Sediment Movement in Alluvial Rivers - River Dynamics and Integrated River Management - page 310

of an island, while a complete vegetation develop with bamboos, shrubs and grass at the downstream end.
Shrubs and grass grow in the middle part of the island and some pioneer plant species begin to colonize
the new created land.
Banks composed of readily erodible material are an important source of sediment as well as being
necessary for the channel widening characteristics of braided reaches. Without erodible banks, any incipient
bar deposits would tend to be destroyed rather than added to. The Turandui River in New Zealand has
changed over a period of years from a braided river to a meandering river as a result of the planting of
willow shrubs at appropriate places.
Rapid fluctuation in discharge is often associated with high rates of sediment supply, bank erosion and
irregular bed load movement, which are important for braided channel formation. High stream power is
also a necessary condition for development of braiding. A river must be sufficiently powerful to erode its
banks and achieve high bed mobility, which is a crucial requirement for braiding.
5.3.4 WanderingRivers
A wandering river is defined as a river with unstable channels. A wandering river carries heavy sediment
load and the discharge and sediment-carrying capacity are unsteady. A wandering river is usually
associated with river aggradation. There are sand bars in the river channel but usually the stream flow
remains in one channel during one period of time but flows in another channel during a second period of
time. This is different from braided rivers, in which the stream flows in multiple channels simultaneously.
One important feature of a wandering river is the high speed of migration of the mainstream channel.
Migration of the main channel is due to the high sediment load and very erodible banks. Most of the
wandering rivers have sediment composition in the bed and banks of sand, silt, and fine gravel.
There are many wandering rivers in the world. The lower reaches of the Brahmaputra River in India
have a width of more than 10 km and moves at a speed of 70 m per year in the transverse direction. The
Pudma River in Bangladesh widened at a rate of 200 m per year in the period 1984-1993. The Ganges River
migrates consecutively resulting in many old mouths and deltas. It merged with the Brahmaputra River
about 200 years ago and created the modern river delta. Such high-speed river migration and geomorphologic
variation are closely related to the high sediment load and low erosion-resistance of the banks.
The Jiamura River in India also is wandering. Figure 5.48 shows the migration of the river channel
near Bahadurabad with contour lines of water depth obtained by bathymetric survey. Fig. 5.48(a) is the
result of the bathymetric survey done in August-September 1993 and Fig. 5.48(b) shows the contour lines
in November 1993. At the fixed point cross 1, the so called Standard Low Water contour curve (the curve
marked with “0”) moved westward by about 400 m. At the upper fixed point cross 2 (+ 2 ), on the other
hand, the Standard Low Water line migrated 520 m eastward.
Xu (1996) studied the wandering braided channel pattern of the Hanjiang River. Unlike the wandering
channels of the lower Yellow River, which are due to the serious sedimentation resulting from the heavy
sediment-laden floods, the wandering braided pattern of the middle Hanjiang River is caused by strong
erosion of the riverbank initiated by the impoundment of the Danjiangkou Reservoir. Large quantities of
sediment are supplied to the channel by bank erosion and deposited at many mid-channel bars during
floods. Thence a wandering braided channel pattern with many unstable mid-channel bars developed under
a condition of sediment transport equilibrium. Scientists place importance on erodible banks for a wandering
braided channel (Knighton, 1984). It is striking that while the river was developing from a braided river
into a wandering-braided river the sediment quantity measured at the upstream station was approximately
equal to the that measured at the downstream station (Xu, 1996). This implies that a huge amount of
sediment on the bed and banks was removed while a small amount of sediment was transported through