Mountain Rivers and Incised Channels - River Dynamics and Integrated River Management - page 137

to chemical weathering and solutions that may directly erode the bedrock, as in carbonate lithologies, or,
more commonly, may weaken the bedrock and render the substrate more susceptible to erosion by
corrasion and cavitation (Carling and Grodek, 1994). Very few estimates have been published of direct
chemical weathering in channels. Most chemical erosion rates are averaged across a basin, and, thus,
incorporate groundwater and soil processes, or are obtained from rock faces exposed on hill slopes,
buildings, or tombstones. Generalized estimates for drainage basins range from 0.005 to 0.2 mm yr -1 for
carbonates and shales and 0.7 mm yr -1 for evaporites (Lerman, 1988). In one of the few studies that
quantified in-channel chemical erosion, Smith et al. (1995) measured rates of 0.022 0.200 mm yr -1 in
carbonate terrains of eastern Australia. Figure 3.16 shows a bedrock channel in the Three Gorges area of
the middle reaches of the Yangtze River, which is suffering from chemical corrosion. The channel bed is
composed of limestone and is likely to be eroded by corrosion. The rate of the corrosion is estimated at
about a few cm/kyr.
Fig. 3.16 The limestone bedrock channel of a stream in the Three Gorges area of the middle reaches of the Yangtze
River is suffering from chemical corrosion
Corrasion is abrasive weathering of bedrock by clusters moving along the channel as bed load. Partly
because it is very difficult to separate erosion caused by abrasion from erosion caused by other processes,
and partly because rates of bedrock channel erosion are commonly slow relative to the duration of most
field studies, no published rates of bedrock channel erosion caused solely by abrasion are available The
most rapid rates of abrasion probably occur during turbulent floods with large and fairly coarse sediment
loads, along channels of weakly resistant bedrock. Channels likely to be dominated by corrasional
erosion have numerous potholes, longitudinal grooves, knickpoints, and similar erosional features along
the channel bed and walls. The bedrock channel of the Yangtze River near the Gezhouba Dam exhibits a
lowest point of elevation of 10 m, 60 70 m lower than the surrounding bed elevation. The main cause
of the incision is abrasion of bed load particles against the relatively soft bedrock.
Perhaps corrasion is the most important process associated with fast bedrock channel incision.
Figure 3.17(a) shows the bedrock channel of the Lijiang River in Guangxi, China. The beautiful
landscape in the area was firstly sculpted by the corrosion process but the bed rock channel is eroded
mainly by corrasion induced by gravel movement on the bed. Figure 3.17(b) shows the incised bedrock
channel of a tributary of the Jinsha River (upper Yangtze River). The bed rock channel has been incised
down by about 20 m mainly due to corrosion. Figure 3.17(c) shows the parallel corrosion slots because
of bed load movement on the bedrock of the Jinsha River. During the flood season gravel bed load was