Fundamentals of Fluvial Geomorphology and Channel Processes
Figure 3.30 Erosion Generated by Vessel Forces
Freeze/thaw is caused by sub-zero temperatures which promote freezing of the bank material.
Ice wedging cleaves apart blocks of soil. Needle-ice formation loosens and detaches grains and crumbs
at the bank face. Freeze/thaw activity seriously weakens the bank and increases its erodibility. Evidence
includes: periods of below freezing temperatures in the river valley; a loose, crumbling surface layer of soil
on the bank; loosened crumbs accumulated at the foot of the bank after a frost event; jumbled blocks of
loosened bank material.
Sheet erosion is the removal of a surface layer of soil by non-channelized surface run-off. It
results from surface water draining over the bank edge, especially where the riparian and bank vegetation
has been destroyed by encroachment of human activities. Evidence includes: surface water drainage down
the bank; lack of vegetation cover, fresh appearance to the soil surface; eroded debris accumulated on the
lower bank/toe area.
Rilling and gullying occurs when there is sufficient uncontrolled surface run-off over the bank to
initialize channelized erosion. This is especially likely where flood plain drainage has been concentrated
(often unintentionally) by human activity. Typical locations might be near buildings and parking lots, stock
access points and along stream-side paths. Evidence includes: a corrugated appearance to the bank surface
due to closely spaced rills; larger gullied channels incised into the bank face; headward erosion of small
tributary gullies into the flood plain surface; and eroded material accumulated on the lower bank/toe in the
form of alluvial cones and fans.