Sediment Movement in Alluvial Rivers - River Dynamics and Integrated River Management - page 307

to excessive sinuosity, which so lowers the channel gradient that the stream cannot transport the load
supplied. Cutoff increases the channel gradient, and, therefore, the local transport capacity.
An artificial cutoff was started in the 19 th century, when Hungarians cut 112 bends on the Tiso River
and reduced the river length from 1,200 km to 745 km. In order to reduce the risk of bank failure at
bends, the middle Yangtze River was artificially cutoff at 2 meanders and the river length was, therefore,
reduced by 78 km in the 1960s. An artificial cutoff has been suggested at the Paizhou Meander, as shown
in Fig. 5.44(c). The meander is located at the downstream of Tongting Lake and upstream of Wuhan City.
There is a big argument between Tongting people and Wuhan people, the former supports the cutoff for
reduction of flood risk. The later worries about that a flood may come to Wuhan quickly and threaten the
safety of the city and is strongly against the cutoff. In general artificial cutoff causes intensive erosion in
the new channel and siltation of the old channel. The new channel is not stable before the finish of an
intensive fluvial process in, at least, several years.
Nowadays, engineers have begun to reconsider the strategy of artificial cutoff from the point of view
of sustainable development and stream ecology protection. A flood wave may propagate through a shortened
channel after cutoff but causes flooding problem to the downstream reaches. Moreover, meandering channel
is better ecologically-sound than straight channel. Therefore, less and less artificial cutoffs have been
implemented in the recent decades.
5.3.2 StraightRivers
The longitudinal profile of a straight river is seldom constant, even over a short reach. Differences in
geology, vegetation patterns, or human disturbances can result in flatter and steeper reaches within an
overall profile. Riffles occur where the stream bottom is higher relative to the streambed elevation
immediately upstream or downstream. These relatively deeper areas are considered pools. At normal flow,
flow velocities decrease in pool areas, allowing deposition of fine sediment to occur, and increase atop
riffles due to the increased bed slope between the riffle crest and the subsequent pool. In fact, the
development of alternating pools and riffles is characteristic of both straight and meandering channels
with heterogeneous bed material in the size range of 2-56 mm (Knighton, 1998)
A significant feature of riffle-pool geometry is the more or less regular spacing of successive pools or
riffles at a distance of 5 to 7 times the channel width. The spacing distance is, thus, scale-related. It describes
at best an average condition. The most extensive data-set has values of pool-to-pool spacing ranging
from 1.5 to 23.3 channel widths, with an overall mean of 5.9 (Keller and Melhorn, 1978). Even in a
channel disturbed by channelization schemes and the introduction of woody debris, the inter-riffle distance
generally falls within the range of 5 to 7 channel widths (Gregory et al., 1994).
A complete explanation of riffle-pool formation needs to consider not only how they develop but also
why they develop within the broader context of stream behavior. Basically, given a flat bed, a riffle-pool
sequence through a combination of scour and deposition, organized spatially to give a more or less
regular spacing between consecutive elements. The regular spacing of pool-riffle affect the bed form and
a meandering thalweg develops within straight channels. Figure 5.45 shows a channelized stream in
Yunnan, in which a regular meandering thalweg bed has formed.
Various mechanisms have been invoked to explain the development of riffle-pool sequences. Keller
and Melhorn (1973) suggested that the regular pattern of scour and deposition required for the formation
of a riffle-pool sequence may be the result of an alternation of convergent and divergent flow along the
channel, combined with secondary circulation currents. Surface flow convergence at the pool induces a
descending secondary current, which increases the bed shear stress and encourages scour, while surface
flow divergence at the riffle produces convergence at the bed and thereby favors deposition. Thompson's
(1986) diagrammatic representation of the process (Fig. 5.46) envisaged a repeated decay and regeneration