jamming and thus is of interest when estimating
Ice cover breakup and movement
ice jam volume. Ice loss during breakup and ice
Well-documented observations of ice cover
cover movement can be described using an ice loss
breakup, movement, and jamming are provided
by Beltaos (1981) for the 1980 Thames River (New
coefficient, cl:
Brunswick) breakup. The candled, relatively weak
ice cover was observed to lift, crack, and move
Vt - Vj
cl =
downstream at a rate of about 0.6 to 0.9 m/s.
(29)
Vt
Downstream movement averaged about 0.8 m/s,
with some consolidation noted at 0.3 m/s. Once
where Vt is the total upstream ice supply and Vj is
jammed, the jam progressed upstream at about 0.9
the volume of ice contained in the jam.
m/s. The velocity of two different jam releases (in
The ice loss coefficient is expected to be low for
different places) was also about 0.9 m/s. In 1981,
shallow, steep rivers with short reaches of contrib-
the ice cover was observed to lift and crack, but
uting ice cover and higher for more gently slop-
not break, for water level rises of 1 to 2 cm/hr, 3 to
ing rivers with longer reaches of contributing ice
5 cm/hr, 5.5 cm/hr, 7 to 9 cm/hr, and 8 cm/hr at
cover, many tributaries, and/or low banks
different times (Beltaos 1983). Surface movement
(USACE 1982). Using a porosity of 0.40, Prowse
of broken ice was observed at velocities of up to
(1986) estimated that loss coefficients ranged from
1.2 m/s. The velocity of an ice run caused by the
0.90 to 0.93 for ice jam volume and ice supply data
failure of an upstream jam was about 1.7 m/s. The
collected by Calkins (1978) on the relatively short,
same velocity was reported by Hicks et al. (1997)
steep White River (Vermont). Based on data from
for an ice run caused by the failure of a 1993 ice
Beltaos (1983) and assuming a porosity of 0.40,
jam on the St. John River, Maine and New
Prowse (1986) estimated a loss coefficient of 0.81
Brunswick.
for the Liard River mainstem (British Columbia
Andres and Doyle (1984) reported average sus-
and Northwest Territories) during the breakup of
tained velocity of the ice cover breakup front on
1983, but he also reported an ice loss of about 40%
the Athabasca River (Alberta) to be 3.7 m/s and
in one day as ice melted, became stranded, or was
3.3 m/s during 1978 and 1979, respectively. Higher
broken into small enough pieces to be transported
values were observed for short durations: 5.5 m/
beneath the jam.
s in 1977 and 1979, and 5.0 m/s in 1978. Observa-
tions of the speed of the peak discharge and the
Shear strength
ice front following the failure of a 1995 breakup
Beltaos and Dean (1981) measured shear
jam on the Porcupine River, Yukon, were reported
strength within a thick (up to 16 m) frazil deposit
by Jasek (1997). He noted that the peak discharge
on the Smoky River, Alberta, using an experimen-
arrived faster (5.4 m/s) than the ice front (4.2 m/
tal shear vane. They found that shear strength
s). Ferrick et al. (1996) used a Doppler radar sys-
varied from year to year and also that it tends to
tem to measure the movement of a breakup ice
increase with height above the bottom of the ac-
run (March 1993) and a frazil ice run (December
cumulation. Reported values ranged up to about
1993) on the Connecticut River, New Hampshire.
80 kPa. According to Dean (1986), the shear
Velocities during the breakup run varied from 1
strength of loose, freshly deposited frazil is on the
m/s to 2.5 m/s with a mean velocity of 1.35 m/s.
order of 10 Pa, while an undisturbed, undegraded
The frazil run was slower, ranging from 0.5 m/s
frazil accumulation will have a shear strength of
to 0.65 m/s with a mean of 0.54 m/s, which com-
about 30 to 60 kPa. Compacted frazil accumula-
pared well with estimates made from video record-
tions are said to attain a shear strength of about 75
ings that averaged 0.52 m/s.
kPa. Thermal degradation will reduce shear
strength in frazil deposits to between 1 and 10 kPa.
Coefficient of ice loss (cl)
Solid ice covers have much higher levels of
The breakup and movement of an ice cover of-
shear strength than frazil deposits. Based on ob-
ten results in the stranding of ice along the banks
servations of the speed of the moving ice front for
in the form of shear walls or ice that was carried
breakup ice jams on the Athabasca River (Alberta)
overbank and deposited in the floodplain. It is also
during 1977, 1978, and 1979, Andres and Doyle
likely that warm temperatures during breakup
(1984) estimated the strength of the ice cover at
may melt ice pieces travelling relatively long dis-
between 310 and 650 kPa for 1-m nominal ice thick-
tances. Ice loss due to stranding and melting can
ness. They reported that these values were reason-
decrease the ice supply available for downstream
21
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