not until initial jamming has occurred. Instead, areal repacking of pieces may be a
characteristic feature. No doubt there is a transition between the two categories.
The free-drift and contiguous-accumulation categories can be classified into sub-
categories that reflect different combinations of merging ice flows:
1. Drifting ice pieces
a. Drift of ice pieces from one channel, while the other channel has no ice,
b. Drift of ice pieces from both channels,
c. Drift of ice pieces from one channel, while the other channel has a station-
ary ice cover,
d. Drift of ice pieces from one channel, while the other channel has sluggish
flow.
2. Ice layer movement
a. A layer of ice discharges from one channel, while the other channel has
no ice,
b. A layer of ice discharges from both channels,
c. A layer of ice discharges from one channel, while the other channel has a
stationary ice cover,
d. A layer of ice discharges from one channel, while the other channel has
sluggish flow,
e. A layer of ice discharges from one channel, while ice drifts freely in the
other channel (same as 1c, if layer movement is slow).
The two sets of subcategories are illustrated generally in Figures 8 and 9. The
categorization does not include a
further level of subcategorization
related to the circumstances of ice
movement in the stem channel
downstream of the confluence. For
the moment, the condition of ice
movement in the outflow channel
will be assumed the same as that
in the larger confluent channel.
Common examples of this situation
occur when the larger confluent
channel and the outflow channel
are subject to a significant backwa-
ter effect, and when one channel
flows into a lake, which is the lim-
iting condition of a channel flow-
ing into a larger confluent channel
with sluggish (or negligible) veloc-
ities.
The categorization is intended
also to assist in defining the actual
confluence conditions for which
jams most usually occur. An impor-
tant issue addressed during this
study is whether ice jams at con-
fluences are attributable to limits in Figure 8. Schematic of drifting ice pieces moving
the capacity of confluent prismatic through a confluence (subcategory 1a).
17
Back to contents page
Previous Page
