ground of the model bed, which is a dull gray. The use of LSPIV was especially
difficult in the region where the model bendway weirs were placed. The model ice
did not stand out sufficiently from the rows of stones used to simulate the weirs.
Repeated runs were needed to get adequate definition of the velocity vectors of
ice movement and water current.
To check the general accuracy of the model, observations of ice movement and
jam formation in the model were compared with aerial photographs of ice move-
ment and ice jams in the actual confluence. In particular, the patterns of ice move-
ment and jamming in the model were compared with the patterns evident in Fig-
ures 35 to 38. In overall terms the patterns observed in the model agreed very well
with the patterns evident in those figures.
Test program and procedure
Once the model was calibrated, the test program followed the sequence of con-
fluence bathymetry conditions listed above. Flow rates and ice discharge rates
were held constant for the sequence of tests.
The first step in a test was to illuminate the main flow patterns in the model
confluence. This was done by means of flow visualization using dye released from
a dye wand. Six experiments focused on the detailed ice flow structure were per-
formed subsequently in the small-scale model.
Results
The results of the model confirmed the findings of the diagnostic model. Conse-
quently, they also confirmed that reduction of the bar size would increase the ice
discharge capacity of confluence of the Missouri and Mississippi Rivers. The use
of bendway weirs, in the locations indicated in Figure 39, should enhance ice move-
ment through the confluence, provided they adequately reduce the size of the bar.
The ensuing discussion briefly describes the main observations and results of
the tests with the model.
Rectangular channels
Flow through the confluence replicated with rectangular channels produced the
flow features typical of a concordant bed confluence (Fig. 4a, c). Ettema et al. (1997)
present LSPIV mapping of the surface velocities of water flow and ice drift for this
case.
Confluence with bar and without bendway weirs
In this case the confluence is fitted with bathymetry, notably a bar, that simu-
lates the bathymetry prior to installation of the bendway weirs (refer to Fig. 41 for
typical bathymetric cross sections). Figure 44 shows the flow features identified
from LSPIV mapping of flow velocities. The separation zone was readily evident
in the model, as was the shear layer between the merging flows. A dividing stream-
line is indicated through the shear layer region. The extent of the flow separation
zone approximately coincides with the bar in the confluence. The nonuniformity of
flow depth across the channel, together with the presence of the bar, significantly
alter the flow field in the confluence. The bar reduces the surface area of flow, as
indicated in Figures 44 and 45. Additionally, the shallower flow depths flanking
the bar and its approach along the southern bank of the Missouri River, concen-
trated the flow in the deeper portion (line of maximum scour) of the confluence.
In consequence, flow velocities in the shallower zones are less than the velocities
in deeper areas. LSPIV measurements of flow field are illustrated in Figure 46.
53
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