RIVER ICE INFLUENCES ON FORT PECK REACH, MISSOURI RIVER
73
Bottom of Probe
Top of Probe
River Bed
Buried Section
Exposed Section
7 April 99
Ice Breakup: 17 March
4 April 99
28 Feb 99
26 Feb 99
20 Feb 99
31 Jan 99
26 Jan 99
14 Jan 99
11 Jan 99
Propagation Time of Reflected Signal
Figure 49. Bed elevation on the north bank of the north channel detected using TDR 1 and the
1503.
tion in drag with the increase in flow area due to enlargement of the channel
caused a net reduction in the bulk velocity of flow. The rate of bed lowering also
diminished, and a transitory bed equilibrium eventually prevailed. The TDR
readings indicate that the bed elevation remained relatively unchanged during the
period of 2028 February, the time of the second survey.
Most of the ice cover at the Culbertson site broke up and departed the site on
17 March (Fig. 46d), leaving no bankfast ice along the channel banks. Ice covers
typically break up because of an increase in stage caused by snowmelt and/or
rain runoff, but the ice cover at Culbertson warmed, weakened in place, and
either melted in place or broke off as a floe and floated downstream. A reading
taken 17 days after the ice cover broke up revealed that 2.8 ft of the probe
remained exposed. We used eq 6 with the change of the inflection points on the
1503 images to determine the changes in elevation at all the TDR locations (Fig.
50).
The waterfall plot of the periodic 1502 images for TDR 1 (Fig. 51) indicates
a trend similar to the one for the 1503 images; the inflection points drift to the
left, stabilize, then suddenly move to the right, followed by a recovery to the left.
Ice cover breakup occurred in this manner along the lower portion of the Fort
Peck reach of the Missouri River. The signals from all the TDR probes indicate
that the channel bed at the downstream end of the site remained stable until the
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