Fundamentals of Fluvial Geomorphology and Channel Processes
concave bank. This depositional feature is often a good indicator of the upstream location to start a bank
protection measure.
3.4.3.2 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
3.24 through 3.30 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 toe/lower bank relative to the bank top;
a fresh, ragged appearance to the bank face; absence of surficial bank vegetation.
Impinging flow erosion is detachment and removal of grains or aggregates of grains by flow
attacking the bank at a steep angle to the long-stream direction. Impinging flow occurs in braided channels
where braid-bars direct the flow strongly against the bank, in tight meander bends where the radius of
curvature of the outer bank is less than that of the channel centerline, and at other locations where an
in-stream obstruction deflects and disrupts the orderly flow of water. Evidence includes: observation of
high flow velocities approaching the bank at an acute angle to the bank; braid or other bars directing the
flow towards the bank; tight meander bends; strong eddying adjacent to the bank; near-bank scouring of
the bed; under-cutting of the toe/lower bank relative to the bank top; a fresh, ragged appearance to the
bank face; absence of surficial bank vegetation.
by the seepage flow (also termed sapping) and may be transported away from the bank face by surface
run-off generated by the seepage, if there is sufficient volume of flow. Piping is especially likely in high
banks or banks backed by the valley side, a terrace, or some other high ground. In these locations the high
head of water can cause large seepage pressures to occur. Evidence includes: pronounced seep lines,
especially along sand layers or lenses in the bank; pipe shaped cavities in the bank; notches in the bank
associated with seepage zones and layers; run-out deposits of eroded material on the lower bank. Note
that the effects of piping erosion can easily be mistaken for those of wave and vessel force erosion
(Hagerty, 1991a,b).
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