Geoscience Reference
In-Depth Information
Table 3.
Velocity and concentration values for the simulated cases of low through a sinuous channel.
Velocity at
inlet (m/s)
Concentration
at inlet (%)
Max. velocity
at Bend 2 (m s
−1
)
Max. concentration
at Bend 2 (%)
Run
1
20
10
2.3
5
2
10
10
5
11
3
5
10
5
11
4
10
5
1.5
3.5
5
5
5
2.5
3.5
6
2
10
4
7
7
2
3
2.3
3.5
8
1
1
1.5
1.5
the overall sediment concentration. The initial
flow velocity and sediment concentration were
varied systematically for each run (Table 3) to
determine their effect on the flow behaviour in
channel bends.
The simulations in this particular case study
have not been tuned to any specific physical
experiments; however, the vertical velocity pro-
file of the turbidity current (Run 2) measured
before entering the first bend has been compared
to previously published experimental and mod-
elled data (Keevil
et al
. (2006), Straub
et al
.
(2008) and Sumner
et al
. (2008)) indicating real-
istic flow properties (Fig. 12B). It should also be
mentioned that the simulated results are inde-
pendent of the grid resolution. Reducing the cell
number of the computational grid by half, and
the resolution by 35%, the model gives almost
identical velocity and concentration values
(Fig. 12C and D).
Run 2 and Run 7 in Fig. 13C to F). As a result, flow
separation zones developed in the downstream
part of the inner bends, where the turbulence
intensified and velocities dropped to near zero
values. The extent and position of separation
zones varied between the runs. Flows with high
velocities and concentrations developed extensive
separation zones, located further downstream of
bends than flows with low velocities and concen-
trations (Fig. 13C and D; 14A and B). Additionally,
overbank flow re-entering the channel in the
downstream part of Bend 2 and the upstream part
of Bend 3 seem to have further accentuated the
turbulence in these areas. The velocity and con-
centration changes also had an effect on the heli-
coidal circulation at channel bends. Flows with
velocities of 5 m s
−1
and higher displayed a weak
basal-inward or basal-outward circulation at Bend
2 (Fig. 14C), whilst currents with velocities below
5 m s
−1
all displayed basal-inward circulation cells
(Fig. 14D). The ratio of maximum velocity versus
flow height for the currents with a basal-inward
helicity ranges from 0.19 to 0.41. The basal flow in
helicoidal cells is generally oriented 2° away from
the channel axis towards the inner bank and there
can be a difference of up to 54° between the basal
and the upper flow direction.
The behaviour of turbidity currents at the higher
end of the velocity spectrum (velocity ≥ 5 m s
−1
)
can be compared to the laboratory experiments of
Peakall
et al
. (2007), Amos
et al
. (2010) and Straub
et al
. (2011). The flows in these experiments
showed significant super elevation at channel
bends, profound overspill and an outward-
directed secondary circulation cell. The high-
velocity core of these currents travels attached
to the inner bank in the upstream part of the
bend and becomes detached from it at or directly
downstream of the bend apex, with excessive
Results and remarks
In each of the simulated runs the turbidity current
experienced significant super elevation at bend
apexes accompanied by flow overspill (Fig. 13A
and B; 14C and D). In flows with initial velocities
of 5 m s
-1
and higher, this effect was so prominent
at the first bend that the velocities directly down-
stream dropped by 50% to 90% (Table 3). Super
elevation and flow overspill decreased with
decreasing initial velocity and increasing concen-
tration, making the slower, higher-concentration
flows more confined to the channel and more effi-
cient in maintaining a constant velocity during the
downstream transport. The high velocity and high
concentration core of the flow travelled down
channel attached to the outer bank in all three
bends during all of the eight runs (see example of
Search WWH ::
Custom Search