Fundamentals of Fluvial Geomorphology and Channel Processes
3.4.3.1 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 migration rates in a variety of streams, and found that the
erosion rate of meanders increases as the radius of curvature to width ratio (r/w) decreased below a value
of about 6, and reached a maximum in the r/w range of 2 to 3. Biedenharn et al. (1989) studied the effects
of r/w and bank material on the erosion rates of 160 bends along the Red River in Louisiana and also
found that the maximum erosion rates were observed in the r/w range of 2 to 3. However, the considerable
scatter in their data indicate that other factors, particularly bank material composition, were also modifying
the meander process.
The severity and location of bank erosion also changes with stage. At low flows, the main thread
of current tends to follow the concave bank alignment. However, as flow increases, the flow tends to cut
across the convex bar to be concentrated against the concave bank below the apex of the bend. Friedkin
(1945) documented this process in a series of laboratory tests on meandering in alluvial rivers. Because
of this process, meanders tend to move in the downvalley direction, and the zone of maximum erosion is
usually in the downstream portion of the bend due to the flow impingement at the higher flows. This explains
why the protection of the downstream portion of the bend is so important in any bank stabilization scheme.
The material eroded from the outer bank is transported downstream and is generally deposited in the next
crossing or point bar. This process also results in the deposition of sediment along the upper portion of the
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