Sediment Movement in Alluvial Rivers - River Dynamics and Integrated River Management - page 273

second. The transition between them is a flat bed stage or a bed with traces of dunes that are about to be
washed out. In ordinary alluvial rivers, the most common bed features are ripples and dunes. Antidunes
occur much less often.
In natural rivers, the process described previously may not occur in a normal progression; various
types of bed forms can exist at the same time, and the process of development may differ from one
instance to another. Even in flume experiments, different bed forms can co-exist in different parts of a
flume; showing that a strong relation exists between the bed forms and the local turbulence structure. For
certain combinations of flow and sediment, the initial flat bed can change directly into a dune or from a
dune to an antidune without the usual transitional stages of ripples or a flat bed. Moreover, ripples and
dunes may develop simultaneously on the riverbed, which was observed in the Liuhe River. The dunes
are not well developed and ripples overlap on the dunes. Dunes need a certain water depth to develop but
the flow in the river has high depth only in flood season, the flow discharge reduces sharply after the
flood season and sometimes is cut off. Therefore, the development of the dunes is disturbed. Ripples are
not affected by the flow depth very much and develop very well on the riverbed. Similarly, ripples
may develop on dunes in sand desert, as shown in Fig. 5.14.
The development of bed forms implies that a plane, cohesionless, granular surface is unstable where
there is sediment transport (Knighton, 1998). Leeder (1983) envisaged a strong interplay between
turbulence, sediment transport, and bed forms. Burst cycles are believed to be an inherent component of
macro-turbulent flow responsible for the initiation of sediment transport, with sweeps exerting high
instantaneous stresses against the bed and ejections carrying sediment away from the bed. Because bed
disturbance is not uniform, such cycles have an important role in the initiation and maintenance of bed
forms. Each bed element represents one “erosion-deposition” sequence with a wavelength dependent on
the length of the burst process. Sediment transport rates vary across individual bed forms as a result of
form-induced accelerations and decelerations, promoting scour in the troughs and deposition toward the
crest.
Fig. 5.14 Ripples on sand dunes in the Kubiqi desert in northwest China, illustrating that the two bed forms may
develop simultaneously on the sand bed
Bagnold (1956) showed theoretically that the formation of ripples and dunes is necessary if some
degree of stability is to be achieved during sediment transport. Without the additional resistance provided
by these bed forms, the channel could be destroyed as a coherent structure for the conveyance of water
and sediment. However, there are few data to suggest how that resistance affects the turbulence structure.
The discussion on ripples, dunes and antidunes refers only to alluvial rivers with bed consisting of