Birth of a subaqueous barchan dune

TL;DR

This study observes the formation of subaqueous barchan dunes from conical heaps in water flow, identifying key timescales for horn development and equilibrium, advancing understanding of underwater dune dynamics.

Contribution

It provides experimental evidence and quantifies the timescales for horn formation and stabilization in subaqueous barchan dunes, a novel insight into underwater dune morphology.

Findings

Horns appear at approximately 0.5 times the characteristic time.

Dunes reach equilibrium horn length around 2.5 times the characteristic time.

The characteristic time depends on grain size, fluid and grain densities, gravity, and flow velocities.

Abstract

Barchan dunes are crescentic shape dunes with horns pointing downstream. The present paper reports the formation of subaqueous barchan dunes from initially conical heaps in a rectangular channel. Because the most unique feature of a barchan dune is its horns, we associate the timescale for the appearance of horns to the formation of a barchan dune. A granular heap initially conical was placed on the bottom wall of a closed conduit and it was entrained by a water flow in turbulent regime. After a certain time, horns appear and grow, until an equilibrium length is reached. Our results show the existence of the timescales $0.5t_c$ and $2.5t_c$ for the appearance and equilibrium of horns, respectively, where $t_c$ is a characteristic time that scales with the grains diameter, gravity acceleration, densities of the fluid and grains, and shear and threshold velocities.