bed, and the cycle continues. It was the transition zone, which covers a wide range of shear
values, that Brooks (1958) was investigating when he concluded that a single-valued function
does not exist between velocity or sediment transport and the shear stress on the bed.
Upper Flow Regime. In the upper flow regime, resistance to flow is small and sediment
transport is large. The usual bed forms are plane bed or antidunes. The water surface is in
phase with bed surface except when an antidune breaks, and normally the fluid does not
separate from the boundary. A small separation zone may exist downstream from the crest
of an antidune prior to breaking. Resistance to flow is the result of grain roughness with the
grains moving, of wave formation and subsidence, and of energy dissipation when the
antidunes break. The mode of sediment transport is for the individual grains to roll almost
continuously downstream in sheets one or two grain diameters thick; however, when
antidunes break, much bed material is briefly suspended, then movement stops temporarily
and there is some storage of the particles in the bed.
3.3.2 Bed Configuration
The bed configurations (roughness elements) that commonly form in sand bed channels are
plane bed without sediment movement, ripples, ripples on dunes, dunes, plane bed with
sediment movement, antidunes, and chutes and pools. These bed configurations are listed
in their order of occurrence with increasing values of stream power (VγyoS) for bed materials
having D50 less than 0.6 mm. For bed materials coarser than 0.6 mm, dunes form instead of
ripples after beginning of motion at small values of stream power. The relation of bed form to
water surface is shown in Figure 3.6.
The different forms of bed-roughness are not mutually exclusive in time and space in a
stream. Different bed-roughness elements may form side-by-side in a cross-section or reach
of a natural stream, giving a multiple roughness; or they may form in time sequence,
producing variable roughness.
Figure 3.6. Relation between water surface and bed configuration, Richardson et al. 1975.