Echo-Friendly Adaptation Design for Stilling Basinof Masudagawa Flood Mitigation DamM. E. Meshkati Shahmirzadi1, T. Sumi1 and S. A. Kantoush1
1

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

Abstract
During past decades the number of huge floods occurred at the urban area worldwide sharply
increased. In order to survive the urban areas against the floods, it could be necessary toconsider Flood Mitigation Dams (FMDs). One of the important energy dissipating structures
is stilling basin (SB) downstream of FMD. Combining sudden enlargement and abrupt drop
would create the non-prismatic type of SB similar to a pool downstream of FMD, named Inground SB. In this paper a new updated design is proposed for SB downstream ofMasudagawa FMD; according to the In-ground SB concept. The relative energy loss of theproposed In-ground SB is 2.5 larger than the original Masudagawa SB design. Moreover, theend-sill height (D) was slightly decreased in the modified design.

Introduction
Urbanization creates the sharp growth in the value of property and the number ofinfrastructures at the flood plain areas. Recently, huge floods experienced even more of pastat the urban area. In order to survive urban areas against the flood, it could be necessary toconsider Flood Mitigation Dams (FMD). The definition of FMD is gateless outlet dam
designed only for the purpose of flood control whose bottom outlets are installed at theoriginal bed level of river.
One of the important energy dissipating structures is stilling basin (SB) downstream of FMD.
There are several problems in the present design of SB: a) low efficiency of energy
dissipation; b) blocking the fish passage and sediment transport by the end-sill; c) high cost.
Therefore, modify and update the SB design of FMD has become important for future
research. To reach the solution for these problematics, a new concept of SB is introduced inthis paper; named In-ground SB.
In-ground SB is such a stilling basin with cross section enlargement and abrupt drop
simultaneously after bottom outlet. In-ground SB is similar to a pool just below the FMD.
Despite of extensive studies on hydraulic jump in SB (Hager and Bretz, 1986; Ohtsu andYasuda, 1991; Moosa et al., 2003; Sumi and Nakanishi, 1991), a few research investigated thehydraulic jump within In-ground SB. The most similar study about the In-ground SB concept
was conducted by Katakam and Rama (1998). Masudagawa FMD completed in 2007 is agood example for FMD in Japan. SB of Masudagawa FMD was designed by hydraulicclassical jump approach. To obtain the required tail-water depth, an end-sill was equipped at
the downstream of SB. Two slits were installed at the end-sill for flushing sediments andachieve fish passage functions (Kantoush and Sumi, 2010).
This paper presents a new concept for SB downstream of FMD, named In-ground Stilling
Basin and the main governing parameters, then, proposes the new updated design for SB
downstream of Masudagawa FMD, by applying the Katakam and Rama’s approach. The new
design and the existing on of Masudagawa SB are compared from the viewpoints of FMD
outlet dimensions and SB dimensions.

In-ground SB concept
The classical SB consists of prismatic, rectangular and nearly horizontal basin which isdesigned by implementing classical hydraulic jump approach. This kind of SB needs arelatively high downstream tail-water depth to ensure transition from supercritical tosubcritical flow and reduce the erosion at the downstream river bed. By combining sudden
enlargement and abrupt drop, the non-prismatic type of SB is created similar to a pooldownstream of FMD as shown in Figure 1. This type of SB can simplify sediment and fishtransports where the SB bed is covered by large rocks and boulders naturally or artificially.
FMD
Embankment
BottomoutletIn-ground SB

Original River
Bed Elevation

Figure 1: The schematic view of In-ground SB downstream of FMD.

Definition of Main Parameters
When a drop is combined with an enlargement, the typical hydraulic phenomena of eachmeasure overlap, reciprocally influencing each other, and produce hydraulic jumps whose
overall characteristics are very complicated (Ferreri and Nasello, 2002). An experimental
research has started at Disaster Prevention Research Institute (DPRI) of Kyoto University,
aiming to study on the flow pattern of hydraulic jump in the In-ground SB. Figure 2 shows theschematic side and plan views of the constructed model and the main hydraulic parameters
involved in this study. Energy loss (HL) in the In-ground SB in clear water phase may depends
on parameters such as: outlet velocity at the bottom outlet (U0), width of outlet (b), height ofoutlet (h0), step depth (s), width of SB (B), length of SB (L), sequent depth (h2), end-sillheight (D), water density (ρw) and gravity acceleration (g):(1)
Thus, the relative energy loss (HL/H1+s) may be written as a function of the following
dimensionless parameters:(2)
where H1 and F1 are respectively total energy and Froude number at the face of bottom outlet
to the SB, y is the ratio of sequent depth to the height of outlet (h2/h0),γ is the ratio of outlet
width to the outlet height (b/h0), β is the expansion ratio (B/b), S is the drop number or inother word ratio of step depth to the bottom outlet height (s/h0), α is the aspect ratio of SB(B/L), and δ is the relative end-sill height (D/B). Moreover, the sequent depth (h2) can bedefined as in Eq. (3) where hc is the critical water depth over the end-sill (Fr =1).
(3)

