ABSTRACT
This chapter deals with the periodic bed forms known as dunes, bars and ripples only, and argues that they develop starting from a flat mobile bed surface: the open-channel is straight. The research on bed forms is not motivated by an academic interest only: the dimensions and geometry of bed forms determine as practically relevant quantities as the effective roughness of a mobile bed and the resistance factor. Dunes and bars are large-scale bed forms, with the length of dunes being proportional to the flow depth h, and that of bars to the flow width B. Many prominent researchers dealing with fluvial processes were convinced since a long time that the large-scale bed forms, i.e. dunes and bars, are caused by large-scale turbulence. The difference appears to be mainly in the “length scale”, with all “lengths” of the large-scale horizontal turbulence being proportional to the flow width B.
