THE EFFECT OF IN-GROUND STILLING BASIN GEOMETRYCOMBINED BY SLIT-TYPE END-SILL ON FLOW PATTERN AND
ENERGY DISSIPATION BELOW THE FLOOD MITIGATION DAMS
M. E. Meshkati Shahmirzadi1, Tetsuya Sumi2 and S. A. Kantoush3

1

Ph.D. Candidate, Disaster Prevention Research Institute, Kyoto University, Goka-sho, Uji-shi,
611-0011, Kyoto, Japan, (e-mail: meshkati.shahmirzadi@gmail.com)

2

Professor, Disaster Prevention Research Institute, Kyoto University, Goka-sho, Uji-shi, 611-0011,
Kyoto, Japan, (e-mail: sumi.tetsuya.2s@kyoto-u.ac.jp)

3

Associate Professor, Faculty of Engineering and Material Science, German University in Cairo, New
Cairo City, Al-Tagamoa Al-Khames, Cairo, Egypt, (e-mail: sameh.kantoush@guc.edu.eg)

Abstract: In this paper, an ecofriendly concept of stilling basin (SB) is introduced; called In-ground
Stilling Basin (ISB), which can be optionally equipped with a slit type (non-continuous) end-sill at its
downstream end. ISB is a non-prismatic SB with a sudden enlargement in transversal cross section andan abrupt drop in the bed at its upstream end, and is accessorized by a positive step at its downstream end,so that it creates a drop box below FMD’s outlet; a geometry similar to an elongated pool below thebottom outlet. Particular interest during our study was to solve the problem of fish and sediment passages
disruption in conventional stilling basin design. Thus, extended series of experiments were carried out toobtain the optimum ISB geometry (length and depth) and optimum end-sill geometry (height and width),
so that be acceptable in terms of hydraulic, economic and environmental aspects. To achieve thispurpose, the experiments were conducted under various hydraulic conditions such as: stilling basin
length (L= 125 and 75cm), step depth (s= 5 and 15cm), end-sill height (he= 0, 4, 8 and 12cm) andend-sill width (be= 50, 40 and 30cm). Observation of the experiments showed that the forced hydraulicjump within ISB has a very complex 3D nature and highly influenced by geometry of ISB as well as
end-sill. In this respect, a higher end-sill can effectively reduce the magnitude of velocity along thecenterline of SB but it may be an obstacle for fish and sediment passage. However, considering two freespaces at the lateral side of end-sill shows almost equal function for velocity reduction to that of fully
end-sill, providing additional positive solution for smooth fish and sediment passing.
Keywords: flood mitigation dam, stilling basin, abrupt drop, sudden enlargement, forced hydraulicjump, slit-type end-sill.

1.

INTRODUCTION

Recently, enormous floods have been experienced worldwide more often than the past and it
causes severe losses and damages for human being properties and civilization. Thus, developing
innovative strategies is vital for protection of urban area against the massive floods. One of thewell-known constructive flood control measures is Flood Mitigation Dam (FMD) which attracted
much attention over past decades. FMD is defined as a dam devoted only to flood retention andretardation which its storage volume is completely dry except for a few weeks per century, while incase of flood events the flood flow can be stored temporary its inside and gradually discharge out
through its gateless bottom outlet (Lempérière, 2006).

In practice, FMD’s design is still facing to several problematic; and need more investigation inorder to improve its design. In particular, design of stilling basin at the downstream of FMDs required
to be modified. Stilling basin (SB) is a hydraulic structure aimed to dissipate the excess energy of flow
and prevent the undesirable scouring at its downstream area by inducing hydraulic jump. Truncate of
hydraulic jump within a limited area is not simply achievable, unless utilizing appurtenances such as
fully width (continuous) end-sill with an adequate height to compact the jump, resulting in reductionof SB length and an economical design. But it should take in to account that, a fully width end-sill can
negatively disrupt the fish migration and sediment transport in river system. Then, presence of fully
width end-sill creates the contradictory goals for SBs.
In this paper, a new innovative concept of SB is introduced; called In-ground Stilling Basin (ISB),
which can be optionally equipped with a slit type (non-continuous) end-sill at its downstream end. ISB canbe defined as a non-prismatic SB with sudden enlargement in the flume width and abrupt drop in the bed atits upstream end and a positive step at its downstream end (with a height equal to abrupt drop), so that it
creates a drop box below FMD’s outlet; a geometry similar to an elongated pool below the bottom outlet.
The schematic view and hydraulic parameters of ISB are shown in Fig. 1. The slit-type end-sill, indeed, issimilar to the conventional fully width (continuous) end-sill as a point of view of shape; and just the lengthof slit-type end-sill is not equal to the width of channel and it has two free spaces in both lateral sides which
can facilitates the fish and sediment passing. Both either positive step only or its combination with slit-type
end-sill are intended to force hydraulic jump within ISB apron without the assistance of tail-water depth
downstream. Present research thus can be considered as a subset of forced hydraulic jump studies.

