Fundamentals of Fluvial Geomorphology and Channel Processes
Evidence includes: composite or layered bank stratigraphy; cohesive layer underlain by less resistant layer;
under-mining; overhanging bank blocks; failed blocks on the lower bank and at the bank toe.
Pop-out failure results from saturation and strong seepage in the lower half of a steep, cohesive
bank. A slab of material in the lower half of the steep bank face falls out, leaving an alcove-shaped
cavity. The over-hanging roof of the alcove subsequently collapses as a cantilever failure. Evidence
includes: cohesive bank materials; steep bank face with seepage area low in the bank; alcove shaped
cavities in bank face.
rates of flow. Such failures are an extension of the piping erosion process described previously, to the point
that there is complete loss of strength in the seepage layer. Sections of bank disintegrate and are entrained
by the seepage flow (sapping). They may be transported away from the bank face by surface run-off
generated by the seepage, if there is sufficient volume of flow. 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; run-out deposits of eroded material on the lower bank or beach. Note that
the effects of piping failure can easily be mistaken for those of wave and vessel force erosion.
Dry granular flow describes the flow-type failure of a dry, granular bank material. Other terms
for the same mode of failure are ravelling and soil avalanche. Such failures occur when a noncohesive bank
at close to the angle of repose is undercut, increasing the local bank angle above the friction angle. A
carpet of grains rolls, slides and bounces down the bank in a layer up to a few grains thick. Evidence
includes: noncohesive bank materials; bank angle close to the angle of repose; undercutting; toe
accumulation of loose grains in cones and fans.
Wet earth flow failure is the loss of strength of a section of bank due to saturation. Such failures
occur when water-logging of the bank increases its weight and decreases its strength to the point that the
soil flows as a highly viscous liquid. This may occur following heavy and prolonged precipitation,
snow-melt or rapid drawdown in the channel. Evidence includes: sections of bank which have failed at very
low angles; areas of formerly flowing soil that have been preserved when the soil dried out; basal
accumulations of soil showing delta-like patterns and structures.
Other failure modes could be significant, but it is impossible to list them all. Cattle trampling is just
one example of a common failure mode.
3.5
CLOSING
In planning a project along a river or stream, awareness of even the fundamentals of
geomorphology and channel processes allows you to begin to see the relationship between form and
process in the landscape. Go into the field and take notes, sketches, pictures - and above all, observe
carefully, think about what you are seeing, and use this information to infer the morphological status of the
river. When you are in the field, look at your surroundings and try to establish a connection between what
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