Geoscience Reference
In-Depth Information
3.3 MOVABLE BED ROUGHNESS IN ALLUVIAL RIVERS
3.3.1 Bed forms in alluvial rivers
Bed forms in alluvial rivers are closely related to flow conditions. As the flow strength
increases, a stationary flat bed may evolve to sand ripples, sand dunes, moving plane
bed, anti-dunes, and chutes/pools (Richardson and Simons, 1967; Zhang
et al
., 1989),
as shown in Fig. 3.10. This process is explained below in more detail:
Figure 3.10
Bed forms in alluvial rivers (Zhang
et al
., 1989).
(a) In the stage of stationary flat bed, the flow is weak and only a small amount of
sediment particles move on the bed.
(b) As the flow strength increases, more and more sediment particles participate in
motion, and sand ripples occur. The generation of sand ripples mainly depends
on the stability of the movable bed under the action of turbulent shear flow. Their
dimension is highly related to the bed-material size
d
, and they are about 100
d
in length and 50-100
d
in height.
(c) Due to the effect of large-scale flow eddies, bed shear stress decreases and increases,
and sediment deposits and erodes at alternate patterns, thus resulting in generation
of sand dunes on the bed. In the upstream slope of a sand dune, flow acceleration
usually causes sediment erosion; in the downstream slope, flow deceleration and
separation cause sediment deposition. Therefore, the sand dunes migrate down-
stream in certain shapes. Their dimension is highly related to the flow depth
h
.
They are usually about 5-10
h
long and 0.1-0.5
h
high.
(d) When the flow strength continually increases, sediment particles may be suspended
and transported far downstream; thus, sand dunes are washed out, and the bed