BottomoutletFMD

h0

End-sillh2

U0
Abrupt
drop

z

he
s

x

Positive
step

L

Figure 1: The schematic side view of ISB concept downstream of FMDs.
Extensive experimental studies have been completed on hydraulic jumps below the expansions which
can be classified into three main categories as: hydraulic jump in case of only sudden enlargement in the
flume width (Bremen and Hager, 1994; Ohtsu et al., 1999; Zare and Doering, 2011), only abrupt drop at thebed (Rajaratnam and Ortiz, 1977; Hager and Bretz, 1986; Ohtsu and Yasuda, 1991; Hotchkiss and Larson,2005), and combination of both sudden enlargement and abrupt drop (Katakam and Rama, 1996; Frerri andNasello, 2002) whose commonly used in the reality. The majority of above-mentioned studies have focusedon the details description on hydraulic characteristics of jump as well as prediction of sequent depth.
Literature review evince that a few number of papers concern the combination of abrupt drop and sudden
enlargement simultaneously. Katakam and Rama (1996), expand an analytical and experimental study ofthe hydraulic jump in stilling basin with abrupt drop and sudden enlargement. They found that the required
tail-water level to ensure the hydraulic jump within stilling basin can be reduced by combining the sudden
enlargement and abrupt drop. They introduce the most common feature of hydraulic jump in case ofsimultaneously abrupt drop and sudden enlargement; called spatial B-jump which has relatively higher
energy loss compared to either the special jump or B-jump. Frerri and Nasello (2002), provide qualitative
physical explanations on sequence of different hydraulic jumps below abrupt drop combined with sudden

enlargement as tail-water depth increases. They stated that, hydraulic jumps at a drop combined withsudden enlargement present very different and complex characteristics whose overall characteristics at time
are mainly referable to those of drop only, at other times to those of enlargement only, while yet other times
the jumps just have autonomous characteristics. Additionally, they emphasize on necessity of specificexperimental investigation for each type of hydraulic jump that can occur in order to design of structure.
There are some differences between the present study and exist studies in the literature. First, the approachsupercritical flow after discharging out through the bottom outlet was plunged into the ISB pool (drop box)
and then was encountered to a positive step as well as an end-sill at the downstream end. Second, there wasno adjustment for tail-water depth at far downstream of flume. Third, the combination of positive step andthe slit-type end-sill had not been examined yet.
Present study experimentally evaluates the functionality of in-ground stilling basin (ISB) as analternative for energy dissipation of high velocity flow exiting from the FMD’s bottom outlet.
Particular motivation during study was to solve the problem of fish and sediment passages disruption
in conventional stilling basin design. Additionally, evaluation of the functionality of ISB pool forresidency of aquatic animals as a desire habitat was one of the other interest point in current research.
Thus, an extended series of experiments were carried out to obtain the optimum ISB geometry (lengthand depth) as well as the necessary end-sill geometry (height and width) that would force and stabilize
hydraulic jump for given discharge and bottom outlet dimension. Proposing a new design procedureby using the successful test outcomes is the ultimate goal of this experimental investigation which it
should be valuable for practicing engineers.

2.

EXPERIMENTAL INVESTIGATION
2.1. Main governing parameter definition

Fig. 2 shows the schematic side and plan views of the constructed model at Disaster Prevention ResearchInstitute of Kyoto University (Japan) including the main hydraulic parameters involved in this study. Based on
the test series, it was found that the energy loss (HL) within in-ground SB to be dependent on the following
parameters: outlet velocity at the outlet exit (U0), outlet width (b0), outlet height (h0), drop height or in otherwords, step depth (s), SB width (B), SB length (L), sequent depth (h2), end sill height (he), the width of freespace at the lateral side of end-sill (2be) water density (ρw) and gravitational acceleration (g):(1)
Since U0, b0, h0, B, ρw and g are constant in present study, thus Eq. (1) could be simplified as Eq.
(2).
(2)
Lastly, the relative energy loss (HL/H1+s) can be written as a function of four dimensionlessparameters as below:(3)
where H1 is the total energy at the face of outlet to the ISB, y is the ratio of sequent depth to theheight of outlet (h2/h0), β is the relative end-sill width ((B-2be)/B), S is the drop number ((s+he)/h0), α isthe aspect ratio of ISB (B/L) and e is the expansion ratio (B/b0). It should be take into account that, thesequent depth (h2) can be defined in Eq. (4) where hc is the critical water depth over the end sill (Fr =1).
(4)

