Fundamentals of Fluvial Geomorphology and Channel Processes
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 3.31 through 3.39). Each of these is discussed
below.
Soil/rock fall occurs only on a steep bank where grains, grain assemblages or blocks fall into the
channel. Such failures are found on steep, eroding banks of low operational cohesion. Soil and rock falls
often occur when a stream undercuts the toe of a sand, gravel or deeply weathered rock bank. Evidence
includes: very steep banks; debris falling into the channel; failure masses broken into small blocks; no
rotation or sliding failures.
Shallow slide is a shallow seated failure along a plane somewhat parallel to the ground surface. Such
failures are common on banks of low cohesion. Shallow slides often occur as
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