Mountain Rivers and Incised Channels - River Dynamics and Integrated River Management - page 130

unstable and the bank failure occurred.

Old flood
plain

Top bank

Arcuate bank
failure

Rotational
failure
Vertical
f
Upper
bank

Failed bank
material

Fluvial
washing

Channel bed

Fig. 3.8 Schematic illustration showing the morphological features of a typical incised river channel (after Darby
and Simon, 1999)

Fig. 3.9 Bank failure during the process of stream bed incision of the Liujia Gully in Lixian, Gansu (See color figure
at the end of this topic)

Channel widening is an extremely important process in accelerating the recovery of incised streams
because it acts to reduce flow depth, available shear stress, and sediment-transport capacity for a given
discharge. Together with the input of volumes of hydraulically controlled sediment from eroding banks,
these processes act as a feedback mechanism to reduce the rate of bed degradation. This helps to promote
establishment of a more stable longitudinal profile through aggradation downstream. Predictive analyses,
such as numerical models of incised channels, that ignore channel widening run the serious risk of
biasing estimates of future channel changes or stable morphologies (Darby and Thornes 1996; Simon and
Darby, 1997a).

3.1.3.3 Evolution of Incised Channels
In general, the morphological process of incised rivers goes through four stages as shown in Fig. 3.10(a)
Rapid degradation of the channel bed ensues as the channel begins to adjust after tectonic motion. The
valley takes a narrow “V” form. There is little space for humans to reclaim the hill slopes; (b) Degradation