could include ice retention to prevent or control
methods that mechanically weaken ice and those
breakup, flow containment or diversion to reduce
that speed ice deterioration.
flood damage, and ice weakening to reduce jam-
ming potential.
Mechanical weakening
Ice weakening has been accomplished by us-
ing mechanical cutting techniques on rivers such
Nonstructural techniques
The most common location for ice jams to form
as the Rideau in Ottawa, Ontario, and the Beau-
is in an area where the slope of the river changes
rivage near Quebec City, Quebec, among others.
from a steeper to a milder slope. Since gravity is
Recently the Finnish National Board of Waters
the driving force for an ice run, when the ice
and Environment, through its District Office in
reaches the milder slope it loses its impetus and
the town of Kokkola, has been developing ice-
stalls against a more stable downstream ice cover.
cutting equipment for the purpose of mechanical
Another common location for ice jam formation
ice weakening.* The prototype device has a cut-
is the point where a tributary stream enters a larg-
ting wheel 2.5 m (8.2 ft) in diameter and 0.2 m
er river. Smaller rivers normally respond to in-
(0.66 ft) wide. It is towed behind a tracked, am-
creasing runoff more quickly than large rivers.
phibious vehicle. In 30-cm- (1-ft-) thick ice it can
The ice cover on a smaller river will typically
cut ice at a speed of 4 km/hr (2.5 mph). With 60-
break up and run until it reaches the strong, intact
cm ice the speed is reduced to about 2 km/hr
ice cover on the larger river, where the slope is
(1.25 mph), and with 120-cm ice the speed is ap-
normally milder as well. The ice run stalls at the
proximately 1 km/hr (0.6 mph). Once testing is
confluence, forming a jam and backing up water
complete, they plan to have these ice cutters man-
on the tributary stream. Riverbends are also fre-
ufactured and marketed commercially.
quently cited as ice jam instigators. While river-
As reported in Deugo (1973), ice cutting is an
bends may contribute to jamming by forcing the
annual activity on the Rideau River in Ottawa,
moving ice to change its direction and by causing
Ontario. A 7.5-mile reach above the confluence
the ice to impact the outer shoreline, slope is
with the Ottawa River is cleared of ice during a
again a factor. Riverbends are nature's way of
two-week period beginning in late February in
controlling river slope, with a straight reach be-
anticipation of the spring freshet. Ten thousand
tween two points being the steepest possible
lineal feet of ice are cut in critical locations during
course, and the slope decreasing as the bend se-
this time using a mechanical saw. They are able to
verity increases.
clear 7.5 miles of river with only 2 miles of actual
On the Missouri River in the BufordTrenton
cutting. The unit is a circular saw powered by a
area, all three of these ice jam instigators are
60-hp engine and is capable of cutting to a depth
present. At the lower end of the District as the
of 2 ft. The ice cutter is used to cut slots parallel to
Missouri enters Lake Sakakawea, the energy
the water flow approximately 50 ft from each
slope decreases as the river enters the still waters
shoreline. Once cutting is complete and spring
of the lake. Near the upper end of the District, the
runoff approaches, explosives are used to dis-
Yellowstone River typically breaks up several
lodge the ice cover throughout the reach, begin-
weeks prior to the Missouri River, and the first ice
ning at the downstream end. The combined costs
jam in the vicinity of the District is often in the
of the cutting and blasting program in the City of
confluence area of the two rivers. Between these
Ottawa is approximately 0,000 in 1985 Cana-
two locations the District is separated by a series
dian dollars.
of riverbends, two of which are common jam lo-
In relatively straight reaches of a river, simply
cations (Ryder and Hurley bends).
cutting slots down both sides of the river to re-
Since jamming in each of these areas arises
lease the ice cover from the riverbanks may be
when the ice run loses a portion of its driving
sufficient, while in curved reaches more elaborate
force due to a reduction in slope, the situation
cutting patterns may be required. Jolicoeur et al.
might be improved either by increasing the driv-
(1984) reported on tests of ice trenching per-
ing forces or by reducing the resisting forces. First
formed in an effort to find a cheaper, safer and
we will look at means of reducing the resisting
more environmentally acceptable alternative to
forces by weakening the ice cover. Weakening the
the explosive demolition of ice. In those tests a
ice cover has been employed in numerous areas
series of five cutting patterns were used to allow
to reduce flood potential or to limit its severity.
Methods of ice weakening can be grouped into
* Personal communication, M. Ferrick, CRREL.
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