Modeling procedure
Each set of tests with a confluence configuration involved establishing how the
variation of confluent flow-rate ratio, Q1/Q2, affected ice conveyance through the
confluence and modified the jamming mechanism.
Before each test, the flow was checked visually by means of dye to determine
that inflow conditions were acceptable (i.e., no asymmetry of inflow or over-
turbulent), and to confirm the uniformity of the flow profile along the confluent
channels. For each test, all independent parameters were constant except for one,
which was varied until a condition of ice jamming resulted in the model. The
parameters most varied were concentrations of model ice flow in the channels,
ratio of water discharge, and extent of bar developed in the simulated confluence.
Typically, once satisfactory flow conditions were confirmed, the following model-
ing procedure entailed releasing model ice at various prescribed rates into both
channels, then observing and recording the patterns of ice movement and accu-
mulation as well as their effect on flow conditions.
Most of the tests were videotaped for subsequent further analysis. The video
records were made with a CCD (charged-coupled device) camera mounted 1.53 m
above the confluence. To enhance the resolution of the recorded images, the
channel liners were painted white, whereas the model ice consisted of black
beads. The video records enabled qualitative analysis of the temporal and spa-
tial behavior of the conveyance of the ice layer in the model. For selected test
cases, subsequent image processing using particle image velocity (PIV) software
facilitated mapping of the whole field velocities of water flow and ice piece drift.
CONFLUENCE JAM PROCESSES
The process modeling confirmed the simple processes that cause ice jams in
confluences and, importantly, revealed the mechanics of four considerably more
complex processes leading to ice jamming. The findings, therefore, are discussed
in five parts:
1.
Simple processes causing ice jams.
2.
Jamming of merging ice runs.
3.
Jamming due to flow impact.
4.
Jamming at a confluence bar.
5.
Jamming at deltaic bars.
The latter four mechanisms are potentially complex. The third jamming process
can be demonstrated in laboratory conditions, but may be highly unlikely to occur
in actual rivers or streams.
Simple jamming processes
Three relatively simple mechanisms result in ice jams at river confluences. They
are sketched in Figures 12 and can be summarized as below:
An ice run in one channel is blocked by stationary or slow-moving ice in the
outflow channel (Fig. 12a).
An ice run in one channel discharges into a larger channel in which flow cur-
rents are sluggish (Fig. 12b); an extreme, though common, example is a river
entering a wide reservoir, lake, or coastal area.
A run of relatively large ice pieces (compared with channel width) arch across
the confluence (Fig. 12c).
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