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ERDC/CRREL TR-02-14
Eq 35, though entailing simplifying assumptions, lead to a clear result. As
the covered flow depth Yi usually exceeds the open water depth Yo, the ratio Sio/So
is less than 1. Therefore, the energy gradient (and stream power) available for
sediment transport and channel forming decreases when a channel has a free-
floating ice cover. For a typical value of, say, Yo/Yi ≈ 0.8, Sio/So ≈ 0.5; in other
words, for a given flow rate in a channel of given length, approximately half the
energy expenditure is available for sediment transport and channel forming. The
effect of an ice cover, therefore, is to trigger a shift in thalweg sinuosity and
alignment so as to balance flow-energy availability and use. Figure 11 suggests,
for instance, that halving the slope of a meandering channel (say, from 0.008% to
0.004%) will reduce thalweg sinuosity; i.e., the thalweg attempts to straighten
and the meander wavelengths shorten, as sketched in Figure 12.
Figure 12. Influence of an ice cover on a meandering channel
of more-or-less uniform flow depth. The cover may cause the
thalweg to straighten and meander loops to shorten.
For sinuous-braided channels, as in Figure 13, ice cover formation and the
associated decrease in the energy gradient may cause flow to concentrate in a
single thalweg of greater sinuosity than the open water thalweg. For braided
channels an ice cover may concentrate the flow into the larger subchannels.
If the ice cover is rigid and attached to the shore or channel structures, the
cross section is constant. An increase in discharge causes a flow regime akin to
pressure flow, with an increase in bulk velocity and shear stresses along the bed.
Ice cover influence on channel bed elevation
A basic issue is an imbalance between the rate of sediment supply to an ice-
covered reach Qs and the sediment transport capacity of that reach Qsc. This issue
involves the complex problem of spatially varied flow and sediment transport,
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