Soil FreezeThaw Effects on
Bank Erodibility and Stability
LAWRENCE W. GATTO
highly weakened soil. Thus, the amount of bank
INTRODUC TION
soil lost in the spring can be the maximum for the
The effects of soil freezing and thawing on the
year if spring high water occurs when bank soil is
erodibility (i.e., susceptibility of in-situ soil par-
still weakened after thaw (Slavin 1977, Wolman
ticles to be detached by flowing water) and stabil-
1959).
ity (i.e., ability of soil particles to resist movement
Thus, soil strength variabilities must be in-
by gravitational forces) of bank soils along rivers,
cluded in bank erosion and recession models be-
lakes and reservoirs have been observed by nu-
cause soil frost and its subsequent thawing so
merous investigators. Lawson (1985) and Lawler
drastically change soil strength. Bank erodibility
(1989, 1993) summarized the results of some of
factors should not be lumped as a single value for
these studies. In this report I summarize research
the entire year, especially in regions with seasonal
on how soil freezethaw disrupts soil structure,
soil frost. Lawler (1993) clearly stated that more
displaces soil particles, and temporarily reduces
research into changes in bank erodibility is needed
soil strength, and I systematically assess the sea-
and emphasizes the need to address soil frost
sonal variations in bank soil strength likely to
effects in that research.
result from these freezethaw actions and relate
those variations to observed bank soil erosion
and failures. Lawson (1983) and Gatto (1984) have
BANK SOIL ERODIBILITY
summarized the effects of permafrost on bank
AND STABILITY
erosion and stability; herein I address only the
effects of seasonal frost, although the processes
Many geotechnical, hydraulic and climatic pro-
described in this report are active in permafrost
cesses and conditions (Fig. 1) interact to reshape
areas as well.
soil banks through erosion and mass failures. The
Many studies have emphasized the importance
patterns of these interactions change with time
of hydraulic tractive force in detaching bank sedi-
and location and produce complex effects and
ments, which leads to bank erosion and instabil-
feedbacks on surface and within subsurface bank
ity. In fact, the hydraulic aspects of detachment
soils (Table 1), resulting in highly variable rates
and sediment transport have generally received
and scales of bank erosion and failure within the
more attention than the bank soil resistance (erod-
same and amongst different locales (Gatto 1987,
ibility/stability) aspects in bank erosion and river
Lawson 1985).
migration prediction models. And yet, in some
These interrelationships are illustrated in Table
instances, bank erosion results not from excessive
1 as follows: if one selects the first process or
hydraulic forces primarily but from very weak
condition in column A (excessive soil pore water),
soils.
there are at least five causes (column B) for that
Bank soils can be highly erodible and unstable
condition. The excessive pore water reduces
during spring thaw due to excessive pore water
granular interlocking and soil cohesion, lowering
and disrupted soil structure. Concurrently, spring
soil strength and stability in bank surface and
subsurface soils (x in columns C and D, respec-
snowmelt and rain on snow often produce max-
imum annual water levels and flow velocities.
tively). This reduction can make the surface soils
These flows and the moving ice often associated
more susceptible to soil water piping (2), ice abra-
with them can easily detach and transport this
sion (5), ice push (6), wave actions (9), water cur-
Previous Page
