京都大学防災研究所年報 第 54 号 B 平成 23 年 6 月
Annuals of Disas. Prev. Res. Inst., Kyoto Univ., No. 54 B, 2011

Eco-friendly Hydraulic Design of In-stream Flood Mitigation Dams

Sameh KANTOUSH, Tetsuya SUMI and Mohammad MESHKATI** Graduate School of Engineering
Synopsis
Recently, huge floods have been experienced more than the past at the urban area.
In order to survive the urban areas against the floods, it could be necessary to deep
attention to Flood Mitigation Dams (FMDs). This paper deals with design andclassification of FMDs worldwide and compares several case studies in Japan andAustria. Finally, a new concept is presented for one of the most important energy
dissipating structures downstream of FMDs, named In-ground stilling basin (SB). As aresult, several unique points which are the outcome of field investigations are presented
to improve performances of FMDs.
Keywords: Flood mitigation dam, Stilling basin, Bottom outlet, Eco-friendly design.

1.

Introduction

1.1 FMDs Definition
Urbanization creates the sharp growth in the valueof property and the number of infrastructures at theflood plain areas. Recently, huge floods
experienced even more of past at the urban area.
There are a wide range of hydraulic engineeringsolutions have been constructed for centuries toprovide flood protections. These traditional
approaches predominately utilize hard engineeringsolution to protect from overflows and ensure quick
outflow of flood volumes. Rivers are channelized,
diverted, straightened and corseted in levees, withlittle or no thought for river dynamics andbiodiversity preservation. Recently, this is widely
criticized (E.U. Commission, 2004). Firstly,
accelerating the flow often results in turbid water
discharge and significant erosions in downstream
reach. Secondly, multipurpose dam interrupt thecontinuity of natural sediment patterns and change
the flow regimes at the downstream reaches ofreservoirs, hence causing erosion or deposits. And
finally, the consequences on ecosystems are often

disastrous. How to both protect citizens from floods
and biodiversity from flood-management schemesis very important issue (Geilen et al., 2004). In
Japan, numbers of new multipurpose dam
construction projects are very limited.
FMD could be a good solution in dam engineering
for sustainable management of reservoirs,
downstream river environment, and sedimenttransport. The definition of FMD is gateless outlet
dam designed only for the purpose of flood control
whose bottom outlets are installed at the original
bed level of river. FMD is expected to beenvironmentally friendly, since almost all incoming
sediment during flood periods can pass through its
bottom outlets that designed at the original river
bed level and there will be fewer impacts todownstream river environment.
Lempérière, (2006) has pointed out that ‘Futuredams may generally be multipurpose, but dams
devoted only to flood mitigation which arecompletely dry except for a few weeks per century
may be very acceptable environmentally; theirdesign may be quite different from multipurpose
dams and their cost much lower for the same

― 735 ―

Fig. 1 Features of designing and operating of flood
mitigation dams (Kantoush and Sumi, 2010)
1.2 Current FMDs Classification
Dams designed only for the purpose of flood
controlhave
different
definitions
andclassifications. In USA they are called “Dry Dam”,
in Europe “Flood Retention Basin”, in Japan “In
stream Flood Control Dam”, in other countries“Flood Mitigation Dam”. Several definitions for thesame hydraulic structure are given in different
countries and languages whose frequently leads tolegal disputes and misunderstandings between
planners, engineers, politicians and people.
Additionally, the absence of a universal definitionand classification scheme for FMD leads toconfusion about the status of individual structures
and their functions. A classification scheme for
FMD is therefore timely and urgently required toassist communication. Scholz and Sadowski (2009)
proposed a conceptual classification model basedon 141 sustainable flood retention basins (SFRB)
including 75 diverse wetland systems in the River
Rhine Valley, Baden, Germany. Six SFRB types
were defined based on the expert judgment ofengineers, scientists and environmentalists. The
Germanflood
retention
basin
guidelines(ATV-DVWK 2001) distinguish between passive
and automated, and small, medium and large sizedflood retention basins. In Germany the complete
design and therefore the design flood discharges(DFQ) and flood water levels (FWL) are dependingon the classification of the basins in the four classes
very small, small, medium and large basins. The
classification according to (DIN 19700-12/2004)
takes into account the height of the dams and thereservoir volume. Eighty flood retention basins
contribute significantly towards minimizing flood
hazard especially in small watersheds in the land of
Styria in the republic of Austria. Basins have beenclassified as small, medium and large according tocapacities range from 5,000 m3 to 1,650,000 m3,
the dam heights are between 2.0 m and 24.5. The
classification for flood retention basins according tomentioned code is given in (Fischer andHaselsteiner, 2008). Flood retention basins may
also be either online or offline with respect to the
local stream. Finally, the type of outlet isoccasionally used as a classification criterion. Sumi
(2008) has classified FMD based on outlet
arrangement: 1) installing regulating gates inbottom outlets or not and 2) securing continuity of
the river through these outlets or not. Fig. 2 shows
the relationship between gross storage capacity anddam height of FMDs in Japan, Switzerland (Ordendam) and USA.
200
Dam Height （m）

