Mountain Rivers and Incised Channels - River Dynamics and Integrated River Management - page 129

more sediment than the same flow (same recurrence-interval flow) prior to degradation (Simon, 1992;
Simon and Darby, 1997a). The dominant discharge, defined by magnitude-frequency analysis of
long-term sediment-discharge rates (Wolman and Miller, 1960; Andrews, 1980; Thorne et al., 1993;
Andrews and Nankervis, 1995) may, therefore, be quite distinct in incised versus non-incised rivers. This
has serious implications for morphological analyses and design tools which are based on regime relations
(Leopold and Maddock, 1953; Hey and Thorne, 1986) or other methods, which rely on specification of
the bankfull or dominant discharge (Rosgen, 1996). Particular care must, therefore, be exercised in
identifying the level of the bankfull discharge, and determining whether the discharge at that level
actually represents the dominant flow.
Fig. 3.7 Factors affecting channel degradation or aggradation. The channel incision (or siltation) is determined by
the stream's energy, slope, and flow of water in balance with the size and quality of the sediment particles (after
FISRWG, 1997)
3.1.3.2 Bank Erosion
An important characteristic of incised alluvial channels is the role of bank erosion and channel-width
adjustment. Depending on the strength of the bank materials, increases of bank heights and angles
resulting from incision may be sufficient to trigger mass movement under gravity (Daniels, 1960; Thorne
et al., 1981; Little et al., 1982; Schumm et al., 1984; Simon and Hupp, 1986; Simon, 1989, 1992; Darby
and Thorne, 1996; Simon and Darby, 1997a). A common feature of the response of incised channels is,
therefore, the sudden switching of the locus of channel instability from deepening to widening. Rates of
channel widening in incised channels range over several orders of magnitude; from less than 0.01 m yr -1
in bedrock canyons, to less than 1.0 m yr -1 in cohesive stream bank materials, to as much as 100 m yr -1
in non-cohesive stream bank materials.
The bank heights of incised channels are greater than those prior to incision, and the upper bank
surfaces are wetted less frequently by rises in stage. Bank failure may occur during the receding limb of a
flood as shown in Fig. 3.8. The onset of widening by mass movement processes results in distinguishable
bank morphologies (Simon, 1989), which in turn depend on the type and mode of bank failure. Planar
failures are generally associated with very steep banks and with the formation of tension cracks, which
develop at the ground surface and extend downward. Rotational failures occur along the highest banks
with shallower angles and tend to occur later in the adjustment sequence. Slab failures are characterized
by the toppling of an upper bank mass after undercutting of the lower part of the bank by fluvial action
(Thorne, 1990) or by pore pressure induced pop-out failures (Bradford and Piest, 1980; Simon and Darby,
1997b). Figure 3.9 shows a bank failure due to bed incision of the Liujia Ravine in Lixian county of
Gansu Province. A recent flood scoured the river bed by about one meter and the banks became very