  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.
