Dunes dominate the bed of sandy rivers and they respond to flow by
changing shape and size, modifying flow and sediment transport dynamics
of rivers. Our understanding and ability to predict dune adaptation,
particularly dune growth and decay, remains incomplete. Here we
investigate dune growth from an initial flatbed in a laboratory setting
by continuously mapping the 3D bed topography using a line laser scanner
combined with a 3D camera. High-resolution profiles of flow velocity and
sediment concentration providing both bedload and suspended sediment
fluxes were obtained by deploying Acoustic Concentration and Velocity
Profiler technology. Our analysis reveals that the magnitude of the dune
slipface angle, which determines flow separation and controls turbulence
production, adjusts to the imposed flow at time scales similar to the
evolution of dune height and length. The initiation of a flow separation
zone intensifies through scour, and results in acceleration of the dune
growth. Gradients in sediment transport and the rate of dune growth are
inherently linked to spatial variations in slipface angles. During dune
growth, the slipface angle evolves differently than the ratio of dune
height to length, which immediately reaches its equilibrium value after
dune initiation.