Main reservoir
Water Surface

Pierced sheets
1.50 m

Bottom outlet

H
U0

h0
s

h2

h3

End-sill

50 cm
heighthe

Side view of model

In-ground SB
L

2.5 m

9 m Length
Lateral free spacebe
Pierced sheets V0

U0b0

50 cm widthbBbeMain reservoir

In-ground SB

Plan view of model

Figure 2: The schematic side and plan view (not to scale) of experimental setup.

2.2. Experimental condition and limitations
The experiments were carried out under different ISB geometries as follow: stilling basin length(L= 125 and 75cm), step depth (s= 5 and 15cm), however, the width of ISB for all experiments was
the same B= 50cm equal to the width of flume. For each ISB geometry configuration, different
geometry of end-sill with various heights and width has been examined systematically to obtain the
optimum case as: end-sill height (he= 0, 4, 8 and 12cm) and end-sill width (be= 50, 40 and 30cm). The
end-sills used in this study were uniform in both height and thickness and were placed vertically above
the positive step downstream end of ISB, perpendicular to the longitudinal axis of flume. In addition to
the given Froude number of supercritical flow at the bottom outlet exist (namely Fr= 5.5), only one
dimension of bottom outlet (h0= 7cm and b0= 10cm) was examined as FMD’s bottom outlet, create an
expansion ratio of 5.

2.3. Experimental setup
The experiments were conducted in a horizontal rectangular flume, 0.5m width, 0.5m deep and 9m
long. This flume had a transparent Plexiglas sidewall at the left side of stream flow direction whichfacilitates the visual access. A centrifugal pump was employed to support the recirculating water
system in this study. The height of opening (h0) was simply changeable by sliding the vertical gate up
or down. Two pierced plates attached with pieces of cotton sheets were installed inside of main
reservoir to attain the laminar flow before discharging out into the in-ground stilling basin (SB). Since
the function of gate in this study was exactly similar to the concept of bottom outlet in FMDs; theterminology of bottom outlet was selected for the gate. The exit of bottom outlet faced to ISB structure
and its entrance was located 50cm inside of main reservoir to guarantee the straight streamlines offlow approaches to exist. Flow was entered into the ISB through bottom outlet, so that a uniform
supercritical flow with a thickness equal to the outlet was fall into the ISB.
An electro magnitude wave gage (EMWG) was used to measure the water depth fluctuation
throughout the in-ground SB as well as the average water depth of stream-wise along the in-ground SB.

Additionally, the instantaneous 3D velocity of water body was measured in different transverse andlongitudinal cross sections using electro magnitude current meter (EMCM) for each test run. For eachtest digital photographs were taken and visual observations were recorded using high resolution digital
camera. The basic data collection procedure was the same for all of the experiments run.

2.4. Experiments program
As previously mentioned, present study aimed to untimely propose a new design guideline forstilling basin downstream of FMDs how to be hydraulically, economically and environmentally
acceptable. To achieve this final purpose and to be able to propose a robust and useful design guideline,
it is necessary to put efforts in organized and systematical manner. Here in the details about theexperiments program was described.
The experiments were conducted in two phases of “clear water” and “clear water with movablebed”. In the first phase of experiments, clear water condition, 50 experiments were carried out to selectsuitable geometries of ISB in which dissipation of energy and stabilization of hydraulic jump weresignificantly higher than others. The hydraulic criteria used to evaluate the overall efficiency of ISB
geometry (from the hydraulic point of view) includes: dissipation of energy between bottom outlet
section and a section at the downstream of end-sill, momentum at the downstream of end-sill, type,symmetry and stabilization of hydraulic jump within ISB, fluctuation of water surface within ISB, themaximum height of standing wave over the end-sill, stream-wise velocity reduction along thecenterline of ISB, and tail water depth at the downstream of end-sill.
It should be noted that precise velocity distribution measurements have been done in several
longitudinal and transversal cross sections. At least 5 minutes, 3D velocity measurement for each point
was considered to create the spatial velocity distribution within ISB and obtains the variation ofvelocity in each point over time. This kind of measurement is needed for environmental analysis of
ISB design. Velocity vectors and magnitude, and its variation in each point over time are threerequired indicators to answer the question of which geometry of ISB is more acceptable from theviewpoint of habitat for aquatic animals.
Moreover, total wetted perimeter and total wetted area of ISB was considered as two mainindicators for selecting the more economical acceptable geometries. Then, the optimal geometry was
selected between the cases that could satisfy all three terms of hydraulic, economic and environmental
requirements.

