reducing the possibility of ice jam flooding
Wh
Wilder Dam
ite
Riv
when the ice finally does release. It may be
e
r
possible to operate a series of river dams to
Gaging Station
encourage a downstream-to-upstream
km 65
Mascoma River
breakup progression and reduce ice jam
potential. Flow regulation for breakup con-
trol has potential benefits at river con-
fluences. One strategy is to delay breakup
km 60
on one stem of a river until the other branch
is ice-free. The following example illus-
trates the opposite approach: regulating
km 55
flow to cause a controlled breakup on the
NEW
main stem of a river to reduce ice jam sever-
HAMPSHIRE
ity when a tributary releases its ice.
Ferrick and Mulherin (1989) developed
VERMONT
a one-dimensional model to simulate
km 50
dynamic ice breakup on rivers. The model
was used to assess the feasibility of regu-
lating flow to control breakup on the
Connecticut River in New Hampshire.
km 45
Figure 12 shows the study reach. The
Windsor
most destructive breakup ice jams on the
Connecticut occur near Windsor, Vermont,
Study
after the uncontrolled White River releases
Reach
km 40
its ice, breaking up the ice cover on the
main stem. The authors theorized that ice
jam severity could be decreased by initiat-
ing an early breakup on the Connecticut
River through planned releases at three
0
2
4
6 km
km 35
dams. Following calibration to an observed
Sugar River
ice-breakup event, the model predicted the
Figure 12. Connecticut River below Wilder Dam.
(From
occurrence and non-occurrence of breakup,
Ferrick and Mulherin 1989.)
as well as the length of the broken ice cover
for a variety of input flow hydrographs and
a range of initial ice thickness and ice
authors used an equilibrium ice jam model to pre-
strength values. The method was validated
dict that lowering the pool during breakup would
through a series of field experiments done on the
cause a slight increase in ice thickness at a refer-
Connecticut River in the early 1990s.*
ence location 3.2 km downstream of Fort Fairfield.
Based on this result and the frazil deposition
Passing ice at structures
analysis, the study concluded that lowering the
during extreme events
winter pool at Tinker Dam would not reduce the
During ice breakup, operators of river dams
ice jam flood risk at Fort Fairfield.
may have to pass the ice and floodwave as it
arrives to avoid a number of problems. These
Controlling timing of breakup
include ice grounding in the pool and upstream
In some cases, flow regulation can influence the
ice jam flooding, thick ice buildups against dam
timing of the final breakup on rivers. Dynamic
gates preventing their operation, and ice block-
and destructive breakups result when rapid run-
ages of lock approaches. Also, if the dam retains
off enters rivers with competent ice covers in late
large quantities of ice, it may be difficult to pass
winter or early spring. Controlling outflow from
this ice once the peak of the water wave has
a dam or system of dams can dampen the
moved downstream.
hydrograph in downstream reaches, delaying or
reducing the severity of breakup. This delay may
* Personal communication with Michael Ferrick, CRREL, Sep-
give the ice a chance to weaken and melt in place,
tember 1998.
17
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