Subaqueous Barchan Dunes

Formation and dynamics of barchan dunes under turbulent water flow — lab experiments

Formation and Dynamics of Subaqueous Barchan Dunes

Barchan dunes are crescent-shaped bedforms that emerge when a limited supply of sediment is transported by a unidirectional flow over a flat bed. While they are commonly observed in deserts, they also form underwater — on riverbeds, continental shelves, and even on Mars.

This project investigated the formation and dynamics of subaqueous barchan dunes through controlled laboratory experiments. An initially conical pile of glass beads (diameter 0.5 mm, density ρ = 2500 kg/m³) was placed at the bottom of a water-filled flume and subjected to turbulent flow. The pile progressively deformed into a fully developed barchan dune.

Experiment

The movie below shows a top-down view of a barchan dune recorded with a high-speed camera. The fluid flow direction is from top to bottom. Black grains are tracers used to identify the trajectories of grains migrating toward the horns (the dune’s tips) as they formed.

Top-down view of a barchan dune forming from a conical pile of glass beads under turbulent water flow (flow direction: top to bottom). Black grains are tracers tracking grain trajectories toward the forming horns.

Key Findings

  • Contrary to what is commonly assumed in the aeolian case, most grains migrating to the horns did not come from the lateral flanks of the initial heap or dune. Instead, they originated from upstream regions of the bedform, exhibiting significant transverse displacements.
  • When the barchan was composed of bidisperse grains, the grains segregated following a diffusion-like mechanism.
  • These results provide new insights into barchan structures found in rivers, coastal environments, and other planetary surfaces.
  • Alvarez, C.A., & Franklin, E.M. (2018). Role of transverse displacements in the formation of subaqueous barchan dunes. Physical Review Letters, 121, 164503. DOI
  • Alvarez, C.A., & Franklin, E.M. (2019). Horns of subaqueous barchan dunes: A study at the grain scale. Physical Review E, 100, 042904. DOI
  • Alvarez, C.A., & Franklin, E.M. (2017). Birth of a subaqueous barchan dune. Physical Review E, 96, 062906. DOI
  • Alvarez, C.A., Cúñez, F.D., & Franklin, E.M. (2021). Growth of barchan dunes of bidispersed granular mixtures. Physics of Fluids, 33, 051705. DOI