pieces may be a characteristic feature. No doubt
channel and the outflow channel are subject to a
there is a transition between the two categories.
significant backwater effect, and when one chan-
The free-drift and contiguous-accumulation cat-
nel flows into a lake, which is the limiting condi-
egories can be classified into sub-categories that
tion of a channel flowing into a larger confluent
reflect different combinations of merging ice flows,
channel with sluggish (or negligible) velocities.
as shown in Table 1.
The foregoing subcategories are intended not
The two sets of subcategories are illustrated in
only to reduce the processes associated with ice
Figures 1 and 2. The foregoing classification does
discharge through confluences so that they are
not include a further level of categorization related
more readily amenable to dimensional analysis,
to the circumstances of ice movement in the stem
but also to help define the actual confluence con-
ditions for which jams are most likely. In this
channel downstream of the confluence; it is
regard, an important issue to be addressed by this
assumed that the condition of ice movement in the
study is whether ice jams at confluences usually
outflow channel will be the same as that in the
are attributable to limits in the capacity of the
larger confluent channel. Common examples of
confluent prismatic channels to convey ice, or
this situation occur when the larger confluent
whether jams usually occur because of
local channel irregularities often found at
confluences (e.g., alluvial bars, flow fea-
tures, channel-control structures). This
point is discussed further below.
Open
Water
Free drift of ice through river
confluences
To describe the characteristics of ice dis-
charge and jamming at river confluences
Ice
Cover
Ice
in general terms, it is necessary to estab-
Cover
lish a group of important nondimensional
parameters that express the proportion-
ate influences of water flow, ice flow, and
confluence-channel characteristics. Figure
3 is a schematic illustration of the variables
associated with water and ice discharge
through a confluence of two fixed-bed
(a)
(b)
channels of rectangular cross
section. The same size ice pieces
are taken to be moving through
both channels. The confluent
inflow channels are designated
Sluggish
with subscripts 1 and 2. The
Open
Water
confluence outflow channel is
designated with subscript c.
The discharge Q, unit dis-
charge q, or a representative
Ice
Ice
velocity V of flow in one of the
Cover
Cover
channels shown in Figure 3 can
Drifting
Ice
be described using depth Y,
Pieces
width b, channel slope S, and
channel roughness k, i.e., by
means of the Manning-Strickler
equation. The terms are defined
(c)
(d)
(e)
in Figure 3. Note that, in this
Figure 2. Categories of ice-layer movement through a confluence (a = II.1; b = discussion, the variables Q, q,
II.2; c = II.5; d = II.3; e = II.4 [Table 1]).
or V can be used (with b, Y, and
3
Previous Page