storage’. There are still several unknown factors in
FMDs design issues such as: sediment trap rates,
patterns and flow regimes in the upstream of thedam, number of bottom outlets, and SB dimensions(height, length, width), depending on floodhydrograph and water level. The features of FMDs
are drawn in Fig. 1 based on different points ofview such as hydraulic design, reservoir sedimentmanagement, ecosystem and clogging of bottom
outlets (Kantoush and Sumi, 2010).

USA
Switzerland

Japan
Austria

Seven Oaks

150
Small dams

Medium dams

Large dams
Asuwagawa

100

TatenoMount Morris

Tatsumi50

Orden
PradoEnglewood
Gabriachbach1
Ligistbach
Masudagawa
Germantown
Felberbach
Lafnizt-Reinbergwiesen
HuffmanSepulveda Lockington TaylorvilleLafnizt-Waldbach

0
0.01

0.10

1.00
10.00
100.00Reservoir Capacity（106m3）

Fig. 2 Gross storage capacity and dam height of
FMD (Sumi, 2008)
Because of geographical conditions, there is large
difference between dams. Reservoir capacities to
dam height of dams in USA are very large since
they constructed in mild river slope and wide
valley. Recently, relatively large dams for flood
mitigation for urban areas have been planned andconstructed. Masudagawa dam is one of them.

― 736 ―

Finally a new concept for SB downstream of FMD
presented, named In-ground Stilling Basin(In-ground SB), then, by considering the newconcept of SB and applying the Katakam andRama’s approach the new updated design for SB
downstream of Masudagawa FMD proposed.

1.3 Objectives and Targets of this researchThis paper deals with three main objectives, firstly
the paper summarized field investigations forseveral flood mitigation structures in Styrian
province (Austria) and comparison between
Austrian and Japanese experiences, from several
points of view such as dam structural and hydraulicdesign,reservoir
sediment
management,
maintaining of ecosystem and land management inreservoir area, clogging problems of bottom outlets.
Secondly Masudagawa FMD well introduced as aconsiderable case study in Japan to highlight its
design strengths and drawbacks.

2.

Field Investigation in Austria

2.1 Key parameters
Table 1 summarizes and characterizes all relevant
key classification variables for FMDs in Japan andAustria, these key points mainly involved: rainfall;

Table 1 Classification and comparison of Flood Mitigation Dams in Austria and Japan
Item

AustriaJapan

Names of field investigation
flood mitigation basin and dams

12 dams (Bärndorfbach, Dobelbach,
Felberbach, Gabriachbach1 & 2,
Labuchbach, Lafnizt-Reinbergwiesen,
Lafnizt-Waldbach, Ligistbach,
Sauhalt-bach, Stullneggbach,Gamlizbach)8 dams (Sotomasuzwe,
Rentaki, Kawachi, Matsuo,
Sagatani, Ootouge,
Sasakura, Takaono)

Dam Height (min-max)

5.8-23.2 m

17-37.7 m

Dam Length (min-max)
Gross Capacity (min-max)

