Fundamentals of Fluvial Geomorphology and Channel Processes
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 secondary failures following rotational slips and/or slab failures. Evidence includes:
weakly cohesive bank materials; thin slide layers relative to their area; planar failure surface;
no rotation or toppling of failure mass.
Rotational slip is the most widely recognized type of mass failure mode. A deep
seated failure along a curved surface results in back-tilting of the failed mass toward the bank.
Such failures are common in high, strongly cohesive banks with slope angles below about 60o.
Evidence includes: banks formed in cohesive soils; high, but not especially steep, banks; deep
seated, curved failure scars; back-tilting of the top of failure blocks towards intact bank;
arcuate shape to intact bank line behind failure mass.
Slab-type block failure is sliding and forward toppling of a deep seated mass into the
channel. Often there are deep tension cracks in the bank behind the failure block. Slab
failures occur in cohesive banks with steep bank angles greater than about 60o. Such banks
are often the result of toe scour and under-cutting of the bank by parallel and impinging flow
erosion. Evidence includes: cohesive bank materials; steep bank angles; deep seated failure
surface with a planar lower slope and nearly vertical upper slope; deep tension cracks behind
the bank-line; forward tilting of failure mass into channel; planar shape to intact bank-line
behind failure mass.
Cantilever failure is the collapse of an overhanging block into the channel. Such
failures occur in composite and layered banks where a strongly cohesive layer is underlain by
a less resistant one. Under-mining by flow erosion, piping, wave action and/or pop-out failure
leaves an overhang which collapses by a beam, shear or tensile failure. Often the upper layer
is held together by plant roots. 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.
Piping failure is the collapse of part of the bank due to high groundwater seepage
pressures and 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.