  We present results of direct numerical simulation of incompressible fluid
flow over a thick bed of mobile, spherically-shaped particles. The algorithm is
based upon the immersed boundary technique for fluid-solid coupling and uses a
soft-sphere model for the solid-solid contact. Two parameter points in the
laminar flow regime are chosen, leading to the emergence of sediment patterns
classified as `small dunes', while one case under turbulent flow conditions
leads to `vortex dunes' with significant flow separation on the lee side.
Wavelength, amplitude and propagation speed of the patterns extracted from the
spanwise-averaged fluid-bed interface are found to be consistent with available
experimental data. The particle transport rates are well represented by
available empirical models for flow over a plane sediment bed in both the
laminar and the turbulent regimes.
