Geoscience Reference
In-Depth Information
3.3 Bifurcation 135
: Run 4a/b/c/d (Q
1
/Q
2
¼
0.2 and 0.5)
The streaklines show the fluid motion in both sides of the branch and indicate a
spiral fluid motion (run 4a/b). The same phenomena can be observed for the
discharge ratio
Q
1
/
Q
2
¼
0.5 (run 4c/d). An analysis of the sediment distribution
for suspended load (Table
1
, run 4b/d) yields
S
1
/
S
2
¼
Q
1
/
Q
2
. Again, the bed load is
attracted into the branch more than proportionally (run 4a/c). However, this ratio is
higher than in channel 3 and less than in channel 2.
4 Analysis of Sediment Distribution
For every simulation, the 1,000 released particles are divided into three groups:
escaped, trapped, and aborted. Particles are defined to distinguish the results, as
escaped when they reach the outflow boundary of the main channel, or trapped
when they reach the outflow boundary of the diverting branch. Particles that reach
neither of the outflow boundaries, because they get into near-stagnant zones or
recirculation areas, are classified as aborted. The results show a small amount of
aborted particles (up to 3%).
Table
1
gives the sediment transport ratios
S
1
/
S
2
based on the results of the
particle tracking computations (runs 2a up to 4d).
Based on the numerical results described, the nodal-point relations for the
bifurcations are sought. The coefficients
and
m
,
n
are found by fitting the
nodal-point relations, (
1
) and (
3
), respectively, to the data found by the numerical
computations. Table
2
shows the determined coefficients.
The sediment transport ratio
S
1
/
S
2
is a function of the discharge ratio
Q
1
/
Q
2
for
the bed-load cases (
D
a
,
b
1, see Table
2
)
show that the sediment ratio is more than proportionally related to the discharge
ratio. The highest value appears for channel 2 (diverting angle 90
). The relatively
low
¼
0.15 mm). The high
a
-values (i.e.,
a >
-values imply that the linear approximations nearly cross the origin of the
axis, which logically corresponds to a zero-sediment intake for a zero-discharge
extraction.
The suspended sediment distribution is a practically proportional relation (i.e.,
S
1
/
S
2
¼
b
Q
1
/
Q
2
) between the discharge ratio and the sediment transport ratio, as can
Table 2 Coefficients
a
,
b
,
m,
and
n
for fitted nodal-point relations (
2
) and (
3
)
Particle D (mm)
Angle (
)
Channel
Equation (
2
)
Equation (
3
)
a
b
m
n
0.15 (bed load)
45
3
1.68
0.06
1.13
0.39
90
2
2.63
0.10
1.15
0.71
135
4
1.85
0.01
0.98
0.44
0.01 (suspended load)
45
3
1.12
0.02
1.05
0.09
90
2
1.07
0.00
0.99
0.05
135
4
1.05
0.00
0.99
0.04
