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ERDC/CRREL TR-02-14
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REVIEW OF RIVER ICE INFLUENCES ON RIVERBANKS
River ice may influence channel bathymetry through several processes that
affect channel riverbanks:
Reduction of riverbank strength by increasing pore water pressure or by
producing rapid drawdown of the riverbank water table during dynamic
ice cover or ice jam breakup;
Thermal weakening of riverbank soils;
Tearing and dislodging of riverbank material and vegetation during the
collapse of bankfast ice; and
Gouging and abrasion of riverbank material and vegetation during an ice
run.
These influences reduce the riverbank resistance to scour, increase the local
supply of sediment, and promote lateral shifting of the channel. The first two
influences are not well studied. The third and fourth have received a little atten-
tion, but the extents to which they affect channel morphology are unclear.
Influence on soil pore pressures
Flow stage and stage fluctuations influence seepage pressures and the freeze
thaw behavior of channel riverbanks. A higher flow stage raises the water table in
a riverbank. A rapid drop in flow stage may quickly reduce the riverbank stability
by increasing seepage pressures, thereby reducing the shearing resistance of the
material comprising the riverbank. Ice cover formation raises the flow stage,
whereas cover breakup may abruptly lower it. River ice formation, therefore, can
weaken riverbanks.
Riverbank freezing is closely linked to bankfast ice formation along a chan-
nel. The details of the relationship between them are unclear, however. They
depend on the condition of the riverbank (material, vegetation, snow, etc.), the
relative elevations of the water table and the flow stage, and the temperatures of
the groundwater and river water. The strength of the attachment of bankfast ice to
a riverbank depends on the relative elevations of the water table and flow stage
and on the water temperatures. A flow of relatively warm groundwater (i.e.,
several degrees above the freezing temperature) into a river will retard border ice
growth and weaken its hold on the riverbank. The growth of a thick fringe of
bankfast ice, on the other hand, may affect seepage flow through the riverbank,
possibly constricting it and slightly raising the water table. This is especially
significant for regulated rivers, for which flows do not diminish during winter.
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