Water Surface

Flap gate
Side View

Supply Tank

Plan View

Figure 2: The schematic side and plan views of the constructed model.

Katakam and Rama’s approach to design of SB
As above mentioned, the most similar research to our In-ground SB concept was carried out
by Katakam and Rama (1988). Equation 4 was proposed by them to calculate the ratio ofsequent depth to the height of outlet:&+%&' ()* ,-.) ' */01' *(+2&,34+(4)Furthermore, Equation 5 was proposed to calculate the relative energy loss in the In-ground
SB. The output of this equation could be one of the reliable criteria to found the optimal
design of SB.
 1 6 78    :;81    :< 2 
2(5)Masudagawa FMD and SB
Masudagawa FMD was completed in 2007, Japan. This dam is designed with probable flowrate of 100 years return period (Qpeak of flood = 640 m3/sec) at the Masuda River. Two gatelessbottom outlet (h03.4×b4.4×2) and overflow spillways are installed. For energy dissipation, aconventional hydraulic jump type SB with an end-sill (D =3 m) was designed where two slits
are installed for self-sediment flushing from the SB as shown in Figure 3. Design ofMasudagawa SB is very close to the presented In-ground SB concept; because the bed level of
SB in Masudagawa FMD is located 4 m lower than the level of its bottom outlet where a pool
below this FMD is formed. But, there are three main differences between Masudagawa SB
design and In-ground SB concept: first, the bottom outlet of Masudagawa FMD is connectedto SB by a ramp (not abrupt drop). Second, there is an end-sill at the downstream end ofMasudagawa SB while in the In-ground SB concept intend to eliminate this end-sill. Third,
the original river bed elevation at the downstream area of In-ground SB concept would levelwith its bottom outlet, in contrast to Masudagawa SB design. This is noteworthy to mention
that, the width of SB was designed according to the width of the downstream river channel(B=30 m). Detail design of Masudagawa FMD is shown in Figure 4.

Upstream side

Downstreamside
Slits

Bottom OutletsSBFigure 3: Bottom outlet and SB of Masudagawa FMD.

Figure 4: Detail design of Masudagawa FMD.

Results and Discussion
In this section, the new design (re-design) for Masudagawa SB is presented by implementing
the Katakam and Rama’s approach. The re-design of SB was conducted by considering flood
peak of 100 years return period (Qpeak of flood=640 m3/sec) passed through one bottom outlet
and not two bottom outlet as the original one. Several scenarios are investigated in this study
including different geometries of SB and various outlet dimensions for four flood discharge
return periods 35, 50, 75 and 100 years. To re-design of SB, Equation 4 was used to calculate
the ratio of sequent depth (h2) to the bottom outlet height (h0). Then, by substituting thesequent depth (h2) in Equation 3, the end-sill height (D) was obtained. The relative energy
loss predicted by Equation 5 and the end-sill height (D) are two key criteria for assessing thedesign.
Figure 5 shows the variation of drop number (s/h0) versus the relative end-sill height (D/B),
for different geometry of SB. As illustrated in Figure 5, for a given geometry of SB andspecific dimension of bottom outlet, by increasing the drop number (s/h0) the relative end-sillheight (D/B) decreased. In other words, increasing the step depth (s) allows reducing the endsill height and consequently facilitates the fish migration. By contrast, the greater drop
number (s/h0) would slightly reduce dissipation of energy in the SB which it could beneglected. The values of relative energy loss (REL) in In-ground SB concept were almost
same for all scenarios which its average was equal to %55. For a given drop number (s/h0) inFigure 5, the lowest relative end-sill height (D/B) occurred when the width of bottom outlet(b) is greater than the height of bottom outlet (h0), horizontal rectangular shape, b/h0>1.

