Journal of Applied Hydrology (1) (2) (2014) 11-17
Journal of Applied Hydrology
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Licensed by; MSRT of I.R .of Iran; No. 3/18/557925 January 29, 2014
Experimental study on control of sedimentary density current by
rough bed and obstacle
Seyed Mahmood Kashefipour a, Mehdi Daryaee b*
a
Dept. of hydraulic structures-Shahid chamran university, Ahwaz, Iran
Candidate of hydraulic structures-Shahid chamran university, Ahwaz, Iran
b Ph.D.
* Corresponding author: [email protected]
Article history:
Received: 9 Sep. 2014
Revised : 3 Oct. 2014
Accepted : 11 Nov. 2014
Abstract
Density or gravity currents have been always one of the main causes for sedimentation along many of the dam
reservoirs. So controlling this phenomenon is very important for increasing useful life of such reservoirs. In this
research study the effect of a combination of rough bed and obstacle on the control of sedimentary density current has
been investigated. Experiments were conducted in a tilting flume with 780 cm length, 35 cm width, and 70 cm depth in
hydraulic laboratory of Ahwaz University, Iran. One obstacle with the height equal to depth of the body of density
current flow (7 cm), width equal to 2 cm and length equal to 35 cm was used. Small cubic pieces with the dimensions of
15×15 mm2 were used to produce the rough bed. Three heights of cubic pieces (0.5, 1.0 and 1.5 cm), three lengths of
bed roughness (0.5, 1.0 and 1.5 m), and three roughness positions (downstream, upstream and both sides of obstacle)
were considered as the main variables in this research study, in total 28 experiments were carried out. Discharge and
concentration (density) of density current flow were kept constant and the bed slope was set to zero during all
experiments. It was found that a combination of obstacle and roughness was able to block the density current up to
90%, whereas obstacle alone with the considered height could only decrease the transport of density current about 50%.
Rough bed at upstream of the obstacle was found to be more effective than downstream of it.
Keywords: Density current management, Dam reservoir, Sedimentation, Turbidity current.
1. Introduction
Density or gravity current is a dense flow
with a density of  t which is different from
the ambient fluid (  a ) and this current exists
inherently due to the density difference and
effective
gravity
acceleration
of

g  g ( t   a ) /  a . Density current is not
only a main cause of reservoir bed erosion but
also sedimentation and reduction of useful
dam life. A schematic form of density current
is shown in Fig.1. As shown a density current
consists of head (unsteady flow) and body
(steady and quasi-uniform flow). The main
parameters of a density current are: U f = head
velocity; H f = head depth; H t = maximum
height of density current in which the velocity
is zero and can be extracted from the
measured velocity profile; H = body depth;
U = average body velocity. H and U are
calculated using the proposed equations by
Ellison and Turner (1959) as:

H
UH  0 udz  0 t udz
(1)

Gij s    rgij (r )e sr dr
0
(2)
Where, u is the velocity at height z of
density current.
A review through the old and recent
literature shows that there are plenty of
research works done on the different views
and specifications of density current. For
example, Altinakar et al. (1996) and Kneller et
al, (1999) analyzed the velocity profile
structure of density currents. Gladstone and
Kashefipour, S.M and Daryaee, M / Journal of Applied Hydrology. 1 (2) (2014) 11-17
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12
Pritchard (2010) studied the patterns of
deposition in turbidity currents. Also there are
many other research works in the literature
regarding the transport mechanism of density
currents (Xu 2010, Wells et al. 2010, Cossu
and Wells 2012).
Although the control of density current is a
very important subject especially for river
engineering and water resources management,
there are a few numerical and experimental
studies in the literature in this regard. It has
been reported in this research study that the
shape of obstacle has not significant effect on
the control of density current. Oehy and
Schleiss (2003) and Schleiss et al. (2008)
experimentally investigated the effect of nonpermeable and permeable obstacles on the
control of turbidity currents.
Fig. 1: Schematic figure of density current and its parameters (Altinakar et al. 1996).
