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ERDC/CRREL TR-02-14
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The survey involved a comprehensive set of instrumentation for measuring
bed bathymetry, flow velocity, and ice thickness.
Bed profiling
Bed profiling was the principal approach taken to determine how the forma-
tion, presence, and breakup of river ice might influence alluvial channel bathym-
etry along the Fort Peck reach. Three or four cross sections were used to charac-
terize the channel bathymetry at each site. The cross section locations established
during the first survey were used for the subsequent surveys. The river mile
locations for the respective cross sections appear in Table 3.
During open water the bed elevations were measured at discrete points using
a conventional differential GPS survey system either by wading in the river or
working from a boat. For the winter survey, we initially planned on using a
ground-penetrating radar (GPR) antenna towed behind an all-terrain vehicle to
continuously profile along the survey alignments and parallel with the channel. It
was thought that GPR could provide detailed bottom and sub-bottom profiles of
the channel. Sellman et al. (1992), for example, described the use of GPR for this
purpose. GPR radar transmits a pulse through the ice into the water and propa-
gates through the water until encountering the bed sediment. In terms of finding
the bathymetric bottom profile, the profile of the ice is not of interest and can be
ignored when attempting to optimize signal processing to best determine the river
bottom profile. At the interface between the water and the bed sediment, a por-
tion of the transmitted energy is reflected back to the water surface. The travel
time for the pulse is directly proportional to the distance from the water surface
to the bed. The portion of the transmitted energy penetrating the bed sediment
propagates through the sediment until it encounters another boundary, such as a
bedrock layer. There the energy again is reflected back towards the radar at the
water surface. The travel time of the radar signal is proportional to the depth to
this interface. The radar transmits thousands of pulses during a traverse, and the
resulting reflected waveforms are used to produce a bed profile along the traverse
line. Profiles along and orthogonal to the survey line could be used to generate a
detail contour plot of the bathymetry in the region of the survey.
For riverbed profiling in open water conditions, an acoustic depth sounder
could be used as an alternative to GPR. However, for bed profiling in ice-covered
conditions, GPR has an advantage since its radio signal can penetrate the ice
cover into the underlying water. Acoustic signals have difficulty penetrating
through an ice cover. The difficulty is especially severe when attempting to
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