84-241 m
14,000-1,100,000 m3

63.6-169 m
186,000-6,500,000 m3

Catchment area

0.8-162 km2

5.5- 16.8 km2

Dam arrangement in river basin

Distributed set of dams

Concentrated dam

Mean Annual Rainfall

865 mm/yr

1700 mm/yr

Utilization of reservoir area

Playground, habitat

Playground

Fish passages

Well design (Stepped
natural sun light)

Screen system design

Bar pitches are designed by guideline

Under development

Design Flood frequency

Return period is 30-50 years

Return period is 80-100
years

Outlet Arrangement

Only one with bypass outlet for
emergency
With gate (Automatic and Fixed
opening)

Usually two bottom outlet

Stilling basin design

In-ground stilling basin and hydraulicjump

Hydraulic jump with end
sillConstruction material

Earth fill with concrete outlet sections

Landscape planning

Well match with nature

Mainlyconcretegravity dams
Under development

River and basin bed gradient

Mild slope

Steep slope

Sediment load

Medium sediment yield

High sediment yield

Reservoir sedimentation

Less deposition

Less deposition

Gate Operation― 737 ―

ladderwith

Under development

Usually gatelessfordam height, dam length; dam arrangementconcentrated or distributed; construction material,fish passage; screen system; basin and channel
connectivity; design flood frequency; outlet
arrangement; gate operation; catchment size;
stilling basin design; and landscape planning andaesthetic. In the republic of Austria, Styrian
government has actively constructed FMD from1960s. More than 100 dams are located at small
tributaries nearby city of GRAZ and mountainregions. In 1992, an interesting guideline forplanning, designing and operation of FMD which
explains engineering, economical and ecologicalaspects is published. The examined Styrian basinscapacities range from 14,000 m3 to 1,100,000 m3,
the dam heights are between 5.8 m and 23.2 m, andthey were all earth fill dam combined with concrete
outlets.
While the Japanese Flood mitigation dam heights
are between 17m to 37.7 m from concrete, thereservoir
volumeisranging
between3
186,000-6,500,000 m . During our visit, we havediscussed several unique points in Styrian casestudies which will be very much valuable toimprove performances of FMDs.

2.2.2 Safeguard for clogging of bottom outlet
Preventing from clogging by floating woods or big
stones, all bottom outlets are covered by screens.
These bar pitches are ranging from 15 to 50 cm
based on design discharge of bottom outlets as Fig.
4. These screens are installed at not only inlet levelbut also on top of the bottom outlets to maintainenough discharge for safeguard Fig. 5. Periodical
cleaning or screen design modification is requested
because unexpected water storage is occurred by
sediment and tree leaves trapping.

(b)

(c)

Fig. 3 Bottom outlet design: (a) Lafinizt-Waldbach,(b) Ligistbach and (c) Labuchbach

Screen bar pitches (m)2.2 Unique points of Styrian flood mitigation
dams
2.2.1 Bottom outlet design
Based on the guideline, bottom outlets are all gated.
They are classified into (a) fixed small gate
opening, (b) closed gate with small gate opening
section, and (c) circular small diameter with
automatic gate as shown in Fig. 3. All gates aredesigned large enough for maintenance.

(a)Design discharge (m3/s)

Fig. 4 Screen bar pitches based on design dischargeof bottom outlets― 738 ―

2.2.4 Reservoir area design
Total landscape design in reservoir area is welldiscussed with local communities and experts.
Biotopes are designed in reservoir area for riverrestoration Fig. 7. In a reservoir, swimming pool iscreated for recreational use.

Fig. 5 screens installed at the inlet of bottom outlet

Fig. 7 Reservoir area and biotopes on the right side

Fig. 6 Natural fish passages in bottom outlet
approach channel equipped by stones and sunlight

2.3 Future Challenges for FMD in Worldwide
Recent flood events in Austria and Japan haveshown the need for improved flood mitigation(retention) dams along the rivers. Therefore, several
further research works is needed to update
planning, designing and operating of flood retention
dams. The FMD individualized and characterized
for three future challenges parts have to be studied,reservoir area and the inlet, outlets and gate
operation, and stilling basin with downstream reachof the dam.

