70
ERDC/CRREL TR-02-14
Bed profiles
The observed changes in channel conditions at the Culbertson site are evident
in the bed profiles obtained from the surveys and the bathymetry contours. Figure
47 shows the bed profiles measured for the cross section at RM 1620.4 during the
four survey periods.
Because ice cover formation around the head of the bars triggered a switch in
the river's thalweg location, it would have been useful to survey additional cross
sections farther upstream; the main changes in channel bathymetry probably
initiated near the upstream end of the diagonal bars that divide the two subchan-
nels. However, the survey cross sections do reveal some of the change, especially
the bed lowering of the north subchannel at the upstream cross section surveyed
(RM 1620.45). Also, the April and October bathymetry contours show how the
thalweg switch to the north subchannel is reflected in the lower bed elevations of
that subchannel.
A further important ice influence, however, occurred in the center of the site.
A low point in the bed (RM 1620.4) deepened once the cover had formed over
the site. The deepening section coincided with a thinner portion of ice cover. One
interpretation is that the flow was concentrated toward the deeper section of the
channel, since that section provided the least flow resistance. The flow velocity
measurements support this interpretation; they show a sustained large unit dis-
charge of water through the section. The TDR measurements indicate that sedi-
ment started to deposit in the vicinity of the probes shortly after ice cover
breakup, and it is safe to assume that sediment was also being deposited in the
deeper portion of the river. By the time of the April survey, the holes detected in
February were backfilled and no longer evident.
Increased flow through the north subchannel scoured and lowered the bed
along the upstream portion of that subchannel by about 56 ft by the time of the
January survey (RM 1620.45). The depth of scouring diminished downstream
along the subchannel; for example, at RM 1620.35, no significant deepening of
the bed is evident for the north subchannel. It is likely that the bed had already
scoured down to a rock layer. A more detailed explanation of this variation is
uncertain.
Reduced flow velocities in the south subchannel appear to have resulted in
sediment deposition along much of that subchannel. Such sediment deposition
would have further increased the flow resistance through the south subchannel.
The amount of deposition was substantial--about 8 ft along the south subchannel
at RM 1620.45 (Fig. 47). By the April survey the entire subchannel had silted up
and a second medial bar had formed (Fig. 46e). At some locations along the
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