tion. Other categories may be fairly rare.
simultaneously from two channels into a
A partial evaluation can be obtained from
confluence is likely to be relatively small; there-
Tuthill and Mamone's (1997) report, which lists
fore, the frequency of these jamming situations is
about 44 sites known to have ice jam problems.
likely to be small. Somewhat of an exception is ice
Eight sites were selected as being potentially ame-
discharge through confluent branches of a
nable to a structural solution to mitigate jamming.
braidedmeandering channel or a river channel
Of the 44 confluence sites, 15 involve rivers enter-
that initially bifurcates around an island (e.g., the
ing a lake or reservoir (e.g., Aroostook River
Upper Niagara River splitting then merging
entering Tinker Dam Reservoir). Of the remain-
around Grand Island) or a large bar. Jamming of
ing river channel confluence sites, several have one
confluent ice discharges may be more likely dur-
channel merging with a channel subject to a sig-
ing freezeup, when channels in an entire water-
nificant backwater condition. Examples include
shed form and convey ice, rather than at breakup,
the Yellowstone River confluent with the Missouri
which arguably is more haphazard because of the
River upstream of Lake Sakakawea, and the
greater diversity of factors affecting it.
Salmon River merging with the Connecticut River
The hypotheses mentioned above, which form
at a reach subject to tidal slowing of river flow. Of
a portion of the findings of this study, point fur-
the eight sites selected for potential structural
ther investigations toward the following two spe-
measures, six have a river entering a lake or a reach
cific objectives:
of flow slowed by a backwater condition. These
Determine the conditions needed for two
sites coincide with categories I.3, I.4, II.3, or II.4
confluent ice discharges to jam. Attaining this
(Table 1), where one confluent channel has slug-
objective entails doing parametric experi-
gish or negligible flow velocities or is covered by
ments to evaluate the jamming limits of the
stationary ice. The seventh and eighth sites
more important parameters identified in eq
selected do not have sluggish flows at the
7 and 12.
confluence. One site is a lake entering a river,
Determine the processes whereby ice (from
which is not so much a confluence as an extreme
one or two channels) moves through a con-
constriction. The other site is the confluence of the
fluence, and assess how confluence bathym-
Missouri and Mississippi rivers. The ice discharge
etry may substantially hamper ice discharge
situation for that site appears to be category I.1;
through confluences. This objective is
ice drifts from the Missouri into the Mississippi,
addressed through a hydraulic model
which releases a negligible amount of ice during
described later in this report.
jamming events at the confluence. Initial assess-
ments of ice transport through the Mississippi
Missouri confluence indicate that local bathymet-
PARTICLE IMAGE VELOCIMETRY
ric irregularities at the confluence play an
important role in jamming at the site.
At the limiting condition of maximum ice dis-
Detailed information on the causes of ice jam-
charge through a confluence, ice movement is at a
ming at the confluence of two channels is not pro-
state of incipient jamming. Ice slows and is on the
vided in the report by Tuthill and Mamone (1997),
verge of accumulating and constricting water flow.
which focuses mainly on the aforementioned eight
Beyond this condition, ice accumulates, jams, and
confluences. Nonetheless, it is possible to hypoth-
constricts flow. To monitor and map incipient jam-
esize that a major proportion of jam problems at
ming in a flow field as potentially complex as a
confluences occur for the following two reasons:
confluence, it is useful to use instrumentation that
maps whole fields of ice and water velocities. One
Ice from one channel discharges into a chan-
such form of instrumentation is particle image
nel that has a sluggish flow (a lake is a limit-
velocimetry (PIV). Its use for mapping ice veloci-
ing example of this situation) or has a station-
ties was examined here.
ary ice cover that initiates jamming.
Bathymetric irregularities in confluence geom-
Background
etry retard ice discharge and initiate jams.
The key components of PIV are a series of
It is possible to hypothesize further that it is
images showing displacements, a personal com-
not common to find ice jam situations caused by
puter with a frame-grabber, and software for pro-
the merging of ice discharged from two confluent
cessing the images. The technique has developed
channels. The joint probability of ice discharging
rapidly in the past few years, making use of
7
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