Sediment Movement in Alluvial Rivers - River Dynamics and Integrated River Management - page 271

Sand dunes on the river bed and in sand desert look similar. Figure 5.12 shows the sand dunes in the
Kubuqi desert in northwest China. The sand has a median diameter around 0.2 mm and the dunes are
about 10 m high and 50-100 m long.
Fig. 5.12 Sand dunes (10 m high and 50 m long) in the Kubuqi Desert in northwest China (See color figure at the
end of this topic)
If the flow velocity is raised progressively, the dune pattern changes in plan from straight lines to
curves; it then has a shape like a moon crescent. The dimension of the straight ridge in the transverse
direction is larger than that along the streamwise direction. Stripe dunes often occur on beaches on the
inner bank of a river bend, and they stretch downstream towards the outer side of the bend. The direction
of the flow near the bed, which brings sediment to the inner side, is perpendicular to the ridge lines of the
dunes; the latter form a larger angle with the direction of the surface flow. If the amplitude of a sand
wave is large compared to the water depth, the sand wave can affect the water surface. The water surface
is usually lower near the dune peak. Many waves of small amplitude form in this vicinity, and they
occasionally make what looks, from a distance, like a stripe on the surface. This stripe reflects, in fact,
the location of the ridge line of the dune. People living near the lower Yellow River call this phenomenon
“Lianzishui” (Chien et al., 1998). If the ratio of the dune height to the water depth is large enough, the vortex
that originates in the separation region downstream of the dune peak is strong enough to carry a high
concentration of sediment up to the water surface, it then has the appearance of water at its boiling point.
If the dune reaches a certain height and the flow velocity is then increased further, the dune decays; its
wave length increases and its height gradually decreases to the form shown in Fig. 5.8(d). With still
further increase in velocity, the bed becomes flat again (Fig. 5.8(e)). Figure 5.13(a) is a plot of the dune
height against the depth of water in the Hankou section of the Yangtze River. The maximum height
occurs if the water stage is 21.5 m. If the water stage is below this level, the dune height increases gradually
with the discharge or with the water stage; and if the stage is above it, the dune height decreases. If the
water stage is 24.5 m or more, the riverbed is flat again.
The sediment transport rate is quite large in the second flat bed phase. If the velocity continues to
increase, the flow approaches or becomes supercritical (Froude number, Fr>1), and the bed form develop
into antidune (Fig. 5.8(f)). Antidune is a type of bed configuration that is in phase with the wave on the
water surface, and these two waves interact strongly. The differences between antidunes and dunes are as
follows: the shape of a dune is non-symmetrical, and the streamlines of the flow separate at the dune peak;
in contrast, an antidune is symmetrical, more like a surface wave, the streamlines are almost parallel to
the river bed and no separation occurs. If antidunes form, the water surface also undulates; this process is