Fundamentals of Fluvial Geomorphology and Channel Processes
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 concave bank. This
depositional feature is often a good indicator of the upstream location to start a bank
protection measure.
2.2.3.2 Streambank Erosion and Failure Processes
The terms streambank erosion and streambank failure are often used to describe the
removal of bank material. Erosion generally refers to the hydraulic process where individual
soil particles at the bank's surface are carried away by the tractive force of the flowing water.
The tractive force increases as the water velocity and depth of flow increase. Therefore, the
erosive forces are generally greater at higher flows. Streambank failure differs from erosion
in that a relatively large section of bank fails and slides into the channel. Streambank failure
is often considered to be a geotechnical process. A detailed discussion of the erosion and
failure processes discussed below is provided by Thorne (1993).
Identifying the processes responsible for bank erosion is not an easy task and often
requires some training. The primary erosion processes are parallel flow, impinging flow,
piping, freeze/thaw, sheet erosion, rilling/gullying, wind waves, and vessel forces. These
erosional forces are illustrated in Figures 2.19 through 2.25 and discussed below.
Parallel flow erosion is the detachment and removal of intact grains or aggregates of
grains from the bank face by flow along the bank. Evidence includes: observation of high
flow velocities close to the bank; near-bank scouring of the bed; under-cutting of the
40
Previous Page
