Annual Journal of HydraulicEngineering,
JSCE, ofVol.58,February
Journalof Japan Society
Civil 2014,
Engineers,Ser.
B1 (Hydraulic Engineering), Vol. 70, No. 4, I_49-I_54, 2014.

INTERDEPENDENCE BETWEEN FLOWPATTERN, GEOMETRY AND FLUSHINGEFFICIENCY OF IN-GROUND STILLING BASIN
M. E. MESHKATI SHAHMIRZADI1, Tetsuya SUMI2, Sameh A. KANTOUSH3
1Member of JSCE, Ph.D. Student, Disaster Prevention Research Institute, Kyoto University (Gokasho, Uji, Kyoto
611-0011, Japan)
2Member of JSCE, Professor, Disaster Prevention Research Institute, Kyoto University (Gokasho, Uji, Kyoto
611-0011, Japan)
3Associated Professor, Civil Engineering Department, The German University in Cairo (New Cairo City, Main
Entrance of Al-Tagamoa Al-Khames, Cairo, Egypt)This paper investigates the interaction between flow patterns and geometry of in-ground stilling basin(ISB) and its influence on flushing efficiency from ISB. The main governing parameters in our study were
the normalized ISB length, drop number, normalized end-sill geometry (height and width), and Froudenumber. Using large scale particle velcocimetry, we classified the flow patterns into four groups ofB-jump, U-jump, Periodic and Steady submerged jump. We found the favorable flow pattern has apositive correlation with drop number, while a negative correlation with relative ISB length. Moreover, thelateral free spaces of the end-sill do not increase the chances for sediment to be flushed out from ISB.
Key Words: Flow patterns, hydraulic jumps, flushing efficiency, in-ground stilling basin.

1. INTRODUCTION
Design of dissipater structures below flood
mitigation dams (FMDs) still face a number ofissues. Included amongst them are; low energy
dissipation due to the effect of sediment deposition,and interruption in fish and sediment passage due tothe presence of a tall and continuous end-sill.
One solution was suggested by MCD (Miami
Conservation District, USA)1) to dig out the apron of
the stilling basin creating a pool-shape stilling basin
below FMDs. This type of stilling basin is called an
in-ground stilling basin (ISB), which potentially canfacilitate migration of aquatic animals2). Althoughdigging out the ground to create ISB is costly, bydeepening the stilling basin we may are allowed toshorten the height of end-sill. A shorter end-sillheight may compensate significant portion of thecost for deepen stilling basin.
An additional proposal investigated in thepresent study is to consider free spaces at bothlateral sides of the end-sill to create a passage forfish and sediment which could be an eco-friendly
alternative for a common continuous end-sill. It may
increase the transparency of the FMD in terms ofboth sediment supply and fish migration. Fig. 1

illustrates a conceptual schematic view of the ISB
downstream of a typical FMD.
There are an enormous number of studies inliterature devoted to the hydraulic characteristics offlow and sediment within stilling basins. Thesestudies mainly concern, firstly, the occurrence of
hydraulic jumps within a confined area; andsecondly, control of the position of hydraulic jumps
within it by: a) adding appurtenances e.g. sills,
baffle blocks and an end-sill 3, 4) and b) modifying
the stilling basin structural design e.g. rise in bed
level (positive step), abrupt drop in bed level(negative step), enlargement in bed width (gradualor sudden) and rough bed 5,6, 7).

FMD

BottomoutletIn-groundstilling basin

End-sill

Positive
step

Fig. 1 Schematic view of in-ground stilling basin (ISB) below
FMD.

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Annual Journal of Hydraulic Engineering, JSCE, Vol.58, 2014, February

Based on the literature review it was found that, alack of information exists on the combination of aforced hydraulic jump with a simultaneous presenceof a sudden enlargement and an abrupt drop. A
forced hydraulic jump within a confined andcomplex structure such as an ISB has yet to besolved in the literature and still future studies areneeded. Moreover, considering two free spaces atboth sides of the end-sill in the presence of a
movable bed increases the complexity of thephenomenon. This study focuses on a small
expansion ratio (k=bottom outlet width/stillingbasin width) of stilling basin equal to 0.2, as it ismore practically common and causes a periodic andunstable flow pattern rather than larger expansion
ratios8).
The major objectives of this paper are: a) toevaluate the interaction between flow patterns,
geometry and flushing efficiency of ISB under
different hydraulic and geometrical conditions. b) todeeply clarify the effect of end-sill height and freespaces at both sides of the end-sill on theperformance of ISB in the presence of movablesediment. The results were tabulated in terms ofstatistical measures and also illustrated throughscatter plots.

