jams in river confluences. Each confluence type has its unique jamming process,
as is discussed subsequently in this report.
Several of the same basic mechanisms cause ice to jam in both types of conflu-
ence. An obvious example is the jam that forms when a stationary (or very slow
moving) ice blocks an ice run from one channel cover in a confluence. The differ-
ences in channel morphology, however, do result in different mechanisms for ice
jam formation when the confluence is not initially blocked with ice. Ice jams in
concordant bed confluences may be initiated because of a ice congestion in the
vicinity of the bar. This mechanism has caused jamming in the confluence of the
Mississippi and Missouri Rivers, for example. Ice jams in discordant bed con-
fluences may initiate in the higher, steeper (and inevitably smaller) channel
due to ice grounding on the sediment fan or ice arching between islands and bars
exposed during low flow. This mechanism has caused ice jamming in the mouth
of the Liard River at its confluence with the Mackenzie confluence, for example.
The present study primarily considers ice jams formed in confluences of concor-
dant bed channels. Arguably, it is the more interesting confluence type to investi-
gate. It also is the more common type for river confluences in the continental
United States. The subject of ice jams triggered by ice grounding or arching in
braided channels is left for a subsequent investigation.
Flow features
Flow through a typical concordant bed channel confluence is illustrated in Fig-
ure 4a. It comprises the following principal features:
A flow separation zone.
A dividing streamline (or stream plane) that delineates the merging flows,
and which actually is a shear layer.
A small zone of flow stagnation at the apex of the confluence.
A flow recovery zone.
These flow features make confluence flows comparatively complex and subject
to the influences of many parameters, such as confluence angle and the relative
magnitudes of discharge in each channel. Consequent to confluence flow com-
plexity, the bathymetry of confluent alluvial channels may also be complex, as is
illustrated in Figure 4b. The bathymetry is notable for the following principal fea-
tures:
A bar, which more or less occupies the flow-separation zone and is formed of
bed sediment deposited in that zone.
A zone of deep scour, which is approximately aligned with a portion of the
dividing streamline.
In actuality, the dividing streamline is not a simple curve as shown in Figure 4a.
It is a time-average representation of the plane between two merging flows. The
plane lies in a shear layer marked by strong vortices that initiate mixing of the
merging flows, as sketched in Figure 4c. The vortices significantly affect the extent
of bed scour in the scour zone.
The potential difficulties for drifting ice to pass through a confluence soon
become apparent when drifting ice is superimposed on the confluent channels.
Not only must the two streams of moving ice merge, they must negotiate the com-
plex flow field and bathymetry of a confluence. The two flow features of special
significance for ice passage through a confluence are the dividing streamline and
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