent a common dune type found on Earth and Mars. Their morphological characteristics are singular and easily recognized. Their formation is favored on relatively immobile substrates with near-unidirectional winds that sculpt the distinctive crescentic, aerodynamic morphology. Barchans often occur isolated from one another, although they can occur in organized sets or barchanoid dune fields.  Long and Sharp (1964) and Bourke and Goudie (2009) measured attributes of barchan morphology and identified four archetypal shapes based on the ratio of the length of the stoss slope to the distance between the ends of the horns.</p><p>            In this study, we report findings based on measurements of 3,406 barchans: 2,686 from 20 terrestrial dune fields and 720 from 10 Martian dune fields. Barchan morphology was characterized by six metrics: body length (L1), measured from the upwind nose of the barchan to the nearest base of the slipface; total length (L2) measured to the (average) ends of the horns; body width (W1), measured on a line perpendicular to L1 and intersecting at the base of the slipface; horn-to-horn width (W2), measured perpendicular to L2 and parallel to W1; and horn lengths (H1 and H2) measured perpendicular to W1. The morphometric data were used to develop three new shape metrics as a basis for barchan shape characterization: 1) a width ratio (WR: W1/W2); 2) a length ratio (LR: L1/L2); and 3) a symmetry ratio (SR: longer horn length/shorter horn length). The barchan stereotype (Type 1) was defined as meeting three criteria: SR between 1.0–1.2, WR between 0.95-1.58 (mean value +/- one standard deviation) and LR between 0.52–0.76. Cluster analysis was used to define three additional characteristic shapes. Type 2 barchans are moderately symmetrical ( =1.47), uniform in width (  = 1.01), and elongated (  = 0.53). Type 3 barchans are moderately symmetrical (  = 1.4), with converging horns (  = 1.56), and compact (  = 0.74). Type 4 barchans are asymmetric (  = 3.46) uniform in width (  = 1.15) and average elongation (  = 0.64).</p><p>            We found that, on average, terrestrial barchans are shorter, proportionately wider, and more symmetric than those on Mars. Most barchans are Type 1, 2, or 3 (26%, 32%, and 35%, respectively), and relatively few are Type 4 (8%). The distributions of types, however, is quite different for the two planets. On Earth, most barchans are Type 2 (38%) and Type 1, stereotypical barchans comprise 30% of our samples. Type 4 barchans are least common (6%). On Mars, most barchans are Type 3 (64%). The distributions of Types 1, 2, and 4 are similar. Type 1, stereotypical barchans, the least common on Mars, comprise 11% of our samples, and Types 2 and 4 each represent 13% of our samples. These results indicate that most barchans do not conform to our idealized morphological image on either Earth or Mars. In our sample, Martian barchans are larger than terrestrial, with shapes characterized largely by asymmetric, converging horns.</p>