Mountain Rivers and Incised Channels - River Dynamics and Integrated River Management - page 166

fill can significantly affect the process of channel incision and adjustment. For example, it was determined
that channels incising into cohesive sediments (silts and clays) deepen rapidly, whereas channels incising
into sandy sediments widen rapidly and deepen much less. Hence, natural control of incised-channels is
primarily sedimentological, which influences the rate, type, and magnitude of incised-channel evolution.
Bed load motion plays an important role in resisting channel incision. Coarse sediment particles, like
gravel, slide or move by saltation on the bed, applying a grain pressure on the stationary bed materials.
The pressure is known as the dispersive pressure defined by Bagnold (1954). It may balance the lift force
or suction force of the flow, which is the main mechanism of scouring of bed materials. With enough bed
load moving on the bed, the channel bed is not scoured and the channel incision is, therefore, stopped.
For cost-effective control of incising channels, timing is most important and, therefore, it is an
important variable in any scheme to control an incised channel and to reduce sediment load. Figure 3.47
shows, in general, how incised channel drainage density (length of incised channels per unit area)
changes with time and how sediment yield follows this trend. In a drainage basin that contains incising
channels, sediment production increases as the length of incising channels increases (Fig. 3.47, Times 1
to 4). However, at Time 4, the maximum, headward growth of the incised channels has ended, and they
begin to stabilize between Times 4 and 7. By understanding this cycle of channel incision from initial
stability (Time 1) to renewed stability (Time 8) it is possible to select times when channel control
practices will be most effective. For example, when incision is just commencing (Times 1 and 2) or when
channels are almost stabilized (Times 6, 7, and 8) control measures will be most successful in preventing
incision and finally stabilizing the channels, but at Times 6, 7, and 8, the most cost effective action would
be to do nothing. At Times 3, 4, and 5, control will be difficult and expensive.
Fig. 3.47 Hypothetical changes of sediment production and incised channel (gully) drainage density at eight times
during incised channel evolution. Dashed lines indicate the effect of gully control structures at various times during
channel evolution (after Schumm, 1999)
3.4.2.2 Knickpoints
Natural knickpoints (e.g. waterfalls) and artificial knickpoints (e.g., dams) control stream bed incision.
Korup and Montgomery (2008) studied the bed incision of the Tsangpo River and concluded that the
knickpoints developed from quake lakes created by glacial landslides inhibit channel bed incision.
Natural knickpoints may be created by the rivers that cross a sharp lithologic contrast (i.e., weak rock
type to strong) in the form of waterfalls, by transient river response to upstream migration of incision in
modes of detachment-limited bedrock river incision (Whipple and Tucker, 2002), or by large scale
landslides damming streams.
Channel incision of numerous streams on the eastern margin of the Qinghai-Tibet Plateau has produced
high-relief, narrow river gorges, and threshold bank hillslopes. The bank slope is so unstable that many