Environmental Engineering Reference
In-Depth Information
train the river by the end of the 19th century. Then the river was channelized by dykes and groins and the
main channel of the river was scoured 2 m (Stevens et al., 1975). The Rhine River was channelized by
German engineers in the 19th century. It has become a human-controlled straight waterway. Sediment
was trapped by dams and weirs in the upstream reaches and tributaries. Consequently, flood flow scoured
the riverbed. The ground water table in the zone by the river was then reduced by 1-2 m and the
navigation conditions of the river channel were worsened. In order to prevent channel bed incision,
German engineers have had to feed gravel to the river at a rate about 170,000 t/yr since 1978 and the
strategy has proved successful (Kuhl, 1992). Incision control strategies will be discussed in detail in
Section 4.8.
Table 3.1 lists the incised channels on the basis of size and location. Rills are small channels that form
on steep slopes (Fig. 3.1(a)). They are ephemeral because they can be obliterated seasonally by frost
action or by the ploughing of fields. They are of little concern except that they increase stream turbidity,
and they can deepen and become permanent gullies. Gullies are incised channels that form where there
was no existing channel. They form on valley sides (Fig. 3.1(b)) and on valley floors. Entrenched streams
are existing channels that have become incised. Figure 3.1(c) shows a stream in the upper Yangtze River
basin in Sichuan Province, which is developing into an entrenched stream in a process of channel
incision. Composite incised channels are composed of reaches that are gullies, as defined above, and
reaches that are entrenched streams. Depending upon the design and construction, a channelized stream
can also be a composite incised channel. Figure 3.1(d) shows the Grand Canyon of the Colorado River,
U.S.. It is a composite incised channel, showing the different varieties of valley-floor gullies. Scarps are
formed by erosion. Headcut occurs in the tributary gullies. The incision of the valley floor forms terraces.
Rill erosion is still occurring on the floor and the rills will eventually develop into gullies and integrate
into the drainage network.
T
able 3.1
Incised channels in different scales (based on Schumm et al., 1984)
Incised channels
Notes
Rill
Very small (centimeters) ephemeral channel on steep slopes
Gully
A relatively deep (meters) incised channel that formed where there was no pre-existing
channel. There are valley-side gullies and valley-floor gullies which can be continuous
or discontinuous
Entrenched stream
Incision of an existing channel produces a deep unstable channel such as a mountain
stream experiencing continuous incision.
Composite incised river
A complex river system with incised main stem river and tributary channels (the middle
reaches of the Yellow River for example)
Incised streams are disturbed ecosystems. Although Brookes (1988) refers to the effects of channelized
streams on stream ecology, the same can be said for incised stream systems in general:”...habitat
diversity and niche potential are reduced, and, the quality and functions of the species occupying the
system are changed.”
Sediment produced from incised channel systems impacts local and downstream water quality.
Populations of fish and benthic macro-invertebrates (a measure of water quality) are severely impacted in
incised channels (Brookes, 1988). Mobile, unstable streambeds destroy spawning habitats and pool-riffle
sequences. Bank failures result in loss of riparian vegetation cover, higher water temperatures, and
increased turbidity. Changes in bed-material composition, or complete burial of gravel substrates by
aggradation, also cause destruction of aquatic habitat. Because the cover provided by riparian vegetation
is lost by mass failure of channel banks, populations of mammals and birds are also reduced along
incised stream corridors (Carothers and Johnson, 1975; Possardt and Dodge, 1978; Barclay, 1980).
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