Luce Malverti

Stability of bedforms in laminar flows with free surface: from bars to ripples

The present paper is devoted to the formation of sand patterns by laminar flows. It focuses on the rhomboid beach pattern, formed during the backswash. A recent bedload transport model, based on a moving-grains balance, is generalized in three dimensions for viscous flows. The water flow is modelled by the full incompressible Navier–Stokes equations with a free surface. A linear stability analysis then shows the simultaneous existence of two distinct instabilities, namely ripples and bars. The comparison of the bar instability characteristics with laboratory rhomboid patterns indicates that the latter could result from the nonlinear evolution of unstable bars. This result, together with the sensibility of the stability analysis with respect to the parameters of the transport law, suggests that the rhomboid pattern could help improving sediment transport models, so critical to geomorphologists.

Erosion structures in laminar flumes

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.