This jamming mechanism
may occur when an ice run in
one river must merge with an
ice run in another river that
is similar or larger in size. The
jam typically will occur in the
smaller river, immediately
upstream of the confluence.
Jamming due to flow impact
When the flow from one
confluent channel is consid-
erably larger than flow from
the second confluent channel,
Figure 15. Jamming
the dividing streamline can
due to flow impact.
be pushed strongly to one
side of the confluence. Ice
drifting from the channel
with the lesser flow is both re-
tarded, because of the impact
pressure exerted by the flow
from the channel with the
larger discharge, and it is con-
strained to move through the
narrowed region of width b3 bd, as depicted in Figure 15. Though feasible, this
jamming process is likely to be rare in nature, because it is unlikely that the differ-
ence in flow velocities in the two confluent channels will differ so much that ice
would be held back in the channel with the lesser discharge.
Jamming at a confluence bar
The model demonstrated the importance of the following additional factors for
confluences of concordant bed, alluvial channels with a large exposed bar:
Aggravated congestion of ice moving in the outflow channel narrowed by the
presence of the bar.
Grounding of ice in the shallow water around the bar.
These two factors, together with factors 1 through 4, contribute to the ice jam pro-
cess illustrated in Figure 16. Ice congestion adjacent to the confluence bar results
in a jam in the confluence. This mechanism may occur when ice enters the conflu-
ence from one inflow channel or from both.
The confluence jamming mechanisms illustrated in Figures 13, 15, and 16 can be
formulated in a relatively simple manner, as described subsequently. It should be
noted that factor 3, flow constriction and backwater development, reinforces the
importance of including unsteady flow conditions when numerically modeling
ice jam formation.
Ice jamming on depositional fan
For completeness of jamming situations, this situation is included here. It was
not investigated with the model. Perhaps, it might be included as a simple (or
obvious) jamming mechanism. As illustrated in Figure 17, during shallow flow
28
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