Fundamentals of Fluvial Geomorphology and Channel Processes
the flood plain surface; and eroded material accumulated on the lower bank/toe in the form
of alluvial cones and fans.
Wind waves cause velocity and shear stresses to increase and generate rapid water
level fluctuations at the bank. They cause measurable erosion only on large rivers with long
fetches which allow the build up of significant waves. Evidence includes: a large channel
width or a long, straight channel with an acute angle between eroding bank and longstream
direction; a wave-cut notch just above normal low water plane; a wave-cut platform or run-up
beach around normal low-water plane. Note that it is easy to mistake the notch and platform
produced by piping and sapping for one cut by wave action (Hagerty, 1991a,b).
Vessel Forces can generate bank erosion in a number of ways. The most obvious way
is through the generation of surface waves at the bow and stern which run up against the bank
in a similar fashion to wind waves. In the case of large vessels and/or high speeds these
waves may be very damaging. If the size of the vessel is large compared to the dimensions
of the channel hydrodynamic effects produce surges and drawdown in the flow. These rapid
changes in water level can loosen and erode material on the banks through generating rapid
pore water pressure fluctuations. If the vessels are relatively close to the bank, propeller wash
can erode material and re-suspend sediments on the bank below the water surface. Finally,
mooring vessels along the bank may involve mechanical damage by the hull. Evidence
includes: use of river for navigation; large vessels moving close to the bank; high speeds and
observation of significant vessel-induced waves and surges; a wave-cut notch just above the
normal low-water plane; a wave-cut platform or "spending" beach around normal low-water
plane. Note that it is easy to mistake the notch and platform produced by piping and sapping
for one cut by vessel forces (Hagerty, 1991a,b).
Ice rafting erodes the banks through mechanical damage to the banks due to the
impact of ice-masses floating in the river and due to surcharging by ice cantilevers during
spring thaw. Evidence includes: severe winters with river prone to icing over; gouges and
disruption to the bank line; toppling and cantilever failures of bank-attached ice masses during
spring break-up.
Other erosion processes (trampling by stock, damage by fishermen, etc.) could be
significant but it is impossible to list them all.
Serious bank retreat often involves geotechnical bank failures as well as direct erosion
by the flow. Such failures are often referred to as "bank sloughing" or "caving," but these
terms are poorly defined and their use is to be discouraged. Examples of different modes of
geotechnical stream bank failure include soil fall, rotational slip, slab failure, cantilever failure,
pop-out failure, piping, dry granular flow, wet earth flow, and other failure modes such as
cattle trampling (Figures 2.26 through 2.34). Each of these is discussed below.
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