Fundamentals of Engineering Design
Bed forms note the presence and type of bed forms that may develop in sand bed channels. The
primary variables that affect bed forms are the slope of the energy grade line, flow depth, bed particle size,
and particle fall velocity (Julien, 1995, p. 138). Julien (1995) or Simons and Sentrk (1992) provide more
extensive information on bedform classifications and prediction. Figure 5.24 presents the basic bed forms.
Bed armoring refers to the presence of a coarse surface layer on the streambed. In noncohesive
Figure 5.24 Bed Forms (after Simons and Richardson, 1966)
sediments, the materials available for transport are essentially those exposed at the bed surface. The active
layer refers to the surface layer from which materials can be entrained by the flow. Below that may be one
or more layers of immobile coarse sediment in which the majority of the finer sediments have been scoured
away. This layer is known as armoring and protects the underlying material from further scour (Chang,
1988, p. 177).
Signs of aggradation or degradation note the presence of features that usually are indicative of
vertical instability. Headcuts or knickpoints (Figure 5.25) are defined as locations on the streambed profile
where there is an abrupt change of elevation and bedslope (Schumm et al., 1984, p. 9). The headcut is
an adjustment by the river to restore equilibrium in the system. Headcuts often migrate upstream along the
channel incision and increased sediment transport.