Mountain Rivers and Incised Channels - River Dynamics and Integrated River Management - page 133

surcharge by increasing the normal force on the potential failure plane. The varying effects of vegetation
on incised stream bank stability have not been addressed in detail although Abernathy and Rutheford
(1998) provide a review of research in North America, Europe, and Australia.
Techniques and results for tropical cut slopes, which are characterized by deep, heterogeneous profiles
may prove useful in analyzing incised channel banks (Anderson et al., 1996; Collison and Anderson,
1996). A modified version of the combined slope hydrology-stability model CHASM (Anderson and
Kemp, 1991) may be particularly useful for testing the role of riparian vegetation on the stability of
banks in incised streams.
Fig. 3.12 The riparian vegetation by a small river in the Mississippi River drainage basin resists bank erosion with
the roots of the trees bonding the bank soil
3.2 Bedrock Channels
3.2.1 Incision Rate of Bedrock Channels
A bedrock channel may be defined as one for which morphology and gradient are directly controlled by
bedrock (Wohl, 1998). A bedrock channel has bedrock exposed along the channel bed or walls for at least
approximately half its length, or has bedrock limits to the magnitude and location of bed scour and bank
erosion during floods. Bedrock exposure along at least half the channel length suggests that alluvium
does not accumulate to a depth at which the active channel is formed entirely in alluvium. The presence
of bedrock as channel boundaries during large discharges will facilitate different patterns of hydraulics,
sediment transport, and channel morphology than those common along alluvial channels. Bedrock-
constrained valley walls may limit floodplain development so that bedrock channels have low-flow and
high-flow portions. A bedrock channel may also have a bedrock surface into which a low-flow inner
channel is incised, thus, having inner channel flow in the dry season and high (flood plain) flow in flood
season (Figure 3.13).
Bedrock channels most commonly occur in regions of high topographic relief. Relief may be a product
of recent tectonic uplift, as in the Himalayan Mountains of central Asia, or the Colorado Plateau of the
southwestern U.S. High topographic relief in a drainage basin tends to produce high stream gradients,
and, thus, the potential for high sediment transport capacity and flow energy per unit of discharge. The
exposed bedrock implies that the channels may be particularly sediment-starved during floods.
Flood flows along channels incised into highly weathered, soft, or thinly bedded or jointed rocks result