48
ERDC/CRREL TR-02-14
shown as Figure 28 using a depth-conversion algorithm that locates the top and
bottom interface reflections and calculates the ice thickness. Physical measure-
ments of ice cover thickness provided ground truth verification of the radar data;
the radar data indicated slightly thicker ice than the physical measurements. The
bias in the radar-calculated ice thickness data is caused by the variations in ice
thickness due to the surface roughness of the refrozen rubble ice sheet or the
thermal effect of the bridge pier.
The difference between the radar data and the measured data is attributable to
the borehole drilling technique. Boreholes were drilled using an 8-in.-diameter,
gasoline-powered ice auger. When the auger broke through the bottom of the ice
sheet, some ice may have spalled from the underside of the ice cover, thinning
the cover immediately adjacent to the borehole. This would have biased the
measurements.
The radar measurements indicated that the ice thickness was relatively uni-
form along the respective cross sections and that the physical measurements were
representative of the region. The only exceptions were adjacent to the bridge
piers and over the land-fast ice, i.e. shorelines and island where the cyclic flood-
ing and freezing thickened the ice or where higher surface velocity suppressed
ice growth, i.e. thalweg areas. Therefore, it was decided not to include radar
profiling of the ice thickness for the February survey.
Velocity profiling
For each survey cross section, once the radar profiling of ice thickness was
completed, boreholes were drilled using the 8-in. auger at premeasured cross-
section stations. The boreholes were used for measuring the water velocity, water
depth, and ice thickness.
Flow velocity was measured using the two-component electromagnetic
(Marsh-McBirney Model 511) velocity meter attached to an extendable pole
(Fig. 30). Velocity measurements were taken at 19.5-in. incremental flow depths,
with the sensor probe oriented such that its X-velocity component was aligned
with the survey line. The velocity probe was attached to a series of 3.3-ft-long
extension rods that combined to a maximum length of 18.05 ft. For the January
survey, 1-in.-diameter fiberglass rods were used, but the assembled rod was too
flexible and bent excessively in high-velocity zones. For the February survey, a
1.25-in.-diameter aluminum rod was used; it was considerably more rigid. The
rod's maximum length was increased to 21.3 ft, requiring a team to launch the
probe for each measurement (Fig. 31).
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