Mountain Rivers and Incised Channels - River Dynamics and Integrated River Management - page 122

Channel incision may cause many problems in navigation. Lowering of local ground water tables as a
result of incision can also have far-reaching effects on floodplain and wetland flora and fauna.
(a) (b)
(c) (d)
Fig. 3.1 (a) Rills developed on slope by erosion on the bank slope of the Yalutsangbu River; (b) A gully developed
from rills in the Yongding River basin; (c) A stream in the upper Yangtze River basin in Sichuan Province developing
into an entrenched stream in a process of channel incision; (d) The Grand Canyon of the Colorado River, U.S. is a
composite incised channel (See color figure at the end of this topic)
Incised channel systems pose particular challenges to engineers, managers, and planners because they
are extremely dynamic. Since incised channels convey flows that are even more erosive than the same
flow within a non-incised channel, river crossings and other in-stream structures must be able to cope
with the maximum amount of morphological change that is likely to occur during the design life of the
structure. It is also essential to investigate the effects that a structure is likely to have on channel processes.
For example, various types of grade-control structures have been successfully used to arrest the upstream
propagation of knickpoints and ensuing degradation. It has been found, however, that if the structure
ponds water as a dam, resulting in sediment deposition upstream from the structure, a new wave of
degradation is induced by clear-water flows downstream (Simon and Darby, 1997c). In alluvial reaches
that are actively widening, maintenance of cross-section shapes at bridges is often counterproductive.
The narrower section at the bridge can cause backwater effects during high flows, resulting in a hydraulic
drop through the bridge opening and the development of enormous scour holes downstream. These scour