Geoscience Reference
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forward scatter transmitting and receiver optics. The transmitting optics features a
20 mW, monochromatic He-Ne laser, capable of detecting positive and negative
velocities. The signal is processedinaDANTEC55N20DopplerFrequency
Tracker and converted into a voltage output ready to be sampled on a personal
computer. The dimensions of the control volume were d x ¼
0.0005 m and
d y ¼
0.0015 m. The sampling software enabled the collection of 12,000 samples
before writing to file. The LDA system was placed at distances from the inlet
ranging from 80 to 125 hydraulic radii, depending on the water depth. The sam-
pling frequency varied with the hydraulic parameters between 200 and 300 Hz
(cf. Table 1 ).
The water depth and the bed elevation were measured with a 1.0 mm precision
point-gage. In order to evaluate the bed texture, a laser bed-profiler was employed
to measure the bed elevation along 60 cm lines with a definition of 4 samples/cm.
Flow discharge was measured on a calibrated triangular weir placed in the
hydraulic circuit. Sediment discharge was computed from samples collected
downstream and dry-weighted. Bed-surface was sampled by scrapping off the
initial 9 mm of core samples. The grain-size distribution of the bed-surface and
bed-load were evaluated by means of dry sieving. For both fixed and mobile beds,
the bed was permeable and composed of cohesionless sediment. The sediment
grains were transported exclusively as bed-load, and no appreciable bed forms
were registered. The flow was subcritical, steady, and approximately uniform for
all flows.
A typical mobile bed test begins with laying out the bed and imposing a selected
bed slope. Uniform flow is imposed by adjusting the downstream vanes. Bed-load
samples are collected and water and bed elevations are monitored for about 8 h to
ensure equilibrium sediment transport. Usable bed-load data is collected from this
Table 1 Conditions of mobile bed experiments
Test
E0
E1
E2
E3
(m 3 s 1 )
Q
0.0138
0.0135
0.0135
0.0130
h
(m)
0.098
0.070
0.065
0.056
i 0
(
)
0.0010
0.0028
0.0033
0.0050
(m s 1 )
u *
0.030
0.041
0.044
0.049
Re
( )
33,173
32,452
32,452
31,250
B/h
( )
4.1
5.7
6.2
7.1
Fr
( )
0.36
0.58
0.65
0.78
t *84
( )
0.0120
0.0220
0.0246
0.0294
t *c84 a
( )
0.0329
0.0329
0.0329
0.0329
d 84b
(mm)
4.74
4.81
4.79
5.02
d 84BL
(mm)
-
2.66
2.86
3.73
f BL
( )
-
23.2
37.8
70.5
f LDA b
(Hz) 200 240 250 300
a Computed from the initial bed mixture; incorporates a hiding-exposure coefficient determined as
in Wu et al. ( 2000 ).
b
Computed from Nezu and Nakagawa ( 1993 ), p. 30.
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