2.2.3 Fish passage
Bottom outlets are also designed for fish passage.
Big stones or stepped pools are used to create
natural stream in the channel by reducing velocity.
Natural sunlight is also introducing to the channel
by mesh opening at both upstream and downstreamsides Fig. 6.

2.3.1 In upstream reservoir and the dam inlet
During the flood discharge retardation, thecharacteristics of sediment (sand and gravel)
outflow rate are changeable and unknown comparedwith the normal stage. The degree of change varies
according to flood control plans, inflow sediment
properties, and scale of the flood, so dams must be― 739 ―

studied individually. Moreover, the development ofa prediction and an optimum management measureof sedimentation in flood mitigation dams should be
investigated.

and flood inundation as well as the human andsocio-economic issues of planning, development
and design.
3.

2.3.2 Outlets and gate operation
As measures to mitigate changes of sedimenttransport properties, the geometry of the outlet
works and stilling basins should be further studied
in order to smooth the fish and sediment passages atthe end of the flood period. The most effective
approach is to accept variability of the reservoir
water level less frequently within a range thatsatisfies flood control plans. By expanding thecross-section of outlet works installed on theelevation of riverbeds, it is possible to raise thereservoir level less frequently. But, in Japan, thepeak cut rate of flood at dam site is generally large,
so in order to achieve a flood control plan, it isnecessary to make the outlet works section small
when the reservoir water level is raised. The
measure that is considered at this time is to installlarge outlet works for sediment discharge andseparate small outlet works for flood control, andswitch over from the former to the latter during
flood control. To rationalize equipment andsimplify its operation during a flood, at normal
times, a large cross-section ensures the movementof sediment, stream, and aquatic life. But for flood
periods, discharge equipment that permits theoperation of gates to reduce the flow section,
thereby controlling the flood discharge, should be
developed. In that sense, automatic gate in Styrian
examples are one of possible solutions for small
discharge.
2.3.3 Stilling basin and downstream reaches
To improve current design method of outlets andstilling basins effective dissipation of energy,
optimal design of stilling basin leading to optimal
geometry is required. The main question is what are
the optimal SB configuration and upstream
conditions to maximize the energy dissipation, fishpassage and minimize the cost? An optimal stillingbasin geometry with acceptable flood risk therefore
requires a holistic approach, addressing the flow
parameters, design flood, upstream water level inthe reservoir, dissipation energy, rivers, ecology

In-ground SB concept

Despite of extensive studies on hydraulic jump in
SB (Hager et al., 1986; Ohtsu and Yasuda, 19991;
Sumi and Nakanishi, 1991; Moosa et al., 2003), a
few research investigated the hydraulic jump within
In-ground SB.The classical SB consists of prismatic,
rectangular and nearly horizontal basin which isdesigned by implementing classical hydraulic jump
approach. This kind of SB needs a relatively highdownstream tail-water depth to ensure transition
from supercritical to subcritical flow and reduce theerosion at the downstream river bed. By combining
sudden enlargement and abrupt drop, thenon-prismatic type of SB is created similar to a pooldownstream of FMD as shown in Fig. 8. This typeof SB can simplify sediment and fish transports
where the SB bed is covered by large rocks andboulders naturally or artificially.

BottomOutletIn-ground SB

Original
Riverbed
Fig. 8 The schematic view of In-ground SB
downstream of FMD
3.1 Definition of Main Parameters
When drops is combined with an enlargement, thetypical hydraulic phenomena of each measureoverlap, reciprocally influencing each other, andproducehydraulic
jumps
whose
overallcharacteristics are very complicated (Ferreri, 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. Fig. 9 shows
the schematic side and plan views of theconstructed model and the main hydraulicparameters involved in this study. Energy loss (HL)

― 740 ―

in the In-ground SB in clear water phase may
depends on parameters such as: outlet velocity atthe bottom outlet (U0), width of outlet (b), height ofoutlet (h0), step depth (s), width of SB (B), lengthof SB (L), sequent depth (h2), end-sill height (D),water density (ρw) and gravity acceleration (g):, ,, , B, ,, ,,3.2 Katakam and Rama’s approach
The most similar research to our In-ground SB
concept was carried out by Katakam and Rama
(1988). Equation 4 was proposed by them tocalculate the ratio of sequent depth to the height ofoutlet:(1)(4)

