Fundamentals of Fluvial Geomorphology and Channel Processes
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
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.
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 you see (form) and why it is there (process).
Then you will begin to have some understanding and can perhaps begin to predict what sort
of changes may result if your project alters the flow patterns. Then you are beginning to think
like a geomorphologist. Dr. Einstein (1972) said in the closing comments of his retirement
It is in the field where we can find out whether our ideas are applicable,
where we can find out what the various conditions are that we have to deal
with, and where we can also find out what the desired improvements are.