for all of the experiments run. A water level meter
was used to measure the water depth and its
fluctuation throughout ISB. The sampling frequency
for water level meter was set to 50 HZ. Using anelectro-magnetic current meter the 3D component ofvelocity was measured in different transversal andlongitudinal cross sections for each test run.
Moreover, for each test digital photographs weretaken from side and plan view of ISB and visualobservations were recorded using high resolution
digital camera.
(3) Experimental condition
Extensive numbers of experiments were carriedout under different geometrical and hydraulicconditions as: ISB length (L= 75, 100 and 125 cm),
step depth (s= 5, 10 and 15 cm), different geometryof end-sill with various heights (he= 0, 4, 8, 12 and13.5 cm) and various widths (be= 50, 40, 30, and 20cm) has been examined. However, the width of
ISB itself for all experiments was the same B= 50
cm equal to the width of flume.
For each ISB geometry configuration, In addition
to the three different Froude number of supercriticalflow at the bottom outlet (namely Fr1= 2.8, 4.9 and5.1), only one dimension of bottom outlet (h1= 5 cm
and b1= 10 cm) was examined, thus, creates an
expansion ratio of k=0.2 (k=b1/B), similar to theaverage practical expansion ratio for FMDs. Table 1
shows the experimental condition considered in thepresent study.

2. MATERIALS AND METHODS(1) Experimental setup
A scale model of stilling basin is constructed at
Disaster Prevention Research Institute, Kyoto
University. This scale model is based on the Froude
similarity of prototype stilling basins downstream ofthree famous FMDs in Japan. The scale ratio oflength in this study is 1/40. Fig. 2 introduces themain governing parameters that govern the energy
dissipation process within an in-ground stillingbasin.
Water surface

BottomoutletU0

K=b1/B0.2

h3

hePositivestepsSide view

L
I

b1

B

s/h1

he/hc

1-(be/B)

Fr1

1.5

1-3

0-4

0-0.6

2.8-5.1

2

1-3

0-4

0-0.6

2.8-5.1

2.5

1-3

0-4

0-0.6

2.8-5.1(1) Possible flow patterns within ISB
The frequently observed flow patterns within
ISB can be summarized into four major families
as: B-jump, U-jump, Periodic submerged jumpand Steady submerged jump. The definition of
the different jumps observed in current study iswell described in the past9). The previous
publication of the authors concerned about theclassification of flow patterns within ISB; where
it was noted that the type of hydraulic jumps
within ISB is less dependent to Froude number at
the face of bottom outlet, Fr1, and more
dependent on the end-sill height at the end
downstream of ISB, he 10); similar findings werereported by Kantoush et al. 4) and Ohtsu et al. as
well 5).

I

U2, h2L/B

3. RESULTS AND DISCUSSION

II

I

h1

Table 1 Experimental condition.

bePlan viewFig. 2 Schematic side and plan view of the in-ground stillingbasin (ISB) and the main governing parameters.

(2) Experimental measurement
The basic data collection procedure was the same

2

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Annual Journal of Hydraulic Engineering, JSCE, Vol.58, 2014, February(2) Flow Pattern Visualization using LSPIV
Large-scale particle image velocimetry(LSPIV) was used to evaluate the variation offlow velocities on the surface of flow within ISB
using flow manager software. A digital camera isused to record images. White particles and lightsources were used for velocity measurements.
Fig. 3a-c illustrates the time average surface
velocity pattern obtained from LSPIV for
U-jump, Periodic submerged jump and Steadysubmerged jump respectively. The velocity
distribution spectrum in this figure is from dark
black to bright white, where, the dark black
indicates high negative velocities and brightwhite shows the area with higher positive
velocities. The velocity vectors are depicted bygreen arrows and the gray lines on the plots are
representative of the velocity contours on thesurface of the flow.
In the case of a relatively short end-sill height
and long ISB length, a narrow and symmetricsuper critical current is observed along thecenterline of the ISB and creates V-shaped steadywaves at the water surface, in which the roller
fronts are continuously undulating over thestagnant water within ISB (U-jump). The toe ofthe jump in this flow pattern is located close to
the bottom outlet (left-hand side of Fig. 3(a)). A
further increase in end-sill height, or decrease in
ISB length leads to submergence of the bottom
outlet, in which, the wall above the bottom outlet
obstructs the jump to move upstream. The
submerged jumps observed in the present
research were mostly periodic submerged jumps
and steady-asymmetric submerged jump8) (Fig.
3(b) and (c)).

