Bed forms in turbulent channel flow

Bed forms in channels result from the interaction between sediment transport, turbulence and gravitational settling. They document mechanisms of self-organization between flow structures and the developing structure of the bed. It is shown that these mechanisms can be characterized by length scales of the sediment, the bed form and the flow structure. Three types of interaction can be distinguished: 1) The first type of mechanisms can be observed at beds of sediment with grain diameter smaller than the typical structural dimension of turbulence. It is shown how with increasing hydraulic loading of the bed a hydraulically smooth surface develops structures, which turn from “orange peel” to stripe and arrowhead patterns and finally into ripples. This group of bed forms is limited to a grain diameter of d+=12.5 in viscous units. In the regime of the stripe structures drag reduction occurs. 2) If grains or bed forms reach a height, which leads to separation, a completely different regime prevails, which is determined by the self-organization of separation zones. A prominent example for these bed forms are dunes. 3) Demixing processes, secondary flows and roughness contrasts finally lead to the development of longitudinal and transverse banks. All three mechanisms are explained on the basis of kinematic models and documented by experimental data. Emphasis is put on the two-dimensionalization of bed forms in a highly 3-dimensional (3D) turbulent flow, which is traced back to the self organization of vortex systems. This review article contains 55 references.