Sediment Movement in Alluvial Rivers - River Dynamics and Integrated River Management - page 272

called “Gan” in the Yellow River, in order to distinguish it from wind-induced surface waves. With dunes,
the bed configuration does not correspond with the surface wave; the surface wave and the bed configuration
are out of phase.
Fig. 5.13 (a) The height of dunes varying with the water stage measured at the Hankou Hydrological Station of the
Yangtze River; (b) A general picture of antidunes (after Chien et al.,1998)
Antidunes can move either in the same direction as the flow, as do ripples and dunes, or in the opposite
direction, this is the reason for the phenomenon is named anti-dune. The former is called a “downstreamward
antidune” and the latter is called an “upstreamward antidune.” Antidunes often form in shallow flows
that are moving at high velocities. The amplitude of an antidune is comparatively large. The flow must
climb the upstream or rising side of the antidune, and usually drops part of its sediment load there. On the
downstream side of the peak, the flow possesses surplus energy and can entrain sediment from the bed. As
a consequence, even though the movement and transport of every particle is in the direction of the flow,
the sand wave as a whole profile, generally moves upstream.
The direction of individual antidune movement should not be confused with the direction of movement
of a series of sand waves. Although the individual surface waves and the corresponding antidunes appear
to be moving against the flow, the whole series of waves is actually moving with the stream; because some
waves disappear from the upstream end of the series and some others are generated at the downstream
edge (Chien et al., 1998).
The crest lines of antidunes in plan are not quite parallel to each other; in most cases, they are similar
to short and wide sea waves in which the length and width are the same order of magnitude. Thus, they
usually occupy only a part of the width of a river. The lower Yellow River is quite wide, and antidunes
that form along its course are generally either near the flood plain or at the confluences of channels. In
transitional and wandering reaches of the Yellow River, especially in the straight sections downstream of
bends, a special phenomenon, “Gan” occurs occasionally, and it is related to the formation of antidunes.
For example, at the Tuchengzi section of the lower Yellow River, antidunes appear over the whole river
cross section, (the width of the river is some 500-600 m), the height of the “Gan” corresponding to the
antidunes is about 1 to 3 m, and its length is about 15 m. Figure 5.13(b) shows a general picture of
antidunes and “Gan”. “Gan” generally occurs during the falling stage of a flood. Initially, the river may
appear calm and be flowing along gently, then suddenly, a series of waves, usually numbering between 6
and 10, but sometimes up to 20, appear on the water surface. These surface waves develop and increase
rapidly in size; then, after about 10 minutes, they gradually decay and disappear. Sometimes they make a
sound like thunder as they break. The wave appears not to move, but with observations related to a
reference point on the bank, one can see that it actually moves slowly upstream.
From the foregoing discussion, the process of bed form development vis-à-vis an increasing flow velocity
can be divided into two distinct stages. Ripples and dunes form in the first stage and antidunes in the