3.

RESULTS AND DISCUSSION

Fig. 3 plots the mean stream-wise velocity (Uave) distribution along the centerline of ISB for
different end-sill heights. As can be seen, in case of experiment without end-sill, the Uave wasremained considerably high until the end of ISB. While for the case of fully end-sill with 10cm heightthe lowest magnitude of Uave was observed. The laboratory observation also proves the results
obtained by flow velocity measurements. In case of the experiment without end-sill, the nappe flow
into the ISB was visible and at the downstream of plunging point a supercritical current observed
which enlarges to the whole width of flume in section 2.
The huge number of minor jumps were observed at the second downstream half of ISB andconsequently considerable amount of air was entrained into the water body that is dragged far
downstream of ISB. It means that the ISB length and end-sill height was not sufficient to force thejump to occupy within ISB. While by increasing the end-sill height to 10cm, the toe of the jump draws
back toward the FMD and the jump stabilize within limited area of ISB.

without end-sill-Section

250 cm/sec

Fully end-sill 6cm height

Fully end-sill 10cm height

Vertical distance from the apron of ISB(mm)300

Section 1

250

Section 2

Section 3

Section 4

Bottomoutlet200
150

Positive
step

100
50
0
0

300

600

900

1200

1500

Horizontal distance from the outlet (mm)Figure 3: The mean stream-wise velocity (Uave) distribution along the centerline of ISB for different
end-sill heights, in case of L= 125cm, s= 15cm, Fr= 5.5.
Fig. 4 depicts the mean transversal velocity (Wave) distribution on four predefined cross sections
along the centerline of ISB. As can be seen, in case of fully end-sill with 6cm height, transversal
reverse flow occurred in deep area of basin where close to the positive step. In this condition, thewater surface was slightly tended to the left side and creates an asymmetric hydraulic jump. The
oriented hydraulic jump to the left side wall hit the wall and deflected back to the center part of ISB.
Then a vertical vortex was created in the section number 4.
Further increase in end-sill height to 10cm, creates two long horizontal eddies as secondary
subcritical flow goes back toward the FMD in both lateral side of flume. Fig. 5 shows the variation of
the maximum stream-wise velocity (Uave) along the ISB centerline. Interestingly, slit-type end-sill with
5 cm free space at both lateral side and 10cm height create a symmetric jump orthogonally to thechannel axis with approximately the same velocity reduction with fully end-sill 10cm height.
Without end-sill

100 cm/sec

Fully end-sill 6cm height
Fully end-sill 10cm height

Vertical distance from the apron of ISB (mm)300

Section 1

250
200

Section 2

Section 3

Section 4

Bottomoutlet150

100

Positive
step

50
0
0

300

600

900

1200

1500

Horizontal distance from the outlet (mm)Figure 4: The mean transversal velocity (Wave) distribution along the centerline of ISB for different
end-sill heights, in case of L= 125cm, s= 15cm, Fr= 5.5.

Without end-sill
Fully end-sill 6cm height
Fully end-sill 10cm height
Slit-type end-sill 10 cm height 5 cm free space width

4

Section 1

3.5

Section 2

Section 3

Section 4

Velcoity (m/sec)

3

2.5
2
1.5
1
0.5
0
0

0.3

0.6

0.9

1.2

1.5

Horizontal Distanc from the Outlet (m)Figure 5: The variation of maximum stream-wise velocity (Umax) along the ISB centerline, in case of
L= 125cm, s= 15cm, Fr= 5.5.

4.

CONCLUSION

The new concept of stilling basin as well as end-sill for FMDs has been proposed in this study,
which could be environmentally acceptable from the point views of fish and sediment passages. The
velocity reduction of stream-wise flow affected by different end-sill heights and width was
experimentally investigated and the following results can be concluded:
1- The presence of end-sill at the end downstream of ISB could stabilize the hydraulic jumpsymmetrically.
2- The taller fully end-sill can effectively reduce the magnitude of velocity within ISB compare to
the shorter one.
3- Considering two free spaces at the lateral side of end-sill (slit-type) shows the almost equal
function for velocity reduction with fully end-sill and positively provides additional effects forfish and sediment passing.
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