5th International Symposium on Hydraulic Structures
Hydraulic Structures and Society: Engineering Challenges and ExtremesISBN 9781742721156 - DOI: 10.14264/uql.2014.33

Brisbane, Australia, 25-27 June 2014

Energy Dissipation within In-Ground Stilling Basin
M.E. Meshkati Shahmirzadi1, T. Sumi1 and S.A. Kantoush2
1

Disaster Prevention Research InstituteKyoto University
Gokasho, Uji, Kyoto, 611-0011
JAPAN
2
Civil Engineering Department
The German University of Cairo
EGYPT
E-mail:meshkati.shahmirzadi@gmail.com
Abstract: We investigated the hydraulic characteristics of an in-ground stilling basin (ISB) below a
flood mitigation dam (FMD). The ISB structure is adapted to be ecologically transparent for fish
migration and sediment transport through consideration of two lateral free spaces at the ISB end. We
attempted to assess the energy dissipation performance of ISB by considering the interdependence
between ISB geometry and flow pattern. The experimental results evidence there is a significant
correlation between flow patterns and ISB geometry.
Keywords: In-ground stilling basin, energy dissipation, flood mitigation dam, eco-hydraulic structures.

1. INTRODUCTION
Flood mitigation dams (FMDs) in the normal condition permits the flow pass through the bottom outlet
unobstructed. Meanwhile, a large percentage of river inflow can be temporarily stored within thereservoir during the flood events (Sumi, 2008). Stilling basin, thus, is vital to dissipate the energy ofoutflow from FMDs to protect downstream reaches from unfavorable degradation and side bankerosions (Hager, 1992). However, the function of stilling basin in FMD is not only to dissipate theexcess energy of the outflow, but also to let the sediment and fish pass through the structure
unopposed (Meshkati Shahmirzadi & Sumi, 2013). Figure 1a and b depict the Masudagawa FMD in
Shimane prefecture, Japan and its stilling basin, respectively. Tall and full width end-sill at the end
downstream of stilling basin brings contradictory goals for FMDs. On the one hand, tall and full width
end-sill can positively confine the hydraulic jumps within a limited area that finally lead to an
economical design. On the other hand, a tall full width end-sill itself negatively acts as an obstacle forsediment supply and fish migration; and it would increase the deposition of sediment within stillingbasin that would build up over time. In order to decrease necessary end-sill height and to keep thesuitable water depth during the normal river condition, an in-ground stilling basin (ISB) can be analternative for the conventional stilling basins. Additionally, lateral free spaces in end-sill may increase
the sediment sluicing and fish migration rate.
a)b)

BottomOutletEnd-sill

Figure 1a and b– Masudagawa FMD and its stilling basin

In spite the fact, there are enormous numbers of researches on hydraulic jumps within stilling basin(Sumi & Nakanishi, 1991; Chanson & Montes,1995; Katakam & Rama, 1998; Ohtsu et al., 1999;
Larson, 2004; Pagliara et al., 2008; Zare & Doering, 2011), 3D nature of energy dissipation process atthe presence of forced hydraulic jumps within simultaneous combination of sudden lateralenlargement, abrupt drop and end-sill is not yet solved in the literature. Fulfilling the gap of knowledge
under this topic was the major motivation to conduct this investigation. Present research is unique due
to covering a various geometrical and hydraulic variables such as Froude number (Fr1), normalized
ISB length (L/B), normalized ISB depth (s/h1), normalized end-sill height (he/hc) and normalized freespace width (1-(be/B)). The governing absolute parameters e.g. h1, hc, be and B are defined in section
2. In the present study we aim to assess the interaction between ISB structure geometry and theenergy dissipation within ISB. The possible hydraulic jumps types within ISB are classified based on
their surface pattern and water surface profile. Finally, the most effective ISB geometry structure waspresented in term of higher velocity reduction, and better stability and symmetry of the flow patterns.

2. EXPERIMENTATION
Figure 2 provides the schematic plan and side view of our physical model and the main governingparameters considered in this study. The range of Froude numbers (Fr1 (Fr1=U1/(gh1)0.5, U1 is theoutflow velocity from the bottom outlet in FMD, g is the gravitational acceleration and h1 is the bottom
outlet opening height) that we examined was from 2.8 to 5.1. The corresponding values of the criticaldepth (hc) were obtained through ((Q/B)2/g)1/3, where Q is the outflow discharge from FMDs bottom
outlet and B is the width of channel. As the width of the bottom outlet in FMDs are considerably
smaller than the width of the downstream river channel, it creates a small but sudden expansion ratio.
The value of this ratio (k=b1/B, b1 is the width of bottom outlet) ranges from 0.1 to 0.3 in real life. Thus,
we selected a given ratio for our physical model equal to 0.2, representing the actual average value ofthe expansion ratios in FMDs. Experiments were carried out under different geometrical and hydraulicconditions as: ISB length (L= 75, 100 and 125 cm), ISB depth (s= 5, 10 and 15 cm), different geometryof end-sill with various heights (he= 0, 4, 8, 12 and 13.5 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; in addition to only one dimension of bottom outlet (h1= 5 cm and b1= 10
cm).

