The second mode of jam failure is called here "complete jam failure." It was the
dominant mode for relatively high initial discharge, for which the entire jam was
close to a condition of instability. Discharge increases simply overwhelmed the
entire jam. As discharge increased, water level rose, and a wave of water traveled
the length of the flume. In these events, however, the entire ice jam (which was up
to 30 m long) mobilized en masse and failed at the downstream screen. The thick-
ening front then progressed upstream from the screen. During some tests with very
large discharge increases, small ridges formed elsewhere within the jam, but the
major thickening took place as the thickening front swept back upstream. The ice
velocities for this type of failure were noticeably higher than for the progressive
failure type, yet still only ranged up to 25% of the bulk water velocity.
A few tests were also conducted in which the discharge was increased and held
constant for a short period. These tests were conducted for initial discharges and
discharge increases previously identified as causing a progressive jam failure. In
these tests, the shoving front was allowed to progress about halfway down the
flume, then the discharge was reduced to its original rate. As the discharge
receded, the entire jam (which was moving upstream of the shoving front) stopped
en masse. This left an ice accumulation that was nearly a single layer thick in the
downstream reaches, but slightly thicker upstream. The discharge was then
increased to the higher value. In all cases, the jam upstream of where the shoving
front had previously progressed mobilized en masse. The shoving front then con-
tinued its progression downstream as if the drop and subsequent increase in dis-
charge had never happened.
While the failure modes observed for the ice pieces and plastic beads were gen-
erally similar, there were a few differences. As expected, the real ice was more
angular and thus had a higher angle of internal resistance. Consequently, the accu-
mulations of real ice were more resistant to increases in downstream load. A com-
plication for the tests using ice was water-temperature regulation. Depending on
air temperature, an accumulation may melt or it may further increase its strength
owing to freeze-bonding of contacting pieces. The ice pieces were also much larger
than the plastic beads, potentially violating assumptions that their behavior could
be treated using continuum or particulate theory.
An interesting finding from the tests was that small increases in discharge do
not necessarily result in shoving and thickening. Sometimes, two or three small
steps in discharge were required to destabilize a jam. This was especially true for
the tests using ice, which involved a single layer of ice pieces with a rather high
piece aspect ratio (L/ηo), whereas the bead experiments involved a layer thickness
of between one and two bead diameters. The high aspect ratio for the ice pieces
invalidates assumptions of continuum theory for treating jam strength behavior.
The initial discharge used for a test likely was substantially less than the discharge
needed to destabilize the jam. For each bead experiment, once a shoving and thick-
ening event was completed, the water discharge was further increased to start a
second shoving and thickening event. Usually, multiple discharge steps were
required for this, especially when the initial event had led to complete jam failure.
Evidently, the collapsed jam had thickened to an extent much greater than the equi-
librium thickness estimated from the existing formulations (e.g., Uzuner and
Kennedy 1976, Beltaos 1983). This preliminary finding strongly suggests the
importance of ice momentum in determining jam thickness. It is this finding that
prompted further experiments to compare thicknesses after shoving to those cal-
culated using equilibrium theory.
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