It must be reiterated that calculations assum-
be greater than the 1.5-ft-thick ice measured dur-
ing an equilibrium ice jam constitute the maxi-
ing 1991-92 but less than the 32-in.-thick ice re-
mum possible water surface elevation for a given
ported in 1952 (USDOC 1953). The ice jam curves
discharge. Actual water levels are most often less.
in Figure 7 represent the maximum water surface
Lower actual water levels can be found at loca-
elevations possible for the range of discharges
tions close to the toe of the jam where the jam may
covered, assuming that an unlimited supply of
not be fully developed. If there is enough ice to
ice has formed an equilibrium jam throughout
form an equilibrium jam, the actual water levels
the study area.
will also be lower than computed. If the discharge
Based on a typical freezeup flow of about
continues to rise or the strength of the ice accum-
10,000 cfs, a spring discharge on the order of
ulation deteriorates, a point can be reached where
25,000 cfs or greater would be required to initially
no stable jam is possible, and a lower, open-water
dislodge a strong ice cover. This value is based on
rating curve again applies. The equilibrium mod-
a rule of thumb that the stage must rise three to
el also assumes that a supply of broken ice is
four ice thicknesses above the freezeup stage to
available. With lower discharges and stronger ice,
initiate the breakup and run of a strong ice cover.
the ice cover may remain intact and not subject to
An increase from 10,000 to 25,000 cfs, with a con-
thickening into a jam.
tinuous ice sheet, would result in an increase in
stage on the order of 5 ft in most areas within the
District. A deteriorated ice cover can release with
Ice-affected water levels
The first step in the analysis of ice-affected wa-
lesser increases in flow, but such events do not
ter levels was a year-by-year review of flow
normally result in significant ice jams. Below that
records to determine the expected breakup dis-
discharge, then, it might be assumed that the
charge. Actual water surface profiles lie some-
stagedischarge relation would follow the sheet
where between the limiting conditions of open
ice curve. Above that discharge, stages would
water, a solid cover of sheet ice and a fully devel-
tend towards the ice jam curve, assuming that
oped equilibrium ice jam. The solid ice cover case
conditions approaching an equilibrium jam were
would represent the minimum ice-affected stage,
possible.
while the equilibrium ice jam case would repre-
At a somewhat higher discharge, the trend of
increasing stage with discharge would begin to
sent the maximum stage possible for a given
flatten out. Because of the wide floodplains
discharge. If we consider the range of possible
throughout most of the area, once the raised later-
Missouri River discharges during the breakup
period, we can categorize ranges of flow from
al ditches were overtopped, the channel stage
would be substantially stabilized. For example, at
discharges too low to cause breakup of the ice
the Borlaug Bridge we know that the 1986 jam
cover to discharges where all ice would move
with an estimated discharge of 59,500 cfs nearly
downstream without jamming. These categories
might be based on personal observations, obser-
overtopped the lateral. The stage would certainly
vations by local residents, notes on nearby gaging
have leveled off near the elevation of the lateral,
or even dropped if the lateral had been breached,
records, sharp breaks in the trend of continuous
had the jam not collapsed and released. Figure 7b
stage measurements or other sources of informa-
tion.
appears to show that the stage would continue to
increase to an elevation of at least 1872 in this
Rating curves have been developed at several
area, but that is due to a fairly coarse HEC2 data
locations for discharges up to 92,000 cfs. This
file in which the apparent low elevation on the
range would include events as large as a ten-year
lateral is somewhat higher than that at the Bor-
spring ice cover breakup period flows, as well as
laug Bridge. By the time the ten-year breakup pe-
two-year open-water flows. Figure 7 shows con-
riod flow (or two-year open water flow) is
ditions near cross sections 1564.48 and 1568.19 in
reached, the ice jam rating curves indicate that
the Middle Bottom, cross section 1578.03 in the
the laterals in both the Middle and West Bottoms
West Bottom and 1581.31 just downstream of the
would be overtopped.
Yellowstone confluence area. The curves repre-
In view of the limited length of historic jams,
sent open-water, ice-covered and ice-jammed
however, actual stages for these flow ranges
conditions in order of increasing stage. Based on
would likely be less than shown, since a fully de-
a review of air temperature records for the win-
veloped equilibrium jam would not exist through-
ters of 1970-71 through 1991-92, it would appear
out the area. Most reported jams were 0.52.5
that the midwinter ice thickness would normally
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