Main
Main
Reservoir
Reservoir

Hw

I

II

U1

U2

h3

h1

h3

he
h2

s
a) Side viewFree
spacesLb1b) Plan view

B

be

ISB

Figure 2 – The schematic side and plan view of physical model3. RESULTS AND DISCUSSION
3.1. Analysis of Non-dimensional Governing Parameters
This section aims to identify the effect of various combinations of ISB geometries on ISB performance.
The effect of different geometrical parameters on velocity reduction has been investigated. It isnoteworthy to mention about the formulation of the naming system for each test configuration in thepresent research. In legend box of each figure three successive capital letters can be seen. The
naming system is very simple to learn and useful to be used for comparison of the different cases
easily. The three capital letters e.g. MMM, give in succession qualitative description of the length of
ISB, height of end-sill and ISB depth respectively. ‘M’ in the example just shown refers to a medium
value. To give a further example, LTD is the case where a Long ISB, Tall end-sill height and Deep ISB
has been examined. The table below provides the definition of each abbreviation used here in thisstudy and its relevant values.
Table 1- The definitions of the qualitative abbreviations used in the present study
Relevant normalized
Abbreviation

Meaning

Example
ISB length

ISB end-sill
ISB depth
heightS

Short, Small,
Shallow

Short ISB length, Small end-sill height,
Shallow ISB depth

1.5

0.8

1

M

MediumMedium ISB length, Medium end-sillheight, Medium ISB depth

2

1.7

2

D

Deep

Deep ISB depth

-

-

3

T

Tall

Tall end-sill height

-

2.8

-

T’

Very tallvery Tall end-sill height

-

3

-

N

Nothing(without)

No end-sill

-

0

-

L

Long

Long ISB length

2.5

-

-

3.1.1. The Effect of Normalized End-sill Height
To identify the effect of end-sill height on performance of ISB, three series of tests were designed inwhich all parameters were constant and the end-sill height and ISB depth varied (Figure 3). In case of
the experiment without an end-sill (e.g. MND or MNS), the normalized velocity reduction remained
very low around 25%; when even increasing the ISB depth could not considerably improve the ISB
performance. While for the cases with a taller end-sill (M, T and T’), the magnitude of normalized
velocity reduction increased up to 60% of the initial velocity.
However, this figure indicates that increasing the height of the end-sill from T to T’ (from tall to verytall) did not enhance the performance of ISB regarding velocity reduction. Thus, it can be concluded
that although a taller end-sill could dissipate more energy than the shorter, increasing the end-sillheight above an optimum value has no enhancement on the performance of ISB. The normalized endsill height, he/hc, greater than 2 may be can provide an acceptable velocity reduction around 60 to 70%
within ISB. Furthermore, as can be seen in Figure 3, for a constant normalized end-sill height, he/hc,
deeper ISB e.g. M(X)D and M(X)M just slightly increased the normalized velocity reduction incomparison with M(X)S. Therefore, deepening the stilling basin may not be economically justified
since it does not lead to a significant impact on energy dissipation within ISB.

1
M(X)D
M(X)M
M(X)S

U(U
1-UU
2/U1
0- 2)/U0

0.75

0.5

0.25

0
0

1

2
hhee/h/hc c

3

4

Figure 3- The variation of normalized velocity reduction versus the normalized end-sill height

