The present paper is devoted to the formation of sand patterns by laminar flows. It focuses on the rhomboid beach pattern, which may be formed during the backswash. The full incompressible Navier-Stokes equations, with a free surface are coupled with an erodible bed with a bedload transport model, based on a moving-grains balance. A linear stability analysis then shows the simultaneous existence of three types of instabilities, namely ripples, bars and longitudinal striations. The comparison of the bar instability characteristics with laboratory rhomboid patterns indicates that the latter could result from the non-linear evolution of unstable bars.