Geoscience Reference
In-Depth Information
stage on. Profiles of instantaneous velocity measurements are performed at four
separate occasions in order to enable ensemble averages of turbulent characteris-
tics. Bed texture and bed surface samples of the final water-worked bed are
collected at the end of the test.
The experimental conditions for the fixed-bed tests were obtained from the
mobile bed ones by letting the water-worked beds undergo a process of armoring.
As a consequence of the armoring process, the bed slope decreased and the water
depth increased. To maintain approximately the same mean bed shear stress, the
downstream vanes were operated and the slope slightly adjusted until the same
u
*
(computed, on a first approach, from the balance of friction and gravity forces) was
obtained. A more accurate value of
u
*
was later obtained from the Reynolds shear
stress profiles.
The remaining procedural steps were identical to the mobile bed ones: velocity
measurements were performed after the attainment of a uniform flow in the LDA
reach, the final bed was sampled and a textural measurement was performed.
3.3 Description of the Experimental Tests
A total of seven experimental tests were performed in accordance to the procedures
above. Test E0 was performed under subthreshold conditions. Tests E1, E2, and
E3 were mobile bed experiments. Their main characteristics are shown in Table
1
.
The symbols in Table
1
are:
Q
¼
water discharge;
h
¼
water depth;
i
0
¼
bed
(t
0
/r)
0.5
slope;
u
*
¼
¼
shear velocity; t
0
¼
mean bed shear stress; r
¼
water
density; Re
¼
hV
/n
¼
flow Reynolds number;
V
¼
depth averaged longitudinal
V
/(
gh
)
0.5
Froude
velocity; n
¼
cinematic viscosity of water;
B
¼
channel width; Fr
¼
number; t
*i
¼
g)
d
i
) nondimensional bed shear stress for the size fraction
represented by
d
i
; t
*ci
¼
t
0
/((g
s
y
c
x
i
¼
critical shear stress; y
c
¼
Shields critical nondi-
mensional shear stress; x
i
¼
hiding-exposure factor;
f
LDA
¼
sampling frequency;
(
Q
s0
*
/
B
)/((
s
1)
gd
50
3
)
0.5
f
BL
¼
nondimensional sediment transport parameter;
Q
s0
*
¼
volumetric equilibrium bed-load discharge;
s
¼
specific gravity of the
sediment particles; and
g
acceleration of gravity.
As stated before, tests E1 to E3 were intended to have a fixed bed counterpart
with the same
u
*
in order to bring to light eventual changes induced by the near-bed
sediment transport in turbulence organization, undetectable by the evaluation of
macroscopic mean parameters. Fixed bed tests E1D to E3D were thus performed
upon the armored beds obtained from their mobile bed counterparts. The main
characteristics of these tests are shown in Table
2
.
The values of t
*84
and t
*c84
in Table
1
are predictions based on the composition of
the initial bed. Tests E1 to E3 were designed to show size-selective sediment transport
conditions. This intention is expressed by the condition t
*84
/t
*c84
<
¼
1.0, which holds
true for all tests. The coarser size fractions should, thus, remain immobile.