50
(a)
40
30
20
10
0
10

0

20

30

40 50

60 70

80

90 100

20

30

40

50

60

70

80

90

20

30 40

50

60

70

80

90 100

50
(b)

40
30
20
10
0

10

0

100

50
(c)
40
30
20
10
0

10

0

Fig. 3 Stationary flow field by using LSPIV; a) U-jump inpresence of he=4 cm, b) Perio. Sub. Jump in presence of he=8
cm and c) Steady Sub. Jump in presence of he=12 cm, and allunder following conditions L=100 cm, s=10 cm, be=50 cm and
Q=15 l/s.
4
B-jump3.5(3) ISB performance under fixed bed conditiona) Interdependence between flow pattern and
ISB geometry
Herein, two flow pattern maps are presented
based on the successful experimental data shownin Table 1. These flow pattern maps consist ofboth geometric and hydraulic parameters (Fig. 4
and 5). Fig. 4 depicts the relationship between
relative end-sill height, he/hc, and relative ISB
length, L/B, for all successful experimental data.
In this figure, all four possible flow patterns
within the ISB are shown. Fig. 5 attempts to plot
the possible flow patterns based on the relative
end-sill height, he/hc, versus the drop number,s/h1.
The variation in drop number, s/h1, andnormalized ISB length, L/B, also showed a minor
correlation with flow patterns within ISB. As it
shown by dot line on Fig. 4, increasing the ISB

U-jump
Perio. Sub. jump

3

Steady Sub. jump

he/hc

2.5
2

1.5
1
0.5
0
1

1.5

2

2.5

3

L/B

Fig. 4 The flow pattern map within ISB, concerning thenormalized ISB length, L/B, and normalized end-sill height,
he/hc.

length negatively impacts on the stability of theflow. Moreover, increase in drop number, s/h1,
slightly improves the flow stability and allows us toshorten the end-sill height accordingly (Fig. 5).

3

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Annual Journal of Hydraulic Engineering, JSCE, Vol.58, 2014, February
4
B-jump3.5

he/hc

U-jump3

Perio. Sub. jump

2.5

Steady Sub. jump

(4) ISB performance under movable-bed
condition
To inspect the performance of ISB in the presenceof movable sediment, all the parameters kept
constant and only end-sill width, be, and end-sillheight, he, varied. In total, eight experiments were
designed as shown in Table 2. The experimental
conditions consist of only on grain sizes of; sandgravel with diameter of 15 mm, uniform andnon-cohesive. Preliminary tests showed that almost
no more sediment with mean diameter of 3 to 7 mm
was remained within ISB in less than half an hour
after the experiment is started. Thus, we run theexperiments with larger diameter of sediment. To
find out the suitable diameter of sediment we used
the scale factor of the sampled sediment obtained
from Masudagwa stilling basin. The experimentswith gravel are named SG1-8, in which the ISB was
filled up with a thickness of 3 cm and each test was
run for 1 hour to reach the equilibrium condition.
Gravels flattered on the apron of stilling basin may
provide a closer condition for ISB to the natural
condition. Moreover, it could help to reduce the costof protection works against abrasion.

2
1.5
1
0.5
0
0

1

2

3

4

s/h1

Fig. 5 The flow pattern map within ISB, concerning the drop
number, s/h1, and normalized end-sill height, he/hc.
.
0.7
MMM-(X)

0.6

MTM-(X)(U0-U2)/U0

0.5
0.4
0.3
0.2
0.1
0

0

0.2

0.4
1-(be/B)

0.6

free space equal to 20% of channel width gives analmost equivalent performance to the no free spacecondition.

