Fundamentals of Fluvial Geomorphology and Channel Processes
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.
and entrained 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).
Freeze/thaw is caused by sub-zero temperatures which promote freezing of the bank
material. Ice wedging cleaves apart blocks of soil. Needle-ice formation loosens and
detaches grains and crumbs at the bank face. Freeze/thaw activity seriously weakens the bank
and increases its erodibility. Evidence includes: periods of below freezing temperatures in
the river valley; a loose, crumbling surface layer of soil on the bank; loosened crumbs
accumulated at the foot of the bank after a frost event; jumbled blocks of loosened bank
material.
Sheet erosion is the removal of a surface layer of soil by non-channelized surface
run-off. It results from surface water draining over the bank edge, especially where the
riparian and bank vegetation has been destroyed by encroachment of human activities.
Evidence includes: surface water drainage down the bank; lack of vegetation cover, fresh
appearance to the soil surface; eroded debris accumulated on the lower bank/toe area.
Rilling and gullying occurs when there is sufficient uncontrolled surface run-off over
the bank to initialize channelized erosion. This is especially likely where flood plain drainage
has been concentrated (often unintentionally) by human activity. Typical locations might be
near buildings and parking lots, stock access points and along stream-side paths. Evidence
includes: a corrugated appearance to the bank surface due to closely spaced rills; larger
gullied channels incised into the bank face; headward erosion of small tributary gullies into
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