**P. A. Jarvis, K. A. Bacik, C. Narteau,
N. M. Vriend**

*Journal of Fluid Mechanics*

If the flow of water over a flat bed of sand is sufficiently fast, then grains
of sand can be picked up, transported and deposited to form sand dunes.
Initially, many small dunes form but, as the flow continues, they grow and
merge to become a smaller number of larger dunes. We have performed
experiments investigating the formation and growth of underwater dunes from an
initially flat bed of sediment. In particular, we vary the speed of the flow,
the depth of the water and the thickness of the sediment bed. We observe that
the dunes initially grow rapidly, before reaching an almost-constant height
that increases with the sediment thickness. In order to relate the initial
dune growth rate to the driving flow, we have also performed numerical
simulations of the water flow in the experiment. This enables us to better
constrain the shear stress on the sediment bed, which is quantified through
the friction velocity u_{∗}. By combining the experimental and
numerical results, we show that the initial dune growth rate is approximately
proportional to u_{∗}^{5}. These results can form a starting
point for models describing the growth of underwater dunes.