0.8

Fig. 6 The variation of normalized velocity reduction within
ISB versus degree of end-sill width, L/B=2, s/h1=2, Fr1= 4.3.

Table 2 The designed variables to investigate the effect ofheight and free spaces at both sides of end-sill.

The previous publication of the authors 10)
demonstrated that taller end-sill could induce thefavorable flow patterns within ISB, which issubmerged jump, and that is the most effective
possible flow patterns in velocity reduction as well.
b) Effect of degree of free space and normalized
end-sill height on velocity reduction
To clarify the effect of free spaces on velocity
reduction two groups of tests are designed; in which
all parameters were constant and only the end-sill
width was varied. The first group of experiments
concerned the effect of free spaces with a medium
end-sill height (MMM: he/hc=1.8) and the second
group of experiments with a taller end-sill (MTM:
he/hc=2.7) (See in Fig. 6). The horizontal axis in Fig.
6 shows the degree of end-sill width, 1-(be/B), in
which be is the width of the end-sill and B is thewidth of the channel. The vertical axis is thenormalized velocity reduction, U0-U2/U0, between
section I and II. U0 is the initial outlet velocity andU2 is the largest average velocity measured just
upstream of end-sill. It is realized that decrease inend-sill width, negatively affects the velocity
reduction within ISB, but considering a width of

Exp.
Case

he/hc

1-(be/B)

d50(mm)

Observed
Flow Pattern

SG1

1.7

0

15

Steady Sub. jump

SG2

1.7

0.2

15

Perio. Sub. jump

SG3

1.7

0.4

15

U-jump

SG4

1.7

0.6

15

U-jump

SG5

2.6

0

15

Steady Sub. jump

SG6

2.6

0.2

15

Steady Sub. jump

SG7

2.6

0.4

15

U-jump

SG8

2.6

0.6

15

U-jumpa) Definition of flushing efficiency
If we define the flushing efficiency from ISB as
illustrated in Eq. 1; by knowing the initial volumeof sediment within ISB (Vi) before being exposed todischarge, and the final remained volume of
sediment within ISB (Vf), we simply could realize
the flushing efficiency.
(1)

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Annual Journal of Hydraulic Engineering, JSCE, Vol.58, 2014, February
(a)
(a)
50

b) Effect of free spaces on flow symmetry
Fig. 7(a) to (d) illustrates the final bed
topography within the ISB for SG5-8. Having freespaces at both sides of end-sill resulted in asymmetric bed topography in equilibrium condition.
While, in case of no free spaces (SG5), the bed
topography in the equilibrium state was asymmetric.
Presence of sediment deposition at the lateral side
and within ISB confined the jump sideward. It
eventually increases the expansion ratio, thus thepossibility for having a symmetric flow pattern ishigher as mentioned by Ohstu et al. 8)

40
30
20
10
0
20

30

40

50

60

70

80

90

100

30

40

50

60

70

80

90

100

30

40

50

60

70

80

90

100

30

40

50

60

70

80

90

100

(b)
50
40
30

c) Effect of free spaces on bed abrasion pattern
By comparing Fig. 7(a) to (d), it can also befound that the larger free spaces caused larger severerosion at the base of end-sill. For instance in SG7-8
the eroded area just upstream of end-sill is
considerably large. Due to larger free spaces inSG7-8, the sequent depth of hydraulic jumps within
ISB becomes much lower. Consequently, the type ofjump from submerged jump changes to U-jump.
U-jump hits the end-sill and positive step at ISB
end. The reflection of flow from end-sill andpositive step creates a large and strong verticalvortex just upstream of end-sill. This vortexes are
the main reason for the sever abrasion just upstreamof the end-sill. (See in Fig. 7(a) to (d)).

