In this paper we report on numerical studies of unsteady, gravity-driven flow of a subaqueous erodible granular bed on an inclined plane. According to our simulations, the evolution of the flow can be partitioned in three phases. In the first phase, due to the onset of an interfacial instability, the material interface deforms into a series of long waves. In the second phase, these waves are transformed to skewed vortex ripples that grow in time and eventually coalesce. The computed wavelengths of these ripples are in good agreement with previously reported experimental measurements. In the third phase of the flow evolution, the high fluid velocities wash out the vortex ripples and a layer of rapidly moving particles is formed at the material interface. The predicted granular velocities comprise two segments: a concave one at the vicinity of the material interface, where the maximum is attained, followed by a slightly convex one, where they decrease monotonically to zero. The same trend has been reported in experimental results for the corresponding steady flows. Finally, we investigate via a parametric study the effect of the configuration stresses, which represent contact forces between grains. As it turns out, such stresses have a stabilizing effect, in the sense that increasing their magnitude inhibits the formation of vortex ripples.