hydroelectric and the navigation industries,
Missouri River: Oahe Dam,
reduce upstream and downstream ice jam flood
Pierre, South Dakota
problems, and minimize disruption of municipal
Oahe Dam, which is operated by the U.S. Army
water supplies and winter fish habitat. This report
Corps of Engineers, is located on the Missouri
defines the midwinter period as the time between
River, 10 km upstream of the city of Pierre, South
the appearance of a stable ice cover and the onset
Dakota. To meet hydroelectric demands, outflow
of the final breakup period.
from Oahe Dam on the Missouri River fluctuates
daily between about 280 and 900 m3/s, within a
maximum range of 0 to 1560 m3/s. Each winter, a
Midwinter flow manipulation:
sheet ice cover forms downstream of Pierre, on the
Hydroelectric production and
pool above Big Bend Dam. During extremely cold
ice jam flood control
periods, a freezeup ice jam may progress from the
After an ice cover has formed, hydroelectric
head of the pool, past Pierre, as far upstream as
operators strive to maintain intact ice covers in
Oahe Dam. The presence of this jam, combined
the reaches upstream and downstream of their
with the daily peaking operations at the dam, can
projects until the onset of the final breakup period.
result in flooding of the low-lying portions of the
In some cases, limiting the magnitude and rate
city.
of flow changes at the dam can preserve a river
Unfortunately, the extreme cold that causes the
ice cover. Careful regulation of the dam outflow
worst ice jam conditions at Pierre coincides with
minimizes the amount of stage change in adja-
regional peak electrical demand, complicating the
cent reaches. As a rough rule of thumb, an ice
decision to cut back releases from Oahe Dam.
cover will break up if the stage increases by
Although no hard and fast rule exists, prior to
three to four times the ice thickness above the
1995, operators typically cut back the peak flow
freezeup water level (Donchenko 1978). Many
to about 710 m3/s once water levels exceeded
other factors are involved, however, such as the
defined "alert stages" at two river gages located
ice's condition and strength before breakup,
within the developed area of the city. It is possible
channel geometry, and the rate of stage rise
that riverbed aggradation at the head of the Big
(Beltaos 1984, Ferrick and Mulherin 1989). The
Bend Pool has exacerbated the ice jam problem at
negative effects of midwinter breakups include
Pierre in recent years.
ice jam flooding and the reappearance of open
Daly et al. (1997) analyzed field data and used
water reaches for frazil production. Once the
numerical hydraulic models to develop separate
midwinter ice cover has formed, the large hydro-
ice-affected stage frequency curves based on cut-
electric projects on the St Lawrence River and
backs to 710 and 990 m3/s once the alert stages at
in northern Canada return to a relatively steady
Pierre were exceeded. The analysis used actual
daily flow, similar to the open water discharge
and adjusted historical hourly flows from selected
level. This flow increase following ice cover for-
"worst-ice" periods during the winters of 1967 to
mation significantly smoothes the underside of
1995. Additional stage frequency relationships
the ice cover, decreasing its hydraulic resistance
were developed for estimated future aggraded
with time.
channel conditions. The study provided the Corps
Where large diurnal fluctuations are required
of Engineers with guidance on flow control as a
to meet hydroelectric peaking demands, it may
tool for ice jam flood mitigation. The work also
be difficult or impossible to maintain an intact
produced estimates of the frequency and duration
ice cover, particularly in the reach downstream
of future flow cutbacks, as shown in Figure 5.
of the project. Operators may be forced to limit
the magnitude of their daily peak discharge to
Yukon River: Whitehorse Rapids,
avoid downstream ice jam flooding. The follow-
Yukon Territory, Canada
ing examples of Oahe Dam on the upper Mis-
The Whitehorse Rapids Power Station located
souri River, and Whitehorse Rapids on the
just upstream of Whitehorse, Yukon Territory,
Yukon River illustrate the difficulties of meet-
provides the city with electricity. To maintain the
ing peak hydroelectric demands while avoiding
downstream ice cover and avoid ice-related flood-
ing, winter outflow from the plant is limited to
avert ice jams at hydroelectric projects that
60% of its 276-m3/s capacity, and daily peaking
withdraw a major portion of the total river flow.
flows are limited to within 10% of the daily aver-
The projects on the Upper Niagara River at
age flow. Through numerical modeling and a pro-
Niagara Falls, New York, are an example.
9
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