RIVER ICE INFLUENCES ON FORT PECK REACH, MISSOURI RIVER
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Thermal influences on riverbank strength
It is well known that the freezing and thawing of soil affect the erosion of
riverbanks adjoining rivers and lakes. Lawson (1983, 1985) and Gatto (1988,
1995), among others, provided extensive reviews of the subject. In short, because
frozen soil is more resistant to erosion than is unfrozen soil, riverbanks are less
erodible while frozen. Freezing and thawing, however, usually weaken soils,
making thawed (or thawing) riverbanks more susceptible to erosion. The net
consequence on the overall rate of riverbank erosion, therefore, remains a matter
of debate but most likely varies regionally and from site to site.
Freezethaw cycles affect the soil structure, porosity, permeability, and den-
sity. These changes in soil properties can substantially reduce soil shear strength
and bearing capacity; strength reductions can be as much as 95% (Andersland
and Anderson 1990). Such adverse effects on soil strength depend on the soil
particle size and gradation, the moisture content, the number and duration of
freezethaw cycles, and several other factors. Though there is no single, standard
test to determine whether a soil is prone to significant weakening due to freeze
thaw (Chamberlain 1981), particle size is commonly used to indicate a soil's
sensitivity to freezethaw weakening. Soils containing fine sands and silts are
especially sensitive because they are permeable and susceptible to change in soil
structure. By virtue of their particle size (about 0.10.06 mm) and the surface-
tension property of water, fine sandy and silty soils absorb moisture more readily
than do coarser or fine sediments. Clayey soils are less sensitive because of their
low permeability. The variability of soil properties along a riverbank and within a
specific riverbank location causes the effects of riverbank freezing to differ along
a reach.
Gatto (1995) suggested that eroding riverbanks are especially subject to deep
penetration of freezing, making more of the riverbank prone to freezethaw
weakening and erosion. The absent or stunted vegetation on many eroding river-
banks results in diminished insulation of the riverbank and increased heat loss to
air. The heat loss is greatest at the crest of a riverbank because it is exposed to air
on at least two surfaces. It is also more exposed to wind, so it may accumulate
less snow, which means deeper frost penetration during the winter and faster
thaw in the spring. However, if the top portion of the riverbank and upland were
dry, the riverbank crest might be the zone of least heat loss, as the distance
between the air and the water table is greatest there.
Spring is when the riverbank is most susceptible to erosion. As the upper
zone of frozen ground thaws, meltwater drains down over the surface of the still
frozen ground. The riverbank, weakened by thaw expansion of the ground and
subject to seepage pressures, is at its least stable annual condition.
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