Thus, the relative energy loss (HL/H1+s) may be
written as a function of the following dimensionlessparameters:, , , , , ,(2)where H1 and F1 are respectively total energy andFroude number at the face of bottom outlet to theSB, y is the ratio of sequent depth to the height ofoutlet (h2/h0), γ is the ratio of outlet width to theoutlet height (b/h0), β is the expansion ratio (B/b), S
is the drop number or in other 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 be defined as in Eq. (3) where hc is thecritical water depth over the end-sill (Fr =1).

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

1

(5)

3.3 Masudagawa FMD and SB
Masudagawa FMD was completed in 2007, Japan.
This dam is designed with probable flow rate of 100
years return period (Qpeak of flood = 640 m3/sec) at
the Masuda River. Two gateless bottom outlet(h03.4×b4.4×2) and overflow spillways are
installed. For energy dissipation, a conventional
hydraulic jump type SB with an end-sill (D =3 m)(3)Water Surface

Side view

In-ground SB
End-sill Flap gate

Plan viewFig. 9 the schematic side and plan views of the constructed model― 741 ―

was designed where two slits are installed for
self-sediment flushing from the SB as shown in Fig.
10. Design of Masudagawa SB is very close to thepresented In-ground SB concept; because the bed
level of SB in Masudagawa FMD is located 4 mlower than the level of its bottom outlet where apool below this FMD is formed. But, there are threemain differences between Masudagawa SB design
and In-ground SB concept: first, the bottom outletof Masudagawa FMD is connected to SB by a ramp
(not abrupt drop).
Upstream side

Slits

SB

Downstream side

Fig. 10 Bottom outlet and SB of Masudagawa
FMD.

Second, there is an end-sill at the downstream endof Masudagawa SB while in the In-ground SB
concept intends to eliminate this end-sill. Third, theoriginal river bed elevation at the downstream areaof In-ground SB concept would level with its
bottom outlet, in contrast to Masudagawa SB
design. This is noteworthy to mention that, thewidth of SB was designed according to the width ofthe downstream river channel (B=30 m). Detaildesign of Masudagawa FMD is shown in Fig. 11.
3.4 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 forfour flood discharge return periods 35, 50, 75 and100 years. To re-design of SB, Equation 4 was used
to calculate the ratio of sequent depth (h2) to thebottom 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 forassessing the design.

Fig. 11 Detail design of Masudagawa FMD― 742 ―

0.25
B/b=8.5, b/h0=0.4

Fr 2.5

B/b=7.5, b/h0=0.5

0.2

B/b=6, b/h0=0.8
B/b=5, b/h0=1.2

D/B

0.15

B/b=4, b/h0=1.8
B/b=3.5, b/h0=2.4

0.1

0.05

0
0

0.5

1

s/h0

1.5

2

2.5

Fig. 12 Variation of drop number (s/h0) versus therelative end-sill height (D/B) for different geometryof SB and different bottom outlet dimensions
0.2
Fr=3.9B/b=3 b/h0=3.3

Fr=3.30.15

D/B

Fr=2.7Fr=2.3

0.1

0.05

0
0

0.6

1.2

1.8

2.4

3

s/h0
Fig. 13 Variation of drop number (s/h0) versus therelative energy loss for different geometry of SB
and different bottom outlet dimensions.

Fig. 12 shows the variation of drop number (s/h0)
versus the relative end-sill height (D/B), fordifferent geometry of SB. As illustrated in Fig. 12,
for a given geometry of SB and specific dimensionof bottom outlet, by increasing the drop number(s/h0) the relative end-sill height (D/B) decreased.
In other words, increasing the step depth (s) allows
reducing the end-sill height and consequently
facilitates the fish migration.

