ABSTRACT

In meandering rivers and bended channels the sediment transport and secondary currents in the transverse direction cannot be neglected, as they are responsible for the highly irregular bed profiles observed around regions of high channel curvature. The main characteristic of the bed deformation in these bends is the formation of a point bar near the inner bank and of a pool at the outer part of the bend. The flow, sediment transport and associated bathymetry evolution are determined to a large degree by the helical motion of the water due to the action of the centrifugal forces present in regions of high channel curvature. As a consequence of the action of these forces, a pressure gradient is induced between the inner and outer banks. This pressure gradient is balanced near the free surface by a secondary current which, near the bed, is oriented from the outer toward the near inner bank.The secondary flow modifies the transversal distribution of the streamwise velocity profile, induces a vertical velocity component in the section, and redistributes the magnitude and direction of the bed shear stresses and sediment transport along the bend (Blanckaert and De Vriend, 2004). In particular, the magnitude of the bed shear stress is amplified compared to the values observed in the straight part of the channel. As a

consequence, under mobile bed conditions, sediment particles move, near the bed, toward the inner bank where, eventually, they deposit and raise the bed level by forming a point bar. Concomitantly, the mean flow velocity and bed shear stress increase in the region close to the outer bend, erosion is initiated and a pool is created. As the scour develops in the outer region, the transversal slope increases until the force induced by the secondary current against the transversal slope is balanced by the (downwards) component of the gravitational force acting on the particle in the same direction. At this point equilibrium is reached and the bathymetry would not change anymore in time. These conditions are observed to be approximately reached in most curved bends, but there are flows for which such an equilibrium state cannot be reached. Non-local effects due to the redistribution of the flow and sediment motion as one advance downstream in the bend can contribute to the local increase of the transverse bed slope.