Geoscience Reference
In-Depth Information
0.44 m
45 m, respectively.
The tests utilised two uniform quartz sediments of respective median sizes
d
0.38 m (deep)
12 m and 1.5 m
1.2 m (deep)
0.24 and 0.8 mm.
For each test, 4D bed development from plane-bed conditions to equilibrium
sand waves was recorded using a “flying-probe” configuration of a 31-transducer
5 MHz ultrasonic ranging system manufactured by Seatek. Individual transducers
were polled at 5 Hz, with one of the transducers being used to map water-surface
elevation, and the remainder being arranged to measure the 3D bed morphology.
Bed- and free-surface elevations were measured to within
¼
1 mm. In the narrow
flume, a chain-and-sprocket driven programmable-speed motorized carriage
enabled bed surfaces measuring 6.25 m long by 0.175 m wide to be recorded
every 60 s. Bed surfaces measuring 18.48 m long by 1.35 m wide were recorded
every 120 s using a rack-and-pinion driven programmable-speed motorized car-
riage for the wide flume tests. For each flume, carriage speeds were sufficiently
large compared to bed-development rates that each recorded bed surface can be
assumed to be representative of a static snapshot of the dynamically changing bed.
For the wide-flume tests, the bed-sensing transducers were arranged in three
rows (ten probes along each row) across the flume (Fig.
9
), the carriage motion
facilitating recording of ten continuous transects parallel to the flume centreline
Resulting
depthsounder
measurement
grid
i) Narrow flume
Depth-sounders
y
Downstream
x
2
mm
25 mm
12.5 mm
Physical
layout
ii) Wide flume
150 mm
Physical
layout
41.2
mm
Ultrasonic
depth-sounders
Downstream
20.6
mm
150 mm
Resulting
depthsounder
measurement
grid
y
20.6
mm
24.8
mm
x
Fig. 9 Flying-probe sensor arrangements