By contrast, the greater drop number (s/h0) wouldslightly reduce dissipation of energy in the SB
which it could be neglected. 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) in
Fig. 12, the lowest relative end-sill height (D/B)
occurred when the width of bottom outlet (b) isgreater than the height of bottom outlet (h0),horizontal rectangular shape, b/h0>1.
In other side, when the width of bottom outlet (b) issmaller than height (h0), the relative energy loss
was increased, vertical rectangular shape, b/h0<1.
Therefore, it is necessary to find the optimal step
depth (s) in order to satisfy both conditions ofsmaller end-sill height (D) and more dissipation ofenergy (REL) within SB. One practical solution
could be equipping SB apron with rocks andboulders 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 2 shows thecomparison between Masudagawa SB design with
its re-design conducted according to Katakam andRama’s approach. As can be seen in Table 2, therelative 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 In-ground SB concept could have moreadvantages than other types of SB.
Fig. 13 shows the variation of drop number (s/h0)
versus the relative end-sill height (D/B), fordifferent flood return period (in other words
different Froude number) when the geometry of
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 isneeded for greater Froude number.

Table 2 Comparison between Masudagawa SB design and re-design proposed in this study

Cases

SB width (B)Bottom outlet
dimensions

End-sill
Step depth (s)height (D)Relativeenergy loss
(REL)

Masudagawa
FMD
Re-design
In-ground SB

30 m[h03.4×b4.4 m]×2

Ramp 4 m

3m

18%

30 m

[h03.5×b8.5 m]×1

Abrupt 6 m

2.2 m

46%

― 743 ―

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 that considered
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. One of the otherdrawbacks of Katakam and Rama’s approach isusing the narrow range of database to develop theirempirical equations. Especially the effect ofgeometry of SB (step depth, width of SB) on flow
pattern field was not well considered.
3.5 Conclusions and future challenges
The new concept of SB has been proposed in thispaper, which it was analogous to a pool below the
FMD and is named In-ground SB. This type of SB
involved sudden expansion and abrupt drop
simultaneously. Katakam and Rama’s approach isused to re-design of Masudagawa 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 of Masaudagawa 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 end-sill height (D).
Taking into account the eco-friendly features of
In-ground SB and unknown factors in this concept,
further study is needed to propose new design
guidelines for SBs. Current experimental researchat DPRI aiming to evaluate several new ideas asfollowing: a) Identifying the effect of wide range ofbottom outlet and SB dimensions on flow pattern
within In-ground SB. b) Installing some separate
piers (cylindrical baffles) instead of end-sill toeliminate the obstacle against fish and sedimentpassage. c) Considering additional outlets above themain bottom outlet to create waterfall into the SB tobreak the hydraulic jump and dissipate energy offlow. d) Increasing the roughness of SB apron and
SB training walls.

Acknowledgements
Authors express their gratitude to Mr. Rudolf
Hornich for his kind guidance in our field trip andparticipation to the symposium on Flood Mitigation
Dams in Tokyo in 2010.
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Vol.33, No.6, pp.21-27.

河川環境に適合した洪水調節専用流水型ダムの水理設計

Sameh KANTOUSH・角 哲也・Mohammad MESHKATI**

京都大学工学研究科

要 旨
近年，都市域においてゲリラ豪雨の発生が顕著である。このような洪水対策として，常用洪水吐
を河床部標高に有し，洪水時に一時的に貯留するだけの洪水調節専用ダム（以下，流水型ダム）の検討事例が増加している。流水型ダムの最大の特徴は，洪水吐きを通じて流砂および生態系の連続性，特に魚類の遡上・降下の連続性を図ることにより，河川環境に対するインパクトを極力小さくすることができることである。本稿では，このような流水型ダムの設計上の課題，世界における分類，さらに日本と同様な規模の施設を数多く有するオーストリアの事例について比較検討を行った。
これら施設の現地調査により，今後の設計を改善するための示唆に富む知見が得られた。最後に，河川環境に対する適合性を高めるために，ダム直下を潜り跳水式とした新しい減勢工形式について，その考え方と水理設計上の課題について検討を行った。

キーワード: 流水型ダム，洪水調節専用ダム，減勢工，洪水吐，環境適合設計― 745 ―