Richards <1980> conjectured that megaripples of the continental shelf are due to a seabed instability in the presence of seabed ripples. Towards a better understanding of this megaripple formation process, we use a morphodynamical numerical model to investigate the bed roughness influence on the bed load--induced linear stability of a sandy bed. We find that the linearly most amplified mode shifts from a dune mode (grain roughness) towards a megaripple mode (large bed roughness). Then, we discuss how megaripples could be generated over a flat bed and over dunes.