Geoscience Reference
In-Depth Information
In preliminary computations using channel 1, various particle tracking experi-
ments were carried out (particle diameters: 0.01, 0.05, 0.1, 0.15, and 0.2 mm). Two
diameters are chosen that characterize bed load and suspended sediment, respec-
tively:
D
1
¼
0.09 mms
1
).
Van Rijn (
1993
) defined, as a criterion for the extent of suspension of the
sediment, the suspension parameter
Z
20.2 mms
1
)and
D
2
¼
0.15 mm (
w
1
¼
0.01 mm (
w
2
¼
¼
w
/
k
u
*
. For the two chosen particle sizes,
this parameter has the values:
z
1
¼
2.0 (bed load and suspended sediment in near-
bed region),
z
2
¼
0.009 (suspended sediment). The Shields parameter is defined
as
y ¼ t
w
/(
r
s
r
)
gD
, whereas the beginning of motion occurs if
y > y
cr
with
y ¼
0.03 ~ 0.06, depending on the Reynolds number. For the chosen particle sizes,
the Shields numbers are
y
1
¼
y
2
¼
5.98. According to Shields diagram
(Simons and Senturk
1992
), these particles will be moved by the flow.
The larger particles (
D
1
¼
0.41 and
0.15 mm) are released in the lowest 0.10 m of the
inlet. The smaller (
D
2
¼
0.01 mm) are released uniformly over the inlet. The
particle motion on the bottom is defined as saltation (jumping); sediment bounces
along the stream bed by the energy and turbulence of the flow. Interaction between
the particles is not modeled.
After each flow computation, two particle tracking computations are executed
using different particle sizes (0.15 and 0.01 mm, respectively). This makes a total of
14 runs, each particle tracking being repeated three times using 1,000 particles so as
to check the replicability of the results.
3 Results
The channel without bifurcation is established as a basic geometry to investigate the
fluid flow and the movement of sediment particles. It allows to check the inlet
conditions at the inlet and the development of the profiles of the flow velocity, the
turbulent kinetic energy, the eddy dissipation, and the particle concentration.
A discharge of
Q
30 m
3
s
1
shows a Reynolds number of
Re
10
6
,
¼
¼
1.5
0.75 ms
1
)
develops over approximately half the length of the channel to an equilibrium
profile, which is in agreement with the theoretical log-profile.
At the inlet, the turbulent kinetic energy and dissipation rate profiles are set in
accordance with function
k
(
z
) and
which indicates turbulent flow. The uniform velocity profile (
u
¼
(
z
), respectively. The adaptation to equilibrium
profiles takes place mainly in the first 40 m, so the channel length is large enough
for the adjustment of these profiles to equilibrium contours.
The turbulent kinetic energy has the highest value in the boundary layer at the
bottom. At the water surface, the value is near zero, because the kinetic energy
of turbulence is very small. For both the turbulent kinetic energy and the eddy
dissipation, the maximum value appears in the first computational cell above the
bottom.
The motion of sediment particles in the basic channel is shown in Fig.
2
. The
particle trajectories in Fig.
2a
represent bed-load transport (
D
1
¼
e
0.15 mm). These
