rent detachment and transport (10), soil failures
served that more than 90% of the total erosion on
(14), raindrop detachment (15), overland sheet
the River Lagan in Northern Ireland occurred dur-
flow (16), rill and gully flow (17), and snow slid-
ing the winter due to needle ice formation and
ing (18). Subsurface soils would be more suscep-
soil heaving, and that after needle ice on a bank
tible to soil failures (14) as a result of the reduced
face melts, a readily erodible skin of loose sedi-
soil strength.
ment remains covering the bank.
Looking across the remaining columns (EG)
Andersland and Anderson (1978) detailed the
in the table, one sees that this type of cause and
complex thermodynamic processes involved in
effect interrelationship between excess pore wa-
soil freezing and thawing. I am unaware, how-
ter and other processes and conditions can occur
ever, of research that addresses these processes
in all climatic zones. The rest of Table 1 illustrates
specifically in banks. Therefore, the following de-
the numerous additional interactions between pro-
scriptions of the sequence of soil freezing and
cesses and conditions and their effects.
thawing in bank soils (Table 2) are drawn from
In the context of this report, a bank is "stable"
research on 1) the processes of freezing and thaw-
when its soils have sufficient strength to resist
ing and resulting soil instabilities in soils below
many of the forces applied by the reshaping pro-
pavements and roads, under and around struc-
cesses and conditions (column A) listed in Table
tures, and in man-made embankments, and on 2)
1. However, even stable banks change shape as
the erodibility of soils in agricultural fields.
some of their soils are displaced or removed, al-
The reader should bear in mind the follow-
beit slowly, resulting in the gradual, almost unno-
ing additional complexities in the soil freezing
ticeable, landward movement (recession) of the
and thawing processes of river and lake bank
bank crest. We are concerned herein with banks
soils: 1) the effects of water level fluctuations,
that lose sediment sufficiently fast that bank re-
which often cause the soil to be alternately ex-
cession is noticeable each year.
posed and inundated, 2) the presence of unfro-
zen soils below the water line, 3) the more ef-
ficient heat loss from bank soils in contact with
moving water, and 4) the heat gained by frozen
SEASONAL CONDITIONS
soils when they are inundated during high wa-
AND PROCESSES
ter periods.
Such factors as soil structure, water content
and bulk density, grain size distribution, shape
Fall
and mineralogy, the degrees of soilgrain inter-
locking, grain cementation and chemical weath-
Soil freezethaw cycles
ering, the particle-bonding mechanisms in clay
As the air temperature fluctuates below and
above 32F in the early fall, some portion of the
soils, and the presence of vegetation determine
soil strength (Gatto 1988). Consequently, processes
bank surface soils cycle between being frozen and
that affect these soil characteristics may reduce
unfrozen. The soil freezes from its surface to some
soil strength (Ogata et al. 1985) and make soils
depth, which is determined by the amount of
geotechnically unstable or more susceptible to re-
heat lost from the soil during the last freezing
moval by water or wind forces. Soil freezethaw
period, and which in turn depends on the sever-
cycles (FTC) usually change soil structure, water
ity of the weather, i.e., air temperature, wind speed,
content and bulk density, and degree of grain
solar insolation. When soil heat loss has progressed
interlocking, thereby reducing soil strength, at
sufficiently, the void or adsorbed soil water will
least temporarily.
freeze and expand in volume by 9% (Jumikis 1962).
Reid (1984, 1985) reports that thaw failures re-
The ice can displace soil particles and separate
sulting from a loss of soil strength when frozen
soil aggregates, often disrupting the interlocking
soils thawed constituted up to nearly 90% of the
of soil grains and changing the soil structure, void
total sediment lost from banks along Orwell Res-
ratio, density, soil fabric, saturated water-holding
ervoir in Minnesota. Slab slide failures and mud-
capacity, and hydraulic conductivity, resulting in
flows that occur during spring thaw account for
decreased soil cohesion and mechanical strength
up to 90% of the soil lost from bluffs along
(Aoyama et al. 1985, Benoit and Voorhees 1990,
Wisconsin's Great Lakes shorelines, while sheet-
Chamberlain and Gow 1979, Frydman et al. 1979,
wash and rill erosion cause up to 50% of the soil
Gifford 1984, Kim and Daniel 1992, Mostaghimi
lost per year (Sterrett 1980). Gardiner (1983) ob-
et al. 1988, Thorne 1982).
4
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