3.1.2. The Effect of Normalized ISB Length
Figure 4 illustrates the variation of normalized velocity reduction versus the normalized ISB length fordifferent combinations of end-sill height and ISB depth where Fr1= 4.3. The normalized velocity
reduction is defined as U1-U2/U1 between sections I and II (see in Figure 1). U1 is the initial bottom
outlet velocity and U2 is the largest average velocity measured just upstream of the end-sill. This figure
illustrates the ISB performance regarding velocity reduction consistently enhanced by increasing the
ISB length, in particular in case of taller end-sill height. In general, in presence of taller end-sill (e.g.
(X)TM and (X)TD) the rate of velocity reduction was relatively larger compared to those cases withshorter end-sill height (e.g. (X)SM and (X)SD). Therefore, by considering a medium height to tall endsill we may are allowed to reduce the ISB length. Reduction of the ISB length is cost effective.
Moreover, the longer the ISB, the higher possibility for formation of periodic and instable submerged
jump within ISB, which is not favorable for hydraulic structure designers. The formation of different flow
pattern within ISB as function of ISB geometry is discussed later in this paper. Considering the
economical aspects in design as well as our desire to have a steady stable flow pattern within ISB, amedium normalized ISB length equal to 2 (L/B=2) is preferred. A very short ISB may not be adequate
to be considered for design, L/B=1.5, due to their relatively low normalized velocity reduction, lessthan 60%, when a medium end-sill height is installed at their downstream (Figure 4). In presence of ashort ISB the plunging jump into ISB has not enough space to be fully developed. Hence, thehydrodynamic force may cause failure of structure in case of extreme flood events.

0.8
(X)SM
(X)SD
(X)MM

0.6

(X)MD

U
(U
1-U
2/U
1 0
0-U
2)/U

(X)TM
(X)TD

0.4

0.2

0
0

1

2
3
L/B
Figure 4- The variation of normalized velocity reduction versus the normalized ISB length
L/B3.1.3. The Effect of Drop Number
The effect of drop number s/h1 has been studied herein. Based on the classification of Ohtsu andYasuda (1991) on drop types, the drop numbers that we examined in this study fall into lowe drops,
0.5~1.5 ≤s/h1≤8~9. Figure 5 depicts the normalized velocity reduction within ISB for different drop
numbers. The ISB performance displayed two different behaviors with an increase in drop number. In
case of a shorter end-sill height e.g. MS(X), by increasing the drop number the ISB performance
regarding velocity reduction is improved. However, for taller end-sills (MM(X) and MT(X)), in general,
by deepening the ISB no significant effect on ISB performance was found.
1
MS(X)
MM(X)
MT(X)

(U
U2)/U
U10--U
/U10

0.75

0.5

0.25

0
0

1

2

3

4

s/hs/h1 1Figure 5- The variation of normalized velocity reduction versus drop number

3.1.4. The Effect of Relative End-sill Width
To clarify the effect of free spaces (be, end-sill width) on velocity reduction along the ISB two groups oftests 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 on performance of those ISB with amedium end-sill height (MMM) and the second group of experiments with a taller end-sill (MTM).
Figure 6 shows the variation of normalized velocity reduction with normalized free space width.
0.7
MMM-(X)

0.6
MTM-(X)

0.4

02

U1-U(U
2/U
1 )/U
-U

0

0.5

0.3
0.2
0.1
0
0

0.2

0.4
1-(b
e/B)
1-(be/B)

0.6

0.8

Figure 6- The variation of normalized velocity reduction versus the degree of end-sill width

Considering two free spaces at both sides of the end-sill weakened the performance of ISB for velocity
reduction; however, the general trend reveals that selecting a taller end-sill with free spaces may
provide a better performance in velocity reduction than a full end-sill with no free spaces. Thus, a taller
end-sill with 20% free space of whole channel width in addition to facilitating the fish passage can offer
better ISB performance.

3.2. Velocity Reduction Performance within ISB
Figures 7a and b presents a general and 3D view of ISB performance with regard to velocity reduction
for all the successful experiments in this study. The horizontal plane in Figure 7a shows thenormalized end-sill height, he/hc, versus the normalized ISB length, L/B. However, the horizontal planein Figure 7b produces the interaction between the normalized end-sill heights, he/hc, with the drop
number, s/h1. The vertical axis in both discussed figures represents the normalized velocity reduction,
U1-U2/U1, between section I and II that is also quantified by the color shading.
Figures 7a illustrates, in an overall view, the combination of medium end-sill height, 1.5<he/hc<2.5,with medium ISB, L/B=1.5, leads to larger reduction of initial velocity about 60 to 70% of U0. However,as can be seen in Figure 7b some amount of drop number can contribute to reduce the necessary
end-sill height since that the combination of (he/hc,s/h1)=(2,2) showed the same performance invelocity reduction as (he/hc, s/h1)=(2.5,1.5).
20%
a)

b)

