Fundamentals of Fluvial Geomorphology and Channel Processes
2.2.3 LOCAL INSTABILITY
For this discussion local instability refers to bank erosion that is not symptomatic of
a dis-equilibrium condition in the watershed (i.e., system instability) but results from site-
specific factors and processes. Perhaps the most common form of local instability is bank
erosion along the concave bank in a meander bend which is occurring as part of the natural
meander process. Local instability does not imply that bank erosion in a channel system is
occurring at only one location or that the consequences of this erosion are minimal. As
discussed earlier, erosion can occur along the banks of a river in dynamic equilibrium. In these
instances the local erosion problems are amenable to local protection works such as bank
stabilization measures. However, local instability can also exist in channels where severe
system instability exists. In these situations the local erosion problems will probably be
accelerated due to the system instability, and a more comprehensive treatment plan will be
necessary.
2.2.3.1 Overview of Meander Bend Erosion
Depending upon the academic training of the individual, streambank erosion may be
considered as either a hydraulic or a geotechnical process. However, in most instances the
bank retreat is the result of the combination of both hydraulic and geotechnical processes.
The material may be removed grain by grain if the banks are non-cohesive (sands and
gravels), or in aggregates (large clumps) if the banks are composed of more cohesive material
(silts and clays). This erosion of the bed and bank material increases the height and angle of
the streambank which increases the susceptibility of the banks to mass failure under gravity.
Once mass failure occurs, the bank material will come to rest along the bank toe. The failed
bank material may be in the form of a completely disaggregated slough deposit or as an
almost intact block, depending upon the type of bank material, the degree of root binding, and
the type of failure (Thorne, 1982). If the failed material is not removed by subsequent flows,
then it may increase the stability of the bank by forming a buttress at the bank toe. This may
be thought of as a natural form of toe protection, particularly if vegetation becomes
established. However, if this material is removed by the flow, then the stability of the banks
will be again reduced and the failure process may be repeated.
As noted above, erosion in meander bends is probably the most common process
responsible for local bank retreat and, consequently, is the most frequent reason for initiating
a bank stabilization program. A key element in stabilization of an eroding meander bend is
an understanding of the location and severity of erosion in the bend, both of which will vary
with stage and plan form geometry.
As streamflow moves through a bend, the velocity (and tractive force) along the outer
bank increases. In some cases, the tractive force may be twice that in a straight reach just
upstream or downstream of the bend. Consequently, erosion in bends is generally much
greater than in straighter reaches. The tractive force is also greater in tight bends than in
longer radius bends. This was confirmed by Nanson and Hickin (1986) who studied the
39
Previous Page