2. Materials and Methods
Sedimentary density current was prepared
using a solution of water and stone powder
with D50 =17 m and uniformity coefficient of
4.5. Experiments were conducted in a tilting
flume with 780 cm length, 35 cm width, and
70 cm depth in hydraulic laboratory of Ahwaz
University, Iran. The flow discharge and
concentration were kept constant and equal to
0.7 lit/s and 20 gr/lit ( t  1017kg / m3 ),
respectively. The slope of the flume was set
to zero. Fig.2 shows a schematic picture of
flume and all related apparatus used in this
research study. The dense fluid was first
pumped from the supply tank to the head tank
and from there to a head tank behind a slide
gate. During each experiment the water
elevation in flume as the ambient fluid was
kept to be equal to surface elevation of
sedimentary dense fluid behind of slide gate
using a source of supply clear water and a
weir installed at the end of flume. Discharge
was measured and controlled by a flow meter
and valve 3 installed between head tank and
reservoir tank (Fig.2). One obstacle with the
height equal to depth of the body of density
current flow was used. Small cubic pieces
with dimensions of 15×15 mm2 were used to
produce the rough bed. Three heights of cubic
pieces ( K S = 0.5, 1.0 and 1.5 cm), three
lengths of bed roughness ( L = 0.5, 1.0 and 1.5
m),
and
three
roughness
positions
(downstream ( D ), upstream ( U ) and both
sides ( UD ) of obstacle) were considered as
the main variables in this research study, in
total 28 experiments were carried out.
In order to quantify the amount of control
of sedimentary density current the sediment
load transported by the head of density current
was calculated by:
QS  10 6  U f  H f  B  C f
(3)
Q
Where, S = sediment discharge (kg/s);
B = flume width (=35 cm); H f =depth of
density current head (cm); C f = average
sediment concentration of head (gr/cm3), and
10-6 is a coefficient to convert the units.
Precise estimates of U f = head velocity
(cm/s) and H f were provided using the
digital photos. To provide the average
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Kashefipour, S.M and Daryaee, M / Journal of Applied Hydrology. 1 (2) (2014) 11-17
concentration of head, samples were taken
from two depths including: 2 cm and 7 cm
from bed, where H f was measured. Each
sample was provided using a syringe installed
in a siphon, and the sample concentration was
calculated using the usual physical laboratory
instructions. For each experiment two points
were specified for measuring the above
parameters, one before beginning the rough
bed and obstacle and the other one after them
13
(Fig.2). The percentage of reduction in
sediment discharge ( QS ) is calculated by:
QS 
QSb  QSa
 100
QSb
(4)
Where, QSb and QSa are sediment
discharge before and after rough bed and
obstacle, respectively.
Fig. 2: Schematic picture of flume and its apparatus.
3. Results and Discussion
The main aim of this study was to evaluate
the combinations of rough bed and obstacle in
control of sedimentary density current. The
measured net value of sediment load
transported by the head of density current (it
can be calculated using Equations 3 and 4)
was the only parameter to quantify the effect
of roughness and obstacle for current
blockage. The velocities, concentrations and
the depth of head were measured at upstream
and downstream of the set of rough bed and
obstacle and are illustrated in Table1. In this
table U , D , and UD state for the position of
rough bed. The measured velocity,
concentration and depth of head of density
current for the experiment with only obstacle
were measured as (3.13cm/s, 3.19gr/lit and
10cm) and (2.08cm/s, 1.7gr/lit and 14.0cm)
for before and after obstacle, respectively.
Calculation of sediment discharges shows that
the obstacle alone was able to block about
50% of density current. It should be noted that
this result is for an obstacle with a height
equal to the body depth of flow (7cm in this
study) and the bed slope was assumed to be
zero.
Kashefipour, S.M and Daryaee, M / Journal of Applied Hydrology. 1 (2) (2014) 11-17
14
K S (mm)
L (m)
Table 1: Measured parameters values for different experiments.
Rough bed
Measured upstream
Measured downstream
Position
Uf
Uf
Cf
Hf
Cf
Hf
Exp. No.
