We have studied experimentally the formation and evolution of sand ripples under water shear flow in a narrow annular channel. The amplitude of the sediment ripples vanishes at a critical value of the shear rate. Near this onset, a band of Fourier modes can be described by exponential growth functions. The growth rates vary linearly with the shear rate. On longer times the sand ripples display complex dynamics. PACS numbers: 47.54. + r, 92.10.Wa

The formation of sand ripples under water shear flow in a narrow annular channel and the approach of the ripple pattern towards a steady state were studied experimentally. Four results are obtained: i) The mean amplitude, the average drift velocity and the mean sediment transport rate of the evolving bed shape are strongly related. A quantitative characterization of this relation is given. ii) The ripple pattern reaches a stationary state with a finite ripple amplitude and wavelength. The time needed to reach the state depends on the shear stress and may be several days. iii) The onset of ripple formation is determined by the bed shear stress, but it seems neither to depend on the grain diameter nor on the depth of the water layer. iv) The ripple amplitude, drift velocity and sediment transport in this stationary state depend on the grain size. This dependency is neither captured by the particle Reynolds number nor by the Shields parameter: an empirical scaling law is presented instead.

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