During frigid and low
discharge conditions, the
extent of the bar can be
increased by the growth of
border ice, and by possi-
ble grounding of drifting
i c e . The extended bar
greatly constricts the con-
fluence, as can be seen
from Figures 35, 36, and
37. The simulated con-
striction in the model is
shown in Figure 47. Also
shown by this figure is the
congestion and jamming Figure 47. Ice congestion for the confluence with border
of model ice in the mod- ice (without bendway weirs).
eled confluence. Ice con-
veyed from the Missouri channel quickly congests at the constricted section of the
confluence. As it does, flow and ice drift velocities upstream of the confluence
decrease, thereby hastening the onset of jamming. Vector plots of ice movement in
the confluence for this condition are shown in Figure 48. The aggravated back-
water effect created by the forming jam is evident in the acutely reduced magni-
tudes of flow velocities upstream of the constriction, especially in the Mississippi
channel.
The findings from the hydraulic model are corroborated by findings from the
numerical simulation reported by Lui and Shen (1998). Though their numerical
simulation simulates smaller flow rates (flow rates in the Missouri and Mississippi
Rivers each are assumed to be about 20,000 cfs) than used in the hydraulic model,
it shows how a jam may readily develop in the confluence when the confluence
Figure 48. Vector plot of velocities for ice and open water flow in Figure 47.
55
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