20
10
0
20

(c)
50
40
30
20
10
0

20

(d)

d) Effect of free spaces on flushing efficiency
In contrary with our expectations, the free space
reduced the flushing efficiency of ISB. This factcomes from the interaction between the geometry ofend-sill and the flow pattern. The wider the freespace, the plunged jump into the ISB is lessdownward. As explained earlier, when the plunged
jump into ISB is less downward, the reverse flow
within ISB is much stronger. Thus, the sedimentfrom the base of end-sill would transport upstream,
then, the chance for sediment for being flushed out
is very low (See in Fig. 8).
However, as can be seen in Fig. 8, there is an
exception. In SG6 a greater flushing ratio has been
observed rather than SG5. The flow pattern in caseof SG5 is an asymmetric submerged jump, and it isoriented to the right side wall. It provides a big
horizontal circulation system within ISB that carrythe entire eroded sediment from right side-wall tothe left sidewall and then deposit them there. Thus,
the mount of sediment deposited is formed only in
one side of the ISB. While, by considering aminimum free space (SG6) the flow pattern within
ISB becomes symmetric. It causes to creation of two
mounts just upstream of end-sill base. Therefore,
more sediment can be flushed out from ISB in this
case.

50
40
30
20
10
0

20

Fig. 7 The bed topography within ISB under he/hc= 2.6; a) SG5:
1-(be/B)=0 , b) SG6: 1-(be/B)=0.2, c) SG7: 1-(be/B)=0.4 and d)
SG8: 1-(be/B)=0.6.

We propose to avoid the asymmetric flow pattern
within ISB, in particular when the expansion ratio,k, is small, by considering a minimum required freespaces at both side of end-sill we can positively
provide a symmetric flow within ISB. Moreover,
32% increase in normalized end-sill height, he/hc,
positively leads to 12.5% increase in sedimentflushing from in-ground stilling basin (see in Fig.
8). It is also showed that the free space width isaround 20% of the channel width has almost same
performance regarding to velocity reduction with nofree spaces condition (See in Fig. 6).

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Annual Journal of Hydraulic Engineering, JSCE, Vol.58, 2014, February

certain geometric and hydraulic conditions.
Through this experimental study we concluded
that deepening the stilling basin allows us to slightly
shorten the end-sill height. However, a taller end-sillprovides better ISB performance with regard to both
energy dissipation and flushing efficiency.
Furthermore, it was found that due to the small
expansion ratio of the stilling basin, even in
combination with a very tall end-sill, a periodic
submerged jump with deflection is induced within
the ISB. This type of flow is typically unfavorable
for design purposes. But the experimental results
evidence that, free lateral spaces at the end-sill canbe utilized to stabilize the symmetry of flow.
Moreover, considering free spaces equal to 20%
of channel width, provides other important
advantages as higher flushing efficiency, a passage
for fish, and improvement of the flow condition
downstream of the ISB.

SG6

SG7
SG5

SG8

SG1
SG2

SG3

SG4

Fig. 8 Flushing efficiency ratio versus degree of free space forSG1-8.

f) Effect of end-sill height on flushing efficiency
We realized that a taller end-sill could finallyresult in a better performance for ISB regarding
flushing (Fig. 8). This finding is in contrary with our
first hypothesis as mentioned in the introduction ofthis paper. The main reason why a taller end-sillleads to a higher flushing ratio is that the taller
end-sill induces a greater sequent depth resulting ina jump directed downwards into the ISB apron.
When the jump hits the ISB apron, it cleans out all
the sediment in the vicinity and transports it to thebase of the end-sill. As shown earlier in these cases(Fig. 7a) this would form a mound just upstream ofthe end-sill. The sediment at the top of the moundhas a higher chance to be flushed out from the ISB.
While, in case of a shorter end-sill (SG1-4),
because of a lower sequent depth the jump is
directed almost horizontal and hits the end-sill andpositive step at the downstream end of the ISB,
resulting in the creation of strong verticalcirculations at the base of the end-sill. The
consequence of these circulations along with intense
reverse flow at the lateral sides of the ISB is totransport back all the sediment and deposit them atthe lateral sides of ISB far from the end-sill,
resulting in less chance for sediment to be flushed
out.

REFERENCES
1)
2)

3)

4)

5)

6)

7)

8)

CONCLUSION

9)Present study systematically investigates theinfluence of in-ground stilling basin (ISB) geometryon the flow and sediment flushing/deposition
processes. It was found that geometrical parameters,such as end-sill height and ISB length, plays keyrole in defining the flow patterns within an ISB thatthen influence on its performance. Two universal
flow pattern maps were proposed to ease theidentification of flow patterns within an ISB under

10)

6

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