was difficult for this confluence orientation of the model, because the large-scale
turbulence generated by the head-on merging of flows in the confluence helped
move the ice pieces through the confluence.
On the basis of the tests, it is possible to conclude that jamming would occur
only for the extreme conditions of either a very wide outflow channel with negli-
gible flow velocity, or an outflow channel whose surface is constricted for some or
other reason (e.g., by an ice cover). Another extreme case that may cause jamming
is for an immensely wide outflow channel (e.g., a lake or reservoir).
α = 90 confluence
This confluence configuration was extensively tested, as it is relatively common.
The full range of values for Q1/Q2 and b1/b2 were tested, with the ice discharges
(G1 and G2) varied within a much narrower range of values near the transport
capacity for both adjacent channels in order to hasten the possible onset of ice jam
formation. The model ice moved in the channels as a bank-to-bank, single layer, or
as closely packed patches with occasional water gaps. The tests showed jam for-
mation to be sensitive to the ice discharge conditions in the confluent channels. Ice
discharge rate in each channel had to be close to the ice discharge capacity of that
channel for the small model ice pieces to jam. Selected cases are described below.
The cases are listed in Table 1.
Case 1: Q2/Q1 = 2/3; b1/b2 = 1.5; b2 = 16 cm. This case illustrates how ice
discharging from one channel can be constricted by an ice run from the sec-
ond channel, which has a larger discharge that pushes the dividing streamline
close to the smaller channel. Jamming on channel 2 occurred, as illustrated in
Figure 19, when the larger discharge and ice run in channel 1 forced the flow
dividing streamline closer to the right (or inner) bank of channel 1 (looking
downstream). In so doing, it choked the movement of ice from channel 2 (Fig.
19a). Consequently, ice jammed in channel 2. Once the ice run on channel 1
had passed, the jam stayed in place in channel 2 (Fig. 19b). Jamming in chan-
nel 2 was hastened by a backwater rise in the water level in channel 2.
Case 2: Q2/Q1 = 3; b1/b2 = 1.5; b2 = 16 cm. This case shows how an ice run from
the smaller confluent channel can cause a jam in the larger confluent channel
when the smaller channel has a larger discharge. The set of three photographs
presented in Figure 20 shows the confluence before, during, and after a jam
develops in channel 1. Ice was released, at a constant rate, first in channel 1 as
depicted in Figure 20a. It can be seen in this figure that the flow-separation
line is well delineated by the drifting model ice. Ice in channel 1 jammed when
the rate of ice release in channel 2 approached the limiting transport capacity
for that channel at its given discharge (Fig. 20b). The jam lasted as long as the
ice moved through channel 2. After the ice supply ceased, the ice jammed in
channel 1 held for a while (Fig. 20c), then gradually released and flowed
through the confluence. Had the jam been simulated in frigid air conditions,
as may likely be the case in nature, the jam in channel 1 would have solidified
and remained in place after the ice run from channel 2 had ended.
Case 3: Q2/Q1 = 2/3; b1/b2 = 1.5; b2 = 16 cm, bar modeled. This example shows
that the presence of a bar in the confluence exacerbates ice jamming. For iden-
tical upstream confluence conditions (both for ice and water) as in Case 1, the
presence of a bar significantly altered the ice-jamming process and caused
jamming to occur more quickly. Observation of the flow before jamming re-
31
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