(cm/s)
(gr/lit)
(cm)
(cm/s)
(gr/lit)
(cm)
U
(1)
4.16
5.10
10.00
2.42
2.50
15.00
D
UD
U
(2)
4.00
4.30
11.00
2.35
2.90
13.00
(3)
4.60
5.80
10.00
1.28
4.40
15.00
(4)
6.00
5.00
10.00
3.00
1.90
16.00
D
UD
U
(5)
4.26
4.2
11.00
2.88
2.00
14.00
(6)
3.71
2.90
10.00
1.17
1.90
14.00
(7)
3.57
4.75
10.00
1.25
2.70
15.00
D
UD
U
(8)
4.00
4.00
11.00
1.88
2.30
15.00
(9)
3.33
3.50
10.00
1.20
1.60
15.00
(10)
4.10
5.20
10.00
2.90
1.80
15.00
D
UD
U
(11)
3.50
2.10
11.00
2.15
1.10
14.00
(12)
3.50
3.60
10.00
1.21
1.80
16.00
(13)
3.85
5.80
10.00
2.20
2.10
14.00
D
UD
U
(14)
3.03
5.40
11.00
1.50
2.80
14.00
(15)
3.57
4.20
10.00
1.55
1.50
16.00
(16)
4.17
4.30
10.00
1.70
1.80
14.00
D
UD
U
(17)
3.23
3.00
11.00
1.15
1.90
14.00
(18)
3.33
4.50
10.00
1.10
1.73
15.00
(19)
4.17
4.0
10.00
1.84
1.80
16.00
D
UD
U
(20)
4.08
3.00
11.00
1.78
1.90
14.00
(21)
3.33
3.90
10.00
1.00
2.10
16.00
(22)
3.85
5.80
10.00
0.90
3.20
20.00
D
UD
U
(23)
3.64
5.00
11.00
1.95
2.00
15.00
(24)
3.85
6.80
10.00
1.20
2.60
16.00
(25)
4.17
5.70
10.00
1.45
2.00
14.00
D
UD
(26)
3.64
4.70
11.00
1.88
1.70
14.00
(27)
3.13
2.60
10.00
1.10
0.60
15.00
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0.50
5.00
1.00
1.50
0.50
10.00
1.00
1.50
0.50
15.00
1.00
1.50
Table 2: Percentage of reduction in sediment load.
Exp. No.
1
2
3
4
5
6
7
8
9
QS
(%)
57.2
53.2
68.3
69.6
59.0
71.1
70.1
63.1
75.3
Exp. No.
10
11
12
13
14
15
16
17
18
QS
(%)
63.3
59.0
72.3
71.0
67.3
75.2
76.1
71.3
81.0
Exp. No.
19
20
21
22
23
24
25
26
27
QS
(%)
68.2
64.8
74.1
74.2
70.8
80.9
82.9
76.2
87.8
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15
Kashefipour, S.M and Daryaee, M / Journal of Applied Hydrology. 1 (2) (2014) 11-17
Extra experiments (not included here)
show that for higher slopes and lower heights
the efficiency of obstacle for control of
density current significantly decreases. QS
for the aforementioned experiments in Table1
are calculated using Equation 4 and
summarized in Table2. The effectiveness of
each considered parameter can be evaluated
using Table2. In general, an average reduction
in sediment discharge was calculated about
71% for rough bed and obstacle. This means
that only 21% of sediment discharge could
transport downstream. In order to evaluate
the effect of K S on QS the results of
experiments 1-9, 10-18, and 19-27 were
separately averaged, with the values being
calculated as: 65.2%, 70.7% and 75.6%
,respectively. This results show that when the
value of K S changes from 5mm to 15mm, the
effectiveness of roughness height improved
about 10%. The average values of QS for
the experiments number of (1-3, 10-12, 1921), (4-6, 13-15, 22-24) and (7-9, 16-18, 2527) are used to check the effect of the
roughness length. These values were
calculated as 64.5% (for L=0.5m), 71% (for
L=1.0m) and 76% (for L=1.5m). As can be
seen for the considered conditions in this
research study the average effect of the
roughness length is marginally more than the
roughness height. The effect of the position of
rough bed may be evaluated by averaging and
QS for
comparing the values of
experiments number (1, 4, 7, 10, 13, 16, 19,
22, 25), (2, 5, 8, 11, 14, 17, 20, 23, 26) and (3,
6, 9, 12, 15, 18, 21, 24, 27). These values
were calculated as: 70.3%, 65.0% and 76.2%
for the positions U , D and UD ,
respectively. Comparison of these values
show that installing rough bed in upstream of
obstacle is more effective than downstream of
it. Table2 shows that the minimum effect of
roughness was for the situation in
which K S =5mm, L=0.5m and the rough bed
position was downstream of obstacle, with a
comparison being revealed that this
experiment improved only about 3% the
amount of blockage of density current flow.