1
0.5

0

0
1

0.5
1

1.5

1.5
2

2

he/hc

2.5

L/B

U1-U2/U1

U1-U2/U1

U0-U2/U0

70%

0

1
0.5
1
0
0.5

1

2

he/hc

2

2.5
3

3
3.5

s/h1

1.5
3
3.5

3

Figure 7a and b- 3D view of normalized velocity reduction within ISB

3.3. Flow Pattern Detection
We utilized electric magnetic velocity meter, LSPIV technique and laboratory observation. The
possible hydraulic jumps types within ISB classified into 4 major flow patterns as: B-jump, U-jump,
Periodic submerged jump and Steady submerged jump. The definition of the different type of jumps
that we observed in the current study is well described in the past (Ferreri and Nasello, 2002). Table 2
summarizes the flow patterns with names, characteristics and ISB structure geometry that is required
to form each flow pattern. Ferreri and Nasello (2002) pointed out through the mutual effect of a drop
and an enlargement, a fully 3D and complex overlapping effect has been detected that in individual
presence of each of these measures has not been detected. Katakam & Rama (1998), but, noted thatin case of low drop number conditions, which is introduced earlier, the flow patterns within ISB is muchreferable to those that is observed in only sudden enlargement. In contrary, we found that due to thepresence of positive step at ISB downstream, the jump types are more referable to those have been
detected in only drop cases and not in an enlargement. Combination of drop and positive step createsa stilling basin homogenous with a box or a pool below the dam. This box increases the complexity offlow field within ISB through providing the condition in which plunging jet into ISB faces to two different
areas with higher and lower shear stress as well as by producing the lateral reverse flow within ISB.
We called this effect as box effect. Box effect solves the huge periodicity issues that stilling basins with

small expansion ratio are generally faced e.g. stilling basin downstream of FMDs. Although, theinfluence of drop number, s/h1, on velocity reduction was found to be not significant, it plays animportant role in defining the flow pattern within ISB with the aid of positive step.
Table 2 – Summary of major flow patterns that observed in ISB and its characteristics
Class
NumberJump Characteristics

ISB geometry
Characteristics

1

B-jump,Symmetric,
In-sufficient velocity
reduction.

When there is no end-sill
downstream of the ISB and
the ISB depth is shallow.

U-jump,Symmetric,
In-sufficient velocity
reduction.

When there is a medium
height of end-sill
downstream of the ISB and
ISB depth is shallow.

Periodic submerged
jump, Unstable,
Medium velocity
reduction.

When there is a medium
end-sill height downstreamof the ISB and ISB length isrelatively long, as well as incase of higher Fr numbers.

Steady submerged jump,
Stable,High velocity reduction.

When there is a tall end-sill
downstream of the ISB and
ISB length is relatively short.

2

3

4

Photograph

Moreover, we realized that the taller end-sill height and the shorter ISB length may be the suitablegeometry; since it could provide steady symmetric submerged jump with a higher dissipation of energy.
A taller end-sill and shorter ISB length confides the jump and provides a sequent depth that leads toformation of a complete jump. It is in consistent with the discussed results in section 3.1.3., where it
was shown that the higher rate of velocity reduction occurred when there was a taller end-sill at ISB
downstream. However, a very tall end-sill didn’t show any benefit in velocity reduction. Regarding ISB
length, the relatively long ISB negatively increased the periodicity and instability of the flow within ISB.
Therefore, a long ISB, in spite the fact that it showed higher velocity reduction performance, is not afavorable option for design purposes.

4. CONCLUSION
We systematically investigated the influence of different geometry of in-ground stilling basin (ISB), e.g.
ISB length, ISB depth, on the flow pattern and its performance in energy dissipation. Following
conclusions are presented as the main outcomes of this study:a) A taller end-sill provides better ISB performance with regard to energy dissipation and it induces thesteady symmetric jump within ISB. However, end-sill height larger than optimum range, 1.5<he/hc<2.5,
showed no more benefit regarding with velocity reduction.
b) The longer ISB, the better velocity reduction. However, long ISBs increase the periodicity andinstability of the flow. Therefore, combination of a medium end-sill height, 1.5<he/hc<2.5, with amedium ISB length, L/B=2, is recommended for design purposes. It should take into account that it isvital to avoid both very tall end-sills and long ISBs, considering the economical aspects.
c) Deepening ISB slightly improves the velocity reduction within ISB while further increase in depth
showed no more benefits, in particular in case of medium to tall end-sill. We highly recommend a
medium ISB depth for design, s/h1=2, since it allows a considerable reduction in end-sill height.
Additionally, drop number in combination with positive step at ISB downstream provides a 3D
influence on flow filed, which is called box effect. Box effect on flow within ISB positively stabilizes theflow.
d) Considering two lateral free spaces with a width up to 10% of the channel width, showedapproximately same performance in velocity reduction with no free space condition. A taller end-sillwith lateral free spaces showed higher velocity reduction than medium end-sill height with no freespace. Hence, considering a relatively tall end-sill height with two lateral free spaces at the end of ISB
may grantee an efficient velocity reduction during flood events and also facilitate fish passage in thenormal river flow condition.

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