Clark Fork River: Montana, 1996
production. A less common, but equally effective,
In February of 1996, a sudden thaw caused
use of early winter flow regulation is to speed ice
rapid runoff and widespread breakup of the
cover formation to reduce frazil production,
region's rivers. Although opinions varied, local
which helps mitigate ice jam flooding. Although
people estimated the recurrence interval for the
research has been reported, flow control has prob-
event at 20 to 40 years. At least two dams were
ably not been used to create ice covers on major
threatened on the Clark Fork River. On 8 Febru-
U.S. rivers to facilitate navigation. A probable rea-
ary, the operators of the Milltown Dam above
son is that nearly all tributary reservoirs in the
Missoula, Montana, prepared for a massive
U.S. are operated primarily for flood control and
breakup ice run that had been clocked at 16 km/hr
few are currently authorized to alter their release
moving down the Big Blackfoot River, a tributary
schedules for the purpose of ice control. Also,
that enters the Clark Fork just upstream of the
retaining tributary flow during the ice-formation
dam. This small hydroelectric project consists of
period might conflict with the objective of main-
a powerhouse, a single radial gate, and a spillway
taining minimum depths for main stem naviga-
section equipped with trippable steel stanchions
tion. Table 4 lists the objectives of early season
that support wooden panels. When the ice run
flow control at the projects discussed in this
was reported to be within 3.2 km of the dam, the
report.
operators cut out the stantions with torches to
There exists considerable operational experi-
remove all potential obstacles to ice passage. The
ence and engineering guidance for controlling
ice run stopped 2.4 km above the dam, against
flow to form ice covers. Operators at large hydro-
thick frazil deposits at the Big BlackfootClark
electric facilities time flow cutbacks for ice cover
Fork Confluence.
formation on the basis of a combination of expe-
The following day, a 14-km-long ice jam
rience, ice observations, water and air tempera-
formed on the Clark Fork River, 1.6 km upstream
tures, and water level measurements. Dam out-
of the Montana Power Dam at Thompson Falls,
flow may be further adjusted on the basis of the
Montana. Ice watchers stationed at the toe of the
position of the upstream edge of the progressing
jam maintained 24-hour contact with dam opera-
ice cover.
tors, who prepared for the jam's release by com-
In addition to experience and observation, ana-
pletely opening the dam's central 12-m-wide
lytical techniques exist for estimating ice cover
gate. On 14 February the jam broke and moved
progression as a function of water discharge and
en masse towards the dam. As ice and debris
air and water temperature. These methods range
passed the gate, operators successfully removed
from simple water velocity and Froude Number
by crane, or sawed in half, large logs that caught
criteria and steady-state hydraulic models, such
in the gate opening. Because of these actions,
as HEC-2, to more sophisticated models such
damages were minor.
as RICE (Shen et al. 1991) and UNET with ice
(Daly et al. 1997). The latter two predict ice thick-
ness and ice edge progression for inputs of time
CONCLUSIONS
series discharge, air, and water temperature data
This section assesses the relative importance of
(Table 3).
existing flow-control methods to manage ice,
identifying valuable areas for further research and
Midwinter period
development. Available guidance on winter flow-
During the midwinter period, flow-control
control methods is discussed, ranging from field
objectives are more diverse than during early win-
observation and operational experience to the use
ter. In the case of hydroelectric facilities, the pri-
of sophisticated numerical models. Table 3 pre-
mary objectives are maximizing generation capac-
sents flow-control objectives and methods for the
ity, while minimizing the chance of midwinter
ice formation, midwinter, and breakup periods,
breakups and ice jam flooding. For navigation
listing some of the available guidance and analyti-
dams on major U.S. rivers, winter operations focus
cal tools.
on passing ice and clearing ice from the upstream
lock approaches. During low-flow winters at large
river storage projects, operators may need to regu-
Ice-formation period
Hydroelectric facilities possess a great deal of
late outflow to compensate for water impounded
operational experience with controlling flow to
by downstream ice jams. Finally, winter releases
create ice covers that help maximize electricity
may need to be adjusted to satisfy in-stream needs
20
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