Also the maximum effect was for the
experiment in which K S =15mm, L=1.5m and
the rough bed position was built along both
upstream and downstream sides of obstacle.
Its value was about QS =88%, which shows
an excellent control of a combination of
roughness and obstacle. The dimensionless
values of K S / H and L/H (H is the height of
body of density current  7cm in this study)
were calculated and plotted against the
percentage reduction of sediment load ( QS )
values (Fig.3). Obstacle and bed roughness
affect and control the density current in two
different ways. Obstacle usually blocks the
current and it has been found by the other
researchers an obstacle with a height more
than two times of body height of flow is
necessary to fully block the density current
(Oehy and Schleiss, 2003). Bed roughness
affects density current by two important
hydraulic phenomena. First reduces the flow
velocity and provides situations for more
sediment particles to be settled. Secondly, it
significantly increases the turbulence and
shear force along the density current surface
and its border with still water. The extra
experiments (not included here) show that the
water entrainment increases more than 4 times
for the rough bed in comparison with the
smooth bed. Thus density current becomes
more diluted and again the sediment particles
would be easily able to deposit along the
flume.
As it was shown in this research study the
rough bed is an effective factor controlling the
density currents, which are usually generated
during the floods. In practice and for the real
conditions producing rough bed and obstacle
in a dam reservoir is not very complicated,
especially during dam construction. For
example, upstream cofferdam can be left after
dam construction to play the role of an
obstacle.
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Kashefipour, S.M and Daryaee, M / Journal of Applied Hydrology. 1 (2) (2014) 11-17
Fig. 3: The percentage reduction of sediment load versus relative roughness for different relative length of rough bed
located at (a): Upstream of obstacle, (b): Downstream of obstacle (c): Both sides of obstacle
4. .Conclusion
In this research study the effect of bed
roughness and one obstacle on sediment
control of sedimentary density current has
been experimentally investigated. The
experiments were carried out in a laboratory
flume with the main variables being the
length, height and position of rough bed in
accordance to the obstacle. In total 28
experiments were conducted, one experiment
with obstacle only and the others 27
experiments include different combinations of
rough bed and obstacle. The main conclusions
drawn from this study are summarized as:
- Obstacle alone was able to control density
current about 50%, however, extra
experiments (are not included here) show that
the effect of obstacle significantly decreased
for higher bed slope.
- A combination of rough bed and obstacle
was able to reduce about 90% of sediment
transported by sedimentary density current.
It was found that the rough bed in
upstream of obstacle is more effective in
controlling of density current in comparison
with the downstream position of rough bed.
However, installing rough bed at upstream
and downstream of obstacle more reduces the
sediment transport.
- Bed roughness not only reduces the flow
velocity increases the water entrainment
significantly and by these two reasons is able
to control sediment transport by density
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1996. Flow structure in turbidity currents.
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Cossu, R., Wells, M.G. 2012. A comparison of
the shear stress distribution in the bottom
boundary layer of experimental density
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of Mechanics-B/Fluids 32, 70-79.
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Kashefipour, S.M and Daryaee, M / Journal of Applied Hydrology. 1 (2) (2014) 11-17
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and
jets
on
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17