0.25

Fr1≃2.5

D/B

0.2

B/b=8.5, b/h0=0.4

0.15

B/b=7.5, b/h0=0.5

0.1

B/b=5, b/h0=1.2

B/b=6, b/h0=0.8
B/b=4, b/h0=1.8

B/b=3.5, b/h0=2.4

0.05
000.5

11.5

s/h02

2.5

Figure 5: Variation of drop number (s/h0) versus the relative end-sill height (D/B) for
different geometry of SB and different bottom outlet dimensions.
0.2

B/b=3, b/h0=3.3

0.15

D/B

Fr1=3.9
Fr1=3.3

0.1

Fr1=2.7
Fr1=2.3

0.05
00

0.6

1.2

s/h0

1.8

2.4

3

Figure 6: Variation of drop number (s/h0) versus the relative energy loss for different
geometry of SB and different bottom outlet dimensions.
In other side, when the width of bottom outlet (b) is smaller than height (h0), the relative
energy loss was increased, vertical rectangular shape, b/h0<1. Therefore, it is necessary tofind the optimal step depth (s) in order to satisfy both conditions of smaller end-sill height (D)and more dissipation of energy (REL) within SB. One practical solution could be equipping
SB apron with rocks and boulders to create greater dissipation of energy, while
simultaneously increasing the drop number (s/h0) and increasing the bottom outlet width (b)to reduce the end-sill height (D). Table 1 shows the comparison between Masudagawa SB
design with its re-design conducted according to Katakam and Rama’s approach. As can beseen in Table 1, the relative energy loss in In-ground SB is 2.5 times larger than the original
SB of Masudagawa FMD, and the end-sill height (D) is slightly decreased. Hence, the Inground SB concept could have more advantages than other types of SB.
Figure 6 shows the variation of drop number (s/h0) versus the relative end-sill height (D/B),
for different flood return period (in other words different Froude number) when the geometryof In-ground SB and dimensions of bottom outlet are constant. As can be seen in this figure,for a given drop number (s/h0), the longer end-sill height is needed for greater Froude number.
Evidently, the flood discharge design, or put differently, flood return period has a key role indesign criteria how shorter flood return period could ensure the smaller end-sill height (D).
As mentioned above, the Katakam and Rama’s approach maybe is the only research thatconsidered the In-ground SB concept. However, the proposed equations cannot directly be
used for design purpose, mainly because no attempt was made to predict the jump length. Oneof the other drawbacks of Katakam and Rama’s approach is using the narrow range of
database to develop their empirical equations. Especially the effect of geometry of SB (stepdepth, width of SB) on flow pattern field was not well considered.

Table 1: Comparison between Masudagawa SB design and re-design proposed in this study.
Cases

SB width(B)Bottom outlet
dimensions

Step depth(s)End-sillheight (D)Relativeenergy loss(REL)

Masudagawa
FMD30 m[h03.4×b4.4 m]×2

Ramp 4 m3m%18Re-design
In-ground SB30 m[h03.5×b8.5 m]×1Abrupt 6 m

2.2 m%46Conclusions and future challenges
The new concept of SB has been proposed in this paper, which it was analogous to a poolbelow the FMD and is named In-ground SB. This type of SB involved sudden expansion andabrupt drop simultaneously. Katakam and Rama’s approach is used to re-design ofMasudagawa SB. Increasing the drop number (s/h0) and installing the wider bottom outlet(horizontal rectangular shape, b/h0>1), positively reduce the end-sill height (D). Re-design ofMasaudagawa FMD according to the In-ground SB concept led to 2.5 times more energy
dissipation and 25% reduction of the end-sill height (D). Similarly, flood return period could
be one of the main design criteria so that short term flood return period may reduce the endsill height (D).
Taking into account the eco-friendly features of In-ground SB and unknown factors in thisconcept, further study is needed to propose new design guidelines for SBs. Current
experimental research at DPRI aiming to evaluate several new ideas as following: a)
Identifying the effect of wide range of bottom outlet and SB dimensions on flow pattern
within In-ground SB. b) Installing some separate piers (cylindrical baffles) instead of end-sillto eliminate the obstacle against fish and sediment passage. c) Considering additional outlets
above the main bottom outlet to create waterfall into the SB to break the hydraulic jump anddissipate energy of flow. d) Increasing the roughness of SB apron and SB training walls.

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