though its field application by Fujita and Komura
ing through the field of view. Radar images at
(1994) and Aya et al. (1995) suggested that the
about 1 minute would have been suitable for PIV
present application was feasible.
analysis.
Field use of PIV
MODELING ICE DISCHARGE AND
Besides its use for the model study described
JAMMING PROCESSES IN CONFLUENCES
here, PIV holds good promise for application with
remote-sensing methods for monitoring ice in vari-
To investigate the processes whereby ice dis-
ous field situations. At present, such methods are
charges through, or jams, in river confluences, a
used extensively to determine the presence of ice,
general hydraulic model was built to accommo-
its general pattern of movement, and the trajecto-
date a two-channel confluence of variable geom-
ries of individual ice masses. PIV, in appropriate
etry. This section describes the model, the
conjunction with remote-sensing methods capable
versatile instrumentation system used to obtain
of producing sequences of images of moving ice,
whole-field mappings of water and ice velocities
would enable whole-field velocities of ice and
in the hydraulic model, and how measurements
possibly water to be determined. The two primary
were made during an investigation of ice discharge
requirements for applying the PIV technique are
through the confluence of the Missouri and
adequate images of moving ice and a set of at least
Mississippi rivers.
four markers of known coordinates in the field of
view. The images could be a direct video record of
Ice movement through the confluence of the
the ice movement through the location, a set of
Mississippi and Missouri rivers
photographs taken at a close interval of time, or
The confluence of the Mississippi and Missouri
they could be of a series of images generated from
rivers is prone to ice jams that are infrequent, but
a radar.
damaging and expensive when they do happen.
Aerial photographs and videos are already used
The conditions at the confluence when jams form
extensively for monitoring ice, identifying obstruc-
typically include almost equal, and low, water dis-
tions to its movement, and assessing damage
charges in both rivers. Jamming at the confluence
caused by it. Ashton (1986) describes their use for
causes some flooding and damage to shoreline
monitoring ice conditions on rivers and lakes. In
structures, but the main difficulty is obstruction
recent years, videos of ice movement have been
to tow-barge traffic on the river, which cannot get
used to obtain more than a visual record of ice and
through the jams.
ice runs in rivers. Dobrowolski et al. (1992), for
The overall orientation and geometry of the
instance, describe the use of video recording and
confluence is depicted in Figure 8, which also pre-
computer software to determine concentrations of
sents the layout of the general hydraulic model.
frazil slush moving in the Vistula River, Poland.
Figure 9 illustrates ice moving through the
If taken at an elevation commensurate with
confluence in January 1994. This case study is an
the requisite areal concentration of ice pieces, and
example of a confluence problem attributable
for a known aircraft speed, not much effort
mainly to local bathymetry, and possibly wind,
is needed to produce areal images that could be
rather than to difficulties in merging ice discharg-
analyzed using the PIV technique described
ing from two confluent channels.
herein.
Discussions with engineers from the U.S. Army
As an alternate form of video image, radar
Engineer District, St. Louis, provided the follow-
images converted to video images could be ana-
ing information about the difficulties with ice dis-
lyzed using the PIV technique. Figure 7, for
charge through the confluence:
instance, shows a marine radar image of ice mov-
Difficulties typically occur when flows in the
ing through the fairly complex bathymetry of the
Grass Island Pool reach of the Upper Niagara
Mississippi and Missouri rivers are low
(nominally 50,000 ft3/s [1415 m3/s] or less in
River. The reach geometry is also shown in Figure
each channel).
7. Once the dimensions have been established,
In recent times, navigation through the
velocity vectors of moving ice could be interpreted
confluence was shut down in 1977, 1979, 1989,
from such video images. However, in the case of
1990, and 1994.
Figure 7, the period between the radar images was
Ice predominantly comes from the Missouri
10 minutes, which is too long to meaningfully map
River. The Mississippi River usually has a
velocity vectors for the ice pieces that were mov-
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